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WO2020195279A1 - Tôle d'acier - Google Patents

Tôle d'acier Download PDF

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Publication number
WO2020195279A1
WO2020195279A1 PCT/JP2020/005390 JP2020005390W WO2020195279A1 WO 2020195279 A1 WO2020195279 A1 WO 2020195279A1 JP 2020005390 W JP2020005390 W JP 2020005390W WO 2020195279 A1 WO2020195279 A1 WO 2020195279A1
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WO
WIPO (PCT)
Prior art keywords
less
steel sheet
annealing
temperature
phase
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Ceased
Application number
PCT/JP2020/005390
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English (en)
Japanese (ja)
Inventor
絵里子 塚本
林　宏太郎
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Nippon Steel Corp
Original Assignee
Nippon Steel Corp
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Nippon Steel Corp filed Critical Nippon Steel Corp
Priority to CN202080019850.XA priority Critical patent/CN113544301B/zh
Priority to JP2021508226A priority patent/JP7063414B2/ja
Publication of WO2020195279A1 publication Critical patent/WO2020195279A1/fr
Anticipated expiration legal-status Critical
Ceased legal-status Critical Current

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Classifications

    • 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
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/60—Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur

Definitions

  • the present disclosure relates to a steel sheet, and specifically to a steel sheet having a high Mn concentration, which has excellent uniform elongation characteristics, high strength, and high yield strength.
  • Residual austenite is obtained by concentrating C in austenite so that austenite does not transform into other tissues even at room temperature.
  • a carbide precipitation inhibitoring element such as Si and Al is contained in the steel sheet to concentrate C in the austenite during the bainite transformation occurring in the steel sheet at the steel sheet manufacturing stage. ing.
  • the austenite can be further stabilized and the amount of retained austenite can be increased, and as a result, a steel sheet having excellent strength and elongation can be produced.
  • Non-Patent Document 1 a steel to which Mn of more than 4.0% is added has been proposed (for example, Non-Patent Document 1). Since the steel contains a large amount of Mn, the weight reduction effect on the members used is also remarkable.
  • Patent Document 2 a steel to which Mn of more than 4.0% is added is cold-rolled, heated for a short time of 300 seconds to 1200 seconds, and ferrite is controlled to 30% to 80% in area%. Discloses steel sheets with significantly improved elongation.
  • Patent Document 3 a steel to which Mn of more than 4.0% is added, held in a temperature range of 740 ° C. or higher for 10 seconds or longer, tempered martensite in an area% of 25% or higher and 90% or lower, and retained austenite. Discloses a steel sheet that ensures excellent uniform elongation characteristics and high strength by containing 10% or more and 75% or less.
  • Patent Document 2 Since the steel sheet described in Patent Document 2 has a structure containing a large amount of ferrite, it is not possible to sufficiently combine tensile strength and formability from the viewpoint of aiming for further increase in strength and weight reduction of the steel sheet for automobiles.
  • the present inventors have 25% or more of the tempered martensite phase in the steel sheet in an area%. 90% or less and retained austenite phase should be contained in an amount of 10% or more and 75% or less, and VC (vanadium carbide) having a circle-equivalent diameter of 10 nm or more and 20 nm or less should be contained in a volume ratio of 0.30% or more and 2.20% or less. Was found to be effective.
  • the steel sheet of the present disclosure was made based on the above findings, and the summary is as follows.
  • the metallographic structure at the position 1/4 of the thickness from the surface in the L cross section contains a tempered martensite phase of 25% or more and 90% or less and a retained austenite phase of 10% or more and 75% or less in area%, and has a circle-equivalent diameter.
  • C is an extremely important element for increasing the strength of steel and ensuring the retained austenite phase.
  • C is also an element necessary for producing VC in this embodiment.
  • a C content of more than 0.18% is required to obtain a sufficient retained austenite content.
  • the upper limit of the C content is set to less than 0.32%.
  • the lower limit of the C content is preferably 0.20% or more, more preferably 0.22% or more.
  • the upper limit of the C content is preferably 0.31% or less, more preferably 0.28% or less, and by setting the upper limit of the C content to the above preferable range, the toughness of the steel sheet can be further enhanced. it can.
  • Si 0.01% or more and less than 3.50%
  • Si is an element effective for strengthening the tempered martensite phase, homogenizing the structure, and improving workability.
  • Si also has an action of suppressing the precipitation of the cementite phase and promoting the residue of the austenite phase.
  • a Si content of 0.01% or more is required.
  • the upper limit of the Si content is set to less than 3.50%.
  • the lower limit of the Si content is preferably 0.05% or more, more preferably 0.30% or more, and further preferably 0.50% or more. By setting the lower limit of the Si content in the above range, the uniform elongation characteristics of the steel sheet can be further improved.
  • the upper limit of the Si content is preferably 3.00% or less, more preferably 2.50% or less.
  • Mn is an element that stabilizes the austenite phase, enhances hardenability, and ensures uniform elongation. Further, in the steel sheet according to the present embodiment, Mn is distributed in the austenite phase to further stabilize the austenite phase. More than 4.20% Mn is required to stabilize the austenite phase at room temperature. On the other hand, if the steel sheet contains Mn excessively, the manufacturability in smelting is lowered, so the upper limit of the Mn content is set to less than 6.50%.
  • the lower limit of the Mn content is preferably 4.40% or more, more preferably 4.80% or more.
  • the upper limit of the Mn content is preferably 6.00% or less, more preferably 5.50% or less.
  • the austenite phase can be further stabilized by setting the lower limit value and the upper limit value of the Mn content in the above range.
  • Al is an antacid and needs to be contained in an amount of 0.001% or more.
  • Al also has an action of improving material stability because it widens the two-phase temperature range at the time of annealing. The larger the Al content, the greater the effect. However, if the Al content is excessive, it becomes difficult to maintain the surface texture, paintability, and weldability.
  • the upper limit of Al is set to less than 1.50%.
  • the lower limit of the Al content is preferably 0.005% or more, more preferably 0.01% or more, and further preferably 0.02% or more. sol.
  • the upper limit of the Al content is preferably 1.25% or less, more preferably 1.00% or less. sol.
  • V More than 0.10% and less than 1.20%)
  • V is an element that increases the yield strength of the steel sheet by forming fine carbides and enhances the collision characteristics, and a V content of more than 0.10% is required. Further, the formation of the fine carbides improves the hydrogen embrittlement resistance. On the other hand, if V is excessively contained, the carbon required to secure the retained austenite phase is insufficient, so the upper limit of the V content is set to 1.20% or less.
  • the lower limit of the V content is preferably more than 0.30%, more preferably 0.32% or more, still more preferably 0.35% or more, still more preferably 0.60% or more.
  • a larger VC content can be obtained, a steel sheet having a very excellent yield strength can be obtained, and hydrogen embrittlement resistance can be obtained. Can be improved.
  • the upper limit of the V content is preferably 1.10% or less, more preferably 1.00% or less.
  • P 0.100% or less
  • the upper limit of the P content is set to 0.100% or less.
  • the upper limit of the P content is preferably 0.050% or less, more preferably 0.030% or less, still more preferably 0.020% or less. Since the steel sheet according to this embodiment does not require P, the lower limit of the P content is 0%.
  • the lower limit of the P content may be more than 0% or 0.001% or more, but the smaller the P content, the more preferable.
  • S is an impurity, and if the steel sheet contains S in excess, MnS stretched by hot rolling is generated, which causes a decrease in moldability. Therefore, the upper limit of the S content is set to 0.010% or less.
  • the upper limit of the S content is preferably 0.007% or less, more preferably 0.003% or less. Since the steel sheet according to this embodiment does not require S, the lower limit of the S content is 0%.
  • the lower limit of the S content may be more than 0% or 0.001% or more, but the smaller the S content, the more preferable.
  • N is an impurity, and if the steel sheet contains 0.050% or more of N, the toughness is lowered. Therefore, the upper limit of the N content is set to less than 0.050%.
  • the upper limit of the N content is preferably 0.010% or less, more preferably 0.006% or less. Since the steel sheet according to this embodiment does not require N, the lower limit of the N content is 0%.
  • the lower limit of the N content may be more than 0% or 0.003% or more, but the smaller the N content, the more preferable.
  • O is an impurity, and if the steel sheet contains 0.020% or more of O, the uniform elongation property deteriorates. Therefore, the upper limit of the O content is set to less than 0.020%.
  • the upper limit of the O content is preferably 0.010% or less, more preferably 0.005% or less, still more preferably 0.003% or less. Since the steel sheet according to this embodiment does not require O, the lower limit of the O content is 0%.
  • the lower limit of the O content may be more than 0% or 0.001% or more, but the smaller the O content, the more preferable.
  • the steel sheet of the present embodiment is further selected from the group consisting of Cr, Mo, W, Cu, Ni, Ti, Nb, B, Ca, Mg, Zr, REM, Sb, Sn and Bi.
  • the above may be contained.
  • the steel sheet according to the present embodiment does not require Cr, Mo, W, Cu, Ni, Ti, Nb, B, Ca, Mg, Zr, REM, Sb, Sn and Bi, Cr, Mo, W, It does not have to contain Cu, Ni, Ti, Nb, B, Ca, Mg, Zr, REM, Sb, Sn and Bi, that is, the lower limit of the content may be 0%.
  • Cr 0% or more and less than 0.50%
  • Mo 0% or more and 2.00% or less
  • W 0% or more and 2.00% or less
  • Cu 0% or more and 2.00% or less
  • Ni 0% or more and 2.00% or less
  • Cr, Mo, W, Cu, and Ni are elements that improve the strength of the steel sheet and may be contained.
  • the steel sheet may contain 0.01% or more of each of one or more elements selected from the group consisting of Cr, Mo, W, Cu, and Ni. ..
  • the upper limit of the Cr content of each of the contents of one or more elements selected from the group consisting of Cr, Mo, W, Cu, and Ni is set to less than 0.50%, and Mo. , W, Cu, and Ni, respectively, shall have an upper limit of 2.00% or less.
  • Ti and Nb do not have to be contained because they are not essential elements for the steel sheet according to the present embodiment, and their respective contents are 0% or more.
  • Ti and Nb are elements that generate fine carbides, nitrides or carbonitrides, they are effective in improving the strength of the steel sheet. Therefore, the steel sheet may contain one or two elements selected from the group consisting of Ti and Nb. In order to obtain the effect of improving the strength of the steel sheet, it is preferable that the lower limit of the content of each of one or two elements selected from the group consisting of Ti and Nb is 0.005% or more.
  • the upper limit of the content of each of one or two elements selected from the group consisting of Ti and Nb is set to 0.300% or less.
  • B 0% or more and 0.010% or less
  • Ca 0% or more and 0.010% or less
  • Mg 0% or more and 0.010% or less
  • Zr 0% or more and 0.010% or less
  • REM 0% or more and 0.010% or less
  • B, Ca, Mg, Zr, and REM rare earth metal
  • B, Ca, Mg, Zr, and REM improve moldability by refining MnS of inclusions.
  • the lower limit of each of one or more elements selected from the group consisting of B, Ca, Mg, Zr, and REM is preferably 0.0001% or more, more preferably 0. .001% or more.
  • the upper limit of the content of each of these elements is set to 0.010% or less, and the element is selected from the group consisting of B, Ca, Mg, Zr, and REM. It is preferable that the total content of one or more elements is 0.030% or less.
  • Sb, Sn, and Bi are not essential elements in the steel sheet of the present disclosure, they do not have to be contained, and their respective contents are 0% or more. However, Sb, Sn, and Bi suppress that easily oxidizing elements such as Mn, Si, and / or Al in the steel sheet are diffused on the surface of the steel sheet to form an oxide, and improve the surface texture and plating property of the steel sheet. Increase.
  • the lower limit of the content of each of one or more elements selected from the group consisting of Sb, Sn, and Bi is preferably 0.0005% or more, more preferably 0.001. % Or more.
  • the content of each of these elements exceeds 0.050%, the effect is saturated, so the upper limit of the content of each of these elements is set to 0.050% or less.
  • the rest is iron and impurities.
  • impurities include elements that are inevitably mixed from the steel raw material, scrap, and / or the steelmaking process, and are allowed as long as they do not impair the characteristics of the steel sheet according to the present embodiment.
  • the impurity is an element other than the components described above, and includes an element contained in the steel sheet at a level at which the action and effect peculiar to the element do not affect the characteristics of the steel sheet according to the embodiment of the present invention. Is what you do.
  • the metallographic structure in the L cross section at a position 1/4 of the thickness (also referred to as 1 / 4t portion) from the surface of the steel sheet according to the present embodiment is a tempered martensite phase of 25% or more and 90% or less in area%, and 10 It contains a retained austenite phase of% or more and 75% or less, and contains a VC having a diameter of 10 nm or more and 20 nm or less in terms of a circle of 0.30% or more and 2.20% or less in volume fraction.
  • the L cross section refers to a surface cut so as to pass through the central axis of the steel sheet in parallel with the plate thickness direction and the rolling direction.
  • the metallographic structure at a position 1/4 of the thickness from the surface in the L cross section of the steel sheet according to the present embodiment contains a tempered martensite phase of 25% or more and 90% or less in area%.
  • the tempered martensite phase is a structure that enhances the strength of the steel sheet and improves the uniform elongation characteristics.
  • the area ratio of the tempered martensite phase is set to 25 to 90 area% in order to preferably maintain both the strength of the steel sheet and the uniform elongation property within the range of the target strength level. If the area ratio of the tempered martensite phase is less than 25% or more than 90%, it becomes difficult to obtain sufficient strength and uniform elongation characteristics.
  • the lower limit of the area ratio of the tempered martensite phase is preferably 35 area% or more, more preferably 50 area% or more. If the area ratio of the tempered martensite phase is within the above preferable range, better uniform elongation characteristics can be maintained even at higher strength.
  • the upper limit of the area ratio of the tempered martensite phase is preferably 70 area% from the viewpoint of hydrogen brittleness.
  • the metal structure at the position of 1/4 of the thickness from the surface in the L cross section of the steel sheet according to the present embodiment contains a retained austenite phase of 10% or more and 75% or less in area%.
  • the retained austenite phase is a structure that enhances the ductility and formability of the steel sheet, particularly the uniform elongation property of the steel sheet, by the transformation-induced plasticity.
  • the retained austenite phase can be transformed into a martensite phase by overhanging, drawing, stretch flangeing, or bending accompanied by tensile deformation, it contributes not only to various workability of the steel sheet but also to improvement of the strength of the steel sheet. .. In order to obtain these effects, the steel sheet according to the present embodiment needs to contain a retained austenite phase having an area ratio of 10% or more in the metal structure.
  • the lower limit of the area ratio of the retained austenite phase is preferably 15% or more, more preferably 18% or more, still more preferably 20% or more.
  • the area ratio of the retained austenite phase is within the above preferable range, better uniform elongation characteristics are maintained even at higher strength.
  • the larger the area ratio of the retained austenite phase the more preferable.
  • the solid solution carbon is reduced by VC precipitation, so that the area ratio of 75% is the upper limit of the area ratio of the retained austenite phase.
  • the metal structure of the 1 / 4t portion of the steel sheet contains VC having a diameter of 10 nm or more and 20 nm or less in terms of circle, which is 0.30% or more and 2.20% or less in volume fraction).
  • VC having a circle-equivalent diameter of 10 nm or more and 20 nm or less is contained in the metal structure in a volume fraction of 0.30% or more and 2.20% or less.
  • tempered martensite contains a large amount of dislocations that become precipitate formation sites as compared with ferrite, so that a larger amount of precipitate can be precipitated.
  • it is effective to deposit VC in the second annealing step described later.
  • VC is deposited in the heating of the steel material (slab) before the hot rolling before the second annealing step, the winding of the hot-rolled steel sheet, and the first annealing step, the VC becomes coarse in the subsequent steps. It may be difficult to obtain the desired fine VC. Therefore, it is important not to precipitate VC in the steps prior to the second annealing step.
  • the steel sheet of the present embodiment contains VC having a circle-equivalent diameter of 10 nm or more and 20 nm or less in an amount of 0.30% or more and 2.20% or less in volume fraction with respect to the matrix phase.
  • the yield strength becomes insufficient. Further, in the component range of the steel sheet of the present embodiment, the upper limit of the volume fraction of the VC having a circle-equivalent diameter of 10 nm or more and 20 nm or less is 2.20%.
  • the volume fraction of the VC having a circle-equivalent diameter of 10 nm or more and 20 nm or less is preferably 0.50% or more, and more preferably 0.80% or more.
  • both uniform elongation and yield strength can be achieved at the same time.
  • the steel sheet according to the present embodiment contains a large amount of fine VC in the metal structure, it is excellent in hydrogen embrittlement resistance.
  • the more diffusible hydrogen inside the steel the worse the hydrogen embrittlement resistance.
  • Diffusible hydrogen is trapped by vacancies, dislocations, grain boundaries or precipitates in the steel. Therefore, a steel sheet containing a large amount of dislocations and precipitates can sufficiently trap diffusible hydrogen inside the steel sheet, and thus hydrogen embrittlement cracking can be suppressed.
  • VCs with a circular equivalent diameter of 10 nm or more and 20 nm or less are 0.30% or more in volume ratio in the metal structure, a sufficient number of fine VC precipitates are present in the metal structure, so that the matching interface or misfit Dislocations increase and the amount of hydrogen trap increases, resulting in improved hydrogen embrittlement resistance.
  • the VC volume fraction having a circle-equivalent diameter of 10 nm or more and 20 nm or less is less than 0.30%, the amount of hydrogen trap is insufficient, and sufficient hydrogen embrittlement resistance may not be obtained.
  • the circle-equivalent diameter of VC is obtained by observing an extracted replica sample of a circular region with a diameter of 3.0 mm at a position 1/4 from the surface of the steel plate with a transmission electron microscope (TEM), and using image software to obtain a TEM image. It is measured by digitizing. As the TEM image, a randomly selected region having an area of 10 ⁇ m 2 is selected. Next, the area of each particle image identified by binarization is obtained, and the circle-equivalent diameter of each particle is calculated based on the area. Then, among the identified particles, the particles having a circle-equivalent diameter in the range of 10 to 20 nm are extracted.
  • TEM transmission electron microscope
  • the steel sheet of the present disclosure was confirmed by energy dispersive X-ray analysis (EDS), all the particles having a circle-equivalent diameter of 10 to 20 nm were VC.
  • EDS energy dispersive X-ray analysis
  • the total area of the particles extracted as described above, that is, the extracted VC having a circle-equivalent diameter of 10 to 20 nm is obtained, and the area ratio of the VC is calculated by dividing it by the area of the binarized image (10 ⁇ m 2 ).
  • the value of the area ratio is regarded as the volume fraction of VC with respect to the matrix phase, and the volume fraction (%) of VC having a diameter of 10 nm or more and 20 nm or less in terms of a circle is calculated.
  • the extraction replica method is a commonly used method for exfoliating precipitates and inclusions from a metal.
  • the remaining structure other than the tempered martensite phase and the retained austenite phase in the metal structure of the steel plate according to the present embodiment includes a ferrite phase, a bainite phase, and a fresh martensite phase (that is, an untempered martensite phase). , Cementite phase, and tempered bainite phase.
  • a ferrite phase may be contained in the metal structure.
  • the area ratio of the ferrite phase in the metal structure is preferably 10% or less, more preferably 3% or less, and further preferably 0%. Therefore, for example, in the steel sheet according to the present embodiment, the area ratio of the ferrite phase in the metal structure may be 0% or more and 10% or less, or 0% or more and 3%.
  • the bainite phase may be contained in the metal structure.
  • Island-like martensite which is a hard structure, may be present in the bainite phase.
  • the area ratio of the bainite phase in the metal structure is preferably 5% or less, more preferably 0%. Therefore, for example, in the steel sheet according to the present embodiment, the area ratio of the bainite phase in the metal structure may be 0% or more and 5% or less.
  • the area% of the retained austenite phase is measured by X-ray diffraction.
  • a test piece having a width of 25 mm (length in the rolling direction), a length of 25 mm (length in the direction perpendicular to rolling), and a thickness in the plate thickness direction of the annealed sample is cut out from the center of the main surface of the steel sheet. This test piece is chemically polished to reduce the plate thickness by 1/4 minute to obtain a test piece having a chemically polished surface.
  • the area% of the retained austenite phase having a plate thickness of 1/4 part can be obtained.
  • the area% of the retained austenite phase at 1/4 part of the plate thickness obtained by this method and the area% of the retained austenite phase at the L cross section are regarded as the same, and the area% obtained by this method. Is the area ratio of the L cross section.
  • the area% of the tempered martensite phase is calculated from microstructure observation with a scanning electron microscope (SEM). After mirror-polishing the L cross section of the steel sheet, it is corroded with 3% nital (3% nitrate-ethanol solution), and with a scanning electron microscope with an acceleration voltage of 15.0 kV and a magnification of 3000 times, 1/4 of the thickness from the surface of the steel sheet.
  • the area% of the tempered martensite phase can be measured by observing the microstructure in the range of 25 ⁇ m (length in the plate thickness direction) ⁇ 40 ⁇ m (length in the rolling direction) of the position.
  • the area% is calculated by determining the white structure recognized by the observation with a scanning electron microscope whose substructure is confirmed in the crystal grains as the tempered martensite phase. ..
  • the area ratios of the ferrite phase, bainite phase, cementite phase and tempered bainite phase can be measured by scanning electron microscope observation in the same manner as the above-mentioned measurement of the area ratio of the tempered martensite phase.
  • the ferrite phase is discriminated as a gray underlying structure and the area% is calculated.
  • the bainite phase is an aggregate of lath-shaped crystal grains when observed with a scanning electron microscope, and is determined as a structure in which carbides extend in the same direction in the lath, and the area% is calculated.
  • the bainite phase may also include a tempered bainite phase, which is not distinguished herein.
  • the area% of the area% of the secondary electron image taken with a brighter contrast than the other areas is calculated as cementite.
  • the TS of the steel sheet according to this embodiment is preferably 1180 MPa or more, more preferably 1470 MPa. This is because when a steel sheet is used as a material for automobiles, the thickness is reduced by increasing the strength, which contributes to weight reduction.
  • the TS ⁇ uEL of the steel sheet according to the present embodiment is preferably 21000 MPa ⁇ % or more, more preferably 24,000 MPa ⁇ % or more, still more preferably 25,000 MPa ⁇ % or more, still more preferably 26000 MPa ⁇ % or more.
  • the yield strength of the steel sheet according to the present embodiment is preferably 800 MPa or more, more preferably 1000 MPa or more.
  • the steel sheet according to this embodiment has high strength, good uniform elongation characteristics, and high yield strength, and is therefore most suitable for use in structural parts of automobiles such as pillars and cross members. Further, since the steel sheet according to the present embodiment has a high Mn concentration, it also contributes to weight reduction of automobiles, so that the industrial contribution is extremely remarkable.
  • the steel sheet according to the present embodiment is obtained by melting steel having the above-mentioned chemical composition by a conventional method, casting it to prepare a steel material (slab), heating the steel, and hot-rolling the steel sheet. Can be manufactured by pickling and then annealing.
  • the molten steel may be melted by a normal blast furnace method, and the raw material is a large amount of scrap like the steel produced by the electric furnace method. It may be included in.
  • the slab may be manufactured by a normal continuous casting process or may be manufactured by thin slab casting.
  • Hot rolling can be performed on a normal continuous hot rolling line.
  • Hot rolling is preferably carried out in a reducing atmosphere, for example, in a reducing atmosphere of 98% nitrogen and 2% hydrogen.
  • Annealing may be carried out in either an annealing furnace or a continuous annealing line as long as the conditions described below are satisfied, but preferably, both the first annealing step and the second annealing step described later are carried out using the continuous annealing line. In that case, productivity can be improved.
  • the first annealing step and the second annealing step are preferably performed in a reducing atmosphere, and may be performed in a reducing atmosphere of, for example, 98% nitrogen and 2% hydrogen.
  • the heat treatment conditions particularly the annealing conditions, within the ranges shown below.
  • the steel material to be subjected to the hot rolling process is preferably heated before the hot rolling.
  • the temperature of the steel material to be subjected to hot rolling (heating temperature before hot rolling) is preferably 1100 ° C. or higher and 1300 ° C. or lower.
  • V can be solid-solved in a shorter time, and the deformation resistance during hot rolling can be further reduced.
  • the temperature refers to the surface temperature of the central portion of the main surface of a steel material (slab), a hot-rolled steel sheet, or a cold-rolled steel sheet.
  • the time for heating to the above-mentioned preferable temperature range of 1100 ° C. or higher and 1300 ° C. or lower before hot rolling is preferably 30 minutes or longer, and more preferably 60 minutes or longer.
  • the upper limit of the time for heating and holding in the above-mentioned preferable temperature range of 1100 ° C. or higher and 1300 ° C. or lower before hot rolling is preferably 10 hours or less in order to suppress excessive scale loss, and is preferably 5 hours or less. Is more preferable.
  • direct rolling or direct rolling is performed, the steel material may be subjected to hot rolling as it is without being heat-treated.
  • finish rolling In hot rolling, it is preferable to perform finish rolling.
  • the finish rolling start temperature By setting the finish rolling start temperature to 1100 ° C. or lower, deterioration of the surface texture of the steel sheet due to intergranular oxidation can be suppressed.
  • the finish rolling end temperature is preferably 900 ° C. or higher and 1050 ° C. or lower. By setting the finish rolling end temperature within the above-mentioned preferable range, it is possible to suppress the precipitation of VC immediately after the finish rolling.
  • the hot-rolled steel sheet obtained by finish rolling can be cooled, wound up, and made into a coil.
  • the winding temperature is preferably 350 ° C or lower. By setting the winding temperature to 350 ° C. or lower, V can be brought into a solid solution state, and VC precipitation in the winding step can be suppressed.
  • the winding temperature is more preferably 200 ° C. or lower, still more preferably 100 ° C. or lower.
  • the lower limit of the winding temperature is not particularly limited, but from the viewpoint of productivity, about room temperature can be the lower limit.
  • cooling from 800 ° C. to 500 ° C. is preferably performed at an average cooling rate of 40 ° C./sec or higher. By setting the lower limit of the average cooling rate from 800 ° C. to 500 ° C.
  • the upper limit of the average cooling rate is not particularly limited, but in consideration of suppressing the occurrence of cooling unevenness and the equipment capacity, it is preferably 1000 ° C./sec or less, more preferably 200 ° C./sec or less, and further preferably 100. It is °C / sec or less.
  • the hot-rolled sheet may be tempered at 300 ° C. or higher and 350 ° C. or lower after being cooled to room temperature and before cold rolling.
  • the hot-rolled plate tempering temperature is within the above temperature range, it is possible to obtain the effect of suppressing breakage during cold rolling without precipitating VC before cold rolling.
  • the hot-rolled steel sheet can be obtained as a cold-rolled steel sheet by being pickled by a conventional method and then cold-rolled.
  • the rolling reduction of cold rolling is preferably 20% or more. From the viewpoint of suppressing fracture during cold rolling, the rolling reduction ratio of cold rolling is preferably 70% or less.
  • light rolling before pickling improves pickling properties, promotes removal of surface-concentrating elements, and has the effect of improving chemical conversion treatment properties and plating treatment properties.
  • the annealing step includes a first annealing step performed after cold rolling and a second annealing step performed after final cooling in the first annealing step.
  • the temperature is raised from 350 ° C. to 820 ° C. or higher and the temperature of Ac3 or higher at an average heating rate of 10 ° C./sec or higher, and 30 seconds or longer in the temperature range of 820 ° C. or higher and Ac3 or higher. Retention)
  • the temperature is preferably raised from 350 ° C. to 820 ° C. or higher and the first annealing temperature of Ac3 or higher at an average heating rate of 10 ° C./sec or higher, and 30 in a temperature range of 820 ° C. or higher and Ac3 point or higher. Hold for more than a second.
  • thermodynamic calculation software Thermo Calc is used, and C, Si, Mn, Al and V, and if any element is contained in the steel plate, the relevant components (however, Bi, Sc, Sb, Sn) are used. , Nb and Zr are excluded), and the values were obtained using TCFE8 as a reference database.
  • the matrix phase can be transformed into an austenite phase to improve uniform elongation characteristics and strength, and VC that can be precipitated during hot rolling can be obtained. It can be dissolved.
  • the upper limit of the annealing temperature in the first annealing step is not particularly limited, but by setting the annealing temperature to 1000 ° C. or lower, damage to the annealing furnace can be suppressed and productivity can be improved.
  • the annealing temperature in the first annealing step is more preferably 850 ° C. or higher, still more preferably 900 ° C. or higher, in order to further promote VC solution formation.
  • the annealing temperature in the first annealing step is more preferably 980 ° C. or lower, still more preferably 950 ° C. or lower.
  • the average heating is preferably 10 ° C./sec or higher, more preferably 15 ° C./sec or higher, preferably in the temperature range of the first annealing temperature (820 ° C. or higher and Ac 3 points or higher) from 350 ° C. It is preferable to raise the temperature at a rate.
  • the lower limit of the average heating rate within the above-mentioned preferable range, precipitation or coarsening of VC during temperature rise can be suppressed, and solution formation in the first annealing step can be promoted.
  • the upper limit of the average heating rate is not particularly limited, but from the viewpoint of suppressing uneven heating of the steel sheet and the equipment capacity, it is 30 ° C./sec or less in the temperature range of 350 ° C. to 820 ° C. or higher and Ac 3 points or higher. It is preferable to do so.
  • the annealing time at the first annealing temperature is 30 seconds or more in order to sufficiently austenite the matrix and dissolve the precipitate.
  • the annealing time is more preferably 40 seconds or more.
  • the upper limit of the annealing time is not particularly limited, but from the viewpoint of productivity, the annealing time is preferably 300 seconds or less.
  • the final cooling temperature after annealing in the first annealing step is less than 100 ° C.
  • the rasmartensite structure immediately after the first annealing step can be increased.
  • the final cooling temperature after annealing in the first annealing step is preferably room temperature (50 ° C. or lower).
  • the temperature range from the annealing temperature in the first annealing step to 350 ° C. is preferably cooled at an average cooling rate of 10 ° C./sec or more. To do.
  • the average cooling rate in the temperature range from the first annealing temperature to 350 ° C. hereinafter, also referred to as the average cooling rate after annealing
  • precipitation of VC during cooling can be suppressed.
  • the average cooling rate after annealing in the first annealing step is preferably 20 ° C./sec or more, more preferably 50 ° C./sec or more, still more preferably 200 ° C./sec or more, still more preferably 250 ° C./sec or more. ..
  • the average cooling rate after annealing within the above-mentioned preferable range, the entire steel material after cooling can be cooled at a critical cooling rate or higher, and the entire steel material after cooling can have a martensite-based structure, so that V can be kept in a solid solution state. It is possible to control the structure after the final heat treatment and improve the material stability.
  • the upper limit of the average cooling rate after annealing in the first annealing step is not particularly limited, but even if a water quenching cooling method or a mist injection cooling method is used, it is difficult to control the temperature to over 2000 ° C./sec, so after annealing.
  • the practical upper limit of the average cooling rate is 2000 ° C./sec.
  • the cooling stop temperature of the average cooling rate in the above range is preferably 350 ° C. or lower, more preferably 200 ° C. or lower, and further preferably 100 ° C. or lower.
  • Cooling condition after cooling stop in the first annealing step Hold for 10 seconds or more and 1000 seconds or less in the temperature range of 350 ° C. or less
  • it is held in a temperature range of 350 ° C. or lower for 10 seconds or more and 1000 seconds or less.
  • C distribution to austenite proceeds sufficiently, and austenite is more generated in the structure before the final heat treatment (before the second annealing step). be able to.
  • it is possible to further suppress the formation of austenite in the structure after the final heat treatment and further suppress the fluctuation of the strength characteristics.
  • the holding time in the above temperature range is more preferably 30 seconds or more. From the viewpoint of productivity, the holding time in the above temperature range is more preferably 300 seconds or less.
  • the lower limit of the holding temperature after cooling is stopped in the above temperature range is not particularly limited, but the holding temperature after cooling is stopped is preferably 50 ° C. or higher, more preferably 100 ° C. or higher, still more preferably 200 ° C. or higher. This makes it possible to improve the efficiency of the continuous annealing line.
  • VC precipitation can be suppressed by setting the holding temperature to 350 ° C. or lower after the cooling is stopped.
  • the holding temperature range is 350 ° C. or lower, the temperature of the steel sheet does not need to be constant. Further, it is not always necessary to hold the product in the holding temperature range after cooling.
  • the steel sheet After cooling after annealing in the first annealing step, the steel sheet is preferably kept in a temperature range of 100 ° C. or higher and 350 ° C. or lower, cooled to less than 100 ° C., preferably room temperature, and then heated again to perform the second annealing step. I do. In the second annealing step, it is preferably held for 50 seconds or more and 360 seconds or less in a temperature range of 640 ° C. or higher and 720 ° C. or lower.
  • VC By setting the second annealing temperature to 640 ° C. or higher, VC can be sufficiently precipitated and the yield strength can be increased. Further, by setting the second annealing temperature to 720 ° C. or lower, a sufficient amount of tempered martensite can be secured, a sufficient amount of VC precipitation can be secured, and a sufficient yield strength and uniform elongation can be secured. ..
  • the second annealing time is set to 50 seconds or more in order to stabilize the retained austenite and secure the amount of VC precipitated.
  • the second annealing time is preferably 100 seconds or longer, more preferably 200 seconds or longer. Further, in order to suppress the coarsening of VC, the second annealing time is set to 360 seconds or less.
  • the temperature range from 500 ° C. to 600 ° C. is set at an average heating rate of 10 ° C./sec or higher and 200 ° C./sec or lower.
  • the average heating rate from 500 ° C to 600 ° C in the second annealing to 10 ° C / sec or more, the formation of cementite in the tissue is suppressed, and C required for stabilization of retained austenite and precipitation of VC is obtained. It can be secured more reliably.
  • by raising the temperature in the temperature range of 500 ° C. to 600 ° C. at an average heating rate of 200 ° C./sec or less temperature unevenness of the steel sheet is less likely to occur, and more stable quality can be ensured.
  • the steel sheet is cooled to 350 ° C. or lower at an average cooling rate of 10 ° C./sec or higher.
  • the average cooling rate is the average cooling rate in the temperature range from the holding temperature in the second annealing step to 350 ° C.
  • Cooling after annealing in the second annealing step may be performed as it is to room temperature when the steel sheet is not plated. Further, when plating a steel plate, the following can be performed.
  • the cooling after annealing in the second annealing step is stopped in the temperature range of 430 to 500 ° C.
  • the hot-dip galvanizing treatment can be performed by immersing in a plating bath. The conditions of the plating bath may be within the normal range. After the plating treatment, it may be cooled to room temperature, preferably to 100 ° C. or lower at an average cooling rate of 30 ° C./sec or more. Alternatively, after cooling after annealing in the second annealing step to a temperature range of 350 ° C.
  • the temperature of the cold-rolled steel sheet is raised to a temperature range of 430 to 500 ° C., and the cold-rolled steel sheet is used in a hot-dip galvanizing bath. It can also be immersed and hot-dip galvanized.
  • hot-dip galvanizing the difference between the annealing temperature in the second annealing step and the final temperature reached by cooling after plating is set to the cooling time from the second annealing step to the start of plating and the final temperature after the end of plating. By dividing by the sum of the cooling time until reaching the point, the average cooling rate after annealing in the second annealing step can be obtained.
  • Hot dip galvanizing can be alloyed at temperature.
  • the alloying treatment conditions may be within the usual range. After the alloying treatment, it may be cooled to room temperature, preferably to 100 ° C. or lower at an average cooling rate of 30 ° C./sec or more.
  • the difference between the annealing temperature in the second annealing step and the final temperature reached by cooling after the alloying process is the cooling time from the second annealing step to the start of plating and alloying.
  • the average cooling rate after annealing in the second annealing step can be obtained by dividing by the sum of the cooling time from the end to the final temperature.
  • the above manufacturing method is an example of the manufacturing method of the steel sheet of the present disclosure, and the manufacturing method of the steel sheet of the present disclosure is not limited to the above manufacturing method.
  • the steel sheet of the present disclosure will be described more specifically with reference to an example.
  • the following examples are examples of the steel sheet of the present disclosure and its manufacturing method, and the steel sheet of the present disclosure and its manufacturing method are not limited to the aspects of the following examples.
  • the obtained steel material (slab) was heat-treated, hot-rolled, wound, and tempered under the conditions shown in Table 2 to obtain a hot-rolled steel sheet. Then, the hot-rolled steel sheet after winding or tempering was cold-rolled. The hot rolling and heat treatment of the hot-rolled steel sheet were carried out in a reducing atmosphere of 98% nitrogen and 2% hydrogen. In all the examples, the holding time at the heating temperature before hot rolling was 60 minutes, and the cold rolling rate was 40%.
  • the obtained cold-rolled steel sheet was annealed twice (first annealing step and second annealing step) under the conditions shown in Table 3 to prepare an annealed cold-rolled steel sheet.
  • the two annealings of the cold-rolled steel sheet were carried out in a reducing atmosphere of 98% nitrogen and 2% hydrogen.
  • annealed cold-rolled steel sheets For some examples of annealed cold-rolled steel sheets, cooling after the second annealing is stopped at 460 ° C., and the cold-rolled steel sheets are immersed in a hot-dip galvanizing bath at 460 ° C. for 2 seconds to perform hot-dip galvanizing treatment. It was.
  • the conditions of the plating bath were the same as those of the conventional one.
  • the alloying treatment described later was not performed, the mixture was cooled to room temperature at an average cooling rate of 30 ° C./sec after holding at 460 ° C.
  • the "average cooling rate from the second annealing temperature Regarding the "average cooling rate from the second annealing temperature to 350 ° C.
  • the difference between the second annealing temperature and room temperature in Table 3 is calculated after the second annealing step. It was calculated by dividing by the sum of the cooling time from the start of plating to the start of plating and the cooling time from after plating to reaching room temperature.
  • annealed cold-rolled steel sheets after hot-dip galvanizing, they were subsequently alloyed without being cooled to room temperature. It was heated to 520 ° C. and held at 520 ° C. for 5 seconds for alloying treatment, and then cooled to room temperature at an average cooling rate of 30 ° C./sec. Regarding the "average cooling rate from the second annealing temperature to 350 ° C. or lower" in the example shown as “alloying" in Table 3, the difference between the second annealing temperature and the room temperature in Table 3 is used as the second annealing step. It was obtained by dividing by the sum of the cooling time from the later to the start of plating and the cooling time from after the alloying treatment until reaching room temperature.
  • the annealed cold-rolled steel sheet thus obtained was temper-rolled at an elongation rate of 0.1% to prepare various evaluation steel sheets.
  • the area ratios of the tempered martensite phase, ferrite phase, retained austenite phase, and bainite phase were calculated from microstructure observation and X-ray diffraction measurement with a scanning electron microscope.
  • the L cross section of a steel plate cut parallel to the thickness direction and the rolling direction is mirror-polished, and then the microstructure is exposed with 3% bainite.
  • a scanning electron microscope with a magnification of 5000 times is used to make a quarter from the surface. The microstructure at the position was observed, and the area ratios of the tempered martensite phase, ferrite phase, and bainite phase were calculated by image analysis (Photoshop®) for a range of 0.1 mm ⁇ 0.3 mm.
  • a test piece having a width of 25 mm (length in the rolling direction), a length of 25 mm (length in the direction perpendicular to rolling), and a thickness in the plate thickness direction of the annealed sample as it is from the center of the main surface of the steel sheet is provided.
  • the test piece was cut out and chemically polished to reduce the plate thickness by 1/4 to obtain a test piece having a chemically polished surface.
  • X-ray diffraction analysis with a measurement range of 2 ⁇ of 45 to 105 degrees was performed three times using a Co tube, the profile of the obtained retained austenite phase was analyzed, and each was averaged. As a result, the area% of the retained austenite phase having a plate thickness of 1/4 part was obtained.
  • the area% of the retained austenite phase at 1/4 of the plate thickness obtained by this method is regarded as the same as the area% of the retained austenite phase at the L cross section, and the area% obtained by this method is the area of the L cross section. It was a rate.
  • the circle-equivalent diameter of VC is obtained by observing an extracted replica sample of a circular region with a diameter of 3.0 mm at a position 1/4 from the surface of the steel plate with a transmission electron microscope (TEM), and using image software to obtain a TEM image. It was measured by digitizing. As the TEM image, a randomly selected region having an area of 10 ⁇ m 2 was selected. Next, the area of each particle image identified by binarization was obtained, and the circle-equivalent diameter of each particle was calculated based on the area. Then, among the identified particles, particles having a circle-equivalent diameter in the range of 10 to 20 nm were extracted.
  • TEM transmission electron microscope
  • JIS No. 5 tensile test pieces were collected from a direction perpendicular to the rolling direction of the steel plate, and tensile strength (TS), uniform elongation (uEL), and yield strength (YS) were measured.
  • the tensile test was carried out by the method specified in JIS-Z2201 using a JIS No. 5 tensile test piece.
  • the uniform elongation test was carried out by the method specified in JIS-Z2201 using a JIS No. 5 test piece having a parallel portion length of 50 mm.
  • Table 4 shows the results of the above evaluation.
  • a steel sheet exhibiting a tensile strength (TS) of 1180 MPa or more, a TS ⁇ uEL of 21000 MPa ⁇ % or more, and a yield strength (YS) of 800 MPa or more is evaluated as a steel sheet having excellent uniform elongation characteristics, high strength, and high yield strength. did.
  • the hydrogen embrittlement resistance of the above example numbers 10, 11, 31, 33 and 47 was evaluated.
  • the evaluation method is as follows.
  • Example Nos. 10, 11, 31, 33 and 47 From the steel plates of Example Nos. 10, 11, 31, 33 and 47, three test pieces punched to 30 mm ⁇ with a clearance of 10% were collected, and the punched test pieces were immersed in a hydrochloric acid aqueous solution of pH 1 for 48 hours to obtain the punched end faces. The presence or absence of cracks was observed with an optical microscope. All three test pieces were accepted if no cracks were observed after 48 hours of immersion.
  • Table 5 shows the results of the above evaluation.
  • a steel sheet that did not show cracks in all three test pieces after being immersed for 48 hours was evaluated as a steel sheet having excellent hydrogen embrittlement resistance, and was shown as " ⁇ " in Table 5 with hydrogen embrittlement resistance.
  • Steel sheets showing cracks even at least one are shown as hydrogen embrittlement resistance “x” in Table 5.

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Abstract

La présente invention concerne une tôle d'acier qui contient, en % en masse, plus de 0,18 % mais moins de 0,32 % de C, pas moins de 0,01 % mais moins de 3,50 % de Si, plus de 4,20 % mais moins de 6,50 % de Mn, pas moins de 0,001 % mais moins de 1,50 % de sol.Al, elle contient des quantités respectives de P, S, N et O limitées à des quantités prédéterminées, et contient un élément arbitrairement sélectionné, le reste étant du fer et des impuretés, une structure métallique à une position 1/4 de l'épaisseur à partir d'une surface dans une section transversale en L comprenant, en % de surface, de 25 à 90 % d'une phase martensite tempérée et de 10 à 75 % d'une phase austénite résiduelle, et comprend, en fraction volumique, de 0,30 à 2,20 % de VC ayant un diamètre de conversion de cercle de 10 à 20 nm.
PCT/JP2020/005390 2019-03-27 2020-02-12 Tôle d'acier Ceased WO2020195279A1 (fr)

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JP2017145469A (ja) * 2016-02-18 2017-08-24 新日鐵住金株式会社 高強度鋼板の製造方法
WO2018151318A1 (fr) * 2017-02-20 2018-08-23 新日鐵住金株式会社 Tôle d'acier

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CA2697226C (fr) * 2007-10-25 2015-12-15 Jfe Steel Corporation Tole d'acier zinguee par immersion a chaud de haute resistance presentant une excellente aptitude au faconnage et son procede de fabrication
CN103710622A (zh) * 2013-12-20 2014-04-09 钢铁研究总院 屈服强度690MPa级低屈强比抗震钢及其制造方法
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KR20190109407A (ko) * 2017-01-16 2019-09-25 닛폰세이테츠 가부시키가이샤 강판 및 그의 제조 방법

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JP2017145469A (ja) * 2016-02-18 2017-08-24 新日鐵住金株式会社 高強度鋼板の製造方法
WO2018151318A1 (fr) * 2017-02-20 2018-08-23 新日鐵住金株式会社 Tôle d'acier

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