Hot-rolled steel sheet and method for producing same

Abstract

Provided is a hot-rolled steel sheet having high strength, excellent bendability, excellent residual bendability, and excellent fatigue resistance, which is suitable as a raw material for automobile components. This hot-rolled steel sheet has a predetermined component composition and has as steel structure containing, by area ratio, 80% or more of bainite and 1-20% of martensite; in the region extending 30 μm from the surface, the pole density D defined as the maximum value of the [100] pole density and the [111] pole density in a plane inclined at 45° in the rolling direction with respect to the sheet surface is 2.5 or less, and the maximum height roughness Rz is 8.0 μm or less.

Description

Hot-rolled steel sheet and method for manufacturing the same

[0001] This invention relates to hot-rolled steel sheets and a method for manufacturing the same.

[0002] From the perspective of improving collision safety and fuel efficiency in automobiles, high strength is required for steel plates used in automotive parts.

[0003] On the other hand, such steel sheets are generally pressed and processed into automotive parts, so excellent bendability is also required. However, increasing the strength of a steel sheet usually reduces its bendability, leading to significant cracking during pressing. In particular, bending cracks become significantly more likely at strength levels of 980 MPa or higher. Therefore, there is a need to achieve both high strength and excellent bendability.

[0004] Furthermore, from the perspective of enhancing collision safety performance, automotive parts are required to deform further and absorb impact when deformed by a collision. Therefore, steel sheets are required to have high residual bending workability that can withstand further deformation even after being processed into the part shape by press working.

[0005] Furthermore, hot-rolled steel sheets are primarily used as materials for automotive undercarriage components. In such applications, excellent fatigue resistance is required to withstand the repeated loads applied by the tires.

[0006] To meet these demands, various types of hot-rolled steel sheets have been developed.

[0007] For example, Patent Document 1 proposes a hot-rolled steel sheet having a predetermined component composition and having reduced anisotropy of Charpy absorption energy.

[0008] Furthermore, Patent Document 2 proposes a high-strength steel sheet having a predetermined component composition and steel structure, and exhibiting excellent residual ductility after hot working.

[0009] Patent Document 3 proposes a high-strength hot-rolled steel sheet in which the hardness distribution in the thickness direction is controlled by varying the Si content in the thickness direction.

[0010] Japanese Patent Publication No. 2010-156016, International Publication No. 2017 / 131052, International Publication No. 2020 / 203934

[0011] However, the conventional technologies proposed in the above-mentioned Patent Documents 1 to 3 were still insufficient.

[0012] For example, while strength and bendability are considered in Patent Document 1, fatigue resistance is not considered at all.

[0013] While Patent Document 2 considers the residual ductility after tensile processing of a steel sheet, it does not consider the residual bendability after bending. Furthermore, the technology described in Patent Document 2 involves controlling the microstructure by performing annealing or cold rolling after hot rolling, and therefore cannot be applied to hot-rolled steel sheets.

[0014] Although Patent Document 3 evaluates fatigue characteristics, it evaluates them after bending deformation has been applied to the steel plate, and does not consider the fatigue characteristics of the original plate (steel plate that has not been bent).

[0015] Thus, a hot-rolled steel sheet possessing high strength, excellent bendability, superior residual bend workability, and excellent fatigue resistance has yet to be realized.

[0016] The present invention aims to solve the above problems and to provide a hot-rolled steel sheet that is suitable as a material for automotive parts, possessing high strength, excellent bendability, excellent residual bend workability, and excellent fatigue resistance.

[0017] In this invention, "high strength" refers to a tensile strength (TS) of 980 MPa or higher. In this invention, "excellent bendability" refers to an R / t of 2.5 or less as measured in the bending test described later. In this invention, "excellent residual bendability" refers to a difference ΔR / t of 1.0 or more between the R / t at which a small crack occurs and the R / t at which a large crack occurs in the bending test described later. In this invention, "excellent fatigue resistance" refers to a fatigue strength of 400 MPa or higher as measured in the fatigue test described later.

[0018] The inventors conducted studies to solve the above problems. As a result, they focused on the crystal orientation of the plane where bending cracks occur along the C axis, that is, the plane tilted 45° in the rolling direction from the surface of the steel sheet, and conceived the idea of ​​improving bendability and residual bend workability by controlling this crystal orientation. Based on this idea, further studies revealed that the above problems can be solved by controlling the component composition and microstructure of the steel sheet, as well as the extreme density and maximum height roughness Rz, which will be described later.

[0019] This invention was completed based on the above findings, and its gist is as follows.

[0020] 1. A hot-rolled steel sheet having a composition in mass%, comprising C: 0.04-0.18%, Si: 0.1-2.0%, Mn: 0.5-3.5%, P: 0.100% or less, S: 0.020% or less, Al: 1.0% or less, and one or both of Ti: 0.005-0.20% and Nb: 0.005-0.20%, with the remainder being Fe and unavoidable impurities; a steel structure having, in area percentage, bainite: 80% or more and martensite: 1-20%, with an extreme density D defined as the maximum value of the {100} extreme density and {111} extreme density on a surface inclined at 45° in the rolling direction with respect to the sheet surface in the region from the surface up to 300 μm, being 2.5 or less, and a maximum height roughness Rz of 8.0 μm or less.

[0021] 2. Hardness HV at a depth of 50 μm from the surface. Ｓ５０ and hardness HV at a depth of 10 μm from the surface Ｓ１０ The difference (HV Ｓ５０ - HV Ｓ１０ The hot-rolled steel sheet described in item 1 above, wherein the ΔHV, defined as ), is 20 or more.

[0022] 3. The hot-rolled steel sheet according to 1 or 2 above, wherein the component composition further comprises at least one selected from the group consisting of, in mass%, Cr: 2.0% or less, Cu: 4.0% or less, Ni: 2.0% or less, Mo: 2.0% or less, V: 1.0% or less, B: 0.0050% or less, Ca: 0.0050% or less, REM: 0.0050% or less, Sb: 0.10% or less, and Sn: 0.50% or less.

[0023] 4. A method for manufacturing a hot-rolled steel sheet as described in any one of items 1 to 3 above, comprising: heating a steel slab having the above component composition; roughly rolling the heated steel slab to form a sheet bar; subjecting the sheet bar to a descaling treatment at a temperature of 1050 to 1150°C and a pressure of 30 MPa or higher; finishing rolling the descaled sheet bar at 900 to 950°C with a total reduction ratio of 30% or less and a final pass reduction ratio of 10% or higher to form a hot-rolled steel sheet; cooling the hot-rolled steel sheet at an average cooling rate of 50°C / s or higher up to 550°C; winding the cooled hot-rolled steel sheet at a winding temperature of 400 to 550°C; and subjecting the hot-rolled steel sheet to temper rolling at an elongation ratio of 0.05% or higher to form a hot-rolled steel sheet.

[0024] 5. The method for manufacturing a hot-rolled steel sheet as described in item 4 above, wherein the entry temperature of the final pass in the finish rolling is 950°C or lower.

[0025] According to the present invention, it is possible to provide a hot-rolled steel sheet that combines high strength, excellent bendability, excellent residual bend workability, and excellent fatigue resistance. The hot-rolled steel sheet of the present invention can be used very suitably as a material for automobile parts.

[0026] The embodiments of the present invention will be described in detail below. The following description illustrates preferred embodiments of the present invention, and the invention is not limited thereto.

[0027] <Hot-rolled steel sheet> The hot-rolled steel sheet in one embodiment of the present invention has a predetermined component composition, steel structure, extreme density, and maximum height roughness Rz. The reasons for each limitation are explained below.

[0028] [Component Composition] The hot-rolled steel sheet in one embodiment of the present invention has the component composition described below. In the following description, "%" as a unit of content refers to "mass%" unless otherwise specified.

[0029] C: 0.04-0.18% Carbon (C) is an element that increases tensile strength by generating and strengthening bainite and martensite. If the C content is less than 0.04%, the above effect is insufficient, and a tensile strength of 980 MPa or more cannot be obtained. Therefore, the C content should be 0.04% or more, preferably 0.05% or more. On the other hand, if the C content exceeds 0.18%, the hardening of martensite becomes significant, and the desired bendability and residual bendability cannot be obtained. Therefore, the C content should be 0.18% or less, preferably 0.16% or less, and more preferably 0.10% or less.

[0030] Si: 0.1-2.0% Si is an element that enhances tensile strength by solid solution strengthening of steel and suppressing tempering softening of martensite. Si is also an effective element for obtaining a structure in which martensite is dispersed in bainite by suppressing the formation of cementite. To obtain these effects, the Si content must be 0.1% or more. Therefore, the Si content should be 0.1% or more, preferably 0.2% or more. On the other hand, if the Si content exceeds 2.0%, excessive scale will remain, and the desired maximum height roughness Rz cannot be obtained. Therefore, the Si content should be 2.0% or less, preferably 1.5% or less, and more preferably 1.0% or less.

[0031] Mn: 0.5-3.5% Mn is an element that increases tensile strength by generating martensite and bainite. If the Mn content is less than 0.5%, the above effect is insufficient, and polygonal ferrite and the like are generated, making it impossible to obtain the desired steel structure. For this reason, the Mn content should be 0.5% or more, preferably 1.0% or more. On the other hand, if the Mn content exceeds 3.5%, the generation of bainite is suppressed, making it impossible to obtain the desired steel structure. For this reason, the Mn content should be 3.5% or less, preferably 3.0% or less, and more preferably 2.3% or less.

[0032] P: 0.100% or less. P is an element that has the effect of reducing bendability. If the P content exceeds 0.100%, the desired bendability cannot be obtained. Therefore, the P content should be 0.100% or less, preferably 0.030% or less. On the other hand, from the viewpoint of improving bendability, the lower the P content, the better, so there is no particular lower limit to the P content. The P content may be 0% or more, or it may be greater than 0%. However, excessive reduction will lead to a decrease in productivity, so it is preferable that the P content be 0.001% or more.

[0033] S: 0.020% or less. S is an element that has the effect of reducing bendability. If the S content exceeds 0.020%, the desired bendability cannot be obtained. Therefore, the S content should be 0.020% or less, preferably 0.0050% or less. On the other hand, from the viewpoint of improving bendability, the lower the S content, the better, so there is no particular lower limit to the S content. The S content may be 0% or more, or it may be greater than 0%. However, excessive reduction will lead to a decrease in productivity, so it is preferable that the S content be 0.0002% or more.

[0034] Al: 1.0% or less. Al is an element that acts as a deoxidizing agent and can be added in the deoxidation process. However, if the Al content exceeds 1.0%, a large amount of polygonal ferrite is formed, and the desired steel structure cannot be obtained. Therefore, the Al content should be 1.0% or less, preferably 0.50% or less. On the other hand, the lower limit of the Al content is not particularly limited and may be 0% or more, or even greater than 0%. From the viewpoint of enhancing the deoxidizing effect, it is preferable to have an Al content of 0.01% or more.

[0035] Ti: 0.005-0.20% Nb: 0.005-0.20% Ti and Nb are elements that strengthen steel. Furthermore, Ti and Nb are elements that refine the crystal grains by suppressing the recrystallization of austenite, thereby improving bendability and residual bendability. In order to obtain these effects, it is necessary to contain 0.005% or more of either Ti or Nb, or both. The content is preferably 0.010% or more, and more preferably 0.030% or more. On the other hand, if the content exceeds 0.20%, the suppression of austenite recrystallization becomes excessive, and elongated grains are generated. As a result, the desired extreme density and bendability cannot be obtained. Therefore, the content is 0.20% or less, preferably 0.160% or less, and more preferably 0.13% or less. Note that the Ti content and Nb content can be adjusted independently.

[0036] The component composition of the hot-rolled steel sheet in one embodiment of the present invention includes the above components, with the remainder being Fe and unavoidable impurities. When N is included as an unavoidable impurity, the N content is preferably 0.001 to 0.01%.

[0037] The component composition of the hot-rolled steel sheet in other embodiments of the present invention may further optionally contain at least one of the following components.

[0038] Cr: 2.0% or less. Cr is an element that promotes the formation of martensite and contributes to further improvement of strength. However, if the Cr content exceeds 2.0%, the formation of bainite may be suppressed, and the desired steel structure may not be obtained. Therefore, when adding Cr, the Cr content should be 2.0% or less, preferably 0.6% or less. On the other hand, the lower limit of the Cr content is not particularly limited and may be 0%. However, in order to enhance the effect of adding Cr, it is preferable to have a Cr content of 0.005% or more, and more preferably 0.10% or more.

[0039] Cu: 4.0% or less. Cu, like Cr, is an element that promotes the formation of martensite and contributes to further improvement in strength. However, when the Cu content exceeds 4.0%, the formation of bainite is suppressed, and a desired steel structure may not be obtained. Therefore, when adding Cu, the Cu content should be 4.0% or less, preferably 0.6% or less. On the other hand, the lower limit of the Cu content is not particularly limited and may be 0%. However, in order to enhance the addition effect of Cu, it is preferable that the Cu content is 0.05% or more, and more preferably 0.10% or more.

[0040] Ni: 2.0% or less. Ni, like Cr, is an element that promotes the formation of martensite and contributes to further improvement in strength. However, when the Ni content exceeds 2.0%, the formation of bainite is suppressed, and a desired steel structure may not be obtained. Therefore, when adding Ni, the Ni content should be 2.0% or less, preferably 0.6% or less. On the other hand, the lower limit of the Ni content is not particularly limited and may be 0%. However, in order to enhance the addition effect of Ni, it is preferable that the Ni content is 0.005% or more, and more preferably 0.1% or more.

[0041] Mo: 2.0% or less. Mo, like Cr, is an element that promotes the formation of martensite and contributes to further improvement in strength. However, when the Mo content exceeds 2.0%, the formation of bainite is suppressed, and a desired steel structure may not be obtained. Therefore, when adding Mo, the Mo content should be 2.0% or less, preferably 0.6% or less. On the other hand, the lower limit of the Mo content is not particularly limited and may be 0%. However, in order to enhance the addition effect of Mo, it is preferable that the Mo content is 0.005% or more, and more preferably 0.10% or more.

[0042] V: 1.0% or less. V, like Cr, is an element that promotes the formation of martensite and contributes to further improvement in strength. However, when the V content exceeds 1.0%, the formation of bainite is suppressed, and a desired steel structure may not be obtained. Therefore, when adding V, the V content should be 1.0% or less, preferably 0.3% or less. On the other hand, the lower limit of the V content is not particularly limited and may be 0%. However, in order to enhance the addition effect of V, it is preferable that the V content is 0.005% or more, and more preferably 0.05% or more.

[0043] B: 0.0050% or less. B is an element that promotes the formation of martensite and contributes to further improvement in strength. However, when the B content exceeds 0.0050%, the amount of B-based compounds increases, the hardenability decreases, and a desired steel structure may not be obtained. Therefore, when adding B, the B content should be 0.0050% or less, preferably 0.0040% or less. On the other hand, the lower limit of the B content is not particularly limited and may be 0%. However, in order to enhance the addition effect of B, it is preferable that the B content is 0.0002% or more, and more preferably 0.0005% or more.

[0044] Ca: 0.0050% or less. Ca is an element that has the effect of further improving the bendability by controlling the form of inclusions. However, when the Ca content exceeds 0.0050%, the amount of inclusions increases, and as a result, the bendability may deteriorate. Therefore, when adding Ca, the Ca content should be 0.0050% or less, preferably 0.0030% or less. On the other hand, the lower limit of the Ca content is not particularly limited and may be 0%. However, in order to enhance the addition effect of Ca, it is preferable that the Ca content is 0.0001% or more, and more preferably 0.0005% or more.

[0045] REM: 0.0050% or less. REM (rare earth metals), like Ca, is a component that further improves flexibility by controlling the morphology of inclusions. However, if the REM content exceeds 0.0050%, the amount of inclusions increases, which may result in a deterioration of flexibility. Therefore, when adding REM, the REM content should be 0.0050% or less, preferably 0.0030% or less. On the other hand, the lower limit of the REM content is not particularly limited and may be 0%. However, in order to enhance the effect of adding REM, it is preferable to have a REM content of 0.0001% or more, and more preferably 0.0005% or more.

[0046] Sb: 0.10% or less. Sb is an element that has the effect of suppressing the decrease in strength caused by denitrification and deboration. However, if the Sb content exceeds 0.10%, it may lead to embrittlement of the steel sheet. For this reason, when adding Sb, the Sb content should be 0.10% or less, preferably 0.050% or less. On the other hand, the lower limit of the Sb content is not particularly limited and may be 0%. However, in order to enhance the effect of adding Sb, it is preferable to have an Sb content of 0.0010% or more, and more preferable to have an Sb content of 0.0050% or more.

[0047] Sn: 0.50% or less. Sn is an element that suppresses the decrease in steel strength caused by the formation of pearlite. However, if the Sn content exceeds 0.50%, it may lead to embrittlement of the steel sheet. Therefore, when adding Sn, the Sn content should be 0.50% or less, preferably 0.050% or less. On the other hand, the lower limit of the Sn content is not particularly limited and may be 0%. However, in order to enhance the effect of adding Sn, it is preferable to have a Sn content of 0.0010% or more, and more preferable to have a Sn content of 0.0050% or more.

[0048] [Structural Structure] Next, the structural structure of the hot-rolled steel sheet of the present invention will be described. The structural structure of the hot-rolled steel sheet of the present invention contains, by area ratio, bainite: 80% or more and martensite: 1-20%. The reason for this will be explained below. In this invention, the structural structure at the 1 / 4 position of the sheet thickness will be used as the structural structure. The area ratio of each structural structure can be measured by the method described in the examples. That is, a secondary electron image is obtained by observing the 1 / 4 position of the hot-rolled steel sheet thickness using a scanning electron microscope (SEM), and the area ratio is calculated by image analysis of the secondary electron image. However, when determining the area ratio of retained austenite, X-ray diffraction is used.

[0049] Bainite: 80% or more In this invention, the area ratio of bainite is set to 80% or more in order to achieve high strength, excellent bendability, and excellent fatigue resistance. If the area ratio of bainite is less than 80%, at least one of the strength, bendability, and fatigue resistance will be insufficient. The area ratio of bainite is preferably 83% or more, and more preferably 88% or more. On the other hand, there is no particular upper limit to the area ratio of bainite, but since the area ratio of martensite is 1% or more, the area ratio of bainite will inevitably be 99% or less.

[0050] In this invention, tempered bainite is also included in the definition of bainite. Furthermore, the area ratio of bainite can be determined as the sum of the area ratio of upper bainite and the area ratio of lower bainite.

[0051] Martensite: 1-20% Martensite is an effective structure for increasing strength. It is also an effective structure for improving bendability by suppressing strain concentration during bending. If the area ratio of martensite is less than 1%, the desired strength and bendability cannot be obtained. Therefore, the area ratio of martensite should be 1% or more, preferably 2% or more, and more preferably 3% or more. On the other hand, if the area ratio of martensite exceeds 20%, the formation of voids caused by martensite becomes significant, and the bendability actually decreases. Therefore, the area ratio of martensite should be 20% or less, preferably 15% or less, and more preferably 12% or less.

[0052] In this invention, all types of martensite, including fresh martensite, auto-tempered martensite, and tempered martensite, are included under the umbrella term "martensite."

[0053] The steel structure may include any other structure as the remainder of the composition other than bainite and martensite. The other structure is not particularly limited, but is typically at least one of ferrite, pearlite, and retained austenite. Desired properties can be obtained if the total area ratio of the other structure is 19% or less. On the other hand, the lower limit of the total area ratio of the other structure is not particularly limited and may be 0%. In other words, the hot-rolled steel sheet in one embodiment of the present invention may have a steel structure consisting of bainite: 80% or more and martensite: 1-20%.

[0054] - Extreme density D: 2.5 or less By setting the extreme density D to 2.5 or less, cracks are less likely to occur on the surface inclined at 45° in the rolling direction relative to the plate surface during bending, and residual bendability is significantly improved. The reason for this is not clear, but it is thought that by reducing the prevalence of the above crystal orientation, dislocation movement on the surface inclined at 45° in the rolling direction relative to the plate surface of the steel sheet is promoted or suppressed, thereby suppressing deformation concentration and fracture on that surface. For this reason, the extreme density D is set to 2.5 or less, preferably 2.2 or less, and more preferably 1.9 or less. On the other hand, the lower limit of the extreme density D is not particularly limited, but typically it may be 1.3 or more, 1.5 or more, or 1.6 or more.

[0055] The extreme density D is defined as the maximum values ​​of the {100} extreme density and the {111} extreme density in a region from the surface of the hot-rolled steel sheet up to 300 μm, on a plane inclined at 45° in the rolling direction with respect to the sheet surface. In other words, the extreme density D is the larger of the {100} extreme density and the {111} extreme density. The extreme density D can be determined from crystal orientation data measured by EBSD (electron backscatter diffraction). More specifically, it can be determined by the method described in the examples.

[0056] - Maximum height roughness Rz: 8.0 μm or less The maximum height roughness Rz in the surface roughness curve of a hot-rolled steel sheet strongly affects the bendability and fatigue strength of the steel sheet. If the maximum height roughness Rz is greater than 8.0 μm, the desired bendability and fatigue resistance cannot be obtained. For this reason, the maximum height roughness Rz is set to 8.0 μm or less, preferably 7.0 μm or less, and more preferably 6.0 μm or less. On the other hand, there is no particular lower limit to the maximum height roughness Rz. However, if Rz is less than 1.0 μm, the manufacturing load becomes significantly higher. For this reason, it is preferable that the maximum height roughness Rz be 1.0 μm or more.

[0057] The maximum height roughness Rz refers to the maximum height (maximum height roughness) in the roughness curve of the steel plate surface. The maximum height roughness Rz can be measured by a roughness test in accordance with the provisions of JIS B 0601:2013.

[0058] • ΔHV Furthermore, in this invention, the hardness HV at a depth of 50 μm from the surface is defined. Ｓ５０ and hardness HV at a depth of 10 μm from the surface Ｓ１０ The difference (HV Ｓ５０ - HV Ｓ１０ It is preferable that ΔHV, defined as ΔHV = 20 or more, bendability can be further improved because stress concentration near the surface during bending deformation is reduced. On the other hand, there is no particular upper limit to ΔHV. However, typically, ΔHV may be 40 or less.

[0059] • Plate Thickness The plate thickness of the hot-rolled steel sheet of the present invention is not particularly limited and may be any thickness. However, from the viewpoint of use as a material for automobile parts and the like, a plate thickness of 0.6 mm or more is preferable, and 1.0 mm or more is preferable. From the same viewpoint, a plate thickness of 10.0 mm or less is preferable, and 6.0 mm or less is preferable.

[0060] • Plate width The plate width of the hot-rolled steel sheet of the present invention is not particularly limited and may be any width. However, from the viewpoint of use as a material for automobile parts and the like, a plate width of 500 mm or more is preferable, and 700 mm or more is preferable. From the same viewpoint, a plate width of 1800 mm or less is preferable, and 1400 mm or less is preferable.

[0061] Furthermore, the hot-rolled steel sheet of the present invention may be a steel sheet in the as-hot-rolled state, or it may be a steel sheet that has been pickled. In other words, the hot-rolled steel sheet may be a black-scale material with mill scale remaining on the surface, or it may be a white-scale material from which the mill scale has been removed by pickling.

[0062] [Manufacturing Method] Next, a method for manufacturing a hot-rolled steel sheet according to one embodiment of the present invention will be described. The hot-rolled steel sheet can be manufactured by subjecting a steel slab having the above-described component composition to hot rolling and temper rolling under predetermined conditions. The specific manufacturing procedure and conditions will be described below. In this invention, unless otherwise specified, the surface temperature at the center of the width of the slab or steel sheet is used. The surface temperature can be measured with a radiation thermometer or the like.

[0063] First, a steel slab having the aforementioned component composition is heated, and the heated steel slab is roughly rolled to form a sheet bar. The conditions for heating and rough rolling are not particularly limited and can be carried out according to conventional methods.

[0064] For example, the temperature during heating (slab heating temperature) is not particularly limited. However, from the viewpoint of removing segregation and solid-solving precipitates, it is preferable to set the slab heating temperature to 1100°C or higher. On the other hand, from the viewpoint of energy efficiency, it is preferable to set the slab heating temperature to 1300°C or lower.

[0065] Next, the sheet bar is subjected to a descaling treatment under conditions of a temperature of 1050 to 1150°C and a pressure of 30 MPa or higher. This descaling treatment can be carried out by so-called high-pressure water descaling, which involves spraying high-pressure water onto the surface of the sheet bar.

[0066] Descaling temperature: 1050 to 1150°C If the descaling temperature after rough rolling is less than 1050°C, scale is likely to remain, and the desired maximum height roughness Rz cannot be obtained. Therefore, the descaling temperature should be 1050°C or higher, preferably 1070°C or higher. On the other hand, if the descaling temperature exceeds 1150°C, scale will be generated again after the descaling process, and the desired maximum height roughness Rz cannot be obtained. Therefore, the descaling temperature should be 1150°C or lower, preferably 1140°C or lower. Here, the temperature (descaling temperature) is defined as the surface temperature of the sheet bar at the start of descaling.

[0067] Descaling pressure: 30 MPa or higher. If the descaling pressure is less than 30 MPa, the scale cannot be sufficiently removed, and the desired maximum height roughness Rz cannot be obtained. Therefore, the descaling pressure should be 30 MPa or higher, preferably 50 MPa or higher, and more preferably 60 MPa or higher. On the other hand, there is no particular upper limit to the descaling pressure. However, if the descaling pressure exceeds 100 MPa, it may damage the shape of the steel plate. Therefore, it is preferable that the descaling pressure be 100 MPa or lower. Here, the pressure (descaling pressure) is defined as the pressure (discharge pressure) when discharging the descaling water.

[0068] Next, the descaled sheet bar is finished-rolled at 900-950°C with a total reduction ratio of 30% or less and a final pass reduction ratio of 10% or more to produce a hot-rolled steel sheet.

[0069] Total reduction ratio at 900-950°C: 30% or less In the finish rolling described above, rolling in the temperature range of 900-950°C tends to generate large shear strains on the surface of the steel sheet, which causes a large change in the crystal orientation in the final structure. If the total reduction ratio at 900-950°C exceeds 30%, the desired extreme density cannot be obtained. For this reason, the total reduction ratio should be 30% or less, preferably 25% or less, and more preferably 20% or less. On the other hand, there is no particular lower limit to the total reduction ratio. However, from the viewpoint of stabilizing the shape of the steel sheet, it is preferable to set the total reduction ratio to 5% or more.

[0070] Reduction ratio of the final pass: 10% or more In the finish rolling process, the surface can be smoothed by setting the reduction ratio of the final pass to 10% or more. If the reduction ratio of the final pass is less than 10%, the desired maximum height roughness Rz cannot be obtained. Therefore, the reduction ratio of the final pass is set to 10% or more, preferably 13% or more, and more preferably 15% or more. On the other hand, there is no particular upper limit to the reduction ratio of the final pass. However, if the reduction ratio of the final pass exceeds 30%, problems such as deterioration of the steel sheet shape may occur. Therefore, it is preferable that the reduction ratio of the final pass is 30% or less.

[0071] - Entry temperature of the final pass The entry temperature of the final pass in the finish rolling process is not particularly limited. However, if the entry temperature is 950°C or lower, the transformation of the outermost layer is promoted, and ΔHV can be set to 20 or higher. For this reason, it is preferable that the entry temperature of the final pass be 950°C or lower, and more preferably 940°C or lower. On the other hand, there is no particular lower limit to the entry temperature of the final pass. However, if the entry temperature is below 850°C, elongation of grains may occur, which may impair bendability. For this reason, from the viewpoint of further improving bendability, it is preferable that the entry temperature of the final pass be 800°C or higher.

[0072] Furthermore, the number of passes in the finish rolling process is not particularly limited. However, from the viewpoint of reducing coarse grains that lead to a decrease in workability, it is preferable to perform the process in four or more passes. The upper limit of the number of passes is also not particularly limited, but typically it may be eight passes or less, or six passes or less.

[0073] Next, the obtained hot-rolled steel sheet is cooled to 550°C at an average cooling rate of 50°C / s or more.

[0074] Average cooling rate up to 550°C: 50°C / s or more If the average cooling rate up to 550°C is less than 50°C / s during the above cooling process, ferrite and pearlite will form, and the desired steel structure will not be obtained. In addition, scale growth will occur, resulting in the inability to obtain the desired maximum height roughness Rz. For this reason, the average cooling rate should be 50°C / s or more, preferably 80°C / s or more. On the other hand, from the viewpoint of controlling the steel structure and Rz, the faster the cooling rate, the better. For this reason, there is no particular upper limit to the average cooling rate. However, from the viewpoint of the shape stability of the steel sheet, it is preferable to set the average cooling rate to 1000°C / s or less, more preferably to 200°C / s or less, and even more preferably to 150°C / s or less.

[0075] Next, the cooled hot-rolled steel sheet is wound up at a winding temperature of 400 to 550°C.

[0076] Winding temperature: 400 to 550°C If the winding temperature is less than 400°C, the amount of martensite increases, and the desired steel structure cannot be obtained. Therefore, the winding temperature should be 400°C or higher. On the other hand, if the winding temperature exceeds 550°C, ferrite and pearlite are formed, and the desired steel structure cannot be obtained. In addition, peeling of the surface layer due to grain boundary oxidation occurs, and as a result, the desired maximum height roughness Rz cannot be obtained. Therefore, the winding temperature should be 550°C or lower, preferably 520°C or lower.

[0077] Subsequently, the hot-rolled steel sheet is subjected to temper rolling under conditions of elongation of 0.05% or more to obtain a hot-rolled steel sheet.

[0078] Elongation rate: 0.05% or more In the temper rolling process, the surface of the steel sheet, which has been prepared in the hot rolling process, is further smoothed. However, if the elongation rate in the temper rolling process is less than 0.05%, the desired maximum height roughness Rz cannot be obtained. For this reason, the elongation rate is set to 0.05% or more, preferably 0.10% or more. On the other hand, there is no particular upper limit to the elongation rate. However, if the elongation rate exceeds 5.0%, the surface roughness becomes excessively small, which may lead to problems such as slipping during handling. For this reason, it is preferable that the elongation rate is 5.0% or less.

[0079] The present invention will be described in more detail below based on examples. However, the present invention is not limited to the following examples.

[0080] First, steel slabs were produced by melting steel having the component composition shown in Table 1 in a vacuum melting furnace. Note that blank spaces in Table 1 indicate intentional omissions, including not only cases where the material is not present (0%) but also cases where it is inevitably present.

[0081] The obtained steel slab was heated to 1250°C, and the heated steel slab was roughly rolled to form a sheet bar. The sheet bar was subjected to descaling treatment with high-pressure water under the conditions shown in Table 2, and then further finished rolling under the conditions shown in Table 2 to form a hot-rolled steel sheet. The total number of passes for the finish rolling was 6.

[0082] The hot-rolled steel sheet was cooled, coiled, and then subjected to temper rolling. The conditions for cooling, coiling, and temper rolling are as shown in Table 2.

[0083] Next, the microstructure, extreme density, maximum height roughness Rz, and ΔHV of the obtained hot-rolled steel sheet were evaluated using the following procedure.

[0084] (Steel structure) The area ratios of bainite and martensite in the steel structure of the hot-rolled steel sheet described above were determined using the following procedure.

[0085] First, a sample was cut from the hot-rolled steel sheet, and the cross-section parallel to the rolling direction was polished. Next, the cross-section was etched with 3% nital. Using a scanning electron microscope (SEM), secondary electron images were obtained by taking three field-of-view images per sample at the 1 / 4 thickness position of the cross-section. The observation magnification was 1500x.

[0086] The area ratio of each tissue was determined by image analysis of the obtained secondary electron images. Image-Pro from Media Cybernetics was used for the image analysis. The average of the area ratios across three fields of view was used as the area ratio for each tissue. The identification of each tissue in the secondary electron images was performed using the following method.

[0087] The area ratio of bainite was determined as the sum of the area ratios of upper bainite and lower bainite. Upper bainite is distinguished as black or dark gray containing carbides or martensite with linear interfaces. Lower bainite is distinguished as black, dark gray, gray, or light gray containing oriented carbides.

[0088] Martensite is distinguished as black, dark gray, gray, or light gray containing carbides in multiple orientations, or as white or light gray without carbides. Retained austenite is distinguished as white or light gray without carbides.

[0089] Since martensite and retained austenite may not be distinguishable, the area ratio of martensite was obtained by dividing the area ratio of retained austenite, determined by the method described later, by the total area ratio of martensite and retained austenite obtained from the secondary electron image above.

[0090] The stronger the tempering, the more the base material appears as a high-contrast black image. Therefore, the above base material color is merely a guideline. In this invention, the amount of carbides, the microstructure morphology, etc., are considered comprehensively, and the material is classified into one of the similarly characterized microstructures, including those described later. The carbides are white, dot-like or linear.

[0091] Furthermore, ferrite can be distinguished as having a black structure, a dark gray structure with little or no carbides internally, or a dark gray structure without linear interfaces with martensite. Pearlite can be distinguished as having a black and white layered or partially discontinuous layered structure. However, the steel structure of hot-rolled steel sheets produced by the method of the present invention contains very little ferrite and pearlite.

[0092] The area ratio of retained austenite was determined by X-ray diffraction. Specifically, the hot-rolled steel sheet was first ground to a position 0.1 mm inward from 1 / 4 of its thickness, and then further polished by 0.1 mm by chemical polishing. The integrated reflectance intensities of the (200), (220), and (311) planes of fcc iron (austenite) and the (200), (211), and (220) planes of bcc iron (ferrite) were measured by X-ray diffraction on the polished surface. Mo's Kα1 rays were used for the measurements.

[0093] The volume fraction was determined from the ratio of the integral reflectance intensity from each surface of fcc iron to the integral reflectance intensity from each surface of bcc iron, and this was taken as the area fraction of retained austenite.

[0094] The measurement results are shown in Table 3. In Table 3, the total area percentage of tissues other than bainite and martensite is shown in the "Other" column.

[0095] (Extreme Density D) The extreme density D, defined as the maximum values ​​of the {100} extreme density and {111} extreme density on a surface inclined at 45° in the rolling direction with respect to the plate surface, in the region from the surface of the hot-rolled steel sheet up to 300 μm, was measured by EBSD (electron backscatter diffraction). The specific procedure was as follows.

[0096] First, samples were cut from the obtained hot-rolled steel sheets. The thickness cross-section of the sample parallel to the rolling direction was polished, and then strain was removed by electropolishing. Subsequently, crystal orientation data was acquired by EBSD for a region of 300 μm in the thickness direction × 300 μm in the rolling direction, centered at 150 μm from the surface. The measurement conditions were an acceleration voltage of 30 kV and a step size of 200 nm. Three fields of view measurements were taken for each sample.

[0097] Next, the obtained crystal orientation data was analyzed to determine the {100} pole density and {111} pole density. OIM Analysis Ver. 7.3.0 from TSL Solutions was used for the analysis.

[0098] Specifically, among the crystal orientation data, data with a Confidence Index (CI) value of 0.1 or less was cut. Thereafter, coordinate transformation was performed so that the ND plane becomes a plane inclined 45° from the rolling direction to the plate thickness direction with respect to the plate surface.

[0099] φ1 = 0 to 90°, φ2 = 45°, Φ = 0 to 5° and 1 = 0 to 90°, φ2 = 45°, Φ = 50 to 60° were set, and the calculation of the Orientation Distribution Function (ODF) with a Resolution of 5° for each was performed, and the average value of that region was taken as the {100} pole density and the {111} pole density for each visual field.

[0100] For each sample, the same measurement was performed in three visual fields, and the average value of the obtained pole densities was taken as the pole density of each sample.

[0101] The maximum value of the {100} pole density and the {111} pole density obtained by the above procedure, that is, the larger value, was taken as the pole density D. The obtained values of the pole density D are shown in Table 3.

[0102] (Maximum height roughness Rz) The maximum height roughness Rz of the above hot-rolled steel sheet was measured by a roughness test in accordance with the provisions of JIS B 0601:2013. Specifically, first, a 30 mm × 100 mm test piece with a longitudinal direction parallel to the rolling direction was taken from the above hot-rolled steel sheet. A roughness test in accordance with the provisions of JIS B 0601:2013 was performed, and the maximum height roughness Rz in the roughness curve of the surface of the test piece was determined.

[0103] Furthermore, the strength, bendability, residual bend formability, and fatigue resistance characteristics of the obtained hot-rolled steel sheet were evaluated by the following procedure. The evaluation results are shown in Table 3.

[0104] (Strength) A tensile test was performed to evaluate the strength of the above hot-rolled steel sheet. Specifically, first, a JIS No. 5 tensile test piece (JIS Z 2241 2022) was taken from the above hot-rolled steel sheet in a direction parallel to the rolling direction. Next, using the above tensile test piece, a tensile test in accordance with the provisions of JIS Z 2241 was performed to obtain the tensile strength TS. The strain rate in the above tensile test was 10 －３The value was set to / s. In this invention, a tensile strength of 980 MPa or higher was considered acceptable.

[0105] (Bendability and Residual Bendability) The bendability and residual bendability of the hot-rolled steel sheet were evaluated by performing bending tests. Specifically, first, a bending test piece measuring 35 mm in width and 100 mm in length was taken from the hot-rolled steel sheet so that the longitudinal direction was the rolling direction. Next, a 90° V bending test was performed using the bending test piece so that the bending ridge was perpendicular to the rolling direction, and the bending radius R at which minute cracks of 0.5 mm or more in length occurred and the bending radius R' at which large cracks of 30 mm or more in length occurred were determined.

[0106] The R / t ratio was calculated from the obtained R and plate thickness t. This R / t value is an indicator of the bendability of the steel plate when it is first press-formed; a smaller value indicates better bendability. Here, an R / t of 2.5 or less was considered to indicate good bendability.

[0107] Furthermore, R' / t was calculated from the obtained R' and plate thickness t. Then, ΔR / t, which is the difference between R / t and R' / t, was calculated using the following formula: ΔR / t = R / t - R / t'

[0108] The R' / t value represents the bendability of a press-formed part when further deformation occurs due to impact or other factors. Therefore, a larger ΔR / t (the difference between R / t and R / t') indicates a greater margin for bending deformation, meaning superior residual bend workability. Here, a ΔR / t of 1.0 or greater was considered to indicate superior residual bend workability.

[0109] (Fatigue Resistance) The fatigue resistance of the hot-rolled steel sheet was evaluated by performing a planar bending fatigue test. Specifically, a planar bending fatigue test specimen with a width of 30 mm, a length of 90 mm, and a radius of R40 mm was first taken from the hot-rolled steel sheet so that the longitudinal direction was the rolling direction. Using the planar bending fatigue test specimen, a planar bending fatigue test was performed under the conditions of stress ratio: -1 and frequency: 25 Hz, and the fatigue strength σw after 2 million cycles was measured. Here, it was determined that the fatigue resistance was excellent if the fatigue strength σw after 2 million cycles was 400 MPa or higher.

[0110]

[0111]

[0112]

[0113] As can be seen from the results shown in Table 3, the hot-rolled steel sheet that satisfies the conditions of the present invention possessed high strength, excellent bendability, excellent residual bend workability, and excellent fatigue resistance. Therefore, the hot-rolled steel sheet of the present invention can be used very suitably as a material for automobile parts and can greatly contribute to improving the collision safety and fuel efficiency of automobiles. In contrast, the hot-rolled steel sheet that did not satisfy the conditions of the present invention was inferior in at least one property.

Claims

1. A hot-rolled steel sheet having a composition in mass%, comprising C: 0.04-0.18%, Si: 0.1-2.0%, Mn: 0.5-3.5%, P: 0.100% or less, S: 0.020% or less, Al: 1.0% or less, and one or both of Ti: 0.005-0.20% and Nb: 0.005-0.20%, with the remainder being Fe and unavoidable impurities; a steel structure having, in area percentage, bainite: 80% or more and martensite: 1-20%, with an extreme density D defined as the maximum value of the {100} extreme density and {111} extreme density on a surface inclined at 45° in the rolling direction with respect to the sheet surface in the region from the surface up to 300 μm, being 2.5 or less, and a maximum height roughness Rz of 8.0 μm or less.

2. Hardness HV at a depth of 50 μm from the surface. Ｓ５０ and hardness HV at a depth of 10 μm from the surface Ｓ１０ The difference (HV Ｓ５０ - HV Ｓ１０ The hot-rolled steel sheet according to claim 1, wherein the ΔHV, defined as ), is 20 or more.

3. The hot-rolled steel sheet according to claim 1 or 2, wherein the component composition further comprises at least one selected from the group consisting of, in mass%, Cr: 2.0% or less, Cu: 4.0% or less, Ni: 2.0% or less, Mo: 2.0% or less, V: 1.0% or less, B: 0.0050% or less, Ca: 0.0050% or less, REM: 0.0050% or less, Sb: 0.10% or less, and Sn: 0.50% or less.

4. A method for manufacturing a hot-rolled steel sheet according to any one of claims 1 to 3, comprising: heating a steel slab having the above-mentioned component composition; roughly rolling the heated steel slab to form a sheet bar; subjecting the sheet bar to a descaling treatment at a temperature of 1050 to 1150°C and a pressure of 30 MPa or higher; finishing rolling the descaled sheet bar at 900 to 950°C with a total reduction ratio of 30% or less and a final pass reduction ratio of 10% or higher to form a hot-rolled steel sheet; cooling the hot-rolled steel sheet at an average cooling rate of 50°C / s or higher up to 550°C; winding the cooled hot-rolled steel sheet at a winding temperature of 400 to 550°C; and subjecting the hot-rolled steel sheet to temper rolling at an elongation ratio of 0.05% or higher to form a hot-rolled steel sheet.

5. The method for manufacturing a hot-rolled steel sheet according to claim 4, wherein the entry temperature of the final pass in the finish rolling is 950°C or lower.

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