High-strength hot-rolled steel sheet and method for manufacturing high-strength hot-rolled steel sheet

Abstract

The high-strength hot-rolled steel sheet of the present application has a specific composition, and the steel structure has 80 to 100% of martensite and bainite as main phases in terms of total area ratio, the total area ratio of the martensite dispersed in the bainite is 2 to 20%, and in the martensite dispersed in the bainite, the area ratio of the martensite having an orientation difference of less than 15° from the crystal orientation of at least one bainite adjacent to the martensite is 50% or more with respect to all the martensite dispersed in the bainite.

Description

TECHNICAL FIELD

[0001] The present application relates to a high-strength hot-rolled steel sheet suitable as a raw material for an automobile part and a manufacturing method of a high-strength hot-rolled steel sheet. BACKGROUND

[0002] From the viewpoint of improving the collision safety of automobiles and improving fuel efficiency, for a steel sheet for an automobile part, high-strength is required. On the other hand, with a high-strength steel sheet, crack generation due to insufficient workability at press forming becomes significant, and thus, it is necessary to improve the press forming process and the workability of the steel sheet. For a hot-rolled steel sheet of more than 980 MPa grade, in order to be applied to a part of a complex shape, particularly, high ductility is required. In addition, since end face cracks at material shearing and cracks at hole expansion processing are likely to occur, excellent end face crack resistance and flange extension are required.

[0003] In response to these requirements, various hot-rolled steel sheets such as those of patent literatures 1 to 3 have been developed.

[0004] Patent literature 1 discloses a technology regarding a hot-rolled steel sheet having a tensile strength (TS) of 780 MPa or more, in which the hot-rolled steel sheet has a specific composition and a structure having a bainite phase of more than 95% by area ratio in the entire region in the thickness direction, and an average grain size of the bainite phase in a region from the surface to a position of 1 / 4 of the thickness in the thickness direction is 5 μm or less in a thickness section parallel to the rolling direction, 4 μm or less in a thickness section in a direction at right angles to the rolling direction, and the number of grains elongated in the rolling direction having an aspect ratio of 5 or more in a region of a width of 1 / 10 of the thickness centered on the center position in the thickness direction is 7 or less, thereby improving the blanking workability.

[0005] Patent literature 2 describes a hot-rolled steel sheet having a specific chemical composition, a number density of grain boundaries of solid solution C of 1 / nm 2 or more and 4.5 / nm 2 Hereinafter, the size of a cementite grain precipitated at a grain boundary in a steel sheet is 1 μm or less. Patent literature 2 discloses a technology regarding a hot-rolled steel sheet having a TS of 540 MPa or more without a fracture surface crack by controlling the solid solution C and the grain boundary cementite.

[0006] Patent Document 3 describes a hot-rolled steel sheet having a specific chemical composition, containing 50% or more by area ratio of grain boundaries of contiguous grains having an orientation difference of 15° or more and an average of 0 to 0.5° within the grains, and a total of 2% or more and 10% or less by area fraction of martensite and tempered martensite and residual austenite, and 40% or more by mass of Ti expressed by a specific formula is present in the form of Ti carbide, and the mass of Ti carbide having an equivalent circle grain diameter of 7 nm or more and 20 nm or less is 50% or more of the mass of the entire Ti carbide. Patent Document 3 discloses a technology relating to a hot-rolled steel sheet having improved ductility by controlling the orientation difference within the grains.

[0007] Prior Art Documents

[0008] Patent Documents

[0009] Patent Document 1: Japanese Patent Application Publication No. 2012-62562

[0010] Patent Document 2: International Publication No. 2008 / 123366

[0011] Patent Document 3: Japanese Patent Application Publication No. 2016-204690 SUMMARY

[0012] PROBLEMS TO BE SOLVED BY THE INVENTION

[0013] However, the technology of Patent Document 1 improves the Ra of the punched fracture surface of the hot-rolled steel sheet (improves the punchability), but does not disclose insights for suppressing the generation of cracks, and does not specifically evaluate the hole expandability, leaving room for improvement. The technology of Patent Document 2 only confirms the presence or absence of cracks on the end surface of the member under specific conditions, and cannot be said to stably improve the cracks on the end surface of the member for variations in the gap, leaving room for improvement. The technology of Patent Document 3 can improve the ductility, but on the other hand, does not make any studies on the end surface crack resistance, leaving room for improvement.

[0014] The present application aims to solve the above problems, and provides a high-strength hot-rolled steel sheet suitable as a raw material for automobile parts, having excellent ductility, excellent end surface crack resistance, and excellent hole expandability, and a method for manufacturing a high-strength hot-rolled steel sheet.

[0015] In the present application, "high strength" means TS of 980 MPa or more. In the present application, "excellent ductility" means uniform elongation of 5.0% or more in a tensile test. In the present application, "excellent end-face crack resistance" means that the range of clearance in which no cracks parallel to the plate surface of the end face of a sample occur is 10% or more in the sample punched at 5% intervals up to a clearance of 5 to 30% in a punching test described later. In the present application, "excellent hole expansion property" means that the hole expansion ratio is 40% or more in a hole expansion test described later.

[0016] Note that, in the present application, the tensile test for measuring TS and uniform elongation, the punching test, and the hole expansion test described above can be performed by the methods described in the Examples described later.

[0017] Method for solving the problem

[0018] The present inventors, in order to solve the above problem, focused on the fact that although ductility is improved, the end-face crack resistance and hole expansion property are reduced by the hard phase, and conceived that the end-face crack resistance can be improved by controlling the fraction and crystal orientation of the hard phase. As a result, on the basis of adjusting the chemical composition of the hot-rolled steel sheet to a specific range, the martensite and bainite are made the main phase, and a certain amount of martensite is dispersed in the bainite, and in the case where the crystal orientation of the martensite dispersed in the bainite is close to the crystal orientation of the bainite around the martensite (the bainite adjacent to the martensite), the end-face crack resistance is not easily reduced, and high hole expansion property can be obtained. The above insight was found, and thus the present application was completed.

[0019] The gist of the present application is as follows.

[0020] [1] A high-strength hot-rolled steel sheet having a composition consisting of, in mass%, C: 0.04 to 0.18%, Si: 0.1 to 3.0%, Mn: 0.5 to 3.5%, P: more than 0% and 0.100% or less, S: more than 0% and 0.020% or less, Al: more than 0% and 1.5% or less, further containing one or two or more selected from Cr: 0.005 to 2.0%, Ti: 0.005 to 0.20%, Nb: 0.005 to 0.20%, Mo: 0.005 to 2.0%, V: 0.005 to 1.0%, the balance consisting of Fe and unavoidable impurities,

[0021] the steel structure having, as the main phase, 80 to 100% of martensite and bainite in terms of total area ratio,

[0022] the total area ratio of the martensite dispersed in the bainite being 2 to 20%,

[0023] The area ratio of the martensite having a crystal orientation difference of less than 15° from the crystal orientation of at least one of the adjacent bainite in the bainite in which the martensite is dispersed is 50% or more with respect to the total martensite dispersed in the bainite.

[0024] [2] The high-strength hot-rolled steel sheet according to the above-mentioned [1], wherein, on the basis of the above-mentioned component composition, one or two or more selected from the group consisting of Cu: 0.05 to 4.0%, Ni: 0.005 to 2.0%, B: 0.0002 to 0.0050%, Ca: 0.0001 to 0.0050%, REM: 0.0001 to 0.0050%, Sb: 0.0010 to 0.10%, Sn: 0.0010 to 0.50% are contained in mass%.

[0025] [3] A method of manufacturing a high-strength hot-rolled steel sheet according to the above-mentioned [1] or [2], wherein,

[0026] a steel billet having the above-mentioned component composition is heated,

[0027] then, at the time of performing hot-rolling,

[0028] coarse rolling is performed at 1100°C or higher with 3 or more passes and a reduction ratio of 15% or more per 1 pass, finish rolling is performed under the conditions that the total reduction ratio at 1000°C or lower is 50% or more and the total number of passes at 1000°C or lower is 3 or more, then, the steel sheet is cooled for 1.0 s or more, then, cooling is performed under the conditions that the average cooling speed from the cooling start temperature to 550°C is 50°C / s or more, and then, coiling is performed at a coiling temperature of (Ms point - 50) °C to 550°C.

[0029] Effects of the Invention

[0030] According to the present application, a high-strength hot-rolled steel sheet and a method of manufacturing a high-strength hot-rolled steel sheet, which are suitable as a raw material for automobile parts, and which are excellent in ductility, end-face crack resistance, and hole expansion property, can be provided. When the high-strength hot-rolled steel sheet of the present application is used as a raw material for automobile parts, a high-strength automobile part or the like can be obtained without generating a processing crack. DETAILED DESCRIPTION

[0031] Hereinafter, the high-strength hot-rolled steel sheet and the method of manufacturing a high-strength hot-rolled steel sheet of the present application will be described in detail. Note that the present application is not limited to the following embodiments.

[0032] <High-strength hot-rolled steel sheet>

[0033] The high-strength hot-rolled steel sheet of the present application is a hot-rolled steel sheet called black skin in a hot-rolled state or white skin further subjected to pickling after hot-rolling. In addition, the high-strength hot-rolled steel sheet of the present application as a target is preferably 0.6 mm or more and 10.0 mm or less in sheet thickness, and more preferably 1.0 mm or more and 6.0 mm or less in the case of use as a raw material for an automobile member. In addition, the sheet width is preferably 500 mm or more and 1800 mm or less, and more preferably 700 mm or more and 1400 mm or less.

[0034] The high-strength hot-rolled steel sheet of the present application has a specific composition and a specific steel structure. Here, the composition and the steel structure are described in order.

[0035] First, the composition of the high-strength hot-rolled steel sheet of the present application is described. Note that "%" indicating the content of the composition means "mass %".

[0036] The composition of the high-strength hot-rolled steel sheet of the present application contains, in mass %, C: 0.04 to 0.18 %, Si: 0.1 to 3.0 %, Mn: 0.5 to 3.5 %, P: more than 0 % and 0.100 % or less, S: more than 0 % and 0.020 % or less, Al: more than 0 % and 1.5 % or less, and further contains one or two or more selected from Cr: 0.005 to 2.0 %, Ti: 0.005 to 0.20 %, Nb: 0.005 to 0.20 %, Mo: 0.005 to 2.0 %, V: 0.005 to 1.0 %, with the balance consisting of Fe and inevitable impurities.

[0037] C: 0.04 to 0.18 %

[0038] C is an element effective for increasing TS by generating and strengthening bainite and martensite. When the C content is less than 0.04 %, such effects cannot be sufficiently obtained, and TS of 980 MPa or more cannot be obtained. On the other hand, when the C content exceeds 0.18 %, hardening of martensite becomes significant, and the end face crack resistance and the hole expandability of the present application cannot be obtained. Therefore, the C content is set to 0.04 to 0.18 %. From the viewpoint of more stably obtaining TS of 980 MPa or more, the C content is preferably set to 0.05 % or more. From the viewpoint of improving the end face crack resistance and the hole expandability, the C content is preferably set to 0.16 % or less, and more preferably set to 0.10 % or less.

[0039] Si: 0.1 to 3.0 %

[0040] Si is an element effective for increasing the TS by solid solution strengthening of the steel or suppressing temper softening of the martensite. In addition, Si is an element effective for suppressing cementite to obtain a structure in which the martensite is dispersed in the bainite. In order to obtain such an effect, the Si content needs to be set to 0.1% or more. On the other hand, when the Si content exceeds 3.0%, the polygonal ferrite is excessively generated and the steel structure of the present application cannot be obtained. Therefore, the Si content is set to 0.1 to 3.0%. The Si content is preferably set to 0.2% or more. In addition, the Si content is preferably set to 2.0% or less, and more preferably set to 1.5% or less.

[0041] Mn: 0.5 to 3.5%

[0042] Mn is an element effective for increasing the TS by generating the martensite and the bainite. When the Mn content is less than 0.5%, such an effect cannot be sufficiently obtained and the polygonal ferrite or the like is generated, and the steel structure of the present application cannot be obtained. On the other hand, when the Mn content exceeds 3.5%, the bainite is suppressed and the steel structure of the present application cannot be obtained. Therefore, the Mn content is set to 0.5 to 3.5%. From the viewpoint of more stably obtaining the TS of 980 MPa or more, the Mn content is preferably set to 1.0% or more. From the viewpoint of stably obtaining the bainite, the Mn content is preferably set to 3.0% or less, and more preferably set to 2.3% or less.

[0043] P: more than 0% and 0.100% or less

[0044] P decreases the end face crack resistance, and thus the amount thereof is preferably reduced as much as possible. In the present application, the P content can be allowed to be 0.100%. Therefore, the P content is set to 0.100% or less, and preferably set to 0.030% or less. The P content is set to more than 0%, and when the P content is less than 0.001%, production efficiency is reduced, and thus is preferably 0.001% or more.

[0045] S: more than 0% and 0.020% or less

[0046] S decreases the end face crack resistance, and thus the amount thereof is preferably reduced as much as possible. In the present application, the S content can be allowed to be 0.020%. Therefore, the S content is set to 0.020% or less, and preferably set to 0.0050% or less, and more preferably set to 0.0020% or less. The S content is set to more than 0%, and when the S content is less than 0.0002%, production efficiency is reduced, and thus is preferably 0.0002% or more.

[0047] Al: more than 0% and 1.5% or less

[0048] Al functions as a deoxidizer and is preferably added in the deoxidizing step. The lower limit value of the Al content is set to be greater than 0%, and the Al content is preferably 0.01% or greater from the viewpoint of use as a deoxidizer. When Al is contained in a large amount, a large amount of polygonal ferrite is generated, and the steel structure of the present application cannot be obtained. In the present application, the Al content can be allowed to be 1.5%. Therefore, the Al content is set to be 1.5% or less. It is preferably set to be 0.50% or less.

[0049] one or two or more kinds selected from Cr: 0.005 to 2.0%, Ti: 0.005 to 0.20%, Nb: 0.005 to 0.20%, Mo: 0.005 to 2.0%, V: 0.005 to 1.0%

[0050] Cr, Ti, Nb, Mo and V are effective elements for obtaining a structure in which martensite is dispersed in bainite. In order to obtain such an effect, the content of one or two or more kinds of elements selected from the above-described elements needs to be the lower limit value or more of each. On the other hand, when the content of one or two or more kinds of elements selected from the above-described elements exceeds the upper limit value of each, such an effect cannot be obtained, and the steel structure of the present application cannot be obtained. Therefore, one or two or more kinds selected from Cr: 0.005 to 2.0%, Ti: 0.005 to 0.20%, Nb: 0.005 to 0.20%, Mo: 0.005 to 2.0%, V: 0.005 to 1.0% are contained. In the case where the above-described elements are contained, it is preferable that Cr: 0.1% or more, Ti: 0.010% or more, Nb: 0.010% or more, Mo: 0.10% or more, V: 0.10% or more be respectively set. The upper limit when the above-described elements are contained is preferably Cr: 1.0% or less, Ti: 0.15% or less, Nb: 0.10% or less, Mo: 1.0% or less, V: 0.5% or less, respectively.

[0051] The balance is Fe and inevitable impurities. As the inevitable impurity elements, for example, N can be listed, and the allowable upper limit of this element is preferably 0.010%.

[0052] The above-described components are the basic component composition of the high-strength hot-rolled steel sheet of the present application. In the present application, the following elements can also be contained as necessary.

[0053] one or two or more kinds selected from Cu: 0.05 to 4.0%, Ni: 0.005 to 2.0%, B: 0.0002 to 0.0050%, Ca: 0.0001 to 0.0050%, REM: 0.0001 to 0.0050%, Sb: 0.0010 to 0.10%, Sn: 0.0010 to 0.50%

[0054] Cu, Ni are effective elements for generating martensite and contributing to high strength. In order to obtain such effects, in the case of containing Cu, Ni, it is preferable to set the content of each of them to be above the above lower limit value. When the content of each of Cu, Ni exceeds the above upper limit value, sometimes the steel structure of the present application is not obtained because the bainite is suppressed. The content of Cu is more preferably set to be 0.10% or more, and more preferably set to be 0.6% or less. The content of Ni is more preferably set to be 0.1% or more, and more preferably set to be 0.6% or less.

[0055] B is an effective element for improving the hardenability of the steel sheet, generating martensite and contributing to high strength. In order to obtain such effects, in the case of containing B, it is preferable to set the content of B to be 0.0002% or more. On the other hand, when the content of B exceeds 0.0050%, B compounds increase and the hardenability decreases, and sometimes the steel structure of the present application is not obtained. Therefore, in the case of containing B, it is preferable to set the content to be 0.0002 to 0.0050%. The content of B is more preferably set to be 0.0005% or more, and more preferably set to be 0.0040% or less.

[0056] Ca, REM (rare earth metal) are effective elements for improving workability through the morphology control of inclusions. In order to obtain such effects, in the case of containing Ca, REM, it is preferable to set the content of each of them to be Ca: 0.0001 to 0.0050%, REM: 0.0001 to 0.0050%. When the content of each of Ca, REM exceeds the above upper limit value, sometimes the amount of inclusions increases and the workability deteriorates. The content of Ca is more preferably set to be 0.0005% or more, and more preferably set to be 0.0030% or less. The content of REM is more preferably set to be 0.0005% or more, and more preferably set to be 0.0030% or less.

[0057] Sb is an effective element for suppressing the decrease in strength of the steel by suppressing denitrogenation, deboronization, etc. In order to obtain such effects, in the case of containing Sb, it is preferable to set the content of Sb to be 0.0010 to 0.10%. When the content of Sb exceeds the above upper limit value, sometimes the steel sheet is embrittled. The content of Sb is more preferably set to be 0.0050% or more, and more preferably set to be 0.050% or less.

[0058] Sn is an effective element for suppressing the decrease in strength of the steel by suppressing the generation of pearlite. In order to obtain such effects, in the case of containing Sn, it is preferable to set the content of Sn to be 0.0010 to 0.50%. When the content of Sn exceeds the above upper limit value, sometimes the steel sheet is embrittled. The content of Sn is more preferably set to be 0.0050% or more, and more preferably set to be 0.050% or less.

[0059] Note that even if the contents of Cu, Ni, B, Ca, REM, Sb, and Sn are less than the above lower limit values, the effects of the present application are not impaired. Therefore, in the case where the contents of these components are less than the above lower limit values, these elements are treated as unavoidable impurities.

[0060] Next, the steel structure of the high-strength hot-rolled steel sheet of the present application will be described.

[0061] The steel structure of the high-strength hot-rolled steel sheet of the present application has 80 to 100% of martensite and bainite as main phases in terms of the total area ratio, and the total area ratio of the martensite dispersed in the bainite is 2 to 20%. In the martensite dispersed in the bainite, the area ratio of the martensite having a crystal orientation difference of less than 15° from the crystal orientation of at least one of the bainites adjacent to the martensite is 50% or more with respect to the entire martensite dispersed in the bainite.

[0062] Total area ratio of martensite and bainite: 80 to 100%

[0063] In the present application, in order to have high TS and excellent end face crack resistance and hole expandability, a steel structure mainly having martensite and bainite (having martensite and bainite as main phases) is made.

[0064] When the total area ratio of the martensite and the bainite is less than 80% with respect to the entire steel sheet structure, at least any one of high TS, end face crack resistance, and hole expandability is not obtained. Therefore, the total area ratio of the martensite and the bainite is set to 80 to 100%, preferably 83 to 100%, and more preferably 88 to 100%.

[0065] Total area ratio of the martensite dispersed in the bainite: 2 to 20%

[0066] The martensite is a steel structure effective for improving TS, and is a steel structure effective for improving uniform elongation by being dispersed in the bainite. In order to obtain such effects, it is necessary to set the total area ratio of the martensite dispersed in the bainite to 2% or more. On the other hand, when the total area ratio of the above-described martensite exceeds 20%, at least any one of uniform elongation, end face crack resistance, and hole expandability is not obtained. Therefore, the total area ratio of the above-described martensite is set to 2 to 20%. The total area ratio of the above-described martensite is preferably set to 3% or more, and more preferably 4% or more. The total area ratio of the above-described martensite is preferably set to 15% or less, and more preferably 12% or less.

[0067] Area ratio of the martensite having a crystal orientation difference of less than 15° from the crystal orientation of at least one of the bainites adjacent to the martensite in the martensite dispersed in the bainite: 50% or more with respect to the entire martensite dispersed in the bainite

[0068] By setting the area ratio of the martensite (hereinafter, sometimes referred to as "martensite dispersed phase") having an orientation difference of less than 15° between the crystal orientation of the martensite and the crystal orientation of at least one of the bainites adjoining the martensite to be 50% or more with respect to the area of all the martensites dispersed in the bainite, the end face crack resistance is improved. As a result, the hole expansion ratio of the present application can be obtained.

[0069] Here, the above-mentioned "martensite having an orientation difference of less than 15° between the crystal orientation of the martensite and the crystal orientation of at least one of the bainites adjoining the martensite" means, for example, when there is a martensite surrounded by a plurality of crystal-oriented bainites, one or more of the plurality of crystal-oriented bainites can have an orientation difference of less than 15° from the martensite.

[0070] The detailed reason is not clear, but in the case where the martensite dispersed in the bainite has a crystal orientation close to the bainite around the martensite (the bainite adjoining the martensite), the deformation of the bainite and the martensite follow each other at the time of shearing, thereby being able to suppress the formation of voids. Therefore, it is considered that the end face crack resistance is improved, etc.

[0071] For such a reason, in the present application, the area ratio of the above-mentioned martensite dispersed phase is set to be 50% or more. By setting the martensite having a small orientation difference that can suppress the formation of voids to be 50% or more, the void connection suppression effect becomes large, and the cracks can be significantly suppressed.

[0072] Therefore, in the martensite dispersed in the bainite, the area ratio of the above-mentioned martensite dispersed phase is set to be 50% or more with respect to all the martensites dispersed in the bainite. It is preferable to be set to be 60% or more, and more preferable to be set to be 70% or more. The upper limit of the above-mentioned area ratio is not particularly specified. It is preferable to be set to be 99% or less, and more preferable to be set to be 98% or less.

[0073] Here, the martensite dispersed phase can be found by the method described in the examples described later. First, the crystal orientations of the bainite and the martensite are found by the electron backscatter diffraction method (EBSD), and by showing the boundary of the orientation difference of 15° or more, the area ratio of the martensite having an orientation difference of less than 15° between the crystal orientation of the above-mentioned martensite and the crystal orientation of at least one of the bainites adjoining the martensite (adjoining bainite) is found.

[0074] Note that the other organization than the above-mentioned martensite and bainite is ferrite, pearlite, and residual austenite. The total area ratio of the other organization than the martensite and the bainite is set to be less than 20%. When the total area ratio is less than 20%, the characteristics of the present application can be achieved.

[0075] In the present application, the area ratio of each of the above-described structures, the crystal orientation of the martensite and the bainite can be measured by the method described in the Examples and the like described later.

[0076] <Manufacturing method of high-strength hot-rolled steel sheet>

[0077] The high-strength hot-rolled steel sheet of the present application is manufactured by heating a steel billet having the above-described composition, and then performing hot-rolling. In the above-described hot-rolling, the heated steel billet is subjected to rough rolling at a reduction ratio of 15% or more per pass at 1100°C or more for 3 passes or more, is subjected to finish rolling under the conditions that the total reduction ratio at 1000°C or less is 50% or more and the total number of passes at 1000°C or less is 3 or more, is then cooled for 1.0 s or more, is then cooled at an average cooling rate of 50°C / s or more from the cooling start temperature to 550°C, and is then coiled at a coiling temperature of (Ms point - 50)°C to 550°C, and is then cooled to room temperature.

[0078] Hereinafter, the manufacturing method will be described in detail. Note that the above-described temperatures are the temperatures of the width central portion of the steel billet and the steel sheet (surface temperature), and the above-described average cooling rate is the average cooling rate of the width central portion of the steel sheet. These temperatures can be measured using a radiation thermometer or the like.

[0079] Number of passes at 1100°C or more: 3 or more

[0080] In the rough rolling of the hot-rolling, by setting the number of passes at 1100°C or more to 3 or more, the austenite grains are fully refined and the inhomogeneity is eliminated, and the area ratio of the martensite having an orientation difference of less than 15° from the crystal orientation of at least one of the bainites adjacent to the martensite among the martensites dispersed in the bainite is stably 50% or more with respect to all of the martensites dispersed in the bainite. When the number of passes at 1100°C or more is less than 3, such an effect cannot be sufficiently obtained. Therefore, the number of passes at 1100°C or more is set to 3 or more. The number of passes at 1100°C or more is preferably set to 4 or more, and more preferably set to 5 or more. The upper limit of the number of passes at 1100°C or more is not particularly limited, but when it exceeds 15, it sometimes leads to an increase in scale loss and the like, which hinders the manufacturability, and therefore it is preferably set to 15 or less.

[0081] Reduction ratio per pass at 1100°C or more: 15% or more

[0082] In the rough rolling of hot rolling, when the reduction ratio per 1 pass at 1100°C or higher is less than 15%, the non-uniformity of austenite grains is not only not eliminated, but rather deteriorated, and the martensite having the characteristics of the crystal orientation cannot be sufficiently obtained. Therefore, the reduction ratio per 1 pass at 1100°C or higher is set to 15% or more. The reduction ratio per 1 pass at 1100°C or higher is preferably set to 18% or more, and more preferably set to 20% or more. The upper limit of the reduction ratio per 1 pass at 1100°C or higher is not particularly specified, but when it exceeds 60%, it sometimes leads to deterioration of the shape of the sheet and manufacturing failure, and thus is preferably 60% or less.

[0083] Total reduction ratio at 1000°C or lower: 50% or more

[0084] By setting the total reduction ratio at 1000°C or lower in the finish rolling of hot rolling to 50% or more, the martensite dispersed phase having the crystal orientation of the above-described application (i.e., the orientation difference between the crystal orientation of the martensite and at least one of the crystal orientations of the bainite adjacent to the martensite is less than 15°) is 50% or more with respect to the total martensite phase dispersed in the bainite.

[0085] The detailed reason is not clear, but it is presumed that each orientation selection at the time of bainite transformation and at the time of martensite transformation in the austenite is limited by the reduction under the above-described condition. Therefore, the total reduction ratio at 1000°C or lower in the finish rolling of hot rolling is set to 50% or more. It is preferably set to 60% or more. The upper limit of the total reduction ratio is not particularly specified. When the total reduction ratio is too large, the texture sometimes develops and the workability such as hole expansion is impaired, and thus it is preferably set to 90% or less.

[0086] Here, the total reduction ratio refers to the percentage of the value obtained by dividing the difference between the entry sheet thickness before the initial pass in the above-described temperature range and the exit sheet thickness after the final pass in the temperature range by the entry sheet thickness before the initial pass.

[0087] That is, it is obtained by (entry sheet thickness before the initial pass in the above-described temperature range - exit sheet thickness after the final pass in the temperature range) / (entry sheet thickness before the initial pass in the above-described temperature range) x 100 (%).

[0088] Total number of passes at 1000°C or lower: 3 or more

[0089] By dispersing the reduction in the finishing rolling of the hot-rolled steel sheet at 1000°C or less into a plurality of times, reducing the reduction per pass, it is easy to generate martensite having a crystal orientation close to that of the bainite (i.e., martensite having an orientation difference of less than 15° from the crystal orientation of at least one of the adjoining bainites). When the total number of passes is 3 or more, the steel structure of the present application (i.e., the area ratio of the martensite having an orientation difference of less than 15° from the crystal orientation of at least one of the adjoining bainites with respect to the total martensite dispersed in the bainite is 50% or more) can be obtained. It is preferable to set it to 4 or more. The upper limit of the total number of passes is not particularly specified. From the viewpoint of production efficiency and the like, it is preferable to set it to 10 or less.

[0090] Finishing rolling end temperature (appropriate condition)

[0091] The finishing rolling end temperature is preferably set to 750 to 1000°C. By controlling it to 750 to 1000°C, it is easy to obtain a stable surface property. It is more preferable to be 780°C or more, and more preferable to be 950°C or less.

[0092] Cooling time after finishing rolling: 1.0 s or more

[0093] When the cooling time after finishing rolling is less than 1.0 s (second), it is not possible to make the area ratio of the crystal-oriented martensite dispersed phase of the present application with respect to the total martensite phase dispersed in the bainite 50% or more. The reason is not clear, but it is considered that, due to cooling, a part of the dislocations introduced in the finishing rolling is recovered, which affects the orientation selection at the time of the subsequent bainite transformation and martensite transformation. Therefore, the cooling time after finishing rolling is set to 1.0 s or more. It is preferably set to 1.5 s or more. The upper limit of the cooling time is not particularly specified. Cooling of 10 s or more sometimes leads to the generation of a structure such as ferrite, which is not preferable in the present application, and therefore the cooling time is preferably set to 10 s or less.

[0094] Average cooling rate from the cooling start temperature to 550°C: 50°C / s or more

[0095] When the average cooling rate from the cooling start temperature to 550°C is less than 50°C / s, ferrite and pearlite are generated, and the steel structure of the present application cannot be obtained. Therefore, the average cooling rate from the cooling start temperature to 550°C is set to 50°C / s or more. It is preferably set to 80°C / s or more. The upper limit of the average cooling rate is not particularly specified, and from the viewpoint of shape stability of the steel sheet and the like, the average cooling rate is preferably set to 1000°C / s or less.

[0096] Note that when the cooling start temperature is lower than 700°C, ferrite is easily generated, and thus the cooling start temperature is preferably set to 700°C or higher. More preferably, it is set to 720°C or higher. Also, since it is technically difficult to set the cooling start temperature higher than the finish rolling end temperature, the cooling start temperature is preferably set to be lower than the finish rolling end temperature.

[0097] Coiling temperature: (Ms point - 50) °C to 550°C

[0098] When the coiling temperature is lower than (Ms point - 50) °C, martensite increases, and the steel structure of the present application cannot be obtained. On the other hand, when it exceeds 550°C, ferrite and pearlite are generated, and the steel structure of the present application cannot be obtained. Thus, the coiling temperature is set to (Ms point - 50) °C to 550°C. Preferably, it is (Ms point - 30) °C or higher, and preferably 520°C or lower.

[0099] Here, the Ms point refers to the martensite transformation start temperature, and can be determined by measurement of thermal expansion and resistance during cooling based on Formaster test and the like.

[0100] Note that the manufacturing method is not particularly limited except for the above conditions, but it is preferably manufactured by adjusting appropriate conditions as follows.

[0101] For example, from the viewpoint of segregation removal, precipitate solid solution, and the like, the billet heating temperature is preferably 1100°C or higher, and from the viewpoint of energy efficiency and the like, the billet heating temperature is preferably 1300°C or lower.

[0102] Also, from the viewpoint of reducing coarse grains and the like that cause a decrease in workability, the finish rolling is preferably set to 4 passes or more. Note that the number of passes of this finish rolling refers to the total number of passes in the finish rolling, including the above "total number of passes at 1000°C or lower".

[0103] Example

[0104] Hereinafter, the present application will be further described based on examples. Note that the present application is not limited to the following examples.

[0105] Steels having the composition shown in Table 1 were melted by a vacuum melting furnace to manufacture billets. Then, these billets were heated to 1200°C, and hot-rolled under the conditions shown in Table 2 to obtain hot-rolled steel sheets. In the hot-rolling, the total number of passes of the finish rolling was set to 7 passes.

[0106] Note that the blank in Table 1 indicates that the element was not intentionally added, and includes not only the case where it is not contained (0%), but also the case where it is inevitably contained. Also, N is an inevitable impurity.

[0107] Using the obtained hot-rolled steel sheet, the following test method was used to perform microstructure observation, tensile property, end face crack resistance property, and hole expansibility evaluation.

[0108] Microstructure Observation

[0109] (Area ratio of each microstructure)

[0110] The area ratio of the martensite and the bainite refers to the proportion of the area of each microstructure in the observation area.

[0111] The area ratio of the martensite was calculated as follows.

[0112] A sample was cut from the obtained hot-rolled steel sheet, and a plate thickness cross section parallel to the rolling direction was polished and etched with 3% nitric acid ethanol solution. At the 1 / 4 position of the plate thickness, 3 fields of view were photographed at 1500 times magnification using SEM (scanning electron microscope). Using Image-Pro manufactured by Media Cybernetics, the area ratio of each microstructure was calculated from the image data of the obtained secondary electron image, and the average area ratio of the fields of view was set as the area ratio of each microstructure.

[0113] In the image data, upper bainite was distinguished as black or dark gray including carbide or martensite having a straight interface. Lower bainite was distinguished as black, dark gray, gray, or light gray including carbide having the same orientation. Martensite was distinguished as black, dark gray, gray, or light gray including carbide having multiple orientations, or white or light gray not including carbide. Retained austenite was distinguished as white or light gray not including carbide.

[0114] Martensite and retained austenite could not be distinguished at times, so the area ratio of the retained austenite calculated by the method described below was subtracted from the total area ratio of the martensite and the retained austenite calculated from the SEM image, and the area ratio of the martensite was calculated.

[0115] Note that in the present application, the martensite can be any one of fresh martensite, self-tempered martensite, tempered martensite, and the like. In addition, the bainite can be any one of upper bainite, lower bainite, tempered bainite, and the like.

[0116] The stronger the degree of tempering of the microstructure, the stronger the contrast of the image in which the matrix becomes black, so the color of the above matrix is the standard, and in the present application, the amount of carbide, the microstructure morphology, and the like are comprehensively judged, including the microstructure described below, and classified into any one of the microstructures having similar characteristics. The carbide is a point-like or linear shape that is white.

[0117] Further, in the present application, although ferrite is substantially not contained, ferrite can be distinguished as black structure, dark gray structure having no carbide or slightly having carbide inside, or dark gray structure having no straight line interface with martensite. Pearlite can be distinguished as black and white lamellar structure or partially interrupted lamellar structure.

[0118] To obtain the area ratio of residual austenite, after annealing the steel sheet, the steel sheet was ground to a position of 1 / 4 + 0.1 mm of the sheet thickness, and further ground by 0.1 mm by chemical polishing, and for the ground surface, the integrated reflection intensity of (200) plane, (220) plane, (311) plane of fcc iron (austenite) and (200) plane, (211) plane, (220) plane of bcc iron (ferrite) was measured using Mo Kα1 ray by an X-ray diffractometer. The volume ratio was obtained from the intensity ratio of the integrated reflection intensity of each plane from fcc iron to that of each plane from bcc iron, and was taken as the area ratio of residual austenite.

[0119] Using the obtained area ratio of each structure, the total area ratio of bainite and martensite, the total area ratio of other structures were obtained, and the total area ratio is shown in Table 3. Note that "V(M)" in Table 3 means the area ratio (%) of martensite, "V(B+M)" means the total area ratio (%) of bainite and martensite, and "V(O)" means the total area ratio (%) of other structures.

[0120] (Crystal orientation)

[0121] For the same field of view of the same sample used in the above structure observation, the crystal orientation of bainite and martensite was obtained by electron backscatter diffraction method (EBSD), and the boundary of the orientation difference of 15° or more was shown. Thereby, the area ratio of the martensite dispersed in the bainite, in which the crystal orientation difference between the martensite and at least one of the bainites adjacent to the martensite is less than 15°, was obtained. The proportion of the area ratio of the martensite in the total area ratio of the martensite was obtained. Note that the measurement of EBSD was performed for a region of 100 μm x 100 μm under the conditions of an acceleration voltage of 30 kV and a step of 0.05 μm.

[0122] The obtained above proportion is shown in Table 3. Note that "the proportion of M having an orientation difference of less than 15° with adjacent B" in Table 3 means the above proportion (%).

[0123] <Stretching test>

[0124] Evaluation of the tensile properties was performed by a stretching test. From the obtained hot-rolled steel sheet, JIS No. 5 tensile test pieces (JIS Z 2201) were cut out in a direction parallel to the rolling direction, and a stretching test was performed at a strain rate of 10-3 TS and the uniform elongation were found from the tensile test according to the regulation of JIS Z 2241.

[0125] Note that, in the present application, TS of 980 MPa or more and uniform elongation of 5.0% or more are respectively evaluated as passing.

[0126] <Blanking Test>

[0127] The evaluation of the end-face crack resistance was performed by a blanking test. From the obtained hot-rolled steel sheet, a test piece having a width of 150 mm and a length of 150 mm was cut out. For the test piece, three times of blanking was performed using a punch of Φ 10 mm under the conditions of a clearance of 5%, 10%, 15%, 20%, 25%, and 30%, and the presence or absence of a crack parallel to the sheet surface (plate surface) at the blanked end face was investigated, and the end-face crack resistance was evaluated. A case where the clearance range without a crack was 10% or more was evaluated as passing the end-face crack resistance.

[0128] For example, in the blanking test performed by the above-described method, in a case where the clearance without a crack was 10%, 15%, 20%, and 25%, the clearance range without a crack was the difference between the largest clearance of 25% and the smallest clearance of 10%, and was 15%.

[0129] <Hole Expanding Test>

[0130] The hole expanding property was evaluated by a hole expanding test. Using three test pieces blanked out under the condition of a clearance of 10% in the above-described blanking test, three times of hole expanding test was performed using a 60° conical punch according to JFST 1001 (Japan Iron and Steel Federation Standard, 2008), and the average hole expanding rate (%) was found as the hole expanding rate. A case where the hole expanding rate was 40% or more was evaluated as passing.

[0131] Various evaluation results are shown in Table 3.

[0132]

[0133]

[0134] [Table 3]

[0135]

[0136] ※ The underlined is outside the scope of the present application.

[0137] As is apparent from Table 3, the examples of the present application are all high-strength hot-rolled steel sheets having excellent uniform elongation, excellent end-face crack resistance, and excellent hole expanding property. On the other hand, the comparative examples outside the scope of the present application do not achieve any one or more of the desired strength, uniform elongation, end-face crack resistance, and hole expanding property.

[0138] Industrial applicability

[0139] According to the present application, a high-strength hot-rolled steel sheet having TS of 980 MPa or more, and excellent ductility, excellent end-face crack resistance, and excellent hole expandability can be obtained. By using the high-strength hot-rolled steel sheet of the present application for automobile part applications, it is possible to greatly contribute to improvement in collision safety and improvement in fuel efficiency of automobiles.

Claims

1. A high-strength hot-rolled steel sheet having a composition consisting of, in mass%, C: 0.04 to 0.18%, Si: 0.1 to 1.5%, Mn: 0.5 to 3.5%, P: more than 0% and 0.100% or less, S: more than 0% and 0.020% or less, Al: more than 0% and 0.50% or less, further containing one or two or more selected from the group consisting of Cr: 0.1 to 1.0%, Ti: 0.010 to 0.15%, Nb: 0.010 to 0.10%, Mo: 0.10 to 1.0%, V: 0.10 to 0.5%, the balance consisting of Fe and unavoidable impurities, the steel structure having, as main phases, 80 to 100% by area ratio of martensite and bainite, the total area ratio of the martensite dispersed in the bainite being 2 to 20%, the area ratio of the martensite dispersed in the bainite, of which crystal orientation has an orientation difference of less than 15° from the crystal orientation of at least one of the bainites adjoining the martensite, being 61% or more relative to the total martensite dispersed in the bainite.

2. The high-strength hot-rolled steel sheet according to claim 1, wherein, on the basis of the composition, one or two or more selected from the group consisting of, in mass%, Cu: 0.05 to 0.6%, Ni: 0.005 to 0.6%, B: 0.0002 to 0.0050%, Ca: 0.0001 to 0.0050%, REM: 0.0001 to 0.0050%, Sb: 0.0010 to 0.050%, Sn: 0.0010 to 0.050%.

3. A method of manufacturing a high-strength hot-rolled steel sheet according to claim 1 or 2, wherein a billet having the composition is heated, then, when hot-rolled, coarsely rolled at 3 or more passes with a reduction ratio of 18% or more per 1 pass at 1100°C or higher, and finish-rolled under the conditions that the total reduction ratio is 50% or more at 1000°C or lower and the total number of passes is 3 or more at 1000°C or lower, and then cooled for 1.0 s or more, and then cooled at an average cooling rate of 50°C / s or more from the cooling start temperature to 550°C, and then coiled at a coiling temperature of (Ms point - 50) °C to 550°C.

4. The method of manufacturing a high-strength hot-rolled steel sheet according to claim 3, wherein the billet is heated at a temperature of 1200°C or higher.

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