WO2019194251A1 - Tôle d'acier et procédé de production de tôle d'acier - Google Patents

Tôle d'acier et procédé de production de tôle d'acier Download PDF

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Publication number
WO2019194251A1
WO2019194251A1 PCT/JP2019/014878 JP2019014878W WO2019194251A1 WO 2019194251 A1 WO2019194251 A1 WO 2019194251A1 JP 2019014878 W JP2019014878 W JP 2019014878W WO 2019194251 A1 WO2019194251 A1 WO 2019194251A1
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steel sheet
phase
rolling
hot
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PCT/JP2019/014878
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English (en)
Japanese (ja)
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力 岡本
林　宏太郎
武 豊田
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日本製鉄株式会社
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Priority to JP2019543869A priority Critical patent/JP6683291B2/ja
Publication of WO2019194251A1 publication Critical patent/WO2019194251A1/fr

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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
    • 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 having a high content of Mn and a manufacturing method thereof.
  • TRIP Transformation Induced Plasticity
  • Residual austenite is obtained by concentrating C in austenite so that austenite does not transform into another structure even at room temperature.
  • a technique for stabilizing austenite it has been proposed to contain carbide precipitation-inhibiting elements such as Si and Al in the steel sheet, and to enrich C in the austenite during the bainite transformation that occurs in the steel sheet during the manufacturing stage of the steel sheet. Yes.
  • the C content contained in the steel plate is large, austenite can be further stabilized and the amount of retained austenite can be increased.
  • a steel plate having excellent strength and elongation properties can be produced.
  • welding is often performed on the steel plate.
  • the C content in the steel plate is large, the weldability is deteriorated, so that use as a structural member is limited. Therefore, it is desired to improve both the formability and strength of the steel sheet with a smaller C content.
  • Non-patent Document 2 steel added with 3.5% or more of Mn
  • Non-patent Document 1 steel added with 3.5% or more of Mn
  • the steel contains a large amount of Mn, the effect of reducing the weight of the member used is also remarkable.
  • the requirements for improving the impact characteristics while enhancing the elongation characteristics were not clear.
  • the inventors have provided an austenitic phase in the steel sheet at an area ratio. 10% or more, tempered martensite phase 5% or more, ferrite phase 10% or more, martensite phase is limited to less than 15%, and the area ratio of unrecrystallized ferrite in ferrite phase is 30% or more It was found that it is effective to set CMn ⁇ / CMn ⁇ , which is the ratio of the average Mn concentration CMn ⁇ in the austenite phase and the average Mn concentration CMn ⁇ in the ferrite phase, to 1.2 or more.
  • the steel sheet and its manufacturing method of the present disclosure have been made on the basis of the above knowledge, and the gist thereof is as follows.
  • the gist of the present disclosure is as follows. (1) In mass%, C: more than 0.10% and less than 0.55%, Si: 0.001% or more and less than 3.50%, Mn: more than 4.20% and less than 9.00%, sol. Al: 0.001% or more and less than 3.00%, P: 0.100% or less, S: 0.010% or less, N: less than 0.050%, O: less than 0.020%, Cr: 0% or more and less than 2.00%, Mo: 0% or more and 2.00% or less, W: 0% to 2.00%, Cu: 0% or more and 2.00% or less, Ni: 0% or more and 2.00% or less, Ti: 0% or more and 0.300% or less, Nb: 0% or more and 0.300% or less, V: 0% or more and 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 0.01
  • B 0.0001% or more and 0.010% or less
  • Ca 0.0001% or more and 0.010% or less
  • Mg 0.0001% or more and 0.010% or less
  • Zr 0.0001% or more and 0.010% or less
  • REM 0.0001% or more and 0.010% or less, comprising one or more selected from the group consisting of The steel sheet according to any one of 1) to (3).
  • Sb 0.0005% or more and 0.050% or less
  • Sn 0.0005% or more and 0.050% or less
  • Bi 0.0005% or more and 0.050% or less, containing one or more selected from the group consisting of: The steel plate according to any one of 1) to (4).
  • the hot-rolled steel sheet is subjected to heat treatment for 1 hour or more in a temperature range where the austenite phase fraction is 20% to 50%, and then subjected to pickling and cold rolling to form a cold-rolled steel sheet,
  • the cold rolling rate in the cold rolling is 30% or more and 70% or less
  • Annealing the cold-rolled steel sheet in a temperature range where the austenite phase fraction is 20% to 50% and holding for 30 seconds or more and 15 minutes or less and after holding the temperature of the annealing, an average cooling rate of 2 ° C. /
  • a method for producing a steel sheet wherein the steel sheet is cooled at a temperature in a range from 100 ° C. to 2000 ° C./second and held in a temperature range from 100 ° C.
  • FIG. 1 is a stress-strain curve of a steel plate.
  • FIG. 2 is a mapping result showing a distribution state of Mn in a steel sheet heat-treated in a ferrite single-phase region and a ferrite / austenite two-phase region.
  • C is an extremely important element for increasing the strength of steel and securing austenite.
  • a C content of more than 0.10% is required.
  • the upper limit of the C content is less than 0.55%.
  • the lower limit of the C content is preferably 0.15% or more, more preferably 0.20% or more.
  • the upper limit value of the C content is preferably 0.40% or less, more preferably 0.35% or less.
  • Si 0.001% or more and less than 3.50%
  • Si is an element effective for strengthening tempered martensite, homogenizing the structure, and improving workability. Si also has the effect of suppressing the precipitation of cementite and promoting the retention of austenite. In order to acquire the said effect, 0.001% or more of Si content is required. On the other hand, if Si is contained excessively, the plating properties and chemical conversion properties of the steel sheet are impaired, so the upper limit of the Si content is set to less than 3.50%.
  • the lower limit of the Si content is preferably 0.005% or more, more preferably 0.010% or more. By setting the lower limit of the Si content in the above range, the elongation characteristics of the steel sheet can be further improved.
  • the upper limit of the Si content is preferably 2.00% or less, more preferably 1.00% or less.
  • Mn is an element that stabilizes austenite and improves hardenability. Moreover, in the steel plate of this indication, Mn is concentrated in austenite and austenite is stabilized more. To stabilize austenite at room temperature, more than 4.20% Mn is required. On the other hand, if the steel sheet contains Mn excessively, the weldability, hole expandability and ductility are impaired, so the upper limit of the Mn content is set to less than 9.00%.
  • the lower limit value of the Mn content is preferably more than 4.50%, more preferably 4.80% or more.
  • the upper limit of the Mn content is preferably 8.50% or less, more preferably 8.00% or less.
  • Al is a deoxidizer and should be contained by 0.001% or more.
  • Al has the effect of increasing the material stability in order to widen the temperature range of the two-phase region during annealing. The effect increases as the Al content increases.
  • excessive addition of Al leads to deterioration of surface properties, paintability, weldability, and the like.
  • the upper limit of Al was made less than 3.00%.
  • 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 2.00% or less, more preferably 1.00% or less. sol.
  • the upper limit of the P content is 0.100% or less.
  • the upper limit of the P content is preferably 0.050% or less, more preferably 0.030% or less, and still more preferably 0.020% or less. Since the steel plate according to this embodiment does not require P, P may not be substantially contained, and the lower limit value 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 lower the P content, the better.
  • S is an impurity, and if the steel sheet contains excessive S, MnS stretched by hot rolling is generated, which causes deterioration of formability such as bendability and hole expansibility. Therefore, the upper limit of the S content is 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 plate according to the present embodiment does not require S, S may not be substantially contained, and the lower limit value 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 lower the S content, the better.
  • N is an impurity, and if the steel sheet contains 0.050% or more of N, the toughness is deteriorated. Therefore, the upper limit of the N content is 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 plate according to the present embodiment does not require N, N may not be substantially contained, and the lower limit value of the N content is 0%.
  • the lower limit of the N content may be more than 0% or 0.005% or more, but the lower the N content, the better.
  • O is an impurity, and if the steel sheet contains 0.020% or more of O, ductility is deteriorated. Therefore, the upper limit of the O content is less than 0.020%.
  • the upper limit of the O content is preferably 0.010% or less, more preferably 0.005% or less, and still more preferably 0.003% or less. Since the steel plate according to the present embodiment does not require O, it may not substantially contain O, and the lower limit value 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 lower the O content, the better.
  • the steel plate of the present embodiment is further selected from the group consisting of Cr, Mo, W, Cu, Ni, Ti, Nb, V, B, Ca, Mg, Zr, REM, Sb, Sn, and Bi, or You may contain 2 or more types. However, the steel sheet according to the present embodiment may not contain Cr, Mo, W, Cu, Ni, Ti, Nb, V, B, Ca, Mg, Zr, REM, Sb, Sn, and Bi. The lower limit of the content may be 0%.
  • each of Cr, Mo, W, Cu, and Ni is not an essential element for the steel sheet according to the present embodiment, and thus may not be contained, and each content is 0% or more. However, since Cr, Mo, W, Cu, and Ni are elements that improve the strength of the steel sheet, they may be contained. In order to obtain the effect of improving the strength of the steel plate, the steel plate 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 steel sheet contains these elements in excess, surface flaws during hot rolling are likely to be generated, and further, the strength of the hot rolled steel sheet becomes too high and cold rolling properties may be deteriorated. Therefore, among the contents of one or more elements selected from the group consisting of Cr, Mo, W, Cu, and Ni, the upper limit value of the Cr content is less than 2.00%, and Mo , W, Cu, and Ni, the upper limit of each content shall be 2.00% or less.
  • Ti, Nb, and V do not have to be contained because they are not essential elements for the steel sheet according to the present embodiment, and each content is 0% or more.
  • the steel sheet may contain one or more elements selected from the group consisting of Ti, Nb, and V.
  • the lower limit value of the content of each of one or more elements selected from the group consisting of Ti, Nb, and V is preferably 0.005% or more.
  • Nb when the Nb content is 0.300% or less, a delay in recrystallization of the ferrite phase can be suppressed, and a desired structure can be obtained more stably. Therefore, it is preferable to set the upper limit of the content of each of one or more elements selected from the group consisting of Ti, Nb, and V to 0.300% or less.
  • B, Ca, Mg, Zr, and REM (rare earth metal) may not be contained because they are not essential elements for the steel sheet of the present disclosure, and each content is 0% or more.
  • B, Ca, Mg, Zr, and REM improve the local elongation and hole expandability of the steel sheet.
  • 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. 0.001% or more.
  • the upper limit of the content of each of these elements is 0.010% or less, and is selected from the group consisting of B, Ca, Mg, Zr, and REM.
  • the total content of one or more elements is preferably 0.030% or less.
  • REM refers to a total of 17 elements of Sc, Y, and lanthanoid
  • the REM content refers to the content when there is one REM, and the total content when there are two or more.
  • Point to. REM is also supplied as misch metal, which is generally an alloy of a plurality of types of REM. For this reason, one or more individual elements may be added so that the REM content falls within the above range. For example, the REM content may be added in the form of misch metal. You may make it contain so that it may become this range.
  • Sb, Sn, and Bi do not need to be contained because they are not essential elements for the steel sheet of the present disclosure, and each content is 0% or more.
  • Sb, Sn, and Bi suppress oxidizable elements such as Mn, Si, and / or Al in the steel sheet from diffusing into the steel sheet surface to form oxides, and improve the surface properties and plating properties 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 preferably 0.050% or less.
  • the balance is iron and impurities. Impurities are inevitably mixed from steel raw materials or scraps and / or from the steel making process, and elements allowed within a range not impairing the characteristics of the steel sheet according to the present embodiment are exemplified.
  • the metal structure at the 1/8 position (also referred to as 1 / 8t part) of the thickness from the surface of the steel sheet according to the present embodiment is an area ratio of 10% or more austenite phase, 5% or more tempered martensite phase, and It contains 10% or more of the ferrite phase, and the martensite phase is limited to less than 15%.
  • the fraction of each structure varies depending on the heat treatment conditions and affects materials such as yield point, strength, elongation characteristics, and hole expandability. Since the required material varies depending on, for example, parts for automobiles, heat treatment conditions may be selected as necessary to control the tissue fraction.
  • the area ratio of each structure can be measured by observing the microstructure at 1/8 position of the thickness from the surface of the steel sheet.
  • the L cross section refers to a surface cut so as to pass through the central axis of the steel plate in parallel with the plate thickness direction and the rolling direction.
  • the steel sheet according to the present embodiment it is important that the amount of austenite phase in the metal structure is in a predetermined range.
  • Austenite is a structure that increases the ductility of a steel sheet by transformation-induced plasticity. Since austenite can be transformed into martensite by stretching, drawing, stretch flange processing, or bending with tensile deformation, it contributes to improving the strength of the steel sheet. In order to obtain these effects, the steel plate according to the present embodiment needs to contain an austenite phase of 10% or more in area ratio in the metal structure.
  • the area ratio of the austenite phase is preferably 15% or more, more preferably 20% or more, and further preferably 25% or more. When the area ratio of the austenite phase is 15% or more, the elongation characteristic is maintained to a higher strength.
  • the upper limit of the area ratio of the austenite phase is not particularly defined, but is substantially 40%.
  • the area ratio of the austenite phase can be measured by backscattered electron diffraction (EBSP: Electron Back Back Scattering Pattern). It is possible to measure the area ratio of the austenite phase by measuring at least 8 visual fields in a range of at least 100 ⁇ m ⁇ 100 ⁇ m at a pitch of 0.1 ⁇ m and averaging the measured values.
  • the martensite (also referred to as fresh martensite) phase is a hard phase containing many dislocations in its structure, and is an effective phase for obtaining the strength of the steel sheet.
  • the area ratio in the metal structure of the martensite phase is set to less than 15% in order to remarkably deteriorate the hole expandability.
  • local elongation can be improved by making the area ratio in the metal structure of a martensite phase less than 15%.
  • the martensite phase may not be contained in the metal structure. That is, the area ratio in the metal structure of the martensite phase may be 0%.
  • the area ratio of the martensite phase is preferably 10% or less, and more preferably 5% or less.
  • the tempered martensite phase is also a hard phase, but has a different structure from the martensite phase, and contributes to the improvement of the hole expanding property while ensuring the strength of the steel sheet.
  • the area ratio in the metal structure of the tempered martensite phase needs to be 5% or more.
  • the area ratio of the tempered martensite phase is preferably 10% or more, more preferably 15% or more, and further preferably 20% or more.
  • the upper limit of the area ratio of the tempered martensite phase is not specified, but is substantially less than 80%.
  • the bainite phase may be included in the metal structure, the bainite phase has the same characteristics as the tempered martensite phase, so when the bainite phase is included in the metal structure, the area ratio of the tempered martensite phase is tempered. It is measured including the bainite phase in addition to the martensite phase.
  • the ferrite phase is an essential structure for ensuring ductility.
  • the area ratio of the ferrite phase in the metal structure is 10% or more, preferably 15% or more.
  • the upper limit of the area ratio of ferrite is not particularly specified, but is substantially less than 85%.
  • the remainder other than the austenite phase, martensite phase, tempered martensite phase (including bainite phase), and ferrite phase can be a structure such as pearlite or cementite.
  • the area ratios of the ferrite phase, the martensite phase, and the tempered martensite phase are calculated from the structure observation with a scanning electron microscope (SEM). After the L section of the steel plate is mirror-polished, it is corroded with 3% nital (3% nitric acid-ethanol solution), and the metal structure at 1/8 position from the surface is observed with a scanning electron microscope with a magnification of 5000 times.
  • the ferrite phase (including unrecrystallized ferrite) is discriminated as a gray background structure, and martensite is discriminated as a white structure. Tempered martensite appears white like martensite, but the one in which the substructure is confirmed in the crystal grains is determined to be tempered martensite.
  • the area ratio of non-recrystallized ferrite is 30% or more, preferably 40% or more.
  • the area ratio of non-recrystallized ferrite is within the above range, a steel plate having a high yield point can be obtained. If there is too much unrecrystallized ferrite, it will lead to a decrease in ductility, so the upper limit of the area ratio is 70%.
  • the upper limit of the area ratio of non-recrystallized ferrite is more preferably 60%.
  • the area ratio of unrecrystallized ferrite As for the area ratio of unrecrystallized ferrite, after discriminating ferrite phase grains as described above, EBSP measurement is performed on this area, and an area with a KAM (Kernel Average Misorientation) value of 1 ° or more is It is calculated by measuring as a crystalline ferrite structure.
  • KAM Kernel Average Misorientation
  • CMn ⁇ / CMn ⁇ which is the ratio of the average Mn concentration CMn ⁇ in the austenite phase and the average Mn concentration CMn ⁇ in the ferrite phase (all ferrite phases including unrecrystallized ferrite phase), is 1.2 or more, preferably 1.5 or more. .
  • CMn ⁇ / CMn ⁇ is within the above range, sufficient Mn distribution to concentrate Mn is obtained at the place where it was an austenite phase during heat treatment, and a stable austenite phase can be obtained even after short-time annealing. Is obtained.
  • CMn ⁇ / CMn ⁇ is less than 1.2, Mn distribution is not sufficient, and it becomes difficult to obtain an austenite phase by short-time annealing.
  • CMn ⁇ / CMn ⁇ is preferably less than 2.0.
  • CMn ⁇ / CMn ⁇ can be measured by EBSP, SEM, and electron beam microanalyzer (EMPA).
  • CMn ⁇ / CMn ⁇ can be calculated by measuring the austenite phase and ferrite phase by EBSP and SEM, and measuring CMn ⁇ and CMn ⁇ by EMPA.
  • the tensile strength (TS) of the steel sheet according to this embodiment is preferably 780 MPa or more, more preferably 980 MPa or more. This is because when a steel plate is used as an automobile material, the plate thickness is reduced by increasing the strength, thereby contributing to weight reduction. Moreover, in order to use the steel plate which concerns on this embodiment for press forming, it is desirable that elongation (El) is excellent. In that case, TS ⁇ El is preferably 28000 MPa ⁇ % or more, more preferably 30000 MPa ⁇ % or more, and further preferably 32000 MPa ⁇ % or more.
  • the steel sheet according to the present embodiment is further excellent in hole expansibility ( ⁇ ), preferably 20% or more, more preferably 25% or more, and further preferably 30% or more. Further, the steel sheet according to the present embodiment preferably has a yield ratio YR of 0.65 or more (obtained by gradually increasing the yield point YP with the tensile strength TS) and exhibits good collision characteristics. Further, the steel sheet according to the present embodiment preferably has a local elongation of 1.5% or more, more preferably 1.8% or more, and further preferably 2.0%, and exhibits good formability.
  • an example of the steel sheet according to the present embodiment is excellent in yield point YP and elongation, and shows a stress-strain curve (SS curve) as shown by “A” in FIG.
  • heat treatment of the hot-rolled steel sheet in the method of the present disclosure that is, annealing the cold-rolled steel sheet for a short time without performing heat treatment for 1 hour or more in a temperature range where the austenite phase fraction is 20% to 50%.
  • the steel sheet obtained by applying only this exhibits a stress-strain curve as indicated by “C” having a small elongation.
  • an example of the steel sheet according to the present embodiment can obtain a higher yield point YP and elongation than a steel sheet obtained by performing only a short time annealing shown in “C”.
  • the yield ratio YR of the steel sheet “A” shown in FIG. 1 is preferably 0.80 or more, more preferably 0.90 or more.
  • a steel sheet according to the present embodiment is “A” as shown as “B” in FIG. 1 by performing a relatively large skin pass rolling with a rolling reduction of 1.0% or more and less than 5.0%. It can also show a stress-strain curve that is slightly lower but has a good yield point and has a lower yield elongation than “A”.
  • a steel sheet showing a stress-strain curve as shown in “A” preferably exhibits a yield ratio YR of 0.80 or more, while a steel sheet showing a stress-strain curve as shown in “B” is preferably 0.8. It exhibits a yield ratio YR of 65 or more, and preferably exhibits a yield elongation (YP-El) of less than 10%.
  • a steel sheet showing a stress-strain curve as indicated by “B” has a smaller yield elongation, and therefore, can be dispersed without localizing the strain than “A”. Thereby, local deformation can be suppressed.
  • the metal structures of the “A” and “B” steel plates will be described later.
  • the steel sheet of the present disclosure has high strength, good elongation characteristics, good hole expandability, and excellent formability.
  • the steel sheet “A” has a higher yield point, the initial absorbed energy at the time of collision can be improved, and the steel sheet “B” has a good yield point and has a small yield elongation. Energy can be suppressed and more energy can be absorbed. Therefore, both the steel sheets “A” and “B” are excellent in collision characteristics.
  • These steel plates are optimal for structural parts of automobiles such as pillars, and any one of the steel plates can be selected according to the usage.
  • the steel sheet of the present disclosure has a high content of Mn and contributes to the weight reduction of automobiles, so that the industrial contribution is extremely remarkable.
  • the steel plate according to the present embodiment is prepared by melting steel having the above-described chemical composition in a conventional manner, casting it to produce a slab or a steel ingot, and heating this to a finishing temperature of 1000 ° C. or less and a winding temperature of After hot rolling at less than 300 ° C., the obtained hot-rolled steel sheet is heat-treated in a ferrite / austenite two-phase region, pickled, and then cold-rolled at a cold rolling rate of 30% to 70%. Then, it is manufactured by annealing for a short time in a ferrite / austenite two-phase region.
  • Hot rolling may be performed on a normal continuous hot rolling line.
  • the heat treatment of the hot-rolled steel sheet after hot rolling can be performed in a batch furnace such as a box annealing furnace (BAF) or a tunnel furnace such as a continuous annealing furnace.
  • Cold rolling may be performed by a normal continuous cold rolling line.
  • the annealing can be performed using a continuous annealing line, and thus the productivity is very excellent.
  • hot rolling conditions heat treatment conditions for hot-rolled steel sheets after hot rolling
  • cold rolling conditions cold rolling conditions
  • annealing conditions are as follows. It is preferable to carry out within.
  • the molten steel may be melted by a normal blast furnace method, and the raw material contains a large amount of scrap like steel produced by the electric furnace method. May be included.
  • the slab may be manufactured by a normal continuous casting process or may be manufactured by thin slab casting.
  • the above-mentioned slab or steel ingot is heated and hot-rolled to obtain a hot-rolled steel sheet.
  • the temperature of the steel material used for hot rolling is preferably 1100 ° C. or higher and 1300 ° C. or lower.
  • the temperature of the steel material used for hot rolling is preferably 1100 ° C. or higher and 1300 ° C. or lower.
  • the time for holding in the temperature range of 1100 ° C. or higher and 1300 ° C. or lower, which is the preferable temperature range before hot rolling, is not particularly specified, but is preferably 30 minutes or longer in order to improve bendability. More preferably, the above is used. Moreover, in order to suppress an excessive scale loss, it is preferable to set it as 10 hours or less, and it is more preferable to set it as 5 hours or less. In addition, it is a case where direct feed rolling or direct rolling is performed, and it may be subjected to hot rolling as it is without performing heat treatment.
  • Finish rolling is performed in hot rolling.
  • Finish rolling start temperature shall be 1000 degrees C or less. If the finishing rolling start temperature exceeds 1000 ° C., the coarsening of the structure in the hot-rolled state cannot be prevented and the subsequent structure control becomes difficult, and the surface properties of the steel sheet are deteriorated due to grain boundary oxidation. It is also difficult to suppress. After finishing rolling, it cools and winds at less than 300 degreeC.
  • the hot-rolled sheet structure When rolled up at 300 ° C or higher, the hot-rolled sheet structure cannot be made into a full martensite structure, and Mn distribution and austenite reverse transformation are efficiently performed in the heat treatment of the hot-rolled steel sheet and the annealing process of the cold-rolled steel sheet, respectively. It becomes difficult to wake up.
  • the finish rolling start temperature is preferably 750 ° C. or higher. When the finish rolling start temperature is 750 ° C. or higher, the deformation resistance during rolling can be reduced, and the structure can be easily controlled.
  • Heat treatment of hot-rolled steel sheet Hold for 1 hour or more in a temperature range where the austenite phase fraction is 20% to 50%
  • the obtained hot-rolled steel sheet is heat-treated for 1 hour or longer in a temperature range where the austenite phase fraction is 20% to 50%.
  • Mn is distributed to the austenite to stabilize the austenite.
  • high ductility can be obtained.
  • the heat treatment is performed at a temperature where the austenite phase fraction is less than 20% or more than 50% in the heat treatment, it becomes difficult to stabilize the austenite.
  • the metal structure at 1/8 of the thickness from the surface in the L cross section of the annealed steel sheet is 10% in area ratio.
  • the above austenite phase can be included.
  • the temperature range in which the area ratio of the austenite phase is 20% to 50% is determined from the volume change during heating by heating at a heating rate of 0.5 ° C / sec from room temperature in an offline preliminary experiment, depending on the components of the steel sheet. It can be obtained by measuring the austenite phase fraction.
  • the lower limit of the heat treatment holding time is preferably 2 hours or more, more preferably 3 hours or more.
  • the upper limit of the heat treatment holding time is preferably within 10 hours, more preferably within 8 hours, from the viewpoint of productivity.
  • FIG. 2 shows that after hot rolling, when heat treatment is performed at 650 ° C. for 6 hours in a temperature range in which the austenite phase fraction is 20% to 50% in the two-phase region, and for 15 minutes at 500 ° C. which is the single-phase region.
  • the example of the mapping result showing the distribution state of Mn in the position of 1/8 of the thickness from the surface of the L cross section of the steel sheet when the heat treatment is performed is shown.
  • Mn is discharged from ferrite and Mn is concentrated in austenite (light-colored region).
  • the Mn concentration in the portion that was ferrite at 650 ° C. becomes low.
  • the hot-rolled steel sheet is pickled by a conventional method, and then cold-rolled at a rolling reduction of 30% to 70% to obtain a cold-rolled steel sheet. If the rolling reduction of cold rolling is less than 30%, the grain size remains large and the austenite reverse transformation is delayed, so that a sufficient austenite phase cannot be obtained. On the other hand, if the rolling reduction is more than 70%, sufficient unrecrystallized ferrite cannot be obtained.
  • the lower limit value of the cold rolling reduction is preferably 40% or more.
  • the upper limit of the cold rolling reduction is preferably 60% or less.
  • the obtained cold-rolled steel sheet is held for 30 seconds to less than 15 minutes, preferably 1 minute to 5 minutes or less, in a temperature range where the austenite phase fraction is 20% to 50% in the two-phase region and is annealed.
  • the Mn distribution has already been completed in the heat treatment of the hot-rolled steel sheet, and Mn is concentrated in the place that was austenite during the heat treatment. Stable austenite can be obtained, and excellent ductility can be obtained by a short annealing treatment.
  • annealing is performed in a temperature range where the austenite phase fraction is 25% to 40%.
  • the difference between the temperature in the heat treatment before cold rolling and the temperature in the annealing after cold rolling is preferably equivalent to 15% or less, more preferably equivalent to 10% or less in terms of the difference in austenite phase fraction. Either the temperature in the heat treatment before cold rolling or the temperature in the annealing after cold rolling may be higher.
  • the austenite phase fraction in the heat treatment before cold rolling and the austenite phase in the annealing after cold rolling Since the fraction can be made closer, austenite can be generated only at the location where Mn is concentrated in the annealing after cold rolling.
  • the temperature in the heat treatment before cold rolling and the temperature in the annealing after cold rolling are substantially the maximum temperatures in the heat treatment profile.
  • the temperature is maintained at 100 ° C. to 500 ° C. for 10 seconds to 1000 seconds.
  • a temperature range of 100 ° C. or more and 500 ° C. or less martensite is generated, and the subsequent holding causes martensite self-tempering.
  • C distribution to austenite is sufficiently advanced, and austenite can be stably generated in the structure before the final heat treatment, As a result, it is possible to suppress the formation of massive austenite in the structure after the final heat treatment, and to suppress fluctuations in strength characteristics.
  • the holding time in the temperature range of 100 ° C. to 500 ° C. is 1000 seconds or less. , Preferably 300 seconds or less, more preferably 180 seconds or less.
  • the efficiency of the continuous annealing line can be improved by setting the holding temperature to 100 ° C. or higher. On the other hand, by setting the holding temperature to 500 ° C. or lower, grain boundary segregation can be suppressed and bendability can be improved.
  • the steel plate After the cooling, it is preferable to cool the steel plate to room temperature.
  • the cooling after the annealing may be performed to room temperature as it is when the steel plate is not plated. Moreover, when plating on a steel plate, it can manufacture as follows.
  • the cooling after the annealing is stopped in a temperature range of 430 to 500 ° C., and then the cold rolled steel sheet is immersed in a hot dip galvanizing bath. Then, hot dip galvanizing is performed.
  • the conditions for the plating bath may be within the normal range. What is necessary is just to cool to room temperature after a plating process.
  • an alloyed hot dip galvanized steel sheet by subjecting the surface of the steel sheet to galvannealed steel sheet, after the hot dip galvanizing treatment is performed on the steel sheet, before the steel sheet is cooled to room temperature, the temperature is changed to 450 to 620 ° C.
  • An alloying treatment of hot dip galvanizing is performed at a temperature.
  • the alloying treatment conditions may be within a normal range.
  • Skin pass rolling may be performed on the steel plate after annealing or the steel plate after plating.
  • the rolling reduction of such skin pass rolling is preferably 0% or more and less than 5.0% (that is, including the case where skin pass rolling is not performed), and more preferably 4.0% or less.
  • skin pass rolling is performed to the steel plate after plating.
  • the rolling reduction of the skin pass rolling can be appropriately selected according to the desired characteristics to be expressed in the steel plate. For example, when skin pass rolling with a rolling reduction of 0% or more and less than 1.0% is performed, finally a steel sheet having a higher yield point and a higher yield elongation is obtained as shown as “A” in FIG. Can do. Those having this characteristic can increase the initial absorption energy at the time of collision.
  • the average dislocation density of the ferrite phase is less than 2 ⁇ 10 12 / m 2 .
  • the average dislocation density is the total dislocation density of fixed dislocations and movable dislocations in the ferrite phase (all ferrite phases including non-recrystallized ferrite phases).
  • the average dislocation density of the ferrite phase is in the range of 2 ⁇ 10 12 / m 2 or more and less than 4 ⁇ 10 12 / m 2 , it is slightly lower than “A” as shown as “B” in FIG.
  • a steel sheet having a good yield point and exhibiting a stress-strain curve with a yield elongation smaller than “A” can be obtained.
  • a steel plate having such characteristics can suppress the localized deformation at the time of a collision, so that the absorbed energy can be increased. In this way, characteristics suitable for the member can be created depending on how the skin pass is applied.
  • the average dislocation density is 2 ⁇ 10 12 / m 2 or more, a steel sheet exhibiting a stress-strain curve having a good yield point and a small yield elongation can be obtained although it is slightly low.
  • the average dislocation density is 4 ⁇ 10 12 / m 2 or more, that is, the rolling reduction is 5.0% or more, it is difficult to ensure sufficient elongation.
  • the rolling reduction of skin pass rolling is preferably more than 2.0%.
  • the average dislocation density of the ferrite phase can be measured by conventional measurement using a TEM (transmission electron microscope).
  • the steel plate according to this embodiment can be obtained.
  • 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 the manufacturing method thereof, and the steel sheet of the present disclosure and the manufacturing method thereof are not limited to the following examples.
  • the obtained slab was hot-rolled at a finishing temperature and a winding temperature shown in Table 2 to produce a 2.6 mm thick hot-rolled steel sheet.
  • the obtained hot-rolled steel sheet is subjected to a heat treatment at a temperature and a holding time at an austenite phase fraction shown in Table 2, then pickled, and further subjected to cold rolling at a cold rolling rate shown in Table 2, A cold rolled steel sheet having a thickness of 1.2 mm was made.
  • the heat treatment of the hot-rolled steel sheet was performed in a reducing atmosphere of 98% nitrogen and 2% hydrogen.
  • the alphabetical characters in the “Steel” column shown in Tables 2 to 7 below correspond to the steel type symbols shown in the “Steel” column of Table 1, respectively.
  • the obtained cold-rolled steel sheet was annealed at a temperature and holding time at which the austenite phase fraction shown in Table 2 was obtained.
  • the cold rolled steel sheet was annealed in a reducing atmosphere of 98% nitrogen and 2% hydrogen.
  • the heat treatment temperature of the hot-rolled steel sheet and the annealing temperature of the cold-rolled steel sheet were a temperature difference corresponding to the austenite phase fraction shown in Table 2.
  • the steel sheet After holding the annealing temperature, the steel sheet was cooled under the conditions of average cooling rate, cooling stop temperature, and holding time shown in Table 2.
  • the example which does not describe the numerical values of the cooling stop temperature and the holding time means an example of cooling after annealing, cooling is not performed in the temperature range of 100 ° C. or higher and 500 ° C. or lower, and is cooled to room temperature as it is after annealing. To do.
  • annealed cold-rolled steel sheets after annealing, cooling after annealing was stopped at 400 ° C., and the cold-rolled steel sheets were immersed in a 400 ° C. hot-dip galvanizing bath for 2 seconds to perform hot dip galvanizing treatment Went.
  • the conditions of the plating bath are the same as the conventional one.
  • the sample was cooled to room temperature at an average cooling rate of 10 ° C./second after maintaining at 400 ° C.
  • Some of the annealed cold-rolled steel sheets were hot-dip galvanized and then subjected to alloying without cooling to room temperature.
  • the mixture was heated to 500 ° C., held at 500 ° C. for 5 seconds to perform alloying treatment, and then cooled to room temperature at an average cooling rate of 10 ° C./second.
  • the area ratio of the austenite phase was measured using backscattered electron diffraction (EBSP: Electron Back Scattering pattern).
  • EBSP Electron Back Scattering pattern
  • the L cross-section obtained by cutting the steel plate in parallel with the plate thickness direction and the rolling direction was subjected to mirror polishing by diamond buffing and alumina polishing, and then the microstructure was revealed with 3% nital, and 100 ⁇ m at 1/8 position from the surface.
  • the x100 ⁇ m range was measured with 8 visual fields at a pitch of 0.1 ⁇ m, and the measured values were averaged.
  • the area ratios of the ferrite phase, the tempered martensite phase, and the martensite phase were calculated from structural observation with a scanning electron microscope (SEM). About the microstructure which carried out the said mirror surface polishing and the nital processing, the range of 0.2 mm x 0.3 mm in the 1/8 position from the surface of the center of the width direction of a steel plate with a scanning electron microscope of 5000 times is carried out at intervals of 0.5 mm. Two visual fields were observed. The area ratio was calculated by measuring 400 to 500 points using the JIS-G0555 point calculation method.
  • the ferrite phase (including non-recrystallized ferrite) was identified as a gray background structure, and martensite was identified as a white structure. Tempered martensite appears white like martensite, but the one in which the substructure was confirmed in the crystal grains was judged to be tempered martensite.
  • the area ratio of non-recrystallized ferrite is determined as follows. 100 to 150 ferrite phase grains are discriminated, and EBSP measurement is performed on the discriminated crystal grains to calculate the KAM value of each crystal grain. And a region of 1 ° or more was calculated as an unrecrystallized ferrite structure.
  • CMn ⁇ / CMn ⁇ was measured by EBSP, SEM, and electron beam microanalyzer (EMPA).
  • EMPA electron beam microanalyzer
  • 10 points each of austenite phase and ferrite phase were selected using EBSP and SEM, and the average values of 10 points were measured as CMn ⁇ and CMn ⁇ , respectively, using EMPA with an acceleration voltage of 15 kV.
  • CMn ⁇ / CMn ⁇ was calculated.
  • the yield point (YP) and the yield elongation (YP-El) were measured by the methods specified in JIS-Z2241.
  • the yield point means a falling yield point when there is a yield phenomenon, and 0.2% proof stress when there is no yield phenomenon.
  • a JIS No. 5 tensile test piece was taken from the direction perpendicular to the rolling direction of the steel sheet, the tensile strength (TS) and the elongation (El) were measured, and TS ⁇ uEL was calculated.
  • the tensile test was performed by the method prescribed
  • the elongation was measured by a method defined in JIS Z2241: 2011 using a JIS No. 5 test piece having a parallel part length of 60 mm and a standard distance of 50 mm as a reference for measuring strain. Uniform elongation is the elongation (strain measured between gauge points) obtained until the maximum test strength (TS) is reached.
  • the measurement of local elongation was calculated by subtracting the value of elongation at the maximum load point (uniform elongation) from the value of elongation (total elongation) when the fractured specimens were butted.
  • Table 5 shows the evaluation results for the steel plates produced under the conditions shown in Table 2.
  • the steel sheet was evaluated as a steel sheet having high properties and high strength.
  • Table 6 shows the evaluation results for the steel sheets produced under the conditions shown in Table 3.
  • a steel plate (invention example) subjected to skin pass rolling at a reduction rate (SPM) of 2.0% and 4.0% is a steel plate (invention example) subjected to skin pass rolling at a reduction rate (SPM) of 0.5%.
  • SPM reduction rate
  • the yield point (YP) and the yield ratio (YR) are slightly decreased, the yield ratio is 0.65 or more, and the yield elongation (YP-El) is suppressed to less than 10%, while 20% or more.
  • a steel plate exhibiting a high TS ⁇ El of 28,000 MPa ⁇ % or more was obtained.
  • Table 7 shows the evaluation results for the steel sheets produced under the conditions shown in Table 4.

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Abstract

L'invention concerne une tôle d'acier qui présente de bonnes caractéristiques de collision, d'excellentes caractéristiques d'allongement, une excellente extensibilité des trous, une résistance élevée et une concentration élevée en Mn. La tôle d'acier contient, en % en masse, plus de 0,10 % et moins de 0,55 % de C, au moins 0,001 % et moins de 3,50 % de Si, plus de 4,20 % et moins de 9,00 % de Mn, et au moins 0,001 % et moins de 3,00 % d'Al soluble. La tôle d'acier est caractérisée en ce que la composition métallique au niveau d'une position au 1/8ème de l'épaisseur à partir de la surface dans une section transversale en L contient, en rapport de surface, au moins 10 % d'une phase austénite, au moins 5 % d'une phase martensite revenue, et au moins 10 % d'une phase ferrite, la phase martensite est limitée à moins de 15 %, le rapport de surface de la ferrite non recristallisée à l'intérieur de la phase ferrite est de 30 à 70 %, et le rapport CMnγ/CMnα de la concentration en Mn moyenne CMnγ dans la phase austénite et de la concentration en Mn moyenne CMnα dans la phase ferrite est supérieur ou égal à 1,2.
PCT/JP2019/014878 2018-04-03 2019-04-03 Tôle d'acier et procédé de production de tôle d'acier WO2019194251A1 (fr)

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TWI703220B (zh) * 2020-01-06 2020-09-01 中國鋼鐵股份有限公司 汽車用鋼及其製造方法

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WO2013061545A1 (fr) * 2011-10-24 2013-05-02 Ｊｆｅスチール株式会社 Procédé de fabrication d'une feuille d'acier haute résistance ayant une aptitude supérieure au façonnage
WO2016067624A1 (fr) * 2014-10-30 2016-05-06 Ｊｆｅスチール株式会社 Tôle d'acier hautement résistante, tôle d'acier galvanisée à chaud hautement résistante, tôle d'acier aluminiée à chaud hautement résistante ainsi que tôle d'acier électrozinguée hautement résistante, et procédés de fabrication de celles-ci
WO2017183349A1 (fr) * 2016-04-19 2017-10-26 Ｊｆｅスチール株式会社 Tôle d'acier, tôle d'acier plaquée et leur procédé de production

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JP5369663B2 (ja) * 2008-01-31 2013-12-18 Ｊｆｅスチール株式会社 加工性に優れた高強度溶融亜鉛めっき鋼板およびその製造方法
BR112014007500A2 (pt) * 2011-09-30 2017-04-04 Nippon Steel & Sumitomo Metal Corp folha de aço galvanizada por imersão a quente e método de fabricação da mesma

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WO2013061545A1 (fr) * 2011-10-24 2013-05-02 Ｊｆｅスチール株式会社 Procédé de fabrication d'une feuille d'acier haute résistance ayant une aptitude supérieure au façonnage
WO2016067624A1 (fr) * 2014-10-30 2016-05-06 Ｊｆｅスチール株式会社 Tôle d'acier hautement résistante, tôle d'acier galvanisée à chaud hautement résistante, tôle d'acier aluminiée à chaud hautement résistante ainsi que tôle d'acier électrozinguée hautement résistante, et procédés de fabrication de celles-ci
WO2017183349A1 (fr) * 2016-04-19 2017-10-26 Ｊｆｅスチール株式会社 Tôle d'acier, tôle d'acier plaquée et leur procédé de production

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JP2022531249A (ja) * 2019-12-09 2022-07-06 ヒュンダイ スチール カンパニー 超高強度および高成形性を有する合金化溶融亜鉛めっき鋼板およびその製造方法
JP7258183B2 (ja) 2019-12-09 2023-04-14 ヒュンダイ スチール カンパニー 超高強度および高成形性を有する合金化溶融亜鉛めっき鋼板およびその製造方法

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