Method For Producing Super High Strength Nano-Precipitation Strengthened, Single Phase Ferritic Steel

Description

BACKGROUND ART

This patent application claims priority to a US provisional patent application having application Ser. No. 63/540,542, filed Sep. 26, 2023, the subject matter of which is expressly incorporated herein by reference.

1. FIELD OF THE INVENTION

The invention relates to a hot rolled steel sheet having a specific chemistry and a method for producing super high strength steel. More particularly, the invention relates to a method for producing super high strength steel while minimizing production steps.

2. DESCRIPTION OF THE RELATED ART

Sheet Steels are thin and are required to have a processing step commonly referred to as cold rolling. After a steel is hot rolled it is usually cold rolled to make it thinner followed by a high temperature continuous heat treatment process (referred to as annealing) to achieve the properties so that they can be used in making autobody structural as well as panel applications. The cold rolling, annealing involves usage of energy, time and hence makes the entire process costly besides increasing CO₂ levels in the atmosphere.

SUMMARY OF THE INVENTION

A hot-rolled steel sheet is defined by the following composition by weight percentage: 0.040%<carbon<0.06%, 1.20%<manganese<1.50%, 0.02%<aluminum<0.04%, 0.09%<titanium<0.15%, 0.15%<silicon<0.40%, 0.00%<copper<0.20%, with the balance being iron and impurities inherent in processing. The hot-rolled steel sheet has a tensile strength equal to or greater than 780 MPa.

BRIEF DESCRIPTION OF THE DRAWINGS

Advantages of the invention will be readily appreciated as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein:

FIG. 1 is a schematic of a production line using the inventive method to produce the hot rolled steel;

FIG. 2 is a schematic of a prior art production line used to make a comparable steel;

FIG. 3 is a photograph of the grain structure of a hot rolled steel using the inventive method;

FIG. 4 is a graph showing the yield and tensile strengths of the hot rolled steel as a function of its thickness;

FIG. 5 is a graph showing the elongation of the hot rolled steel as a function of its thickness; and

FIG. 6 is a graph showing hole expansion of the hot rolled steel as a function of tensile strength.

DETAILED DESCRIPTION OF THE DRAWINGS

Referring to FIG. 1, one embodiment of a mill line is generally indicated at 10. From the perspective when viewing FIG. 1, the mill line 10 begins on the left side and finishes on the right side. The mill line 10 begins with a ladle 12 where steel (not shown) is melted at a temperature within the range of 40° F. to 70° F. above liquidus temperature. At the appropriate time, the melted steel (not shown) is poured into a tundish 14, where the melted steel is collected.

The melted steel then is poured through a mold 16. The mold 16 casts the melted steel to create cast steel 20 as it exits the mold 16. The cast steel 20 enters a segment section 22. The segment section 22 includes water cooled lines to cool the cast steel 20 from the outside in. This forms a case around the molten steel 20 and allows the molten steel 20 to continue to move through the mill line 10. The cooling of the cast steel 20 by the segment section 22 reduces the temperature to within a range of 1950° F.-1650° F.

A sheer station 26 cut the cast steel 20 based on downstream activity: namely, upon the determination that enough cast steel 20 has passed the sheering station 26 to produce a complete roll of steel 30. At this stage in the mill line 10, the cast steel 20 is in range of 55 mm and 85 mm thick.

A furnace 32 maintains the temperature of the cast steel 20 to within a range of 2050° F.-2100° F. for a period within a range of 15 minutes to 35 minutes as the cast steel 20 passes therethrough. Finishing mill stands 34 hot roll the cast steel 20 into a hot-rolled steel 36. The hot-rolled steel 36 is rolled out to a thickness approximately 1.4 mm. In the current process, sheet steels are hot rolled down to 1.4 mm and achieve the high strength mechanical properties after hot rolling itself.

A laminar cooling structure 40 cools the hot-rolled steel 36 before it is coiled by the coiling station 42 to create the steel coil 30. The laminar cooling structure 40 cools the temperature of the hot-rolled steel 36 to within a range of 1100° F.-1225° F. for a period within a range of approximately six seconds to 15 seconds.

Referring to FIG. 2, wherein like prime numerals represent similar elements as those described in FIG. 1, a prior art mill line 10′ is shown. It is similar to the mill line 10 of FIG. 1, but it includes additional elements. The first additional element is a slab cutting station 27, which cuts the cast steel 20′ at predetermined lengths for storage. When stored, the cast steel segments 29 are stored at room temperature.

Once retrieved from storage, the cast steel segments 29 are heated a second time in a second furnace 31. This second heating of the cast steel segments 29 allows the thickness of the cast steel segments 29 to be reduced by a rougher 33. The rougher 33 reduces the thickness of the cast steel segments 29 to approximately 35 mm-45 mm. Once the cast steel segments 29 pass through the rougher 33, they are hot-rolled by the finishing mill stands 34′ into hot-rolled steel 36′ and coiled by the coiling station 42′.

By using the processing route set forth by the mill line 10 of FIG. 1, a slab cutting station (27 of FIG. 2), a second furnace (31 of FIG. 2) and a rougher (33 of FIG. 2) are not needed. This provides the distinct advantage of reducing the stations required to create a coil 30 of hot-rolled steel 36, which translates directly into a smaller footprint for the mill line 10 as well as reduced energy consumption in producing the same coil 30 of hot-rolled steel 36. An added advantage to the processing route of the mill line 10 is that there is one less level of inventory of material needed than with the prior art. This is because an inventory of the cast steel slabs 29 is not needed.

The steel used in the mill line 10 (of FIG. 1) has a chemistry and shortest processing route for the continuous production from casting to coiling of a super high strength nano-precipitation strengthened, single phase ferritic steel with essential elemental composition in wt. % of carbon0.040 to 0.06%, Mn 1.2 to 1.5%, Al 0.02 to 0.04%, Ti 0.09 to 0.15%, Si 0.15-0.40%, Cu 0-0.20% and balance iron and impurities inherent in processing. The essentially ferritic steel exhibits a tensile strength minimum of 780 MPa with an enhanced hole expansion ratio (a measure of edge stretchability) of a minimum of 40% after hot rolling. The steel retains the same strength and forming properties after pickling and Zn-coating (galvanizing). The steel, the composition of which is described subsequently, in combination with the above-described method produces a super high strength and formable steel. The steel is produced in a continuous manner from casting of slab of minimum slab thickness between 55 mm to 110 mm. The steel is then hot rolled and coiled, thereby saving a significant amount of process time and energy, all of which directly results in a significant reduction in CO₂ emissions.

When needed to be galvanized, the steel 36 can be pickled and galvanized. The steel 36 manifests a minimum tensile strength of 780 MPa with an outstanding hole expansion ratio of at least 40%. There is no need for cold rolling and high temperature annealing as the thickness can be brought down to 1.4 mm during hot rolling itself. The steel processing eliminates the need for cold rolling and high temperature annealing used in conventional processing. The processing eliminates multiple processing steps thereby saving significant amounts of energy and production times while reducing overall CO₂ emission.

The composition of the steel 36 includes the following composition by weight percentage; 0.040%<carbon<0.06%, 1.20%<manganese<1.50%, 0.02%<aluminum<0.04%, 0.09%<titanium<0.15%, 0.15%<silicon<0.40%, 0.00%<copper<0.20%, with the balance being iron and impurities inherent in processing.

The hot-rolled steel 36 that results in the steel roll 30, the composition of steel 36 as set forth above, in combination of with the temperature ranges and time ranges set forth above, results in a very fine ferrite grain size of approximately two to five microns as is shown in FIG. 3. Such a consistent ferrite grain size throughout the hot-rolled steel 36 is a desired characteristic because it guarantees ultra-high strength (ultra-high strength is any yield strength greater than 550 MPa).

The super high strength combined with enhanced formability properties makes this steel 36 applicable for automotive structural components thereby savings in steel total weight, increasing fuel efficiency and reduced CO₂ emission.

Referring to FIG. 4, the strength of the hot-rolled steel 36 as a function of thickness is shown. The yield strength data points are the circles, whereas the tensile strength data points are the squares. For example, it can be seen that with a thickness of approximately 2.0 mm, the yield strength is approximately 750 MPa and the tensile strength is approximately 840 MPa.

Referring to FIG. 5, data points represent the elongation of the hot-rolled steel 36 as a function of sheet thickness. Referring to FIG. 6, hole expansion (measured as a percentage) is measured as a function of tensile strength.

By eliminating the steps of cooling the steel slab and reheating it, the composition of the steel 36 set forth above be processed using the method illustrated in FIG. 1 manifests higher elongation, toughness and formability and very low in carbon equivalence (lower carbon dioxide emission during processing), which makes this steel easily cold formable and weldable. These are unexpected results that are highly desirable. In addition, the elimination of the thermomechanical rolling (using the rougher 33 of FIG. 2) further quickens the processing of the hot-rolled steel 36.

The invention has been described in an illustrative manner. It is to be understood that the terminology, which has been used, is intended to be in the nature of words of description rather than of limitation.

Many modifications and variations of the invention are possible in light of the above teachings. Therefore, within the scope of the appended claims, the invention may be practiced other than as specifically described.