JP2024072572A - Hot rolled steel sheet and method of producing the same - Google Patents

Abstract

To provide a hot rolled steel sheet which has a yield strength (YS) of 500 MPa or more and has excellent bending workability and toughness, and to provide a method of producing the same.SOLUTION: Provided is a hot rolled steel sheet of a composition comprising C, Si, Mn, P, S, Al, N, Nb, and optionally selected components, which has, in terms of areal fraction of the metallic structure, 0 to 85% of ferrite, 3% or less of retained austenite, 5% or less of lath morphology, 15% or more of a structure having a KAM value of 1.0, and which has a yield strength of 500 MPa or more with carbides containing Nb with an average particle size of 8 nm or less. Also provided is method producing a hot rolled steel sheet, including; a rough rolling step of heating or not heating a steel raw material having the above-mentioned component composition; a finish rolling step in which the starting temperature is 950°C or lower, the total draft from the first pass to the fifth pass is 75% or more, and the finishing temperature is 860°C or higher or 910°C or lower; a cooling step; and a coiling step.SELECTED DRAWING: None

Description

The present invention relates to a hot-rolled steel sheet having a yield strength of 500 MPa or more and excellent bending workability and toughness, and a method for manufacturing the same.

In recent years, from the perspective of protecting the global environment, the entire automobile industry has been oriented toward improving automobile fuel efficiency in order to regulate _CO2 emissions. The most effective way to improve automobile fuel efficiency is to reduce the weight of automobiles by making the parts thinner, so in recent years the amount of high-strength steel sheets used as materials for automobile parts has been increasing.
Generally, as the strength of a steel plate increases, its formability and toughness tend to deteriorate. Therefore, in order to further expand the use of high-strength steel plates, it is essential that high strength, formability and toughness are achieved simultaneously.

Therefore, to solve these problems, various technologies have been proposed to increase the strength and improve the workability of steel plates.

For example, Patent Document 1 discloses a hot-rolled steel sheet in which Ti carbides with an average grain size of less than 6 nm and TiS with an average grain size of 0.5 μm or less are dispersed in ferrite crystals with an area ratio of 95% or more. This results in a high-tensile hot-rolled steel sheet with a tensile strength of 780 MPa to 900 MPa and good bending workability.

Patent Document 2 discloses a technology in which a steel slab containing one or more of Ti and Nb is heated, roughly hot rolled to produce a steel plate, which is then joined to the rear end of the preceding roughly rolled steel plate and hot finish rolled in the temperature range of Ar3 to Ar3+50°C. This is said to produce a hot-rolled steel plate for processing with good toughness.

International Publication No. 2013/099196 Japanese Patent Application Laid-Open No. 09-227949

However, the prior art disclosed in the above patent document has the following problems:

The technology described in Patent Document 1 does not allow the structure required by the present invention to be obtained, as seen in the example of steel plate No. 5. For this reason, it is not possible to achieve both good bending workability and toughness at a yield strength of 500 MPa or more.

In addition, the technology described in Patent Document 2 does not consistently produce steel sheets with a yield strength of 500 MPa or more, and Patent Document 2 does not suggest any requirements for obtaining good bendability. Furthermore, narrow range control of the hot rolling temperature to obtain high toughness significantly hinders manufacturability, and in some cases was not feasible depending on the manufacturing size.

The present invention was developed in consideration of the above-mentioned problems with the conventional technology, and aims to provide a hot-rolled steel sheet having a yield strength (YS) of 500 MPa or more and excellent bending workability and toughness, and a manufacturing method thereof.

In order to solve the above problems, the inventors have thoroughly studied the requirements for combining bending workability and toughness in a hot-rolled steel sheet with a yield strength of 500 MPa or more. Considering the recent trend toward higher strength hot-rolled steel sheets for automobiles, it is preferable to set the yield strength to 500 MPa or more, and it is preferable to add Nb and Ti in combination, as described below. Since high ductility is necessary to obtain good workability, a process that assumes a high coiling temperature that can obtain high total elongation was examined. In order to obtain a strength of 500 MPa or more at a coiling temperature of 600°C or more, it was decided to strengthen the hot-rolled steel sheet with extremely fine nano-sized carbides containing Nb.

However, it was previously common knowledge that when the coiling temperature of hot-rolled steel sheet, which precipitates carbides containing Nb, is 600°C or higher, a ferrite phase that does not contain many dislocations is formed in the steel sheet. The fracture surface unit that has a significant effect on the toughness of this ferrite structure is said to be equivalent to the ferrite grain size. And, after investigating ways to refine the ferrite phase, it was concluded that it would be difficult to stably obtain the desired toughness.

Therefore, the inventors conducted extensive research into the possibility of the formation of a crystal structure other than ferrite when the coiling temperature of this hot-rolled steel sheet is 600°C or higher, and as a result, they obtained a new structure that cannot be classified as either ferrite or bainite, and found that this structure has good strength, bending workability, and toughness.
It was discovered that this new structure is formed from fine recrystallized austenite grains that recrystallize during the finish rolling process of hot rolling.

The hot-rolled steel sheet according to the present invention, which has been developed based on the above findings, has the following configuration.
[1] In mass%, C: 0.030% to 0.100%, Si: 1.5% or less, Mn: 1.3% or less, P: 0.05% or less, S: 0.010% or less, Al: 0.005% to 0.080% or less, N: 0.0060% or less, Nb: 0.03% to 0.20% or less,
Optionally, it further contains at least one component from group A to group C below:
Group A: Ti: 0.010% or more and 0.150% or less,
Group B: B: 0.0002% or more and 0.0050% or less,
Group C: A hot-rolled steel sheet having a component composition containing one or more of V, Mo, Sb, REM, Mg, Ca, Sn, Ni, Cu, Co, As, Cr, W, Ta, Pb, Cs, Zr, Hf, Te, Bi, and Se in total at 1% or less, with the balance being Fe and unavoidable impurities, in which, in terms of area ratio of the metal structure, ferrite is 0% to 85% and retained austenite is 3% or less, a lath-type structure is 5% or less, and a structure with a KAM value of 1.0 or more is 15% or more, and which has carbides containing Nb and having an average particle size of 8 nm or less, and has a yield strength of 500 MPa or more.
[2] In the above [1], the hot-rolled steel sheet has a plating layer on a surface thereof.

The manufacturing method of the hot-rolled steel sheet according to the present invention, which was developed based on the above findings, is configured as follows: [3] A rough rolling process in which a steel material having the component composition described in the above [1] is heated to a heating temperature of 1200 ° C. or more or not heated, and then roughly rolled to a sheet bar; a finish rolling process in which the sheet bar is finish-rolled to a hot-rolled steel sheet at a rolling start temperature of 950 ° C. or more, a total reduction rate from the first pass to the fifth pass of 75% or more, and a rolling completion temperature of 860 ° C. to 910 ° C.; a cooling process in which the hot-rolled steel sheet is cooled at an average cooling rate of 40 ° C./s or more to a cooling stop temperature of 600 ° C. to 700 ° C.; and a coiling process in which the cooled hot-rolled steel sheet is coiled at a coiling temperature of 600 ° C. to 700 ° C. This is a manufacturing method of the hot-rolled steel sheet.
[4] In the above [3], the method includes a casting step of casting a steel material having a thickness of 35 mm to 200 mm and having the component composition described in [1] before the rough rolling step or the finish rolling step, and the method is to produce a sheet bar by applying or not applying the rough rolling step.
[5] The method for producing a hot-rolled steel sheet according to the above item [3] further includes a joining step of joining the roughly rolled sheet bar and a preceding sheet bar at 1050°C or higher between the rough rolling step and the finish rolling step, and in the finish rolling step, the joined sheet bar is finish-rolled.
[6] In any one of the above [3] to [5], the method for producing a hot-rolled steel sheet further includes a hot-rolled sheet annealing step of annealing the hot-rolled steel sheet at an annealing temperature of 720°C or less, and a plating step of plating the annealed hot-rolled steel sheet.
[7] The method for producing a hot-rolled steel sheet according to the above [6], further comprising an alloying step of subjecting the plated hot-rolled steel sheet to an alloying treatment at a temperature of 480°C or higher and 600°C or lower.

According to the present invention, it is possible to manufacture a hot-rolled steel sheet having high strength with a yield strength (YS) of 500 MPa or more, and excellent bending workability and toughness. If the hot-rolled steel sheet according to the present invention is applied to automobile parts, further weight reduction of the automobile parts can be realized.

Hereinafter, the hot-rolled steel sheet according to this embodiment will be described.
<Chemical composition of hot-rolled steel sheet>
The composition of the hot-rolled steel sheet is, in mass%, C: 0.030% to 0.100%, Si: 1.5% or less, Mn: 1.3% or less, P: 0.05% or less, S: 0.010% or less, Al: 0.005% to 0.080% or less, N: 0.0060% or less, and Nb: 0.03% to 0.20% or less. Each component is explained below. In the following explanation, "%" representing the content of a component means "mass%".

C: 0.030% or more and 0.100% or less C combines with Ti to contribute to increasing the strength of the steel sheet and forming a highly dislocated structure during isothermal transformation. To obtain a steel sheet with a yield strength of 500 MPa or more, the C content is set to 0.030% or more. On the other hand, if the C content exceeds 0.100%, coarse cementite precipitates, increasing the risk of reducing bending workability and toughness. Therefore, the C content is set to 0.030% or more and 0.100 or less. It is preferably 0.035% or more and 0.090% or less.

Si: 1.5% or less Si is an effective element for improving workability, since it increases the elongation of the steel sheet and suppresses cementite precipitation. On the other hand, if the Si content exceeds 1.5%, the effect of improving bending workability is reduced, the surface properties and weldability are deteriorated, and the adverse effects of the large amount of Si added are increased. Therefore, the Si content is set to 1.5% or less. Preferably, the Si content is set to 1.2% or less. Note that even if the Si content is 0%, the effect of this embodiment is not impaired, but in order to generate a structure that does not have a lath structure stably and has large crystal strain, the Si content is preferably set to 0.15% or more.

Mn: 1.3% or less Mn increases hardenability and suppresses the formation of ferrite with small crystal strain during the cooling process after hot rolling. In order to stably manufacture hot-rolled steel sheets, the Mn content is preferably 0.2% or more. In addition, in order for austenite to recrystallize during hot rolling, it is preferable that hot working strain exists, and it is effective to control the contents of Si and Mn, which are substitutional solid solution elements, within a narrow range. For this purpose, it is preferable to satisfy the following formula (1).
0.9≦0.8[%Si]+[%Mn]≦2.7... (1)
Here, [%Si] and [%Mn] are the Si content and the Mn content in mass%.
On the other hand, if the Mn content exceeds 1.3%, the driving force for the transformation from austenite to ferrite is excessively reduced, and a structure with small crystal strain cannot be obtained. Therefore, the Mn content is set to 1.3% or less. Preferably, the Mn content is 1.2% or less.

P: 0.05% or less P is a harmful element that segregates at grain boundaries and reduces toughness, so it is reduced as much as possible, with the P content being 0.05% or less. The P content is preferably 0.04% or less, but when used in an environment where stricter toughness is required, it is more preferable to keep it 0.02% or less. On the other hand, 0.002% P may be inevitably mixed in during manufacturing.

S: 0.010% or less S forms coarse sulfides in steel, which expand during hot rolling to become wedge-shaped inclusions, adversely affecting toughness. Therefore, since S is also a harmful element, it is reduced as much as possible, and the S content is set to 0.010% or less. Preferably, the S content is 0.003% or less, but for use in environments where stricter toughness is required, it is more preferable to set it to 0.001% or less. In manufacturing, 0.0001% S may be inevitably mixed in.

Al: 0.005% to 0.080% When Al is added as a deoxidizer at the steelmaking stage, the Al content is 0.005% or more. Al forms oxides, which reduces bending workability and toughness. Therefore, the Al content is set to 0.080% or less. Preferably, the Al content is 0.010% to 0.070%.

N: 0.0060% or less N is a harmful element that combines with Nb to form coarse Nb-containing nitrides, thereby reducing strength, bending workability, and toughness. Therefore, the N content is reduced as much as possible to 0.0060% or less. Preferably, the N content is 0.0050% or less. In manufacturing, 0.0005% N may be inevitably mixed in.

Nb: 0.03% or more and 0.20% or less Nb combines with C and contributes to increasing the strength of the steel plate. In order to obtain a yield strength of 500 MPa or more, the Nb content is 0.03% or more. On the other hand, if the Nb content exceeds 0.20%, it becomes impossible to dissolve the coarse Nb-containing carbonitrides in the heating process before hot rolling, and not only does the effect of increasing the strength become saturated, but it also has a negative effect on bending workability and toughness. Therefore, the Nb content is set to 0.03% or more and 0.20% or less. Preferably, the Nb content is 0.035% or more and 0.20% or less.

As described above, C contributes to the formation of a structure with large crystal strain, while it is also used to form carbides containing Nb by combining with Nb. Therefore, in order to stably obtain the metal structure required for the hot-rolled steel sheet according to this embodiment, it is preferable to satisfy the following formula (2). In particular, if the formula (2) is below 1.5, the concentration of C deposited at the grain boundaries during isothermal transformation decreases, and a structure with large crystal strain cannot be stably obtained. Therefore, it is preferable that the formula (2) is 1.5 or more.
On the other hand, in order to obtain a steel sheet with a yield strength of 500 MPa or more, it is necessary to strengthen the steel sheet with fine carbides containing Nb of the nano-order. However, if the formula (2) exceeds 3.5, the coarse NbC cannot be dissolved during reheating of the slab, which leads to a decrease in the strength of the steel sheet and a decrease in the bending workability of the steel sheet. Therefore, it is preferable that the formula (2) is 3.5 or less.
1.5≦([%C]/12)/([%Nb]/93)≦3.5 (2)
Here, [%C] and [%Nb] are the C content and Nb content in mass%.

The above is the basic composition of the components of the hot-rolled steel sheet according to this embodiment, but optionally, at least one component selected from the following groups A to C may be further contained.
Group A: Ti: 0.010% or more and 0.15% or less Group B: B: 0.0002% or more and 0.0050% or less Group C: V, Mo, Sb, REM, Mg, Ca, Sn, Ni, Cu, Co, As, Cr, W, Ta, Pb, Cs, Zr, Hf, Te, Bi, and Se, any one or more of which is 1% or less in total

Ti: 0.010% to 0.150% Ti combines with C to form fine carbides containing Ti, thereby contributing to the strength of the steel plate. In order to obtain a yield strength of 500 MPa or more, the Ti content is 0.010% or more. On the other hand, if the Ti content exceeds 0.150%, the coarse carbides containing Ti cannot be dissolved in the heating process before hot rolling, and not only is the effect of increasing the strength saturated, but it also has a negative effect on bending workability and toughness. Therefore, the range of the Ti content is 0.010% to 0.150%. Preferably, the Ti content is 0.02% to 0.14%.

Considering the need to increase the strength of automotive components, it is sometimes preferable to obtain a yield strength of 500 MPa or more. In this case, it is effective to add Nb and Ti in combination, and a hot-rolled steel sheet having a yield strength of 500 MPa or more can be obtained by satisfying the following formula (3).
0.10≦([% Nb]/2)+[% Ti*] (3)
Here, [% Ti*] = [% Ti] - 48 [% N] / 14, and [% Nb], [% Ti] and [% N] are the Nb content, Ti content and N content in mass%.
When Nb and Ti are added in combination, not only Nb but also Ti is bonded to C, and C is utilized. Therefore, it is preferable to satisfy the following formula (4) instead of formula (2).
1.5≦([%C]/12)/([%Nb]/93+[%Ti*]/48)≦3.5
...(4)

B: 0.0002% or more and 0.0050% or less B is an element effective for improving hardenability, and it is necessary to ensure hardenability in order to obtain a structure with large crystal strain. By setting the B content to 0.0002% or more, it is possible to stably obtain a desired structure. On the other hand, if the B content exceeds 0.0050%, the effect on the hardenability of the steel is saturated, so the B content is set to 0.0050% or less. More preferably, the B content is 0.0004% or more and 0.0030% or less.

Any one or more of V, Mo, Sb, REM, Mg, Ca, Sn, Ni, Cu, Co, As, Cr, W, Ta, Pb, Cs, Zr, Hf, Te, Bi, and Se are contained in a total content of 1% or less. If any one or more of them are contained in a total content of 1% or less, there is little effect on the properties of the hot-rolled steel sheet according to the present embodiment. On the other hand, preferably, the content of each element is limited to 0.03% or less.

The chemical composition of the hot-rolled steel sheet according to this embodiment contains the above elements, with the remainder being Fe and unavoidable impurities.

<Metal structure of hot-rolled steel sheet>
Next, the metal structure of the hot-rolled steel sheet will be described.
The metal structure of the hot-rolled steel sheet according to this embodiment has an area ratio of ferrite of 0% or more and 85% or less, an area ratio of retained austenite of 3% or less, an area ratio of a structure having a lath morphology of 5% or less, an area ratio of a structure having a KAM value of 1.0 or more of 15% or more, and has carbides containing Nb having an average particle size of 8 nm or less.
In the following description, the "%" representing the metal structure means the "area ratio".

Ferrite is 0% or more and 85% or less Ferrite is a structure with inferior toughness because the fracture surface unit at the time of brittle fracture is larger than the new structure with large crystal strain in the hot-rolled steel sheet according to this embodiment. In order to obtain the desired toughness, the area ratio of ferrite needs to be limited to 85% or less. Preferably, the area ratio of ferrite is 80% or less, more preferably 70% or less.

Retained austenite is 3% or less (including 0%)
The bainite and tempered martensite defined in this embodiment are those in which a lath structure is observed within the grains. Martensite is a structure observed as a white contrast on an SEM, but since it may be cementite, it is sufficient to separate it by crystal structure using electron backscatter diffraction (EBSD) analysis. For example, it is possible to determine whether or not bainite, martensite, and tempered martensite that satisfy the Kurdjumov-Sachs relationship with the parent phase are applicable by obtaining a (001) α pole figure of a single prior γ grain region. The retained austenite can be obtained by XRD analysis using a sample that has been chemically polished 0.1 mm or more after grinding the steel sheet surface to 1/4 of the sheet thickness from the surface. In the hot-rolled steel sheet of this embodiment, these structures reduce strength, workability, and toughness. It is preferable to reduce these structures as much as possible, and the retained austenite is 3% or less. Preferably, the total amount of bainite, martensite, tempered martensite, martensite and retained austenite is 5% or less, and more preferably, 3% or less.

The largest technical feature of this embodiment is that the structure having a large crystal strain and not having a lath structure is strengthened with carbides containing Nb with an average particle size of 8 nm or less. Ferrite has small crystal strain, that is, the KAM value is less than 1. Low-temperature transformation phases such as bainite, martensite, and tempered martensite have a lath structure. Therefore, a structure having a large crystal strain and not having a lath structure is a structure that cannot be classified as ferrite or bainite. Lath is a structure observed as a plate-like form within a grain by transmission electron microscope (TEM) or EBSD analysis. A structure having this lath structure is hard, but has poor workability and does not obtain the desired bendability. The structure having a large crystal strain in the present invention is one having a KAM value of 1.0 or more as determined by EBSD analysis. The KAM value indicates a disorder in the crystal structure, and this disorder in the crystal makes the effective fracture surface unit finer, achieving toughness greater than that of ferritic structure steel. From the above, this structure makes it possible to obtain a steel sheet with good workability and toughness. Therefore, a structure without a lath structure has an area ratio of lath-shaped structure of 5% or less, and a structure with large crystal strain has a structure with a KAM value of 1.0 or more of 15% or more. More preferably, the structure with a KAM value of 1.0 or more of 20% or more.

Carbide containing Nb with an average particle size of 8 nm or less In this embodiment, the steel sheet is strengthened by carbide containing Nb. In order to obtain a high-strength hot-rolled steel sheet with a yield strength of 500 MPa or more, it is necessary to set the average particle size of the carbide containing Nb dispersed in the steel to 8 nm or less. In order to stably obtain a strength with a yield strength of 500 MPa or more, it is preferable to set the average particle size of the carbide containing Nb to 5 nm or less. The carbide containing Nb may be a composite carbide with Ti.
Furthermore, in the production of the hot-rolled steel sheet of this embodiment, when the coiling temperature is 600°C or higher, Nb, even though it is a substitutional element, diffuses sufficiently in the steel. By utilizing this property of Nb and diffusing and precipitating Nb in the steel, a steel sheet having a yield strength of 500 MPa or more can be obtained even if the structure of bainite, martensite, and tempered martensite, which are often used in high-strength steel sheets, is small. To obtain a steel sheet having a yield strength of 500 MPa or more, 80% or more of the contained Nb is utilized for precipitation. Preferably, 85% or more of the contained Nb is utilized for precipitation.

The hot-rolled steel sheet according to the present embodiment preferably has a plating layer on the surface. Even if the plating layer is formed, the function of the hot-rolled steel sheet is not impaired. The composition of the plating layer is preferably one or more selected from Zn, Si, Al, Ni, and Mg.
In this embodiment, the plated steel sheet includes any of those that have been subjected to a hot-dip galvanizing treatment (GI), those that have been subjected to an alloying treatment after hot-dip galvanizing treatment (GA), and those that have been subjected to an electrolytic galvanizing treatment (EG).

Next, a first embodiment of the method for producing a hot-rolled steel sheet according to the present embodiment will be described.
<Method of manufacturing hot-rolled steel sheet>
In general, hot-rolled steel sheets are manufactured by casting a slab (steel material), loading the slab (steel material) whose temperature has been reduced to 1000° C. or less into a heating furnace, heating it for a short time, and then reducing the thickness to a predetermined thickness in a hot rolling line and winding it into a coil. Alternatively, after casting a slab (steel material), the slab (steel material) that has once cooled to room temperature is heated for a long time in a heating furnace, and then reducing the thickness to a predetermined thickness in a hot rolling line and winding it into a coil. There is also a manufacturing method in which the cast slab (steel material) is directly sent to a hot rolling line without being heated in a heating furnace, and reduced to a predetermined thickness and wound into a coil.
The manufacturing method of the hot-rolled steel sheet according to this embodiment can be applied not only to a process in which the steel material is heated after casting, but also to a process in which the steel material is directly sent to a hot rolling line without being heated after casting.

<First form steel material>
The smelting method for producing the steel material of this embodiment is not particularly limited, and known smelting methods such as converters and electric furnaces can be adopted. Secondary refining may also be performed in a vacuum degassing furnace. The molten steel thus adjusted to the above-mentioned composition is then preferably made into a slab (steel material) by a continuous casting method, taking into consideration productivity and quality. Alternatively, the slab may be made into a slab by an ingot casting-blooming rolling method or other known casting methods.

<First form of rough rolling step>
In this embodiment, the steel material is heated to a heating temperature of 1200° C. or higher, or is not heated, and is roughly rolled to form a sheet bar.
<Finish rolling process of the first embodiment>
Next, hot rolling is performed in which the start temperature of finish rolling is 950°C or higher, the total rolling reduction from the first pass to the fifth pass is 75% or higher, and the completion temperature of finish rolling is 860°C to 910°C, to produce a hot-rolled steel sheet.
<Cooling step of the first embodiment>
Next, the hot-rolled steel sheet is cooled to a cooling stop temperature of 600° C. or more and 700° C. or less at an average cooling rate of 40° C./s or more.
<Winding process of the first embodiment>
Thereafter, the cooled hot-rolled steel sheet is coiled at a coiling temperature of 600°C or higher and 700°C or lower.

Heating of steel material: heating to 1200°C or higher, or not heating Coarse carbides containing Nb precipitated in the slab (steel material) are dissolved in a heating process before hot rolling, so that fine carbides containing Nb are precipitated after hot rolling. Therefore, in order to obtain carbides containing Ti with an average particle size of 8 nm or less, the slab (steel material) is heated to 1200°C or higher. It is preferably 1220°C or higher, and it is more preferable to heat the slab (steel material) to 1240°C or higher when the Nb content is 0.12% or more. Although there is no particular upper limit, in order to avoid thermal damage of the heating furnace, the heating of the slab (steel material) is restricted to 1300°C in manufacturing.
When the steel material held at 1200° C. or higher after casting is directly sent to the hot rolling line, the cast steel material is not heated.

Finish rolling start temperature: 950°C or higher, total reduction rate from the first pass to the fifth pass of 75% or higher, finish rolling end temperature: 860°C to 910°C In order to generate a structure with large crystal strain, which is characteristic of the hot-rolled steel sheet according to this embodiment, it is necessary to precisely control the hot rolling conditions. Specifically, fine austenite is formed by recrystallization of austenite during finish rolling. For this purpose, the start temperature of finish rolling is 950°C or higher, and the total reduction rate from the first pass to the fifth pass of 75% or higher. If the chemical composition of the hot-rolled steel sheet according to this embodiment is within the range, austenite recrystallization occurs in the finish rolling stand after the fifth pass.
Therefore, the finish rolling is performed in 5 passes or more. If the finish rolling start temperature is lower than 950°C, austenite will recrystallize early in the finish rolling, and the recrystallized austenite will be rolled again. This will result in the formation of ferrite, and a structure with large crystal strain will not be obtained.
If the finish rolling start temperature exceeds 1100°C, there is a high possibility that austenite recrystallization will not occur during the finish rolling, so the finish rolling start temperature is preferably 1100°C or lower.

If the finish rolling completion temperature falls below 860°C, there is a high risk of ferrite being generated during rolling. On the other hand, if the finish rolling completion temperature exceeds 910°C, austenite cannot be recrystallized during finish rolling. Therefore, the finish rolling completion temperature is set to 860°C or higher and 910°C or lower. To stably obtain austenite recrystallization, it is preferable to set the finish rolling completion temperature to 890°C or lower.

As mentioned above, in order to recrystallize austenite by finish rolling, it is necessary to accumulate strain from the first pass to the fifth pass of finish rolling. For this reason, if the rolling interval from the first pass to the fifth pass is long, the strain imparted by rolling will recover, and austenite cannot be recrystallized stably by finish rolling. Therefore, from the viewpoint of avoiding the adverse effects of this austenite recovery, it is preferable to set the rolling interval time from the first pass to the fifth pass to at least 1.5 seconds or less.

Cooling stop temperature after finish rolling is 600°C to 700°C at an average cooling rate of 40°C/s or more. If the cooling rate to 700°C or less after hot rolling is slow, polygonal ferrite (ferrite) that is coarse at high temperature and has small crystal strain within the grains is generated. To suppress the generation of this ferrite, the material is cooled at an average cooling rate of 40°C/s or more after hot rolling. Preferably, the material is cooled at an average cooling rate of 50°C/s or more to 700°C or less within 2 seconds after hot rolling.
On the other hand, if the cooling stop temperature is below 600° C., it becomes difficult to obtain carbides containing Nb, and a steel sheet having a yield strength of 500 MPa or more cannot be obtained.
Therefore, the cooling stop temperature is set to 600° C. or more and 700° C. or less. Preferably, the cooling stop temperature is 610° C. or more and 690° C. or less. Here, the average cooling rate may be calculated by {(cooling start temperature)-(cooling completion temperature)}/(forced cooling time other than natural cooling) after hot rolling, in which forced cooling other than natural cooling is performed. An example of the forced cooling method is water cooling.

Coiling temperature: 600° C. or higher and 700° C. or lower For the same reason as the cooling stop temperature, the coiling temperature is set to 600° C. or higher and 700° C. or lower. The coiling temperature is preferably 610° C. or higher and 690° C. or lower. If coiling is performed in this temperature range, the generation of ferrite, bainite, martensite, and retained austenite can be suppressed.

Next, a second embodiment of the method for producing a hot-rolled steel sheet according to the present embodiment will be described. In this embodiment, the difference from the first embodiment will be described.
<Second type casting process>
The hot rolled steel sheet according to the present embodiment can also be produced by a thin slab continuous casting method. When produced by the thin slab continuous casting method, a steel material having a thickness of 35 mm to 200 mm is cast.
<Second type rough rolling step>
The cast steel material is heated to a heating temperature of 1200° C. or higher, or is not heated, and is roughly rolled as necessary to form a sheet bar.
The process after the finish rolling step is the same as that of the first embodiment.

Here, we will explain the slab (steel material) thickness that is unique to the thin slab continuous casting method.

Slab thickness (steel material): 35 mm to 200 mm Unlike continuous casting, the thin slab before hot rolling is thin in thin slab continuous casting, so the degree of austenite processing in hot rolling is low. If the slab thickness is less than 35 mm, a total reduction rate of 75% or more from the first pass to the fifth pass cannot be obtained. On the other hand, if the slab thickness exceeds 200 mm, the casting speed becomes slow, and the productivity advantage of thin slab continuous casting is lost compared to continuous casting. Therefore, the slab thickness in thin slab continuous casting is 35 mm to 200 mm.

Next, a third embodiment of the method for producing a hot-rolled steel sheet according to the present embodiment will be described. In this embodiment, the difference from the first and second embodiments will be described. In the third embodiment, a continuous hot rolling technique can be applied.
<Joining process of the third embodiment>
The sheet bar obtained in the first or second embodiment is joined to the preceding sheet bar at 1050° C. or higher before finish rolling. If the temperature is lower than 1050° C., it becomes difficult to perform rolling at the finish rolling start temperature of 950° C. or higher. The preferred heating temperature of the sheet bar during joining is 1070° C. or higher.
The steps after the cooling step are the same as those in the first embodiment.

In the manufacturing method of the hot-rolled steel sheet according to this embodiment, an annealing process in which annealing is performed in a continuous annealing line at an annealing temperature of 720°C or less, and a plating process in which plating is performed in a continuous plating line can be applied. Furthermore, an alloying process in which the plated hot-rolled steel sheet is heated to 480°C or more and 600°C or less and alloyed is performed may be included. This annealing process or this plating process does not affect the material of the hot-rolled steel sheet according to this embodiment. Therefore, it is possible to further plate the surface of the hot-rolled steel sheet to provide a plating layer on the steel sheet surface.

As described above, the plating process and the composition of the plating bath do not affect the material of the hot-rolled steel sheet according to this embodiment, and therefore any of hot-dip galvanizing, alloyed hot-dip galvanizing, and electrolytic galvanizing processes can be applied as the plating process. The composition of the plating bath can include one or more of Zn, Al, Mg, Si, and Ni. In other words, the composition of the plating layer formed on the surface of the hot-rolled steel sheet in the plating process can include one or more of Zn, Al, Mg, Si, and Ni.

The embodiments of the present invention will be further explained using examples. Note that the present invention is not limited to the manufacturing conditions and product performance shown in the examples below. As long as the embodiments are within the scope of the present invention, they can achieve the desired performance.

<First form using continuous casting method>
Steel materials having a thickness of 250 mm and having the chemical compositions shown in Tables 1-1 and 1-2 were hot rolled under the rough rolling and finish rolling conditions shown in Table 2, and then temper rolled at an elongation rate of 0.1 to 0.5% and pickled to produce steel plates to be evaluated.

<Second method using thin slab continuous casting method>
Steel having the chemical composition shown in Table 1-1 was hot-rolled into thin slabs under the conditions shown in Table 3, and then the steel was temper-rolled to an elongation rate of 0.1 to 0.5% and pickled to produce steel sheets for evaluation.

<Third form by hot continuous rolling method>
Steels having the chemical compositions shown in Table 1-1 were joined into sheet bars under the conditions shown in Table 4, and the joined sheet bars were hot-rolled, temper-rolled to an elongation rate of 0.1 to 0.5%, and pickled to produce steel sheets for evaluation.

<Manufacturing method for imparting a plating layer to a hot-rolled steel sheet>
The hot-rolled coil produced under the conditions shown in Table 2 was pickled, and then the hot-rolled steel sheet was plated in a continuous hot-dip galvanizing line (CGL) under the conditions shown in Table 5. In this manner, a continuous hot-dip galvanized steel sheet (GI) and an alloyed hot-dip galvanized steel sheet (GA) were produced.

The hot-rolled steel sheets obtained under the conditions shown in Tables 2 to 5 were evaluated in terms of metal structure, tensile properties, bending workability, and toughness using the following methods. The results are shown in Table 6.

(i) Area ratio of metal structure A test piece was cut out from a hot-rolled steel sheet so that a cross section parallel to the rolling direction was the observation surface, and the center part of the sheet thickness was etched with 1% nital to reveal the structure. Then, images of 10 fields of view of the ¼t part of the sheet thickness were taken with a scanning electron microscope (SEM) at a magnification of 2000 times and an acceleration voltage of 15 kV.
Ferrite is a crystal grain that does not show any corrosion marks within the grain and is observed with a lower brightness than martensite (gray in SEM). Bainite and tempered martensite are crystal grains in which three or more adjacent lath-shaped corrosion marks with a width of 500 nm or less are observed within the grain. Martensite is a crystal grain that does not show any corrosion marks within the grain, but is observed with a higher brightness than ferrite (white in SEM). The area ratio of the metal structure of the structure separated in the above manner was determined using image analysis software (Photoshop elements and Image J).
The retained austenite was measured by grinding the surface of the test piece to 3/4 of the total thickness, chemically polishing it to 0.1 mm or more, and measuring the polished surface by X-ray diffraction. The volume fraction of the retained austenite was measured from the peaks of (200)α, (211)α, (220)α, (200)γ, (220)γ, and (311)γ using MoKα radiation as the incident radiation source. The volume fraction of the retained austenite phase obtained in this manner was taken as the area fraction of the retained austenite.

The area ratio of the structure with large crystal strain that does not have a lath structure was measured using SEM and EBSD. Before observation, the test piece was marked with a Vickers tester or the like so that the same field of view could be obtained by SEM and EBSD. When observed with SEM, the structure with large crystal strain that does not have a lath structure has corrosion marks in the grains. In this case, depending on the shape of the corrosion marks, there may be cases where the corrosion marks are not laths but look like laths. In this case, in order to distinguish between the structure that looks like laths and the lath structure, a rectangular structure that occurs in a grain with a width of the short side of the crystal grain exceeding 500 nm and two or less adjacent grains is not considered to be a lath structure. A structure with a width of the short side of the crystal grain being 500 nm or less and three or more adjacent grains was considered to be a lath structure observed in bainite or tempered martensite. This lath structure can be more clearly distinguished by observation with a transmission electron microscope (TEM). Then, EBSD analysis was performed using OIM Analysis software (TSL). In the EBSD analysis, within the grains surrounded by high-angle grain boundaries with an angle difference of 15° or more, the structure with a KAM value exceeding 1.0 and having no lath structure was determined as a structure with large crystal distortion without a lath structure, and the area ratio of the structure was calculated in a field of view of ¹ mm2 or more. The analysis of the KAM value was performed under the condition of 1st nearest neighbor.

(ii) Average particle size of Nb-containing carbides A thin film for observation was taken from a location equivalent to 1/4 of the plate thickness of the hot-rolled steel plate, and 300 or more Nb-containing carbides were photographed at a magnification of 600,000 times or more using a transmission electron microscope. The circle-equivalent diameters of the photographed Nb-containing carbides were calculated, and the average value was taken as the average particle size. The Nb-containing carbides can be identified by checking the presence or absence of a peak derived from Nb using EDX attached to the TEM.

(iii) Analysis of Precipitation Amount of Carbide Containing Nb The front and back surfaces of the test piece were ground by 25% of the plate thickness, and then dissolved in a 10% AA electrolytic solution. The solution was filtered through a filter with a mesh size of 0.2 μm, and the Nb concentration in the electrolytic solution after filtration was analyzed using ICP-MS. The amount of precipitated carbide containing Nb was determined by subtracting the Nb concentration in the electrolytic solution from the amount of Nb contained.

(iv) Tensile test From the hot-rolled steel sheets obtained under the conditions shown in Tables 2 to 5, tensile test pieces of JIS No. 5 were prepared in the direction perpendicular to the rolling direction, and tensile tests in accordance with the provisions of JIS Z2241 (2011) were performed five times to obtain the average yield strength (YS) and tensile strength (TS). The crosshead speed of the tensile test was 10 mm/min. In Table 6, the steel sheets with a yield strength of 500 MPa or more were considered to be examples of the invention.

(v) Bending test From the hot rolled steel sheets obtained under the conditions shown in Tables 2 to 5, test pieces with a width of 35 mm and a length of 100 mm were taken by grinding the end faces, and a bending test was performed five times by the V-block method described in JIS Z 2248. For test pieces with a yield strength of 500 MPa or more and less than 680 MPa, R/t was 0.1 or less, and for test pieces with a yield strength of 680 MPa or more, R/t was 0.5 or less, which are the properties required in the present invention, and the results in Table 6 were marked with "◯". For test pieces in which cracks were found on the surface of the test piece at least once under the above conditions out of the five tests, they were marked with "X" as they do not have the properties required in the present invention.

(vi) Charpy impact test From the hot-rolled steel sheets obtained under the conditions shown in Tables 2 to 5, V-notch test pieces according to JIS Z2242 were taken so that the longitudinal direction was normal to the rolling direction. When the thickness of the hot-rolled steel sheet was less than 10 mm, a plurality of test pieces were stacked, holes were drilled at the ends of the test pieces, and the thickness was adjusted to 10±1 mm by connecting them with bolts. For test pieces having a yield strength of 500 MPa or more and less than 680 MPa, the test was performed by a method conforming to JIS Z 2242 after immersing them in a bath adjusted to −80° C. for 10 minutes or more. The test results are shown in Table 6. When the absorbed energy was 30 J/cm ² or more, it was marked with “◯” as the characteristic required by the present invention, and when it was less than 30 J/cm ² , it was marked with “×” as it was not the characteristic required by the present invention. Test pieces with a yield strength of 680 MPa or more were immersed in a bath adjusted to -40°C for 10 minutes or more, and then tested according to a method conforming to JIS Z2242. Absorbed energy of 30 J/cm2 or ^more was marked with "◯" as it is a characteristic required by the present invention, and a level below 30 J/ ^cm2 was marked with "X" as it is not a characteristic required by the present invention.

All of the examples of the present invention had a yield strength (YS) of 500 MPa or more, and good bending workability and toughness were obtained. On the other hand, the comparative examples outside the range of the present invention either had a yield strength not reaching 500 MPa or did not obtain the bending workability or toughness required by the present invention.

Claims (7)

In mass percent,
C: 0.030% or more and 0.100% or less,
Si: 1.5% or less,
Mn: 1.3% or less,
P: 0.05% or less,
S: 0.010% or less,
Al: 0.005% or more and 0.080% or less,
N: 0.0060% or less,
Nb: 0.03% or more and 0.20% or less;
Optionally, it further contains at least one component from group A to group C below:
Group A:
Ti: 0.010% or more and 0.150% or less,
Group B:
B: 0.0002% or more and 0.0050% or less,
Group C:
V, Mo, Sb, REM, Mg, Ca, Sn, Ni, Cu, Co, As, Cr, W,
Ta, Pb, Cs, Zr, Hf, Te, Bi and Se, total of 1%
below,
The balance is Fe and unavoidable impurities.
The area ratio of the metal structure is
Ferrite is 0% or more and 85% or less.
Retained austenite is 3% or less.
Lath-shaped structure is 5% or less,
The KAM value of the tissue is 1.0 or more and 15% or more of the tissue is
A hot-rolled steel sheet having a yield strength of 500 MPa or more and containing Nb-containing carbides with an average grain size of 8 nm or less. The hot-rolled steel sheet according to claim 1, characterized in that the surface of the hot-rolled steel sheet has a plating layer. A rough rolling process in which the steel material having the composition according to claim 1 is roughly rolled into a sheet bar with or without heating at a heating temperature of 1200 ° C. or more;
a finish rolling step of finish rolling the sheet bar at a rolling start temperature of 950° C. or more, a total rolling reduction rate of 75% or more from the first pass to the fifth pass, and a rolling completion temperature of 860° C. to 910° C. to obtain a hot-rolled steel sheet;
A cooling step of cooling the hot-rolled steel sheet to a cooling stop temperature of 600° C. or more and 700° C. or less at an average cooling rate of 40° C./s or more;
a coiling step of coiling the cooled hot-rolled steel sheet at a coiling temperature of 600° C. or more and 700° C. or less;
A method for producing a hot-rolled steel sheet, comprising: A casting process is included prior to the rough rolling process or the finish rolling process, in which a steel material having a thickness of 35 mm or more and 200 mm or less and having the component composition according to claim 1 is cast,
The method for producing a hot-rolled steel sheet according to claim 3, characterized in that the rough rolling step is applied or not applied to produce a sheet bar. Between the rough rolling step and the finish rolling step, a joining step of joining the roughly rolled sheet bar and a preceding sheet bar at 1050 ° C. or more is included;
The method for producing a hot-rolled steel sheet according to claim 3, characterized in that, in the finish rolling step, the joined sheet bar is finish-rolled. Further, a hot-rolled steel sheet annealing process is performed in which the hot-rolled steel sheet is annealed at an annealing temperature of 720° C. or less.
A plating process for plating the annealed hot-rolled steel sheet;
The method for producing a hot-rolled steel sheet according to any one of claims 3 to 5, further comprising the steps of: The method for producing a hot-rolled steel sheet according to claim 6, further comprising an alloying step of subjecting the plated hot-rolled steel sheet to an alloying treatment at a temperature of 480°C or higher and 600°C or lower.