Hot-rolled steel sheet and its manufacturing method

Abstract

To provide a hot-rolled steel sheet having high strength, excellent ductility and excellent toughness and having small anisotropy in the ductility and the toughness.SOLUTION: A hot-rolled steel sheet has a predetermined chemical composition. Micro structure of the hot-rolled steel sheet at a position having the depth of one fourth of the sheet thickness from the surface of a section parallel to the rolling direction and the direction of the sheet thickness is comprised of, by volume fraction, 90% or more of martensite and 0-10%, in total, of retained austenite of and ferrite, wherein the average aspect ratio of old austenite grains in the micro structure at the position is less than 3.0; the ratio of sulfide having an aspect ratio of more than 3.0 out of the sulfide having an area of 1.0 μm2 or more is 20.0% or less; and in the center portion of the sheet thickness, a pole density in the {211}<011>orientation is 5.0 or less, and wherein a tensile strength of the hot-rolled steel sheet is 950 MPa or more.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a hot-rolled steel sheet and a method for manufacturing the same. [Background technology]

[0002] The booms of construction machinery cranes tend to be longer as the height of the buildings being constructed increases. Therefore, in order to reduce the weight of the boom itself and increase the lifting and carrying capacity, the steel plates used as the boom materials are required to have higher strength as well as excellent ductility and toughness. Furthermore, in recent years, there has been a demand for such steel plates to have smaller anisotropy in ductility and toughness in order to ensure a high level of component performance.

[0003] In response to such demands, for example, Patent Document 1 discloses a steel sheet containing, by mass%, C: 0.08 to 0.25%, Si: 0.01 to 1.0%, Mn: 0.8 to 1.5%, P: 0.025% or less, S: 0.005% or less, Al: 0.005 to 0.10%, Nb: 0.001 to 0.05%, Ti: 0.001 to 0.05%, Mo: 0.1 to 1.0%, Cr: 0.1 to 1.0%, and B: 0.0005 to 0.005%, in which the martensite phase or tempered martensite phase accounts for 90% or more of the main phase by volume, and the aspect ratio of the prior austenite grains is 3 to 18, and which has a high strength of yield strength YS: 960 MPa or more, and vE -40 A hot-rolled steel sheet having a high toughness of 40J or more and a manufacturing method thereof have been reported.

[0004] Furthermore, as a method for reducing the anisotropy of a hot-rolled steel sheet, for example, Patent Document 2 reports a hot-rolled steel sheet containing, in mass%, C: 0.04 to 0.15%, Si: 0.01 to 0.25%, Mn: 0.1 to 2.5%, P: 0.1% or less, S: 0.01% or less, Al: 0.005 to 0.05%, N: 0.01 or less, Ti: 0.01 to 0.12%, and B: 0.0003 to 0.0050%, in which 90% or more of the structure is martensite, the amount of TiC precipitates is 0.05% or less, and the cleanliness of A-type inclusions as defined in JIS G0202 is 0.010% or less, and a manufacturing method thereof. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Patent No. 5609383 [Patent Document 2] Japanese Patent Application Laid-Open No. 2014-47414 Summary of the Invention [Problem to be solved by the invention]

[0006] The steel plate in Patent Document 1 has an aspect ratio of the prior austenite grains of 3 or more, which causes problems with large anisotropy in ductility and toughness. Anisotropy makes it difficult to maintain high levels of component performance and reduces dimensional accuracy during processing, posing challenges for application to the booms of construction machinery cranes.

[0007] Although the steel plate of Patent Document 2 has reduced anisotropy in bending workability, yield strength, and toughness at -20°C, the anisotropy in ductility is not necessarily reduced. Furthermore, the absorbed energy and anisotropy at -40°C are not disclosed.

[0008] As described above, it has been difficult with conventional techniques to obtain a hot-rolled steel sheet that has high strength, excellent ductility, and excellent toughness, and also has small anisotropy in ductility and toughness.

[0009] The present invention is intended to solve the above-mentioned problems, and has an object to provide a hot-rolled steel sheet having high strength, excellent ductility, and excellent toughness, and having small anisotropy of ductility and toughness. [Means for solving the problem]

[0010] The inventors have investigated various methods for obtaining the required strength, ductility, and toughness by melting and hot-rolling various steels in a laboratory, each containing different amounts of C, Si, and Mn. As a result, they have found that in order to reduce the anisotropy of ductility and toughness while maintaining a high tensile strength of 950 MPa or more, it is important to reduce the anisotropy of the structure and the shape anisotropy of sulfides. Specifically, 1) the S content is to be 0.0060% or less, preferably 0.0010% or less, 2) the microstructure at a depth of 1 / 4 of the plate thickness from the surface is to be a structure containing martensite (including fresh martensite and tempered martensite) at a volume fraction of 90% or more, the average aspect ratio of prior austenite grains is to be less than 3.0, and the area is to be 1.0 μm 2 The ratio of sulfides with an aspect ratio of more than 3.0 among the above sulfides must be 20.0% or less, and 3) the {211} <011> It was found that it is important to keep the pole density of the orientation at 5.0 or less.

[0011] The present invention has been made based on the above findings. The gist of the present invention is as follows. (1) In mass %, C: 0.08~0.25%, Si: 0.01~1.00%, Mn: 0.8~2.0%, P: 0.020% or less, S: 0.0060% or less, Al: 0.005~1.000%, N: 0.0010~0.0100%, Ti:0 .005~0.300%, Ca:0.0005~0.0100%, Nb:0~0.300%, V:0~0.50%, Cr:0~3.0%, Mo:0~3.00%, Ni:0~5.0%, Cu:0~3.00%, B:0~0.0100%, M The steel sheet has a chemical composition consisting of Zr: 0-0.0100%, Zr: 0-0.0500%, REM: 0-0.0500%, and the balance: Fe and impurities, and the microstructure at a position at a depth of 1 / 4 of the sheet thickness from the surface of a cross section parallel to the rolling direction and parallel to the sheet thickness direction is composed of 90% or more martensite and 0-10% in total of retained austenite and ferrite, in terms of volume fraction, and the average aspect ratio of prior austenite grains in the microstructure at the position is less than 3.0, and the area is 1.0 μm 2Among the above sulfides, the proportion of sulfides with an aspect ratio of more than 3.0 is 20.0% or less, and the {211} <011> A hot-rolled steel sheet having a pole density of 5.0 or less and a tensile strength of 950 MPa or more. (2) The hot-rolled steel sheet according to (1), wherein the chemical composition contains, in mass%, one or more elements selected from the group consisting of Nb: 0.005 to 0.300%, V: 0.01 to 0.50%, Cr: 0.05 to 3.0%, Mo: 0.05 to 3.00%, Ni: 0.05 to 5.0%, Cu: 0.10 to 3.00%, B: 0.0003 to 0.0100%, Mg: 0.0005 to 0.0100%, Zr: 0.0010 to 0.0500%, and REM: 0.0010 to 0.0500%. (3) A method for producing a hot-rolled steel sheet according to (1) or (2), a hot rolling step of hot-rolling the cast slab after the heating step to a temperature of 1250°C or higher and 1350°C or lower; a hot rolling step of hot-rolling the cast slab after the heating step to form a hot-rolled steel sheet; and a coiling step of coiling the hot-rolled steel sheet after the hot rolling step at a coiling temperature of 100°C or lower, wherein in the hot rolling step, the cast slab is rolled to a finish rolling temperature of 1000°C or higher, and cooling is started within 0.10 seconds after the completion of the rolling, and first cooling is performed to reduce the temperature by 50°C or more at an average cooling rate of 100°C / second or higher, and after the first cooling, light reduction rolling is performed at a temperature of Ar3 point or higher with a rolling reduction of 5% to 20%, and second cooling is performed from the completion of the light reduction rolling to the coiling temperature at an average cooling rate of 50°C / second or higher. [Effects of the Invention]

[0012] According to the present invention, it is possible to provide a hot-rolled steel sheet having high strength, excellent ductility, and excellent toughness, and small anisotropy of ductility and toughness, and a method for manufacturing the same. This hot-rolled steel sheet can be suitably applied to booms of construction machinery cranes, etc., and by applying it, it can contribute to weight reduction of booms of construction machinery cranes, etc., and therefore it is a very significant contribution to industry. DETAILED DESCRIPTION OF THE INVENTION

[0013] Hereinafter, a hot-rolled steel sheet according to one embodiment of the present invention (a hot-rolled steel sheet according to this embodiment) and a method for manufacturing the same will be described. The hot-rolled steel sheet according to this embodiment has a chemical composition containing, by mass%, C: 0.08 to 0.25%, Si: 0.01 to 1.00%, Mn: 0.8 to 2.0%, P: 0.020% or less, S: 0.0060% or less, Al: 0.005 to 1.000%, N: 0.0010 to 0.0100%, Ti: 0.005 to 0.300%, Ca: 0.0005 to 0.0100%, and optionally containing one or more of Nb, V, Cr, Mo, Ni, Cu, B, Mg, Zr, and REM, with the balance being Fe and impurities; The microstructure at a depth of 1 / 4 of the plate thickness from the surface of a cross section parallel to the rolling direction and the plate thickness direction is composed of 90% or more martensite and 0 to 10% in total of retained austenite and ferrite, in terms of volume fraction; The average aspect ratio of prior austenite grains in the microstructure at the position is less than 3.0 and the area is 1.0 μm 2 Among the above sulfides, the proportion of sulfides with an aspect ratio of more than 3.0 is 20.0% or less, and the {211} <011> The orientation pole density is 5.0 or less, The tensile strength is 950 MPa or more.

[0014] First, the reasons for limiting the range of the content of each element contained in the chemical composition of the hot-rolled steel sheet according to this embodiment will be explained. Hereinafter, % in the content of each element is % by mass.

[0015] C: 0.08 to 0.25% C is an element that increases the strength of steel. If the C content is less than 0.08%, it is difficult to ensure a tensile strength of 950 MPa or more. Therefore, the C content is set to 0.08% or more. The C content is preferably 0.10% or more. On the other hand, if the C content exceeds 0.25%, ductility, weldability, toughness, etc. are significantly deteriorated. Therefore, the C content is set to 0.25% or less, and preferably 0.20% or less.

[0016] Si: 0.01 to 1.00% Si is an element useful for increasing the strength of steel sheets through solid solution strengthening. Si is also an element useful for suppressing the formation of cementite. If the Si content is less than 0.01%, these effects cannot be fully obtained. Therefore, the Si content is set to 0.01% or more. On the other hand, if the Si content exceeds 1.00%, the peelability of scale formed during hot rolling and the chemical conversion treatability are significantly deteriorated. Also, the desired structure may not be obtained. Therefore, the Si content is set to 1.00% or less.

[0017] Mn: 0.8 to 2.0% Mn is an element effective in improving hardenability. If the Mn content is less than 0.8%, the effect of improving hardenability cannot be sufficiently obtained. Therefore, the Mn content is set to 0.8% or more. On the other hand, if the Mn content exceeds 2.0%, toughness deteriorates. Therefore, the Mn content is set to 2.0% or less. Furthermore, if the Mn content is less than 0.8% or exceeds 2.0%, sulfides with large aspect ratios are likely to form, although this depends on the contents of S, Ti, Ca, etc.

[0018] P:0.020% or less P is an impurity element that segregates at grain boundaries, reducing grain boundary strength and toughness. Therefore, it is desirable to reduce the P content. Taking into account current refining technology and manufacturing costs, the P content is set to 0.020% or less. Since a low P content is preferable, 0% is acceptable, but in consideration of steelmaking costs, it may be set to 0.001% or more.

[0019] S:0.0060% or less S is an impurity element that deteriorates hot workability and toughness and further promotes anisotropy in ductility and toughness. Therefore, it is desirable to reduce the S content. If the S content exceeds 0.0060%, the anisotropy in ductility and toughness increases. Therefore, taking into consideration current refining technology and manufacturing costs, the S content is set to 0.0060% or less. The S content is preferably 0.0010% or less. By setting the S content to 0.0010% or less, the anisotropy in ductility and toughness can be further reduced. Since a low S content is preferable, 0% is acceptable, but in consideration of steelmaking costs, it may be set to 0.0001% or more.

[0020] Al: 0.005 to 1.000% Al is a useful element as a deoxidizer. Furthermore, Al is an element that forms AlN and contributes to suppressing grain coarsening. If the Al content is less than 0.005%, these effects cannot be obtained. Therefore, the Al content is set to 0.005% or more. On the other hand, if the Al content exceeds 1.000%, the toughness deteriorates, so the Al content is set to 1.000% or less.

[0021] N: 0.0010 to 0.0100% N is an element that is effective in suppressing deterioration of toughness by forming nitrides and contributing to suppression of grain coarsening. If the N content is less than 0.0010%, this effect cannot be obtained. Therefore, the N content is set to 0.0010% or more. On the other hand, if the N content exceeds 0.0100%, the toughness deteriorates, so the N content is set to 0.0100% or less.

[0022] Ti: 0.005 to 0.300% Ti is an element that forms TiN and is effective in suppressing the coarsening of crystal grains. If the Ti content is less than 0.005%, this effect cannot be sufficiently obtained. Therefore, the Ti content is set to 0.005% or more. The Ti content is preferably 0.010% or more. On the other hand, if the Ti content exceeds 0.300%, TiN becomes coarse and the toughness may deteriorate, so the Ti content is set to 0.300% or less.

[0023] Ca: 0.0005 to 0.0100% Ca is an element that is effective in suppressing the deterioration of hot workability and toughness caused by S by controlling the morphology of sulfides. If the Ca content is less than 0.0005%, this effect cannot be sufficiently obtained. Therefore, the Ca content is set to 0.0005% or more. On the other hand, if Ca is contained in excess, not only will the effect saturate but the cost will also increase, so the Ca content is set to 0.0100% or less.

[0024] The above are the basic components of the hot-rolled steel sheet according to this embodiment. Other than the above, the steel sheet typically contains Fe and impurities. However, depending on the desired strength level and other required properties, optional elements may be added, such as one or more elements selected from the group consisting of Nb, V, Cr, Mo, Ni, Cu, B, Mg, Zr, and REM, within the ranges shown below. Since the optional elements do not have to be added, the lower limit of their content is 0%. Furthermore, in this embodiment, "impurities" refers to elements that are mixed in from raw materials such as ore or scrap, or from the manufacturing environment, and are acceptable within a range that does not adversely affect the hot-rolled steel sheet according to this embodiment. The optional elements are described in detail below.

[0025] Nb: 0 to 0.300% Nb is an element that forms fine carbonitrides and is effective in suppressing the coarsening of crystal grains. Therefore, Nb may be added. In order to improve toughness, the Nb content is preferably 0.005% or more. On the other hand, if the Nb content is excessive, the precipitates become coarse and the toughness may deteriorate, so if Nb is added, the Nb content is set to 0.300% or less.

[0026] V: 0 to 0.50% V is an element that forms fine carbonitrides like Nb, and therefore may be added. In order to suppress the coarsening of crystal grains and increase toughness, the V content is preferably 0.01% or more. On the other hand, if the V content exceeds 0.50%, the toughness may deteriorate, so if V is added, the V content is set to 0.50% or less.

[0027] Cr: 0 to 3.0% Mo: 0 to 3.00% Ni: 0 to 5.0% Cu: 0 to 3.00% Cr, Mo, Ni, and Cu are elements effective in improving ductility and toughness. Therefore, they may be added. To improve ductility and toughness, the Cr content is preferably 0.05% or more, the Mo content is 0.05% or more, the Ni content is 0.05% or more, and the Cu content is preferably 0.10% or more. More preferably, the Cr content is 0.1% or more, the Mo content is 0.10% or more, the Ni content is 0.1% or more, and the Cu content is 0.20% or more. On the other hand, if the Cr content exceeds 3.0%, the Mo and Cu contents exceed 3.00%, and the Ni content exceeds 5.0%, the toughness may decrease due to the increase in strength. Therefore, if these elements are contained, the Cr content should be 3.0% or less, the Mo content should be 3.00% or less, the Ni content should be 5.0% or less, and the Cu content should be 3.00% or less.

[0028] B: 0 to 0.0100% B is an element that segregates at grain boundaries and suppresses the grain boundary segregation of P and S. It is also an element that is effective in improving the hardenability of steel. Therefore, B may be added. In order to improve ductility, toughness, and hot workability by strengthening the grain boundaries, and to improve hardenability, the B content is preferably 0.0003% or more. On the other hand, if the B content exceeds 0.0100%, coarse precipitates may form at the grain boundaries, impairing hot workability and toughness. Therefore, if B is added, the B content is set to 0.0100% or less.

[0029] Mg: 0 to 0.0100% Zr: 0 to 0.0500% REM: 0 to 0.0500% Mg, Zr, and REM are elements that control the morphology of sulfides and are effective in suppressing the deterioration of hot workability and toughness caused by S. Therefore, they may be added. To improve toughness, it is preferable that the Mg content be 0.0005% or more, the Zr content be 0.0010% or more, and the REM content be 0.0010% or more. On the other hand, even if Mg, Zr and / or REM are contained in excess, the effect saturates. Therefore, when these elements are contained, the Mg content should be 0.0100% or less, the Zr content should be 0.0500% or less, and the REM content should be 0.0500% or less. Here, REM refers to a total of 17 elements consisting of Sc, Y, and lanthanides, and the REM content refers to the total content of these elements. In the case of lanthanides, they are industrially added in the form of misch metal.

[0030] The chemical composition of the above-mentioned hot-rolled steel sheet may be measured by a common analytical method. For example, it may be measured using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry). C and S may be measured using a combustion-infrared absorption method, and N may be measured using an inert gas fusion-thermal conductivity method. B may be measured using absorptiometry or ICP-MS.

[0031] Next, the microstructure of the hot-rolled steel sheet according to this embodiment will be described. In this embodiment, the microstructure at a position 1 / 4 of the sheet thickness depth from the surface (a range of 1 / 8 to 3 / 8 of the sheet thickness from the surface is acceptable) is specified because the microstructure at this position is a representative structure of the steel sheet and has a strong correlation with the properties of the steel sheet.

[0032] <Contains 90% or more martensite by volume fraction, with the remaining structure consisting of retained austenite and ferrite> In order to increase the homogeneity of the structure and reduce anisotropy, the hot-rolled steel sheet according to this embodiment has a microstructure in which martensite (including fresh martensite and tempered martensite) is contained as the main phase at a volume fraction of 90% or more, with the remainder consisting of 0 to 10% in total of retained austenite and ferrite. The distribution of retained austenite and ferrite differs between the rolling direction and the direction perpendicular to the rolling direction, so anisotropy increases as their content increases. Therefore, the total content of these elements is set to 10% or less, and a homogeneous martensite structure is set to 90% or more. Since retained austenite and ferrite are not necessary, the lower limit is 0%. There is no need to limit the volume fractions of fresh martensite and tempered martensite, but as the volume fraction of tempered martensite increases, strength decreases. Therefore, from the perspective of ensuring strength, it is desirable that the volume fraction of tempered martensite be less than 5%.

[0033] The volume fraction of each structure in the microstructure is determined by the following method. First, a sample is taken from an arbitrary position on the hot-rolled steel sheet so that the cross section parallel to the rolling direction and the sheet thickness direction becomes the observation surface. The microstructure at a depth of 1 / 4 of the plate thickness from the surface of the observation surface in the plate thickness direction is revealed by Lepera etching or nital etching, and a 100 × 100 μm area (field of view) is observed with an optical microscope or SEM at 1000x magnification. Each phase is then identified based on the microstructural morphology, carbide precipitation state, dislocation density, etc., and the area fractions of martensite (fresh martensite and tempered martensite) and ferrite are measured using an image analyzer or similar. The obtained area fractions of each phase are taken as volume fractions. In this embodiment, it is not necessary to distinguish between fresh martensite and tempered martensite, but if necessary, they are distinguished by Vickers hardness (Hv) and C concentration (mass%). The Vickers hardness (HvM) of martensite is measured at three points within a martensite grain at a test force of 5 gf in accordance with JIS Z 2244:2009, and the average Vickers hardness is calculated. Next, the C concentration (CM: mass%) of the martensite is measured. In this embodiment, when cementite is present in martensite grains, the C concentration of the martensite is determined by adding the C concentration of the cementite to the C concentration of the martensite. The C concentration (CM) of the martensite is obtained by measuring the C concentration at intervals of 0.5 μm or less using an electron probe microanalyzer (EPMA) attached to an FE-SEM and calculating the average value of the obtained C concentrations. Tempered martensite and fresh martensite are distinguished from each other based on the Vickers hardness (HvM) and C concentration (CM) of the obtained martensite. Specifically, if the obtained HvM and CM satisfy the following formula 1, the martensite is determined to be tempered martensite; otherwise, it is determined to be fresh martensite. HvM / (-982.1×CM 2 +1676×CM+189)≦0.60…Formula 1 The value obtained by substituting the C concentration (CM) of martensite into the denominator of the left side of the above formula 1 (-982.1 × CM 2 The hardness (C + 1676 × CM + 189) represents the hardness of the original martensite at that C concentration. The tempered martensite contained in the microstructure of the hot-rolled steel sheet according to this embodiment has a lower hardness than the original martensite due to cementite precipitation within the martensite grains caused by tempering. On the other hand, the fresh martensite contained in the hot-rolled steel sheet according to this embodiment is not tempered and has a hardness close to that of the original martensite. Therefore, in this embodiment, the tempered martensite and the fresh martensite are distinguished by calculating the ratio between the hardness of the original martensite and the hardness of the martensite obtained by actual measurement.

[0034] The volume fraction of retained austenite is measured by the following method. A sample is taken from any position on the hot-rolled steel sheet so that the cross section parallel to the sheet surface is the observation surface. The surface of the sample is ground to a 1 / 4 depth, then chemically polished. X-ray diffraction is then performed using a Mo tube. The volume fraction of retained austenite is calculated from the intensity ratio of the ferrite (200) diffraction intensity Iα(200), the ferrite (211) diffraction intensity Iα(211), the austenite (200) diffraction intensity Iγ(220), and the austenite (311) diffraction intensity Iγ(311) based on the following formula: Vγ in the formula below represents the volume fraction of retained austenite. Vγ=0.25×{Iγ(220) / (1.35×Iα(200)+Iγ(220))+Iγ(220) / (0.69×Iα(211)+Iγ(2 20))+Iγ(311) / (1.5×Iα(200)+Iγ(311))+Iγ(311) / (0.69×Iα(211)+Iγ(311))}

[0035] <Average aspect ratio of prior austenite grains: less than 3.0> In the hot-rolled steel sheet according to this embodiment, the average aspect ratio of the prior austenite grains in a cross section parallel to the rolling direction is set to less than 3.0. If the average aspect ratio of the prior austenite grains is 3.0 or more, the anisotropy of ductility and toughness increases.

[0036] The average aspect ratio of the prior austenite grains is determined by the following method. First, a sample is taken from an arbitrary position on the hot-rolled steel sheet so that the cross section parallel to the rolling direction and the thickness direction serves as the observation surface. The sample size is not particularly limited, but it must contain 100 or more prior austenite grains. The structure of the observation surface (cross section in the rolling direction) at a depth of 1 / 4 of the plate thickness from the surface is etched using an etching solution (ethanol, 2% picric acid, 1% iron (II) chloride) that reveals prior austenite grain boundaries, and observed at 400x magnification using an optical microscope or SEM. In the obtained image, the aspect ratios of 100 or more prior austenite grains are measured using an image analyzer or the like, and the average value is taken as the average aspect ratio of the prior austenite grains. Here, the aspect ratio of the prior austenite grain is calculated by (aspect ratio) = (diameter (major diameter) in the rolling direction) / (diameter (minor diameter) in the plate thickness direction).

[0037] <1.0μm area 2 Of the above sulfides, the percentage of sulfides with an aspect ratio of more than 3.0 is 20.0% or less. Area is 1.0 μm 2 If the ratio of the number of sulfides having an aspect ratio of more than 3.0 among the above sulfides exceeds 20.0%, these sulfides will act as origins for the generation of voids, resulting in increased anisotropy in ductility and toughness. 2 The ratio of the number of sulfides having an aspect ratio of more than 3.0 among the above sulfides is set to 20.0% or less. The target area is 1.0 μm 2 Sulfides with an aspect ratio of 3.0 or more are defined as those with an area of ​​1.0 μm 2 This is because sulfides with an aspect ratio of less than 3.0 are unlikely to become the starting point of voids.

[0038] Area is 1.0 μm 2 The proportion of sulfides having an aspect ratio of more than 3.0 among the above sulfides is determined by the following method. In this embodiment, sulfides are defined as inclusions with an S mass fraction of 5% or more. Therefore, when determining the proportion of sulfides with an aspect ratio of more than 3.0, first, a sample is taken from an arbitrary position on the steel sheet so that the cross section parallel to the rolling direction and the sheet thickness direction becomes the observation surface. The sample size is not particularly limited, but a sample with an area of ​​1.0 μm 2The steel plate must contain 1,000 or more sulfides. The as-polished structure at a depth of 1 / 4 of the plate thickness from the surface of the steel plate at the observation surface is observed with an SEM at a magnification of 500x, and the composition of each inclusion is measured using an EDX attached to the SEM to identify sulfides. The area of ​​the sulfides is also measured using an image analyzer, and if the area is 1.0 μm or more, the sulfides are judged to be 1,000 or more. 2 The aspect ratio of the above sulfides is measured. 2 The aspect ratios of 1,000 or more of the above sulfides are measured, and the proportion of sulfides with an aspect ratio of more than 3.0 is determined. Here, the aspect ratio of sulfides is calculated by (aspect ratio) = (major axis in the rolling direction) / (minor axis in the plate thickness direction).

[0039] <{211} at the center of the plate thickness <011> Orientation polar density: 5.0 or less The hot-rolled steel sheet according to this embodiment has a {211} <011> The pole density of the orientation is 5.0 or less. <011> If the texture has a polar density of orientation exceeding 5.0, the structural anisotropy increases, and the anisotropy of ductility and toughness also increases. Regarding pole density, the center of the plate thickness has a strong correlation with the properties of the steel plate. Therefore, in this embodiment, the {211} <011> The orientation pole density is used as an index.

[0040] The pole density can be obtained from the crystal orientation information by EBSD analysis, and is synonymous with the X-ray random intensity ratio. <011> The pole density of the orientation is determined by the following method. First, a sample is taken from an arbitrary position on the steel sheet so that the cross section parallel to the rolling direction and the thickness direction serves as the observation surface. Using a device combining a scanning electron microscope and an EBSD analyzer and OIM Analysis (registered trademark) manufactured by AMETEK, orientation information for 1,000 or more bcc crystal grains is measured by EBSD analysis at the center of the sheet thickness (within a range of approximately 1 / 10 of the sheet thickness from the center of the sheet thickness toward the front and back of the steel sheet), distinguishing between fcc and bcc, and the orientation information is then obtained by ODF analysis using harmonic series expansion.

[0041] <Tensile strength: 950 MPa or more> Considering the contribution to weight reduction of the boom of a crane for construction machinery, the hot-rolled steel sheet according to this embodiment is assumed to be a high-strength steel sheet having a tensile strength of 950 MPa or more. There is no need to specify an upper limit for the tensile strength, but since there is a concern that a higher tensile strength will result in a decrease in elongation, the tensile strength may be set to 1570 MPa or less.

[0042] Next, the reasons for limiting the manufacturing conditions will be described. The hot-rolled steel sheet according to this embodiment can be manufactured by a manufacturing method including the following steps. (I) a heating step of heating a cast slab having a predetermined chemical composition to 1250°C or more and 1350°C or less; (II) a hot rolling step of hot rolling the cast slab after the heating step to obtain a hot-rolled steel sheet; (III) A coiling step of coiling the hot-rolled steel sheet after the hot rolling step at a coiling temperature of 100°C or less. Preferred conditions for each step will be explained below.

[0043] In the production of the hot-rolled steel sheet according to this embodiment, the production process preceding the heating process is not particularly limited. That is, following melting in a blast furnace or electric furnace, various secondary smelting processes may be performed, and then casting may be performed by a method such as ordinary continuous casting, ingot casting, or thin slab casting. In the case of continuous casting, the cast slab may be cooled to a low temperature once and then reheated before hot rolling, or the cast slab may be hot rolled directly after casting without being cooled to a low temperature. Scrap may also be used as the raw material.

[0044] <Heating process> In the heating step, the cast slab is heated to 1250° C. or more and 1350° C. or less. However, when the cast slab is at a temperature of 1250° C. or more, the heating step can be omitted and the slab can be subjected to the hot rolling step. If the heating temperature is less than 1250°C, the dissolution of sulfides will be insufficient, and undissolved sulfides will remain, causing the material to elongate in the rolling direction during hot rolling, resulting in increased anisotropy. Therefore, the heating temperature is set to 1250°C or higher. The heating temperature is preferably higher than 1250°C. On the other hand, if the heating temperature exceeds 1350°C, scale formation will be intense, the surface quality will deteriorate, and the crystal grains will become coarse, reducing the strength and low-temperature toughness of the hot-rolled steel sheet. Therefore, the heating temperature is set to 1350°C or less. The heating temperature is preferably less than 1350°C.

[0045] <Hot rolling process> <Winding process> In the hot rolling process, the cast slab is rolled to a finish rolling temperature (finish rolling completion temperature) of 1000°C or higher, and then cooling (first cooling) begins within 0.10 seconds after the completion of rolling. In the first cooling, cooling is performed so that the temperature drops by 50°C or more at an average cooling rate of 100°C / second or higher. After the first cooling, light reduction rolling is performed at a temperature of the Ar3 point or higher with a reduction ratio of 5% to 20%, and then second cooling is performed from the temperature at the completion of the light reduction rolling to a coiling temperature of 100°C or lower at an average cooling rate of 50°C / sec or higher. After second cooling is completed, coiling is performed. This converts the slab into a hot-rolled steel sheet.

[0046] If the finish rolling temperature is less than 1000°C, the texture will develop and the anisotropy of the structure will increase. Therefore, the finish rolling temperature is set to 1000°C or higher. On the other hand, if the finish rolling temperature exceeds 1100°C, the crystal grains become coarse. Therefore, the finish rolling temperature is preferably 1100°C or less.

[0047] If the time from the completion of rolling (completion of the final pass of finish rolling) to the start of cooling exceeds 0.10 seconds, or the average cooling rate of the first cooling is less than 100°C / second, or the temperature drop due to the first cooling is less than 50°C, the desired sulfides will not be obtained and toughness will decrease. Therefore, in the first cooling, cooling is started within 0.10 seconds after finish rolling, and cooling is performed by 50°C or more at an average cooling rate of 100°C / second or more (the temperature drop will be 50°C or more). In the first cooling, the cooling stop temperature is preferably set to the Ar3 point or higher because the subsequent soft reduction is performed at the Ar3 point (°C) or higher. There is no need to limit the upper limit of the average cooling rate, but it may be set to 1000°C / sec or lower in consideration of equipment, etc. An example of a method for starting cooling within 0.10 seconds after the completion of rolling is to use a cooling device between stands of a tandem rolling mill. The term "cooling" used here does not mean a temperature drop due to natural cooling, but rather means cooling at a faster cooling rate than natural cooling.

[0048] In the method for producing a hot-rolled steel sheet according to this embodiment, after the completion of the first cooling described above, rolling (light reduction rolling) is performed at a temperature of the Ar3 point (°C) or higher with a reduction ratio of 5% to 20% inclusive in order to finely precipitate sulfides. If the soft-reduction rolling temperature is lower than the Ar3 point, ferrite will form. Therefore, the soft-reduction rolling temperature is set to the Ar3 point or higher to suppress the formation of ferrite. Furthermore, if the soft-reduction rolling reduction rate is less than 5%, the effect of finely precipitating sulfides will not be sufficiently obtained. Furthermore, if the soft-reduction rolling reduction rate exceeds 20%, anisotropy will increase. Therefore, the soft-reduction rolling reduction rate is set to 5% or more and 20% or less. Here, the Ar3 point can be measured by using a fully automatic transformation recording and measuring device manufactured by Fuji Electric Industrial Co., Ltd., for example, to heat a test piece of a predetermined shape to 950°C for 30 minutes, then cool it at a rate of 30°C / second, and measure the expansion curve.

[0049] After soft reduction rolling, the steel sheet is cooled from the soft reduction rolling completion temperature to a coiling temperature of 100°C or less at an average cooling rate of 50°C / second or more (secondary cooling), and then coiled. If the average cooling rate is less than 50°C / second or the coiling temperature exceeds 100°C, large amounts of retained austenite, ferrite, and bainite are formed, and it is not possible to achieve a martensite volume fraction of 90% or more.

[0050] <Other processes> The hot-rolled steel sheet after the coiling step may be subjected to temper rolling for shape correction, elimination of yield point elongation, and homogenization of hardness distribution in the sheet thickness direction. When temper rolling is performed, if the elongation is less than 0.2%, the effect is insufficient, and if the elongation exceeds 3.0%, the yield ratio increases significantly and the elongation deteriorates. Therefore, it is desirable to set the elongation to 0.2% to 3.0%.

[0051] Furthermore, in order to remove scale formed during hot rolling, pickling may be carried out after hot rolling or temper rolling. The pickling conditions may be known conditions. [Example]

[0052] The effects of the present invention will be explained in more detail below with reference to examples. These examples are examples for confirming the effects of the present invention, and are not intended to limit the present invention.

[0053] Steel having the chemical composition shown in Table 1 (the balance being Fe and impurities) was cast, and heated, rolled, first cooled, soft-reduction rolled, and second cooled under the conditions shown in Table 2, and coiled under the conditions shown in Table 2 to obtain a hot-rolled steel sheet with a thickness of 5.0 mm. The soft-reduction rolling was performed in one pass. In Example No. 8, temper rolling with an elongation of 1.0% was also performed. In Table 2, SRT indicates the heating temperature of the slab, FT indicates the finish rolling temperature of the hot rolling before the first cooling, and CT indicates the coiling temperature.

[0054] The volume fraction of martensite, retained austenite, ferrite and other structures at a depth of 1 / 4 of the plate thickness from the surface of the obtained hot-rolled steel plate, the average aspect ratio of prior austenite grains, and the area of ​​1.0 μm2 The number of sulfides with an aspect ratio of over 3.0 among the above sulfides, and the {211} <011> The orientation pole density was evaluated by the method described above. The results are shown in Tables 3-1 and 3-2.

[0055] Furthermore, to evaluate the mechanical properties, JIS No. 5 tensile test pieces were taken from the L direction (rolling direction) and C direction (direction perpendicular to the rolling direction) and subjected to tensile tests in accordance with JIS Z 2241:2011. The tensile strength (TS) and total elongation (EL) were determined from the stress-strain curve of the tensile test. The toughness was evaluated by taking 5 mm wide sub-size V-notch Charpy test pieces from the L and C directions and conducting Charpy tests in accordance with JIS Z 2242:2018 at a test temperature of -40°C. Tensile strength of 950 MPa or more, total elongation of 10.0% or more, and Charpy absorbed energy (vE -40 ) is 50J / cm 2 If the strength was equal to or greater than this, it was determined that the steel had high strength, excellent ductility, and excellent toughness. Furthermore, if the ratio of the value in the L direction to the value in the C direction of each characteristic value (value in the L direction / value in the C direction) was 0.90 or more and 1.10 or less, the anisotropy was determined to be small. The results are shown in Tables 3-1 and 3-2.

[0056] [Table 1]

[0057] [Table 2]

[0058] [Table 3-1]

[0059] [Table 3-2]

[0060] Tables 3-1 and 3-2 show that the target properties were achieved in all of the invention examples Nos. 1 to 9. On the other hand, the comparative examples Nos. 10 to 25, whose chemical compositions or manufacturing methods were outside the scope of the invention, were inferior in one or more properties.

Claims

1. In mass%, C: 0.08-0.25%, Si: 0.01-1.00%, Mn: 0.8 to 2.0%, P: 0.020% or less, S: 0.0060% or less, Al: 0.005-1.000%, N: 0.0010-0.0100%, Ti: 0.005-0.300%, Ca: 0.0005-0.0100%, Nb: 0-0.300%, V: 0-0.50%, Cr: 0-3.0%, Mo: 0-3.00%, Ni: 0 to 5.0%, Cu: 0-3.00%, B: 0 to 0.0100%, Mg: 0 to 0.0100%, Zr: 0 to 0.0500%, REM: 0 to 0.0500%, and The balance is Fe and impurities. and a chemical composition consisting of The microstructure at a depth of 1 / 4 of the plate thickness from the surface of a cross section parallel to the rolling direction and the plate thickness direction is composed of 90% or more martensite and 0 to 10% in total of retained austenite and ferrite, in terms of volume fraction; In the microstructure at the location, The average aspect ratio of prior austenite grains is less than 3.0, Area is 1.0 μm ２ Among the above sulfides, the proportion of the number of sulfides having an aspect ratio of more than 3.0 is 20.0% or less, At the center of the plate thickness, The pole density of the {211}<011> orientation is 5.0 or less, Tensile strength is 950 MPa or more A hot-rolled steel sheet characterized by:

2. The chemical composition is, in mass %, Nb: 0.005-0.300%, V: 0.01-0.50%, Cr: 0.05-3.0%, Mo: 0.05-3.00%, Ni: 0.05-5.0%, Cu: 0.10-3.00%, B: 0.0003 to 0.0100%, Mg: 0.0005-0.0100%, Zr: 0.0010 to 0.0500%, and REM: 0.0010-0.0500%, Contains one or more selected from the group consisting of The hot-rolled steel sheet according to claim 1 .

3. A method for manufacturing a hot-rolled steel sheet according to claim 1 or 2, A heating step of heating a cast slab having the chemical composition according to claim 1 or 2 to 1250°C or higher and 1350°C or lower; a hot rolling step of hot rolling the cast slab after the heating step to form a hot-rolled steel sheet; a coiling step of coiling the hot-rolled steel sheet after the hot rolling step at a coiling temperature of 100°C or less; and In the hot rolling step, The cast slab is subjected to a finish rolling process at a temperature of 1000°C or higher. After the rolling is completed, cooling is started within 0.10 seconds, and a first cooling is performed so that the temperature decreases by 50°C or more at an average cooling rate of 100°C / second or more; After the first cooling, Ar ３ Light rolling is performed at a temperature of 5% or more and 20% or less at a temperature of 5% or more and 20% or less. a second cooling is performed from the completion of the soft reduction rolling to the coiling temperature so that the average cooling rate is 50°C / sec or more; A method for manufacturing a hot-rolled steel sheet.

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