Hot-rolled steel sheet and method for producing same

Abstract

The purpose of the present invention is to provide a hot-rolled steel sheet which has a tensile strength of 980 MPa or more, while having excellent ductility, fatigue resistance, and shear workability.　This hot-rolled steel sheet has a predetermined component composition and has a microstructure that includes: an upper bainite phase; and one or both of a fresh martensite phase and a retained austenite phase, wherein the area ratio of the upper bainite phase is 90% or more, the total area ratio of the fresh martensite phase and the retained austenite phase is 10% or less, the average aspect ratio of prior austenite grains is 6.0 or less, the number density of MnS having a major axis of 10 μm or more in a cross section parallel to the rolling direction is 20 per mm2 or less, and the average particle diameter of Ti-based sulfides is 2.0 μm or less.

Description

Hot-rolled steel sheet and method for manufacturing the same

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

[0002] In recent years, from the perspective of global environmental conservation, CO2 ２ Reducing emissions is a pressing need. In particular, there is a strong demand for improved fuel efficiency in automobiles, and the lightweighting of vehicle bodies is a key focus. Increasing the strength of steel sheets used in automobile components and making them thinner is one effective way to reduce weight without compromising the strength of the vehicle body. Steel sheets with a tensile strength of 980 MPa or higher are especially promising as materials that can dramatically improve automobile fuel efficiency through thinning.

[0003] On the other hand, increasing the tensile strength of steel sheets reduces their ductility, thus worsening their press-formability. Automotive parts, especially undercarriage components such as suspension parts, need to have complex shapes to ensure rigidity. To enable press forming of such complex shapes, steel sheets used as materials for automotive parts require high ductility.

[0004] Furthermore, thinning automotive parts reduces their durability. Therefore, to compensate for this decrease in durability, it is necessary to improve the fatigue resistance of the steel sheet. Automotive parts, especially undercarriage parts such as suspension components, are subjected to repeated loads from the tires, so if their fatigue strength is low, they will not be able to achieve the durability intended in the design. However, increasing the strength of the steel sheet does not necessarily increase fatigue strength. Therefore, there is a demand for steel sheets that combine high strength with excellent fatigue resistance.

[0005] Furthermore, when steel plates have high strength, shearing them can cause defects with irregularities on the sheared end surface, and these defects can lead to a deterioration in fatigue properties. Therefore, steel plates also require excellent shear workability.

[0006] Thus, steel plates are required to possess not only strength but also excellent properties in various other areas. Therefore, various technologies have been proposed to improve the properties of steel plates.

[0007] For example, Patent Document 1 proposes a technology for hot-rolled steel sheets that are excellent in formability, fracture characteristics, and fatigue characteristics. Specifically, the aforementioned characteristics are improved by controlling the hot-rolling conditions to make ferrite the main phase and controlling the shape and dispersion form of inclusions.

[0008] Patent Document 2 proposes a technology for hot-rolled steel sheets that have excellent punching fatigue characteristics and workability. Specifically, these characteristics are improved by controlling the shape and hardness of martensite in the center of the sheet thickness.

[0009] Patent Document 3 proposes a technology relating to hot-rolled steel sheets with excellent bendability. Specifically, the aforementioned properties are improved by controlling the hot-rolling conditions to make ferrite the main phase and controlling the precipitation state of Ti carbides and sulfides.

[0010] Patent Document 4 proposes a technology for high-strength cold-rolled steel sheets with excellent delayed fracture resistance. Specifically, this property is improved by controlling the annealing conditions after cold rolling to control the precipitation state of inclusions.

[0011] Patent document 5 proposes a technology relating to hot-rolled steel sheets with excellent hole-expanding and punching properties. Specifically, the aforementioned properties are improved by controlling the hot-rolling manufacturing conditions and controlling the size and number of inclusions.

[0012] Japanese Patent Publication No. 2014-031560, Japanese Patent Publication No. 2015-214718, International Publication No. 2013 / 099196, International Publication No. 2020 / 045219, International Publication No. 2017 / 017933

[0013] However, the prior art described in Patent Documents 1 to 5 had the following problems.

[0014] The technologies described in Patent Documents 1 to 3 cannot achieve a tensile strength of 980 MPa or higher.

[0015] The technology described in Patent Document 4 yields a steel sheet with a tensile strength of 980 MPa or more and excellent delayed fracture resistance. However, Patent Document 4 controls inclusions by performing annealing under predetermined conditions after cold rolling, and does not relate to hot-rolled steel sheets.

[0016] The technology described in Patent Document 5 yields a hot-rolled steel sheet with a tensile strength of 980 MPa or more, excellent punchability, and excellent hole-expanding properties. However, shear workability is not considered.

[0017] Thus, conventional technology has not established a method for producing hot-rolled steel sheets with a tensile strength of 980 MPa or higher, as well as excellent ductility, fatigue resistance, and shear workability.

[0018] Therefore, the present invention aims to provide a hot-rolled steel sheet having a tensile strength of 980 MPa or more, and excellent ductility, fatigue resistance, and shear workability.

[0019] The inventors of the present invention conducted studies to solve the above problems. As a result, they found that the above problems can be solved by appropriately controlling the component composition and microstructure of hot-rolled steel sheets. The present invention was completed based on the above findings, and its gist is as follows.

[0020] 1. The composition has the following components in mass%, containing C: 0.03-0.15%, Si: 0.5-3.0%, Mn: 1.0-3.0%, P: 0.1% or less, S: 0.02% or less, Ti: 0.03-0.1%, Al: 0.01-1.0%, and N: 0.01% or less, with the remainder being Fe and unavoidable impurities, and having an A value of 0-0.05 as defined by the following formula (1), comprising an upper bainite phase and either or both of fresh martensite and retained austenite phases, with the area ratio of the upper bainite phase being 90% or more, the total area ratio of the fresh martensite and retained austenite phases being 10% or less, and the average aspect ratio of the prior austenite grains being 6.0 or less. The number density of MnS particles with a major axis of 10 μm or more in a cross-section parallel to the rolling direction is 20 particles / mm². ２A hot-rolled steel sheet having the following microstructure, wherein the average particle size of Ti-based sulfides is 2.0 μm or less: A = Ti - 48 / 14 × N - 48 / 32 × S …(1) Here, each element symbol in equation (1) above represents the content (mass %) of each element.

[0021] 2. The hot-rolled steel sheet according to 1 above, wherein the component composition further comprises at least one selected from the group consisting of, in mass%, V: 0.005 to 0.5%, Nb: 0.005 to 0.1%, Cu: 0.005 to 0.5%, Ni: 0.005 to 0.5%, Cr: 0.005 to 1.0%, Mo: 0.005 to 0.5%, B: 0.0002 to 0.005%, Sb: 0.001 to 0.1%, Sn: 0.001 to 0.1%, Ca: 0.0005 to 0.01%, Mg: 0.0005 to 0.01%, and REM: 0.0005 to 0.01%.

[0022] 3. A method for manufacturing a hot-rolled steel sheet as described in 1 or 2 above, comprising: casting a molten steel material having the above component composition with an induction electromagnetic stirring device while rotating it in a horizontal plane relative to a mold at a rotational speed of 10 cm / s or more to obtain a steel material; heating the steel material to a heating temperature of 1150°C or higher; rough rolling the heated steel material to obtain a steel sheet; finishing rolling the steel sheet under the conditions of a total reduction ratio of 50% or less in a temperature range of (RC + 200°C) or less, and a finishing rolling completion temperature of RC or higher and (RC + 200°C) or lower; cooling the steel sheet after finishing rolling under the conditions of an average cooling rate of 30°C / s or more from the finishing rolling completion temperature to Bs, and a cooling stop temperature of (Bs - 150°C) or higher and Bs or lower; and winding the cooled steel sheet under the conditions of a winding temperature of (Bs - 150°C) or higher and Bs or lower. A method for manufacturing hot-rolled steel sheets, comprising cooling the rolled steel sheet at an average cooling rate of 1°C / s or less to a cooling stop temperature of (Bs - 450°C) or less. Here, RC and Bs are defined by the following equations (2) and (3), respectively. RC (°C) = 750 + 120 × C + 100 × N + 10 × Mn + 250 × Ti + 5000 × B + 10 × Cr + 50 × Mo + 750 × Nb + 150 × V ... (2) Bs (°C) = 830 - 270 × C - 90 × Mn - 70 × Cr - 37 × Ni - 83 × Mo - 20 × Cu ... (3) In the above equations (2) and (3), each element symbol represents the content (mass%) of the element, and 0 is used if the element is not contained.

[0023] According to the present invention, a hot-rolled steel sheet having a tensile strength (TS) of 980 MPa or more, and excellent ductility, fatigue resistance, and shear workability can be obtained. When the hot-rolled steel sheet of the present invention is applied to automobile parts (for example, automobile undercarriage parts such as suspensions, structural parts, frame parts, and truck frame parts), safety can be ensured and the weight of the automobile body can be reduced. Therefore, the hot-rolled steel sheet of the present invention can be used very suitably as a material for automobile parts.

[0024] In this invention, excellent ductility refers to having a uniform elongation (U.EL) of 6% or more.

[0025] Excellent fatigue resistance is defined as a fatigue limit ratio of 0.5 or higher. Here, the fatigue limit ratio is the stress ratio of 2 × 10 in a plane bending test with a stress ratio of -1. ６ It is defined as the ratio (σw / TS) of the plane bending fatigue strength (σw) to the tensile strength (TS).

[0026] Excellent shear workability means that both of the following conditions (A) and (B) are met: (A) When sheared with a clearance of 15 ± 1%, there are no defects (cracks, chips, brittle fracture surfaces, secondary shear surfaces) on the sheared end face. (B) The increase in end face hardness at the above sheared end face is 100 HV or less. Here, the increase in end face hardness is defined as the difference (increase) between the Vickers hardness at a position 0.1 mm away from the end face and the Vickers hardness of the matrix phase in a hardness measurement sample taken so that the cross section perpendicular to the sheared end face becomes the hardness measurement cross section.

[0027] This is a schematic diagram of the test specimen used in the planar bending fatigue test.

[0028] Embodiments of the present invention will be described in detail below. The following description is merely an example of preferred embodiments of the present invention, and the invention is not limited thereto. Furthermore, in this specification, the upper bainite phase may be referred to as the main phase. Also, the fresh martensite and retained austenite phases may be referred to as the hard second phase or simply the second phase.

[0029] <Hot-rolled steel sheet> The hot-rolled steel sheet in one embodiment of the present invention has a predetermined component composition and microstructure. The reasons for each limitation are explained below.

[0030] [Component Composition] The hot-rolled steel sheet in one embodiment of the present invention has the component composition described below. In the following description, "%" as a unit of content refers to "mass%" unless otherwise specified.

[0031] C: 0.03-0.15% C is an element that promotes the formation of bainite by improving hardenability and thereby improving strength. If the C content is less than 0.03%, the above effect is insufficient, and the desired strength cannot be obtained. Therefore, the C content should be 0.03% or more, preferably 0.04% or more, and more preferably 0.05% or more. On the other hand, if the C content exceeds 0.15%, fresh martensite and retained austenite phases as hard second phases are excessively formed, and the desired ductility cannot be obtained. Therefore, the C content should be 0.15% or less, preferably 0.14% or less, and more preferably 0.13% or less.

[0032] Si: 0.5-3.0% Si is an element that improves the strength of steel sheets through solid solution strengthening. To obtain the above effect, the Si content should be 0.5% or more, preferably 0.6% or more, and more preferably 0.7% or more. On the other hand, Si also has the effect of promoting ferrite formation. If the Si content exceeds 3.0%, ferrite is formed, resulting in an insufficient area ratio of the upper bainite phase, and thus the desired fatigue resistance cannot be obtained. For this reason, the Si content should be 3.0% or less, preferably 2.7% or less, and more preferably 2.5% or less.

[0033] Mn: 1.0-3.0% Mn is an element that has the effect of stabilizing austenite. If the Mn content is less than 1.0%, the above effect is insufficient, ferrite is formed, and the desired strength and fatigue resistance cannot be obtained. For this reason, the Mn content should be 1.0% or more, preferably 1.2% or more, and more preferably 1.5% or more. On the other hand, if the Mn content exceeds 3.0%, fresh martensite and retained austenite phases as hard second phases are formed in excess, and the desired ductility cannot be obtained. For this reason, the Mn content should be 3.0% or less, preferably 2.8% or less, and more preferably 2.5% or less.

[0034] P: 0.1% or less. Since P degrades weldability, it is desirable to reduce the P content as much as possible. In this invention, a P content of up to 0.1% is acceptable. Therefore, the P content is set to 0.1% or less. On the other hand, the lower limit of the P content is not particularly limited and may be 0%. However, a P content of less than 0.03% leads to an increase in refining costs. Therefore, from the viewpoint of cost, it is preferable to have a P content of 0.03% or more, and more preferable to have a P content of 0.05% or more.

[0035] S: 0.02% or less. Since sulfur (S) degrades weldability, it is desirable to reduce the S content as much as possible. In this invention, an S content of up to 0.02% is acceptable. Therefore, the S content should be 0.02% or less. On the other hand, the lower limit of the S content is not particularly limited and may be 0%. However, an S content of less than 0.0001% leads to a decrease in production efficiency. Therefore, from the viewpoint of production efficiency, it is preferable to have an S content of 0.0001% or more, and more preferable to have an S content of 0.0005% or more.

[0036] Ti: 0.03-0.1% Ti refines the crystal grains by precipitating as sulfides and nitrides, thereby improving the strength of the steel sheet. In addition, by forming sulfides, Ti suppresses the formation of coarse MnS, improving shear resistance. If the Ti content is less than 0.03%, these effects are insufficient. Therefore, the Ti content should be 0.03% or more, preferably 0.04% or more, and more preferably 0.05% or more. On the other hand, if the Ti content exceeds 0.1%, coarse sulfides are formed, which actually reduces shear workability. Therefore, the Ti content should be 0.1% or less, preferably 0.09% or less, and more preferably 0.08% or less.

[0037] Al: 0.01 - 1.0% Al acts as a deoxidizer and improves the cleanliness of steel. If the amount of Al is too small, the above effect will be insufficient. Therefore, the Al content should be 0.01% or more, preferably 0.015% or more, more preferably 0.02% or more. On the other hand, Al is an element that promotes ferrite formation. If the Al content rate exceeds 1.0%, ferrite will be generated and the desired fatigue resistance characteristics cannot be obtained. Therefore, the Al content should be 1.0% or less, preferably 0.8% or less, more preferably 0.5% or less.

[0038] N: 0.01% or less N is an element that precipitates as nitrides and contributes to the refinement of crystal grains. However, N is likely to combine with Ti at high temperatures to form coarse nitrides, and excessive content will reduce the shear resistance processing property. Therefore, the N content should be 0.01% or less, preferably 0.008% or less, more preferably 0.006% or less. On the other hand, the lower limit of the N content is not particularly limited and may be 0%. However, if the N content is less than 0.001%, it will lead to a decrease in production efficiency. Therefore, from the perspective of production efficiency, it is preferable that the N content is 0.001% or more.

[0039] The component composition of the hot-rolled steel sheet in one embodiment of the present invention includes the above components, and the balance is Fe and inevitable impurities.

[0040] The component composition of the hot-rolled steel sheet in another embodiment of the present invention can further optionally contain at least one of the following components.

[0041] V: 0.005 - 0.5% V is an element that forms carbides and has the effect of further improving the strength of the steel sheet by precipitation strengthening. When adding V, in order to obtain the above effect, the V content should be 0.005% or more, preferably 0.05% or more, more preferably 0.1% or more. On the other hand, if the V content exceeds 0.5%, the carbides will coarsen and the shear resistance processing property may deteriorate. Therefore, the V content should be 0.5% or less, preferably 0.3% or less.

[0042] Nb: 0.005 - 0.1% Nb, like V, is an element that forms carbides and has the effect of further improving the strength of the steel sheet by precipitation strengthening. When adding Nb, in order to obtain the above effect, the Nb content should be 0.005% or more, preferably 0.01% or more. On the other hand, when the Nb content exceeds 0.1%, the carbides may coarsen, and the shear resistance processing property may deteriorate. Therefore, the Nb content should be 0.1% or less, preferably 0.08% or less.

[0043] Cu: 0.005 - 0.5% Cu is an element that stabilizes austenite. By adding Cu, the formation of ferrite can be suppressed, and the strength can be further improved. When adding Cu, in order to obtain the above effect, the Cu content should be 0.005% or more, preferably 0.01% or more, more preferably 0.05% or more. On the other hand, when the Cu content exceeds 0.5%, excessive fresh martensite and retained austenite phases as hard second phases may be generated, and the desired microstructure may not be obtained. Therefore, the Cu content should be 0.5% or less, preferably 0.3% or less.

[0044] Ni: 0.005 - 0.5% Ni, like Cu, is an element that stabilizes austenite. By adding Ni, the formation of ferrite can be suppressed, and the strength can be further improved. When adding Ni, in order to obtain the above effect, the Ni content should be 0.005% or more, preferably 0.01% or more, more preferably 0.05% or more. On the other hand, when the Ni content exceeds 0.5%, excessive fresh martensite and retained austenite phases as hard second phases may be generated, and the desired microstructure may not be obtained. Therefore, the Ni content should be 0.5% or less, preferably 0.3% or less.

[0045] Cr: 0.005-1.0% Cr, like Cu, is an element that stabilizes austenite. By adding Cr, the formation of ferrite can be suppressed and the strength can be further improved. When adding Cr, in order to obtain the above effect, the Cr content should be 0.005% or more, preferably 0.01% or more, and more preferably 0.3% or more. On the other hand, if the Cr content exceeds 1.0%, fresh martensite and retained austenite phases as hard second phases will be excessively produced, and the desired microstructure may not be obtained. For this reason, the Cr content should be 1.0% or less.

[0046] Mo: 0.005-0.5% Mo, like Cu, is an element that stabilizes austenite. By adding Mo, the formation of ferrite can be suppressed and the strength can be further improved. When adding Mo, in order to obtain the above effect, the Mo content should be 0.005% or more, preferably 0.01% or more, and more preferably 0.05% or more. On the other hand, if the Mo content exceeds 0.5%, fresh martensite and retained austenite phase as hard second phases may be excessively produced, and the desired microstructure may not be obtained. For this reason, the Mo content should be 0.5% or less, preferably 0.3% or less.

[0047] B: 0.0002-0.005% B is an element that segregates at austenite grain boundaries and suppresses the formation of ferrite. By adding B, the formation of upper bainite is promoted, and the strength of the steel sheet can be further improved. When adding B, in order to obtain the above effect, the B content should be 0.0002% or more, preferably 0.0005% or more, and more preferably 0.0007% or more. On the other hand, if the B content exceeds 0.005%, the above effect saturates. Therefore, the B content should be 0.005% or less, preferably 0.004% or less, and more preferably 0.003% or less.

[0048] Sb: 0.001-0.1% Sb is an element that contributes to further improvement of steel sheet strength by suppressing the de-elementation from the surface of the steel material when the steel material is heated. When adding Sb, the Sb content should be 0.001% or more, preferably 0.005% or more, in order to obtain the above effect. On the other hand, if the Sb content exceeds 0.1%, it may lead to embrittlement of the steel sheet. Therefore, the Sb content should be 0.1% or less, preferably 0.05% or less.

[0049] Sn: 0.001-0.1% Sn is an element that contributes to further improvement of steel sheet strength by suppressing the formation of pearlite. When adding Sn, the Sn content should be 0.001% or more, preferably 0.005% or more, in order to obtain the above effect. On the other hand, if the Sn content exceeds 0.1%, it may lead to embrittlement of the steel sheet. Therefore, the Sn content should be 0.1% or less, preferably 0.05% or less.

[0050] Ca: 0.0005 to 0.01% Ca is an element that contributes to further improvement of shear workability by controlling the morphology of inclusions. When adding Ca, the Ca content should be 0.0005% or more, preferably 0.001% or more, in order to obtain the above effect. On the other hand, if the Ca content exceeds 0.01%, the amount of inclusions increases and the shear workability decreases. Therefore, the Ca content should be 0.01% or less, preferably 0.005% or less.

[0051] Mg: 0.0005-0.01% Mg, like Ca, is an element that contributes to further improvement of shear workability by controlling the morphology of inclusions. When adding Mg, the Mg content should be 0.0005% or more, preferably 0.001% or more, in order to obtain the above effect. On the other hand, if the Mg content exceeds 0.01%, the amount of inclusions increases and the shear workability decreases. Therefore, the Mg content should be 0.01% or less, preferably 0.005% or less.

[0052] REM: 0.0005-0.01% REM (rare earth metals), like Ca and Mg, are elements that contribute to improving shear workability by controlling the morphology of inclusions. When adding REM, the REM content should be 0.0005% or more, preferably 0.001% or more, in order to obtain the above effect. On the other hand, if the REM content exceeds 0.01%, the amount of inclusions increases and the shear workability decreases. Therefore, the REM content should be 0.01% or less, preferably 0.005% or less.

[0053] A value: 0 to 0.05 Furthermore, in this invention, it is important to adjust the content of Ti, N, and S so that the A value defined by the following equation (1) is between 0 and 0.05. The reason for this is explained below. A = Ti - 48 / 14 × N - 48 / 32 × S …(1) Here, each element symbol in the above equation (1) represents the content (mass %) of each element.

[0054] As mentioned above, Ti contributes to improved shear resistance by suppressing the formation of coarse MnS through the formation of sulfides. To obtain the above effect, it is necessary to add a sufficient amount of Ti relative to the amount of S contained in the steel. In addition, since Ti also forms nitrides, if it is not added in an amount greater than or equal to the atomic weight ratio of the amount of N present in the steel, the effect of suppressing the formation of coarse MnS will be insufficient. For this reason, the A value should be 0 or greater. If the A value is less than 0, the effect of adding Ti will be insufficient, and the desired shear workability cannot be obtained. On the other hand, if the A value exceeds 0.05, the nitrides and sulfides will become coarse, and the shear workability will actually decrease. For this reason, the A value should be 0.05 or less.

[0055] [Microstructure] Next, the reasons for limiting the microstructure of the hot-rolled steel sheet of the present invention will be explained. In this invention, the microstructure at the 1 / 4 position of the sheet thickness will be used as the microstructure.

[0056] The hot-rolled steel sheet of the present invention has a microstructure comprising an upper bainite phase and either or both of the fresh martensite and retained austenite phases.

[0057] UB: 90% or more The microstructure of the hot-rolled steel sheet of the present invention includes upper bainite phase (UB) as the main phase. However, if the area ratio of the upper bainite phase is less than 90%, excellent shear workability cannot be obtained. For this reason, the area ratio of the upper bainite phase is set to 90% or more, preferably 92% or more, and more preferably 94% or more. On the other hand, there is no particular upper limit to the area ratio of the upper bainite phase, and it may be 100%. However, as will be described later, from the viewpoint of further increasing the strength, it is preferable that the total area ratio of fresh martensite and retained austenite phase be 1% or more. For this reason, it is preferable that the area ratio of the upper bainite phase be 99% or less.

[0058] The area ratio of the upper bainite phase can be determined by analyzing images obtained from scanning electron microscopy (SEM) observation. More specifically, it can be determined by the method described in the examples.

[0059] FM + γ: 10% or less The microstructure of the hot-rolled steel sheet of the present invention further includes either or both of the following as a hard second phase: fresh martensite (FM) and retained austenite phase (γ). However, if the total area ratio of fresh martensite and retained austenite phase (FM + γ) exceeds 10%, excellent ductility cannot be obtained. For this reason, the total area ratio is set to 10% or less, preferably 8% or less, and more preferably 6% or less. On the other hand, the lower limit of the total area ratio is not particularly limited and may be 0%. However, from the viewpoint of further increasing strength, 1% or more is preferred.

[0060] The area ratios of fresh martensite and retained austenite phases can be determined by electron beam reflection diffraction. More specifically, they can be determined by the method described in the examples.

[0061] In this invention, it is sufficient that the area ratios of upper bainite, fresh martensite, and retained austenite satisfy the above conditions, and the presence of other structures as the remainder is acceptable. Examples of these other structures include lower bainite (LB) and ferrite (F). The total area ratio of these other structures should be low, preferably 0%. In other words, the microstructure of the hot-rolled steel sheet in one embodiment of the present invention may consist of an upper bainite phase and either or both of the fresh martensite and retained austenite phases.

[0062] Furthermore, the microstructure of the hot-rolled steel sheet of the present invention requires that the prior austenite grains, MnS, and Ti-based sulfides each satisfy the following conditions.

[0063] Average aspect ratio of prior austenite grains: 6.0 or less. If the average aspect ratio of prior austenite grains exceeds 6.0, the dislocation density of the phase generated from the austenite grains is high, and excellent ductility cannot be obtained. Therefore, the average aspect ratio of prior austenite grains should be 6.0 or less, preferably 5.5 or less, and more preferably 5.0 or less. On the other hand, the lower limit of the average aspect ratio is not particularly limited, but may be, for example, 2.0 or more. The average aspect ratio of prior austenite grains can be determined by analyzing images obtained by optical microscopy observation. More specifically, it can be determined by the method described in the examples.

[0064] Number density of MnS: 20 particles / mm ２ The following shows that the number density of MnS particles with a major axis of 10 μm or more in a cross-section parallel to the rolling direction is 20 particles / mm². ２ If the number density exceeds 20 particles / mm², the shear workability deteriorates. This is because the difference in hardness between MnS and the matrix structure causes stress to concentrate macroscopically at the phase interface, making it easier for cracks to form during shearing. Therefore, the number density should be set to 20 particles / mm². ２ Preferably 18 pieces / mm ２ More preferably, 15 pieces / mm ２ The following applies. On the other hand, the lower limit of the number density is not particularly limited, but may be, for example, 1 or more.

[0065] Average particle size of Ti-based sulfides: 2.0 μm or less As described above, Ti has the effect of suppressing the formation of coarse MnS by forming sulfides and improving shear resistance. However, if the average particle size of Ti-based sulfides is greater than 2.0 μm, the shear workability deteriorates. This is because stress concentrates at the phase interface macroscopically, making it easier for cracks to occur during shearing. Therefore, the average particle size of Ti-based sulfides is set to 2.0 μm or less, preferably 1.8 μm or less, and more preferably 1.5 μm or less. On the other hand, the lower limit of the average particle size is not particularly limited, but may be, for example, 0.1 μm or more.

[0066] The number density of MnS and the average particle size of the Ti-based sulfide can be determined by SEM (scanning electron microscope)-EDX (energy-dispersive spectroscopy). More specifically, they can be determined by the method described in the examples.

[0067] The hot-rolled steel sheet of the present invention has a tensile strength of 980 MPa or more, and possesses excellent ductility, excellent fatigue resistance, and excellent shear workability. Therefore, even when thinned, the hot-rolled steel sheet of the present invention exhibits excellent formability and fatigue fracture resistance, making it suitable for use as a material for automotive components such as trucks and passenger cars. The area ratio, particle size, number density, and mechanical properties of each of the above structures are based on values ​​measured by the method described in the examples.

[0068] The thickness of the hot-rolled steel sheet of the present invention is not particularly limited, but is typically preferably 1.5 to 6.0 mm.

[0069] [Manufacturing Method] Next, a method for manufacturing a hot-rolled steel sheet according to one embodiment of the present invention will be described. In the following description, unless otherwise specified, temperature refers to the surface temperature of the object (steel material or steel sheet).

[0070] The high-strength steel sheet of the present invention can be manufactured by sequentially carrying out the following steps (1) to (7). Each step will be described in detail below. (1) Casting (2) Heating (3) Rough rolling (4) Finish rolling (5) Cooling (first cooling) (6) Winding (7) Cooling (second cooling)

[0071] Casting: First, molten steel having the above-mentioned component composition is cast to produce steel material. The molten steel can be manufactured according to conventional methods. Scrap can be used as a raw material in this process.

[0072] Swirling speed: 10 cm / s or more In the above casting, stirring is performed using an induction electromagnetic stirring device. By casting while swirling the molten steel material by induction electromagnetic stirring, columnar crystals growing from the mold are divided, and equiaxed crystals can be generated at the 1 / 4 to 1 / 2 position of the plate thickness. As a result, segregation of components generated during solidification is reduced, and the localized increase in the number of MnS is suppressed. However, if the swirling speed during electromagnetic stirring is less than 10 cm / s, the columnar crystals are not sufficiently divided, and the desired effect cannot be obtained. For this reason, the swirling speed is set to 10 cm / s or more, preferably 15 cm / s or more, and more preferably 20 cm / s or more in the horizontal plane. On the other hand, from the above viewpoint, the faster the swirling speed, the better, so there is no particular upper limit to the swirling speed. However, if the swirling speed is increased excessively, the effect will saturate. For this reason, from the viewpoint of efficiency, it is preferable to set the swirling speed to 100 cm / s or less, more preferably 50 cm / s or less, and even more preferably 30 cm / s or less.

[0073] A steel material is produced by casting under the above conditions. The casting may typically be continuous casting. The steel material may typically be a steel slab.

[0074] The resulting steel material may be subjected to the next heating step directly after casting, or it may be subjected to the heating step after being cooled to become a hot or cold sheet. The final hot-rolled steel sheet will have the same composition as the steel material used.

[0075] Next, the steel material is heated. The heating temperature is as follows:

[0076] Heating temperature: 1150°C or higher. In steel materials, most precipitate-forming elements exist non-uniformly as coarse precipitates. If the added elements exist as coarse and non-uniform precipitates, the effect of the added elements cannot be fully obtained, and the desired microstructure cannot be obtained. Therefore, it is necessary to heat the steel material prior to hot rolling to solidify the coarse precipitates. For this reason, the heating temperature of the steel material should be 1150°C or higher, preferably 1180°C or higher, and more preferably 1200°C or higher. On the other hand, there is no particular upper limit to the heating temperature. However, if the heating temperature is too high, it can lead to the occurrence of slab defects and a decrease in yield due to scale-off. For this reason, the heating temperature should preferably be 1350°C or lower, more preferably 1300°C or lower, and even more preferably 1280°C or lower.

[0077] From the viewpoint of uniformizing the temperature of the steel material, it is preferable to raise the steel material to the heating temperature and then hold it at that heating temperature. The holding time at the heating temperature (holding time) is not particularly limited, but from the viewpoint of improving the uniformity of the temperature of the steel material, it is preferable to set it to 1800 seconds or more. On the other hand, if the holding time exceeds 10000 seconds, the amount of scale generated increases. As a result, scale inclusion and other problems are more likely to occur in the subsequent hot rolling, leading to a decrease in yield due to surface defects. For this reason, it is preferable to set the holding time to 10000 seconds or less, and more preferably to 8000 seconds or less. On the other hand, there is no particular lower limit to the holding time. Since the holding is not essential, the lower limit of the holding time may be 0.

[0078] Next, the heated steel material is roughly rolled to form a steel sheet. The conditions for rough rolling are not particularly limited and can be carried out under general conditions. Before performing finish rolling on the obtained steel sheet, descaling may be performed. This descaling can be carried out, for example, by spraying high-pressure water onto the surface of the steel sheet (high-pressure water descaling). This descaling can be carried out at the entrance of the finish rolling mill.

[0079] Next, the steel sheet is subjected to finish rolling. In the finish rolling process, the reduction ratio and the finish rolling completion temperature are controlled as follows.

[0080] Total reduction ratio below (RC + 200°C): 50% or less In the above finish rolling, the total reduction ratio in the temperature range below (RC + 200°C) shall be 50% or less. Here, RC is defined by the following equation (2) and represents the lower limit temperature of austenite recrystallization estimated from the composition of the steel. RC (°C) = 750 + 120 × C + 100 × N + 10 × Mn + 250 × Ti + 5000 × B + 10 × Cr + 50 × Mo + 750 × Nb + 150 × V ... (2) In the above equation (2), each element symbol represents the content (mass%) of each element, and if the element is not contained, it shall be 0.

[0081] If the total reduction ratio exceeds 50%, the aspect ratio of the prior austenite grains cannot be kept within the desired range, and as a result, excellent ductility cannot be obtained. For this reason, the total reduction ratio in the temperature range below (RC + 200°C) should be 50% or less, preferably 45% or less, and more preferably 40% or less. On the other hand, the lower limit of the total reduction ratio is not particularly limited, but from the viewpoint of further increasing strength, it is preferable to set it to 5% or more.

[0082] Finish rolling completion temperature: RC or higher (RC + 200°C) or lower. If the finish rolling completion temperature in the above finish rolling is lower than RC, the aspect ratio of the prior austenite grains cannot be kept within the desired range, and as a result, excellent ductility cannot be obtained. This is because strain relaxation due to recrystallization is difficult to occur. For this reason, the finish rolling completion temperature is set to be RC or higher, preferably (RC + 20°C) or higher, and more preferably (RC + 50°C) or higher. On the other hand, if the finish rolling completion temperature is higher than (RC + 200°C), excessive ferrite is generated, and the area ratio of the upper bainite phase cannot be kept within the desired range. As a result, the desired strength and fatigue resistance cannot be obtained. For this reason, the finish rolling completion temperature is set to be (RC + 200°C) or lower, preferably (RC + 180°C) or lower, and more preferably (RC + 150°C) or lower.

[0083] • Cooling (First Cooling) Next, the obtained steel plate is cooled (first cooling). At this time, the average cooling rate and the cooling stop temperature are controlled as follows.

[0084] Average cooling rate: 30°C / s or more If the average cooling rate from the finish rolling completion temperature to Bs is less than 30°C / s, excessive ferrite is generated, making it impossible to achieve the desired area ratio of the upper bainite phase. As a result, the desired strength and fatigue resistance cannot be obtained. Therefore, the average cooling rate should be 30°C / s or more, preferably 40°C / s or more, and more preferably 50°C / s or more. On the other hand, there is no particular upper limit to the average cooling rate, but if it is too fast, it becomes difficult to control the cooling stop temperature. Therefore, the average cooling rate should preferably be 500°C / s or less, more preferably 300°C / s or less, and even more preferably 150°C / s or less.

[0085] In the aforementioned cooling process, forced cooling should be performed to achieve the above-mentioned average cooling rate. The cooling method is not particularly limited, but typically it can be performed by water cooling.

[0086] Cooling stop temperature: (Bs-150°C) or higher and Bs or lower The cooling stop temperature shall be (Bs-150°C) or higher and Bs or lower. If the cooling stop temperature is below (Bs-150°C), the microstructure will become lower bainite. Lower bainite is a high-strength structure but does not have good ductility. For this reason, the cooling stop temperature shall be (Bs-150°C) or higher, preferably (Bs-140°C) or higher, and more preferably (Bs-130°C) or higher. On the other hand, if the cooling stop temperature is higher than Bs, an excessive amount of hard second phase will be formed, and excellent fatigue resistance cannot be obtained. For this reason, the cooling stop temperature shall be Bs or lower, preferably (Bs-10°C) or lower, and more preferably (Bs-20°C) or lower.

[0087] Furthermore, Bs is defined by the following equation (3): Bs (°C) = 830 - 270 × C - 90 × Mn - 70 × Cr - 37 × Ni - 83 × Mo - 20 × Cu ... (3) In the above equation (3), each element symbol represents the content (mass %) of the element, and 0 is used if the element is not contained.

[0088] Next, the cooled steel sheet is wound up. At that time, the winding temperature is controlled as follows.

[0089] Winding temperature: (Bs - 150°C) or higher and Bs or lower. If the winding temperature is below (Bs - 150°C), lower bainite will form, and excellent ductility cannot be obtained. Therefore, the winding temperature should be (Bs - 150°C) or higher, preferably (Bs - 140°C) or higher, and more preferably (Bs - 130°C) or higher. On the other hand, if the winding temperature is higher than Bs, excessive hard second phase will be formed, and the desired ductility cannot be obtained. Therefore, the winding temperature should be Bs or lower, preferably (Bs - 10°C) or lower, and more preferably (Bs - 20°C) or lower.

[0090] • Cooling (Second Cooling) Next, the steel sheet after winding is cooled. At this time, the average cooling rate and the cooling stop temperature are controlled as follows.

[0091] Average cooling rate: 1°C / s or less If the average cooling rate from the winding temperature to the cooling stop temperature described later exceeds 1°C / s, lower bainite will form, and excellent ductility cannot be obtained. For this reason, the average cooling rate should be 1°C / s or less, preferably 0.8°C / s or less, and more preferably 0.5°C / s or less. On the other hand, the lower limit of the average cooling rate is not particularly limited, but from the viewpoint of production efficiency, it is preferably 0.01°C / s or more, and more preferably 0.1°C / s or more.

[0092] Cooling stop temperature: (Bs - 450°C) or lower. If the cooling stop temperature exceeds (Bs - 450°C), the transformation to UB will not be completed, and a sufficient amount of UB will not be obtained. Therefore, the cooling stop temperature should be (Bs - 450°C) or lower. On the other hand, the lower limit is not limited, and it can be performed down to any temperature below (Bs - 450°C). From the viewpoint of productivity, it is preferable that the cooling stop temperature be 20°C or higher, more preferably 30°C or higher, and even more preferably 40°C or higher. Note that cooling can be performed in any form, for example, in the state of a wound coil.

[0093] The hot-rolled steel sheet of the present invention can be manufactured by following the above procedure. After winding and subsequent cooling, for example, temper rolling may be performed, or pickling may be performed to remove scale formed on the surface.

[0094] The present invention will be described in more detail below based on examples. However, the present invention is not limited to the following examples.

[0095] First, molten steel with the composition shown in Table 1 was melted in a converter, and steel slabs (steel materials) were manufactured by continuous casting. In the casting process, the molten steel was rotated using an induction electromagnetic stirring device at the rotation speed shown in Table 2. For convenience, Table 1 shows the A value obtained by equation (1) rounded to the fourth decimal place. Similarly, the values ​​for RC, Bs, etc., are shown rounded to the first decimal place.

[0096] The obtained steel slabs were heated to the heating temperatures shown in Table 2, and then the heated steel material was subjected to hot rolling consisting of rough rolling and finish rolling to produce hot-rolled steel sheets. The total reduction ratio in the temperature range of (RC + 200°C) or lower during the finish rolling, and the finish rolling completion temperature are as shown in Table 2.

[0097] Next, the obtained hot-rolled steel sheet was cooled under the conditions of average cooling rate and cooling stop temperature shown in Table 2 (first cooling). The cooled hot-rolled steel sheet was wound at the winding temperature shown in Table 2, and the wound steel sheet was cooled at the average cooling rate shown in Table 2 (second cooling) to obtain a hot-rolled steel sheet. After the above cooling, temper rolling was performed, followed by pickling. The pickling was carried out using a 10% by mass hydrochloric acid aqueous solution at a temperature of 85°C.

[0098] (Microstructure) The microstructure of each obtained hot-rolled steel sheet was evaluated using the procedure described below.

[0099] From the obtained hot-rolled steel sheet, a specimen for microstructural observation was taken so that the cross-section parallel to the rolling direction would serve as the observation surface. The surface of the obtained specimen was polished, and the microstructure was revealed by further etching the surface with an etching solution (3% nital solution). Next, the microstructure at the 1 / 4 thickness position was observed using a scanning electron microscope (SEM), and an SEM image was obtained. The observation was performed at a magnification of 5000x and in 10 fields of view. The obtained SEM images were analyzed by image processing, and the area ratios of upper bainite (UB), polygonal ferrite (F), and lower bainite (LB) were quantified.

[0100] Furthermore, from the obtained hot-rolled steel sheet, test specimens for microstructural observation were taken so that the cross-section parallel to the rolling direction would serve as the observation surface. The surface of the obtained test specimens was polished, and the prior austenite structure was revealed using an etching solution (an aqueous solution containing picric acid, a surfactant, and oxalic acid). Next, the microstructure at the 1 / 4 thickness position was observed using an optical microscope, and microscopic images were obtained. The observation was performed at a magnification of 500x and in 5 fields of view. The obtained microscopic images were analyzed by image processing, and the aspect ratio of the prior austenite grains was calculated. In the above analysis, the prior austenite grains were approximated as ellipses. That is, the longest part of the prior austenite grain was defined as the major axis, and the shortest part as the minor axis, and the aspect ratio (major axis / minor axis) of each prior austenite grain was determined. The average aspect ratio was obtained by arithmetic mean of the obtained aspect ratios of each prior austenite grain.

[0101] Furthermore, since fresh martensite (FM) and retained austenite (γ) are difficult to distinguish using SEM, they were identified using electron beam reflection diffraction, and their respective area fractions and average grain sizes were determined.

[0102] The upper bainite phase is an aggregate of bainite ferrites, and typically has a structure containing Fe-based carbides and / or retained austenite phases between the bainite ferrites. However, in the present invention, the upper bainite phase is also included in cases where there are no Fe-based carbide retained austenites between the bainite ferrites.

[0103] Unlike lamellar (layered) ferrites and polygonal ferrites in pearlite, bainitic ferrites have a lath-like shape and a relatively high dislocation density internally, making them distinguishable using scanning electron microscopes (SEM) and transmission electron microscopes (TEM). If retained austenite is present between the laths, only the bainitic ferrite portion is considered upper bainite and distinguished from the retained austenite.

[0104] Fresh martensite is martensite that does not have Fe-based carbides. Also, the fresh martensite and / or retained austenite phase has a brighter contrast in the SEM image compared to the upper bainite phase, lower bainite phase, and polygonal ferrite phase. Therefore, the fresh martensite phase and / or retained austenite phase can be distinguished from these microstructures using SEM. The fresh martensite phase and the retained austenite phase have a similar contrast in SEM, but can be distinguished from each other using the electron backscatter diffraction method.

[0105] MnS and Ti-based sulfides can be distinguished using EDX (energy dispersive spectroscopy). Therefore, the number density of MnS and the average particle size of Ti-based sulfides were determined using SEM-EDX. Specifically, first, a range of 79.4 μm × 5000 μm was observed using SEM, and the MnS and Ti-based sulfides present within this range were identified using EDX.

[0106] Next, the length in the major axis direction of MnS was measured, and the number density of MnS with a length in the major axis direction of 10 μm or more was calculated. Furthermore, image analysis was performed to calculate the equivalent circle diameter of each Ti-based sulfide, and the average value thereof was taken as the average particle size of the Ti-based sulfides.

[0107] The measurement results are shown in Table 3. In Table 3, the total area ratio (FM + γ) of fresh martensite and retained austenite is also shown.

[0108] Next, the strength, ductility, fatigue resistance characteristics, and shear resistance workability of the obtained hot-rolled steel sheet were evaluated by the following procedure. The evaluation results are shown in Table 3.

[0109] (Strength, Ductility) A tensile test was conducted to evaluate the strength and ductility of the above hot-rolled steel sheet. Specifically, first, a JIS No. 5 tensile test piece (JIS Z 2241) was taken from the above hot-rolled steel sheet in a direction parallel to the rolling direction. Next, using the above tensile test piece, a tensile test was conducted in accordance with the provisions of JIS Z 2241, and the tensile strength (TS) and uniform elongation (U.EL) were determined. The strain rate in the above tensile test was 10 －３ / s.

[0110] In this invention, for strength, a tensile strength (TS) of 980 MPa or higher was deemed acceptable. For ductility, a uniform elongation (U.EL) of 6% or higher was deemed acceptable.

[0111] (Fatigue Resistance Characteristics) The fatigue resistance characteristics of the hot-rolled steel sheet were evaluated by performing a planar bending fatigue test. Specifically, first, a test specimen with the dimensions and shape shown in Figure 1 was taken from the hot-rolled steel sheet so that the longitudinal direction of the test specimen was perpendicular to the rolling direction. Next, a planar bending fatigue test was performed using the test specimen in accordance with the provisions of JIS Z 2275. The stress loading mode was set to a stress ratio R = -1 and a frequency f = 25 Hz. The load stress amplitude was changed in six steps, and the stress cycle until fracture was measured. An S-N curve was obtained from the measurement results, and 2 × 10⁻⁶ ６ The fatigue intensity (fatigue limit) σw for each cycle was determined.

[0112] The fatigue limit ratio (σw / TS) was calculated by dividing the obtained fatigue limit σw by the tensile strength TS obtained in the tensile test described above. If the fatigue limit ratio (σw / TS) is 0.50 or higher, it was determined that the material has excellent fatigue resistance.

[0113] (Shear workability) The shear workability of the hot-rolled steel sheet was evaluated by actually performing shear processing. Specifically, first, a test piece was taken from the hot-rolled steel sheet. The dimensions of the test piece were 30 mm in width and 30 mm in length. The thickness of the test piece was the same as the original thickness of the hot-rolled steel sheet.

[0114] Next, the test specimen was sheared with a clearance of 15 ± 1% to form a sheared end surface. The clearance is a percentage of the plate thickness of the test specimen. The obtained sheared end surface was checked for defects such as cracks, chips, brittle fracture surfaces, and secondary shear surfaces.

[0115] Furthermore, hardness measurement samples were taken from shear end faces that did not contain the aforementioned defects, including the hardness measurement cross-section. The hardness of the portion at a distance of 0.1 mm or less from the end face and the matrix portion at a distance of 5.0 mm or more from the end face were measured. The hardness measurement conditions were a load of 100 g and a holding time of 10 s. Measurements were taken at five points with measurement intervals of 250 μm or more in the thickness direction of the plate, and the average value was evaluated.

[0116] In the above tests, if there were no end face defects and the increase in end face hardness was 100 HV or less, it was determined that the material had excellent shear workability.

[0117]

[0118]

[0119]

[0120] As can be seen from the results shown in Table 3, the hot-rolled steel sheets that met the conditions of the present invention had a tensile strength of 980 MPa or more and excellent ductility, fatigue resistance, and shear workability. In contrast, the hot-rolled steel sheets that did not meet the conditions of the present invention were inferior in at least one of the properties.

Claims

1. The composition has the following components in mass%, containing C: 0.03-0.15%, Si: 0.5-3.0%, Mn: 1.0-3.0%, P: 0.1% or less, S: 0.02% or less, Ti: 0.03-0.1%, Al: 0.01-1.0%, and N: 0.01% or less, with the remainder being Fe and unavoidable impurities, and having a component composition where the A value defined by the following formula (1) is 0-0.05, comprising an upper bainite phase and either or both of fresh martensite and retained austenite phases, wherein the area ratio of the upper bainite phase is 90% or more, the total area ratio of the fresh martensite and retained austenite phases is 10% or less, and the average aspect ratio of the prior austenite grains is 6.0 or less. The number density of MnS particles with a major axis of 10 μm or more in a cross-section parallel to the rolling direction is 20 particles / mm². ２ A hot-rolled steel sheet having the following microstructure, wherein the average particle size of Ti-based sulfides is 2.0 μm or less: A = Ti - 48 / 14 × N - 48 / 32 × S …(1) Here, each element symbol in equation (1) above represents the content (mass %) of each element.

2. The hot-rolled steel sheet according to claim 1, wherein the component composition further comprises at least one selected from the group consisting of, in mass%, V: 0.005 to 0.5%, Nb: 0.005 to 0.1%, Cu: 0.005 to 0.5%, Ni: 0.005 to 0.5%, Cr: 0.005 to 1.0%, Mo: 0.005 to 0.5%, B: 0.0002 to 0.005%, Sb: 0.001 to 0.1%, Sn: 0.001 to 0.1%, Ca: 0.0005 to 0.01%, Mg: 0.0005 to 0.01%, and REM: 0.0005 to 0.01%.

3. A method for manufacturing a hot-rolled steel sheet according to claim 1 or 2, comprising: casting molten steel having the above-mentioned component composition with an induction electromagnetic stirring device while rotating it in a horizontal plane relative to a mold at a rotational speed of 10 cm / s or more to obtain a steel material; heating the steel material to a heating temperature of 1150°C or higher; rough rolling the heated steel material to obtain a steel sheet; finishing rolling the steel sheet under the conditions of a total reduction ratio of 50% or less in the temperature range of (RC + 200°C) or lower, and a finishing rolling completion temperature of RC or higher and (RC + 200°C) or lower; cooling the steel sheet after finishing rolling under the conditions of an average cooling rate of 30°C / s or higher from the finishing rolling completion temperature to Bs, and a cooling stop temperature of (Bs - 150°C) or higher and Bs or lower; and winding the cooled steel sheet under the conditions of a winding temperature of (Bs - 150°C) or higher and Bs or lower. A method for manufacturing hot-rolled steel sheets, comprising cooling the rolled steel sheet at an average cooling rate of 1°C / s or less to a cooling stop temperature of (Bs - 450°C) or less. Here, RC and Bs are defined by the following equations (2) and (3), respectively. RC (°C) = 750 + 120 × C + 100 × N + 10 × Mn + 250 × Ti + 5000 × B + 10 × Cr + 50 × Mo + 750 × Nb + 150 × V ... (2) Bs (°C) = 830 - 270 × C - 90 × Mn - 70 × Cr - 37 × Ni - 83 × Mo - 20 × Cu ... (3) In the above equations (2) and (3), each element symbol represents the content (mass%) of the element, and 0 is used if the element is not contained.

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