CN118056028A

CN118056028A - Hot rolled steel sheet

Info

Publication number: CN118056028A
Application number: CN202280066932.9A
Authority: CN
Inventors: 荻巢靖之; 豊田武
Original assignee: Nippon Steel and Sumitomo Metal Corp
Current assignee: Nippon Steel Corp
Priority date: 2021-10-14
Filing date: 2022-10-12
Publication date: 2024-05-17
Also published as: MX2024004346A; WO2023063347A1; KR20240065114A; JPWO2023063347A1; EP4417333A1

Abstract

The hot-rolled steel sheet has a predetermined chemical composition and a predetermined metal structure, and has a polar density of {001} <110>, {111} <110> and {112} <110> orientation groups of 2.0 or more in the texture of the surface layer region, a polar density of {110} <112> orientation of 5.0 or less in the texture of the inner region, and a tensile strength of 1180MPa or more.

Description

Hot rolled steel sheet

Technical Field

The present invention relates to a hot rolled steel sheet.

The present application claims priority based on japanese patent application No. 2021-168623, 10 and 14 of 2021, the contents of which are incorporated herein by reference.

Background

In recent years, for the purpose of improving fuel efficiency and collision safety of automobiles, a high-strength steel sheet is applied to reduce the weight of the automobile body. However, if the strength of the steel sheet is increased, the toughness generally deteriorates. Therefore, in the development of high-strength steel sheets, it is an important issue to increase the strength without deteriorating the toughness.

In general, in order to improve toughness, a method is known in which rolling is performed at a low temperature, and toughness is improved by imparting a high cumulative strain in a state of unrecrystallized austenite. However, if the reduction ratio in the unrecrystallized austenite state is increased, there are problems that the aspect ratio of the prior austenite grains is high and the anisotropy of toughness is high.

For example, patent document 1 discloses a hot-rolled steel sheet characterized by having the following texture in a sheet thickness center portion, which is a steel sheet portion divided by a 3/8 thickness position and a 5/8 thickness position from a surface of the steel sheet: the average value of the X-ray random intensity ratios of {100} <011> to {223} <110> orientation groups of the plate surface is 6.5 or less, and the X-ray random intensity ratio of {332} <113> crystal orientation is 5.0 or less, and has the following microstructure: the tempered martensite, martensite and lower bainite have a total area ratio exceeding 85% and an average crystal grain size of 12.0 μm or less.

Prior art literature

Patent literature

Patent document 1: japanese patent No. 5621942

Disclosure of Invention

Problems to be solved by the invention

However, in the technique disclosed in patent document 1, there is room for further improvement in terms of improvement of fuel efficiency and improvement of collision safety of automobiles, regarding reduction of anisotropy of toughness in high-strength steel sheets.

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a hot-rolled steel sheet having high strength and excellent toughness, and reduced toughness anisotropy.

Means for solving the problems

The inventors of the present invention examined the relationship between the texture and mechanical properties of a hot-rolled steel sheet, and found that: the anisotropy of toughness can be further reduced even in a hot-rolled steel sheet having a tensile strength of 1180MPa or more. The inventors of the present invention recognized that: the rolled steel sheet has different textures developed in the surface and the interior. Furthermore, the inventors of the present invention recognized that: in order to reduce the anisotropy of toughness, it is effective to control the texture in the austenite region as compared with the texture of martensite after quenching. Further, the inventors of the present invention recognized that: in order to obtain a texture having a desired crystal orientation, it is effective to preferably control the hot rolling conditions.

The gist of the present invention based on the above knowledge is as follows.

(1) The hot rolled steel sheet according to an embodiment of the present invention has a chemical composition comprising, in mass%:

C：0.100～0.500％、

Si：0.100～3.000％、

Mn：0.50～3.00％、

P:0.100% or less,

S:0.0100% or less,

Al: less than 1.000 percent,

N:0.0100% or less,

Ti：0～0.20％、

Nb：0～0.100％、

Ca：0～0.0060％、

Mo：0～0.50％、

Cr：0～1.00％、

V：0～0.50％、

Cu：0～0.50％、

Ni:0 to 0.50 percent

Sn：0～0.050％，

The remainder comprising Fe and impurities,

The metal structure in the region from 1/8 depth to 3/8 depth from the surface to the plate thickness contains 90 to 100% martensite and 0 to 10% of the remaining structure in area%,

In the texture of the region from the surface to a depth of 1/8 of the plate thickness, the polar density of the {001} <110>, {111} <110>, and {112} <110> orientation groups is 2.0 or more,

In the texture of the region from 1/8 depth to 1/2 depth of the plate thickness, the polar density of the {110} <112> orientation is 5.0 or less,

The tensile strength of the hot-rolled steel sheet is 1180MPa or more.

(2) The hot-rolled steel sheet according to the above (1), wherein the chemical composition may contain 1 or 2 or more kinds of elements selected from the following elements in mass%:

Ti：0.02～0.20％、

Nb：0.010～0.100％、

Ca：0.0001～0.0060％、

Mo：0.01～0.50％、

Cr：0.01～1.00％、

V：0.01～0.50％、

Cu：0.01～0.50％、

Ni:0.01 to 0.50 percent

Sn：0.001～0.050％。

Effects of the invention

According to the above aspect of the present invention, a hot rolled steel sheet having high strength and excellent toughness and reduced anisotropy of toughness can be provided.

Detailed Description

Hereinafter, the hot-rolled steel sheet according to the present embodiment will be specifically described.

First, the reason why the chemical composition of the hot-rolled steel sheet according to the present embodiment is limited will be described. The numerical values described in the "to" are limited to the ranges, and the lower limit value and the upper limit value are included in the ranges. With respect to values expressed as "below", "above", the values are not included in the numerical range. In addition, "%" with respect to the chemical composition means "% by mass".

The chemical composition of the hot-rolled steel sheet according to the present embodiment includes C:0.100 to 0.500 percent of Si:0.100 to 3.000 percent of Mn:0.50 to 3.00 percent of P:0.100% or less, S: less than 0.0100%, al: less than 1.000%, N: less than 0.0100% and the remainder: fe and impurities. Hereinafter, each element will be described in detail.

C：0.100～0.500％

C is an important element for improving the strength of the hot rolled steel sheet. If the C content is less than 0.100%, the strength of the hot rolled steel sheet is lowered. Therefore, the C content is set to 0.100% or more. The C content is preferably 0.150% or more, 0.170% or more, 0.200% or more, or 0.220% or more.

On the other hand, if the C content exceeds 0.500%, the toughness of the hot-rolled steel sheet deteriorates. Therefore, the C content is set to 0.500% or less. The C content is preferably 0.450% or less, 0.400% or less, or 0.370% or less.

Si：0.100～3.000％

Si is an element having an effect of improving the strength of the hot rolled steel sheet. If the Si content is less than 0.100%, the strength of the hot rolled steel sheet is deteriorated. Therefore, the Si content is set to 0.100% or more. The Si content is preferably 0.200% or more, 0.300% or more, 0.400% or more, or 0.500% or more. The Si content is more preferably more than 1.000%, and still more preferably 1.100% or more.

On the other hand, if the Si content exceeds 3.000%, the toughness of the hot-rolled steel sheet deteriorates. Therefore, the Si content is set to 3.000% or less. The Si content is preferably 2.700% or less, 2.500% or less, or 2.300% or less.

Mn：0.50～3.00％

Mn is an element effective for improving the strength of a hot-rolled steel sheet by improving hardenability and solid solution strengthening. If the Mn content is less than 0.50%, the strength of the hot-rolled steel sheet is lowered. Therefore, the Mn content is set to 0.50% or more. The Mn content is preferably 1.00% or more, 1.20% or more, or 1.50% or more.

On the other hand, if the Mn content exceeds 3.00%, anisotropic MnS is generated which improves the toughness of the hot-rolled steel sheet. Therefore, the Mn content is set to 3.00% or less. The Mn content is preferably 2.50% or less, 2.30% or less, or 2.00% or less.

P: less than 0.100%

P is an impurity element, and the lower the P content is, the more preferable. If the P content exceeds 0.100%, deterioration of workability and weldability of the hot-rolled steel sheet becomes remarkable, and fatigue characteristics are also deteriorated. Therefore, the P content is set to 0.100% or less. The P content is preferably 0.070% or less, 0.050% or less, or 0.030% or less.

The lower limit of the P content is not particularly limited, but if the P content is excessively reduced, the manufacturing cost increases, and therefore the P content may be set to 0.001% or more or 0.005% or more.

S:0.0100% or less

S is an impurity element, and the lower the S content is, the more preferable. If the S content exceeds 0.0100%, inclusions such as MnS, which improve the toughness of the hot-rolled steel sheet, are produced in large amounts. Therefore, the S content is set to 0.0100% or less. The S content is preferably 0.0080% or less, 0.0060% or less, or 0.0040% or less.

The lower limit of the S content is not particularly limited, but if the S content is excessively reduced, the manufacturing cost increases, and thus the S content may be set to 0.0005% or more or 0.0010% or more.

Al: less than 1.000 percent

Al is an element effective for improving the cleanliness of steel, which acts as a deoxidizer in the steelmaking stage. However, if the Al content exceeds 1.000%, alumina precipitated in clusters is formed, and the toughness of the hot-rolled steel sheet deteriorates. Therefore, the Al content is set to 1.000% or less. The Al content is preferably 0.700% or less, 0.500% or less, or 0.400% or less.

The lower limit of the Al content is not particularly limited, but if the Al content is excessively reduced, the manufacturing cost increases, and thus the Al content may be set to 0.001% or more or 0.005% or more.

N:0.0100% or less

N is an impurity element. If the N content exceeds 0.0100%, coarse Ti nitrides are formed at high temperature, and the toughness of the hot-rolled steel sheet deteriorates. Therefore, the N content is set to 0.0100% or less. The N content is preferably 0.0080% or less, 0.0060% or less, or 0.0040% or less.

The lower limit of the N content is not particularly limited, but if the N content is excessively reduced, the manufacturing cost increases, and thus the N content may be set to 0.0010% or more.

The hot-rolled steel sheet according to the present embodiment may contain the above elements, and the remainder may contain Fe and impurities. Examples of the impurities include elements inevitably mixed from steel raw materials or scraps and/or during the steelmaking process, and elements which are allowed within a range that does not hinder the properties of the hot-rolled steel sheet according to the present embodiment.

In order to improve various properties, the hot-rolled steel sheet according to the present embodiment may contain the following optional elements in place of a part of Fe. In order to reduce the alloy cost, it is not necessary to intentionally contain these optional elements in the steel, and therefore the lower limit of the content of these optional elements is 0%.

Ti：0.02～0.20％

Ti is an element effective for suppressing recrystallization of austenite between hot rolled frames and grain growth. By suppressing recrystallization of austenite between frames, strain can be further accumulated. As a result, the texture of the hot rolled steel sheet can be preferably controlled. In order to reliably obtain the above-described effects, the Ti content is preferably set to 0.02% or more.

On the other hand, if the Ti content exceeds 0.20%, inclusions due to TiN are formed, and the toughness of the hot-rolled steel sheet deteriorates. Therefore, the Ti content is set to 0.20% or less.

Nb：0.010～0.100％

Nb is an element effective for suppressing recrystallization and grain growth of austenite between hot rolled frames. By suppressing recrystallization of austenite between frames, strain can be further accumulated. As a result, the texture of the hot rolled steel sheet can be preferably controlled. In order to reliably obtain the above-described effect, the Nb content is preferably set to 0.010% or more.

On the other hand, when the Nb content exceeds 0.100%, the effect is saturated. Therefore, the Nb content is set to 0.100% or less.

Ca：0.0001～0.0060％

Ca is an element having the following effects: during deoxidation of molten steel, a large amount of fine oxides are dispersed, and the structure of the hot-rolled steel sheet is refined. In addition, ca is also the following element: s in the steel is fixed in the form of spherical CaS, thereby suppressing the generation of extended inclusions such as MnS and reducing the anisotropy of toughness of the hot-rolled steel sheet. In order to reliably obtain these effects, the Ca content is preferably set to 0.0001% or more.

On the other hand, when the Ca content exceeds 0.0060%, the above effect is saturated. Therefore, the Ca content is set to 0.0060% or less.

Mo：0.01～0.50％

Mo is an element effective for precipitation strengthening of ferrite. In order to reliably obtain this effect, the Mo content is preferably set to 0.01% or more.

On the other hand, when the Mo content exceeds 0.50%, the cracking sensitivity of the slab increases and the handling of the slab becomes difficult. Therefore, the Mo content is set to 0.50% or less.

Cr：0.01～1.00％

Cr is an element effective for improving the strength of the hot rolled steel sheet. In order to reliably obtain this effect, the Cr content is preferably set to 0.01% or more.

On the other hand, if the Cr content exceeds 1.00%, the ductility of the hot-rolled steel sheet deteriorates. Therefore, the Cr content is set to 1.00% or less.

V：0.01～0.50％

V the strength of the hot-rolled steel sheet is improved by strengthening by the precipitates and grain refining of ferrite grains. In order to reliably obtain this effect, the V content is preferably set to 0.01% or more.

On the other hand, when the V content exceeds 0.50%, carbonitrides are precipitated in large amounts, and the formability of the hot-rolled steel sheet is deteriorated. Therefore, the V content is set to 0.50% or less.

Cu：0.01～0.50％

Cu is an element that is solid-dissolved in steel and contributes to the improvement of the strength of the steel. Cu is also an element for improving hardenability. In order to reliably obtain these effects, the Cu content is preferably set to 0.01% or more.

On the other hand, if the Cu content exceeds 0.50%, the surface properties of the hot-rolled steel sheet may be lowered, and the chemical conversion treatability and corrosion resistance may be deteriorated. Therefore, the Cu content is set to 0.50% or less.

Ni：0.01～0.50％

Ni is an element that is solid-dissolved in steel and contributes to the strength increase of steel. Ni is also an element that improves hardenability. In order to reliably obtain these effects, the Ni content is preferably set to 0.01% or more.

On the other hand, since Ni is high in alloy cost, if Ni is contained in a large amount, an increase in cost is caused. In addition, when the Ni content exceeds 0.50%, there is a possibility that the weldability of the hot rolled steel sheet deteriorates. Therefore, the Ni content is set to 0.50% or less.

Sn：0.001～0.050％

Sn has an effect of suppressing internal oxidation and an effect of improving strength. In order to reliably obtain this effect, the Sn content is preferably set to 0.001% or more.

On the other hand, if Sn is contained in a large amount, defects may occur during hot rolling. Therefore, the Sn content is set to 0.050% or less.

The chemical composition may be measured by a general analytical method. For example, the measurement may be performed by using ICP-AES (inductively coupled plasma-atomic emission Spectrometry; inductively Coupled Plasma-Atomic Emission Spectrometry). The measurement of C and S may be performed by a combustion-infrared absorption method, and the measurement of N may be performed by an inert gas fusion-thermal conductivity method.

When the hot-rolled steel sheet has a coating on the surface, the coating on the surface may be removed by mechanical grinding and then the chemical composition may be analyzed.

Next, the microstructure of the hot-rolled steel sheet according to the present embodiment will be described.

The hot-rolled steel sheet of the present embodiment has a metal structure in a region of 1/8 depth from a surface to 3/8 depth from the surface, the region including 90 to 100% of martensite and 0 to 10% of a remaining structure in area%, and the {001} <110>, {111} <110>, and {112} <110> orientation groups have a polar density of 2.0 or more, and the {110} <112> orientation groups have a polar density of 5.0 or less in a region of 1/8 depth from the surface to 1/2 depth from the surface.

In the present embodiment, the reason why the area% of martensite and the remaining structure in the region from 1/8 depth to 3/8 depth from the surface is defined is that: the microstructure at this location represents a representative microstructure of the hot rolled steel sheet. Each of the specifications is described in detail below.

Area ratio of martensite: 90 to 100 percent

If the area ratio of martensite is less than 90%, the strength of the hot-rolled steel sheet is deteriorated, and a desired strength cannot be obtained. Therefore, the area ratio of martensite is set to 90% or more. The area ratio of martensite is preferably 92% or more, 95% or more, or 97% or more, and more preferably 100%.

In the present embodiment, martensite means primary martensite and tempered martensite. In this embodiment, since it is not necessary to distinguish between primary martensite and tempered martensite, both are collectively referred to as martensite.

The tempered martensite is tempered primary martensite, and has a dislocation density lower than that of primary martensite. In the preferred method for producing a hot-rolled steel sheet according to the present embodiment described below, heat treatment for tempering is not included after quenching, but tempered martensite may be generated by heat returning during cooling after hot rolling or after coiling.

Area ratio of remaining tissue: 0 to 10 percent

The microstructure of the hot-rolled steel sheet according to the present embodiment may contain bainite as a residual microstructure. If the area ratio of the excess structure exceeds 10%, the strength of the hot-rolled steel sheet is lowered, and the desired strength cannot be obtained. Therefore, the area ratio of the remaining tissue is set to 10% or less. The area ratio of the remaining tissue is preferably 8% or less, 5% or less, or 3% or less, and more preferably 0%.

The area ratio of each tissue was obtained by the following method.

Test pieces for tissue observation were collected from a position 1/4 of the plate thickness of the hot-rolled steel plate (a region from 1/8 depth to 3/8 depth from the surface) and a plate width center position so that a plate thickness cross section parallel to the rolling direction became an observation surface. After mirror polishing the observation surface, the observation surface was etched with 3 vol% nitric acid ethanol solution. The etched observation surface was photographed at 2000 x magnification for 3 fields of view using an optical microscope and a Scanning Electron Microscope (SEM). Each imaging field was set to 500 μm×500 μm. And (5) carrying out image analysis on the shot pictures, and calculating the area ratio of each organization. The area ratio of each tissue was obtained by calculating an average value of the area ratios obtained for the 3 fields of view.

Since martensite is a structure having a lower structure such as a slab and a lath in a crystal grain, it can be distinguished from other metal structures by electron channel contrast imaging using a scanning electron microscope.

The following structure is considered as bainite: the structure is composed of a set of lath-shaped crystal grains, a structure which is not martensitic in the structure and does not contain Fe-based carbide having a length of 20nm or more in the structure, and a structure which contains Fe-based carbide having a length of 20nm or more in the structure and has a single modification, that is, fe-based carbide elongated in the same direction. Here, the Fe-based carbide extending in the same direction means that the difference in the extending direction of the Fe-based carbide is within 5 °.

Average grain size of prior austenite grains: more than 5.0 μm and less than 30.0 μm

In the hot-rolled steel sheet according to the present embodiment, the average grain size of the prior austenite grains may be more than 5.0 μm and 30.0 μm or less at 1/4 of the plate thickness (a region from 1/8 depth of the plate thickness to 3/8 depth of the plate thickness from the surface). By setting the average grain size of the prior austenite grains to be more than 5.0 μm, the predetermined texture required in the present embodiment can be stably obtained, and the anisotropy of toughness of the hot-rolled steel sheet can be further reduced. The average grain size of the prior austenite grains is preferably 6.0 μm or more, 7.0 μm or more, 8.0 μm or more, or 9.0 μm or more.

On the other hand, if the average grain size of the prior austenite grains exceeds 30.0. Mu.m, the desired strength may not be obtained. Therefore, the average grain size of the prior austenite grains is preferably set to 30.0 μm or less.

The average grain size of the prior austenite grains is obtained by the following method.

Test pieces for tissue observation were collected from a position 1/4 of the plate thickness of the hot-rolled steel plate (a region from 1/8 depth to 3/8 depth from the surface) and a plate width center position so that a plate thickness cross section parallel to the rolling direction became an observation surface. After mirror polishing the observation surface, the metal structure was observed by a Scanning Electron Microscope (SEM) by etching with 3 vol% nitric acid ethanol solution. The range in which about 10000 grains can be observed in 1 field of view was subjected to 3 field of view photographing by SEM observation. For the obtained photograph, image analysis was performed using image analysis software (windof), and the average grain size of the prior austenite grains was calculated. For 1 prior austenite grain included in the observation field, an average value of the shortest diameter and the longest diameter was calculated, and the average value was used as the grain size of the prior austenite grain. The above-described operations were performed on all prior austenite grains except that the entire crystal grains such as the end of the shot field were not included in the shot field, and the grain size of all prior austenite grains in the shot field was determined. The average grain size of the prior austenite grains in the shot view field is obtained by calculating a value obtained by dividing the sum of the grain sizes of the prior austenite grains obtained by the measurement by the total number of prior austenite grains of the measured grain size. By performing this operation for each of all the photographed fields, the average grain size of the prior austenite grains for all the photographed fields is calculated, and the average grain size of the prior austenite grains is obtained.

The {001} <110>, {111} <110>, and {112} <110> orientation groups in the texture of the region having a depth of 1/8 of the plate thickness from the surface: 2.0 or more

If the polar densities of {001} <110>, {111} <110>, and {112} <110> orientation groups in the texture of a region (hereinafter, sometimes referred to as a surface layer region) having a depth of 1/8 of the plate thickness from the surface to the surface are lower than 2.0, the occurrence of minute cracks in the surface layer region cannot be suppressed. As a result, the anisotropy of toughness of the hot-rolled steel sheet increases. Therefore, the polar densities of {001} <110>, {111} <110> and {112} <110> orientation groups in the texture of the surface layer region are set to 2.0 or more. Preferably 2.2 or more, 2.5 or more, or 2.7 or more.

The upper limits of the polar densities of the {001} <110>, {111} <110> and {112} <110> orientation groups in the texture of the surface layer region are not particularly limited, but may be set to 9.0 or less, 8.0 or less, 7.0 or less, or 5.0 or less from the viewpoint of suppressing deterioration of ductility.

The {110} <112> orientation in the texture of the region from 1/8 depth to 1/2 depth from the surface to the plate: 5.0 or less

If the polar density of the {110} <112> orientation in the texture of the region (hereinafter, may be referred to as the internal region) having a depth of 1/8 to 1/2 of the plate thickness from the surface exceeds 5.0, the anisotropy of the toughness of the hot-rolled steel sheet becomes high. Therefore, the {110} <112> orientation in the texture of the inner region has a polar density of 5.0 or less. Preferably 4.6 or less, 4.2 or less, or 4.0 or less.

The lower limit of the polar density of the {110} <112> orientation in the texture of the internal region is not particularly limited, but may be set to 2.0 or more or 2.5 or more from the viewpoint of suppressing deterioration in strength.

As the polar density, an OIM Analysis (registered trademark) manufactured by ameteek corporation, which is a combination of a scanning electron microscope and an EBSD Analysis device, was used. From a crystal orientation distribution function (ODF: orientation Distribution Function) representing a three-dimensional texture calculated by calculation using orientation data and spherical harmonics measured by an EBSD (electron back scattering diffraction; electron Back Scattering Diffraction) method, the polar densities of {001} <110>, {111} <110> and {112} <110> orientation groups in the texture of the surface layer region and {110} <112> in the texture of the inner region are obtained.

The measurement range was set to a region having a depth of 1/8 of the plate thickness from the surface to the surface of the surface layer region, and a region having a depth of 1/8 of the plate thickness from the surface to 1/2 of the plate thickness from the surface of the inner region. The measurement pitch was set to 5 μm/step.

{ Hkl } represents a crystal plane parallel to the rolling surface, and < uvw > represents a crystal direction parallel to the rolling direction. That is, { hkl } < uvw > represents a crystal in which { hkl } is oriented in the plate surface normal direction and < uvw > is oriented in the rolling direction.

The rolling direction of the hot rolled steel sheet can be determined by the following method.

First, test pieces were collected so that the thickness and cross section of the hot-rolled steel sheet could be observed. The plate thickness cross section of the test piece thus collected was finished by mirror polishing, and then observed with an optical microscope. The observation range was set to the entire thickness of the sheet thickness, and the area with dark brightness was determined as an inclusion. Among the inclusions, those having a long axis length of 40 μm or more were identified as rolling directions in parallel with the direction in which the inclusions were stretched.

Tensile strength: 1180MPa or more

The tensile strength of the hot-rolled steel sheet according to the present embodiment is 1180MPa or more from the viewpoint of improvement of collision safety of automobiles and the like or weight reduction of the vehicle body. Preferably 1250MPa or more, 1300MPa or more, 1350MPa or more, or 1400MPa or more.

The upper limit of the tensile strength is not particularly limited, but is preferably 2000MPa or less, 1600MPa or less, 1500MPa or less, or 1400MPa or less.

Tensile strength according to JIS Z2241: 2011. Test piece was set as JIS Z2241: 2011, the test direction is set to be perpendicular to the rolling direction.

The thickness of the hot-rolled steel sheet according to the present embodiment is not particularly limited, but may be set to 1.2 to 8.0mm. When the plate thickness of the hot rolled steel sheet is less than 1.2mm, there is a possibility that the securing of the rolling completion temperature becomes difficult, and the rolling load becomes excessive and the hot rolling becomes difficult.

Further, if the plate thickness exceeds 8.0mm, control of the texture may become difficult, and it may become difficult to obtain the texture. Therefore, the plate thickness may be set to 8.0mm or less.

The hot-rolled steel sheet according to the present embodiment may have a plating layer on the surface. Examples of the plating layer include an aluminum plating layer, an aluminum-zinc plating layer, an aluminum-silicon plating layer, a hot dip galvanization layer, an electro-galvanization layer, and an alloyed hot dip galvanization layer.

Next, a preferred method for producing the hot-rolled steel sheet according to the present embodiment will be described. The method for producing a hot-rolled steel sheet according to the present embodiment preferably includes the following steps (a) to (d). The temperature in the following description refers to the surface temperature of the steel sheet unless otherwise specified.

(A) And a heating step of heating the slab having the chemical composition to a temperature range of 1100 ℃ or higher and 1350 ℃ or lower.

(B) And a finish rolling step of finish rolling the heated slab using a rolling mill having a plurality of stands, wherein the following conditions (I) to (V) are satisfied.

(I) The finish rolling start temperature is set to 800 ℃ or higher.

(II) in the last 4 frames among the plurality of frames, rolling was performed so that the sigma represented by the following formula (1) becomes 40 to 80.

σ＝exp(0.753+3000/T)×ε ^0.21×ε' ^0.13 (1)

Wherein T is the temperature (DEG C) immediately before entering each rack, epsilon is the equivalent plastic strain, and epsilon' is the strain rate.

(III) the inter-pass time between the last 4 frames is set to 0.2-10.0 seconds.

(IV) the cumulative rolling reduction of the last 4 frames is set to 60% or more.

(V) the finish rolling completion temperature is set to 800 to 950 ℃.

(C) And a cooling step of starting cooling within 1.0 second after finishing finish rolling and cooling to a temperature region of 300 ℃ or lower so that the average cooling rate in the temperature region of from finishing finish rolling temperature to 300 ℃ is 100 ℃/sec or more.

(D) And a coiling step of coiling after cooling.

Hereinafter, each step will be described.

(A) Heating process

In the heating step, the slab having the chemical composition described above is preferably heated to a temperature range of 1100 ℃ or higher and lower than 1350 ℃. The method for producing the slab is not particularly limited, and the following general methods can be applied: molten steel having the chemical composition described above is melted by a converter or the like, and a slab is produced by a casting method such as continuous casting. The ingot-cogging method may be used.

In the slab, most of the carbonitride forming elements such as Ti exist as coarse carbonitrides in the slab in a non-uniform distribution. Coarse precipitates (carbonitrides) present in an uneven distribution deteriorate various properties (for example, tensile strength, toughness, hole expansibility, etc.) of the hot-rolled steel sheet. Therefore, the slab before hot rolling is heated to dissolve coarse precipitates. In order to sufficiently dissolve the coarse precipitates before hot rolling, the heating temperature of the slab is preferably set to 1100 ℃ or higher. However, if the heating temperature of the slab becomes too high, the occurrence of surface flaws and scale peeling may cause a decrease in yield. Therefore, the heating temperature of the steel raw material is preferably set to be lower than 1350 ℃.

The slab was heated to a temperature range of 1100 ℃ or more and 1350 ℃ or less and held for a predetermined time, but if the holding time exceeds 4800 seconds, the amount of scale generation increases. As a result, scale biting or the like may easily occur in the subsequent finish rolling step, and the surface quality of the hot-rolled steel sheet may be deteriorated. Therefore, the holding time in the temperature range of 1100 ℃ or more and 1350 ℃ or less is preferably set to 4800 seconds or less.

Rough rolling process

The slab may be rough rolled between the heating step and the finish rolling step. The rough rolling is not particularly limited as long as the desired sheet bar size can be obtained.

(B) Finish rolling process

In the finish rolling step, the heated slab is finish rolled by using a rolling mill having a plurality of stands. In this case, the following conditions (I) to (V) are preferably satisfied.

It is preferable to remove the scale before finish rolling or during rolling between rolling stands of finish rolling.

(I) Finish rolling start temperature: 800 ℃ above

The finish rolling start temperature (the temperature at the inlet side of the initial pass of finish rolling) is preferably set to 800 ℃ or higher. If the finish rolling start temperature is lower than 800 ℃, rolling in a part of the plurality of rolling stands (particularly the stands of the first half) becomes performed in a ferrite+austenite two-phase region temperature. As a result, the processed structure may remain after finish rolling, and the strength and toughness of the hot-rolled steel sheet may deteriorate. Accordingly, the finish rolling start temperature is preferably set to 800 ℃ or higher.

In order to suppress coarsening of austenite, and to preferably control texture of the surface layer region and the inner region, the finish rolling start temperature is preferably set to 1100 ℃ or lower.

(II) Sigma represented by the following formula (1) in the last 4 frames: 40 to 80 percent

σ＝exp(0.753+3000/T)·ε ^0.21·ε' ^0.13 (1)

Where T is the temperature immediately before entering each rack (C.) (i.e., the inlet side temperature), ε is the equivalent plastic strain, ε' is the strain rate.

Sigma between 40 and 80 in the last 4 frames may be in other words: the sigma of the 4 th last frame, the sigma of the 3 rd last frame, the sigma of the 2 nd last frame and the sigma of the final frame are all 40-80.

If there are even 1 machine frame with σ less than 40, the strain required for development of texture of the surface layer region may not be appropriately imparted in the last 4 machine frames. As a result, in the texture of the region having a depth of 1/8 of the plate thickness from the surface to the surface, the polar densities of the {001} <110>, {111} <110> and {112} <110> orientation groups may not be controlled preferably. Therefore, σ in the last 4 frames is preferably set to 40 or more.

Further, if there are even 1 machine frame with σ exceeding 80, the texture of the inner region may not be preferably controlled, and the anisotropy of the toughness of the hot-rolled steel sheet may be improved. Therefore, σ in the last 4 frames is preferably set to 80 or less.

When the thickness of the inlet side plate is set to H and the thickness of the outlet side plate is set to H, the equivalent plastic strain ε can be obtained by ε= (2 /) × (H/H). When the rolling time is set to t(s), the strain rate, that is, ε 'can be obtained by ε' =ε/t. The rolling time t is a time when the steel sheet is brought into contact with the roll and strain is applied to the steel sheet.

(III) inter-pass time between last 4 frames: 0.2 to 10.0 seconds

In the last 4 frames, if there is more than 10.0 seconds of inter-pass time even 1 inter-pass time, the recovery and recrystallization between passes proceeds. As a result, the accumulation of strain may become difficult, and a desired structure may not be obtained in the hot-rolled steel sheet. Therefore, the inter-pass time between the last 4 frames is preferably set to 10.0 seconds or less.

The inter-pass time between the last 4 frames is preferably short, but there is a limitation in terms of installation space and rolling speed of each frame with respect to shortening of the inter-pass time. Further, if the inter-pass time becomes less than 0.2 seconds between the last 4 frames, there is a possibility that the grain size is significantly increased without recrystallization, and the desired texture cannot be obtained. Therefore, the time is preferably set to 0.2 seconds or longer.

The inter-pass time between the last 4 frames of 0.2 to 10.0 seconds may be: the inter-pass time between the 4 th and 3 rd frames, the inter-pass time between the 3 rd and 2 nd frames, and the inter-pass time between the 2 nd and final frames are all 0.2-10.0 seconds.

(IV) cumulative reduction of last 4 frames: more than 60 percent

When the cumulative reduction of the last 4 frames is less than 60%, there is a possibility that the dislocation density introduced into unrecrystallized austenite becomes small. If the dislocation density introduced into unrecrystallized austenite becomes small, it may become difficult to obtain a desired structure, and the strength and toughness of the hot-rolled steel sheet may deteriorate. Therefore, the cumulative rolling reduction of the last 4 frames is preferably set to 60% or more.

If the cumulative reduction of the last 4 frames exceeds 97%, there is a possibility that the shape of the hot rolled steel sheet may deteriorate. Therefore, the cumulative rolling reduction of the last 4 frames can also be set to 97% or less.

When the inlet plate thickness of the 4 th last frame is set to t0 and the outlet plate thickness of the final frame is set to t1, the cumulative rolling reduction of the last 4 frames can be expressed as {1- (t 1/t 0) } ×100 (%).

(V) finish finishing temperature: 800-950 DEG C

When the finish rolling finish temperature (outlet side temperature of the final stand) is lower than 800 ℃, rolling proceeds in a ferrite+austenite duplex region temperature. Therefore, there is a possibility that the worked structure remains after rolling and the strength and toughness of the hot-rolled steel sheet decrease. Therefore, the finishing temperature is preferably set to 800 ℃.

In the slab having the chemical composition of the present embodiment, the unrecrystallized austenite region is substantially a temperature region of 950 ℃ or lower. Therefore, if the finish rolling completion temperature exceeds 950 ℃, austenite grains grow, and the grain length of martensite of the hot-rolled steel sheet obtained after cooling becomes large. As a result, it may become difficult to obtain a desired texture, and the strength and toughness of the hot-rolled steel sheet may be lowered. Therefore, the finishing temperature is preferably set to 950 ℃ or lower.

(C) Cooling process

In the cooling step, it is preferable that the cooling is started within 1.0 second after completion of the finish rolling, and the temperature range is cooled to 300 ℃ or lower so that the average cooling rate in the temperature range from the finish rolling completion temperature to 300 ℃ is 100 ℃/sec or higher.

In the present embodiment, it is preferable that a cooling device is provided at the rear stage of the finish rolling device, and the finish rolled steel sheet is cooled while passing through the cooling device. The cooling device is preferably set to be a device capable of cooling the steel sheet at an average cooling rate of 100 ℃/sec or more. As the cooling device, for example, a water cooling device using water as a cooling medium can be exemplified.

The average cooling rate in the cooling step is set to a value obtained by dividing the temperature decrease width of the steel sheet from the start of cooling to the end of cooling by the time required from the start of cooling to the end of cooling. The start of cooling is set when the steel sheet is introduced into the cooling facility; the cooling end is set to be when the steel sheet is led out from the cooling device.

Further, there are apparatuses having no air cooling section in the middle and apparatuses having 1 or more air cooling sections in the middle. In this embodiment, any cooling device may be used. Even when a cooling device having an air cooling section is used, the average cooling rate from the start of cooling to the end of cooling may be 100 ℃/sec or more.

The reason for limiting the cooling conditions will be described below. The cooling stop temperature was 300 ℃ or lower, and the winding step was described under this condition.

Cooling start time: within 1.0 second after finishing rolling

After finishing the finish rolling, it is preferable to start cooling immediately. If the cooling start time exceeds 1.0 seconds, there is a possibility that recrystallization progresses, cooling is performed in a state where strain is released, and a desired texture cannot be obtained in the hot-rolled steel sheet. Therefore, it is preferable to start cooling within 1.0 second after finishing finish rolling.

Average cooling rate in the temperature range of finish rolling completion temperature to 300 ℃): 100 ℃/s or more

If the average cooling rate in the temperature range from the finish rolling completion temperature to 300 ℃ is less than 100 ℃/sec, bainite and ferrite may be easily formed, and a desired amount of martensite may not be obtained. Therefore, the average cooling rate in the temperature range from the finish rolling temperature to 300 ℃ is preferably set to 100 ℃/sec or more.

(D) Winding process

In the coiling step, the steel sheet cooled to a temperature range of 300 ℃ or lower is preferably coiled into a coil shape. Since coiling of the steel sheet is performed immediately after cooling, the coiling temperature is substantially equal to the cooling stop temperature. If the coiling temperature exceeds 300 ℃, polygonal ferrite or bainite is generated, and therefore, there is a possibility that the strength of the hot-rolled steel sheet is lowered. Therefore, the winding temperature is preferably set to a temperature range of 300 ℃ or less.

After coiling, the hot rolled steel sheet may be temper rolled according to a conventional method, or may be pickled to remove scale formed on the surface. Or further plating treatment such as aluminizing, aluminizing-zincing, aluminizing-silicozing, hot dip galvanizing, electrogalvanizing, alloyed hot dip galvanizing, and chemical conversion treatment may be performed.

The hot-rolled steel sheet according to the present embodiment can be stably produced by the preferred production method described above.

Examples

Next, an embodiment of the present invention will be described, but the conditions in the embodiment are one example of conditions adopted for confirming the operability and effect of the present invention, and the present invention is not limited to this one example of conditions. The present invention can employ various conditions as long as the object of the present invention is achieved without departing from the gist of the present invention.

Molten steel having the chemical composition shown in table 1 was melted in a converter, and a slab was obtained by a continuous casting method. Next, these slabs were heated under the conditions shown in table 2A and table 2B, rough rolled, and then finish rolled under the conditions shown in table 2A and table 2B. After completion of finish rolling, hot rolled steel sheets having the sheet thicknesses shown in tables 3A and 3B were obtained by cooling and coiling under the conditions shown in tables 3A and 3B.

In the heating step, the holding time at the heating temperature described in table 2A and table 2B was set to 4800 seconds or less.

The cooling after finish rolling is set to be cooling by water cooling, and the steel sheet is passed through a water cooling facility having no air cooling section in the middle. The average cooling rates in tables 3A and 3B are values obtained by dividing the temperature decrease width of the steel sheet from the time of introduction into the water cooling facility to the time of discharge from the water cooling facility by the passage time required for the steel sheet to pass through the water cooling facility.

Test pieces were collected from the obtained hot-rolled steel sheet, and the area ratio of each structure and the polar density of the texture were measured and the tensile test was performed by the above-described method.

The results obtained are shown in tables 4A and 4B.

When the tensile strength obtained was 1180MPa or more, the steel sheet was judged to be satisfactory as having high strength. On the other hand, when the obtained tensile strength is lower than 1180MPa, the steel sheet is judged to be unacceptable as having no high strength.

As an evaluation of toughness of the hot-rolled steel sheet, the brittle transition temperature was measured by performing the charpy impact test. Determination of the brittle transition temperature according to JIS Z2242: 2018, a C-direction notch charpy impact test was performed using a 2.5mm small-sized V-notch test piece. The brittle fracture rate was set to 50% at the brittle transition temperature. Further, the test piece having a final sheet thickness of less than 2.5mm was measured as the whole thickness of the hot rolled steel sheet.

When the obtained brittle transition temperature was-50℃or lower, the obtained product was judged to be satisfactory as excellent in toughness. On the other hand, when the obtained brittle transition temperature exceeds-50 ℃, the toughness is judged as poor and is judged as unacceptable.

Further, the anisotropy of toughness was evaluated by the following method. According to JIS Z2242: 2018, the absorption energy of the C-direction notch and the absorption energy of the L-direction notch were measured by the Charpy impact test using a 2.5mm small-sized V-notch test piece. The Charpy impact test was carried out at-60 ℃. The difference between the absorption energy of the L-direction notch and the absorption energy of the C-direction notch was calculated, and if the difference was + -15J or less, the difference was judged to be acceptable as a decrease in the anisotropy of toughness. On the other hand, when the difference between the absorption energy of the L-direction notch and the absorption energy of the C-direction notch exceeds ±15j, the anisotropy as toughness is determined to be unacceptable without decreasing.

TABLE 1

[ Table 2A ]

[ Table 2B ]

[ Table 3A ]

TABLE 3B

[ Table 4A ]

TABLE 4B

From an examination of Table 4A and Table 4B, it can be seen that: the hot rolled steel sheet of the present invention has high strength and excellent toughness, and the anisotropy of toughness is reduced. On the other hand, it can be seen that: some characteristics of the hot rolled steel sheet of the comparative example were deteriorated.

Claims

1. A hot-rolled steel sheet characterized by comprising, in mass%, the chemical composition:

C：0.100～0.500％、

Si：0.100～3.000％、

Mn：0.50～3.00％、

P:0.100% or less,

S:0.0100% or less,

Al: less than 1.000 percent,

N:0.0100% or less,

Ti：0～0.20％、

Nb：0～0.100％、

Ca：0～0.0060％、

Mo：0～0.50％、

Cr：0～1.00％、

V：0～0.50％、

Cu：0～0.50％、

Ni:0 to 0.50 percent

Sn：0～0.050％，

The remainder comprising Fe and impurities,

The metallic structure in the region from 1/8 depth to 3/8 depth from the surface to the plate thickness contains 90 to 100% of martensite and 0 to 10% of the remaining structure in area%,

In the texture of the region from the surface to a depth of 1/8 of the plate thickness, the polar density of {001} <110>, {111} <110>, and {112} <110> orientation groups is 2.0 or more,

In the texture of the region from 1/8 depth to 1/2 depth of the plate thickness from the surface, the polar density of the {110} <112> orientation is 5.0 or less,

The tensile strength of the hot rolled steel sheet is 1180MPa or more.

2. The hot-rolled steel sheet according to claim 1, wherein the chemical composition contains 1 or 2 or more elements selected from the following elements in mass%:

Ti：0.02～0.20％、Nb：0.010～0.100％、Ca：0.0001～0.0060％、Mo：0.01～0.50％、Cr：0.01～1.00％、V：0.01～0.50％、Cu：0.01～0.50％、Ni：0.01～0.50％、 Sn:0.001 to 0.050 percent.