WO2024105999A1

WO2024105999A1 - Hot-rolled steel sheet and method for producing same

Info

Publication number: WO2024105999A1
Application number: PCT/JP2023/033820
Authority: WO
Inventors: 典晃 ▲高▼坂; 広志松田
Original assignee: Ｊｆｅスチール株式会社
Priority date: 2022-11-16
Filing date: 2023-09-19
Publication date: 2024-05-23

Abstract

The present invention provides a hot-rolled steel sheet having a yield strength of not less than 650 MPa, and having excellent bending workability, stretch flange formability, and shrink flange formability. The present invention also provides a method for producing the same. Provided is a hot-rolled steel sheet having a component composition arbitrarily selected of C, Si, Mn, P, S, Al, N, B, Ti, and Nb, having a metal structure which, in terms of area ratio, is 0-75% ferrite, with the total of bainite, martensite, tempered martensite, and residual austenite being not more than 3%, and crystal grains including a small angle boundary being not less than 25%, said hot-rolled steel sheet comprising a carbide including Ti or Nb with a mean particle size of not more than 8 nm, wherein yield strength is not less than 650 MPa. Also provided is a method for producing a hot-rolled steel sheet, said method comprising: a rough rolling step in which a steel raw material having the above component composition is heated or not heated; a finish rolling step in which a starting temperature is higher than 1000°C, the rolling reduction in each of a first pass and a second pass is not less than 35%, and the total rolling reduction from a third pass to rolling completion is not more than 85%; a cooling step; and a winding step.

Description

HOT-ROLLED STEEL SHEET AND ITS MANUFACTURING METHOD

The present invention relates to a hot-rolled steel sheet having a yield strength of 650 MPa or more and excellent stretch flange formability and shrink flange formability, and a method for manufacturing the same. The hot-rolled steel sheet of the present invention is suitable as a material for automobile parts.

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, the formability of steel sheets tends to deteriorate as the strength of the steel sheets increases, so that improvement of the formability is essential to further expand the use of high-strength steel sheets. In particular, hot-rolled steel sheets are often used for suspension arm parts that are formed into complex shapes, and therefore excellent bendability, stretch flangeability, and shrink flangeability are required.

In order 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 the main phase of the matrix is a ferrite phase with an area ratio of more than 95%, and in which Ti carbides with an average particle size of less than 10 nm are finely precipitated within the ferrite crystal grains. This is said to result in a high-strength hot-rolled steel sheet with excellent workability and a tensile strength of 780 MPa or more.

Patent Document 2 discloses a method for manufacturing a hot-rolled steel sheet, which includes hot rolling consisting of rough rolling at a rolling start temperature of 1200°C or higher and finish rolling at a rolling end temperature of 900°C or higher, and coiling at 580°C or higher. The hot-rolled steel sheet has a ferrite phase with an area ratio of 95% or higher and fine carbides containing TiN with an average particle size of 20 nm or more and Ti with an average particle size of less than 6 nm dispersed in the metal structure. As a result, it is said that a high-tensile hot-rolled steel sheet with a tensile strength of 590 MPa to 750 MPa and excellent punchability and stretch flangeability can be obtained.

Patent Document 3 discloses a hot-rolled steel sheet in which 1.0% or more of Mn is added for the purpose of improving hardenability, upper bainite having an area ratio of 75.0% or more and less than 97.0% as the main phase, and the number density of second phase particles of 0.5 μm or more is 150,000 particles/ ^mm2 or less. As a result, it is said that a high-strength hot-rolled steel sheet having a tensile strength of 980 MPa or more can be obtained.

JP 2013-95996 A JP 2013-133525 A International Publication No. 2018/150955

However, the conventional technology disclosed in the above patent document has the following problems:

The technologies proposed in Patent Documents 1 and 2 have a ferrite phase as the main phase, in which a large amount of finely dispersed Ti-containing carbides are used, which results in poor shrink flangeability and may cause specific cracking at the shrink flange area.

In addition, in Patent Document 3, a large amount of Mn is added, which has a negative effect on shrink flangeability. In addition, the main phase is an upper bainite structure (including structures without Fe-based carbides and retained austenite) that has Fe-based carbides and/or retained austenite between bainitic ferrite with a lath-like morphology. The lath-like structure means that the transformation is carried out at low temperatures, and there has been no precedent for the precipitation of Ti-containing carbides into a structure with this lath-like structure. Furthermore, bainite structures obtained at low temperatures in the past often contained a mixture of martensite and retained austenite, resulting in poor stretch flangeability.

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 of 650 MPa or more and excellent bending workability, stretch flangeability, and shrink flangeability, as well as a manufacturing method thereof.

In order to solve the above problems, the inventors have thoroughly studied the requirements for hot-rolled steel sheets to have both stretch flangeability and shrink flangeability as well as high strength. The thickness of the hot-rolled steel sheets concerned in this case is 1.0 mm or more and 35.0 mm or less. They have found that shrink flangeability is correlated with toughness, and have concluded that good shrink flangeability cannot be obtained with ferrite-based phase steels that have poor toughness. On the other hand, conventional bainite-based phase steels inevitably produce hard martensite and retained austenite, so they do not obtain the required bending workability or stretch flangeability.

Therefore, they attempted to precipitate carbides containing Ti or Nb in the steel, and then modify the parent phase structure to a structure other than ferrite, which has been mainly used in the past, at a coiling temperature of 600°C or higher, which is the temperature range in which the carbides precipitate. B was added as a component that sufficiently increases the driving force for the transformation from austenite to ferrite, and to further improve hardenability, and conditions were set in which the degree of processing of austenite during hot rolling was not excessively increased. It was found that this resulted in a new structure that differed from conventional ferrite, and that the bendability, stretch flangeability, and shrink flangeability of hot-rolled steel sheets with this new structure were good.

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: 0.045% or more and less than 0.110%, Si: 1.5% or less, Mn: 0.7% 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, B: 0.0002% or more and 0.0050% or less, and further, one or two of Ti and Nb are contained within a range satisfying the following formula (1),
Optionally, the composition further contains 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 a total amount of 1% or less, with the balance being Fe and unavoidable impurities;
The hot-rolled steel sheet has a metallographic area ratio of 0% or more and 75% or less of ferrite, a total of 3% or less of bainite, martensite, tempered martensite and retained austenite, and 25% or more of crystal grains containing small-angle grain boundaries, and has carbides containing Ti or Nb with an average grain size of 8 nm or less, and has a yield strength of 650 MPa or more.
0.09≦([% Nb]/2)+[% Ti*] (1)
Here, [% Ti*] = [% Ti] - 48 [% N] / 14, and [% M] (M = Nb, Ti, N) is the content of Nb, Ti, and N in mass%.
[2] In the above [1], the hot-rolled steel sheet has a plating layer on a surface thereof.

The method for producing a hot-rolled steel sheet according to the present invention, which was developed based on the above findings, is configured as follows.
[3] A method for producing a hot-rolled steel sheet, comprising: a rough rolling step in which a steel material having the composition described in [1] above is heated to a heating temperature of 1200°C or higher, or is not heated after casting, and is rough-rolled to a sheet bar; a finish rolling step in which the sheet bar is finish-rolled to a hot-rolled steel sheet at a rolling start temperature exceeding 1000°C, with rolling reductions of 35% or more in the first and second passes, and a total rolling reduction of 85% or less from the third pass to the end of rolling; a cooling step in which the hot-rolled steel sheet is cooled at an average cooling rate of 40°C/s or higher to a cooling stop temperature of 600°C to 700°C; and a coiling step in which the cooled hot-rolled steel sheet is coiled at a coiling temperature of 600°C to 700°C.
[4] In the above [3], the method for producing a hot-rolled steel sheet includes a casting step of casting a steel material having a thickness of 35 mm or more and 200 mm or less and having the component composition described in [1] before the rough rolling step or the finish rolling step, and the hot-rolled steel sheet is produced into a sheet bar with or without 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 1070°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 460°C or higher and 600°C or lower.

According to the present invention, it is possible to manufacture hot-rolled steel sheets that have high strength, with a yield strength of 650 MPa or more, and excellent bendability, stretch flangeability, and shrink flangeability. By applying the hot-rolled steel sheets according to the present invention to automobile parts, further weight reduction of the automobile parts can be achieved.

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: 0.045% or more and less than 0.110%, Si: 1.5% or less, Mn: 0.7% 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, B: 0.0002% or more and 0.0050% or less, and further contains one or both of Ti and Nb in a range that satisfies the following formula (1): 0.09≦([%Nb]/2)+[%Ti*]...(1)
Here, [% Ti*] = [% Ti] - 48 [% N] / 14, and [% M] (M = Nb, Ti, N) is the content of Nb, Ti, and N in mass%.
Each component will be described below. In the following description, "%" representing the content of a component means "% by mass."

C: 0.045% or more and less than 0.110% C contributes to increasing the strength of the steel sheet by combining with Ti and Nb. In order to obtain a steel sheet with a yield strength of 650 MPa or more, it is necessary to ensure sufficient hardenability to suppress the progress of ferrite transformation after the end of finish rolling, and the C content is set to 0.045% or more. On the other hand, if the C content is 0.110% or more, coarse cementite precipitates, making it impossible to obtain good bending workability and stretch flangeability. Therefore, the C content is set to 0.045% or more and less than 0.110%. Preferably, it is 0.050% or more and 0.100% or less.

Si: 1.5% or less Si is an element that increases the driving force for the transformation from austenite to ferrite and is effective in modifying the parent phase structure at a coiling temperature of 600°C or more. However, if the Si content exceeds 1.5%, the driving force for the transformation from austenite to ferrite becomes excessively high, and the risk of ferrite formation during the cooling process after hot rolling increases. Therefore, the Si content is set to 1.5% or less. It is preferably 0.15% or more and 1.1% or less.

Mn: 0.7% or less Mn reduces the driving force for transformation from austenite to ferrite. It is also an element that is prone to segregation, and this segregation reduces shrink flangeability. Therefore, the Mn content must be reduced as much as possible, and is set to 0.7% or less. The Mn content is preferably less than 0.50%. In manufacturing, 0.05% is inevitably mixed in, but even if it is 0%, the effect of the present invention is not impaired.
In addition, from the viewpoint of improving the driving force for transformation from austenite to ferrite and hardenability, it is preferable that the following formula (2) is satisfied.
0.35≦0.8[%Si]+[%Mn]≦1.35 (2)
Here, [%M] (M=Si, Mn) is the content of Si and Mn in mass %.

P: 0.05% or less P is a harmful element that segregates at grain boundaries and reduces shrink flangeability, 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 for use under more severe shrink flange processing conditions, it is more preferable to keep it 0.02% or less. On the other hand, there are cases where 0.002% P is 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 stretch flangeability. Therefore, since S is also a harmful element, it is preferable to reduce it as much as possible, and the S content is 0.010% or less. Preferably, the S content is 0.003% or less, but for use under more severe stretch flange processing conditions, it is more preferable to make it 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 stretch flangeability. 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 Ti and Nb to form coarse carbonitrides, thereby adversely affecting workability, stretch flangeability, and strength. 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, about 0.0005% of N may be inevitably mixed in.

B: 0.0002% or more and 0.0050% or less B is considered to contribute to the formation of crystal grains including small angle grain boundaries, and is also an effective element for improving hardenability, and in order to obtain a bainitic ferrite structure, the B content is 0.0002% or more. However, even 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. Preferably, the B content is 0.0004% or more and 0.0030% or less.

One or both of Ti and Nb are contained within a range satisfying the formula (1): 0.09≦([%Nb]/2)+[%Ti*]...(1)
Here, [% Ti*] = [% Ti] - 48 [% N] / 14, and [% M] (M = Nb, Ti, N) is the content of Nb, Ti, and N in mass %.
Ti and Nb combine with C and contribute to increasing the strength of the steel sheet. Ti and Nb need to satisfy formula (1) in order to stably obtain a yield strength of 650 MPa or more.
In order to obtain a yield strength of 650 MPa or more, one or both of Ti and Nb are contained, and preferably, the Ti content is 0.06% or more and 0.18% or less, and the Nb content is 0.02% or more and 0.18% or less.

While C contributes to the formation of a structure with large crystal strain, it is also used to combine with Ti and Nb to form carbides. This use of C to form carbides reduces the hardenability of the steel. However, if C is contained in excess of Ti and Nb, coarse TiC cannot be dissolved when the slab is reheated, and the strength and bending workability are reduced. From the above viewpoints, it is preferable that C, Ti, and Nb satisfy formula (3).
1.8≦([%C]/12)/([%Nb]/93+[%Ti*]/48)≦4.0
...(3)
Here, [% Ti*] = [% Ti] - 48 [% N] / 14, and [% M] (C, Nb, Ti, N) is the content of C, Nb, Ti, and N in mass%.

The above is the basic configuration of the composition of the hot-rolled steel sheet according to this embodiment, but optionally, 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 may be further contained in a total amount of 1% or less.
This is because if the total content of any one or more elements is 1% or less, the effect on the properties of the hot-rolled steel sheet according to the present embodiment is small. Preferably, the content of each element is 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 75% or less, a total area ratio of bainite, martensite, tempered martensite and retained austenite of 3% or less, crystal grains containing low-angle grain boundaries of 25% or more, and carbides containing Ti or Nb with an average particle size of 8 nm or less.
In the following description, "%" representing the metal structure means "area ratio".

Ferrite is 0% or more and 75% or less Ferrite is a structure that reduces shrink flangeability. Ferrite does not contain small angle grain boundaries and is a structure with poor toughness, which also has a negative effect on shrink flangeability. In order to obtain the desired shrink flangeability, the area ratio of ferrite is 75% or less. Preferably, the area ratio of ferrite is 0% or more and 65% or less.

The total of bainite, martensite, tempered martensite, and retained austenite is 3% or less (including 0%)
Bainite, martensite, tempered martensite, and retained austenite deteriorate bending workability and stretch flangeability, so it is preferable to reduce them as much as possible, and the total content of the above structures is set to 3% or less. The total content of the above structures may be 0%, and is preferably 0% or more and 2% or less.
Bainite, martensite, and tempered martensite may be separated by crystal structure using electron backscatter diffraction pattern (EBSD) analysis. For example, whether or not bainite, martensite, and tempered martensite that satisfy the Kurdjumov-Sachs relationship with the parent phase are applicable is determined from the (001) α pole figure of a single prior γ (austenite) grain region.

The heat-rolled steel sheet according to the present embodiment is characterized in that nano-sized carbides containing Ti or Nb are precipitated in the grains containing the low-angle grain boundaries. The grains containing the low-angle grain boundaries make it possible to achieve both high strength with a yield strength of 650 MPa or more and workability, stretch flangeability, and shrink flangeability. Therefore, the grains containing the low-angle grain boundaries are 25% or more. Preferably, the grains containing the low-angle grain boundaries are 30% or more.

The average particle size of the carbide containing Ti or Nb is 8 nm or less In the hot-rolled steel sheet according to the present embodiment, the steel sheet is strengthened by the carbide containing Ti or Nb. In order to obtain a high-strength hot-rolled steel sheet having a yield strength of 650 MPa or more, it is necessary to set the average particle size of the carbide containing Ti or Nb dispersed in the steel to 8 nm or less. In order to stably obtain a yield strength of 650 MPa or more, it is preferable to set the average particle size of the carbide containing Ti or Nb to 5 nm or less. The carbide containing Ti or Nb may be a composite carbide, or may be a single carbide containing either Ti or Nb.
Furthermore, in the manufacture of the hot-rolled steel sheet according to this embodiment, when the coiling temperature of the hot-rolled steel sheet is 600° C. or higher, Nb, even though it is a substitutional element, diffuses sufficiently in the steel. By utilizing this property of Ti or Nb and diffusing and precipitating Ti or Nb in the steel, a steel sheet having a yield strength of 650 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 650 MPa or more, 80% or more of the contained Ti or Nb is utilized for precipitation. Preferably, 85% or more of the contained Ti or 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.
The plated steel sheet in the present invention includes steel sheets that have been subjected to hot-dip galvanizing treatment (GI), steel sheets that have been subjected to hot-dip galvanizing treatment followed by alloying treatment (GA), and steel sheets that have been subjected to 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.
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 after casting, and is roughly rolled to form a sheet bar.
<Finish rolling process of the first embodiment>
Next, finish rolling is performed with a starting temperature of over 1000°C, a rolling reduction of 35% or more in the first and second passes, and a total rolling reduction of 85% or less from the third pass to the end of rolling, 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 Ti or Nb precipitated in the slab (steel material) are dissolved in a heating process before hot rolling, so that fine carbides containing Ti or Nb are precipitated after hot rolling. Therefore, in order to obtain carbides containing Ti or Nb with an average particle size of 8 nm or less, the slab (steel material) is heated to 1200°C or higher. The temperature is preferably 1220°C or higher, and when the Nb content is 0.12% or more, it is more preferable to heat the slab (steel material) to 1240°C or higher. There is no particular upper limit, but 1300°C is a manufacturing constraint in order to avoid thermal damage to the heating furnace.
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.

Starting temperature of finish rolling: over 1000°C The hot-rolled steel sheet according to this embodiment has a steel composition with an increased driving force for the transformation from austenite to ferrite, so that under normal hot rolling conditions, ferrite is generated at a high temperature range of 600°C or higher coiling temperature, and the desired steel structure cannot be obtained. Therefore, it is necessary to increase the hot rolling temperature and avoid rolling in the austenite non-recrystallization temperature range. Therefore, the starting temperature of finish rolling is over 1000°C. Preferably, the starting temperature of finish rolling is 1010°C or higher. Due to the nature of the steel, there is no particular upper limit for the starting temperature of finish rolling. Unless there is a heating device in the hot rolling line, the slab heating temperature is the substantial upper limit temperature for the starting temperature of finish rolling, and is often 1200°C or lower.

The reduction ratios of the first and second passes are each 35% or more. By increasing the reduction ratio in the high temperature region where austenite recrystallizes, the degree of austenite processing at the end of finish rolling is reduced, and as a result, the nucleation of ferrite is suppressed, and a metal structure with crystal grains containing low-angle grain boundaries is obtained. To obtain a metal structure with crystal grains containing low-angle grain boundaries, the reduction ratios of the first and second passes are each 35% or more. Preferably, the reduction ratios of the first and second passes are each 38% or more.
The rolling reductions in the first and second passes can be calculated by the following formulas (4) and (5), respectively.
Reduction rate of first pass=(t ₀ −t ₁ )/t ₀ (4)
Reduction rate of second pass=(t ₁ −t ₂ )/t ₁ (5)
Here, t ₀ , t ₁ , and t ₂ are the sheet thickness before finish rolling, the sheet thickness after one pass, and the sheet thickness after two passes, respectively.

Total reduction ratio from the third pass to the end of rolling: 85% or less By controlling the degree of austenite processing until the end of finish rolling and reducing the density of ferrite nucleation sites, the generation of polygonal ferrite is suppressed and the formation of a structure containing low-angle grain boundaries is promoted. To obtain a structure containing low-angle grain boundaries, the total reduction ratio from the third pass to the end of rolling is 85% or less. Preferably, the total reduction ratio from the third pass to the end of rolling is 80% or less.
In addition, the total rolling reduction from the third pass to the end of rolling can be calculated by formula (6).
Total reduction rate from the third pass to the end of rolling=(t ₂ −t _f )/t ₂ (6)
Here, _tf is the plate thickness after the completion of finish rolling.

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. In order to suppress the generation of this ferrite, it is necessary to cool at an average cooling rate of 40°C/s or more after hot rolling, and it is preferable to cool 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 is difficult to obtain carbides containing Ti or Nb, and a steel sheet having a yield strength of 650 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 after casting, 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 (steel material) thickness: 35 mm to 200 mm Unlike the continuous casting method, the thin slab before hot rolling is thin in the thin slab continuous casting method, so the degree of austenite processing in the hot rolling is low. If the slab thickness is less than 35 mm, the specified reduction rate cannot be obtained from the first pass to the second pass, or from the third pass to the completion of rolling.
On the other hand, if the slab thickness exceeds 200 mm, the casting speed of the steel material becomes slow, and the productivity advantage of the thin slab continuous casting method compared to the continuous casting method is lost. Therefore, the slab thickness in the thin slab continuous casting method is set to 35 mm or more and 200 mm or less.

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 1070° C. or higher before finish rolling. If the temperature is lower than 1070° C., it becomes difficult to perform rolling at the finish rolling start temperature of 1000° C. or higher. The preferred heating temperature of the sheet bar during joining is 1100° C. or higher.
The steps after the cooling step are the same as those in the first embodiment.

The manufacturing method for the hot-rolled steel sheet according to this embodiment can apply an annealing process in which annealing is performed in a continuous annealing line where the annealing temperature is 720°C or less, and a plating process in which plating is performed in a continuous plating line. In addition, an alloying process may be included in which the plated hot-rolled steel sheet is heated to 460°C or more and 600°C or less and alloyed. This annealing process or this plating process does not affect the material properties 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 composition shown in Table 1 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 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 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.

<Production of applying 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 Zn-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, stretch flangeability, and shrink flangeability 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 procedure for determining the area fraction was as follows: first, the fractions of bainite, martensite, and cementite, which are lath-shaped structures, were obtained from SEM images; next, the fraction of retained austenite was obtained from XRD; and next, the fraction of crystal grains that did not have a lath morphology and contained low-angle grain boundaries was obtained using EBSD. The remainder of these crystal grains was calculated as the ferrite fraction, thereby deriving the fraction of each structure.

The area ratio of the structure of crystal grains containing low-angle grain boundaries was obtained by analyzing a visual field of 1 ^mm2 or more by electron backscatter diffraction (EBSD). Using OIM Analysis software manufactured by TSL, grain boundaries with an angle difference of 15° or more were defined as high-angle grain boundaries, and grain boundaries with an angle difference of 2° or more and less than 15° were defined as low-angle grain boundaries, and the area ratio of crystal grains containing low-angle grain boundaries in grain boundaries surrounded by high-angle grain boundaries was obtained. For bainite and martensite, low-angle grain boundaries are also included in high-angle grain boundaries. Therefore, the area ratio of crystal grains containing low-angle grain boundaries in the hot-rolled steel sheet according to this embodiment was obtained by subtracting the area ratios of bainite, tempered martensite, and martensite from the area ratio of crystal grains containing low-angle grain boundaries obtained by EBSD analysis.

(ii) Average particle size of Ti or Nb-containing carbides A thin film for observation was taken from a location equivalent to 1/4 of the plate thickness of the test piece, and 300 or more Ti or 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 Ti or Nb-containing carbides were calculated, and the average value was taken as the average particle size. The Ti or 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 the amount of carbide precipitated containing Ti or Nb The front and back surfaces of the test pieces 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 Ti and Nb concentrations in the filtered electrolytic solution were analyzed using ICP-MS. The amount of carbide precipitated containing Ti and Nb was determined by subtracting the Ti and Nb concentrations in the electrolytic solution from the amount of Ti and 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 Z 2241 (2011) were performed five times to obtain the average 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 650 MPa or more were considered to be inventive examples.

(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 carried out five times by the V-block method described in JIS Z 2248. Test pieces with an R/t of 0.5 or less are marked with "◯" in the results in Table 6, as they have the characteristics required by the present invention, and test pieces in which cracks were found on the surface of the test piece at least once out of the five tests under the above conditions are marked with "X", as they do not have the characteristics required by the present invention.

(vi) Stretch flangeability Three test pieces of 120 mm x 120 mm were taken from the hot-rolled steel sheets obtained under the conditions shown in Tables 2 to 5, and a hole expansion test in accordance with JFST1001 (Japan Iron and Steel Federation standard) was carried out five times to determine the average hole expansion ratio λ (%) and evaluate the stretch flangeability. In Table 6, λ of 80% or more is regarded as an invention example, and is the mechanical property of the steel sheet required for the steel of the present invention.

(vii) Shrinkage flangeability Five samples for deep drawing with a diameter of 110 mm were taken from the hot-rolled steel sheets obtained under the conditions shown in Tables 2 to 5, and a cylindrical drawing process was performed using a punch with a diameter of 50 mm and a radius of the punch shoulder of 5 mm. The cylindrical drawing process was then immersed in ice water for 30 minutes. Immediately thereafter, the cylindrical drawing process was placed under a weight of 10 kg with a truncated cone punch with a tip angle of 45°, which was adjusted so as not to hit the bottom surface of the cylindrical deep drawing process, located 1 m above. The cylindrical drawing process was placed so that the opening of the cylindrical drawing process was at the top, at a position where the center of the truncated cone punch and the center of the cylindrical drawing process were aligned. The upper weight was immediately allowed to fall freely in the vertical direction of the cylindrical drawing process, and the presence or absence of cracks in the cylindrical drawing process after the weight was dropped was confirmed. In Table 6, cylindrical drawn products that did not crack in any of the five repeated tests are marked with "O" as the characteristics required by the present invention, and cylindrical drawn products that cracked in any of the five repeated tests or that cracked at least once due to the impact of the falling weight are marked with "X".

All of the examples of the present invention had a yield strength (YS) of 650 MPa or more, and good bending workability, stretch flangeability and shrink flangeability were obtained. On the other hand, the comparative examples outside the range of the present invention either had a yield strength not reaching 650 MPa or did not obtain the bending workability, stretch flangeability or shrink flangeability required in the present invention.

Claims

In mass percent,
C: 0.045% or more and less than 0.110%;
Si: 1.5% or less,
Mn: 0.7% 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,
B: 0.0002% or more and 0.0050% or less,
Furthermore, one or both of Ti and Nb are contained in a range satisfying the following formula (1),
Optionally, the composition further contains 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 a total amount of 1% or less;
The balance is Fe and unavoidable impurities.
The area ratio of the metal structure is
Ferrite is 0% or more and 75% or less.
The total of bainite, martensite, tempered martensite and retained austenite is 3% or less;
The percentage of crystal grains containing small angle boundaries is 25% or more,
A hot-rolled steel sheet having a yield strength of 650 MPa or more and containing carbides containing Ti or Nb with an average grain size of 8 nm or less.
0.09≦([% Nb]/2)+[% Ti*] (1)
Here, [% Ti*] = [% Ti] - 48 [% N] / 14, and [% M] (M = Nb, Ti, N) is the content of Nb, Ti, and N in mass%.
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 by heating the steel material to a heating temperature of 1200 ° C. or more, or by not heating the steel material after casting;
a finish rolling process in which the sheet bar is finish-rolled at a rolling start temperature exceeding 1000°C, with rolling reductions of 35% or more in the first and second passes, and with a total rolling reduction of 85% or less from the third pass to the end of rolling, 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:
The method includes a casting step of casting 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, prior to the rough rolling step or the finish rolling step;
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 1070 ° 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 460°C or higher and 600°C or lower.