Slab for High-Strength Steel Sheet and Cooling Method Thereof, Producing Method of High-Strength Hot-Rolled Steel Sheet, Producing Method of High-Strength Cold-Rolled Steel Sheet, and Producing Method of High-Strength Plated Steel Sheet

This is the U.S. National Phase application of PCT/JP2023/012741, filed Mar. 29, 2023 which claims priority to Japanese Patent Application No. 2022-077128, filed May 9, 2022, the disclosures of these applications being incorporated herein by reference in their entireties for all purposes.

The present invention relates to a slab for a high-strength steel sheet and a cooling method thereof that prevent cracking during cooling. In addition, the present invention relates to methods of producing a high-strength hot-rolled steel sheet from that slab for a high-strength steel sheet, a high-strength cold-rolled steel sheet from that high-strength hot-rolled steel sheet, and producing a high-strength plated steel sheet from that high-strength cold-rolled steel sheet.

In recent years, to further reduce the thickness of vehicle bodies and secure collision safety at the same time, the automotive field has been making progress toward further enhancing the strength of high-strength steel and increasing alloy contents to that end. Increasing the alloy contents causes a significant decrease in the toughness of slabs.

As the toughness of a slab decreases with an increase in the alloying degree, cracking in the slab during cooling, known as season cracking, has occurred more frequently. Such season cracking may cause the slab to fracture while being conveyed, preventing the slab from being hot rolled. Even if the slab does not fracture, the cracks in the slab may open during hot rolling, causing the resulting hot-rolled steel sheet to fracture. Meanwhile, small cracks in a slab may appear as surface defects, such as scabs or slivers, on the resulting steel sheet after hot rolling, cold rolling, annealing, or plating. Typically, cracks in the surface of a slab are removed with a grinder. However, in a case where the toughness of the slab has decreased with an increase in the alloying degree and the cracks in the slab develop due to the stress applied by the grinder, it may be impossible to remove the cracks in the slab completely. Furthermore, small cracks in the slab may be overlooked and appear as surface defects on the resulting steel sheet after hot rolling, cold rolling, annealing, or plating. For the above reasons, it is necessary to suppress cracking in slabs.

In this regard, measures against cracking of slabs, for example, like those described in Patent Literatures 1 and 2 have been explored. Patent Literature 1 discloses a method in which slow cooling is performed from 700 to 500° C. which is a temperature range where austenite transforms into ferrite to thereby inhibit bainite-martensite transformation and reduce the stress exerted by expansion during the transformation. Patent Literature 2 discloses a method for reducing a temperature difference and reducing stress due to transformation by starting slow cooling of a slab immediately after the slab is cast, then slowly cooling the slab at a temperature of 700° C. or higher for 10 hours or longer and further from 700 to 500° C.

Patent Literature 1: JP-2020-139209A

Patent Literature 2: JP-2019-167560A

However, the conventional techniques have the following problem.

When it comes to more highly alloyed slabs that have low toughness, the techniques of Patent Literatures 1 and 2 cannot completely inhibit season cracking of these slabs.

An object of aspects of the present invention, which has been made in view of these circumstances, is to provide a slab for a high-strength steel sheet and a cooling method thereof that, even when the slab is a low-toughness, highly alloyed slab, do not cause slab cracking during cooling of the slab. Another object is to provide producing methods of a high-strength hot-rolled steel sheet, a high-strength cold-rolled steel sheet, and a high-strength plated steel sheet using this slab.

To achieve the above objects, the present inventors have vigorously conducted studies. The inventors analyzed the fracture morphology of slab cracking and found that in the fracture surface, there was at least one type of fracture surface selected from an intergranular fracture surface along a prior austenite grain boundary and a transgranular fracture surface (cleavage fracture surface) that crosses a prior austenite grain boundary. We conducted further studies and gained the following insights:

Aspects of the present invention have been completed after further studies were conducted based on these insights.

Aspects of the present invention are as follows:

[1] A slab for a high-strength steel sheet which has been continuously cast, characterized in that an average prior austenite grain size at a position 10 mm from a slab surface layer is 2.0 mm or less, and that the slab has a microstructure in which a total area ratio of bainitic ferrite and tempered martensite is 50% or more but 97% or less, an area ratio of residual austenite is 3% or more but 30% or less, an area ratio of ferrite is 20% or less, and a total area ratio of pearlite and quenched martensite is 20% or less.

[2] The slab for a high-strength steel sheet according to 1 above, wherein the slab for a high-strength steel sheet has an ingredient composition containing, in mass %: C: 0.10% or more but 0.50% or less, Si: 0.10% or more but 2.50% or less, Mn: 1.00% or more but 5.00% or less, P: 0.100% or less, S: 0.0200% or less, Al: 0.005% or more but 2.500% or less, N: 0.0100% or less, and O: 0.0100% or less, and further optionally containing at least one type of element selected from the following: Ti: 0.200% or less, Nb: 0.200% or less, V: 0.200% or less, Ta: 0.10% or less, W: 0.10% or less, B: 0.0100% or less, Cr: 1.00% or less, Mo: 1.00% or less, Co: 1.00% or less, Ni: 1.00% or less, Cu: 1.00% or less, Sn: 0.200% or less, Sb: 0.200% or less, Ca: 0.0100% or less, Mg: 0.0100% or less, REM: 0.0100% or less, Zr: 0.100% or less, Te: 0.100% or less, Hf: 0.10% or less, and Bi: 0.200% or less, with the balance being Fe and unavoidable impurities.

[3] The slab for a high-strength steel sheet according to 1 above, wherein the slab for a high-strength steel sheet has an ingredient composition containing, in mass %: C: 0.10% or more but 0.50% or less, Si: 0.70% or more but 2.50% or less, Mn: 1.00% or more but 5.00% or less, P: 0.100% or less, S: 0.0200% or less, Al: 0.005% or more but 2.500% or less, N: 0.0100% or less, and O: 0.0100% or less, and further optionally containing at least one type of element selected from the following: Ti: 0.200% or less, Nb: 0.200% or less, V: 0.200% or less, Ta: 0.10% or less, W: 0.10% or less, B: 0.0100% or less, Cr: 1.00% or less, Mo: 1.00% or less, Co: 1.00% or less, Ni: 1.00% or less, Cu: 1.00% or less, Sn: 0.200% or less, Sb: 0.200% or less, Ca: 0.0100% or less, Mg: 0.0100% or less, REM: 0.0100% or less, Zr: 0.100% or less, Te: 0.100% or less, Hf: 0.10% or less, and Bi: 0.200% or less, with the balance being Fe and unavoidable impurities.

[4] A cooling method of a slab for a high-strength steel sheet, characterized in that the slab for a high-strength steel sheet with the ingredient composition according to 2 above is cooled such that a retention time in a temperature range of 1200° C. or higher but 1450° C. or lower at the position of a widthwise center 10 mm below a surface layer of the slab is 130 seconds or less;

then cooled such that an average cooling rate while a surface temperature at a widthwise center of the slab is in the range of 700° C. or higher but 850° C. or lower is 25° C./hr or more; cooled such that an average cooling rate while the surface temperature is in the range of 550° C. or higher but lower than 700° C. is 20° C./hr or more; cooled such that an average cooling rate while the surface temperature is in the range of 400° C. or higher but lower than 550° C. is 15° C./hr or more; cooled such that an average cooling rate until the surface temperature reaches a cooling stop temperature of 250° C. or higher but lower than 400° C. is 10° C./hr or more; heated so that the surface temperature reaches a reheating temperature of higher than the cooling stop temperature but 450° C. or lower; and then cooled such that an average cooling rate while the surface temperature is in the range of 200° C. or higher but the reheating temperature or lower is 30° C./hr or less.

[5] A cooling method of a slab for a high-strength steel sheet, characterized in that the slab for a high-strength steel sheet with the ingredient composition according to 3 above is cooled such that a retention time in the temperature range of 1200° C. or higher but 1450° C. or lower at the position of a widthwise center 10 mm below a surface layer of the slab is 130 seconds or less; cooled such that an average cooling rate while a surface temperature at a widthwise center of the slab is in the range of 700° C. or higher but 850° C. or lower is 25° C./hr or more; cooled such that an average cooling rate while the surface temperature is in the range of 550° C. or higher but lower than 700° C. is 20° C./hr or more; cooled such that an average cooling rate while the surface temperature is in the range of 400° C. or higher but lower than 550° C. is 10° C./hr or more; and then cooled such that an average cooling rate while the surface temperature is in the range of 200° C. or higher but lower than 400° C. is 30° C./hr or less.

[6] A producing method of a high-strength hot-rolled steel sheet, characterized in that the slab for a high-strength steel sheet according to any one of 1 to 3 above is heated such that a slab heating temperature is 1000° C. or higher but 1300° C. or lower, rough rolled, and then finish rolled such that a finish rolling end temperature is 750° C. or higher but 1000° C. or lower, and wound such that a winding temperature is room temperature or higher but 750° C. or lower.

[7] A producing method of a high-strength cold-rolled steel sheet, characterized in that: a high-strength hot-rolled steel sheet produced by the producing method according to 6 above is pickled and then cold-rolled such that a rolling reduction is 30% or more but 80% or less; and optionally, one process selected from the following is further performed: (a) a process in which the high-strength cold-rolled steel sheet obtained by the cold rolling is heated such that an annealing temperature is 750° C. or higher but 950° C. or lower, cooled to a cooling stop temperature of 300° C. or higher but 600° C. or lower, and then cooled to 100° C. or lower; (b) a process in which the high-strength cold-rolled steel sheet obtained by the cold rolling is heated such that an annealing temperature is 750° C. or higher but 950° C. or lower, cooled to a cooling stop temperature of 130° C. or higher but 400° C. or lower, reheated to a temperature of 200° C. or higher but 450° C. or lower, and then cooled to 100° C. or lower, and (c) a process in which the high-strength cold-rolled steel sheet obtained by the cold rolling is heated such that an annealing temperature is 750° C. or higher but 950° C. or lower, followed by water quenching at 500° C. or higher, cooled by water to 100° C. or lower, and then reheated at 100° C. or higher but 300° C. or lower.

[8] A producing method of a high-strength plated steel sheet, characterized in that: a high-strength cold-rolled steel sheet obtained by the cold rolling according to 7 above is heated such that an annealing temperature is 750° C. or higher but 950° C. or lower; the high-strength cold-rolled steel sheet is subjected to a molten-metal plating process and turned into a plated steel sheet; the plated steel sheet is then cooled under a condition of a cooling stop temperature of 150° C. or lower; and the molten-metal plating process optionally adopts one type selected from zinc plating, zinc-based-alloy plating, zinc-Al-alloy plating, and Al plating.

[9] The producing method of a high-strength plated steel sheet according to 8, wherein the plated steel sheet having been subjected to the molten-metal plating process is subjected to an alloying process.

[10] A producing method of a high-strength plated steel sheet, characterized in that: using a high-strength cold-rolled steel sheet produced by the producing method according to 7 above, an electroplating process is performed on a surface; and optionally, the electroplating process adopts one type selected from zinc plating, zinc-based-alloy plating, zinc-Al-alloy plating, and Al plating.

Aspects of the present invention can provide a slab that, even when composed of ingredients for a high-strength steel sheet with a high alloy content, does not develop cracks during a cooling process. In addition, aspects of the present invention can provide producing methods of a high-strength hot-rolled steel sheet, a high-strength cold-rolled steel sheet, and a high-strength plated steel sheet using this slab that have few surface defects.

Embodiments of the present invention will be specifically described below. The following embodiments illustrate steel structures and methods for embodying the technical idea of aspects of the present invention and are not intended to restrict the configuration to the one to be described below. Thus, various changes can be made to the technical idea of aspects of the present invention within the technical scope described in the claims.

First, appropriate ranges of the microstructure of a slab and reasons for restriction will be described. In the following description, “%” representing a constituent ratio in the microstructure means “area %” unless otherwise indicated.

The average prior austenite grain size is a factor that determines the unit of fracture. As the grain size increases, the toughness of the slab decreases, resulting in slab cracking that exhibits an intergranular fracture surface. To inhibit this slab cracking, it is necessary that the average prior austenite grain size at the position 10 mm from the slab surface layer is 2.0 mm or less. It is preferably 1.8 mm or less, more preferably 1.5 mm or less. The average prior austenite grain size can be measured by a method described in Examples to be described later.

Since bainitic ferrite and tempered martensite have high toughness compared with pearlite and quenched martensite, they are important constituent elements in this embodiment and can enhance the toughness of the steel and inhibit slab cracking. To achieve this effect, the total of the area ratios of bainitic ferrite and tempered martensite at the position 10 mm from the surface layer of the slab needs to be 50% or more. It is preferably 60% or more, more preferably 70% or more, and even more preferably 80% or more. When the total area ratio exceeds 97%, the toughness-improving effect of residual austenite may not be produced; therefore, the total area ratio should be 97% or less. The area ratios of bainitic ferrite and tempered martensite can be measured by a method described in Examples to be described later.

Residual austenite, which is an extremely important constituent element in this embodiment, has a face-centered cubic lattice (FCC) crystal structure with no cleavage plane, and can therefore dramatically improve the toughness of the steel. When subjected to a high stress, residual austenite undergoes martensitic transformation. Even when a crack develops due to slab cracking, as martensite forms in a stress-concentrated part at a leading end of the crack, the stress concentration is mitigated and the crack can be stopped from growing. Thus, surface defects of the steel sheet after hot rolling, cold rolling, annealing, or plating can be inhibited. To achieve this effect, it is necessary that the area ratio of residual austenite at the position 10 mm from the slab surface layer is 3% or more. Preferably, it is 5% or more, more preferably 7% or more. When residual austenite exceeds 30%, unstable residual austenite increases and undergoes martensitic transformation under low stress, which may result in reduced toughness; therefore, the area ratio should be 30% or less. It is preferably 25% or less, more preferably 20% or less. The area ratio of residual austenite can be measured by a method described in Examples to be described later.

Compared with bainitic ferrite, tempered martensite, quenched martensite, residual austenite, and pearlite, ferrite has a large grain size and low strength. Therefore, when stress is applied, the stress may be concentrated on ferrite and cracking originating from ferrite may occur. To inhibit such cracking, the area ratio of ferrite at the position 10 mm from the slab surface layer needs to be 20% or less. Preferably, it is 15% or less, more preferably 10% or less, and even more preferably 0%. The area ratio of ferrite can be measured by a method described in Examples to be described later.

Pearlite and quenched martensite are inferior in toughness to residual austenite, bainitic ferrite, and tempered martensite, and the presence of large amounts of pearlite and quenched martensite may result in cracking originating from these structures. To inhibit such cracking, the total of the area ratios of pearlite and quenched martensite at the position 10 mm from the slab surface layer needs to be 20% or less. Preferably 15% or less. More preferably 10% or less. Even more preferably 0%. The area ratios of pearlite and quenched martensite can be measured by a method described in Examples to be described later.

Next, appropriate ranges of the ingredient composition and reasons for limitation thereof will be described. In the following description, “%” representing a content of an ingredient element of the steel means “mass %” unless otherwise indicated.

C is an essential element that affects the fraction of residual austenite in the slab and increases the strength of the steel sheet. When the C content is less than 0.10%, it may be difficult to secure sufficient residual austenite in the slab. Or it may be difficult to achieve the tensile strength (TS) required for the steel sheet. On the other hand, when the C content exceeds 0.50%, the fraction of quenched martensite in the slab may become excessive. Therefore, the C content is preferably 0.10% or more but 0.50% or less. More preferably 0.12% or more. More preferably 0.45% or less. Even more preferably 0.15% or more. Even more preferably 0.40% or less.

Si inhibits the formation of carbide during cooling of the slab and promotes the formation of residual austenite, and thus is an element that affects the fraction of residual austenite.

When a slab cooling method to be described later involves reheating, the Si content is preferably 0.10% or more. When the Si content is less than 0.10%, the fraction of residual austenite decreases, which may lead to slab cracking. More preferably 0.15% or more. Even more preferably 0.20% or more.

When the slab cooling method to be described later does not involve reheating but uniformly lowers the temperature, the Si content is preferably 0.70% or more. When the Si content is less than 0.70%, the fraction of residual austenite decreases, resulting in slab cracking. More preferably 0.90% or more. Even more preferably 1.00% or more.

On the other hand, when the Si content exceeds 2.50%, firm scale forms on the hot-rolled steel sheet, which may result in surface defects. Therefore, the Si content is preferably 2.50% or less. More preferably 2.00% or less. Even more preferably 1.80% or less.

Mn is an essential element that affects the fraction of residual austenite and enhances the strength of the steel sheet. When the Mn content is less than 1.00%, it may be difficult to secure sufficient residual austenite in the slab. Or it may be difficult to achieve the tensile strength (TS) required for the steel sheet. On the other hand, when the Mn content exceeds 5.00%, the fraction of quenched martensite in the slab may become excessive. Therefore, the Mn content is preferably 1.00% or more but 5.00% or less. More preferably 1.20% or more. More preferably 4.50% or less. Even more preferably 1.40% or more. Even more preferably 4.00% or less.

P segregates at prior austenite grain boundaries to cause the embrittlement of these grain boundaries, which may result in slab cracking. Therefore, the P content is preferably 0.100% or less. While the lower limit of the P content is not defined, since P is a solid-solution strengthening element and can increase the strength of the steel sheet, it is more preferably 0.001% or more. More preferably 0.070% or less.

S is an element that is present as sulfide and causes the embrittlement of the slab. Therefore, the S content is preferably 0.0200% or less. While the lower limit of the S content is not defined, in view of restrictions in production technique, it is more preferably is 0.0001% or more. More preferably 0.0050% or less.

Al inhibits the formation of carbide during cooling of the slab and promotes the formation of residual austenite, and thus is an element that affects the fraction of residual austenite in the slab. For deoxidation, adding 0.005% or more Al is preferable. On the other hand, when the Al content exceeds 2.500%, the slab may become brittle. Therefore, the Al content is preferably 0.005% or more but 2.500% or less. More preferably 0.010% or more. More preferably 1.000% or less. Even more preferably 0.100% or less.

N is an element that is present as nitride and causes the embrittlement of the slab. Therefore, the N content is preferably 0.0100% or less. While the lower limit of the N content is not defined, in view of restrictions in production technique, the N content is more preferably 0.0001% or more. More preferably 0.0050% or less.

O is an element that is present as oxide and causes the embrittlement of the slab. Therefore, the O content is preferably 0.0100% or less. While the lower limit of the O content is not defined, in view of restrictions in production technique, the O content is more preferably 0.0001% or more. More preferably 0.0050% or less.

It is preferable that the slab for a high-strength steel sheet according to one embodiment of the present invention have an ingredient composition containing the above-described ingredients, with the balance including Fe and unavoidable impurities. It is favorable that the slab for a high-strength steel sheet according to one embodiment of the present invention should have an ingredient composition containing the above-described ingredients, with the balance being Fe and unavoidable impurities. Here, examples of unavoidable impurities include Zn, Pb, and As. Containing 0.100% or less these impurities in total is allowable.

In addition to the above-described ingredient composition, the slab for a high-strength steel sheet according to aspects of the present invention may further contain, in mass %, at least one type of element selected from the following alone or in combination: Ti: 0.200% or less, Nb: 0.200% or less, V: 0.200% or less, Ta: 0.10% or less, W: 0.10% or less, B: 0.0100% or less, Cr: 1.00% or less, Mo: 1.00% or less, Ni: 1.00% or less, Co: 1.00% or less, Cu: 1.00% or less, Sn: 0.200% or less, Sb: 0.200% or less, Ca: 0.0100% or less, Mg: 0.0100% or less, REM: 0.0100% or less, Zr: 0.100% or less, Te: 0.100% or less, Hf: 0.10% or less, and Bi: 0.200% or less.

When the content of each of Ti, Nb, and V is 0.200% or less, coarse precipitates or inclusions are not produced in large amounts and the toughness of the slab does not decrease. Therefore, the content of each of Ti, Nb, and V is preferably 0.200% or less. While the lower limit of the content of Ti, Nb, and V is not defined, as these elements raise the strength of the steel sheet by forming fine carbide, nitride, or carbonitride during hot rolling or continuous annealing, the content of each of Ti, Nb, and V is more preferably 0.001% or more. Therefore, when Ti, Nb, and V are contained, the content of each element should be 0.200% or less. More preferably 0.001% or more. Even more preferably 0.100% or less.

When the content of each of Ta and W is 0.10% or less, coarse precipitates or inclusions are not generated in large amounts and the toughness of the slab does not decrease. Therefore, the content of each of Ta and W is preferably 0.10% or less. While the lower limit of the content of Ta and W is not defined, as these elements raise the strength of the steel sheet by forming fine carbide, nitride, or carbonitride during hot rolling or continuous annealing, the content of each of Ta and W is more preferably 0.01% or more. Therefore, when Ta and W are contained, the content of each element should be 0.10% or less. More preferably 0.01% or more. Even more preferably 0.08% or less.

When B is 0.0100% or less, the toughness of the slab is not affected. Therefore, the B content is preferably 0.0100% or less. While the lower limit of the B content is not defined, since B is an element that segregates at austenite grain boundaries during hot rolling or annealing and improves hardenability, the B content is more preferably 0.0003% or more. Therefore, when B is contained, the content thereof should be 0.0100% or less. More preferably 0.0003% or more. Even more preferably 0.0080% or less.

When the content of each of Cr, Mo, and Ni is 1.00% or less, coarse precipitates or inclusions do not increase and the toughness of the slab does not decrease. Therefore, the content of each of Cr, Mo, and Ni is preferably 1.00% or less. While the lower limit of the content of Cr, Mo, and Ni is not defined, as these are elements that improve hardenability, the content of each of Cr, Mo, and Ni is more preferably 0.01% or more. Therefore, when Cr, Mo, and Ni are contained, the content of each element should be 1.00% or less. More preferably 0.01% or more. Even more preferably 0.80% or less.