CN117295837A - High-strength hot-rolled steel sheet and method for producing high-strength hot-rolled steel sheet - Google Patents

High-strength hot-rolled steel sheet and method for producing high-strength hot-rolled steel sheet Download PDF

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CN117295837A
CN117295837A CN202280034028.XA CN202280034028A CN117295837A CN 117295837 A CN117295837 A CN 117295837A CN 202280034028 A CN202280034028 A CN 202280034028A CN 117295837 A CN117295837 A CN 117295837A
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martensite
less
steel sheet
bainite
rolled steel
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长谷川宽
木村英之
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JFE Steel Corp
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JFE Steel Corp
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    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/18—Hardening; Quenching with or without subsequent tempering
    • C21D1/19—Hardening; Quenching with or without subsequent tempering by interrupted quenching
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0205—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips of ferrous alloys
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    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
    • C21D8/0226—Hot rolling
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    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
    • C21D8/0263—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment following hot rolling
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    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
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    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
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    • C22C38/005—Ferrous alloys, e.g. steel alloys containing rare earths, i.e. Sc, Y, Lanthanides
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    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
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    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
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    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/34—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of silicon
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    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/38—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of manganese
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    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/50—Ferrous alloys, e.g. steel alloys containing chromium with nickel with titanium or zirconium
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    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/54—Ferrous alloys, e.g. steel alloys containing chromium with nickel with boron
    • C—CHEMISTRY; METALLURGY
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    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese
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    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/60—Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur
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    • C21—METALLURGY OF IRON
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    • C21D2201/00—Treatment for obtaining particular effects
    • C21D2201/05—Grain orientation
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
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    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/002—Bainite
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    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/008—Martensite

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Abstract

The high-strength hot-rolled steel sheet of the present invention has a specific composition, the steel structure has, as a main phase, 80 to 100% by total area ratio of martensite and bainite, the total area ratio of martensite in bainite is 2 to 20%, and the area ratio of martensite having a difference in crystal orientation between the martensite and at least one of the bainite adjacent to the martensite of 15 DEG or more with respect to the total martensite is more than 50%, and when a region surrounded by a boundary having a difference in crystal orientation of 15 DEG or more between adjacent crystals is defined as crystal grains, the average aspect ratio of the crystal grains existing from the surface of the steel sheet to a region having a depth of 5 [ mu ] m is 2 or less.

Description

High-strength hot-rolled steel sheet and method for producing high-strength hot-rolled steel sheet
Technical Field
The present invention relates to a high-strength hot-rolled steel sheet suitable as a material for automobile parts and a method for producing the high-strength hot-rolled steel sheet.
Background
From the viewpoints of improving collision safety and fuel efficiency of automobiles, steel sheets used for automobile parts are required to have high strength. On the other hand, in the case of a steel sheet having a high strength, cracks generated due to insufficient workability at the time of pressing become remarkable, and therefore, improvement in the pressing process and workability of the steel sheet is demanded. In hot-rolled steel sheets having a TS of greater than 980MPa, high ductility is particularly required for application to parts of complex shape such as lower arms. In addition, in many cases, the member formed into a complex shape through a plurality of steps requires formability in different deformation histories. Bending-bending recovery processing is a particularly common processing mode, and excellent bending-bending recovery is required.
In response to such a demand, various hot rolled steel sheets have been developed, for example, as in patent documents 1 to 3.
Patent document 1 discloses a technique related to zn—al-based coated steel sheet, wherein Al is provided on the surface of the steel sheet: in a coated steel sheet having a coating layer consisting essentially of Zn in the balance of 50 to 60 mass% and a coating film on the upper layer of the coating layer, bending recovery resistance is improved by satisfying HM > HP and HP > 90 by making the section hardness HM (HV) of a base material and the section hardness HP (HV) of the coating layer.
Patent document 2 describes a hot-rolled steel sheet having a structure in which ferrite is the main phase and retained austenite is the second phase, wherein the retained austenite is contained in an average of 5% by volume or more, the difference (Vmax-Vmin) between the maximum content Vmax and the minimum content Vmin of retained austenite at each position between 0.1mm from the front surface of the steel sheet and 0.1mm from the back surface of the steel sheet in the sheet thickness direction is 3.0% by volume or less, and the total elongation corresponding to a sheet thickness of 2mm is 34% or more. Patent document 2 discloses a technique related to a hot-rolled steel sheet having high total elongation and improved bend-bend recovery properties by forming a structure containing ferrite as a main phase and retained austenite.
Patent document 3 describes a hot-rolled steel sheet having a specific chemical composition, which contains grains having a grain boundary difference of 50% or more in terms of area ratio and adjacent grains of 15 ° or more and an average of orientation differences within the grains of 0 to 0.5 °, and in which the total of martensite, tempered martensite and retained austenite is 2% or more and 10% or less in terms of area ratio, and in which Ti represented by a specific formula, which is 40% or more by mass of Ti, is present in the form of Ti carbide, and the equivalent circle diameter of the Ti carbide is 7nm or more and 20nm or less, is 50% or more by mass of the total Ti carbide. Patent document 3 discloses a technique related to a hot-rolled steel sheet having improved ductility by controlling the difference in orientation within the crystal grains.
Prior art literature
Patent literature
Patent document 1: japanese patent laid-open No. 2008-156729
Patent document 2: japanese patent laid-open No. 2001-32041
Patent document 3: japanese patent laid-open publication 2016-204690
Disclosure of Invention
Problems to be solved by the invention
However, the technique of patent document 1 only researches bending recovery cracks at the plating start point, and does not research bending recovery cracks generated in a hot-rolled steel sheet without a plating layer. Patent document 2 discloses that strength of 900MPa or less is not improved in ductility and bending recovery at 980MPa level, which are more strictly required. The technique of patent document 3 can improve ductility, but on the other hand, there is room for improvement in bending-bending recovery without any study.
The present invention has been made to solve the above problems, and an object of the present invention is to provide a high-strength hot-rolled steel sheet and a method for producing the same, which are suitable as a material for automobile parts and have excellent ductility and excellent bend-bend recovery properties.
In the present invention, "high strength" means that TS (tensile strength) is 980MPa or more. In the present invention, "excellent ductility" means that the uniform elongation in the tensile test is 5.0% or more. In the present invention, the term "excellent bending-bending recovery" means that in the bending-bending recovery test described later, after a 90 ° V-shaped bending process is performed by a punch having a bending radius of 5mm, the bending recovery process is performed by a flat bottom punch to a bending angle of 10 ° or less, and no crack is generated in the ridge line of the test piece.
In the present invention, the tensile test for measuring the TS and the uniform elongation and the bend-bend recovery test can be performed by the methods described in examples described below.
Means for solving the problems
The present inventors have focused on the hard phase to improve the uniform elongation by controlling the fraction thereof to promote work hardening.
Further, it is conceivable that the bending-bending recovery is improved by controlling the crystal orientation of the hard phase and the aspect ratio of the crystal grains in the surface layer of the steel sheet when the region surrounded by the boundary where the orientation difference between adjacent crystals is 15 ° or more is defined as the crystal grains.
As a result, it has been found that, in addition to adjusting the chemical composition of a hot-rolled steel sheet to a specific range, martensite and bainite are used as main phases, martensite is dispersed in the bainite, the aspect ratio of crystal grains in the steel sheet surface layer is further reduced, and the crystal orientation of martensite in the bainite is controlled to be different from the crystal orientation of bainite around the martensite (bainite adjacent to the martensite). Thus, the present invention has been accomplished by improving both ductility and bend-bend recovery properties even for hot rolled steel sheets of 980MPa grade or more.
The gist of the present invention is as follows.
[1] A high-strength hot-rolled steel sheet having a composition containing, in mass%, C:0.04 to 0.18 percent of Si:0.1 to 3.0 percent of Mn: 0.5-3.5%, P: greater 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 comprises a metal selected from Cr: 0.005-2.0%, ti: 0.005-0.20%, nb: 0.005-0.20%, mo: 0.005-2.0%, V: 0.005-1.0% of one or more of Fe and unavoidable impurities in balance,
the steel structure takes martensite and bainite with the total area ratio of 80-100 percent as the main phase,
the total area ratio of martensite in the bainite is 2-20%,
among martensite in the bainite, martensite having a difference in crystal orientation between the martensite and at least one of the bainite adjacent to the martensite of 15 DEG or more has an area ratio of more than 50% with respect to the whole martensite,
when a region surrounded by a boundary where the orientation difference between adjacent crystals is 15 ° or more is defined as a crystal grain, the average aspect ratio of the crystal grain existing in a region from the surface of the steel sheet to a depth of 5 μm is 2.0 or less.
[2] The high-strength hot-rolled steel sheet according to the above [1], wherein the steel sheet contains, in mass%, at least one component selected from the group consisting of Cu:0.05 to 4.0 percent of Ni: 0.005-2.0%, B:0.0002 to 0.0050 percent, ca: 0.0001-0.0050%, REM:0.0001 to 0.0050 percent of Sb:0.0010 to 0.10 percent of Sn:0.0010 to 0.50% of one or more than two of the following components.
[3] A method for producing a high-strength hot-rolled steel sheet according to [1] or [2], wherein,
the billet having the above composition is heated,
next, when the hot rolling is carried out,
rough rolling is performed under conditions in which the total rolling reduction is 3 times or more at 1000 ℃ or more and less than 50% at 1000 ℃ or less and the total rolling reduction is 35% or less at the final pass rolling temperature to the final pass rolling temperature +50℃ and then cooling is started at less than 1.0s, cooling is performed under conditions in which the average cooling rate from the cooling start temperature to 550 ℃ is 50 ℃/s or more, and then coiling is performed at the coiling temperature of (Ms point-50) DEG C to 550 ℃.
Effects of the invention
According to the present invention, a high-strength hot-rolled steel sheet and a method for producing a high-strength hot-rolled steel sheet, which are suitable as a material for automobile parts and are excellent in ductility and bending-bending recovery properties, can be provided. When the high-strength hot-rolled steel sheet of the present invention is used as a material for automobile parts, products such as high-strength automobile parts having a complicated shape can be obtained.
Drawings
Fig. 1 is a schematic view for explaining the aspect ratio of the crystal grains of the present invention.
Detailed Description
Hereinafter, the high-strength hot-rolled steel sheet and the method for producing the high-strength hot-rolled steel sheet according to the present invention will be described in detail. The present invention is not limited to the following embodiments.
< high-Strength Hot rolled Steel sheet >)
The high-strength hot-rolled steel sheet of the present invention is a hot-rolled steel sheet called a black skin in a hot-rolled state or a white skin further subjected to pickling after hot rolling. The high-strength hot-rolled steel sheet according to the present invention preferably has a sheet thickness of 0.6mm or more and 10.0mm or less, and more preferably 1.0mm or more and 6.0mm or less when used as a material for automobile parts. The plate width is preferably 500mm to 1800mm, more preferably 700mm to 1400 mm.
The high-strength hot-rolled steel sheet of the present invention has a specific composition and a specific steel structure. The composition and the steel structure will be described in order.
First, the composition of the high-strength hot-rolled steel sheet according to the present invention will be described. The "%" indicating the content of the component composition means "% by mass".
The composition of the components of the high-strength hot-rolled steel sheet of the present invention contains, in mass%, C:0.04 to 0.18 percent of Si:0.1 to 3.0 percent of Mn: 0.5-3.5%, P: greater 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 comprises a metal selected from Cr: 0.005-2.0%, ti: 0.005-0.20%, nb: 0.005-0.20%, mo: 0.005-2.0%, V: 0.005-1.0% of one or more than two of Fe and unavoidable impurities in balance.
C：0.04～0.18％
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 an effect cannot be sufficiently obtained, and TS of 980MPa or more cannot be obtained. On the other hand, when the C content exceeds 0.18%, hardening of martensite becomes remarkable and the bend-bend recovery property of the present invention is not obtained. Therefore, the C content is set to 0.04 to 0.18%. From the viewpoint of obtaining a TS of 980MPa or more stably, the C content is preferably set to 0.05% or more. From the viewpoint of improving the bend-bend recovery property, the C content is preferably set to 0.16% or less, more preferably to 0.10% or less.
Si：0.1～3.0％
Si is an element effective for increasing TS by solid solution strengthening steel or suppressing tempering softening of martensite. Si is an element effective for suppressing cementite to obtain a structure in which martensite is dispersed in bainite. In order to obtain such an effect, the Si content needs to be set to 0.1% or more. On the other hand, if the Si content exceeds 3.0%, polygonal ferrite is excessively generated, and the steel structure of the present invention is not obtained. Therefore, the Si content is set to 0.1 to 3.0%. The Si content is preferably set to 0.2% or more. The Si content is preferably set to 2.0% or less, more preferably 1.5% or less.
Mn：0.5～3.5％
Mn is an element effective for forming martensite and bainite and increasing TS. When the Mn content is less than 0.5%, such effects cannot be sufficiently obtained, polygonal ferrite or the like is generated, and the steel structure of the present invention cannot be obtained. On the other hand, when the Mn content exceeds 3.5%, bainite is suppressed, and the steel structure of the present invention is not obtained. Therefore, the Mn content is set to 0.5 to 3.5%. From the viewpoint of obtaining a TS of 980MPa or more stably, the Mn content is preferably set to 1.0% or more. From the viewpoint of stably obtaining bainite, the Mn content is preferably set to 3.0% or less, and more preferably set to 2.3% or less.
P: more than 0% and less than 0.100%
P reduces the bend-bend recovery property, so that the amount thereof is preferably reduced as much as possible. In the present invention, the P content may be allowed to be 0.100%. Therefore, the P content is set to 0.100% or less, preferably 0.030% or less. When the P content is set to be more than 0% and less than 0.001%, the production efficiency is lowered, and thus it is preferably 0.001% or more.
S: more than 0% and less than 0.020%
S reduces the bend-bend recovery, so that the amount is preferably reduced as much as possible, and in the present invention, the S content may be allowed to be 0.020%. Therefore, the S content is set to 0.020% or less, preferably 0.0050% or less, and more preferably 0.0020% or less. When the S content is set to be more than 0% and less than 0.0002%, the production efficiency is lowered, and thus, it is preferably 0.0002% or more.
Al: more than 0% and less than 1.5%
Al functions as a deoxidizer and is preferably added in the deoxidizing step. The lower limit of the Al content is set to be more than 0%, and the Al content is preferably 0.01% or more from the viewpoint of use as a deoxidizer. When Al is contained in a large amount, polygonal ferrite is generated in a large amount, and the steel structure of the present invention is not obtained. In the present invention, the Al content may be allowed to be 1.5%. Therefore, the Al content is set to 1.5% or less. Preferably, the content is set to 0.50% or less.
Selected from Cr: 0.005-2.0%, ti: 0.005-0.20%, nb: 0.005-0.20%, mo: 0.005-2.0%, V: 0.005-1.0% of one or more than two kinds of
Cr, ti, nb, mo and V are elements effective for obtaining a structure in which martensite is dispersed in bainite. In order to obtain such effects, the content of one or more elements selected from the above elements needs to be equal to or higher than the respective lower limit values. On the other hand, when the content of one or more elements selected from the above elements exceeds the respective upper limit values, such an effect is not obtained, and the steel structure of the present invention is not obtained. Therefore, it contains Cr selected from: 0.005-2.0%, ti: 0.005-0.20%, nb: 0.005-0.20%, mo: 0.005-2.0%, V: 0.005-1.0% of one or more than two kinds of the components. When the above elements are contained, cr is preferably set to: more than 0.1 percent of Ti:0.010% or more, nb:0.010% or more, mo:0.10% or more, V:0.10% or more. The upper limits of the case where the above elements are contained are preferably set to Cr: less than 1.0%, ti: less than 0.15%, nb: less than 0.10%, mo: less than 1.0%, V: less than 0.5%.
The balance being Fe and unavoidable impurities. As the inevitable impurity element, for example, N is cited, and the allowable upper limit of the element is preferably 0.010%.
The above components are essential components of the high-strength hot-rolled steel sheet of the invention. In the present invention, the following elements may be contained as needed.
Selected from Cu:0.05 to 4.0 percent of Ni: 0.005-2.0%, B:0.0002 to 0.0050 percent, ca: 0.0001-0.0050%, REM:0.0001 to 0.0050 percent of Sb:0.0010 to 0.10 percent of Sn:0.0010 to 0.50% of one or more than two kinds of
Cu and Ni are effective elements for forming martensite and contributing to high strength. In order to obtain such an effect, when Cu and Ni are contained, the respective contents are preferably set to the above lower limit value or more. When the contents of Cu and Ni exceed the upper limit values, bainite may be suppressed, and the steel structure of the present invention may not be obtained. The Cu content is more preferably set to 0.10% or more, and still more preferably set to 0.6% or less. The Ni content is more preferably set to 0.1% or more, and still more preferably set to 0.6% or less.
B is an effective element for improving hardenability of the steel sheet and promoting formation of martensite to thereby contribute to high strength. In order to obtain such an effect, when B is contained, the B content is preferably set to 0.0002% or more. On the other hand, if the B content exceeds 0.0050%, the B-based compound increases, hardenability decreases, and the steel structure of the present invention may not be obtained. Therefore, in the case of containing B, the content is preferably set to 0.0002 to 0.0050%. The B content is more preferably set to 0.0005% or more, and still more preferably set to 0.0040% or less.
Ca. REM (rare earth metal) is an element effective for improving workability by controlling the morphology of inclusions. In order to obtain such an effect, when Ca and REM are contained, the respective contents are preferably set to Ca: 0.0001-0.0050%, REM:0.0001 to 0.0050 percent. Ca. When the REM content exceeds the upper limit, the inclusion content may increase to deteriorate workability. The Ca content is more preferably set to 0.0005% or more, and still more preferably set to 0.0030% or less. The REM content is more preferably set to 0.0005% or more, and still more preferably set to 0.0030% or less.
Sb is an element effective in suppressing the strength decrease of steel by suppressing denitrification, boron removal, and the like. In order to obtain such an effect, when Sb is contained, the Sb content is preferably set to 0.0010 to 0.10%. If the Sb content exceeds the upper limit, embrittlement of the steel sheet may occur. The Sb content is more preferably set to 0.0050% or more, and still more preferably set to 0.050% or less.
Sn is an element effective for suppressing the strength decrease of steel by suppressing pearlite generation. In order to obtain such an effect, when Sn is contained, the Sn content is preferably set to 0.0010 to 0.50%. When the Sn content exceeds the upper limit, embrittlement of the steel sheet may occur. The Sn content is more preferably set to 0.0050% or more, and still more preferably set to 0.050% or less.
Even if the content of Cu, ni, B, ca, REM, sb, sn is smaller than the above lower limit, the effect of the present invention is not impaired. Therefore, in the case where the content of these components is less than the above-described lower limit value, these elements are contained as unavoidable impurities to be treated.
Next, the steel structure of the high-strength hot-rolled steel sheet according to the present invention will be described.
The steel structure of the high-strength hot-rolled steel sheet of the present invention has, as a main phase, 80 to 100% by total area ratio of martensite and bainite, wherein the total area ratio of martensite in the bainite is 2 to 20%, and wherein the area ratio of martensite having a difference in crystal orientation between the martensite crystal orientation and the crystal orientation of at least one of the bainite adjacent to the martensite of 15 DEG or more is more than 50% with respect to the total martensite, and wherein when a region surrounded by a boundary having a difference in crystal orientation of 15 DEG or more between adjacent crystals is defined as crystal grains, the average aspect ratio of crystal grains existing from the surface of the steel sheet to a region having a depth of 5 [ mu ] m is 2.0 or less.
Aggregate area ratio of martensite and bainite: 80 to 100 percent
In the present invention, in order to have high TS and excellent bend-bend recovery properties, a steel structure mainly having martensite and bainite (mainly martensite and bainite) is produced.
When the total area ratio of martensite and bainite is less than 80% relative to the whole steel sheet structure, neither high TS nor bend-bend recovery properties can be obtained. Therefore, the total area ratio of martensite and bainite is set to 80 to 100%. Preferably, the content is set to 90 to 100%, more preferably 94 to 100%.
Total area ratio of martensite in bainite: 2 to 20 percent
Martensite is a steel structure effective for increasing TS, and is a steel structure effective for increasing uniform elongation by being dispersed in bainite. In order to obtain such an effect, the total area ratio of martensite in bainite needs to be set to 2% or more. On the other hand, when the total area ratio of the martensite exceeds 20%, the uniform elongation and the bend-bend recovery property are reduced. Therefore, the total area ratio of the martensite is set to 2 to 20%. The total area ratio of the martensite is preferably set to 3% or more, and more preferably set to 4% or more. The total area ratio of the martensite is preferably set to 15% or less, more preferably 12% or less.
Among martensite in the bainite, martensite having a difference in crystal orientation between the crystal orientation of the martensite and the crystal orientation of at least one of the bainite adjacent to the martensite of 15 ° or more has an area ratio: greater than 50% relative to total martensite
The bending-bending recovery property of the present invention can be obtained by setting the area ratio of martensite (hereinafter, also referred to as "martensite dispersed phase") having a difference of 15 ° or more between the crystal orientation of the martensite in the martensite and the crystal orientation of at least one of the bainite adjacent to the martensite to be more than 50% with respect to the area of the entire martensite.
Here, the phrase "the difference between the crystallographic orientation of the martensite and the crystallographic orientation of at least one of the bainite adjacent to the martensite is 15 ° or more" means that, for example, when there is martensite surrounded by a plurality of crystallographic-oriented bainite, the difference between the orientation of one or more of the plurality of crystallographic-oriented bainite and the martensite is 15 ° or more.
The reason for this is not clear, but it is assumed that the difference in crystal orientation between martensite in the bainite and bainite surrounding the martensite (bainite adjacent to the martensite) is large, and this is likely to be an obstacle to crack propagation during bending-bending recovery.
For this reason, the area ratio of the martensite dispersed phase is set to be more than 50% in the present invention. The more martensite that becomes an obstacle to crack propagation, the more suppressed the crack propagation in bending-bending recovery. The bend-bend recovery of the present invention can be achieved by making the area ratio larger than 50%.
Therefore, the area ratio of the martensite dispersed phase in the bainite is set to be more than 50% with respect to the total martensite. Preferably, the content is set to 60% or more, and more preferably, to 70% or more. The upper limit of the area ratio of the martensite dispersed phase is not particularly limited. Since it is substantially difficult to be 100%, it is preferably less than 100%.
Here, the "martensite dispersed phase" can be measured by the method described in examples described later. Specifically, the crystal orientations of bainite and martensite were obtained by Electron Back Scattering Diffraction (EBSD), and boundaries of orientation differences of 15 ° or more were shown. Then, the area ratio of martensite having a difference in crystal orientation of 15 ° or more between the martensite and at least one of the bainite adjacent to the martensite (adjacent bainite) among the martensite dispersed in the bainite was obtained.
The steel structure of the present invention may have ferrite, pearlite, and retained austenite as other structures than martensite and bainite. The total area ratio of the other structures than martensite and bainite is set to be less than 20% (including 0%). When the total area ratio is less than 20%, the characteristics of the present invention can be achieved.
Average aspect ratio of crystal grains present in a region from the surface of the steel sheet to a depth of 5 μm: 2.0 or less
The grains on the surface layer of the steel sheet become the starting points of cracks in bending-bending recovery, and the larger the aspect ratio of the grains, the more likely the cracks are generated. In order to obtain the bending-bending recovery property targeted in the present invention, it is necessary to set the average aspect ratio of the crystal grains existing in the region from the surface of the steel sheet to a depth of 5 μm to 2.0 or less. The average aspect ratio of the crystal grains is preferably 1.7 or less, more preferably 1.5 or less.
Here, as shown in fig. 1, the "crystal grains" refer to regions surrounded by boundaries where the orientation difference between adjacent crystals is 15 ° or more. The above-mentioned "aspect ratio" is obtained by the ratio of the maximum length RL in the rolling direction to the maximum length TL in the plate thickness direction (maximum length RL in the rolling direction/maximum length TL in the plate thickness direction) when the maximum length RL in the rolling direction of the crystal grain and the maximum length TL in the plate thickness direction of the crystal grain are defined as RL. The "average aspect ratio of crystal grains" refers to an average of aspect ratios of crystal grains existing in a region from the surface of the steel sheet to a depth of 5 μm.
In the present invention, the area ratio, crystal orientation, and aspect ratio of each structure can be measured by the methods described in examples described below.
Method for producing high-strength hot-rolled steel sheet
The high-strength hot-rolled steel sheet of the present invention is produced by heating a steel slab having the above-described composition and then hot-rolling the steel slab. In the hot rolling, rough rolling is performed on the heated billet, finish rolling is performed under the conditions that the total rolling time is 3 times or more and the total rolling reduction is less than 50% at 1000 ℃ or less and the total rolling reduction is 35% or less at the final pass rolling temperature to the final pass rolling temperature +50 ℃, cooling is started at less than 1.0s, cooling is performed under the conditions that the average cooling rate is 50 ℃/s or more from the cooling start temperature to 550 ℃, and then coiling is performed at the coiling temperature of (Ms point-50) DEG C to 550 ℃ and cooling is performed to room temperature.
The following describes the manufacturing method in detail. The temperature is a temperature (surface temperature) of a steel billet or a widthwise central portion of a steel sheet, and the average cooling rate is an average cooling rate of a widthwise central portion of a steel sheet. These temperatures can be measured using a radiation thermometer or the like.
Total channel number at 1000 ℃ or higher: 3 times or more
In finish rolling in hot rolling, austenite can be recrystallized by pressing at 1000 ℃ or higher for 3 or more times, and grains having a small aspect ratio can be formed in the surface layer of the steel sheet. Therefore, the total number of passes at 1000 ℃ or higher is set to 3 or more. Preferably, the number of times is set to 4 or more. The upper limit of the total number of lanes at 1000℃or higher is not particularly defined. From the viewpoint of productivity, etc., it is preferably set to 20 times or less.
Total reduction at 1000 ℃ or less: less than 50%
In finish rolling in hot rolling, when the total reduction ratio is 50% or more at 1000 ℃ or less, crystal grains having a large aspect ratio are formed on the surface layer of the steel sheet, and martensite having a crystal orientation close to that of adjacent bainite is easily formed, and the steel structure of the present invention cannot be obtained. Therefore, the total reduction ratio at 1000 ℃ or lower is set to be less than 50%. Preferably, the content is set to less than 40%, and more preferably, less than 30%. The lower limit of the total reduction ratio at 1000 ℃ or lower is not particularly limited. Abnormal crystal grains may be generated when the pressure is reduced, and thus, the content is preferably set to 10% or more.
The total reduction ratio is a percentage of a value obtained by dividing a difference between an inlet plate thickness before the initial pass and an outlet plate thickness after the final pass in the temperature range by the inlet plate thickness before the initial pass.
That is, the thickness is obtained by (the inlet thickness before the initial pass in the above temperature range-the outlet thickness after the final pass in the above temperature range)/(the inlet thickness before the initial pass in the above temperature range) ×100 (%).
Total reduction ratio at final pass rolling temperature to final pass rolling temperature +50℃: less than 35 percent
When the reduction ratio in the vicinity of the final pass temperature (hereinafter also referred to as FT) exceeds 35%, stretched grains are formed in the vicinity of the surface layer, and the average aspect ratio of the grains present in the region from the surface of the steel sheet to the depth of 5 μm according to the present invention cannot be obtained. The amount of strain introduced into austenite becomes excessive, and martensite having the crystal orientation relationship of the present invention is not obtained. Therefore, the total reduction ratio at the final pass rolling temperature to the final pass rolling temperature +50℃isset to 35% or less, preferably 30% or less. The lower limit is not particularly limited, but if the reduction ratio is too low, surface defects and the like may be caused, and therefore, the lower limit is preferably set to 5% or more, more preferably 10% or more.
Cooling time after finish rolling: less than 1.0s
After finish rolling, cooling is started at less than 1.0s (second). When the cooling time after finish rolling is 1.0s or more, the dispersed phase of the crystal-oriented martensite according to the present invention is not obtained. The reason for this is not clear, but it is considered that the recovery of dislocations introduced during finish rolling is suppressed by reducing the cooling time, and the subsequent bainite transformation and orientation selection during martensite transformation are affected. Therefore, the cooling time after finish rolling is set to less than 1.0s. Preferably, the time is set to 0.7s or less.
The lower limit of the cooling time is not particularly limited. The cooling time is preferably set to 0.01s or more because it is difficult to start cooling immediately after rolling due to restrictions on the equipment configuration or the like.
Average cooling rate from the cooling start temperature to 550 ℃): 50 ℃/s or more
When the average cooling rate from the cooling start temperature to 550 ℃ is less than 50 ℃/s, ferrite and pearlite are generated, and the steel structure of the present invention is not obtained. Therefore, the average cooling rate from the cooling start temperature to 550 ℃ is set to 50 ℃/s or more. Preferably, the temperature is set to 80 ℃/s or more. The upper limit of the average cooling rate is not particularly limited, and the average cooling rate is preferably set to 1000 ℃/s or less from the viewpoint of shape stability of the steel sheet, and the like.
Coiling temperature: (Ms point-50) DEG C-550 DEG C
When the coiling temperature is lower than (Ms point-50) DEGC, martensite increases, and the steel structure of the present invention is not obtained. On the other hand, when the temperature exceeds 550 ℃, ferrite and pearlite are generated, and the steel structure of the present invention is not obtained. Therefore, the winding temperature is set to (Ms point-50) to 550 ℃. Preferably (Ms point-30) DEG C or higher, and preferably 520℃ or lower.
Here, the Ms point is the martensite start temperature, and can be determined by actual measurement based on thermal expansion measurement and resistance measurement at the time of cooling by the formater test or the like.
The conditions of the production method are not particularly limited, but the production is preferably performed by adjusting appropriate conditions as follows.
For example, the billet heating temperature is preferably 1100 ℃ or higher from the viewpoints of segregation removal, solid solution of precipitates, and the like, and 1300 ℃ or lower from the viewpoints of energy efficiency, and the like.
In addition, from the viewpoint of reducing coarse particles that cause a decrease in workability, the finish rolling is preferably set to 4 or more passes. The number of passes in finish rolling is the total number of passes in finish rolling, and includes the "total number of passes at 1000 ℃ or less".
Examples
The present invention will be further described below based on examples. The present invention is not limited to the following examples.
Steel having the composition shown in table 1 was melted in a vacuum melting furnace to produce a billet. These billets were then heated to 1200 ℃ and hot rolled under the conditions shown in table 2 to obtain hot rolled steel sheets. In the hot rolling, the total number of finish rolling passes was 7 passes. The blank in table 1 indicates that no element was intentionally added, and includes not only the case where it was not contained (0%), but also the case where it was inevitably contained. In addition, N is an unavoidable impurity.
Using the obtained hot-rolled steel sheet, structure observation, tensile properties, and evaluation of bending-bending recovery were performed according to the following test methods.
< tissue observations >
(area ratio of tissues)
The area ratio of martensite and bainite refers to the ratio of the area of each structure to the observation area.
The area ratio of martensite was determined as follows.
Samples were cut from the obtained hot-rolled steel sheet, and after polishing a plate thickness cross section parallel to the rolling direction, corrosion was performed with a 3% nitric-acid ethanol solution, and 3 views were taken at 1500 x magnification from a SEM (scanning electron microscope) for each 1/4 position of the plate thickness. The area ratio of each tissue was obtained from the Image data of the obtained secondary electron Image using Image-Pro manufactured by Media Cybernetics company, and the average area ratio of the field of view was set as the area ratio of each tissue.
In the image data, upper bainite is distinguished by black or dark gray containing carbides or martensite or retained austenite with a straight line interface. Lower bainite is distinguished by black, dark gray, gray or light gray containing carbide of uniform orientation. Martensite is distinguished as black, dark gray, gray or light gray containing multiple orientations of carbide, or white or light gray without carbide. The retained austenite is distinguished in white or bright gray without carbides.
Since martensite and retained austenite may not be distinguished from each other, the area ratio of retained austenite obtained by a method described later is subtracted from the total area ratio of martensite and retained austenite obtained from SEM images, and the area ratio of martensite is obtained.
In the present invention, the martensite may be any of fresh martensite, self-tempered martensite, and the like. The bainite may be any of upper bainite, lower bainite, tempered bainite, and the like.
The texture with a stronger tempering degree is an image with a stronger contrast than black, and therefore, the color of the matrix is a standard, and the amount of carbide, the texture morphology, and the like are comprehensively determined in the present invention, and the texture described later is classified as any texture having similar characteristics. The carbide is white dot-like or linear.
In the present invention, ferrite is substantially not contained, but ferrite may be distinguished by a black or dark gray structure having no carbide or a little carbide inside and being surrounded mainly by a boundary of a curve. Pearlite can be distinguished in black and white lamellar or partially terminal near lamellar organization.
To determine the area ratio of retained austenite, the annealed steel sheet was ground to a position of 1/4+0.1mm of the sheet thickness, and then further polished by chemical polishing for 0.1mm, and the (200), (220), (311) and (200), (211) and (220) integrated reflection intensities of fcc iron (austenite) were measured on the polished surface by using kα1 rays of Mo by an X-ray diffraction apparatus. The volume fraction was obtained from the intensity ratio of the integrated reflection intensity from each surface of fcc iron to the integrated reflection intensity from each surface of bcc iron, and was used as the area fraction of retained austenite.
Using the area ratios of the respective obtained structures, the total area ratio of bainite and martensite and the total area ratio of other structures were obtained, and the total area ratio is shown in table 3. In table 3, "V (M)" means an area ratio (%) of martensite, V (b+m) "means a total area ratio (%) of bainite and martensite, and V (O)" means a total area ratio (%) of other structures.
(Crystal orientation)
For the same field of view of the same sample used for the above-described structure observation, the crystal orientations of bainite and martensite were obtained by Electron Back Scattering Diffraction (EBSD), and boundaries of orientation differences of 15 ° or more were shown, whereby the area ratio of martensite having a crystal orientation difference of 15 ° or more between the martensite in the martensite dispersed in the bainite and at least one of the bainite adjacent to the martensite (adjacent bainite) was obtained. Then, the proportion of the area of the martensite to the total area of the martensite is obtained. The EBSD was measured for a region of 100 μm X100 μm under conditions of an acceleration voltage of 30kV and a step size of 0.05. Mu.m.
The obtained ratios are shown in table 3. In table 3, "the ratio (%) of M having an orientation difference of 15 ° or more from the adjacent B" means the above ratio (%).
(aspect ratio of grains)
The surface layer portion of the same sample used for the above-described structure observation was found to have a crystal orientation by EBSD, and boundaries of orientation differences of 15 ° or more in adjacent crystals were shown, and regions surrounded by these boundaries were defined as crystal grains. The maximum length RL in the rolling direction and the maximum length TL in the plate thickness direction were obtained for each crystal grain existing in the region from the surface of the steel plate to 5 μm in the depth direction (plate thickness direction) (see fig. 1). The aspect ratio of each grain is calculated from the ratio (RL/TL) of the maximum length RL in the rolling direction to the maximum length TL in the plate thickness direction among these grains, and the average of the calculated values is taken as the average aspect ratio of the grains. The ratio of the maximum length RL in the rolling direction to the maximum length TL in the sheet thickness direction was obtained so that the minimum value of the aspect ratio was 1.0.
The number of grains spanning the position 5 μm from the steel sheet surface in the depth direction was counted as grains in the region ranging from the steel sheet surface to 5 μm in the depth direction.
The EBSD was measured for a region of 100 μm by 100 μm under an acceleration voltage of 30kV and a step size of 0.10. Mu.m, and the aspect ratio of the crystal grains was measured for all the corresponding crystal grains in the region (region of 100 μm by 100 μm).
< tensile test >)
The tensile properties were evaluated by a tensile test. A JIS No. 5 tensile test piece (JIS Z2201) was cut from the hot-rolled steel sheet obtained in a direction parallel to the rolling direction, and the strain rate was 10 -3 TS and uniform elongation were obtained by a tensile test in accordance with JIS Z2241.
In the present invention, the TS was evaluated as being equal to or greater than 980MPa and the uniform elongation was evaluated as being equal to or greater than 5.0%.
< bend-bend recovery test >
The evaluation of the bend-bend recovery characteristic was performed by a bend-bend recovery test. Test pieces having a width of 30mm and a length of 100mm were cut from the obtained hot-rolled steel sheet so that the longitudinal direction became parallel to the rolling direction. The test piece was used to perform 90V-bend processing at a stroke speed of 10 mm/min, a bending radius of 5mm, and a maximum pressing load of 10 tons. Then, the test piece was reversed, the flat bottom punch was pressed at a stroke speed of 10 mm/min, and stopped at a stroke with a bending angle of 10 ° or less, and after unloading, the sample was taken out. Then, the curved ridge portion of the sample was visually observed.
In the present invention, a sample having no crack on the surface corresponding to the inner side of the bend at the time of the initial bending (V-bend processing) was evaluated as being qualified in the bend-bend recovery property. The "none" of the "bend-bend recovery cracks" in table 3 means pass.
Table 3 shows various evaluation results.
TABLE 2
The underline indicates that the present invention is out of the scope of the present invention.
TABLE 3
The underline indicates that the present invention is out of the scope of the present invention.
As is clear from Table 3, the examples of the present invention are high strength hot rolled steel sheets having excellent uniform elongation and excellent bend-bend recovery. On the other hand, the comparative examples outside the range of the present invention did not obtain any one or more of the desired strength, uniform elongation and bend-bend recovery.
Industrial applicability
According to the present invention, a high strength hot rolled steel sheet having a TS of 980MPa or more and excellent ductility and excellent bend-bend recovery can be obtained. By using the high-strength hot-rolled steel sheet of the present invention for automotive parts applications, it is possible to greatly contribute to improvement of collision safety of automobiles and improvement of fuel efficiency.

Claims (3)

1. A high-strength hot-rolled steel sheet having a composition containing, in mass%, C:0.04 to 0.18 percent of Si:0.1 to 3.0 percent of Mn: 0.5-3.5%, P: greater 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 comprises a metal selected from Cr: 0.005-2.0%, ti: 0.005-0.20%, nb: 0.005-0.20%, mo: 0.005-2.0%, V: 0.005-1.0% of one or more of Fe and unavoidable impurities in balance,
the steel structure takes martensite and bainite with the total area ratio of 80-100 percent as the main phase,
the total area ratio of martensite in the bainite is 2-20%,
among martensite in the bainite, martensite having a difference in crystal orientation between the martensite and at least one of the bainite adjacent to the martensite of 15 DEG or more has an area ratio of more than 50% with respect to the whole martensite,
when a region surrounded by a boundary where the orientation difference between adjacent crystals is 15 ° or more is defined as a crystal grain, the average aspect ratio of the crystal grain existing in a region from the surface of the steel sheet to a depth of 5 μm is 2.0 or less.
2. The high-strength hot-rolled steel sheet as claimed in claim 1, wherein the composition comprises, in mass%, a composition selected from the group consisting of: 0.05 to 4.0 percent of Ni: 0.005-2.0%, B:0.0002 to 0.0050 percent, ca: 0.0001-0.0050%, REM:0.0001 to 0.0050 percent of Sb:0.0010 to 0.10 percent of Sn:0.0010 to 0.50% of one or more than two of the following components.
3. A method for producing a high-strength hot-rolled steel sheet according to claim 1 or 2, wherein,
heating the steel billet with the composition of the components,
next, when the hot rolling is carried out,
rough rolling is performed under conditions in which the total rolling reduction is 3 times or more at 1000 ℃ or more and less than 50% at 1000 ℃ or less and the total rolling reduction is 35% or less at the final pass rolling temperature to the final pass rolling temperature +50℃ and then cooling is started at less than 1.0s, cooling is performed under conditions in which the average cooling rate from the cooling start temperature to 550 ℃ is 50 ℃/s or more, and then coiling is performed at the coiling temperature of (Ms point-50) DEG C to 550 ℃.
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JP3546266B2 (en) 1999-07-26 2004-07-21 Ｊｆｅスチール株式会社 High strength hot rolled steel sheet excellent in workability and method for producing the same
JP2008156729A (en) 2006-12-26 2008-07-10 Nisshin Steel Co Ltd Zn-Al-BASED PLATING-COATED STEEL SHEET EXCELLENT IN UNBENDING RESISTANCE, AND ITS PRODUCTION METHOD
JP6528522B2 (en) 2015-04-17 2019-06-12 日本製鉄株式会社 High strength hot rolled steel sheet excellent in ductility, fatigue characteristics and corrosion resistance and manufacturing method thereof
MX2018001082A (en) * 2015-07-27 2018-06-06 Jfe Steel Corp High strength hot rolled steel sheet and manufacturing method for same.
US11603571B2 (en) * 2017-02-17 2023-03-14 Jfe Steel Corporation High-strength hot-rolled steel sheet and method for producing the same
EP3868903A4 (en) * 2018-10-19 2022-05-18 Nippon Steel Corporation Hot-rolled steel sheet and method for manufacturing same
MX2022004885A (en) * 2019-11-06 2022-05-16 Nippon Steel Corp Hot rolled steel sheet and production method thereof.

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