Hot-rolled steel sheet and method for manufacturing the same

Abstract

The present invention provides a hot-rolled steel sheet with a tensile strength of 400 MPa or less and excellent ductility and work hardening properties. [Solution] A hot-rolled steel sheet having a composition in mass% of C: 0.005% or more and less than 0.050%, Si: 0.10% or less, Mn: 0.05% or more and less than 0.50%, P: 0.05% or less, S: 0.008% or less, Al: 0.01% or more and less than 0.08%, and N: 0.0080% or less, wherein the content of Mn, Cu, Ni, Cr, and Mo satisfies a predetermined relational expression, and the remainder consists of Fe and unavoidable impurities, and having ferrite with an area ratio of ferrite grains with a particle size of less than 15 μm: 20% or less, an area ratio of ferrite grains with a particle size of 15 μm or more and less than 35 μm: 50% or more and less than 100%, and an area ratio of ferrite grains with a particle size exceeding 35 μm: 30% or less, and having a tensile strength of 400 MPa or less.

Description

Technical Field

[0001] The present invention relates to a hot-rolled steel sheet and a method for manufacturing the same, and particularly to a hot-rolled steel sheet for which workability is required and a method for manufacturing the same. Since the hot-rolled steel sheet of the present invention is soft and excellent in ductility and work hardening property, it is easy to form and is suitable as a material for automotive members, for example.

Background Art

[0002] Among the hot-rolled steel sheets used for automobiles, there are those that are subjected to bulging forming or deep drawing forming and are used as parts. For hot-rolled steel sheets used for such applications, excellent workability is required. What tends to be a problem in bulging forming and deep drawing forming is insufficient ductility or cracking due to stress concentration when a high amount of strain is applied to the steel sheet. Since stress concentration in the high strain region is caused by necking that occurs when the amount of work hardening is small, a hot-rolled steel sheet having both ductility and work hardening property is required.

[0003] For example, in Patent Document 1, it is stated that a hot-rolled steel sheet having good chemical conversion treatment property can be obtained by setting the ratio of measurement points on the steel sheet surface having a Ni content of 0.5 mass% or more to 10 to 70%. In Patent Document 2, it is stated that a hot-rolled steel sheet having good elongation flange property can be obtained by controlling the amount of oxygen in the steel to a certain level or less, making the main phase ferrite, and setting the maximum orientation density of crystal grains to 2.1 or less. In Patent Document 3, it is stated that a hot-rolled steel sheet excellent in elongation and local deformation ability and having little formability orientation dependence can be obtained by controlling the microstructure, crystal grain size, and morphology.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Patent Document 2

Patent Document 3

[0005] However, the prior art disclosed in the above-mentioned patent document has the following problems.

[0006] The technology proposed in Patent Document 1 does not take into consideration the processability of soft steel sheets with a tensile strength of 400 MPa or less. In the technology proposed in Patent Document 2, the growth of ferrite grains is not properly controlled, resulting in a large amount of coarse ferrite, and the desired ductility and work hardening properties cannot be obtained. The technology proposed in Patent Document 3 does not take into account the cooling pattern after hot rolling, and therefore the desired microstructure and properties cannot be obtained.

[0007] The present invention has been made in view of the above circumstances, and aims to provide a hot-rolled steel sheet having a tensile strength of 400 MPa or less and excellent ductility and work hardening properties, and a method for manufacturing the same. [Means for solving the problem]

[0008] To solve the above problems, the inventors diligently investigated the requirements for improving the processability of hot-rolled steel sheets with a tensile strength of 400 MPa or less.

[0009] The thickness of the hot-rolled steel sheet targeted by this invention is between 1.0 mm and 12.0 mm. To meet the demands of rigorous stretching and deep drawing processes, ductility is necessary, and for this, a high uniform elongation value is required to prevent necking at high strain levels.

[0010] On the other hand, we found that even steel sheets that exhibit high uniform elongation may have poor work hardening properties, resulting in poor strain distribution and a tendency for necking due to stress concentration. To avoid this necking, it is effective to set the n-value to 0.19 or higher in the region where the strain is between 15% and 20% of the nominal strain. Therefore, our goal was to achieve both uniform elongation of 20% or more of the nominal strain and high work hardening properties (n-value: 0.19 or higher) in the region where the strain is between 15% and 20% of the nominal strain.

[0011] Analysis of properties and microstructure revealed a tendency for optimal ferrite grains to achieve both uniform elongation and work hardening properties, with ferrite grains in the particle size range of 15 μm to 35 μm exhibiting the best properties. Ferrite grains smaller than 15 μm have a limited capacity for dislocation accumulation; therefore, microstructures with a high area ratio of ferrite grains smaller than 15 μm exhibit reduced uniform elongation and work hardening properties. When ferrite grains larger than 35 μm are formed, the microstructure becomes a mixture of fine and coarse ferrite. Strain concentrates in the coarse ferrite grains larger than 35 μm, causing microvoids and reducing uniform elongation. Furthermore, the localization of strain prevents good work hardening properties from being obtained in the high-strain region.

[0012] Next, in order to determine the manufacturing conditions for obtaining the desired ferrite grains, we investigated conditions for suppressing fine ferrite grains with a particle size of less than 15 μm and coarse ferrite grains with a particle size exceeding 35 μm, which should be reduced as much as possible. As a result, we found that it is effective to control the austenite-to-ferrite transformation driving force (hereinafter also referred to as the "austenite-to-ferrite transformation driving force") and to promote the ferrite transformation in a temperature range where ferrite grain nuclei are uniformly generated and the migration speed of the ferrite interface is fast.

[0013] This invention was completed based on the above findings, and its gist is as follows. [1] In mass%, C: 0.005% or more and less than 0.050% Si: 0.10% or less, Mn: 0.05% or more and less than 0.50% P: 0.05% or less, S: 0.008% or less, Al: 0.01% to 0.08%, and N: 0.0080% or less It contains Mn, and the content of Cu, Ni, Cr, and Mo satisfies the following formula (1), with the remainder being Fe and unavoidable impurities, Area percentage of ferrite particles with a particle size of less than 15 μm: 20% or less. Area ratio of ferrite grains with a particle size of 15 μm or more and 35 μm or less: 50% or more and 100% or less. The ferrite contains ferrite with a particle size exceeding 35 μm and an area ratio of 30% or less. Hot-rolled steel sheet with a tensile strength of 400 MPa or less. 0.15≦[%Mn]+0.21[%Cu]+0.46[%Ni]+0.78[%Cr]+1.8[%Mo]≦0.65 (1) Here, in equation (1), [%M] represents the content (mass%) of element M, and is set to 0 if it is not present. [2] The hot-rolled steel sheet according to [1], wherein the component composition further contains, by mass%, one or more selected from the following groups A to D. Group A: One or more elements selected from Cu: 0.40% or less, Ni: 0.40% or less, Cr: 0.40% or less, and Mo: 0.1% or less. Group B: One or more elements selected from V: 0.05% or less, Nb: 0.05% or less, Ti: 0.05% or less, Zr: 0.05% or less, Hf: 0.05% or less, and W: 0.05% or less. Group C: One or more elements selected from Mg: 0.01% or less, REM: 0.01% or less, Co: 0.2% or less, Ca: 0.01% or less. Group D: One or more selected from Sb: less than 0.01%, Sn: less than 0.05%, As: less than 0.01%, Ta: less than 0.01%, Pb: less than 0.01%, Cs: less than 0.01%, Te: less than 0.01%, Bi: less than 0.01%, Zn: less than 0.01%, Ge: less than 0.01%, Sr: less than 0.01%, Se: less than 0.0020%, B: less than 0.0020%, O: less than 0.0030%, H: less than 0.0010% [3] The hot-rolled steel sheet according to [1] or [2], having a plating layer on the surface. [4] A method for manufacturing the hot-rolled steel sheet according to [1] or [2], comprising: A heating step of heating a steel material having the above composition to 1050 °C or higher; A hot-rolling step of performing finish rolling on the steel material after the heating step at a finish rolling end temperature of 840 °C or higher and 930 °C or lower to obtain a hot-rolled steel sheet; A primary cooling step of cooling the hot-rolled steel sheet after the hot-rolling step to a cooling stop temperature of 670 °C or higher and 800 °C or lower at an average cooling rate of 30 °C / s or higher within 2.5 seconds after the finish rolling; A secondary cooling step of cooling the hot-rolled steel sheet after the primary cooling step at an average cooling rate of 1 °C / s or higher and 10 °C / s or lower for 2 seconds or longer and 15 seconds or shorter; A tertiary cooling step of cooling the hot-rolled steel sheet after the secondary cooling step to a cooling stop temperature of 550 °C or higher and 640 °C or lower at an average cooling rate of 30 °C / s or higher; A winding step of winding the hot-rolled steel sheet after the tertiary cooling step. A method for manufacturing a hot-rolled steel sheet. [5] A method for manufacturing the hot-rolled steel sheet according to [3], comprising: A heating step of heating a steel material having the above composition to 1050 °C or higher; A hot-rolling step of performing finish rolling on the steel material after the heating step at a finish rolling end temperature of 840 °C or higher and 930 °C or lower to obtain a hot-rolled steel sheet; A primary cooling step of cooling the hot-rolled steel sheet after the hot-rolling step to a cooling stop temperature of 670 °C or higher and 800 °C or lower at an average cooling rate of 30 °C / s or higher within 2.5 seconds after the finish rolling; A secondary cooling step of cooling the hot-rolled steel sheet after the primary cooling step at an average cooling rate of 1 °C / s or higher and 10 °C / s or lower for 2 seconds or longer and 15 seconds or shorter; A tertiary cooling step is performed to cool the hot-rolled steel sheet after the secondary cooling step to a cooling stop temperature of 550°C to 640°C at an average cooling rate of 30°C / s or more, A winding process for winding the hot-rolled steel sheet after the tertiary cooling process, A pickling step in which the hot-rolled steel sheet after the winding step is pickled, A method for manufacturing a hot-rolled steel sheet, comprising a plating step of applying a plating treatment to the hot-rolled steel sheet after the pickling step. [Effects of the Invention]

[0014] According to the present invention, a hot-rolled steel sheet with a tensile strength (TS) of 400 MPa or less and excellent ductility and work hardening properties can be obtained.

[0015] The hot-rolled steel sheet of the present invention can be used to form parts with complex shapes. Applying the hot-rolled steel sheet of the present invention to, for example, automotive components, contributes to improving the safety of automobile bodies. [Modes for carrying out the invention]

[0016] One embodiment of the present invention is described below. However, the present invention is not limited to the following embodiments.

[0017] <Component composition of hot-rolled steel sheet> First, the component composition of the hot-rolled steel sheet according to one embodiment of the present invention will be described. Note that all units in the component composition are in "mass%", but unless otherwise specified, they will be simply referred to as "%".

[0018] C: 0.005% or more and less than 0.050% Carbon (C) is an element that, during ferrite transformation, is discharged to the austenite side of the ferrite / austenite interface, significantly altering the austenite-to-ferrite transformation driving force near the interface. In this invention, it is necessary to appropriately control the austenite-to-ferrite transformation driving force, and if the C content is less than 0.005%, it leads to ferrite nucleation and uneven growth. From this viewpoint, the C content is set to 0.005% or more. On the other hand, carbon (C) is an element that facilitates the formation of microvoids at the interface between ferrite and cementite or between ferrite and pearlite during molding by forming cementite or pearlite when the ferrite transformation is complete. To obtain the desired processability, the C content needs to be less than 0.050%. Furthermore, carbon (C) is an element that lowers the austenite-to-ferrite transformation driving force and transformation point, and if the C content is 0.050% or more, ferrite growth is inhibited, and the desired structure cannot be obtained. Therefore, from this viewpoint as well, the C content is set to less than 0.050%. The C content is preferably 0.008% or more. Furthermore, the C content is preferably 0.035% or less.

[0019] Si: 0.10% or less (including 0%) Si is an element that strengthens steel sheets through solid solution strengthening. In this invention, it is important to control the austenite-to-ferrite transformation driving force. Si is an element that changes the austenite-to-ferrite transformation driving force, and if the Si content exceeds 0.10%, the transformation behavior from austenite to ferrite assumed in this invention will be deviated from, and the desired structure cannot be obtained. For this reason, Si is not actively added, but a content of up to 0.10% is acceptable. Therefore, the Si content should be 0.10% or less. Preferably, the Si content is 0.05% or less. Note that even if the Si content is 0%, the effects of this invention will not be impaired, so the Si content may be 0%. However, since there is a possibility of contamination of about 0.005% depending on the raw materials used, the Si content may be 0.005% or more as an example.

[0020] Mn: 0.05% or more and less than 0.50% Mn is an effective element for controlling the driving force of the austenite-ferrite transformation, and it also has the effect of fixing sulfur and suppressing hot cracking. For this reason, the Mn content needs to be 0.05% or more. On the other hand, if the Mn content exceeds 0.50%, localized areas of high hardness will occur due to the effect of central segregation, which will adversely affect machinability. For this reason, the Mn content was kept below 0.50%. The Mn content is preferably 0.08% or more. Furthermore, the Mn content is preferably 0.35% or less.

[0021] P: 0.05% or less (including 0%) Since phosphorus (P) is a harmful element that causes microvoid formation during molding by segregating at grain boundaries, it is preferable to reduce its content as much as possible. In this invention, a P content of up to 0.05% is acceptable. Therefore, the P content should be 0.05% or less. Preferably, the P content is 0.04% or less. On the other hand, the lower the P content, the better, and it may even be 0%. However, since approximately 0.002% may inevitably be mixed in during manufacturing, the P content may be 0.002% or more as an example.

[0022] S: 0.008% or less (including 0%) S (S) acts as a wedge-shaped inclusion, negatively affecting processability. Therefore, since S is a harmful element, it is preferable to reduce its content as much as possible. In this invention, S content is acceptable up to 0.008%, so the S content was set to 0.008% or less. Preferably, the S content is 0.006% or less. On the other hand, the lower the S content, the better, and it may even be 0%. However, since approximately 0.0001% may inevitably be mixed in during manufacturing, the S content may be 0.0001% or more as an example.

[0023] Al: 0.01% or more and 0.08% or less Al acts as a deoxidizing agent during the steelmaking process. To achieve this effect, the Al content should be 0.01% or higher. On the other hand, Al is an element that reduces formability by forming oxides. In this invention, an Al content of up to 0.08% is acceptable, so the Al content was set to 0.08% or less. Preferably, the Al content is 0.07% or less.

[0024] N: 0.0080% or less (including 0%) N is a harmful element that reduces aging properties at room temperature and adversely affects moldability. Therefore, it is preferable to reduce N as much as possible, but in this invention, N content up to 0.0080% is acceptable. Thus, the N content is 0.0080% or less. Preferably, the N content is 0.0060% or less. On the other hand, the lower the N content, the better, and it may even be 0%. However, since about 0.0005% may inevitably be mixed in during manufacturing, the N content may be 0.0005% or more as an example.

[0025] The content of Mn, Cu, Ni, Cr, and Mo satisfies the following equation (1). 0.15≦[%Mn]+0.21[%Cu]+0.46[%Ni]+0.78[%Cr]+1.8[%Mo]≦0.65 (1) Here, in equation (1), [%M] (M: Mn, Cu, Ni, Cr, Mo) represents the content (mass%) of element M, and is set to 0 if it is not present. Mn, Cu, Ni, Cr, and Mo are all elements that alter the austenite-to-ferrite transformation driving force. In this invention, in order to control the austenite-to-ferrite transformation driving force within an appropriate range, the content of Mn, Cu, Ni, Cr, and Mo must satisfy equation (1). If the value of the middle side of equation (1) ([%Mn] + 0.21[%Cu] + 0.46[%Ni] + 0.78[%Cr] + 1.8[%Mo]) is less than 0.15, the ferrite transformation will start at high temperatures, and some of the ferrite grains generated at high temperatures will grow rapidly, making it impossible to obtain the desired structure. On the other hand, if the value of the middle side of equation (1) exceeds 0.65, the ferrite transformation will not proceed properly after primary cooling, and the number of fine ferrite grains will increase. Therefore, the content of Mn, Cu, Ni, Cr, and Mo must satisfy equation (1). The coefficients for the content of each element in equation (1) are related to the austenite-to-ferrite transformation driving force in the component composition of the steel specified in this invention, which was determined experimentally.

[0026] The remainder of the components other than those mentioned above may consist of Fe and unavoidable impurities.

[0027] The basic component composition (hereinafter also referred to as the basic component composition) of the hot-rolled steel sheet according to one embodiment of the present invention has been described above. Furthermore, the hot-rolled steel sheet according to one embodiment of the present invention may optionally contain one or more groups selected from the following groups A to D in addition to the above basic component composition.

[0028] ·Group A One or more elements selected from Cu: 0.40% or less, Ni: 0.40% or less, Cr: 0.40% or less, and Mo: 0.1% or less. ·B group One or more elements selected from V: 0.05% or less, Nb: 0.05% or less, Ti: 0.05% or less, Zr: 0.05% or less, Hf: 0.05% or less, and W: 0.05% or less. ·C group One or more of the following: Mg: 0.01% or less, REM: 0.01% or less, Co: 0.2% or less, Ca: 0.01% or less. ·D group One or more elements selected from the following: Sb: 0.01% or less, Sn: 0.05% or less, As: 0.01% or less, Ta: 0.01% or less, Pb: 0.01% or less, Cs: 0.01% or less, Te: 0.01% or less, Bi: 0.01% or less, Zn: 0.01% or less, Ge: 0.01% or less, Sr: 0.01% or less, Se: 0.0020% or less, B: 0.0020% or less, O: 0.0030% or less, H: 0.0010% or less.

[0029] The elements in Group A (Cu, Ni, Cr, Mo) are all elements that alter the austenite-to-ferrite transformation driving force. However, if the content of each of Cu, Ni, Cr, and Mo exceeds the above upper limit, the austenite-to-ferrite transformation driving force may decrease excessively, and the ferrite grains may become excessively fine. Therefore, when Cu, Ni, Cr, and Mo are included, it is preferable to keep the content of each below the above upper limit. On the other hand, the lower limit of the content of each of Cu, Ni, Cr, and Mo is not particularly limited and may be 0%. However, these elements may be mixed in as unavoidable impurities, and when these elements are included, the content of each may be 0.01% or more. Furthermore, in the present invention, as described above, in order to control the austenite-to-ferrite transformation driving force within an appropriate range, the content of Mn and Cu, Ni, Cr, and Mo included in the basic component composition must satisfy equation (1) above.

[0030] The elements in Group B (V, Nb, Ti, Zr, Hf, W) are elements that bond with carbides. However, if the content of each of V, Nb, Ti, Zr, Hf, and W exceeds the above upper limit, a large amount of carbides may be formed, which may lead to a decrease in uniform elongation and work hardening properties. Therefore, when V, Nb, Ti, Zr, Hf, and W are included, it is preferable to keep the content of each below the above upper limit. Furthermore, when elements of Group B are included, it is more preferable that the total content of elements of Group B is 0.05% or less. On the other hand, the lower limit of the content of each of V, Nb, Ti, Zr, Hf, and W is not particularly limited and may be 0%. However, these elements may be present as unavoidable impurities, and when these elements are included, their content may be 0.0001% or more.

[0031] Elements in group C (Mg, REM, Co, Ca) are elements that can change the morphology of inclusions and are expected to improve the workability of hot-rolled steel sheets. However, if Mg, REM, Co, and Ca are included in amounts exceeding the above upper limits, the effect will saturate. Furthermore, it may cause deterioration of various properties such as weldability. For this reason, when Mg, REM, Co, and Ca are included, it is preferable to keep the content of each below the above upper limits, and more preferably Mg: 0.005% or less, REM: 0.005% or less, Co: 0.1% or less, and Ca: 0.005% or less. Also, when elements in group C are included, it is more preferable that the total content of elements in group C be 0.01% or less. On the other hand, the lower limit of the content of each of Mg, REM, Co, and Ca is not particularly limited and may be 0%. However, these elements may be mixed in as unavoidable impurities, and when these elements are included, their content may be 0.0001% or more. REM (Rare Earth Elements) is a collective term for 17 elements: the 15 lanthanides from lanthanum (La, atomic number 57) to lutetium (Lu, atomic number 71), scandium (Sc, atomic number 21), and yttrium (Y, atomic number 39). These 17 elements can be included individually or in combination. The REM content refers to the total amount of these 17 elements.

[0032] The elements in group D (Sb, Sn, As, Ta, Pb, Cs, Te, Bi, Zn, Ge, Sr, Se, B, O, H) are mainly elements that are present as unavoidable impurities, and the content of each of these elements may be 0%. If these elements are present, their content is acceptable as long as it is within the above range. Therefore, when elements in group D are present, it is preferable to keep the content of each element below the above upper limit. Furthermore, if elements of group D are included, it is more preferable to limit the total content of group D elements to 0.05% or less, and further restrict the content to Sb: 0.005% or less, Sn: 0.04% or less, As: 0.005% or less, Ta: 0.005% or less, Pb: 0.005% or less, Cs: 0.005% or less, Te: 0.005% or less, Bi: 0.005% or less, Zn: 0.005% or less, Ge: 0.005% or less, Sr: 0.005% or less, Se: 0.0015% or less, B: 0.0015% or less, O: 0.0020% or less, and H: 0.0005% or less. On the other hand, the lower limit of the content of each element of group D is not particularly limited and may be 0%, however, if elements of group D are included, the content of each element may be 0.0001% or more.

[0033] <Metal structure of hot-rolled steel sheet> Next, the reason for limiting the microstructure of the hot-rolled steel sheet according to one embodiment of the present invention will be explained. The area ratio of ferrite grains of each particle size shown below represents the area ratio occupied by ferrite grains of each particle size relative to the entire microstructure.

[0034] Area percentage of ferrite particles with a particle size of less than 15 μm: 20% or less To improve uniform elongation and the n-value, it is necessary to increase the total amount of dislocations generated during molding as much as possible and suppress microvoids generated by the localization of dislocations. Fine ferrite particles with a particle size of less than 15 μm accumulate less dislocations during molding and are prone to microvoid formation. Therefore, if the area ratio of ferrite particles with a particle size of less than 15 μm in the structure becomes too high, the desired high uniform elongation and n-value cannot be obtained. For this reason, it is preferable to suppress the generation of ferrite particles with a particle size of less than 15 μm as much as possible, but in the present invention, an area ratio of ferrite particles with a particle size of less than 15 μm is acceptable up to 20%. Thus, the area ratio of ferrite particles with a particle size of less than 15 μm is set to 20% or less. The area ratio is preferably 18% or less, and more preferably 16% or less. On the other hand, the lower limit of the area ratio is not particularly limited, and the area ratio may be 0%, but since it is difficult to completely suppress the generation of fine ferrite, the area ratio may be 3% or more, or 6% or more, as an example.

[0035] Area ratio of ferrite grains with a particle size of 15 μm or more and 35 μm or less: 50% or more and 100% or less In this invention, in order to improve uniform elongation and work hardening properties, the area ratio of ferrite grains with a particle size of 15 μm or more and 35 μm or less is increased as much as possible. To obtain the desired uniform elongation (uniform elongation: 20% or more) and the desired work hardening properties (n value in the region of strain amount of 15% or more and 20% or less: 0.19 or more), the area ratio of ferrite grains with a particle size of 15 μm or more and 35 μm or less must be 50% or more and 100% or less. The area ratio is preferably 52% or more, and more preferably 55% or more. In addition, the area ratio may be 80% or less as an example.

[0036] Area percentage of ferrite grains with a particle size exceeding 35 μm: 30% or less In coarse ferrite grains with a particle size exceeding 35 μm, strain concentrates, and as the area ratio of coarse ferrite grains increases, the strain becomes localized, reducing uniform elongation and work hardening properties. To avoid this adverse effect, the area ratio of ferrite grains with a particle size exceeding 35 μm must be 30% or less. The area ratio is preferably 28% or less, and more preferably 27% or less. On the other hand, the lower limit of the area ratio is not particularly limited, and the area ratio may be 0%. The area ratio may be 5% or more as an example.

[0037] The ferrite grain size distribution is obtained by electron backscatter diffraction (EBSD). The analysis position is set to (1 / 4)t of the plate thickness t, and the field of view of the analysis target is 0.225 mm². 2 Data was acquired with a step size of 0.2 μm. The obtained image data was analyzed using OIM Analysis software (TSL Corporation). The grain size distribution of ferrite was acquired in "Grain Size (diameter)" mode, and the area ratio of ferrite grains of each grain size was calculated from this grain size distribution.

[0038] The microstructure of a hot-rolled steel sheet according to one embodiment of the present invention consists of ferrite composed of ferrite grains of the above-mentioned particle sizes and the remaining microstructure. The remaining microstructure consists of cementite, pearlite, and pseudo-pearlite, and the area ratio of the remaining microstructure is, for example, 0 to 5%. The remaining microstructure consists of one or more of cementite, pearlite, and pseudo-pearlite, and does not include bainite, martensite, or retained austenite. The area ratio of cementite and pearlite (including pseudo-pearlite) is determined using optical microscope images or SEM (scanning electron microscope) images taken by conventional methods. Specifically, 20 test lines with an actual length of 35 μm are drawn in the longitudinal direction (thickness direction) and transverse direction (rolling direction), and the area ratio of cementite and pearlite (including pseudo-pearlite) is determined by the conventional point counting method. Although cementite may occur at grain boundaries, in this invention, cementite that occurs along grain boundaries is excluded from measurement because it is difficult to distinguish it from the grain boundaries themselves.

[0039] Tensile strength: 400 MPa or less In this invention, we investigated how to improve the processability of hot-rolled steel sheets with a tensile strength of 400 MPa or less. For hot-rolled steel sheets with a tensile strength exceeding 400 MPa, the desired processability cannot be obtained. Therefore, in this invention, the tensile strength of the hot-rolled steel sheet is limited to 400 MPa or less. Furthermore, the tensile strength of the hot-rolled steel sheet in this invention is substantially 220 MPa or more.

[0040] The hot-rolled steel sheet of the present invention preferably has a uniform elongation of 20% or more at nominal strain. On the other hand, the upper limit of the uniform elongation is not limited. For example, the uniform elongation may be 30% or less. For a method of measuring the uniform elongation, please refer to the description in the examples.

[0041] The hot-rolled steel sheet of the present invention preferably has an n-value of 0.19 or higher in the region where the nominal strain is between 15% and 20%. On the other hand, the upper limit of the n-value is not limited. For example, the n-value may be 0.27 or lower. For the method of measuring the n-value, please refer to the description in the examples.

[0042] The hot-rolled steel sheet of the present invention may have a plating layer on the surface of the steel sheet (one or both sides). The type of plating layer is not particularly limited and may be, for example, a hot-dip galvanized layer or an electroplated layer. The plating layer may also be an alloyed plating layer. A zinc plating layer is preferred as the plating layer. The zinc plating layer may contain Al or Mg.

[0043] Furthermore, the composition of the plating layer is not particularly limited and can be, for example, a known composition. In the case of a hot-dip galvanized layer or an alloyed hot-dip galvanized layer, the composition of the plating layer can be, for example, one containing Fe: 20% by mass or less, Al: 0.001% by mass or more and 1.0% by mass or less, and further containing one or more selected from Pb, Sb, Si, Sn, Mg, Mn, Ni, Cr, Co, Ca, Cu, Li, Ti, Be, Bi, and REM in a total of 0% by mass or more and 3.5% by mass or less, with the remainder being Zn and unavoidable impurities. Also, the amount of plating adhesion is not particularly limited, but for example, the amount of plating adhesion per side of the steel sheet can be 20 to 80 g / m². 2 It can be done this way.

[0044] <Manufacturing method for hot-rolled steel sheets> Next, a method for manufacturing a hot-rolled steel sheet according to one embodiment of the present invention will be described. The method for manufacturing a hot-rolled steel sheet according to one embodiment of the present invention includes a heating step of heating a steel material having the above-mentioned component composition to 1050°C or higher. It also includes a hot-rolling step of performing finish rolling on the steel material after the heating step at a finish rolling completion temperature of 840°C to 930°C to produce a hot-rolled steel sheet. It also includes a primary cooling step of cooling the hot-rolled steel sheet after the hot-rolling step to a cooling stop temperature of 670°C to 800°C at an average cooling rate of 30°C / s or higher within 2.5 seconds after the completion of finish rolling. It also includes a secondary cooling step of cooling the hot-rolled steel sheet after the primary cooling step to a cooling stop temperature of 2 seconds to 15 seconds at an average cooling rate of 1°C / s to 10°C / s. It also includes a tertiary cooling step of cooling the hot-rolled steel sheet after the secondary cooling step to a cooling stop temperature of 550°C to 640°C at an average cooling rate of 30°C / s or higher. It also includes a winding step of winding the hot-rolled steel sheet after the tertiary cooling. Furthermore, if necessary, a pickling process can be performed on the hot-rolled steel sheet. Unless otherwise specified, the above temperatures refer to the surface temperatures of the steel material and steel sheets (hot-rolled steel sheets). The cooling start temperature may be, for example, the temperature at the end of the immediately preceding process. The average cooling rate refers to the average cooling rate of the steel sheet surface. Unless otherwise specified, the average cooling rate in each process is [(cooling start temperature - cooling stop temperature) / cooling time from the cooling start temperature to the cooling stop temperature] for each process. Hot-rolled steel sheets include hot-rolled steel strips.

[0045] First, a steel material such as a slab having the above-mentioned component composition is prepared. The method for manufacturing the steel material such as a slab is not particularly limited, and commonly used methods can be used. Examples of methods for manufacturing the steel material include melting molten steel having the above-mentioned component composition using known methods in a converter or electric furnace, and then manufacturing slabs by continuous casting, ingot casting, or thin slab casting. Scrap may also be used as a raw material for the steel material.

[0046] [Heating process] Heating temperature for steel materials: 1050℃ or higher If the heating temperature of the steel material, such as a slab, falls below 1050°C, it is not possible to complete the finish rolling at 840°C or higher. For this reason, the heating temperature of the steel material should be 1050°C or higher. Preferably, the heating temperature is 1080°C or higher. On the other hand, there is no particular upper limit to the heating temperature, but from the viewpoint of suppressing damage to the furnace wall of the heating furnace, it is preferable that the heating temperature be 1350°C or lower.

[0047] [Hot rolling process] Next, the steel material heated to 1050°C or higher (including materials shipped directly at high temperature after casting) is subjected to hot rolling consisting of rough rolling and finish rolling. The rough rolling only needs to ensure the desired sheet bar dimensions are achieved, and the conditions are not particularly limited. The steel material is roughly rolled to obtain a rough-rolled plate. Then, the obtained rough-rolled plate is subjected to finish rolling.

[0048] Finishing rolling completion temperature: 840°C to 930°C If the finish rolling completion temperature exceeds 930°C, ferrite nucleation during the austenite-ferrite transformation will not be uniform, and the desired microstructure cannot be obtained. Therefore, the finish rolling completion temperature must be 930°C or lower. On the other hand, if the finish rolling completion temperature is below 840°C, a work structure will be formed on the surface of the steel sheet, and there is a risk that the area ratio of fine ferrite grains with a particle size of less than 15 μm will increase. Therefore, the finish rolling completion temperature must be 840°C or higher. Preferably, the finish rolling completion temperature is 850°C or higher. Also, preferably, the finish rolling completion temperature is 920°C or lower.

[0049] [Cooling process] Cooling at an average cooling rate of 30°C / s or higher within 2.5 seconds after the completion of finish rolling (primary cooling process). In this invention, cooling immediately after the completion of finish rolling is referred to as "primary cooling." If this primary cooling is not started within 2.5 seconds (s) immediately after the completion of finish rolling, ferrite nucleation and growth will not be uniform, and the desired structure will not be obtained. Therefore, the time from immediately after the completion of finish rolling until the start of cooling (cooling start time) is set to within 2.5 seconds. Preferably, the cooling start time is within 2.0 seconds. There is no particular lower limit to the cooling start time, and it may be 0 seconds or more than 0 seconds. Furthermore, if the average cooling rate of primary cooling is less than 30°C / s, ferrite transformation will start at a high temperature, resulting in uneven ferrite nucleation and growth, and the desired structure will not be obtained. Therefore, the average cooling rate of primary cooling is set to 30°C / s or higher. Preferably, the average cooling rate is 40°C / s or higher. On the other hand, there is no particular upper limit to the average cooling rate of primary cooling, but from the viewpoint of controllability of the cooling stop temperature, the average cooling rate is preferably 200°C / s or lower.

[0050] Cool down to a cooling stop temperature between 670°C and 800°C. To promote uniform and appropriate ferrite grain growth, primary cooling must be stopped in the temperature range of 670°C to 800°C. If the primary cooling is stopped at a temperature below 670°C, ferrite grain growth will be insufficient, and the number of fine ferrite grains will increase. On the other hand, if the cooling is stopped at a temperature above 800°C, the growth rate of ferrite grains will be excessive, and the number of coarse ferrite grains will increase. The cooling stop temperature is preferably 680°C or higher. Furthermore, the cooling stop temperature is preferably 780°C or lower.

[0051] Cooling for 2 to 15 seconds at an average cooling rate of 1°C / s to 10°C / s (secondary cooling process) After the primary cooling process, it is necessary to stop the forced rapid cooling and apply air cooling conditions to form ferrite grains with a particle size of 15 μm to 35 μm. In this invention, cooling at a cooling rate of 1°C / s to 10°C / s after primary cooling is referred to as "secondary cooling". For plate thicknesses of 1.0 mm to 12.0 mm targeted in this invention, the air cooling rate is in the range of 1°C / s to 10°C / s. If the time spent cooling at an average cooling rate of 1°C / s to 10°C / s (secondary cooling time) is less than 2 seconds, the austenite-to-ferrite transformation will not proceed sufficiently, and the desired structure cannot be obtained. On the other hand, if the secondary cooling time exceeds 15 seconds, the ferrite grains will undergo Ostwald growth, resulting in a mixed-grain structure, and the desired structure cannot be obtained. The secondary cooling time is preferably 3 seconds or more. Furthermore, the secondary cooling time is preferably 12 seconds or less.

[0052] Cooling to between 550°C and 640°C at an average cooling rate of 30°C / s or higher (tertiary cooling). After the secondary cooling process, further cooling is necessary to suppress the growth of ferrite grains. In this invention, this recooling is referred to as "tertiary cooling." To suppress the growth of ferrite grains, the average cooling rate of the tertiary cooling must be 30°C / s or higher. Preferably, the average cooling rate of the tertiary cooling is 40°C / s or higher. On the other hand, there is no particular upper limit to the average cooling rate of the tertiary cooling, but from the viewpoint of controllability of the cooling stop temperature, the average cooling rate is preferably 200°C / s or lower. Furthermore, to suppress the growth of ferrite grains, the tertiary cooling must be cooled to 640°C or lower. On the other hand, if the cooling stop temperature of the tertiary cooling is below 550°C, the risk of formation of structures other than cementite and pearlite, such as bainite and martensite, increases, so the cooling stop temperature of the tertiary cooling is set to 550°C or higher and 640°C or lower. Preferably, the cooling stop temperature is 560°C or higher. Also, preferably, the cooling stop temperature is 630°C or lower.

[0053] Subsequently, the hot-rolled steel sheet after the tertiary cooling process may be wound into a coil. The winding temperature is preferably 560°C to 630°C, considering constraints on the ROT (Run-Out-Table) length and the need to ensure the coil winding shape. The obtained hot-rolled steel sheet may also be subjected to a pickling process. The pickling process can be carried out by known methods. Furthermore, temper rolling may be performed before or after the pickling process. Temper rolling can be carried out by known methods.

[0054] The above describes a method for manufacturing a hot-rolled steel sheet according to one embodiment of the present invention. The properties of the hot-rolled steel sheet of the present invention are not impaired even when a plating layer is formed on its surface. When the hot-rolled steel sheet of the present invention has a plating layer, for example, the plating layer can be formed by performing a plating process on the hot-rolled steel sheet manufactured as described above, after which a pickling process is performed. The plating process can be carried out, for example, under the following conditions.

[0055] Annealing at a temperature of 750°C or less on a continuous plating line. In one embodiment of the present invention, when forming a plating layer on the surface of a hot-rolled steel sheet, the formation of the plating layer can be carried out in a continuous plating line that allows annealing and plating to be performed in a single pass through the sheet. In this case, since ferrite grains do not grow significantly at temperatures up to 750°C, it is preferable that the annealing temperature in the continuous plating line be 750°C or lower. More preferably, the annealing temperature is 730°C or lower. There is no lower limit to the annealing temperature in order to obtain the characteristics required in the present invention, but for manufacturing purposes, it is preferable to set the annealing temperature to 630°C or higher.

[0056] In a continuous plating line, hot-rolled steel sheets annealed at the aforementioned annealing temperature can be plated, or further plated alloying can be performed after the plating. The conditions for the plating and plated alloying are not particularly limited, and known conditions can be applied, for example. As a plating treatment, for example, hot-dip galvanizing can be applied, which can be carried out by immersing the hot-rolled steel sheet (base steel sheet) in a zinc plating bath at 440 to 550°C. The composition of the plating bath can include one or more of Zn, Al, Mg, Si, and Ni. That is, the composition of the plating layer formed on the surface of the hot-rolled steel sheet during the plating treatment can include one or more of Zn, Al, Mg, Si, and Ni. Specifically, a zinc plating bath can be mentioned as a plating bath. The zinc plating bath consists of Zn, Al, and unavoidable impurities, and its components are not particularly specified, but as an example, the Al concentration in the bath can be 0.001% by mass or more and 1.0% by mass or less. Furthermore, the plating alloying treatment can be carried out, for example, by heating a steel sheet that has been subjected to hot-dip galvanizing to an alloying temperature of 450 to 550°C. [Examples]

[0057] The present invention will be described in more detail with reference to examples. However, the present invention is not limited to the following examples.

[0058] Steel having the component composition shown in Table 1 was melted in a vacuum melting furnace to produce a 250 mm thick steel slab. Subsequently, the steel slab was heated (heating process), hot-rolled, cooled (primary cooling, secondary cooling, tertiary cooling), and wound under the conditions shown in Table 2 to produce hot-rolled steel sheets. The produced hot-rolled steel sheets were subjected to temper rolling with an elongation rate of 0.1% to 0.5% and pickling to obtain steel sheets for evaluation. Some of these steel sheets (No. 2, 3, 15, 16) were passed through a continuous plating line to form a plating layer under the conditions shown in Table 3 to obtain hot-dip galvanized steel sheets (GI) and alloyed hot-dip galvanized steel sheets (GA).

[0059] The metal structure and tensile properties of the steel sheets obtained as described above were evaluated using the following methods. The results are shown in Table 4.

[0060] (i) Methods for analyzing metal microstructure A section parallel to the rolling direction was cut from the steel plate to serve as the observation surface. SEM observation and EBSD analysis were performed on the (1 / 4)t portion of the plate thickness t using the method described above, and the area ratio of ferrite grains of each grain size was determined. Specifically, corrosion was induced with 1 vol% nital, and 10 fields of view of the 1 / 4t portion of the plate thickness t were photographed under an optical microscope at 100x magnification. No corrosion traces were observed within the ferrite grains, and they were observed as crystalline grains with gray contrast. Cementite and pearlite (including pseudo-pearlite) could be observed as black contrast. The area ratios of cementite and pearlite (including pseudo-pearlite) were determined using the point counting method described above.

[0061] (ii) Tensile test Tensile test specimens conforming to JIS No. 5 were prepared from steel plates in a direction parallel to the rolling direction, and tensile tests were performed five times in accordance with the provisions of JIS Z 2241 (2011) to determine the average tensile strength and uniform elongation. The crosshead speed for the tensile tests was set to 10 mm / min. The n-value was calculated using a method in accordance with JIS Z 2253 (2011), from the region where the strain amount is between 15% and 20% of the nominal strain. Specifically, the n-value was calculated using the least squares method from the true stress-true strain relationship across the entire aforementioned region. For steel plates with uniform elongation and a strain amount of less than 20% of the nominal strain, the n-value was determined in the region from 15% strain to uniform elongation at the nominal strain.

[0062] [Table 1]

[0063] [Table 2]

[0064] [Table 3]

[0065] [Table 4]

[0066] As shown in Table 4, all of the hot-rolled steel sheets of the present invention examples had a tensile strength (TS) of 400 MPa or less, and obtained the high uniform elongation of 20% or more and a high n-value of 0.19 or more required by the present invention. On the other hand, the hot-rolled steel sheets of the comparative examples, which did not have the desired structure, did not obtain the desired properties. Steel sheet No. 19, which had a Mn content of 0.50% or more, showed reduced workability due to the effect of central segregation of Mn.

Claims

1. In mass percent, C: 0.005% or more and less than 0.050% Si: 0.10% or less, Mn: 0.05% or more and less than 0.50% P: 0.05% or less, S: 0.008% or less, Al: 0.01% or more and 0.08% or less, N: 0.0080% or less It contains Mn, and the content of Cu, Ni, Cr, and Mo satisfies the following formula (1), with the remainder being Fe and unavoidable impurities, Area percentage of ferrite particles with a particle size of less than 15 μm: 20% or less. Area ratio of ferrite grains with a particle size of 15 μm or more and 35 μm or less: 50% or more and 100% or less. The ferrite has a particle size exceeding 35 μm, with an area ratio of 30% or less. Hot-rolled steel sheet having a tensile strength of 400 MPa or less. 0.15≦[%Mn]+0.21[%Cu]+0.46[%Ni]+0.78[%Cr]+1.8[%Mo]≦0.65...(1) Here, in equation (1), [%M] represents the content (mass%) of element M, and is set to 0 if it is not present.

2. The hot-rolled steel sheet according to claim 1, wherein the aforementioned component composition further contains, by mass%, one or more selected from the following groups A to D. Group A: One or more elements selected from Cu: 0.40% or less, Ni: 0.40% or less, Cr: 0.40% or less, and Mo: 0.1% or less. Group B: One or more selected from V: 0.05% or less, Nb: 0.05% or less, Ti: 0.05% or less, Zr: 0.05% or less, Hf: 0.05% or less, and W: 0.05% or less. Group C: One or more elements selected from Mg: 0.01% or less, REM: 0.01% or less, Co: 0.2% or less, Ca: 0.01% or less. Group D: One or more elements selected from the following: Sb: 0.01% or less, Sn: 0.05% or less, As: 0.01% or less, Ta: 0.01% or less, Pb: 0.01% or less, Cs: 0.01% or less, Te: 0.01% or less, Bi: 0.01% or less, Zn: 0.01% or less, Ge: 0.01% or less, Sr: 0.01% or less, Se: 0.0020% or less, B: 0.0020% or less, O: 0.0030% or less, H: 0.0010% or less.

3. A hot-rolled steel sheet according to claim 1 or 2, having a plating layer on its surface.

4. A method for manufacturing a hot-rolled steel sheet according to claim 1 or 2, A heating step of heating a steel material having the above-mentioned component composition to 1050°C or higher, The steel material after the heating process is subjected to a hot rolling process in which it is finished rolling at a finish rolling completion temperature of 840°C to 930°C to produce a hot-rolled steel sheet. A primary cooling step is performed in which the hot-rolled steel sheet after the hot-rolling process is cooled to a cooling stop temperature of 670°C to 800°C at an average cooling rate of 30°C / s or more within 2.5 seconds after the completion of finish rolling, A secondary cooling step is performed in which the hot-rolled steel sheet after the primary cooling step is cooled for 2 to 15 seconds at an average cooling rate of 1°C / s to 10°C / s, A tertiary cooling step is performed to cool the hot-rolled steel sheet after the secondary cooling step to a cooling stop temperature of 550°C to 640°C at an average cooling rate of 30°C / s or more. A method for manufacturing a hot-rolled steel sheet, comprising a winding step of winding the hot-rolled steel sheet after the tertiary cooling step.

5. A method for manufacturing a hot-rolled steel sheet according to claim 3, A heating step of heating a steel material having the above-mentioned component composition to 1050°C or higher, The steel material after the heating process is subjected to a hot rolling process in which it is finished rolling at a finish rolling completion temperature of 840°C to 930°C to produce a hot-rolled steel sheet. A primary cooling step is performed in which the hot-rolled steel sheet after the hot-rolling process is cooled to a cooling stop temperature of 670°C to 800°C at an average cooling rate of 30°C / s or more within 2.5 seconds after the completion of finish rolling, A secondary cooling step is performed in which the hot-rolled steel sheet after the primary cooling step is cooled for 2 to 15 seconds at an average cooling rate of 1°C / s to 10°C / s, A tertiary cooling step is performed to cool the hot-rolled steel sheet after the secondary cooling step to a cooling stop temperature of 550°C to 640°C at an average cooling rate of 30°C / s or more. A winding process for winding the hot-rolled steel sheet after the tertiary cooling process, A pickling step in which the hot-rolled steel sheet after the winding step is pickled, A method for manufacturing a hot-rolled steel sheet, comprising a plating step of applying a plating treatment to the hot-rolled steel sheet after the pickling step.

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