WO2023218785A1

WO2023218785A1 - Slab for high-strength steel sheet and cooling method thereof, method for producing high-strength hot-rolled steel sheet, method for producing high-strength cold-rolled steel sheet, and method for producing high-strength plated steel sheet

Info

Publication number: WO2023218785A1
Application number: PCT/JP2023/012741
Authority: WO
Inventors: 和彦山崎; 智也小田垣; 大輝川▲崎▼; 一輝遠藤
Original assignee: Ｊｆｅスチール株式会社
Priority date: 2022-05-09
Filing date: 2023-03-29
Publication date: 2023-11-16
Also published as: JP7396544B1; JPWO2023218785A1

Abstract

Provided are a slab for a high-strength steel sheet and a cooling method thereof, wherein slab cracking is prevented during slab cooling. Also provided are methods for producing a high-strength hot-rolled steel sheet, a high-strength cold-rolled steel sheet, and a high-strength plated steel sheet by using said slab. The slab for a high-strength steel sheet is a slab continuously cast, and is characterized in that the average prior austenite grain size at a 10 mm depth from the surface layer of the slab is 2.0 mm or less, and the slab has a microstructure in which the total area ratio of bainitic ferrite and tempered martensite is 50-97%, the area ratio of retained austenite is 3-30%, the area ratio of ferrite is 20% or less, and the total area ratio of pearlite and quenched martensite is 20% or less.

Description

Slabs for high-strength steel plates and their cooling methods, methods for producing high-strength hot-rolled steel plates, methods for producing high-strength cold-rolled steel plates, and methods for producing high-strength plated steel plates.

The present invention relates to a high-strength steel plate slab that prevents cracking during cooling, and a method for cooling the same. In addition, high-strength hot-rolled steel sheets are manufactured from the high-strength steel plate slabs, high-strength cold-rolled steel sheets are manufactured from the high-strength hot-rolled steel sheets, and high-strength plated steel sheets are manufactured from the high-strength cold-rolled steel sheets. Regarding how to.

In recent years, in the field of automobiles, in order to achieve both further thinning of car bodies and ensuring collision safety, progress has been made in increasing the strength of high-strength steels and using higher alloys for this purpose. High alloying greatly reduces the toughness of the slab.

With the decrease in toughness due to high alloying, cracking during cooling of the slab, so-called cracking, has become more frequent. When cracks occur, there is a risk that the slab will break during slab transportation, making it impossible to use it for hot rolling. Further, even if the slab does not break, cracks may open during hot rolling and the hot rolled steel plate may break. Alternatively, if the cracks are small, they appear as surface defects such as sludge marks and sliver marks on the steel plate after hot rolling, cold rolling, annealing, or plating. Usually, cracks on the slab surface are removed using a grinder. However, the toughness of the slab decreases due to high alloying, and cracks may propagate due to the stress of the grinder, making it impossible to completely remove them. On the other hand, small cracks may be overlooked and appear as surface defects on the steel sheet after hot rolling, cold rolling, annealing, or plating. For these reasons, it is necessary to suppress cracking of slabs.

Regarding this point, countermeasures against slab cracking are being considered, for example, as described in Patent Documents 1 and 2. Patent Document 1 discloses a method of suppressing bainite/martensite transformation and reducing stress caused by the transformation expansion by slowly cooling the material to 700 to 500°C, which is the temperature range at which austenite transforms to ferrite. There is. Patent Document 2 discloses that slow cooling is started immediately after casting, and further slow cooling is carried out at a temperature of 700 to 500 °C for 10 hours or more at a temperature of 700 °C or higher to reduce temperature differences and stress during transformation. A method is disclosed.

Japanese Patent Application Publication No. 2020-139209 Japanese Patent Application Publication No. 2019-167560

However, the conventional technology has the following problems.
Slabs made of higher alloys have low toughness, and the techniques disclosed in Patent Documents 1 and 2 have a problem in that they cannot completely suppress cracking in slabs.

The present invention has been made in view of the above-mentioned circumstances, and provides a high-strength steel plate slab and a cooling method thereof that do not cause slab cracking during cooling of the slab, even in the case of a high-alloy slab with low toughness. It is to be. Another object of the present invention is to provide a method for producing high-strength hot-rolled steel sheets, high-strength cold-rolled steel sheets, and high-strength plated steel sheets using the slabs.

The inventors have made extensive studies to achieve the above object. The fracture morphology of slab cracking was analyzed, and the fracture surface included at least one type of fracture among intergranular fracture surfaces along prior austenite grain boundaries and intragranular fracture surfaces (cleavage fracture surfaces) across prior austenite grain boundaries. I discovered that there is a surface. After further investigation, we discovered the following.
(1) By setting the average prior austenite grain size at a position 10 mm from the surface layer of the slab to 2.0 mm or less, grain boundary fracture along prior austenite grain boundaries can be suppressed.
(2) The microstructure at a position 10 mm from the surface layer of the slab has a total area ratio of bainitic ferrite and/or tempered martensite of 50% to 97%, and retained austenite of 3% to 30% in area ratio. By setting the area ratio of ferrite to 20% or less and the total area ratio of pearlite and hardened martensite to 20% or less, it is possible to suppress intragranular fractures that cross within the prior austenite grains.
(3) By suppressing slab cracking, surface defects of the steel plate after hot rolling, cold rolling, annealing, or plating can be suppressed.

The present invention was completed based on the above findings and further studies.
That is, the gist of the present invention is as follows.
[1] A continuously cast slab for high-strength steel plate, in which the average prior austenite grain size at a position 10 mm from the slab surface is 2.0 mm or less, and the microstructure is composed of bainitic ferrite and tempered martensite. is 50% or more and 97% or less in total area ratio, retained austenite is 3% or more and 30% or less in area ratio, ferrite is 20% or less in area ratio, and pearlite and hardened martensite are in the area ratio A slab for high-strength steel plate characterized by a total strength of 20% or less.
[2] The high-strength steel plate slab contains, in mass %, C: 0.10% or more and 0.50% or less, Si: 0.10% or more and 2.50% or less, and Mn: 1.00% or more.5. Contains 0.00% or less, P: 0.100% or less, S: 0.0200% or less, Al: 0.005% or more and 2.500% or less, N: 0.0100% or less, and O: 0.0100% or less. Optionally, 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 remainder consisting of Fe and inevitable impurities. The high-strength steel plate slab according to 1 above.
[3] The high-strength steel plate slab contains, in mass %, C: 0.10% or more and 0.50% or less, Si: 0.70% or more and 2.50% or less, and Mn: 1.00% or more. Contains 0.00% or less, P: 0.100% or less, S: 0.0200% or less, Al: 0.005% or more and 2.500% or less, N: 0.0100% or less, and O: 0.0100% or less. Optionally, 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, and has a component composition of which the remainder is Fe and inevitable impurities. Slabs for high-strength steel plates.
[4] Cool the high-strength steel plate slab having the composition described in 2 above so that the residence time at 1200 ° C. or more and 1450 ° C. or less at the center of the slab width and 10 mm below the surface layer is 130 s or less, and then , the average cooling rate is 25°C/hr or more when the surface temperature at the center of the slab width is 700°C or more and 850°C or less, and then the average cooling rate is 20°C/hr or more when the surface temperature is 550°C or more and less than 700°C. Then, the average cooling rate is 15°C/hr or more at a temperature of 400°C or more and less than 550°C, and then the average cooling rate is 10°C/hr or more until the cooling stop temperature is 250°C or more and less than 400°C. Then, it is heated to a reheating temperature exceeding the cooling stop temperature and 450°C or less, and then heated so that the average cooling rate at 200°C or more and below the reheating temperature is 30°C/hr or less. A method for cooling a slab for high-strength steel plate, which is characterized by cooling.
[5] The high-strength steel plate slab having the composition described in 3 above is cooled so that the residence time at 1200 ° C. or more and 1450 ° C. or less at the center of the slab width and 10 mm below the surface layer is 130 s or less, and then , the average cooling rate is 25°C/hr or more when the surface temperature at the center of the slab width is 700°C or more and 850°C or less, and then the average cooling rate is 20°C/hr or more when the surface temperature is 550°C or more and less than 700°C. Cooled so that the average cooling rate at 400°C or higher and lower than 550°C is 10°C/hr or higher, and then cooled so that the average cooling rate at 200°C or higher and lower than 400°C is 30°C/hr or lower. A method for cooling a slab for high-strength steel plate, which is characterized by cooling.
[6] The high-strength steel plate slab according to any one of 1 to 3 above is heated so that the slab heating temperature is in the range of 1000°C or more and 1300°C or less, and after rough rolling, the finishing rolling temperature is 750°C. A method for producing a high-strength hot-rolled steel sheet, characterized in that finish rolling is carried out so that the temperature is above 1000°C and winding is carried out so that the coiling temperature is above room temperature and below 750°C.
[7] After pickling the high-strength hot rolled steel sheet manufactured by the manufacturing method described in 6 above, cold rolling is performed so that the rolling reduction is 30% or more and 80% or less, and optionally, a) The high-strength cold-rolled steel sheet obtained by the cold rolling is heated to an annealing temperature of 750°C or higher and 950°C or lower, cooled to a cooling stop temperature of 300°C or higher and 600°C or lower, and then heated to 100°C. b) heating the high-strength cold-rolled steel sheet obtained by the cold rolling to an annealing temperature of 750°C or more and 950°C or less until a cooling stop temperature of 130°C or more and 400°C or less; cooling, then reheating to 200°C or higher and 450°C or lower, and then cooling to 100°C or lower; and c) the high-strength cold rolled steel sheet obtained by the cold rolling is annealed at an annealing temperature of 750°C or higher and 950°C or lower. ℃ or less, water quenching is started at 500℃ or higher, water cooling to 100℃ or lower, and then one treatment selected from the following is performed: reheating at 100℃ or higher and 300℃ or lower. A method for manufacturing high-strength cold-rolled steel sheets.
[8] The high-strength cold-rolled steel sheet obtained by the cold rolling described in 7 above is heated so that the annealing temperature is 750°C or more and 950°C or less, and then molten metal is added to the high-strength cold-rolled steel sheet. The plated steel sheet is subjected to a plating treatment, and then the plated steel sheet is cooled at a cooling stop temperature of 150° C. or lower, and optionally, the hot-dip metal plating treatment is applied to zinc plating, zinc-based alloy plating, or zinc-based plating. A method for producing a high-strength plated steel sheet, characterized by plating with one type selected from Al alloy plating and Al plating.
[9] The method for producing a high-strength plated steel sheet according to item 8, characterized in that the plated steel sheet that has been subjected to the hot-dip metal plating treatment is subjected to an alloying treatment.
[10] Using the high-strength cold-rolled steel sheet manufactured by the manufacturing method described in 7 above, the surface is subjected to electroplating treatment, and optionally, the electroplating treatment is applied to zinc plating, zinc-based alloy plating, or zinc plating. - A method for producing a high-strength plated steel sheet, characterized in that it is coated with one type selected from Al alloy plating and Al plating.

According to the present invention, it is possible to provide a slab that does not crack during the cooling process even if it is made of a high-alloy component system for high-strength steel sheets. Furthermore, it is possible to provide a method for producing a high-strength hot-rolled steel sheet, a high-strength cold-rolled steel sheet, and a high-strength plated steel sheet with few surface defects using the slab.

Hereinafter, embodiments of the present invention will be specifically described. Further, the following embodiments are intended to illustrate the steel structure and method for embodying the technical idea of the present invention, and the configuration is not limited to the following. That is, the technical idea of the present invention can be modified in various ways within the technical scope described in the claims.

First, the appropriate range of the slab microstructure and the reason for its limitation will be explained. In the following description, "%" indicating the composition ratio of the microstructure means "area %" unless otherwise specified.

[Average prior austenite grain size: 2.0 mm or less]
The average prior austenite grain size is a factor that determines the unit of fracture; the larger it is, the lower the toughness of the slab is, and slab cracks exhibiting intergranular fracture surfaces occur. In order to suppress this slab cracking, the average prior austenite grain size at a position 10 mm from the surface layer of the slab needs to be 2.0 mm or less. Preferably it is 1.8 mm or less, more preferably 1.5 mm or less. Incidentally, the average prior austenite grain size can be measured by the method described in Examples described later.

[Total area ratio of bainitic ferrite and tempered martensite: 50% or more and 97% or less]
Bainitic ferrite and tempered martensite, which are important constituent elements in this embodiment, have higher toughness than pearlite and hardened martensite, can increase the toughness of steel, and suppress slab cracking. I can do it. In order to obtain this effect, the total area ratio of bainitic ferrite and tempered martensite at a position 10 mm from the slab surface layer needs to be 50% or more. Preferably it is 60% or more, more preferably 70% or more, and still more preferably 80% or more. If it exceeds 97%, there is a risk that the effect of improving toughness due to retained austenite cannot be obtained, so it is set to 97% or less. Incidentally, the area ratio of bainitic ferrite and tempered martensite can be measured by the method described in Examples described below.

[Area ratio of retained austenite: 3% or more and 30% or less]
In this embodiment, retained austenite, which is an extremely important constituent element, has a face-centered cubic lattice (FCC) crystal structure and no cleavage planes, so it can dramatically improve the toughness of the steel. In addition, retained austenite undergoes martensitic transformation when subjected to high stress. Even if a crack occurs due to slab cracking, martensite is generated at the stress concentration area at the tip of the crack, which alleviates the stress concentration and stops the crack from propagating. Therefore, surface defects of the steel sheet after hot rolling, cold rolling, annealing, or plating can be suppressed. In order to obtain the above effect, the area ratio of retained austenite at a position 10 mm from the slab surface layer needs to be 3% or more. Preferably it is 5% or more, more preferably 7% or more. If the retained austenite exceeds 30%, the unstable retained austenite increases, causing martensitic transformation under small stress, which may reduce toughness, so it is set to 30% or less. Preferably it is 25% or less, more preferably 20% or less. In addition, the area ratio of retained austenite can be measured by the method described in Examples described later.

[Ferrite area ratio: 20% or less]
Ferrite has a larger grain size and lower strength than bainitic ferrite, tempered martensite, hardened martensite, retained austenite, and pearlite. Therefore, when stress is applied, stress concentration occurs in the ferrite, and cracks may occur starting from the ferrite. In order to suppress the above cracks, the area ratio of ferrite at a position 10 mm from the surface layer of the slab needs to be 20% or less. Preferably it is 15% or less, more preferably 10% or less, and still more preferably 0%. Note that the area ratio of ferrite can be measured by the method described in Examples described later.

[Total area ratio of pearlite and hardened martensite: 20% or less]
Pearlite and hardened martensite have inferior toughness compared to retained austenite, bainitic ferrite, and tempered martensite, and when present in large amounts, cracks may occur starting from these structures. In order to suppress the above cracks, the total area ratio of pearlite and hardened martensite at a position 10 mm from the slab surface layer must be 20% or less. Preferably it is 15% or less, more preferably 10% or less, and even more preferably 0%. Incidentally, the area ratio of pearlite and hardened martensite can be measured by the method described in Examples described later.

Next, the appropriate range of component composition and the reason for its limitation will be explained. In the following description, "%" representing the content of component elements of steel means "mass %" unless otherwise specified.

[C: 0.10% or more and 0.50% or less]
C is an important element that affects the retained austenite fraction of the slab and increases the strength of the steel sheet. If the C content is less than 0.10%, there is a risk that sufficient retained austenite cannot be ensured in the slab. Alternatively, it may become difficult to achieve the tensile strength (TS) required for the steel plate. On the other hand, if the C content exceeds 0.50%, the quenched martensite fraction of the slab may become excessive. Therefore, the content of C is preferably 0.10% or more and 0.50% or less. More preferably, it is 0.12% or more. More preferably, it is 0.45% or less. More preferably, it is 0.15% or more. More preferably, it is 0.40% or less.

[Si: 0.10% or more and 2.50% or less or 0.70% or more and 2.50% or less]
Si is an element that affects the fraction of retained austenite because it suppresses the generation of carbides during slab cooling and promotes the generation of retained austenite.
When the slab cooling method described below involves reheating, the Si content is preferably 0.10% or more. If the Si content is less than 0.10%, the fraction of retained austenite decreases and slab cracking may occur. More preferably, it is 0.15% or more. More preferably, it is 0.20% or more.
When the slab cooling method described below uniformly lowers the temperature without reheating, the Si content is preferably 0.70% or more. If the Si content is less than 0.70%, the fraction of retained austenite decreases and slab cracking may occur. More preferably, it is 0.90% or more. More preferably, it is 1.00% or more.
On the other hand, if the Si content exceeds 2.50%, strong scale may occur in the hot-rolled steel sheet, which may cause surface defects. Therefore, the Si content is preferably 2.50% or less. More preferably, it is 2.00% or less. More preferably, it is 1.80% or less.

[Mn: 1.00% or more and 5.00% or less]
Mn is an important element that affects the fraction of retained austenite and increases the strength of steel sheets. If the Mn content is less than 1.00%, there is a risk that sufficient retained austenite cannot be ensured in the slab. Alternatively, it may become difficult to achieve the tensile strength (TS) required for the steel plate. On the other hand, if the Mn content exceeds 5.00%, the quenched martensite fraction of the slab may become excessive. Therefore, the Mn content is preferably 1.00% or more and 5.00% or less. More preferably, it is 1.20% or more. More preferably, it is 4.50% or less. More preferably, it is 1.40% or more. More preferably, it is 4.00% or less.

[P: 0.100% or less]
P segregates at prior austenite grain boundaries and embrittles the grain boundaries, so there is a risk of slab cracking. Therefore, the content of P is preferably 0.100% or less. Although the lower limit of the P content is not particularly specified, it is more preferably 0.001% or more since P is a solid solution strengthening element and can increase the strength of the steel sheet. More preferably, it is 0.070% or less.

[S: 0.0200% or less]
S is an element that exists as a sulfide and causes slab embrittlement. Therefore, the S content is preferably 0.0200% or less. Although the lower limit of the S content is not particularly defined, it is more preferably 0.0001% or more due to production technology constraints. More preferably, it is 0.0050% or less.

[Al: 0.005% or more and 2.500% or less]
Al is an element that affects the fraction of retained austenite in the slab because it suppresses the generation of carbides during cooling of the slab and promotes the generation of retained austenite. Further, it is preferable to add 0.005% or more for deoxidation. On the other hand, if the Al content exceeds 2.500%, there is a risk of slab embrittlement. Therefore, the Al content is preferably 0.005% or more and 2.500% or less. More preferably, it is 0.010% or more. More preferably, it is 1.000% or less. More preferably, it is 0.100% or less.

[N: 0.0100% or less]
N is an element that exists as a nitride and causes embrittlement of the slab. Therefore, the N content is preferably 0.0100% or less. Note that although the lower limit of the N content is not particularly specified, it is more preferable that the N content is 0.0001% or more due to constraints on production technology. More preferably, it is 0.0050% or less.

[O: 0.0100% or less]
O is an element that exists as an oxide and causes embrittlement of the slab. Therefore, the content of O is preferably 0.0100% or less. Although the lower limit of the O content is not particularly defined, it is more preferable that the O content is 0.0001% or more due to production technology constraints. More preferably, it is 0.0050% or less.

It is preferable that the high-strength steel plate slab according to an embodiment of the present invention has a composition containing the above-mentioned components, with the remainder containing Fe and inevitable impurities. Preferably, the high-strength steel plate slab according to an embodiment of the present invention contains the above-mentioned components, with the remainder consisting of Fe and unavoidable impurities. Here, unavoidable impurities include Zn, Pb, and As. The total content of these impurities is allowed to be 0.100% or less.

In addition to the above-mentioned composition, the high-strength steel plate of the present invention further includes, in mass %, 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% Below, 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, Contains at least one element selected from Zr: 0.100% or less, Te: 0.100% or less, Hf: 0.10% or less, and Bi: 0.200% or less, singly or in combination. It's okay.

If the content of Ti, Nb, and V is 0.200% or less, large amounts of coarse precipitates and inclusions will not be generated and the toughness of the slab will not be reduced. Therefore, it is preferable that the contents of Ti, Nb, and V are each 0.200% or less. Note that the lower limits of the content of Ti, Nb, and V are not particularly specified, but the strength of the steel sheet can be increased by forming fine carbides, nitrides, or carbonitrides during hot rolling or continuous annealing. Therefore, it is more preferable that the contents of Ti, Nb, and V are each 0.001% or more. Therefore, when Ti, Nb, and V are contained, their contents are each 0.200% or less. More preferably, it is 0.001% or more. More preferably, it is 0.100% or less.

If the content of Ta and W is 0.10% or less, large amounts of coarse precipitates and inclusions will not be generated and the toughness of the slab will not be reduced. Therefore, it is preferable that the contents of Ta and W are each 0.10% or less. Note that there is no particular lower limit to the content of Ta and W, but the strength of the steel sheet is increased by forming fine carbides, nitrides, or carbonitrides during hot rolling or continuous annealing. It is more preferable that the contents of Ta and W are each 0.01% or more. Therefore, when Ta and W are contained, their contents are each 0.10% or less. More preferably, it is 0.01% or more. More preferably, it is 0.08% or less.

If B is 0.0100% or less, it does not affect the toughness of the slab. Therefore, the content of B is preferably 0.0100% or less. Note that the lower limit of the B content is not particularly specified, but since it is an element that segregates at austenite grain boundaries during hot rolling and annealing and improves hardenability, the B content should be 0.0003% or more. It is more preferable to do so. Therefore, when B is contained, its content should be 0.0100% or less. More preferably, it is 0.0003% or more. More preferably, it is 0.0080% or less.

If each of Cr, Mo, and Ni is 1.00% or less, coarse precipitates and inclusions do not increase and the toughness of the slab does not deteriorate. Therefore, it is preferable that the contents of Cr, Mo, and Ni are each 1.00% or less. Note that the lower limit of the content of Cr, Mo, and Ni is not particularly specified, but since they are elements that improve hardenability, it is more preferable that the content of Cr, Mo, and Ni is each 0.01% or more. . Therefore, when Cr, Mo, and Ni are contained, their contents are each 1.00% or less. More preferably, it is 0.01% or more. More preferably, it is 0.80% or less.

If Co is 1.00% or less, coarse precipitates and inclusions do not increase and the toughness of the slab does not deteriorate. Therefore, the Co content is preferably 1.00% or less. Although the lower limit of the Co content is not particularly specified, since it is an element that improves hardenability, the Co content is more preferably 0.001% or more. Therefore, when Co is contained, the content should be 1.00% or less. More preferably, it is 0.001% or more. More preferably, it is 0.80% or less.

If Cu is 1.00% or less, coarse precipitates and inclusions will not increase and the toughness of the slab will not deteriorate. Therefore, the Cu content is preferably 1.00% or less. Although the lower limit of the Cu content is not particularly specified, since it is an element that improves hardenability, the Cu content is preferably 0.01% or more. Therefore, if Cu is contained, the content should be 1.00% or less. More preferably, it is 0.01% or more. More preferably, it is 0.80% or less.

If Sn is 0.200% or less, it does not affect the toughness of the slab. Therefore, the content of Sn is preferably 0.200% or less. Although the lower limit of the Sn content is not particularly defined, since Sn is an element that improves hardenability, the Sn content is more preferably 0.001% or more. Therefore, if Sn is contained, the content should be 0.200% or less. More preferably, it is 0.001% or more. More preferably, it is 0.100% or less.

If Sb is 0.200% or less, coarse precipitates and inclusions will not increase and the toughness of the slab will not deteriorate. Therefore, the content of Sb is preferably 0.200% or less. Although the lower limit of the Sb content is not particularly defined, it is more preferable that the Sb content is 0.001% or more, since it is an element that suppresses decarburization and enables the strength adjustment of steel sheets. . Therefore, if Sb is contained, the content should be 0.200% or less. More preferably, it is 0.001% or more. More preferably, it is 0.100% or less.

If each of Ca, Mg and REM is 0.0100% or less, coarse precipitates and inclusions will not increase and the toughness of the slab will not deteriorate. Therefore, the content of Ca, Mg and REM is preferably 0.0100% or less. Note that the lower limits of the contents of Ca, Mg, and REM are not particularly stipulated, but since they are elements that spheroidize the shape of nitrides and sulfides and improve the toughness of slabs, the contents of Ca, Mg, and REM are More preferably, each content is 0.0005% or more. Therefore, when Ca, Mg and REM are contained, their contents are each 0.0100% or less. More preferably, it is 0.0005% or more. More preferably, it is 0.0050% or less.

If Zr and Te are each 0.100% or less, coarse precipitates and inclusions will not increase and the toughness of the slab will not deteriorate. Therefore, the contents of Zr and Te are preferably 0.100% or less. Note that the lower limits of the contents of Zr and Te are not particularly specified, but since they are elements that spheroidize the shape of nitrides and sulfides and improve the toughness of the slab, the contents of Zr and Te are each 0.001. % or more is more preferable. Therefore, when Zr and Te are contained, their contents are each 0.100% or less. More preferably, it is 0.001% or more. More preferably, it is 0.080% or less.

If Hf is 0.10% or less, coarse precipitates and inclusions will not increase and the toughness of the slab will not deteriorate. Therefore, the Hf content is preferably 0.10% or less. Note that there is no particular lower limit to the Hf content, but since it is an element that spheroidizes the shape of nitrides and sulfides and improves the ultimate deformability of steel sheets, the Hf content should be 0.01% or more. It is more preferable to do so. Therefore, if Hf is contained, the content should be 0.10% or less. More preferably, it is 0.01% or more. More preferably, it is 0.08% or less.

If Bi is 0.200% or less, coarse precipitates and inclusions will not increase and the toughness of the slab will not deteriorate. Therefore, the Bi content is preferably 0.200% or less. Although the lower limit of the Bi content is not particularly defined, since it is an element that reduces segregation, the Bi content is more preferably 0.001% or more. Therefore, when Bi is contained, the content should be 0.200% or less. More preferably, it is 0.001% or more. More preferably, it is 0.100% or less.

In addition, each content of Ti, Nb, V, Ta, W, B, Cr, Mo, Ni, Co, Cu, Sn, Sb, Ca, Mg, REM, Zr, Te, Hf and Bi is preferable. If it is less than the lower limit, the effect of the present invention will not be impaired, and therefore it is included as an unavoidable impurity.

<Method for cooling high-strength steel plate slab according to the first embodiment>
Next, a suitable slab cooling method for achieving the above-mentioned microstructure will be explained. The method for cooling a high-strength steel plate slab according to the first embodiment involves reheating during cooling. Among the above-mentioned component compositions, a case involving reheating during cooling of the slab is selected, and a steel material having the selected component composition is melted to produce a steel slab. In this embodiment, the method of melting the steel material is not particularly limited, and any known melting method such as a converter or an electric furnace is suitable. Moreover, in order to prevent macro segregation, the steel slab (slab) is preferably manufactured by a continuous casting method, but it can also be manufactured by a thin slab casting method or the like.

[Residence time from 1200°C to 1450°C from completion of casting at the center of the slab width and 10mm below the surface layer: 130s or less]
The prior austenite grain size is a factor that determines the unit of fracture, and the larger it is, the lower the toughness is. The factor that determines the austenite grain size is the residence time at 1200° C. or more and 1450° C. or less, and the longer the residence time, the coarser the prior austenite grain size. If the residence time at 1200° C. or higher and 1450° C. or lower exceeds 130 seconds, the average prior austenite grain size may exceed 2.0 mm, and slab cracking may occur. Therefore, the residence time at 1200° C. or higher and 1450° C. or lower is set to 130 seconds or less. Preferably it is 120 seconds or less. More preferably, it is 110 seconds or less. More preferably, the time is 100 seconds or less. Note that the lower limit of the residence time at 1200° C. or higher and 1450° C. or lower is not particularly specified, but if the residence time is too short, there is a high risk of breakout during continuous casting due to uneven solidification, so it is set to 40 seconds or more. More preferably, the time is 50 seconds or more. Since it is difficult to actually measure the above temperature, the temperature history at a position 10 mm below the surface of the slab was calculated by heat transfer analysis. The analysis position was set at the center of the slab width, where the residence time in the above temperature range is the longest within the slab.

[Average cooling rate when the surface temperature at the center of the slab width is 700°C or more and 850°C or less: 25°C/hr or more]
A temperature range of 700° C. or more and 850° C. or less is a temperature range in which ferrite transformation occurs, and if the cooling rate in this temperature range is low, the area ratio of ferrite in the slab increases and the toughness of the slab decreases. In order to control the area ratio of ferrite to a low level, it is necessary to increase the cooling rate. In order to obtain such an effect, the average cooling rate at 700° C. or higher and 850° C. or lower is set to 25° C./hr or higher. Preferably it is 30°C/hr or more, more preferably 40°C/hr or more, and still more preferably 50°C/hr or more. Although the upper limit of the average cooling rate is not particularly specified, it is set to 1000° C./hr or less since it becomes difficult to control the cooling stop at 250° C. or more and less than 400° C. Preferably it is 500°C/hr or less, more preferably 200°C/hr or less. The cooling rate was measured using a thermocouple. After the slab came out of the continuous casting machine, a thermocouple was installed at the center of the upper surface of the wide side (long side) of the slab, and the cooling rate was determined from the measured temperature. All cooling rates of the slab described below are cooling rates determined by the method described above.

[Average cooling rate at 550°C or higher and lower than 700°C: 20°C/hr or higher]
The temperature range of 550°C or more and less than 700°C is the temperature range where ferrite transformation and pearlite transformation occur, and if the cooling rate in this temperature range is low, the ferrite and/or pearlite fraction of the slab will increase, and the toughness of the slab will decrease. descend. Therefore, the average cooling rate at 550°C or higher and lower than 700°C is set to 20°C/hr or higher. Preferably it is 30°C/hr or more, more preferably 40°C/hr or more, and still more preferably 50°C/hr or more. Although the upper limit of the average cooling rate is not particularly specified, it is set to 1000° C./hr or less since it becomes difficult to control the cooling stop at 250° C. or more and less than 400° C. Preferably it is 500°C/hr or less, more preferably 200°C/hr or less.

[Average cooling rate at 400°C or higher and lower than 550°C: 15°C/hr or higher]
The temperature range of 400°C or more and less than 550°C is the temperature range where pearlite transformation and bainite transformation occur, and if the cooling rate in this temperature range is low, the pearlite fraction of the slab will increase and the toughness of the slab will decrease. Therefore, the average cooling rate at 400°C or higher and lower than 550°C is set to 15°C/hr or higher. Preferably it is 20°C/hr or more, more preferably 30°C/hr or more, and still more preferably 50°C/hr or more. Although the upper limit of the average cooling rate is not particularly specified, it is set to 500° C./hr or less since it becomes difficult to control the cooling stop at 250° C. or more and less than 400° C. Preferably it is 300°C/hr or less, more preferably 100°C/hr or less.

[Average cooling rate until cooling stop temperature of 250°C or more and less than 400°C: 10°C/hr or more]
The temperature range of 250°C or more and less than 400°C is a temperature range in which bainite transformation and martensitic transformation occur. In order to concentrate carbon in the untransformed austenite and generate residual austenite in the subsequent reheating, it is necessary to leave the untransformed austenite without completing the transformation during cooling to the cooling stop temperature. Therefore, the cooling rate to the cooling stop temperature is set to 10° C./hr or more. Preferably it is 15°C/hr or more, more preferably 20°C/hr or more, and still more preferably 30°C/hr or more. Although the upper limit of the average cooling rate is not particularly specified, it is set to 500° C./hr or less since it becomes difficult to control the cooling stop temperature. Preferably it is 300°C/hr or less, more preferably 100°C/hr or less. Furthermore, if the cooling stop temperature is lower than 250°C, the transformation will be completed, making it impossible to ensure residual austenite. Therefore, the cooling stop temperature is set to 250°C or higher. The temperature is preferably 270°C or higher, more preferably 300°C or higher. If the cooling stop temperature is 400° C. or higher, pearlite transformation will proceed, making it impossible to ensure retained austenite. Therefore, the cooling stop temperature is set to less than 400°C. The temperature is preferably 380°C or lower, more preferably 350°C or lower.

[Heating to a reheating temperature exceeding the cooling stop temperature and below 450°C]
Reheating from the cooling stop temperature promotes bainitic transformation, and when bainitic ferrite is formed, carbon is concentrated in untransformed austenite, promoting the formation of retained austenite. Furthermore, if martensitic transformation has occurred before cooling is stopped, reheating causes carbon to be distributed and concentrated from martensite to untransformed austenite, promoting the formation of retained austenite. Below the cooling stop temperature, carbon concentration in untransformed austenite becomes insufficient and residual austenite cannot be secured. Therefore, the reheating temperature is set to exceed the cooling stop temperature. Preferably, the cooling stop temperature is +20°C or higher, more preferably the cooling stop temperature +40°C or higher. If the reheating temperature exceeds 450° C., decomposition of untransformed austenite occurs and residual austenite cannot be secured. Therefore, the reheating temperature is set to 450°C or less. The temperature is preferably 430°C or lower, more preferably 410°C or lower.

[Average cooling rate at 200°C or higher and below reheating temperature is 30°C/hr or less]
The temperature range of 200° C. or higher and lower than the reheating temperature is a temperature range in which bainite transformation and martensitic transformation occur, and bainite transformation occurs at a higher temperature than martensitic transformation. If the cooling rate is high in this temperature range, quenched martensite will be generated and the toughness of the slab will be reduced. Furthermore, in bainitic transformation, carbon is concentrated in untransformed austenite during the formation of bainitic ferrite, promoting the formation of retained austenite. In order to ensure the residual austenite fraction and bainite fraction of the slab, it is necessary to allow bainite transformation to occur for a sufficient period of time. Furthermore, if martensitic transformation has occurred before the cooling is stopped, carbon is distributed and concentrated from martensite to untransformed austenite, promoting the formation of retained austenite. Therefore, the average cooling rate at 200° C. or higher and lower than the reheating temperature is set to 30° C./hr or lower. Preferably it is 25°C/hr or less, more preferably 20°C/hr or less. Although the lower limit of the average cooling rate is not particularly specified, it is preferably 5° C./hr or more from the viewpoint of productivity.

<Method for cooling high-strength steel plate slab according to second embodiment>
The method for cooling a high-strength steel plate slab according to the second embodiment uniformly lowers the temperature without reheating during cooling. Common parts with the first embodiment will be omitted, and different parts will be explained. Among the above-mentioned component compositions, a case without reheating during cooling of the slab is selected, and a steel material having the selected component composition is melted to produce a steel slab in the same manner as in the first embodiment.

[Residence time at 1200°C or more and 1450°C or less at the center of the slab width and 10mm below the surface layer] 130s or less]
This is common to the first embodiment.

[Average cooling rate when the surface temperature at the center of the slab width is 700°C or more and 850°C or less: 25°C/hr or more]
This is common to the first embodiment.

[Average cooling rate at 550°C or higher and lower than 700°C is 20°C/hr or higher]
This is common to the first embodiment.

[Average cooling rate at 400°C or higher and lower than 550°C is 10°C/hr or more]
The temperature range of 400°C or more and less than 550°C is the temperature range where pearlite transformation and bainite transformation occur, and if the cooling rate in this temperature range is low, the pearlite fraction of the slab will increase and the toughness of the slab will decrease. Therefore, the average cooling rate at 400°C or higher and lower than 550°C is set to 10°C/hr or higher. Preferably it is 15°C/hr or more, more preferably 20°C/hr or more, and still more preferably 30°C/hr or more. Although the upper limit of the average cooling rate is not particularly specified, it is set to be 500°C/hr or less since it becomes difficult to control the cooling rate at 200°C or more and less than 400°C. Preferably it is 300°C/hr or less, more preferably 100°C/hr or less.

[Average cooling rate at 200°C or higher and lower than 400°C is 30°C/hr or less]
The temperature range of 200°C or higher and lower than 400°C is a temperature range in which bainite transformation and martensitic transformation occur, and bainite transformation occurs at a higher temperature than martensitic transformation. If the cooling rate is high in this temperature range, quenched martensite will be generated and the toughness of the slab will be reduced. Furthermore, in bainitic transformation, carbon is concentrated in untransformed austenite during the formation of bainitic ferrite, promoting the formation of retained austenite. In order to ensure the residual austenite fraction and bainite fraction of the slab, it is necessary to allow bainite transformation to occur for a sufficient period of time. Therefore, the average cooling rate at 200°C or higher and lower than 400°C is set to 30°C/hr or less. Preferably it is 25°C/hr or less, more preferably 20°C/hr or less. Although the lower limit of the average cooling rate is not particularly specified, it is preferably 5° C./hr or more from the viewpoint of productivity.

<Manufacturing method of high strength steel plate>
With the high-strength steel plate slab of the above embodiment, there will be no slab cracking after continuous casting, and there will be no sludge or sliver scratches on the steel plate after hot rolling, cold rolling, annealing, or plating. It becomes possible to manufacture high-strength hot-rolled steel sheets, high-strength cold-rolled steel sheets, and high-strength plated steel sheets without generating surface defects. The manufacturing method for these is as follows.

First, one of the above-mentioned high-strength steel plate slabs is used. For high-strength hot rolled steel sheets, the slab heating temperature is 1000°C or higher and 1300°C or lower, and after rough rolling, finish rolling is performed at a finish rolling temperature of 750°C or higher and 1000°C or lower, and the coiling temperature is room temperature or higher and 750°C or lower. Take up the winding. From the completion of finish rolling to the time of winding, rapid cooling, maintaining/insulating sheet temperature, or air cooling may be performed. After winding, it may be rolled at an elongation rate of 0.05% or more and 1.00% or less. In addition, pickling may be performed. Pickling may be carried out once or in multiple steps. In this way, a high-strength hot-rolled steel sheet is manufactured.

In the case of a high-strength cold-rolled steel sheet, the above-mentioned high-strength hot-rolled steel sheet is used, and after pickling, cold rolling is performed at a rolling reduction of 30% or more and 80% or less. Since pickling can remove oxides on the surface of the steel sheet, it is important for ensuring good chemical conversion treatability and plating quality in the final high-strength steel sheet. Further, the pickling may be carried out once or in multiple steps. Cold rolling introduces strain uniformly and efficiently, resulting in a uniform structure. It is preferable to perform rolling. In this way, the high-strength cold-rolled steel sheet 1 is manufactured.

The high-strength cold-rolled steel sheet 1 is heated at an annealing temperature of 750°C or higher and 950°C or lower, cooled to a cooling stop temperature of 300°C or higher and 600°C or lower, and then cooled to 100°C or lower. After the cooling, it may be rolled at an elongation rate of 0.05% or more and 1.00% or less. In this way, high-strength cold-rolled steel sheet 2 is manufactured.

Using the high-strength cold-rolled steel sheet 1, the annealing temperature is heated at 750°C or higher and 950°C or lower, cooled to a cooling stop temperature of 130°C or higher and 400°C or lower, then reheated at 200°C or higher and 450°C or lower, and then heated to 100°C or higher and 450°C or lower. Cool to below ℃. After the cooling, it may be rolled at an elongation rate of 0.05% or more and 1.00% or less. In this way, the high-strength cold-rolled steel sheet 3 is manufactured.　

Using the high-strength cold-rolled steel sheet 1, heat at an annealing temperature of 750°C or higher and 950°C or lower, start water quenching at 500°C or higher, water cool to 100°C or lower, and then reheat at 100°C or higher and 300°C or lower. After the reheating, rolling may be performed at an elongation rate of 0.05% or more and 1.00% or less. In this way. In this way, high-strength cold-rolled steel sheet 4 is manufactured.

The high-strength cold-rolled steel sheet 1 is heated at an annealing temperature of 750° C. or higher and 950° C. or lower, and then the high-strength cold-rolled steel sheet is subjected to a hot-dip metal plating treatment to obtain a plated steel sheet, and then the plated steel sheet is Cooling is performed at a cooling stop temperature of 150°C or less. In the hot-dip metal plating process, for example, a cold-rolled steel sheet is immersed in a hot-dip galvanizing bath at a temperature of 440° C. or higher and 500° C. or lower. Further, it is preferable to use a hot-dip galvanizing bath having a composition in which the amount of Al is 0.10% by mass or more and 0.23% by mass or less, and the balance is Zn and unavoidable impurities. Further, the amount of plating deposited is preferably 20 to 80 g/m ² per side (both sides plated). Note that the amount of plating deposited can be adjusted by performing gas wiping or the like after the hot-dip galvanizing process. After the cooling, it may be rolled at an elongation rate of 0.05% or more and 1.00% or less. In this way, a high-strength hot-dip galvanized steel sheet is manufactured. In addition to zinc plating, it can also be applied to zinc-based alloy plating, zinc-Al alloy plating, Al plating, etc.

After the hot-dip metal plating process, the plated steel sheet is subjected to an alloying process. For example, in the case of hot-dip galvanizing, alloying treatment is preferably carried out in a temperature range of 460° C. or higher and 600° C. or lower. In this way, a high strength alloyed galvanized steel sheet is produced.

Using the high-strength cold-rolled steel sheets 1 to 4, electroplating is performed on the surface to produce a high-strength plated steel sheet. In addition to zinc plating, electroplating can also be applied to zinc-based alloy plating, zinc-Al alloy plating, Al plating, etc.

Manufacturing conditions other than those mentioned above are not particularly limited and may be according to conventional methods.

[Measurement of average prior austenite grain size]
Here, the method for measuring the average prior austenite grain size is as follows. A sample was cut out from the width center position of the slab after cooling, so that the slab thickness cross section parallel to the slab width direction served as the observation surface. Next, the observation surface was mirror polished using diamond paste, then finished polished using colloidal silica, and further polished using 3vol. % nital to reveal the structure on the observation surface. Using an optical microscope, 5 fields of view are observed at 10x magnification at a position 10 mm below the surface layer of the slab to obtain a tissue image. The prior austenite grain size was determined for five fields of view by cutting the obtained structure image in accordance with JIS G 0551:2020, and the values were averaged to determine the average prior austenite grain size.

[Measurement method of area ratio of ferrite]
To measure the ferrite area ratio, prepare the observation surface of the slab in the same manner as the above-mentioned method for measuring the average prior austenite grain size. Next, the observation surface was mirror polished using diamond paste, then finished polished using colloidal silica, and further polished using 3vol. Etch with % nital to reveal the tissue. Using an SEM (Scanning Electron Microscope) at an accelerating voltage of 15 kV, 10 fields of view were observed at a magnification of 50 times at a position 10 mm below the slab surface layer, and the obtained tissue images were displayed on Adobe's PHOTOSHOP. (registered trademark), the area ratio of ferrite was calculated for 10 fields of view, and the values were averaged to determine the area ratio of ferrite. In addition, ferrite has a larger grain size than other structures (pearlite, bainitic ferrite, tempered martensite, hardened martensite, retained austenite), has a smooth surface, and has a dark contrast, so the magnification is 50x. can be easily distinguished.

[Method for measuring the area ratio of bainitic ferrite, tempered martensite, quenched martensite, retained austenite and pearlite]
The method for measuring the area ratio of these structures causes the structure to appear on the observation surface of the slab, similar to the method for measuring ferrite described above. Using an SEM, 10 fields of view were observed at a magnification of 10,000 times with the ferrite removed from the field of view at a position 10 mm below the surface layer of the slab under the condition of an accelerating voltage of 15 kV. ), calculate the area ratio of bainitic ferrite, tempered martensite, hardened martensite, retained austenite, and pearlite for 10 fields, average those values, and calculate the area ratio of ferrite measured using the method described above. The total was calculated to be 100%, and the area ratio of each tissue was determined. Bainitic ferrite is a concave structure, tempered martensite is a concave structure that contains fine carbides, hardened martensite is a convex structure with fine irregularities inside, and retained austenite is a convex structure with fine irregularities inside. Pearlite is a structure with a flat interior, and pearlite is a structure with concave portions and contains lamellar carbides. Note that bainitic ferrite and tempered martensite do not need to be distinguishable from each other since the total area ratio of bainitic ferrite and tempered martensite is determined.

[Evaluation method of slab cracking]
The evaluation method for slab cracking was based on the penetrant test specified in JIS Z 2343:2017, and the presence or absence of cracks on the wide and narrow sides of the slab was evaluated. After applying the developer, the appearance of the penetrating solution was visually observed to visually check for cracks and flaws on the surface.

[Method for evaluating surface defects in steel sheets]
The surface defects of the steel sheet after hot rolling, cold rolling, annealing, or plating were evaluated visually by visual inspection. A steel plate with no sludge defects, sliver defects, or unplated surfaces over the entire length and width of the steel sheet is considered to be ``no'' surface defects, and a steel sheet with some sludge defects, sliver defects, or unplated surfaces observed as ``presence'' of surface defects. did.

[Mechanical properties]
The method for measuring mechanical properties (tensile strength TS) is as follows. The tensile test was conducted in accordance with JIS Z 2241:2011 using a JIS No. 5 test piece taken so that the length of the tensile test piece was perpendicular to the rolling direction of the steel plate (direction C). (Tensile strength) was measured. In the present invention, a case where the TS is 590 MPa or more is defined as high strength and passed.

(Example 1)
Table 1 shows the composition of the steel slabs used in the test. The steel slabs were cooled under the cooling conditions including reheating shown in Tables 2-1 and 2-2. In Tables 2-1 and 2-2, t1 is the residence time (s) at 1200°C or more and 1450°C or less at the center of the slab width and 10 mm below the surface layer, and v1 is the residence time at the center of the slab width of 700°C or more. It is the average cooling rate (°C/hr) at 850°C or lower, v2 is the average cooling rate (°C/hr) at 550°C or higher and lower than 700°C, and v3 is the average cooling rate (°C/hr) at 400°C or higher and lower than 550°C. hr), v4 is the average cooling rate (°C/hr) to a cooling stop temperature of 250°C or more and less than 400°C, Tf is the cooling stop temperature (°C), and Th is the reheating temperature (°C). and v5 is the average cooling rate (°C/hr) at 200°C or higher and lower than the reheating temperature. In addition, in the "Cracks" column, the presence or absence of cracks during cooling of the slab was recorded. d (γ) is the average prior austenite grain size (mm), BF + TM is the total area ratio (%) of bainitic ferrite and tempered martensite, Rγ is the area ratio (%) of retained austenite, F is the area ratio (%) of ferrite, and P+FM is the total area ratio (%) of pearlite and hardened martensite. By subjecting the steel slab composition used in the experiment to appropriate cooling conditions including reheating, it was possible to satisfy the prior austenite grain size and microstructure according to the present invention, and no cracks were observed in the steel slab. Cracks were observed in the steel slab under conditions outside the scope of the present invention.

(Example 2)
Tables 3-1 and 3-2 list the compositions of the steel slabs used in the test. The steel slabs were cooled under the cooling conditions shown in Tables 4-1 and 4-2, in which the temperature decreased uniformly. In Tables 4-1 and 4-2, t1 is the residence time (s) at 1200°C or higher and 1450°C or lower at the center of the slab width and 10 mm below the surface layer, and v1 is the residence time at the center of the slab width of 700°C or higher. It is the average cooling rate (°C/hr) at temperatures below 850°C, v2 is the average cooling rate (°C/hr) at temperatures above 550°C and below 700°C, and v3 is the average cooling rate (°C/hr) at temperatures above 400°C and below 550°C. hr), and v6 is the average cooling rate (°C/hr) at 200°C or higher and lower than 400°C. In addition, in the "Cracks" column, the presence or absence of cracks during cooling of the slab was recorded. d (γ) is the average prior austenite grain size (mm), BF + TM is the total area ratio (%) of bainitic ferrite and tempered martensite, Rγ is the area ratio (%) of retained austenite, F is the area ratio (%) of ferrite, and P+FM is the total area ratio (%) of pearlite and hardened martensite. The steel slab with the composition used in the experiment was able to satisfy the prior austenite grain size and microstructure according to the present invention by applying appropriate cooling conditions that uniformly lowered the temperature, and no cracks were observed in the steel slab. Ta. Cracks were observed in the steel slab under conditions outside the scope of the present invention.

The cooled steel slabs were then hot rolled under the conditions listed in Tables 5-1 and 5-2, and then subjected to cold rolling, annealing, and plating. In Tables 5-1 and 5-2, THs is the slab heating temperature (°C), THf is the finish rolling temperature of hot rolling (°C), and THr is the coiling temperature of the hot rolled steel strip (°C). ℃). CR is the rolling ratio (%) of cold rolling, and represents the percentage of the plate thickness after rolling with respect to the plate thickness before rolling. TA is the annealing temperature (°C) of the cold rolled steel plate, TW is the water quenching start temperature (°C), TAf is the cooling stop temperature after annealing (°C), and TAh is the subsequent reheating temperature (°C). ℃). In the product type column, CR is cold-rolled steel sheet, GI is hot-dip galvanized steel sheet, GA is alloyed hot-dip galvanized steel sheet, EG is electrogalvanized steel sheet, Al is hot-dip aluminized steel sheet, HR is a hot rolled steel plate. When the steel slab according to the present invention, in which no cracks were observed in the steel slab, was hot rolled, no surface defects were observed in the steel plate, and a tensile strength (TS) of 590 MPa or more was achieved.

By using the method for cooling slabs of the present invention, even high-alloy slabs with low toughness do not suffer from slab cracking after casting, and the yield during manufacturing can be greatly improved. In addition, by using the high-strength steel plate slab of the present invention, it is possible to obtain high-strength hot-rolled steel plates, high-strength cold-rolled steel plates, and high-strength plated steel plates that have no surface defects even if they are made of a high alloy, and these steel plates When applied to automobile suspension parts, structural parts, and frame parts, it is industrially useful because the weight of the car can be reduced while ensuring the reliability of the car.

Claims

A continuously cast slab for high-strength steel plate, in which the average prior austenite grain size at a position 10 mm from the slab surface layer is 2.0 mm or less, and the microstructure has an area ratio of bainitic ferrite and tempered martensite. The total area ratio of retained austenite is 3% or more and 30% or less, the area ratio of ferrite is 20% or less, and the total area ratio of pearlite and hardened martensite is 20% or less. % or less.
The above-mentioned slab for high-strength steel plate is
In mass%,
C: 0.10% or more and 0.50% or less,
Si: 0.10% or more and 2.50% or less,
Mn: 1.00% or more and 5.00% or less,
P: 0.100% or less,
S: 0.0200% or less,
Al: 0.005% or more and 2.500% or less,
Contains N: 0.0100% or less and O: 0.0100% or less,
Additionally, optionally,
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,
Contains at least one element selected from Hf: 0.10% or less and Bi: 0.200% or less,
The high-strength steel plate slab according to claim 1, having a composition in which the remainder is Fe and unavoidable impurities.
The above-mentioned slab for high-strength steel plate is
In mass%,
C: 0.10% or more and 0.50% or less,
Si: 0.70% or more and 2.50% or less,
Mn: 1.00% or more and 5.00% or less,
P: 0.100% or less,
S: 0.0200% or less,
Al: 0.005% or more and 2.500% or less,
Contains N: 0.0100% or less and O: 0.0100% or less,
Additionally, optionally,
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,
Contains at least one element selected from Hf: 0.10% or less and Bi: 0.200% or less,
The high-strength steel plate slab according to claim 1, having a composition in which the remainder is Fe and unavoidable impurities.
The high-strength steel plate slab having the composition according to claim 2 is cooled so that the residence time at 1200° C. or higher and 1450° C. or lower at the center of the slab width and 10 mm below the surface layer is 130 seconds or less, and then the slab is cooled. Cool so that the average cooling rate is 25°C/hr or more when the surface temperature at the center of the width is 700°C or higher and 850°C or lower, and then cool so that the average cooling rate at 550°C or higher and lower than 700°C is 20°C/hr or higher. Cool, then cool so that the average cooling rate at 400°C or higher and lower than 550°C is 15°C/hr or higher, and then the average cooling rate is 10°C/hr or higher until the cooling stop temperature is 250°C or higher and lower than 400°C. Then, it is heated to a reheating temperature exceeding the cooling stop temperature and 450°C or less, and then cooled so that the average cooling rate at 200°C or more and below the reheating temperature is 30°C/hr or less. A method for cooling a slab for high-strength steel plate, characterized by:
The high-strength steel plate slab having the composition according to claim 3 is cooled so that the residence time at 1200° C. or higher and 1450° C. or lower at the center of the slab width and 10 mm below the surface layer is 130 seconds or less, and then the slab is cooled. Cool so that the average cooling rate is 25°C/hr or more when the surface temperature at the center of the width is 700°C or higher and 850°C or lower, and then cool so that the average cooling rate at 550°C or higher and lower than 700°C is 20°C/hr or higher. Cool, then cool so that the average cooling rate at 400°C or higher and lower than 550°C is 10°C/hr or higher, and then cool so that the average cooling rate at 200°C or higher and lower than 400°C is 30°C/hr or lower. A method for cooling a slab for high-strength steel plate, characterized by:
The slab for high-strength steel plate according to any one of claims 1 to 3 is heated so that the slab heating temperature is in the range of 1000°C or more and 1300°C or less, and after rough rolling, the finish rolling end temperature is 750°C or more. A method for producing a high-strength hot-rolled steel sheet, which comprises performing finish rolling at a temperature of 1000°C or lower, and winding at a coiling temperature of at least room temperature and at most 750°C.
After pickling the high-strength hot rolled steel sheet produced by the production method according to claim 6, cold rolling is performed so that the rolling reduction is 30% or more and 80% or less,
Additionally, optionally,
a) The high-strength cold-rolled steel sheet obtained by the cold rolling is heated to an annealing temperature of 750°C or higher and 950°C or lower, cooled to a cooling stop temperature of 300°C or higher and 600°C or lower, and then heated to 100°C. The process of cooling down to
b) The high-strength cold rolled steel sheet obtained by the cold rolling is heated so that the annealing temperature is 750°C or more and 950°C or less, cooled to a cooling stop temperature of 130°C or more and 400°C or less, and then 200°C. A process of reheating above to 450°C or lower and then cooling to 100°C or lower, and
c) The high-strength cold-rolled steel sheet obtained by the cold rolling is heated so that the annealing temperature is 750°C or higher and 950°C or lower, water quenching is started at 500°C or higher, and after water cooling to 100°C or lower, A method for producing a high-strength cold-rolled steel sheet, the method comprising performing one treatment selected from reheating at a temperature of 100°C or higher and 300°C or lower.
The high-strength cold-rolled steel sheet obtained by the cold rolling according to claim 7 is heated so that the annealing temperature is 750° C. or higher and 950° C. or lower, and then the high-strength cold-rolled steel sheet is subjected to molten metal plating treatment. to obtain a plated steel plate, and then cool the plated steel plate at a cooling stop temperature of 150°C or less,
Optionally, the hot-dip metal plating treatment is one type selected from zinc plating, zinc-based alloy plating, zinc-Al alloy plating, and Al plating. A method for producing a high-strength plated steel sheet.
9. The method for manufacturing a high-strength plated steel sheet according to claim 8, wherein the plated steel sheet that has been subjected to the hot-dip metal plating treatment is subjected to an alloying treatment.
Using a high-strength cold-rolled steel sheet manufactured by the manufacturing method according to claim 7, electroplating is applied to the surface,
Optionally, the electroplating process is one type selected from zinc plating, zinc-based alloy plating, zinc-Al alloy plating, and Al plating. A method for producing a high-strength plated steel sheet.