COLD ROLLED STEEL SHEET AND METHOD FOR MANUFACTURING SAME

Description

TECHNICAL FIELD

The present disclosure relates to a cold rolled steel sheet having excellent strength and formability and a method for manufacturing the same.

BACKGROUND ART

The development of steel plates having high strength has been continuously promoted for reducing the weight of automobiles and enhancing safety thereof, and recently, the importance of ultra-high strength steel with a tensile strength of 1,500 MPa or more has been increasing to improve the driving range of electric vehicles and protect batteries. Meanwhile, in the case of existing MART steel, since the elongation and formability are insufficient, it is expected that the economic value will be higher if an ultra-high strength steel plate for cold forming with formability by overcoming this is developed. In addition, a method of using a TRansformation Induced Plasticity (TRIP) phenomenon by introducing retained austenite is widely used as a method of increasing the elongation to improve the formability of steel. However, in the case of such TRIP steel plates, the addition of Si and Al is necessary for the introduction of retained austenite, and a larger amount of retained austenite may be obtained when accompanied by bainite transformation. However, since the bainite phase is transformed at relatively high temperatures, the tensile strength (TS) is low and the yield strength (YS) is also low for use as ultra-high strength steel.

Therefore, the recent trend is to adopt the quenching and partitioning process to utilize the TRIP phenomenon while increasing the strength of the steel plate. In the case of the so-called Q&P steel, the main structure of the matrix is the tempered martensite phase, and it has excellent yield strength and hole expansion ratio (HER; Hole Expansion Ratio), and the elongation may also be increased because the formation of retained austenite is possible. However, since it becomes difficult to obtain strength in the case in which the Q&P process temperature is increased to further increase the elongation, it is still difficult to obtain sufficient elongation along with high strength in ultra-high strength steel of 1.5 GPa.

SUMMARY OF INVENTION

Technical Problem

An aspect of the present disclosure is to provide a cold rolled steel sheet having excellent strength and formability and a method for manufacturing the same.

The subject matter of the present disclosure is not limited to the aforementioned contents. Anyone with ordinary knowledge in the technical field to which the present disclosure belongs will have no difficulty in understanding the additional subject matter of the present disclosure from the contents throughout the specification of the present disclosure.

Solution to Problem

According to an aspect of the present disclosure, a cold rolled steel sheet includes,

• • in weight %, C: 0.20% or more and less than 0.30%, Si: 1.0 to 3.0%, Al: 0.01 to 0.3%, Mn: 2.0 to 3.0%, Cr: 0.001 to 0.5%, Mo: 0.08 to 0.32%, B: 0.0001 to 0.0050%, Nb: 0.001 to 0.05%, Ti: 0.08 to 0.25%, P: 0.04% or less (excluding 0%), S: 0.01% or less (excluding 0%), N: 0.01% or less (excluding 0%), with a remainder of Fe and other unavoidable impurities, and
  • as a microstructure, in area %, ferrite: 10% or less (excluding 0%), retained austenite: more than 2% and 15% or less, fresh Martensite: less than 5%, a remainder being tempered martensite and bainite,
  • wherein an average number of carbides per unit area, in which a total ratio of Ti, Mo and C atoms to total carbide atoms exceeds 75%, is 10¹³/m² or more.

The cold rolled steel sheet may have a size of carbide, in which the total ratio of Ti, Mo and C atoms to the total carbide atoms exceeds 75% is 10 to 200 nm.

The cold rolled steel sheet may have a yield strength of 1000 MPa or more.

The cold rolled steel sheet may have a tensile strength of 1470 MPa or more.

The cold rolled steel sheet may have an elongation of 12% or more.

The cold rolled steel sheet may have a hole expansion ratio (HER) of 25% or more.

According to another aspect of the present disclosure, a method for manufacturing a cold rolled steel sheet includes,

• • an operation of reheating a slab containing, in wt %, C: 0.20% or more and less than 0.30%, Si: 1.0 to 3.0%, Al: 0.01 to 0.3%, Mn: 2.0 to 3.0%, Cr: 0.001 to 0.5%, Mo: 0.08 to 0.32%, B: 0.0001 to 0.0050%, Nb: 0.001 to 0.05%, Ti: 0.08 to 0.25%, P: 0.04% or less (excluding 0%), S: 0.01% or less (excluding 0%), N: 0.01% or less (excluding 0%), with a remainder of Fe and other unavoidable impurities;
  • an operation of finishing hot rolling the reheated slab at 830 to 950° C. and obtaining a hot-rolled steel sheet;
  • an operation of coiling the hot-rolled steel sheet at 400 to 550° C.;
  • a heat treatment operation of maintaining the coiled hot-rolled steel sheet at a temperature in a range of 550 to 650° C. for 5 to 15 hours;
  • an operation of cold-rolling the heat-treated hot-rolled steel sheet and obtaining a cold-rolled steel sheet;
  • an operation of continuously annealing the cold-rolled steel sheet at a temperature of 830 to 900° C.;
  • an operation of first cooling the continuously annealed cold-rolled steel sheet to a first cooling end temperature of 500 to 700° C. at an average cooling rate of less than 10° C./s;
  • an operation of second cooling the first-cooled cold-rolled steel sheet to a second cooling end temperature of 150 to 350° C. at an average cooling rate of 10° C./s or more; and
  • an operation of reheating the second-cooled cold-rolled steel sheet to a temperature range of 200 to 400° C. and then maintaining it at a temperature range of 200 to 400° C. for 300 to 1,000 seconds.

When reheating the slab, a reheating temperature may be 1150 to 1250° C.

At the time of the cold-rolling, a cold reduction ratio may be 30 to 60%.

After the finishing hot rolling, cooling may be performed at an average cooling rate of 10 to 100° C./s to a coiling temperature.

In the maintaining, an end point temperature of a reheating section after the second cooling may be 285 to 348° C.

In the maintaining, an end point temperature of a maintaining section of reheating after the second cooling may be 273 to 342° C.

Advantageous Effects of Invention

According to an aspect of the present disclosure, a cold rolled steel sheet having excellent strength and formability and a method for manufacturing the same may be provided.

The various advantageous advantages and effects of the present disclosure are not limited to the above-described contents, and it will be easier to understand in the process of explaining detailed embodiments of the present disclosure.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is an image of a Ti—Mo-based precipitate observed on a specimen obtained from Invention Example 1, taken using SEM.

BEST MODE FOR INVENTION

Hereinafter, preferred embodiments of the present disclosure will be described. However, the embodiments of the present disclosure may be modified in various other forms, and the scope of the present disclosure is not limited to below. In addition, the embodiments described embodiments of the present disclosure are provided to more completely explain the present disclosure to those with average knowledge in the relevant technical field.

Meanwhile, the terms used in this specification are for the describing specific embodiments, and are not intended to limit the present disclosure. For example, the singular forms used in this specification also include the plural forms unless the relevant definition clearly indicates a meaning contrary thereto. In addition, the meaning of “including” used in the specification is to specify a configuration, and does not exclude the presence or addition of other configurations.

First, the alloy composition of the cold rolled steel sheet of the present disclosure will be described. The content of the alloy composition mentioned below refers to weight %.

C: 0.20% or More and Less than 0.30%

Carbon (C) is an element that secures the strength of steel through solid solution strengthening and precipitation strengthening. If the content of C is less than 0.20%, it is difficult to secure a tensile strength (TS) of 1.5 GPa. On the other hand, if the content of C is 0.30% or more, arc weldability and laser weldability deteriorate, and the risk of cracking occurrence due to the formation of coarse carbides increases. Therefore, it is preferable that the content of C is in the range of 0.20% or more and less than 0.30%. It is more preferable that the lower limit of the content of C is 0.22%. It is more preferable that the upper limit of the content of C is 0.28%, and it is more preferable that it is 0.26%.

Si: 1.0 to 3.0%

Silicon (Si) is a key element of TRIP (Transformation Induced Plasticity) steel that inhibits the precipitation of cementite and thereby increases the retained austenite fraction and elongation. If the content of Si is less than 1.0%, almost no retained austenite remains, resulting in an excessively low elongation. On the other hand, if the content of Si exceeds 3.0%, the deterioration of weld zone properties due to the formation of LME cracks cannot be prevented, and the surface characteristics and plating properties of the steel deteriorate. Therefore, it is preferable that the content of Si be in the range of 1.0 to 3.0%. Alternatively, the lower limit of the Si content may be 1.48%, or the upper limit of the Si content may be 2.23%.

Al: 0.01 to 0.3%

Aluminum (Al) is not only an element included for deoxidation of steel, but also an element that is effective in stabilizing retained austenite by suppressing precipitation of cementite. If the content of Al is less than 0.01%, deoxidation of the steel is not sufficiently performed, and the cleanliness of the steel is impaired. On the other hand, if the content of Al exceeds 0.3%, the castability of the steel is impaired. Therefore, it is preferable that the content of Al has a range of 0.01 to 0.3%. Alternatively, the lower limit of the content of Al may be 0.02%, or the upper limit of the content of Al may be 0.092%.

Mn: 2.0 to 3.0%

Manganese (Mn) is an element added to secure strength. If the content of the Mn is less than 2.0%, it may be difficult to secure strength, and on the other hand, if the content exceeds 3.0%, the phase transformation speed is slowed down, so that too much fresh martensite is formed, making it difficult to obtain excellent formability. In addition, a band structure is formed due to the segregation of Mn, which impairs the material uniformity and formability of a material. Therefore, the content of the Mn is preferably in the range of 2.0 to 3.0%. The lower limit of the Mn content is more preferably 2.2%, and even more preferably 2.3%. The upper limit of the Mn content is more preferably 2.8%, and even more preferably 2.7%.

Cr: 0.001 to 0.5%

Chromium (Cr) is an element added to secure strength and hardenability. In the case in which Mn is added alone, a very large amount of Mn should be added exceeding the Mn content range of the present disclosure, but this problem may be solved by adding Cr in an amount of 0.001% or more. If the Cr content exceeds 0.5%, local corrosion resistance deteriorates and oxides are formed on the surface, thereby impairing phosphate treatment properties. Therefore, it is preferable that the Cr content has a range of 0.001 to 0.5%. Alternatively, the lower limit of the Cr content may be 0.01%, or it is more preferable that the upper limit of the Cr content is 0.3%.

Mo: 0.08 to 0.32%

Molybdenum (Mo) is an element added to secure strength and hardenability, and when added together with Ti, it forms carbide together with Ti. To obtain the effect of strengthening the structure due to the formation of such carbide, the content of Mo should be added in the amount of 0.08% or more. However, since it is an expensive element, the economic feasibility of the steel sheet is poor and phase transformation may be delayed too much, causing the formation of fresh martensite, it is preferable that the content of Mo does not exceed 0.32%. Alternatively, the lower limit of the Mo content may be 0.132%, or the upper limit of the Mo content may be more preferably 0.27%, and even more preferably 0.22%.

B: 0.0001 to 0.0050%

Boron (B) is an element added to secure hardenability. When Mn is added alone, a very large amount of Mn should be added exceeding the Mn content range of the present disclosure, but this problem may be solved by adding B of 0.0001% or more. However, when the content of B exceeds 0.0050%, boron-based carbides are formed at the grain boundaries, which impairs rather the hardenability. Therefore, it is preferable that the content of B has a range of 0.0001 to 0.0050%. Alternatively, the lower limit of the B content may be 0.0005%, or it is more preferable that the upper limit of the B content is 0.0025%.

Nb: 0.001 to 0.05%

Niobium (Nb) is an element added to secure the strength of the steel sheet and refine the structure. If the Nb is added in an amount of less than 0.001%, it is difficult to obtain the effects of strength improvement and structure refinement, and if the content of the Nb exceeds 0.05%, recrystallization is delayed due to local grain fixation, thereby damaging the uniformity of the structure. Therefore, it is preferable that the content of the Nb has a range of 0.001 to 0.05%. Alternatively, the lower limit of the Nb content may be 0.002%, or more preferably, the upper limit of the Nb content is 0.03%.

Ti: 0.08 to 0.25%

Titanium (Ti) is an element added to secure the strength of the steel plate and to refine the structure. In addition, in the present disclosure, it is a major element that forms carbides by adding 0.08% or more. If the content of the Ti exceeds 0.25%, castability is damaged due to excessive formation of TiN, and the impact properties of the steel are damaged due to excessive formation of carbides. Therefore, it is preferable that the content of Ti is in the range of 0.08 to 0.25%. Alternatively, the lower limit of the Ti content may be 0.096%, or it is more preferable that the upper limit of the Ti content is 0.22%.

P: 0.04% or Less (Excluding 0%)

Phosphorus (P) exists as an impurity in steel, and it is advantageous to control its content as low as possible, but it is also intentionally added to increase the strength of the steel. However, if the P is excessively added, the toughness of the steel deteriorates, so in the present disclosure, it is preferable to limit the upper limit thereof to 0.04% to prevent this. It is more preferable that the content of P is 0.015% or less, more preferably 0.010% or less, and most preferably 0.007% or less.

S: 0.01% or Less (Excluding 0%)

Sulfur(S), like the P, exists as an impurity in steel, and it is advantageous to control its content as low as possible. In addition, since the S deteriorates the ductility and impact properties of the steel, it is preferable to limit its upper limit to 0.01%. Alternatively, the content of the S is more preferably 0.005% or less, more preferably 0.003% or less, and most preferably 0.0015% or less.

N: 0.01% or Less (Excluding 0%)

In the present disclosure, nitrogen (N) is included in the steel as an impurity, and its upper limit is preferably limited to 0.01%. Alternatively, the content of the N is more preferably 0.007% or less, more preferably 0.005% or less, and most preferably 0.003% or less.

In addition to the steel composition described above, the remainder may contain Fe and unavoidable impurities. Unavoidable impurities are those that may be unintentionally mixed in during the normal steel manufacturing process, and they cannot be completely excluded, and those skilled in the field of normal steel manufacturing may easily understand the meaning. In addition, the present disclosure does not completely exclude the addition of other compositions other than the steel composition described above.

Meanwhile, the cold rolled steel sheet according to an embodiment of the present disclosure may additionally include at least one selected from the group consisting of Cu: 0.1% or less and Ni: 0.1% or less in weight %.

Cu: 0.1% or Less, Ni: 0.1% or Less

The copper (Cu) and nickel (Ni) are elements that increase the strength of steel. However, although the elements are elements that increase the strength and hardenability of steel, if they are added in excessive amounts, the target strength grade may be exceeded, and since they are expensive elements, it is preferable to limit upper limits thereof to the levels mentioned above from an economical perspective. Meanwhile, since the Cu and Ni act as solid solution strengthening elements, if they are added in amounts of less than 0.03%, the solid solution strengthening effect may be minimal, and thus it is preferable to add them in amounts of 0.03% or more, respectively.

In addition, the cold rolled steel sheet according to an embodiment of the present disclosure may additionally include V: 0.05% or less (excluding 0%) in weight %.

V: 0.05% or Less (Excluding 0%)

Vanadium (V) may increase the strength of steel even with a small amount of addition, but its effect on improving elongation is not significant, so it is preferable to control its content to 0.05% or less. The content of V is more preferably 0.04% or less, and even more preferably 0.03% or less.

The microstructure of the cold rolled steel sheet according to an embodiment of the present disclosure includes, in area %, ferrite: 10% or less (excluding 0%), retained austenite: more than 2% and 15% or less, fresh martensite: less than 5%, and the remainder includes tempered martensite and bainite.

The cold rolled steel sheet of the present disclosure aims to secure excellent formability at a tensile strength (TS) of 1470 MPa or more, and to obtain particularly high local formability, the hardness difference between the microstructure phases constituting the steel sheet should be reduced. If the ferrite fraction exceeds 10%, the yield strength decreases and the formability such as hole expansion ratio and the like deteriorates.

Retained austenite is a structure that increases the elongation of steel through the TRIP effect, and the higher the fraction, the higher the elongation may be obtained. To obtain the required level of elongation, it is desirable that the fraction of retained austenite exceeds 2%. However, to obtain retained austenite exceeding 15%, a phase transformation should be obtained at a high temperature, and a high strength of 1470 MPa or higher cannot be obtained.

The fresh martensite phase is formed during the final cooling and has a high contribution to strength, but since it is not tempered and greatly impairs formability, the fraction should be controlled so as not to be 5% or more to obtain the high elongation of the present disclosure.

Except for the structures, the remaining microstructure of the steel sheet of the present disclosure is obtained as tempered martensite and bainite.

Meanwhile, according to an embodiment of the present disclosure, the microstructure may include, in terms of area %, ferrite: 1.7 to 10%, retained austenite: 3 to 15%, fresh martensite: 1.0 to 4.5%, and a remainder of tempered martensite and bainite.

In addition, the cold rolled steel sheet according to the present disclosure may have an average number of carbides of 10¹³/m² or more per unit area, in which the total ratio of Ti, Mo, and C atoms to total carbide atoms exceeds 75% (for example, exceeds 75% and 100% or less, or 75.1% or more and 100% or less). The cold rolled steel sheet according to the present disclosure contains 0.08% or more of Ti and Mo, and has a high carbon content of 0.2% or more and less than 0.3%, so that Ti—Mo-based carbides are formed during annealing. These carbides have small particles of 10 to 25 nm and large particles of 100 to 200 nm in size in mixing, and the large particles are first formed during the hot rolling and hot-rolled sheet heat treatment processes, and the small particles are mainly formed during annealing. Through comparison of the tensile material and microstructure observation results, it was found that when the average number of carbides per unit area, in which the total ratio of Ti, Mo and C atoms to total carbide atoms with a size of 10 nm or more (or in the range of 10 to 200 nm) exceeds 75%, is at least 10¹³/m², or more, the TS-El product of the material is improved. Such carbide particles are formed by the addition of Ti, and when the components are analyzed by EDS in TEM, it can be clearly confirmed that carbon (C) and Ti and Mo are the main components in atomic ratio. At this time, in this specification, the size of the carbide means the diameter equivalent to a circle.

Meanwhile, although not particularly limited, in terms of further improving the aforementioned effect, the upper limit of the average number of carbides per unit area, in which the total ratio of Ti, Mo, and C atoms to the total carbide atoms exceeds 75%, may be 5×10¹⁴/m².

The cold rolled steel sheet provided according to an embodiment of the present disclosure has a tensile strength (TS) of 1470 MPa or more, a yield strength (YS) of 1000 MPa or more, an elongation (El) of 12% or more, and a hole expansion ratio (HER) of 25% or more, thereby simultaneously securing excellent strength and formability.

In addition, according to an embodiment of the present disclosure, in addition to the above-described effects, a uniform elongation of 7.5% or more may be additionally secured.

Hereinafter, a method for manufacturing a cold rolled steel sheet having excellent strength and formability according to an embodiment of the present disclosure will be described.

First, a slab having the above-described alloy composition is reheated. The reheating temperature during the slab reheating is preferably 1150 to 1250° C. If the slab reheating temperature is less than 1150° C., the next operation, hot rolling, may not be performed, while if it exceeds 1250° C., a lot of energy is unnecessarily consumed to increase the slab temperature. Therefore, the slab reheating temperature is preferably in the range of 1150 to 1250° C. The lower limit of the slab reheating temperature is more preferably 1170° C., and more preferably 1180° C. The upper limit of the slab reheating temperature is more preferably 1230° C., and even more preferably 1220° C.

Thereafter, the reheated slab is subjected to finishing hot rolling at 830 to 950° C. to obtain a hot-rolled steel sheet. If the finishing hot rolling temperature (hereinafter referred to as ‘FDT’) is less than 830° C., the rolling load is large and shape defects increase, resulting in poor productivity. On the other hand, if the finishing hot rolling temperature exceeds 950° C., the surface quality deteriorates due to an increase in oxides caused by excessive high-temperature work. Therefore, the finishing hot rolling temperature is preferably in the range of 830 to 950° C. The lower limit of the finishing hot rolling temperature is more preferably 880° C. The upper limit of the finishing hot rolling temperature is more preferably 930° C., and even more preferably 910° C.

Meanwhile, according to an embodiment of the present disclosure, after the finishing hot rolling, it is preferable to cool to the coiling temperature described later at an average cooling rate of 10 to 100° C./s. If the average cooling rate is less than 10° C./s, there may be a disadvantage in that the hot rolling productivity is reduced and a cooling medium with reduced capacity cooling during actual production should be deliberately adopted. In addition, if the average cooling rate exceeds 100° C./s, the temperature deviation inside the steel sheet becomes uneven, and there is a risk that the shape will deteriorate and the strength of the steel sheet will increase excessively. Therefore, it is preferable that the average cooling rate has a range of 10 to 100° C./s. Alternatively, the lower limit of the average cooling rate after the hot rolling may be 30° C./s, or the upper limit of the average cooling rate after the hot rolling may be 80° C./s.

Thereafter, the hot-rolled steel sheet is coiled at 400 to 550° C. If the coiling temperature (hereinafter also referred to as ‘CT’) exceeds 550° C., coarse internal hot-rolled oxidation occurs, which has the disadvantages of deteriorating surface characteristics, and Ti—Mo-based precipitates coarsely during cooling from the FDT temperature, often having a final size of 250 nm or more, which reduces the precipitation hardening effect. If the coiling temperature is less than 400° C., it corresponds to the transition boiling range, which has the disadvantages of deteriorating the controllability of the coiling temperature and deteriorating the shape of the steel sheet. The lower limit of the coiling temperature is more preferably 440° C., and even more preferably 480° C. The upper limit of the coiling temperature is more preferably 530° C., and even more preferably 520° C.

After that, the coiled hot-rolled steel sheet is heat-treated at a temperature range of 550 to 650° C. for 5 to 15 hours. The heat treatment is performed using a batch-type furnace in which 1 to 3 coils may be loaded at a time, and the temperature is slowly increased at an average temperature increase rate of 1° C./sec or less from room temperature, and cooling may be performed by selecting air cooling or furnace cooling, and the average cooling rate to room temperature is slowly cooled to less than 1° C./sec.

If the heat treatment temperature of the hot-rolled steel sheet is less than 550° C., the strength after the heat treatment of the hot-rolled steel sheet may be excessively high and the final precipitation amount of Ti—Mo-based carbide may be insufficient, and if the heat treatment temperature exceeds 650° C., the surface properties of the material deteriorate and the size of the Ti—Mo-based precipitate becomes coarse to 250 nm or more. Alternatively, the lower limit of the heat treatment temperature of the hot-rolled steel sheet may be 580° C., or the upper limit of the heat treatment temperature of the hot-rolled steel sheet may be 620° C.

In addition, if the holding time in the range of 550 to 650° C. is shorter than 5 hours, the strength after the heat treatment of the hot-rolled steel sheet may be excessively high, the final precipitation amount of Ti—Mo-based carbides may be insufficient, and the temperature deviation within the coil may be too large, which may also increase the material deviation. Conversely, if the holding time exceeds 15 hours, the surface properties of the material may deteriorate and the Ti—Mo-based precipitation may become coarse. Alternatively, the lower limit of the holding time in the range of 550 to 650° C. may be 12 hours, or the upper limit of the holding time in the range of 550 to 650° C. may be 14 hours.

Thereafter, the coiled hot-rolled steel sheet is cold-rolled to obtain a cold-rolled steel sheet. During the cold rolling, the cold reduction ratio may be 30 to 60%. If the cold reduction ratio is less than 30%, it is difficult to secure the target thickness precision and the shape correction of the steel plate may also become difficult. On the other hand, if the cold reduction ratio exceeds 60%, the possibility of cracks occurring at the edge of the steel plate increases and the cold rolling load may become excessively large. Therefore, it is preferable that the cold reduction ratio has a range of 30 to 60%. Alternatively, the lower limit of the cold reduction ratio may be 33%, or the upper limit of the cold reduction ratio may be 55%.

Thereafter, the cold rolled steel plate is continuously annealed in the range of 830 to 900° C. The continuous annealing operation is performed to heat the steel plate to the austenite single-phase region to form austenite close to 100% and use it for subsequent phase transformation. If the continuous annealing temperature (hereinafter also referred to as ‘SS’) is less than 830° C., sufficient austenite reverse transformation may not occur, and thus there is a concern that 10% or more of the ferrite phase may be formed after annealing. On the other hand, if the continuous annealing temperature exceeds 900° C., the surface quality and productivity may deteriorate, and coarse austenite may be formed, resulting in material deterioration. Alternatively, the lower limit of the continuous annealing temperature may be 841° C., or the upper limit of the continuous annealing temperature may be 877° C.

Thereafter, the continuously annealed cold rolled steel sheet is first cooled to a first cooling end temperature (hereinafter, also referred to as ‘SCS’) of 500 to 700° C. at an average cooling rate of less than 10° C./s. The first cooling end temperature may be defined as the point in time at which a rapid cooling facility that was not applied in the first cooling is additionally applied and second cooling (rapid cooling) is initiated. If the cooling process is divided into first and second cooling and performed in stages, the temperature distribution of the steel sheet may be made uniform in the slow cooling stage, thereby reducing the final temperature and material deviation. If the first cooling end temperature is less than 500° C., there is a concern that soft bainite transformation may be induced, and it is also difficult to cool to below 500° C. at a cooling rate of less than 10° C./s due to the actual equipment length. If the first cooling end temperature exceeds 700° C., the cooling amount to the second cooling end temperature increases, resulting in a poor shape of the steel sheet. Meanwhile, if the first cooling speed is less than 1° C./s, a ferrite phase is formed during cooling, making it difficult to obtain high-strength steel, and if it exceeds 10° C./s, the cooling amount in the second cooling increases, which increases the final temperature deviation and material deviation. Alternatively, the lower limit of the first cooling end temperature may be 580° C., or the upper limit of the first cooling end temperature may be 620° C.

Thereafter, the first-cooled cold-rolled steel sheet is secondarily cooled to a second cooling end temperature (hereinafter referred to as ‘RCS’) of 150 to 350° C. at an average cooling speed of 10° C./s or more. The second cooling end temperature is set to be the Ms or lower temperature of the steel sheet so that martensite transformation occurs during cooling, and this martensite ultimately becomes a tempered martensite phase through a post-process reheating operation. If the second cooling end temperature is less than 150° C., the amount of martensite transformation is too large, so the tensile strength becomes excessively high and the elongation is insufficient, and the yield strength also increases, making forming difficult. However, when most of the structure is composed of tempered martensite, the hole expansion ratio may be maintained high. On the other hand, f the second cooling end temperature exceeds 350° C., martensite is not sufficiently generated during cooling, making it difficult to obtain sufficient yield strength, tensile strength, and hole expansion ratio. In addition, if the fresh martensite fraction ultimately increases, the elongation and hole expansion ratio are greatly impaired. If the second cooling speed is less than 10° C./s, even if the target second cooling end temperature is reached, a high-temperature phase such as upper bainite is mixed during cooling, and thus the target tempered martensite fraction and high strength cannot be obtained. Alternatively, the lower limit of the second cooling end temperature may be 200° C., or the upper limit of the second cooling end temperature may be 325° C.

Thereafter, the secondary cooled cold rolled steel sheet is heated to 200 to 400° C. and then maintained in the temperature range of 200 to 400° C. for a time range of 300 to 1000 seconds. Through the above process, interphase carbon distribution and additional phase transformation necessary for stabilizing the retained austenite are obtained. In the present disclosure, the end point temperature of the heating section is conveniently referred to as the reheating temperature (hereinafter, also referred to as ‘RHS’), and the end point temperature of the maintenance section is conveniently referred to as the over-aging temperature (hereinafter, also referred to as ‘OAS’). If the RHS or OAS temperature is less than 200° C., the strength becomes excessively high and the elongation becomes poor, while, if the RHS and OAS temperatures exceed 400° C., it is difficult to obtain the high strength corresponding to the steel of the present disclosure. The elongation of the steel of the present disclosure is related to the retained austenite, and if the second cooling end temperature is too low and the tempered martensite transformation occurs too much, the space for the austenite to remain may be absolutely insufficient, and if the carbon is not sufficiently distributed into the austenite during the reheating operation, the stability of the austenite deteriorates and it is difficult to obtain an elongation of 12% or more.

According to an aspect of the present disclosure, the end point temperature (RHS) of the reheating section after the second cooling may be 285 to 348° C. At this time, it should be noted that the end point temperature of the reheating section after the second cooling is higher than the second cooling end temperature described above.

In addition, according to an aspect of the present disclosure, the end point temperature (OAS) of the maintenance section of the reheating after the second cooling may be 273 to 342° C. In this case, it should be noted that the end point temperature of the maintenance section of the reheating after the second cooling is lower than the end point temperature of the reheating section after the second cooling described above.

MODE FOR INVENTION

Hereinafter, the present disclosure will be described in more detail through examples. However, it should be noted that the following examples are only intended to explain the present disclosure through examples and are not intended to limit the scope of the rights of the present disclosure. This is because the scope of the rights of the present disclosure is determined by the matters described in the patent claims and matters reasonably inferred therefrom.

Example

After preparing a slab having the alloy composition (weight %) described in Table 1 below, it was reheated at 1180 to 1220° C., and a cold rolled steel sheet was manufactured by performing hot rolling, coiling, annealing, first cooling, second cooling, reheating, and maintenance processes under the conditions described in Table 2 below. At this time, the average cooling rate after the finishing hot rolling was 30-50° C./s, the cold reduction ratio was 33-55%, the average cooling rate of the first cooling was 4.5-6.5° C./s, and also the average cooling rate of the second cooling was 20-40° C./s. In addition, the holding time in the maintenance process was 450-700 seconds(s).

The tensile property evaluation results of the steel plate manufactured in this manner are illustrated in Table 3 below. The tensile strength (TS), yield strength (YS), and total elongation (t-El) were measured through a tensile test in the direction perpendicular to the rolling. The total elongation (t-El) was divided into the uniform elongation (u-El) until again reaching the tensile strength and the local elongation (p-El) until the final fracture thereafter. For the tensile test, a No. 5 tensile test specimen of the KS B0801 standard was used, and the gauge length is 50 mm and the width of the tensile test section is 25 mm. In the tensile test, the elongation was measured by the so-called nominal strain, which is calculated by dividing the amount of elongation of the specimen by the initial gauge length (50 mm), and the total nominal strain until the specimen breaks is referred to the total elongation (t-El). The strength was measured by the so-called nominal stress, which is calculated by dividing the load measured during the tensile test by the initial cross-section of the specimen, and the stress value when this nominal stress reaches a maximum value is referred to the tensile strength (TS). Here again, the uniform elongation (u-El) refers to the nominal strain value when the nominal stress reaches the tensile strength (TS), and the local elongation (p-El) refers to the nominal strain amount, going forward after reaching the tensile strength, until the final fracture occurs. In detail, the total elongation is the sum of the uniform elongation and the local elongation.

The hole expansion ratio (HER) was measured according to the ISO 16630 standard. First, a square specimen with a side length of 120 mm was prepared. Then, a hole of size 10 mm (Do) was punched in the center, and the clearance at this time was set to 12%. Then, the punched hole was pushed up with a cone-shaped punch of a 60° angle to expand the hole until a crack penetrating the entire thickness was formed. If the expanded hole diameter when a through-thickness crack occurred is Df, the hole expansion ratio value is obtained by the following equation.

HER ⁢ ( % ) = D f - D 0 D 0 × 100

In addition, the results of measuring the microstructure fraction are illustrated together in Table 3. The microstructure fraction was measured by the Point Counting method from the scanning electron microscope (SEM) image, and the fraction of retained austenite was measured by XRD and is illustrated in Table 3 below.

In addition, for the cold-rolled steel sheet manufactured by the above-mentioned method, five specimens were manufactured, and then the number (pieces/m²) of carbides per unit area, in which the total ratio of Ti, Mo, and C atoms to the total carbide atoms with the size range of 10 to 200 nm in each specimen exceeds 75%, was measured through TEM observation, and then the average value was obtained and is illustrated as D in Table 3 below.

TABLE 1
Steel
Grade	C	Si	Mn	Al	Mo	Ti	B	Cr	Nb	P	S	N

A	0.188	0.482	2.59	0.018	0.102	0.018	0.0022	0.01	0.019	0.0085	0.0018	0.0053
B	0.193	0.964	2.59	0.021	0.03	0.023	0.0022	0.02	0.003	0.0065	0.0019	0.0029
C	0.197	1.370	2.65	0.025	0.109	0.022	0.0024	0.11	0.018	0.0112	0.0008	0.0043
D	0.193	1.911	2.61	0.045	0.109	0.021	0.0023	0.05	0.020	0.0099	0.0025	0.0050
E	0.245	0.515	2.47	0.013	0.106	0.017	0.0019	0.03	0.018	0.0072	0.0018	0.0028
F	0.243	0.962	2.50	0.024	0.210	0.019	0.0019	0.16	0.002	0.0053	0.0022	0.0028
G	0.240	1.912	2.47	0.023	0.01	0.021	0.0020	0.01	0.018	0.0085	0.0012	0.0030
H	0.237	1.911	2.57	0.039	0.103	0.023	0.0024	0.02	0.001	0.0083	0.0023	0.0044
I	0.290	2.011	2.70	0.161	0.150	0.021	0.0002	0.22	0.021	0.0062	0.0013	0.0035
J	0.249	1.93	2.59	0.0438	0.210	0.0962	0.0021	0.01	0.002	0.0065	0.0031	0.0044
K	0.24	2.23	2.57	0.092	0.132	0.096	0.0021	0.02	0.012	0.0082	0.0011	0.0021
L	0.24	1.48	2.56	0.088	0.208	0.102	0.0018	0.01	0.003	0.0080	0.0009	0.0033
M	0.25	1.56	2.72	0.032	0.01	0.110	0.0022	0.32	0.021	0.0077	0.0032	0.0052

TABLE 2
		Hot	Cold
		Rolled	Rolled	Reduction			Heat
	Steel	Thickness	Thickness	Ratio	FDT	CT	Treatment	SS	SCS	RCS	RHS	OAS
Classification	Grade	[mm]	[mm]	[%]	[° C.]	[° C.]	Condition	[° C.]	[° C.]	[° C.]	[° C.]	[° C.]

Comparative	A	3.5	2.0	43%	902	525	630° C.	831	612	155	203	212
Example 1							9 hr
Comparative	B	3.3	1.6	52%	927	523	630° C.	823	598	202	251	243
Example 2							10 hr
Comparative	C	2.5	1.2	52%	933	497	620° C.	819	562	199	249	232
Example 3							12 hr
Comparative	D	2	1.1	45%	895	542	610° C.	823	548	196	248	222
Example 4							13 hr
Comparative	E	2	1.0	50%	872	488	560° C.	810	667	204	252	253
Example 5							12 hr
Comparative	F	2	0.9	55%	866	432	580° C.	825	668	202	302	311
Example 6							11 hr
Comparative	G	2.1	1.2	43%	851	512	570° C.	833	612	245	298	279
Example 7							12 hr
Comparative	H	2.3	1.4	39%	899	475	610° C.	841	623	248	296	272
Example 8							10 hr
Comparative	I	2.5	1.6	36%	902	433	600° C.	825	574	252	352	312
Example 9							15 hr
Comparative	B	2.4	1.2	50%	913	412	580° C.	782	589	152	211	206
Example 10							12 hr
Invention	J	2.5	1.3	48%	897	452	610° C.	841	592	302	323	312
Example 1							14 hr
Invention	J	2.1	1.2	43%	886	442	590° C.	866	612	248	348	342
Example 2							12 hr
Invention	K	2.6	1.4	46%	902	505	590° C.	853	602	225	285	273
Example 3							13 hr
Invention	L	2.9	1.5	48%	917	515	600° C.	877	598	252	305	297
Example 4							13 hr
Comparative	M	2.1	1.1	45%	933	521	570° C.	863	605	152	202	206
Example11							13 hr
Comparative	L	2.6	1.3	50%	875	652	670° C.	868	602	235	303	299
Example 12							12 hr

	TABLE 3
	Characteristics

	Total		Uniform	Microstructure Fraction
	Elongation		Elongation	[area %]	D*

	Steel	YS	TS	(t-El)	HER	(U-El)				Retained	[pieces/
Classification	Grade	[MPa]	[MPa]	[%]	[%]	[%]	F	TM + B	FM	Y	m2]

Comparative	A	1083	1509	8.7	38%	5.1	3.0	90.8	4.2	2.0	—
Example1
Comparative	B	1108	1504	10.1	41%	5.4	2.5	91.6	3.8	2.1	—
Example2
Comparative	C	1057	1525	9.3	30%	5.2	4.8	90.1	2.7	2.4	—
Example3
Comparative	D	1105	1531	10.1	24%	5.6	3.5	90.9	3.3	2.3	—
Example4
Comparative	E	1075	1539	8.9	33%	5.6	2.1	93.5	2.5	1.9	—
Example5
Comparative	F	1219	1501	9.2	28%	5.0	6.5	84.8	3.7	5.0	—
Example6
Comparative	G	1055	1483	11.6	27%	6.6	1.5	83.7	3.3	11.5	—
Example7
Comparative	H	1114	1513	10.9	24%	6.4	0.6	90.5	2.2	6.7	—
Example8
Comparative	I	1137	1507	11.9	26%	6.6	1.7	87.2	3.3	7.8	—
Example9
Comparative	B	819	1536	9.8	29%	5.5	25.6	71.7	2.1	0.6	—
Example10
Invention	J	1066	1502	13.5	33%	8.5	2.3	82.7	4.5	10.5	7.3 × 1013
Example 1
Invention	J	1235	1513	14.1	28%	8.9	3.1	79.9	3.7	13.3	1.1 × 1014
Example 2
Invention	K	1099	1507	12.2	29%	8.0	1.7	85.2	2.8	10.3	2.2 × 1013
Example 3
Invention	L	1152	1493	12.7	31%	8.2	2.1	83.2	3.5	11.2	4.3 × 1013
Example 4
Comparative	M	1052	1497	10.4	24%	6.2	2.2	91.9	2.7	3.2	4.7 × 1012
Example11
Comparative	L	1108	1448	11.7	24%	6.7	3.1	86.3	3.4	7.2	7.7 × 1012
Example12

(F: ferrite, TM: tempered martensite, B: bainite, FM: fresh martensite, retained y: retained austenite)

(D*: average number of carbides per unit area in which the total ratio of Ti, Mo, and C atoms to the total carbide atoms exceeds 75%)

As can be seen from the Tables 1 to 4, Invention Examples 1 to 4 satisfied the Ti, Mo addition range of the present disclosure and also satisfied the phase fraction, thereby exhibiting a yield strength (YS) of 1000 MPa or more, a tensile strength (TS) of 1470 MPa or more, an elongation (El) of 12% or more, and a hole expansion ratio (HER) of 25% or more.

In particular, an image of the Ti—Mo-based precipitate observed on the specimen obtained from Invention Example 1 by SEM is illustrated in FIG. 1.

On the other hand, Comparative Examples 1 to 9 manufactured using steel grades A to I did not satisfy the lower limit of Si and the lower limit of the addition amounts of Ti and Mo proposed by the present disclosure, and thus, although they satisfied all other manufacturing conditions and phase fraction conditions, it can be seen that the elongation did not reach 12% or the hole expansion ratio (HER) did not meet 25% or more even if they achieves a yield strength of 1000 MPa or more and a tensile strength of 1470 MPa or more.

In addition, it was found that in the case of Comparative Example 10, in which the ferrite phase fraction exceeded 10%, the yield strength did not reach 1000 MPa.

In addition, in the case of Comparative Example 12, although it satisfied the component range of the present disclosure, the coiling temperature was high and the heat treatment temperature was high, so that a large amount of Ti and Mo were consumed due to coarse Ti—Mo-based carbides of 250 nm or more, and the required material was not obtained.