CLAD STEEL PLATE AND METHOD OF PRODUCING SAME

Description

TECHNICAL FIELD

The present disclosure relates to a clad steel plate and a method of producing the same. The present disclosure especially relates to a high-strength clad steel plate excellent in low-temperature toughness and ammonia-induced SCC resistance and suitable for structural parts such as tanks used in low-temperature and liquid ammonia environments.

BACKGROUND

In liquid ammonia environments, carbon steel is likely to suffer stress corrosion cracking (SCC) due to liquid ammonia (hereafter referred to as ammonia-induced SCC). Hence, for structures such as carbon steel pipes, storage tanks, tank cars, and line pipes that handle liquid ammonia, steel materials with low ammonia-induced SCC susceptibility are used and operational measures are taken to suppress ammonia-induced SCC.

For example, it is known that ammonia-induced SCC is correlated with the strength and hardness of the material. In detail, it is considered desirable to use, as carbon steel, a material with a tensile strength of less than 600 MPa. Thus, in the case of using high-strength carbon steel (hereafter also referred to as high-strength steel) in a liquid ammonia environment, measures such as adjusting tensile strength by performing post-weld heat treatment by total annealing are necessary.

Liquid ammonia does not emit CO₂ when burned. Liquid ammonia is therefore attracting attention as a clean energy source in recent years, and is expected to see huge demand. This increases the need for larger equipment for transporting and storing liquid ammonia.

Typically, for larger tanks, the use of thinner steel materials is preferred from the viewpoint of weight reduction and construction cost reduction, so that it is desirable to use high-strength steel.

For efficient operation of transportation equipment and storage equipment, these equipment may be used for both liquid ammonia and LPG. Liquefied gases such as liquid ammonia and LPG are transported and stored at low temperatures. Steel materials used for such applications are accordingly required to have excellent low-temperature toughness.

Technologies related to steel materials used for such applications are disclosed in, for example, JP S55-30062 B2 (PTL 1), JP S57-139493 A (PTL 2), and JP H8-269537 A (PTL 3). PTL 1 describes a method of softening the steel material surface. PTL 2 and PTL 3 each describe a method of producing a clad steel plate having a mild steel layer on one side.

CITATION LIST

Patent Literature

• • PTL 1: JP S55-30062 B2
  • PTL 2: JP S57-139493 A
  • PTL 3: JP H8-269537 A

SUMMARY

Technical Problem

However, the method described in PTL 1 requires long heat treatment in order to soften the surface layer homogeneously and sufficiently, and has difficulty in controlling the strength at the center of the steel plate. The method thus has a problem in terms of strength.

The methods described in PTL 2 and PTL 3 produce clad steel by casting, weld overlaying, or continuous casting. Clad steel plates produced by the methods described in PTL 2 and PTL 3 cannot be considered as having all of excellent low-temperature toughness, excellent ammonia-induced SCC resistance, and high strength. The methods described in PTL 2 and PTL 3 also have an economic problem as the costs of production lines and energy are high.

It could therefore be helpful to provide a high-strength clad steel plate excellent in ammonia-induced SCC resistance and low-temperature toughness and suitable for use in tanks, etc. for transportation and storage of liquid ammonia, and a method of producing the same.

Solution to Problem

Upon careful examination on various factors influencing the ammonia-induced SCC resistance, low-temperature toughness, and tensile property (strength property) of steel plates, we discovered the following.

Since ammonia-induced SCC occurs inside the product (tank), ammonia-induced SCC resistance depends on the properties of the surface layer of the steel plate on the inside. In view of this, we came up with a clad steel plate obtained by bonding a base metal and a cladding metal, specifically, a clad steel plate formed using a steel plate with excellent strength and low-temperature toughness as a base metal and a steel plate with low hardness as a cladding metal in order to improve ammonia-induced SCC resistance.

We found that such a clad steel plate is excellent in all of ammonia-induced SCC resistance, low-temperature toughness, and tensile property.

We also found that, if the cladding metal in the clad steel plate is excessively thin, corrosion-induced wear occurs and the ammonia-induced SCC resistance of the clad steel plate degrades, and if the cladding metal in the clad steel plate is excessively thick, the strength of the clad steel plate decreases.

A clad steel plate according to the present disclosure takes significantly less alloy costs and production costs than clad steel plates using stainless steel or non-ferrous alloys as cladding metals.

The present disclosure is based on these discoveries. We thus provide:

1. A clad steel plate comprising: a base metal; and a cladding metal that is carbon steel on at least one side of the base metal, wherein the base metal has a chemical composition containing (consisting of), in mass %, C: 0.010% to 0.200%, Si: 0.01% to 1.00%, Mn: 0.50% to 2.50%, Al: 0.001% to 0.060%, P: 0.0200% or less, and S: 0.0100% or less, with a balance consisting of Fe and inevitable impurities, CE_B/CE_C is 2.000 or more, where CE_B and CE_C are respectively an equivalent carbon content of the base metal and an equivalent carbon content of the cladding metal, the base metal has a metallic microstructure in which a volume fraction of bainite is 90% or more and an average grain size of bainite is 25 μm or less, the cladding metal has a Vickers hardness of 210 HV10 or less, and t_C1 is 2.0 mm or more, t_C2 is 0.0 mm or more, and (t_C1+t_C2)/(t_B+t_C1+t_C2) is 0.30 or less, where t_B is a thickness of the base metal, t_C1 is a thickness of the cladding metal on one side of the base metal, and t_C2 is a thickness of the cladding metal on the other side of the base metal.

2. The clad steel plate according to 1., wherein the chemical composition of the base metal further contains, in mass %, one or more selected from Cu: 1.00% or less, Ni: 2.00% or less, Cr: 1.00% or less, Mo: 1.00% or less, V: 0.500% or less, Ti: 0.100% or less, Nb: 0.100% or less, Ca: 0.0200% or less, Mg: 0.0200% or less, and REM: 0.0200% or less.

3. The clad steel plate according to 1, or 2., wherein the cladding metal has a chemical composition containing, in mass %, C: 0.100% or less, and Mn: 0.01% to 1.50%, further containing, in mass %, one or more selected from Cu: 0.01% to 0.50%, Cr: 0.01% to 0.50%, Sb: 0.01% to 0.50%, and Sn: 0.01% to 0.50%, with a balance consisting of Fe and inevitable impurities, and a CR value calculated by the following formula (1) is 0.30 or more:

CR ⁢ value = 2.3 [ Cu ] + 2.8 [ Cr ] + 7.3 [ Sb ] + 3.6 [ Sn ] ( 1 )

where [X] denotes a content of element X in the chemical composition of the cladding metal in mass %.

4. A method of producing a clad steel plate including a base metal and a cladding metal that is carbon steel on at least one side of the base metal, the method comprising: heating a clad raw material plate to 1000° C. to 1250° C., the clad raw material plate being obtained by overlapping a base metal raw material plate having the chemical composition of the base metal according to 1, or 2. with a cladding metal raw material plate that is the carbon steel according to 1.; thereafter subjecting the clad raw material plate to hot rolling with a cumulative rolling reduction ratio of 20% or more in a non-recrystallization temperature range of the base metal raw material plate and a rolling finish temperature of an Ar₃ transformation point or more, to obtain a hot-rolled steel plate; and thereafter subjecting the hot-rolled steel plate to cooling with a cooling start temperature of the Ar₃ transformation point or more, an average cooling rate of 20° C./s to 120° C./s, and a cooling stop temperature of 500° C. or less.

5. The method of producing a clad steel plate according to 4., wherein the cladding metal raw material plate has a chemical composition containing, in mass %, C: 0.100% or less, and Mn: 0.01% to 1.50%, further containing, in mass %, one or more selected from Cu: 0.01% to 0.50%, Cr: 0.01% to 0.50%, Sb: 0.01% to 0.50%, and Sn: 0.01% to 0.50%, with a balance consisting of Fe and inevitable impurities, and a CR value calculated by the following formula (1) is 0.30 or more:

CR ⁢ value = 2.3 [ Cu ] + 2.8 [ Cr ] + 7.3 [ Sb ] + 3.6 [ Sn ] ( 1 )

where [X] denotes a content of element X in the chemical composition of the cladding metal raw material plate in mass %.

6. The method of producing a clad steel plate according to 4, or 5., comprising tempering the hot-rolled steel plate in a temperature range of 650° C. or less, after the cooling.

Advantageous Effect

It is thus possible to obtain a high-strength clad steel plate excellent in ammonia-induced SCC resistance and low-temperature toughness and suitable for use in tanks, etc. for transportation and storage of liquid ammonia. The clad steel plate does not use stainless steel or a non-ferrous alloy as the cladding metal and can be produced by a simple process, and therefore is very advantageous in terms of cost.

DETAILED DESCRIPTION

A clad steel plate according to one embodiment of the present disclosure comprises: a base metal; and a cladding metal that is carbon steel on at least one side of the base metal.

The clad steel plate according to one embodiment of the present disclosure has excellent ammonia-induced SCC resistance and low-temperature toughness, and therefore is suitable for structural parts such as tanks used in a liquid ammonia environment. The operating environment is not limited to liquid ammonia, and may be other liquefied gases such as LPG and liquified CO₂. In the present disclosure, the expression “ammonia, etc.” means all liquified gases including not only liquid ammonia but also LPG, liquified CO₂, and so on.

In the clad steel plate according to one embodiment of the present disclosure, the cladding metal may be provided on either side of the base metal. When the clad steel plate is used in a tank or the like, however, the cladding metal is provided on the side expected to come into contact with ammonia, etc. (hereafter also referred to as a first side (i.e. surface) or inner side (i.e. surface) of the base metal). This imparts ammonia-induced SCC resistance and low-temperature toughness to the tank (structure). In the present disclosure, the cladding metal provided on the first side of the base metal is also referred to as a first cladding metal. The other side of the base metal than the first side is also referred to as a second side (i.e. surface) or outer side (i.e. surface) of the base metal. The cladding metal provided on the second side of the base metal is also referred to as a second cladding metal. The second cladding metal is optional. In other words, the clad steel plate according to one embodiment of the present disclosure encompasses a clad steel plate having the first cladding metal on one side of the base metal, and a clad steel plate having the first cladding metal on one side of the base metal and the second cladding metal on the other side of the base metal. The thickness of the base metal is denoted as t_B, the thickness of the first cladding metal as t_C1, and the thickness of the second cladding metal as t_C2.

The clad steel plate according to one embodiment of the present disclosure will be described in more detail below. In the following description, “%” representing the content of each element means “mass %” unless otherwise specified.

(1) Chemical Composition of Base Metal

C: 0.010% to 0.200%

C is the most effective element for enhancing the strength of the clad steel plate according to one embodiment of the present disclosure. In order to achieve this effect, the C content is 0.010% or more. The C content is preferably 0.030% or more from the viewpoint of reducing the content of other alloying elements and lowering production costs. If the C content is more than 0.200%, toughness and weldability degrade. The C content is therefore 0.200% or less. The C content is preferably 0.170% or less from the viewpoint of toughness.

Si: 0.01% to 1.00%

Si is added for deoxidation. In order to achieve this effect, the Si content is 0.01% or more. The Si content is preferably 0.03% or more. If the Si content is more than 1.00%, toughness and weldability degrade. The Si content is therefore 1.00% or less. The Si content is preferably 0.40% or less from the viewpoint of toughness.

Mn: 0.50% to 2.50%

Mn is an element that has the effect of increasing the quench hardenability of steel. Mn is thus one of the important elements for obtaining high strength. In order to achieve this effect, the Mn content is 0.50% or more. The Mn content is preferably 0.70% or more from the viewpoint of reducing the content of other alloying elements and lowering production costs. If the Mn content is more than 2.50%, toughness decreases. The Mn content is therefore 2.50% or less. The Mn content is preferably 2.30% or less from the viewpoint of suppressing the decrease in toughness.

Al: 0.001% to 0.060%

Al acts as a deoxidizer. In order to achieve this effect, the Al content is 0.001% or more. If the Al content is more than 0.060%, oxide-based inclusions increase and cleanliness decreases. In addition, toughness decreases. The Al content is therefore 0.060% or less. The Al content is preferably 0.050% or less from the viewpoint of suppressing the decrease in toughness.

P: 0.0200% or Less

P is an element contained as an inevitable impurity. P causes adverse effects such as decreasing toughness and weldability by segregating at grain boundaries. Hence, it is desirable to reduce the P content as much as possible, although a P content of 0.0200% or less is acceptable. No lower limit is placed on the P content, and the lower limit may be 0%. Since P is an element that is usually inevitably contained in steel as an impurity, the P content may be more than 0% industrially. Moreover, excessive reduction of P leads to an increase in refining costs. The P content is therefore preferably 0.0005% or more.

S: 0.0100% or Less

S is an element contained as an inevitable impurity. Moreover, S is present in steel as sulfide-based inclusions such as MnS, and causes adverse effects such as forming a fracture origin and decreasing toughness. Hence, it is desirable to reduce the S content as much as possible, although a S content of 0.0100% or less is acceptable. No lower limit is placed on the S content, and the lower limit may be 0%. Since S is an element that is usually inevitably contained in steel as an impurity, the S content may be more than 0% industrially. Moreover, excessive reduction of S leads to an increase in refining costs. The S content is therefore preferably 0.0005% or more from the viewpoint of cost.

The chemical composition of the base metal in the clad steel plate according to one embodiment of the present disclosure may optionally contain the following elements (hereafter also referred to as optionally added elements). In the chemical composition of the base metal in the clad steel plate according to one embodiment of the present disclosure, the balance other than the foregoing elements and the following optionally added elements consists of Fe and inevitable impurities.

Cu: 1.00% or Less

Cu is an element effective in improving the strength of the clad steel plate. If the Cu content is less than 0.01%, the effect is insufficient. Accordingly, in the case of adding Cu, the Cu content is preferably 0.01% or more. If the Cu content is more than 1.00%, toughness degrades. Accordingly, in the case of adding Cu, the Cu content is preferably 1.00% or less.

Ni: 2.00% or Less

Ni is not only effective in improving the strength of the clad steel plate but also effective in improving toughness. If the Ni content is less than 0.01%, the effect is insufficient. Accordingly, in the case of adding Ni, the Ni content is preferably 0.01% or more. If the Ni content is more than 2.00%, the effect is saturated and the alloy costs increase. Accordingly, in the case of adding Ni, the Ni content is preferably 2.00% or less.

Cr: 1.00% or Less

Cr is an element effective in improving the strength of the clad steel plate. If the Cr content is less than 0.01%, the effect is insufficient. Accordingly, in the case of adding Cr, the Cr content is preferably 0.01% or more. If the Cr content is more than 1.00%, toughness degrades. Accordingly, in the case of adding Cr, the Cr content is preferably 1.00% or less.

Mo: 1.00% or Less

Mo is an element effective in improving the strength of the clad steel plate. If the Mo content is less than 0.01%, the effect is insufficient. Accordingly, in the case of adding Mo, the Mo content is preferably 0.01% or more. If the Mo content is more than 1.00%, toughness degrades. Accordingly, in the case of adding Mo, the Mo content is preferably 1.00% or less.

V: 0.500% or Less

V is an element that has the effect of improving the strength of clad steel plate. In order to achieve this effect, in the case of adding V, the V content is preferably 0.005% or more. If the V content is more than 0.500%, weldability degrades and alloy costs increase. Accordingly, in the case of adding V, the V content is preferably 0.500% or less. The lower limit of the V content is more preferably 0.010%. The upper limit of the V content is more preferably 0.100%.

Ti: 0.100% or Less

Ti is an element that has a strong tendency to form nitrides and has the effect of fixing N and reducing solute N. Thus, the addition of Ti can improve the toughness of the base metal and weld portion. In order to achieve this effect, in the case of adding Ti, the Ti content is preferably 0.005% or more. The Ti content is more preferably 0.007% or more. If the Ti content is more than 0.100%, toughness decreases. Accordingly, in the case of adding Ti, the Ti content is preferably 0.100% or less. The Ti content is more preferably 0.090% or less.

Nb: 0.100% or Less

Nb is an element that has the effect of reducing the prior austenite grain size and improving toughness by precipitating as carbonitrides. In order to achieve this effect, in the case of adding Nb, the Nb content is preferably 0.005% or more. The Nb content is more preferably 0.007% or more. If the Nb content is more than 0.100%, a large amount of NbC precipitates and toughness decreases. Accordingly, in the case of adding Nb, the Nb content is preferably 0.100% or less. The Nb content is more preferably 0.060% or less.

Ca: 0.0200% or Less

Ca is an element that has the effect of, by combining with S, suppressing the formation of MnS and the like which elongate in the rolling direction. Thus, adding Ca enables morphological control so that sulfide-based inclusions will be spherical, with it being possible to improve the toughness of the weld portion, etc. In order to achieve this effect, in the case of adding Ca, the Ca content is preferably 0.0005% or more. If the Ca content is more than 0.0200%, the cleanliness of the steel decreases. The decrease in cleanliness leads to the decrease in toughness. Accordingly, in the case of adding Ca, the Ca content is preferably 0.0200% or less. The Ca content is more preferably 0.0020% or more. The Ca content is more preferably 0.0100% or less.

Mg: 0.0200% or Less

Mg is an element that has the effect of, by combining with S, suppressing the formation of MnS and the like which elongate in the rolling direction, as with Ca. Thus, adding Mg enables morphological control so that sulfide-based inclusions will be spherical, with it being possible to improve the toughness of the weld portion, etc. In order to achieve this effect, in the case of adding Mg, the Mg content is preferably 0.0005% or more. If the Mg content is more than 0.0200%, the cleanliness of the steel decreases. The decrease in cleanliness leads to the decrease in toughness. Accordingly, in the case of adding Mg, the Mg content is preferably 0.0200% or less. The Mg content is more preferably 0.0020% or more. The Mg content is more preferably 0.0100% or less.

REM: 0.0200% or Less

REM (rare earth metal) is an element that has the effect of, by combining with S, suppressing the formation of MnS and the like which elongate in the rolling direction, as with Ca and Mg. Thus, adding REM enables morphological control so that sulfide-based inclusions will be spherical, with it being possible to improve the toughness of the weld portion, etc. In order to achieve this effect, in the case of adding REM, the REM content is preferably 0.0005% or more. If the REM content is more than 0.0200%, the cleanliness of the steel decreases. The decrease in cleanliness leads to the decrease in toughness. Accordingly, in the case of adding REM, the REM content is preferably 0.0200% or less. The REM content is more preferably 0.0020% or more. The REM content is more preferably 0.0100% or less.

(2) Chemical Composition of Cladding Metal

In the clad steel plate according to one embodiment of the present disclosure, carbon steel with which CE_B/CE_C is 2.000 or more is used as the cladding metal

Herein, carbon steel is a steel that is an alloy of iron (Fe) and carbon (C) and has a chemical composition in which the C content is 2.2 mass % or less, the total content of elements (Si, Mn, P, S, etc., including elements corresponding to inevitable impurities) other than C and Fe is 5.0 mass % or less, and the balance consists of Fe.

CE_B/CE_C: 2.000 or More

CE_B/CE_C is 2.000 or more, where CE_B and CE_C are respectively the equivalent carbon content of the base metal and the equivalent carbon content of the cladding metal. If CE_B/CE_C is less than 2.000, the equivalent carbon content of the cladding metal is excessively high and the hardness of the clad steel plate increases excessively, leading to degradation in ammonia-induced SCC resistance. CE_B/CE_C is preferably 2.050 or more, and more preferably 2.100 or more. No upper limit is placed on CE_B/CE_C. For example, CE_B/CE_C is preferably 5.000 or less.

CE_B and CE_C can be calculated by the following formulas.

CE B = [ C ] B + [ Mn ] B / 6 + [ Si ] B / 24 + [ Ni ] B / 40 + [ Cr ] B / 5 + [ Mo ] B / 4 + [ V ] B / 14

where [X]_B denotes the content (mass %) of element X in the chemical composition of the base metal. The content of each element that is not contained is assumed to be 0. The same applies to the formulas below.

C ⁢ E C = [ C ] + [ Mn ] / 6 + [ Si ] / 24 + [ Ni ] / 40 + [ Cr ] / 5 + [ Mo ] / 4 + [ V ] / 14

where [X] denotes the content (mass %) of element X in the chemical composition of the cladding metal.

A preferred chemical composition of the cladding metal in the clad steel plate according to one embodiment of the present disclosure is as follows.

C: 0.100% or Less

C is an element that enhances the hardness of steel. When hardness is higher, liquid ammonia-induced SCC susceptibility is higher. The C content of the cladding metal is therefore preferably 0.100% or less. A lower C content of the cladding metal is better, but excessive reduction of C leads to an increase in refining costs. The C content is therefore preferably 0.0005% or more.

Mn: 0.01% to 1.50%

Mn is an element that has the effect of increasing the quench hardenability of steel. Accordingly, if the Mn content is excessively high, the hardness of the cladding metal increases excessively. The excessive increase in the hardness of the cladding metal leads to degradation in ammonia-induced SCC resistance. The Mn content is therefore preferably 1.50% or less. The Mn content is more preferably 1.25% or less, and further preferably 1.00% or less. Reducing Mn to less than 0.01% requires high cost. The Mn content is therefore preferably 0.01% or more.

Cu, Cr, Sb, and Sn can further improve the ammonia-induced SCC resistance of the steel plate. Hence, it is preferable to add one or more of these elements in the below-described amounts and set the CR value calculated by the following formula (1) to 0.30 or more.

The CR value is a formula devised to estimate ammonia-induced SCC resistance from the content of each element, and a higher CR value corresponds to higher ammonia-induced SCC resistance. By setting the CR value to 0.30 or more, it is possible to effectively suppress stress corrosion cracking in a liquid ammonia environment.

CR ⁢ value = 2.3 [ Cu ] + 2.8 [ Cr ] + 7.3 [ Sb ] + 3.6 [ Sn ] ( 1 )

where [X] denotes the content (mass %) of element X in the chemical composition of the cladding metal. Since the chemical composition of the cladding metal and the chemical composition of the cladding metal raw material plate (in other words, raw material for cladding metal) are substantially the same, [X] can be regarded as also denoting the content (mass %) of element X in the chemical composition of the cladding metal raw material plate. Likewise, [X]_B mentioned above can be regarded as also denoting the content (mass %) of element X in the chemical composition of the base metal raw material plate (in other words, raw material for base metal).

The use of a low-hardness steel plate as the cladding metal can suppress ammonia-induced SCC. However, if there are dents or scratches on the surface of the cladding metal, stress concentration may occur at such dents or scratches, causing degradation in ammonia-induced SCC resistance. By setting the CR value to 0.30 or more, it is possible to prevent degradation in ammonia-induced SCC resistance even if there are dents or scratches on the surface of the cladding metal. The CR value is therefore preferably 0.30 or more. The CR value is more preferably 0.32 or more, and further preferably 0.35 or more. No upper limit is placed on the CR value. For example, the CR value is preferably 1.50 or less.

Cu, Cr, Sb, and Sn have the effect of suppressing stress corrosion cracking by quickly forming protective corrosion products in a liquid ammonia environment. In order to achieve this effect, in the case of adding Cu, the Cu content is preferably 0.01% or more. In the case of adding Cr, the Cr content is preferably 0.01% or more. In the case of adding Sb, the Sb content is preferably 0.01% or more. In the case of adding Sn, the Sn content is preferably 0.01% or more.

Meanwhile, excessive addition of Cu, Cr, Sb, and Sn degrades weldability and toughness, and is also disadvantageous from the viewpoint of alloy costs. Accordingly, in the case of adding Cu, the Cu content is preferably 0.50% or less. In the case of adding Cr, the Cr content is preferably 0.50% or less. In the case of adding Sb, the Sb content is preferably 0.50% or less. In the case of adding Sn, the Sn content is preferably 0.50% or less. More preferably, the Cu content is 0.40% or less, the Cr content is 0.40% or less, the Sb content is 0.40% or less, and the Sn content is 0.40% or less.

In the preferred chemical composition of the cladding metal in the clad steel plate according to one embodiment of the present disclosure, the balance other than the above-described elements consists of Fe and inevitable impurities.

(3) Metallic Microstructure

[Metallic Microstructure of Base Metal: Volume Fraction of Bainite: 90% or More, Average Grain Size of Bainite: 25 μm or Less]

The volume fraction of bainite in the metallic microstructure of the base metal needs to be 90% or more, in order to satisfy the tensile property and low-temperature toughness of the base metal. If the volume fraction of bainite is less than 90%, the volume fractions of other microstructures such as ferrite, martensite austenite constituent, martensite, pearlite, and austenite increase, so that sufficient strength and low-temperature toughness cannot be obtained. No upper limit is placed on the volume fraction of bainite, and the upper limit may be 100%.

Here, bainite includes microstructures called bainitic ferrite and granular ferrite, as well as microstructures obtained by tempering them. The microstructures called bainitic ferrite and granular ferrite are microstructures formed during or after cooling subsequent to hot rolling which contributes to transformation strengthening.

The residual microstructure with a volume fraction of 10% or less may include martensite in addition to ferrite, pearlite, and austenite. The volume fraction of each microstructure in the residual microstructure is not limited. The residual microstructure is preferably pearlite. The volume fraction of the residual microstructure may be 0%.

The average grain size of bainite is 25 μm or less, in order to obtain excellent low-temperature toughness. If the average grain size of bainite is more than 25 μm, crack propagation resistance decreases and sufficient low-temperature toughness cannot be obtained. The average grain size of bainite is preferably 22 μm or less. No lower limit is placed on the average grain size of bainite. For example, the average grain size of bainite is preferably 1 μm or more.

The volume fraction and average grain size of bainite can be measured by the methods described in the EXAMPLES section below.

The metallic microstructure of the cladding metal is not limited. For example, the metallic microstructure of the cladding metal may be a conventionally known metallic microstructure of carbon steel, such as ferrite and bainite.

[Vickers Hardness of Cladding Metal: 210 HV10 or Less]

The Vickers hardness of the cladding metal is 210 HV10 or less. If a high hardness region exists in the surface layer of the clad steel plate, ammonia-induced SCC is promoted. In detail, if the Vickers hardness of the cladding metal is more than 210 HV10, the desired ammonia-induced SCC resistance cannot be obtained. The Vickers hardness of the cladding metal is preferably 200 HV10 or less. No lower limit is placed on the Vickers hardness of the cladding metal. For example, the Vickers hardness of the cladding metal is preferably 100 HV10 or more.

The Vickers hardness can be measured by the method described in the EXAMPLES section below.

The Vickers hardness of the base metal is not limited, and may be 210 HV10 or less, or more than 210 HV10.

(4) Thicknesses of Base Metal and Cladding Metal

[t_C1: 2.0 mm or More, t_C2: 0.0 mm or More, and (t_C1+t_C2)/(t_B+t_C1+t_C2): 0.30 or Less]

t_C1 is 2.0 mm or more, t_C2 is 0.0 mm or more, and (t_C1+t_C2)/(t_B+t_C1+t_C2) (hereafter also referred to as a clad ratio) is 0.30 or less, where t (mm) is the thickness of the base metal, t_C1 (mm) is the thickness of the cladding metal on one side of the base metal, i.e. the thickness of the first cladding metal, and t_C2 (mm) is the thickness of the cladding metal on the other side of the base metal, i.e. the thickness of the second cladding metal.

If t_C1 is less than 2.0 mm, the low-hardness region wears due to corrosion and ammonia-induced SCC resistance degrades. t_C1 is preferably 2.1 mm or more, and more preferably 2.2 mm or more. t_C1 is preferably 5.0 mm or less.

If the clad ratio is more than 0.30, the proportion of the low-hardness cladding metal relative to the base metal that provides strength is excessively high, and sufficient strength cannot be obtained. The clad ratio is preferably 0.20 or less. The clad ratio is preferably 0.05 or more.

Since the second cladding metal may be omitted (i.e. the second cladding metal is optional), t_C2 may be 0.0 mm or more. t_C2 is preferably 0.5 mm or more, and more preferably 1.0 mm or more. t_C2 is preferably 5.0 mm or less.

The total thickness of the clad steel plate, which is typically t_B+t_C1+t_C2, is preferably 8 mm or more, and more preferably 15 mm or more. The total thickness of the clad steel plate is preferably 50 mm or less, and more preferably 40 mm or less.

(5) Production Method

Next, a method of producing the clad steel plate according to one embodiment of the present disclosure will be described. For example, first, a base metal raw material plate having the chemical composition of the base metal described above and a cladding metal raw material plate that is the carbon steel (with CE_B/CE_C of 2.000 or more) described above, preferably a cladding metal raw material plate having the chemical composition of the cladding metal described above, are overlapped to form a clad raw material plate. The method of preparing the base metal raw material plate and the cladding metal raw material plate is not limited. For example, the base metal raw material plate and the cladding metal raw material plate can each be prepared using a conventionally known production method. Specifically, molten steel adjusted to a predetermined chemical composition by a typical steelmaking method (converter, electric furnace, or the like) is cast by a typical casting method (continuous casting or ingot casting) to obtain a cast steel material. The obtained cast steel material is then subjected to hot rolling and the like to obtain a base metal raw material plate. Following this, a cladding metal raw material plate is placed on (overlapped with) at least one side of the base metal raw material plate, particularly on the side expected to come into contact with ammonia, etc., to prepare a two-layer clad raw material plate. Alternatively, respective cladding metal raw material plates are placed on (overlapped with) both sides of the base metal raw material plate to prepare a three-layer clad raw material plate (a clad raw material plate in which a cladding metal raw material plate, a base metal raw material plate, and a cladding metal raw material plate are stacked in this order).

The clad raw material plate is then pressure-bonded and also heat-treated under predetermined conditions to control the microstructure.

In detail, the clad raw material plate is heated to 1000° C. to 1250° C., and then subjected to hot rolling with a cumulative rolling reduction ratio of 20% or more in the non-recrystallization temperature range of the base metal raw material plate and a rolling finish temperature of Ar₃ transformation point or more to obtain a hot-rolled steel plate. The hot-rolled steel plate is then subjected to cooling with a cooling start temperature of Ar₃ transformation point or more, an average cooling rate of 20° C./s to 120° C./s, and a cooling stop temperature of 500° C. or less. The foregoing clad steel plate according to one embodiment of the present disclosure can be produced in this way.

After the cooling, the hot-rolled steel plate may be tempered in a temperature range of 650° C. or less.

The temperatures in the production conditions are each the temperature at a position of ½ of the thickness of the base metal or the base metal raw material plate. The temperature at the position may be measured directly. Alternatively, the temperature at the position may be determined, for example, through differential calculation using a process computer from the surface temperature of the clad steel plate or the clad raw material plate measured with a radiation thermometer.

[Heating Temperature: 1000° C. to 1250° C.]

If the heating temperature of the clad raw material plate (hereafter also referred to as the heating temperature) is less than 1000° C., the dissolution of carbides is insufficient and the required strength cannot be obtained. In addition, a high heating temperature is preferable from the viewpoint of the bondability between the base metal raw material plate and the cladding metal raw material plate. The heating temperature is therefore 1000° C. or more. If the heating temperature is more than 1250° C., the crystal grains of the base metal coarsen, leading to degradation in toughness. The heating temperature is therefore 1250° C. or less.

[Cumulative Rolling Reduction Ratio in Non-Recrystallization Temperature Range of Base Metal Raw Material Plate: 20% or More]

As a result of the cumulative rolling reduction ratio in the non-recrystallization temperature range of the base metal raw material plate (hereafter also simply referred to as the cumulative rolling reduction ratio) being 20% or more, deformation bands that serve as nucleation sites are introduced into the austenite grains of the base metal raw material plate. This refines bainite that is formed through transformation during cooling after hot rolling, and improves the toughness of the clad steel plate. The cumulative rolling reduction ratio is therefore 20% or more. The cumulative rolling reduction ratio is preferably 30% or more, and more preferably 40% or more. The cumulative rolling reduction ratio is preferably 85% or less, and more preferably 80% or less.

The non-recrystallization temperature range of the base metal raw material plate is a temperature range less than or equal to Tnr (° C.). Tnr (° C.) can be calculated by the following formula.

Tnr ⁢ ( ° ⁢ C . ) = 174 × log ⁡ ( [ Nb ] B × ( [ C ] B + 12 / 14 [ N ] B ) ) + 1444

where [X]_B denotes the content (mass %) of element X in the chemical composition of the base metal raw material plate, and log is the common logarithm.

The cumulative rolling reduction ratio can be calculated by the following formula.

[ Cumulative ⁢ rolling ⁢ reduction ⁢ ratio ⁢ ⁠⁠⁠ ( % ) ] = [ ⁠ Total ⁢ thickness ⁢ reduction ⁢ ⁢ quanity ⁢ of ⁢ base ⁢ metal ⁢ raw ⁢ material ⁢ plate ⁢ in ⁢ non - recrystalization ⁢ ⁢ temperature ⁢ range ⁢ of ⁢ base ⁢ metal ⁢ raw ⁢ material ⁢ plate ⁢ ( mm ) ] ⁢ ⁠ / [ Thickness ⁢ of ⁢ base ⁢ metal ⁢ raw ⁢ material ⁢ plate ⁢ in ⁢ clad ⁢ raw ⁢ material ⁢ plate ⁢ before ⁢ start ⁢ of ⁢ hot ⁢ rolling ⁢ ( mm ) ] × 100.

Whether each pass of hot rolling is performed in the non-recrystallization temperature range of the base metal raw material plate (in other words, whether the thickness reduction quantity of the base metal raw material plate in each pass of hot rolling is included in the total thickness reduction quantity of the base metal raw material plate in the non-recrystallization temperature range of the base metal raw material plate) is determined based on the finish temperature of the pass.

The reason why the cumulative rolling reduction ratio is based on the thickness of the base metal raw material plate in the clad raw material plate is because the toughness of the clad steel plate is greatly influenced particularly by the toughness of the base metal.

[Rolling Finish Temperature: Ar₃ Transformation Point or More]

If the rolling finish temperature of hot rolling is less than Ar₃ transformation point, ferrite formed is affected by machining, so that toughness degrades. The rolling finish temperature is therefore Ar₃ transformation point or more. No upper limit is placed on the rolling finish temperature. For example, the rolling finish temperature is preferably less than or equal to (Ar₃ transformation point+90° C.).

Ar₃ transformation point can be calculated by the following formula.

Ar 3 ⁢ transformation ⁢ point ⁢ ( ° ⁢ C . ) = 910 - 310 [ C ] B - 80 [ Mn ] B - 20 [ Cu ] B - 15 [ Cr ] B - 55 [ Ni ] B - 80 [ Mo ] B

where [X]_B denotes the content (mass %) of element X in the chemical composition of the base metal raw material plate.

[Cooling Start Temperature: Ar₃ Transformation Point or More]

The hot-rolled steel plate obtained as a result of hot rolling is cooled from a temperature more than or equal to Ar₃ transformation point. If the cooling start temperature is less than Ar₃ transformation point, ferrite forms excessively. The formed ferrite coexists with bainite and martensite, which differ greatly in strength from ferrite. This results in insufficient strength and toughness degradation. The cooling start temperature after hot rolling is therefore Ar₃ transformation point or more. No upper limit is placed on the cooling start temperature. For example, the cooling start temperature is preferably less than or equal to (Ar₃ transformation point+70° C.).

[Average Cooling Rate: 20° C./s to 120° C./s]

By setting the average cooling rate to 20° C./s or more, it is possible to obtain a clad steel plate with high strength and high toughness. In particular, cooling at a high rate has the effect of increasing strength by transformation strengthening. If the average cooling rate is less than 20° C./s, the grain size of bainite increases. Moreover, ferrite and pearlite may form, causing insufficient strength and toughness degradation. If the average cooling rate is more than 120° C./s, the volume fraction of martensite increases excessively, and toughness decreases. The average cooling rate is therefore 20° C./s or more and 120° C./s or less.

The average cooling rate here is the average value of the cooling rate from the cooling start temperature to the cooling stop temperature, and is based on the temperature at the position of ½ of the thickness of the base metal. For example, the temperatures at the position of ½ of the thickness of the base metal at the start and end of cooling are determined through differential calculation using a process computer based on the surface temperatures at the start and end of cooling measured with a radiation thermometer, respectively. The average cooling rate can then be calculated by the following formula.

[ Average ⁢ cooling ⁢ rate ⁢ ( ° ⁢ C . / s ) ] = ( [ Temperature ⁢ at ⁢ position ⁢ of ⁢ 1 / 2 ⁢ of ⁢ thickness ⁢ of ⁢ base ⁢ metal ⁢ at ⁢ start ⁢ of ⁢ cooling ⁢ ( ° ⁢ C . ) ] - [ Temperature ⁢ at ⁢ position ⁢ of ⁢ 1 / 2 ⁢ of ⁢ thickness ⁢ of ⁢ base ⁢ metal ⁢ at ⁢ end ⁢ of ⁢ cooling ⁢ ( ° ⁢ C . ) ] ) ⁢ / [ Cooling ⁢ times ⁢ ( s ) ] .

[Cooling Stop Temperature: 500° C. Or Less]

By setting the cooling stop temperature to 500° C. or less, it is possible to obtain a predetermined volume fraction of bainite in the metallic microstructure of the base metal. If the cooling stop temperature is more than 500° C., ferrite and pearlite form excessively, causing insufficient strength and toughness degradation. The cooling stop temperature is therefore 500° C. or less. No lower limit is placed on the cooling stop temperature. For example, the cooling stop temperature may be room temperature, but is preferably 150° C. or more from the viewpoint of production efficiency, etc.

[Tempering Temperature: 650° C. Or Less]

Tempering may be optionally performed in order to recover the toughness of the base metal. If the tempering temperature, i.e. the temperature (temperature at the position of ½ of the thickness of the base metal) of the clad steel plate during reheating by tempering is more than 650° C., there is a possibility that dislocation recover and the strength of the base metal decreases. Accordingly, in the case of performing tempering, the tempering temperature is 650° C. or less. The lower limit of the tempering temperature is preferably 350° C. from the viewpoint of recovering the toughness of the base metal.

The clad steel plate according to one embodiment of the present disclosure can be produced as described above. The clad steel plate according to one embodiment of the present disclosure thus obtained has excellent tensile property and toughness.

Herein, “excellent tensile property” means that the yield strength YS (yield point YP when there is a yield point, 0.2% proof stress @0.2 when there is no yield point) is 490 MPa or more and the tensile strength (TS) is 610 MPa or more as measured by a tensile test in accordance with JIS Z 2241 (2022). Moreover, “excellent toughness” means that the fracture appearance transition temperature (hereafter also referred to as vTrs) measured by a Charpy impact test in accordance with JIS Z 2242 (2018) is −30° C. or less. Details are as described in the EXAMPLES section below.

Conditions other than those described above are not limited and may be in accordance with conventional methods.

EXAMPLES

Example 1

Table 1 shows the chemical compositions of base metals (the balance consisting of Fe and inevitable impurities). In the table, steel sample IDs A to P are conforming steels that satisfy the chemical composition of the base metal in the clad steel plate according to one embodiment of the present disclosure, and steel sample IDs Q to X are comparative steels outside the range of the chemical composition of the base metal in the clad steel plate according to one embodiment of the present disclosure.

Base metal raw material plates having the chemical compositions shown in Table 1 were overlapped with cladding metal raw material plates having CE_C shown in Table 2 to prepare clad raw material plates, and clad steel plates (Nos. 1 to 34) were produced under the conditions shown in Table 2. For each of the obtained clad steel plates, the measurement of the volume fraction and average grain size of bainite in the metallic microstructure of the base metal, the measurement of the Vickers hardness of the cladding metal, the evaluation of the tensile property and toughness, and the evaluation of the ammonia-induced SCC resistance in a liquid ammonia environment were conducted. The test methods are as follows.

[Measurement of Volume Fraction of Bainite in Metallic Microstructure of Base Metal]

A sample was taken so that the thickness center (position of ½ of the thickness) of the base metal in the clad steel plate would be the observation plane. The sample was then mirror-polished and further etched with nital. Next, an area of 10 mm×10 mm of the sample was photographed using a scanning electron microscope (SEM) at 500 to 3000 magnifications. The photographed image was analyzed using an image analyzer to determine the volume fraction of bainite in the metallic microstructure of the base metal. Here, the area fraction was regarded as the volume fraction, because the area fraction was equivalent to the volume fraction when the anisotropy of the metallic microstructure of the base metal was small.

[Measurement of Average Grain Size of Bainite in Metallic Microstructure of Base Metal]

The same sample was used to measure the average grain size of bainite as in the measurement of the volume fraction of bainite in the metallic microstructure of the base metal. First, the surface of the sample was mirror-polished. Next, the crystal orientation was measured from an electron backscatter diffraction image using an electron backscattering pattern (EBSP) device attached to the SEM. Specifically, the crystal orientation was measured with 0.3 μm intervals within a 200 μm square region of the sample. Following this, assuming a region surrounded by grain boundaries with a crystal orientation difference between adjacent crystal grains of 15° or more to be one crystal grain, for each of the crystal grains determined to be bainite, the equivalent circular diameter of the crystal grain was calculated from the area of the crystal grain. The average value of the equivalent circular diameters of the crystal grains determined to be bainite was then taken to be the average grain size of bainite.

Here, among the crystal grains, each crystal grain having long and thin grown lath-like ferrite was determined to be bainite.

[Measurement of Vickers Hardness of Cladding Metal]

A sample was taken from the clad steel plate so that a section orthogonal to the rolling direction, namely, T-section, would be the measurement plane. The sample was then mirror-polished. Following this, the Vickers hardness (HV10, measurement load: 10 kgf) was measured at 20 points with 1 mm intervals in a direction orthogonal to the rolling direction (direction orthogonal to the rolling direction and the thickness direction) at a position of ½ of the thickness of the cladding metal, in accordance with JIS Z 2244 (2020). The average value of the measured values was taken to be the Vickers hardness of the cladding metal. For example, in the case where the Vickers hardness measured under the condition of a measurement load of 10 kgf is 210, the Vickers hardness is normally expressed as 210 HV10.

[Tensile Property]

A JIS Z 2241 (2022) No. 1B test piece was taken from the clad steel plate so that a direction orthogonal to the rolling direction would be the longitudinal direction, and a tensile test was conducted in accordance with JIS Z 2241 (2022) to measure the yield strength YS (yield point YP when there is a yield point, 0.2% proof stress 00.2 when there is no yield point) and the tensile strength (TS). The tensile property was evaluated as excellent when the yield strength was 490 MPa or more and the tensile strength was 610 MPa or more. The initial strain rate was 1×10⁻³/s.

[Toughness]

A JIS Z 2242 (2018) V-notch test piece was taken from the base metal in the clad steel plate so that the rolling direction would be the longitudinal direction, and a Charpy impact test was conducted in accordance with JIS Z 2242 (2018) to measure vTrs. The toughness was evaluated as excellent when vTrs was −30° C. or less.

[Ammonia-Induced SCC Resistance]

The ammonia-induced SCC resistance was evaluated by a four-point bending anodic electrolysis test according to the following procedure.

A 15 mm×115 mm test piece with a thickness of 5 mm was taken from the clad steel plate so that the first side (inner side) of the clad steel plate would be the evaluation plane. The test piece was then subjected to ultrasonic degreasing in acetone for 5 minutes. Next, stress equivalent to the yield strength of the test piece was applied to the test piece by four-point bending. After this, the test piece was placed in a test cell while still under stress. The test cell was then filled with a test solution prepared by mixing 12.5 g of ammonium carbamate and 1 L of liquid ammonia. Next, constant-potential anodic electrolysis was performed with the potential difference from a reference electrode being controlled to +2.0V vs Pt by a potentiostat. The temperature of the test atmosphere was set to room temperature (25° C.). The test piece was held in this state for 720 hours from the start of immersion (current application). After the holding, the test piece was visually inspected. The ammonia-induced SCC resistance was evaluated as excellent (pass) when no cracks were found in the test piece, and evaluated as poor when cracks were found in the test piece. The evaluation results are shown in Table 2.

TABLE 1
Steel
sample	Chemical composition (mass %)		Ar3

ID

C

Si

Mn

Al

P

S

Cu

Ni

Cr

Mo

V

Ti

Nb

Ca

Mg

REM

CEB

(° C.)

A	0.080	0.24	1.96	0.019	0.0043	0.0023	—	—	—	—	—	—	—	—	—	—	0.417	728
B	0.104	0.32	1.85	0.040	0.0039	0.0046	—	—	—	—	—	—	—	—	—	—	0.426	730
C	0.068	0.41	1.92	0.022	0.0038	0.0035	0.22	—	—	—	—	—	—	—	—	—	0.405	731
D	0.070	0.48	1.96	0.048	0.0051	0.0005	—	0.98	—	—	—	—	—	—	—	—	0.441	678
E	0.075	0.47	1.16	0.031	0.0057	0.0048	—	—	0.55	—	—	—	—	—	—	—	0.398	786
F	0.097	0.65	1.23	0.009	0.0043	0.0049	—	—	—	0.33	—	—	—	—	—	—	0.412	755
G	0.099	0.13	1.78	0.035	0.0030	0.0026	—	—	—	—	0.027	—	—	—	—	—	0.403	737
H	0.083	0.22	1.84	0.028	0.0062	0.0010	—	—	—	—	—	0.080	—	—	—	—	0.399	737
I	0.081	0.65	1.69	0.029	0.0031	0.0010	—	—	—	—	—	—	0.066	—	—	—	0.390	750
J	0.084	0.07	1.85	0.038	0.0056	0.0016	—	—	—	—	—	—	—	0.0049	—	—	0.395	736
K	0.091	0.49	1.69	0.043	0.0029	0.0029	—	—	—	—	—	—	—	—	0.0022	—	0.393	747
L	0.097	0.67	1.82	0.027	0.0049	0.0022	—	—	—	—	—	—	—	—	—	0.0096	0.428	734
M	0.095	0.35	1.86	0.042	0.0043	0.0038	0.08	0.71	—	—	0.020	—	0.046	—	0.0085	—	0.439	691
N	0.075	0.45	1.32	0.042	0.0034	0.0031	—	—	0.52	0.12	—	0.034	—	0.0078	—	—	0.448	764
O	0.126	0.53	0.69	0.024	0.0069	0.0031	0.64	0.77	0.68	—	0.016	—	0.075	0.0025	—	—	0.419	750
P	0.046	0.67	1.02	0.023	0.0040	0.0044	0.57	0.97	0.26	0.37	0.093	0.047	0.067	0.0076	0.0032	0.0076	0.419	716
Y	0.015	0.32	2.45	0.054	0.0100	0.0080	—	—	—	—	—	—	—	—	—	—	0.437	709
Z	0.182	0.11	0.61	0.003	0.0044	0.0033	0.21	0.21	0.34	0.12	—	—	0.021	—	—	—	0.392	774
Q	0.008	0.14	1.79	0.025	0.0056	0.0029	0.17	0.69	—	—	—	0.081	—	—	—	—	0.329	723
R	0.211	0.58	1.15	0.005	0.0057	0.0012	—	—	0.69	0.23	—	—	0.019	0.0077	—	—	0.622	724
S	0.100	1.09	1.41	0.031	0.0059	0.0049	0.30	0.79	—	—	—	—	—	—	—	—	0.400	717
T	0.086	0.27	0.46	0.007	0.0054	0.0018	0.32	0.51	—	—	—	—	—	—	—	—	0.187	812
U	0.058	0.18	2.54	0.008	0.0051	0.0045	0.63	0.72	—	—	—	—	—	—	—	—	0.507	637
V	0.057	0.37	1.43	0.061	0.0052	0.0044	0.48	1.38	—	—	—	—	—	—	—	—	0.345	692
W	0.057	0.20	1.87	0.027	0.0224	0.0041	0.41	1.07	—	—	—	—	—	—	—	—	0.404	676
X	0.082	0.60	1.11	0.019	0.0067	0.0118	0.21	1.45	—	—	—	—	—	—	—	—	0.328	712
* Underlines indicate outside the range of the present disclosure.

	TABLE 2
	Hot rolling	Cooling

Cumulative

Rolling

Cooling

Average

Steel

Thickness (mm)

Heating

rolling

finish

start

cooling

	sample			Total			Clad	temperature	reduction	temperature	temperature	rate
No.	ID	CEC	CEB/CEC	thickness	tc1	tc2	ratio	(° C.)	ratio (%)	(° C.)	(° C.)	(° C./s)
1	A	0.110	3.772	38	3.5	0.0	0.09	1200	60	770	750	55
2	B	0.134	3.169	38	4.4	0.0	0.12	1120	63	780	770	49
3	C	0.103	3.933	25	2.4	0.0	0.10	1120	68	760	740	46
4	D	0.134	3.281	38	3.7	0.0	0.10	1120	66	730	720	45
5	E	0.120	3.307	38	2.5	0.0	0.07	1190	60	830	810	52
6	F	0.179	2.294	38	3.7	0.0	0.10	1010	44	820	800	50
7	G	0.115	3.514	38	3.6	0.0	0.09	1180	73	810	790	54
8	H	0.144	2.774	38	4.7	0.0	0.12	1140	73	800	790	49
9	I	0.122	3.191	38	2.8	0.0	0.07	1090	50	790	770	46
10	J	0.187	2.114	38	4.4	0.0	0.12	1200	75	780	760	53
11	K	0.119	3.307	38	3.2	3.2	0.17	1050	45	790	780	45
12	L	0.113	3.785	38	3.8	0.0	0.10	1120	41	800	780	25
13	M	0.136	3.236	38	5.0	0.0	0.13	1030	71	730	720	47
14	N	0.151	2.963	38	4.1	0.0	0.11	1160	59	830	810	35
15	O	0.117	3.571	38	3.3	0.0	0.09	1030	42	780	770	54
16	P	0.194	2.163	38	2.8	0.0	0.07	1110	60	760	740	53
35	Y	0.126	3.466	38	4.5	0.0	0.12	1100	65	750	730	55
36	Z	0.185	2.116	38	4.5	0.0	0.12	1120	75	800	790	58
17	A	0.232	1.794	38	4.5	0.0	0.12	1060	70	780	760	51
18	A	0.202	2.067	38	1.5	0.0	0.04	1100	61	770	750	50
19	B	0.153	2.781	38	6.0	6.0	0.32	1010	54	800	790	55
20	B	0.102	4.191	38	3.8	0.0	0.10	1300	62	750	730	49
21	B	0.156	2.724	38	4.2	0.0	0.11	1160	13	770	750	50
22	B	0.151	2.825	38	3.6	0.0	0.09	1170	51	710	700	49
23	B	0.167	2.556	38	4.3	0.0	0.11	1090	44	780	700	50
24	B	0.137	3.118	38	5.0	0.0	0.13	1200	75	760	740	11
25	B	0.142	2.998	38	4.2	0.0	0.11	1140	72	770	750	135
26	B	0.155	2.752	38	5.0	0.0	0.13	1050	68	750	730	48
27	Q	0.087	3.776	38	3.5	0.0	0.09	1200	46	760	740	45
28	R	0.159	3.922	38	2.5	0.0	0.07	1010	64	780	770	50
29	S	0.185	2.164	38	4.0	0.0	0.11	1110	50	770	760	52
30	T	0.081	2.302	38	3.2	0.0	0.08	1010	52	830	820	46
31	U	0.134	3.770	38	4.4	0.0	0.12	1150	47	670	650	50
32	V	0.122	2.831	38	3.2	0.0	0.08	1020	50	740	720	54
33	W	0.159	2.541	38	2.5	0.0	0.07	1030	69	720	710	51
34	X	0.108	3.047	38	3.2	0.0	0.08	1100	72	740	720	47

Metallic microstructure

Cooling

of base metal

Vickers

	Cooling	Tempering	Average	Volume	hardness	Tensile		Ammonia-
	stop	Tempering	grain size	fraction	of	property	Toughness	induced

	temperature	temperature	of bainite	of bainite	cladding	YS	TS	vTrs	SCC
No.	(° C.)	(° C.)	(μm)	(%)	metal	(MPa)	(MPa)	(° C.)	resistance	Remarks
1	282	—	8	95	176	536	709	−75	Pass	Example
2	268	—	8	93	165	527	728	−73	Pass
3	320	—	5	96	152	501	657	−101	Pass
4	300	—	5	94	172	522	673	−83	Pass
5	311	—	7	93	170	553	761	−73	Pass
6	276	—	10	93	175	550	751	−72	Pass
7	279	—	8	94	179	557	745	−73	Pass
8	263	—	11	93	168	521	715	−70	Pass
9	304	—	5	93	175	498	682	−85	Pass
10	324	—	5	96	191	525	699	−87	Pass
11	270	—	3	95	163	517	679	−76	Pass
12	280	—	22	93	159	556	748	−35	Pass
13	294	—	7	94	167	511	660	−83	Pass
14	330	—	5	93	186	563	772	−77	Pass
15	254	—	9	98	168	558	725	−71	Pass
16	232	560	5	98	188	520	628	−93	Pass
35	270	—	5	96	161	584	720	−58	Pass
36	230	—	15	92	189	499	653	−82	Pass
17	301	620	6	93	221	543	655	−85	Poor	Comparative
18	266	—	5	97	201	517	695	−74	Poor	Example
19	271	—	11	91	175	462	561	−70	Pass
20	250	—	38	95	177	514	721	−26	Pass
21	307	—	33	93	183	515	708	−23	Pass
22	289	—	9	75	181	498	625	−28	Pass
23	280	—	9	73	187	398	559	−28	Pass
24	314	—	28	60	184	417	569	−19	Pass
25	293	—	14	83	180	687	907	−19	Pass
26	560	—	13	71	183	456	629	−22	Pass
27	265	—	10	91	145	427	572	−102	Pass
28	300	—	7	91	170	613	821	−8	Pass
29	269	—	10	93	180	503	688	−21	Pass
30	285	—	16	98	185	431	561	−47	Pass
31	280	—	7	97	178	514	663	−19	Pass
32	252	—	7	99	175	452	586	−13	Pass
33	282	—	4	95	183	463	635	0	Pass
34	261	—	8	98	173	397	551	−3	Pass
* Underlines indicate outside the range of the present disclosure.
* tc2 = 0.0 indicates that cladding metal is on only one side.

As can be seen from Table 2, all of Examples had a yield strength YS of 490 MPa or more and a tensile strength of 610 MPa or more. Moreover, all of Examples had vTrs of −30° C. or less. Furthermore, all of Examples had excellent ammonia-induced SCC resistance. Thus, all of Examples had excellent ammonia-induced SCC resistance and low-temperature toughness as well as high strength.

Regarding Comparative Examples, in No. 17, CE_B/CE_C and the Vickers hardness of the cladding metal were outside the appropriate ranges. In Nos. 18 and 19, t_C1 and the clad ratio were outside the appropriate ranges, respectively. In Nos. 20 to 26, part of the production conditions was outside the appropriate range, so that the desired metallic microstructure of the base metal was not obtained. Consequently, these Comparative Examples were inferior in at least one of the yield strength YS, the tensile strength TS, the low-temperature toughness, and the ammonia-induced SCC resistance.

In Nos. 27 to 34, part of the chemical composition of the base metal was outside the appropriate range. Consequently, these Comparative Examples were inferior in at least one of the yield strength YS, the tensile strength TS, the low-temperature toughness, and the ammonia-induced SCC resistance.

Example 2

Base metal raw material plates having the chemical compositions shown in Table 1 (the balance consisting of Fe and inevitable impurities) were overlapped with cladding metal raw material plates having the chemical compositions shown in Table 3 (the balance consisting of Fe and inevitable impurities) to prepare clad raw material plates, and clad steel plates (Nos. 1 to 17) were produced under the conditions shown in Table 4. For each of the obtained clad steel plates, the measurement of the volume fraction and average grain size of bainite in the metallic microstructure of the base metal, the measurement of the Vickers hardness of the cladding metal, and the evaluation of the tensile property and toughness were conducted in the same manner as in Example 1. Moreover, the evaluation of the ammonia-induced SCC resistance in a liquid ammonia environment was conducted in the following manner.

[Ammonia-Induced SCC Resistance]

Two 15 mm×115 mm test pieces with a thickness of 5 mm were taken from the clad steel plate so that the first side (inner side) of the clad steel plate would be the test plane, as in Example 1. Assuming a clad steel plate having a dent on its surface (first side), a notch with a depth of 0.3 mm and a diameter of 0.2 mm was provided on the test plane of one test piece. Hereafter, the unnotched test piece is referred to as a first test piece, and the notched test piece as a second test piece. Next, a four-point bending anodic electrolysis test was conducted using the first and second test pieces in the same manner as in Example 1. The test pieces after the test were visually inspected, and the ammonia-induced SCC resistance was evaluated according to the following criteria.

Superior (pass, particularly excellent): No cracks were found in both the first and second test pieces.

Pass (excellent): No cracks were found in the first test piece, but cracks were found in the second test piece.

Poor: Cracks were found in both the first and second test pieces.

TABLE 3
Steel	Chemical composition (mass %)	CR

sample ID	C	Mn	Cu	Cr	Sb	Sn	value

A′	0.023	0.33	0.15	—	—	—	0.35
B′	0.041	0.41	—	0.21	—	—	0.59
C′	0.065	0.39	—	—	0.10	—	0.73
D′	0.031	0.93	—	—	—	0.28	1.01
E′	0.010	0.75	0.08	0.15	—	—	0.60
F′	0.033	0.81	—	0.18	0.12	—	1.38
G′	0.093	0.55	—	—	0.04	0.14	0.80
H′	0.048	0.69	0.09	0.07	0.05	0.05	0.95
I′	0.042	0.82	0.02	—	0.03	—	0.27

	TABLE 4
	Hot rolling	Cooling

Cumulative

Rolling

Cooling

Steel sample ID

Thickness (mm)

Heating

rolling

finish

start

	Base	Cladding			Total				temperature	reduction	temperature	temperature
No.	metal	metal	CEC	CEB/CEC	thickness	tc1	tc2	Clad ratio	(° C.)	ratio (%)	(° C.)	(° C.)
1	F	A′	0.103	3.996	35	3.1	0.0	0.09	1150	68	780	760
2	B	B′	0.158	2.694	35	2.4	0.0	0.07	1110	52	790	770
3	O	C′	0.148	2.844	35	2.5	0.0	0.07	1040	57	800	790
4	C	D′	0.198	2.049	35	2.1	0.0	0.06	1040	48	810	790
5	K	E′	0.167	2.353	35	1.5	0.0	0.04	1010	42	780	760
6	B	F′	0.210	2.029	35	1.7	0.0	0.05	1000	56	770	760
7	A	G′	0.205	2.036	35	2.3	0.0	0.07	1070	54	790	770
8	C	H′	0.199	2.039	35	2.5	2.5	0.14	1020	57	780	770
9	A	A′	0.103	4.045	35	1.9	0.0	0.05	1130	43	770	750
10	F	A′	0.103	3.996	35	1.5	0.0	0.04	1160	64	770	760
11	E	B′	0.158	2.518	35	1.4	0.0	0.04	1030	67	800	790
12	H	B′	0.158	2.524	35	2.3	0.0	0.07	1130	61	790	780
13	P	C′	0.148	2.843	35	3.1	0.0	0.09	1120	48	770	750
14	K	C′	0.148	2.665	35	2.2	0.0	0.06	1120	60	820	800
15	J	C′	0.148	2.680	35	2.6	0.0	0.07	1120	64	820	800
16	M	C′	0.148	2.975	35	1.5	0.0	0.04	1160	63	780	760
17	B	I′	0.195	2.179	35	1.6	0.0	0.05	1070	64	810	790

Metallic

microstructure

Cooling

of base metal

Vickers

		Cooling	Tempering	Average	Volume	hardness		Tensile		Ammonia-
	Average	stop	Tempering	grain size	fraction	of		property	Toughness	induced

	cooling rate	temperature	temperature	of bainite	of bainite	cladding	YS	TS	vTrs	SCC
No.	(° C./s)	(° C.)	(° C.)	(μm)	(%)	metal	(MPa)	(MPa)	(° C.)	resistance	Remarks
1	48	271	—	3	98	149	525	692	−96	Superior	Example
2	47	228	—	5	98	178	535	722	−83	Superior
3	46	249	—	3	94	172	541	737	−85	Superior
4	53	202	—	8	98	188	568	745	−75	Superior
5	51	202	—	6	93	174	528	698	−82	Superior
6	50	254	—	5	97	191	522	707	−89	Superior
7	46	223	—	5	94	185	548	709	−85	Superior
8	48	211	—	8	93	181	528	696	−86	Superior
9	51	252	—	5	93	162	508	686	−93	Superior
10	52	254	—	8	97	159	508	700	−91	Superior
11	51	238	—	6	98	177	542	725	−86	Superior
12	53	245	—	5	96	180	544	712	−87	Superior
13	50	210	—	11	99	184	524	700	−83	Superior
14	46	266	—	7	98	173	538	718	−88	Superior
15	51	227	—	9	97	180	550	742	−83	Superior
16	48	228	530	5	95	175	534	665	−88	Superior
17	49	274	—	3	95	197	555	746	−89	Pass

As can be seen from Table 4, all of Examples had a yield strength YS of 490 MPa or more and a tensile strength of 610 MPa or more. Moreover, all of Examples had vTrs of −30° C. or less. Furthermore, all of Examples had excellent ammonia-induced SCC resistance. Thus, all of Examples had excellent ammonia-induced SCC resistance and low-temperature toughness as well as high strength. The ammonia-induced SCC resistance was particularly excellent when the CR value of the cladding metal was 0.30 or more.