WO2015198582A1 - Tôle d'acier à haute résistance - Google Patents

Tôle d'acier à haute résistance Download PDF

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Publication number
WO2015198582A1
WO2015198582A1 PCT/JP2015/003114 JP2015003114W WO2015198582A1 WO 2015198582 A1 WO2015198582 A1 WO 2015198582A1 JP 2015003114 W JP2015003114 W JP 2015003114W WO 2015198582 A1 WO2015198582 A1 WO 2015198582A1
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steel sheet
phase
steel
strength
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PCT/JP2015/003114
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English (en)
Japanese (ja)
Inventor
長谷川　浩平
義彦 小野
村井　剛
櫻井　理孝
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Ｊｆｅスチール株式会社
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Priority to JP2016529072A priority Critical patent/JP6048623B2/ja
Publication of WO2015198582A1 publication Critical patent/WO2015198582A1/fr

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    • 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
    • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
    • 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
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese
    • 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

Definitions

  • the present invention relates to a high-strength steel plate having a tensile strength of 780 MPa or more, and particularly to a high-strength steel plate excellent in bending workability suitable for manufacturing machine structural parts such as automobile structural members and reinforcing members.
  • the chemical component is mass%, C: 0.08 to 0.20%, Si: 0.1 to 1.5%, Mn: 1.5 to 2.5%, P: 0.02% or less, S: 0.002% or less, Al: 0.02 to 0.06%, N: 0.0005% or less, Ca: 0.0005% or less, O: 0.0007% or less
  • the balance is composed of Fe and inevitable impurities, and the structure is composed of a ferrite phase and a low-temperature transformation generation phase.
  • the size of inclusions in the structure is expressed by the diameter of a circle corresponding to the area, the diameter is 5 ⁇ m or more.
  • An ultrahigh strength cold-rolled steel sheet excellent in bending workability is proposed, characterized by having 25 inclusions / mm 2 or less and a tensile strength of 780 MPa or more.
  • the cause of cracks caused by bending is oxide inclusions
  • Ca is 0.0005% or less for forming oxide inclusions
  • 0 for easily forming relatively large inclusions is 0. It is disclosed that excellent bending workability is obtained by reducing N to 0.0005% or less to a very low level while reducing the bending workability to an extremely low level of .0007% or less.
  • a steel layer has a soft layer with a ferrite volume fraction of 90% or more and a thickness of 10 to 100 ⁇ m on the surface layer of the steel sheet, the central structure has a tempered martensite volume ratio of 30% or more, and the remainder is a ferrite phase.
  • An ultra-high strength cold-rolled steel sheet having excellent bendability and stretch flangeability is disclosed.
  • Patent Document 3 discloses a high-strength cold-rolled steel sheet in which the metal structure and the amount of inclusions are limited for the purpose of improving stretch flangeability.
  • a tempered martensite having a hardness of 380 Hv or less includes an area ratio of 50% or more (including 100%), and the balance has a structure made of ferrite, and is present in the tempered martensite.
  • the number of cementite particles having a diameter of 0.1 ⁇ m or more is 2.3 or less per 1 ⁇ m 2 of the tempered martensite, and the inclusion having an aspect ratio of 2.0 or more present in the entire structure is 200 or less per 1 mm 2.
  • a high-strength cold-rolled steel sheet excellent in certain stretch flangeability has been proposed.
  • Patent Document 4 the total of one or two of Ce or La is 0.001 to 0.04%, and (Ce + La) / acid-soluble Al ⁇ 0.1 on a mass basis.
  • a high-strength steel sheet having a chemical component having (Ce + La) / S of 0.4 to 50 and excellent in stretch flangeability and fatigue characteristics has been proposed.
  • MnS, TiS, and (Mn, Ti) S are deposited on fine and hard Ce oxide, La oxide, cerium oxysulfide, and lanthanum oxysulfide generated by deoxidation by addition of Ce and La.
  • Patent Document 1 requires a great deal of time and cost in the steel making process in order to reduce Ca, N, and O to the ranges described above.
  • Patent Document 2 since the surface layer of the steel sheet is made soft, there is a problem that the fatigue characteristics that are significantly affected by the surface layer hardness are remarkably deteriorated.
  • Patent Document 3 is to improve the stretch flangeability by controlling the form of MnS inclusions, etc., but has suggestions regarding the control of oxide inclusions that greatly affect bending workability. Not give.
  • Patent Document 4 is not necessarily effective for improving the bending workability to be improved by the present invention. Further, since the addition of special elements such as Ce and La is necessary, the manufacturing cost is remarkably increased.
  • the present invention has been made as a result of earnest research in view of such circumstances, and has a tensile strength of 780 MPa or more, suitable for the production of automobile structural members, reinforcing members, and all other mechanical structural members, and has bending workability.
  • An object is to provide a high-strength steel sheet excellent in resistance at a low cost.
  • the present inventors have studied the factors governing the bending workability of high-strength steel sheets. As a result, it was found that the starting point of cracking during processing was an oxide inclusion having a particle major axis of 2 ⁇ m or more existing within 100 ⁇ m from the steel sheet surface. In order to ensure excellent bending workability, it is effective to reduce the number of inclusions to 100 or less per 100 mm 2, and to progress of micro cracks generated during bending, It was clarified that the chemical composition (component composition) of the steel sheet and the metal structure of the steel sheet determined by the heat treatment are affected.
  • the gist of the present invention is as follows.
  • a high-strength steel sheet having a tensile strength of 780 MPa or more and excellent in bending workability, which is optimal for the manufacture of machine structural members such as automobile structural members and reinforcing members, at low cost. Very useful.
  • C 0.05 to 0.18% C is an important element for strengthening the martensite of the quenched structure. If the C content is less than 0.05%, the effect of increasing the strength is insufficient. For this reason, the amount of C is made into 0.05% or more. Preferably, the amount of C is 0.10% or more. On the other hand, if the amount of C exceeds 0.18%, the strength becomes too high and the bending workability is remarkably deteriorated. Further, since the welded portion is broken in the cross tension test in spot welding, the joint strength is significantly reduced. For this reason, the amount of C is made into 0.18% or less. Preferably, the amount of C is 0.15% or less.
  • Si 0.8 to 3.0% Si is effective for increasing the ductility of the high strength composite steel sheet. If the Si amount is less than 0.8%, the effect is not sufficient. When the Si content is less than 0.8%, the bending workability improving effect by controlling the composition of oxide inclusions, which is a feature of the present invention, is not recognized. For this reason, the amount of Si shall be 0.8% or more. Preferably, the amount of Si is 1.2% or more. On the other hand, when the amount of Si exceeds 3.0%, a large amount of Si oxide is formed on the surface of the steel sheet in the hot rolling process, and surface defects are generated. For this reason, the amount of Si shall be 3.0% or less. From the viewpoint of chemical conversion properties, the Si content is preferably 2.3% or less.
  • Mn 1.5 to 3.0% Mn is an important element for suppressing the formation of ferrite during cooling to the quenching start temperature in continuous annealing. If the amount of Mn is less than 1.5%, the effect of suppressing such ferrite formation is not sufficient. For this reason, the amount of Mn is 1.5% or more. On the other hand, if the amount of Mn exceeds 3.0%, so-called slab cracking occurs in which the steel piece (slab) is cracked in the continuous casting process, so the amount of Mn is set to 3.0% or less. In order to improve the production stability in the continuous annealing process, the Mn content is preferably 1.8% or more, and preferably 2.5% or less.
  • P 0.02% or less
  • P is an impurity in the steel of the present invention, and is desirably removed by a steel making process as much as possible in order to deteriorate spot weldability.
  • the amount of P needs to be 0.02% or less.
  • the amount of P is 0.01% or less.
  • S 0.01% or less S is an impurity in the steel of the present invention, and is desirably removed in the steelmaking process as much as possible in order to deteriorate spot weldability and bending workability.
  • the amount of S exceeds 0.01%, the spot weldability deteriorates significantly, so the amount of S needs to be 0.01% or less.
  • the amount of S is 0.002% or less.
  • Sol. Al 0.01 to 0.1% Al is added to deoxidize and precipitate N as AlN. Sol. If the amount of Al is less than 0.01%, the effect of deoxidation / denitrification is not sufficient. For this reason, Sol. The amount of Al is 0.01% or more. Preferably, Sol. The amount of Al is 0.03% or more. On the other hand, Sol. If the amount of Al exceeds 0.1%, the effect of adding Al becomes saturated and uneconomical. For this reason, Sol. The Al content is 0.1% or less. Preferably, Sol. The amount of Al is 0.06% or less.
  • Sol. Al is acid-soluble aluminum.
  • the amount of Al is the amount of Al excluding Al existing as an oxide out of the total amount of Al in steel.
  • N 0.0015 to 0.0050%
  • N is an impurity contained in the crude steel, and the N content needs to be 0.0050% or less in order to deteriorate the formability of the steel sheet.
  • the N content is 0.0035% or less.
  • the N content is made less than 0.0015%, the refining cost increases significantly.
  • the N amount is set to 0.0015% or more.
  • the N amount is 0.0025% or more.
  • O is a metal oxide or the like produced during refining that remains as inclusions in the steel.
  • the amount of O exceeds 0.0020%, bending workability is remarkably deteriorated. For this reason, the amount of O is made 0.0020% or less.
  • the amount of O is 0.0015% or less.
  • the amount of O is set to 0.0008% or more.
  • the steel of the present invention may further contain the following chemical components depending on the purpose.
  • One or more of Cr: 0.01 to 1.0%, Mo: 0.01 to 0.5%, B: 0.0001 to 0.0030% Cr, Mo, and B are continuously annealed.
  • one or more of these elements can be contained. Since such effects can be obtained at 0.01% or more, 0.01% or more, and 0.0001% or more, the Cr amount is 0.01% or more, the Mo amount is 0.01% or more, B The amount is 0.0001% or more.
  • the Cr amount is 0.1% or more
  • the Mo amount is 0.05% or more
  • the B amount is 0.0003% or more.
  • Cr, Mo, and B exceed 1.0%, 0.5%, and 0.0030%, respectively, ductility deteriorates.
  • the Cr content is 1.0% or less, the Mo content is 0.5% or less, and the B content is 0.0030% or less.
  • the Cr content is 0.7% or less, the Mo content is 0.3% or less, and the B content is 0.0020% or less.
  • Ti, Nb, V, and Zr have the effect of improving stretch flangeability by forming carbides and nitrides in steel in the casting and hot rolling processes and suppressing the coarsening of the crystal grain size.
  • one or more of these elements can be contained. Such an effect can be acquired by making the content of any additive element 0.001% or more. Therefore, the Ti amount is 0.001% or more, the Nb amount is 0.001% or more, the V amount is 0.001% or more, and the Zr amount is 0.001% or more.
  • the Ti amount is 0.1% or less
  • the Nb amount is 0.1% or less
  • the V amount is 0.1% or less
  • the Zr amount is 0.1% or less.
  • One or more of Cu: 0.01 to 0.5%, Ni: 0.01 to 0.5%, Sn: 0.001 to 0.01% Cu, Ni, and Sn increase the corrosion resistance of the steel sheet. There is an effect, and in order to obtain such an effect, one or more of these elements can be contained. Since such effects can be obtained at 0.01% or more, 0.01% or more, or 0.001% or more, respectively, the Cu content is 0.01% or more, the Ni content is 0.01% or more, Sn The amount is 0.001% or more. On the other hand, if Cu, Ni, and Sn exceed 0.5%, 0.5%, and 0.01%, surface defects are generated due to embrittlement during casting and hot rolling. Therefore, the Cu content is 0.5% or less, the Ni content is 0.5% or less, and the Sn content is 0.01% or less.
  • components other than the above are Fe and inevitable impurities.
  • the number of oxide inclusions having a particle length of 2 ⁇ m or more in a steel sheet within 100 ⁇ m from the surface of the steel sheet is 100 or less per 100 mm 2.
  • the alumina content is: The number ratio of those having a composition of 50% by mass or more, silica content: 20% by mass or less, and calcia content: 40% by mass or less is 80% or more.
  • Oxide inclusions present on the center side of the plate thickness from the surface of the steel sheet from 100 ⁇ m or oxide inclusions having a particle length of less than 2 ⁇ m have little influence on the bending workability, and therefore need not be controlled in the present invention. . For this reason, the oxide inclusions within 100 ⁇ m from the steel sheet surface and having a particle major axis of 2 ⁇ m or more are limited as follows.
  • the number of oxide inclusions within 100 ⁇ m from the steel plate surface and the particle major axis is 2 ⁇ m or more exceeds 100 per 100 mm 2 , the bending workability is remarkably deteriorated. For this reason, the number of inclusions is 100 or less per 100 mm 2 .
  • the size of inclusions is evaluated by a cross section including the rolling direction.
  • the evaluation position may be an arbitrary cross section within 100 ⁇ m from the steel sheet surface. However, in the case of non-uniform distribution in the plate thickness direction, the evaluation is made at the depth with the largest number of distributions.
  • the evaluation area should just be 100 mm ⁇ 2 > or more.
  • alumina is inevitably contained as a deoxidation product, but alumina alone has a small effect on bending workability.
  • the oxide content in the oxide inclusions is less than 50% by mass, the oxide has a low melting point, and the oxide inclusions tend to extend during rolling and become a crack starting point during bending. For this reason, the alumina content rate in an oxide inclusion is 50 mass% or more.
  • silica and calcia coexist with alumina, the oxide has a low melting point, and oxide inclusions extend during rolling and become a starting point of cracking during bending, which deteriorates the bending workability of the steel sheet. .
  • the silica content is 20% by mass or less and the calcia content is 40% by mass or less.
  • the average composition of oxides in steel in molten steel is mass%, alumina content: 60% or more, silica content: 10% or less, and calcia content: 20%. (The average composition in the molten steel, the average composition of the intermediate product (for example, slab), and the average composition in the high-strength steel sheet are almost the same).
  • the number ratio of oxide inclusions satisfying the above composition is 80% or more. That is, the number ratio of oxide inclusions having a composition of alumina content: 50% by mass or more, silica content: 20% by mass or less, and calcia content: 40% by mass or less is 80%. That's it. That is, in the technique of the present invention, the melting point of the oxide inclusions can be increased to 1600 ° C.
  • the composition of the oxide inclusions as described above.
  • Such high-temperature oxide inclusions do not have extensibility in the hot rolling process in the steel sheet manufacturing process, and therefore the number ratio of oxide inclusions characterized in the present invention is controlled by the above composition control. It can be within the scope of the invention, and further, the bending workability intended by the present invention can be improved. In order to further improve the bending workability, the number ratio is preferably 90% or more. Adjustment of the oxide composition is achieved by adjusting the slag composition of the converter or secondary refining process.
  • the Ca concentration in the molten steel tends to be close to within the oxide composition range of the present invention. is there. That is, when the oxide composition is out of the range of the present invention, it tends to be adjusted within the oxide composition range of the present invention by reducing the Ca concentration.
  • the average composition of the oxide in steel can be quantitatively determined by cutting out a sample from the slab and using an extraction residue analysis method (for example, Kuraho et al .: Iron and Steel, Vol. 82 (1996), 1017).
  • the upper limit of the alumina content is not particularly limited, but 95% or less is preferable from the viewpoint of melting cost, and the lower limit of the silica content is not particularly limited, but 3% or more is preferable from the viewpoint of melting cost, Although a minimum is not specifically limited, 5% or more is preferable from the point of melting cost.
  • the steel sheet of the present invention needs to have a substantially two-phase structure of ferrite and martensite.
  • the definition of the metal structure in the present invention will be described.
  • Martensite volume ratio 20-65%
  • the volume ratio of the martensite phase is set to 65% or less.
  • the volume ratio of the martensite phase is 60% or less, more preferably 50% or less.
  • the martensite phase includes a tempered martensite phase that has been tempered.
  • the metal structure of the steel sheet of the present invention is preferably substantially a two-phase structure of ferrite and martensite. That is, it is preferable that the remainder other than the martensite phase is substantially the ferrite phase. For this reason, the volume fraction of the ferrite phase is preferably 40% or more, and preferably 80% or less. By making the remainder other than the above-described martensite phase substantially a ferrite phase, it is possible to industrially facilitate the control of the structure ratio and stabilize the mechanical characteristics.
  • the present invention preferably has a substantially two-phase structure of ferrite and martensite. It is preferable that a phase containing iron other than these two phases as a main constituent element, that is, a bainite phase and an austenite phase are not included in the metal structure. Because there is, it may be included.
  • the austenite phase is transformed into hard martensite at the time of bending and becomes the starting point of bending cracking.
  • the bainite phase is more preferably 3% or less.
  • a compound phase containing Fe that is, a cementite phase, etc. may be contained in the ferrite phase, the martensite phase, and their interfaces.
  • a compound phase derived from an additive element such as AlN or MnS or an impurity element is substantially harmless within the chemical component range of the present invention, and may be included in the metal structure.
  • the volume fraction of the bainite phase is less than 5% and the volume fraction of the austenite phase is less than 5%, and the bainite phase and the austenite phase are included, except for the martensite phase and the ferrite phase.
  • the total volume fraction of the metal structure is 10% or less, the influence is small. In this case, it can be said that the metal structure is substantially a two-phase structure of ferrite and martensite. Therefore, the bainite phase is less than 5% (including 0%), the austenite phase is less than 5% (including 0%), and the total volume ratio of the metal structure other than the martensite phase and the ferrite phase is 10% or less (including 0%). ) Is preferable. More preferably, the total volume ratio of the metal structure other than the martensite phase and the ferrite phase is 5% or less.
  • the steel sheet of the present invention has the above-described component composition, and the average composition of oxides in steel is, in mass%, alumina content: 60% or more, silica content: 10% or less, calcia content: 20% or less.
  • the molten steel adjusted to be a steel slab the steel slab is hot-rolled, cold-rolled at a rolling rate of 60% or more to form a cold-rolled sheet, and then the cold-rolled sheet is heated to 750 to 870 ° C. Hold for 10 sec or more in the temperature range, then cool to a quenching start temperature of 550 to 750 ° C., cool from the quenching start temperature to an quenching stop temperature of 300 ° C. or less at an average cooling rate of more than 100 ° C./sec, and then 150 It is preferable to manufacture by holding at ⁇ 450 ° C. for 100 to 1000 seconds.
  • preferable production conditions will be described.
  • the molten steel in which the composition of steel and the average composition of oxides in steel are adjusted as described above is used as a steel slab such as a slab, and the steel slab is subjected to hot rolling.
  • the molten steel is preferably made into a steel piece by continuous casting. What is necessary is just to perform a hot rolling according to a conventional method.
  • the steel slab once lowered in temperature is heated to a predetermined temperature before hot rolling.
  • the heating temperature before the hot rolling is preferably set to 1190 ° C. or lower in order to suppress the extension due to high-temperature deformation of the oxide inclusions to make it harmless and further improve the bending workability.
  • the heating temperature of the steel slab before hot rolling is preferably 1100 ° C. or higher.
  • the steel slab obtained as described above is hot-rolled, cooled and wound up according to a conventional method to obtain a hot-rolled sheet.
  • the winding temperature is preferably 550 ° C. or lower. This is to reduce the distribution of solid solution elements such as Mn during cooling after the hot rolling is completed, thereby reducing the unevenness of the structure.
  • the coiling temperature is less than 450 ° C., it is difficult to control the temperature by water cooling in the cooling after hot rolling, which is normally performed, and the material variation including bending workability increases. For this reason, it is preferable that winding temperature shall be 450 degreeC or more.
  • cold rolling with a rolling rate of 60% or more is performed to obtain a cold rolled sheet.
  • the oxide inclusions in the hot-rolled sheet can be discontinuously divided by cold working, and can be made harmless to bending workability.
  • the cold rolling reduction rate is preferably 60% or more. From the viewpoint of bending workability, the higher the cold rolling rate, the better. On the other hand, when the rolling rate of cold rolling is increased, the productivity is remarkably lowered. Therefore, the rolling rate is preferably 80% or less.
  • the obtained cold-rolled sheet is heated to 750 to 870 ° C., held in this temperature range for 10 seconds or more, then cooled to a quenching start temperature of 550 to 750 ° C., and an average cooling rate from the quenching start temperature: Cooling is performed to a quenching stop temperature of 300 ° C. or less at a temperature exceeding 100 ° C./sec, and then heat treatment is performed at 150 to 450 ° C. for 100 to 1000 sec. This heat treatment is preferably continuous annealing performed by a continuous annealing furnace. This heat treatment condition will be described in detail below.
  • the heating / holding temperature range is 750 to 870 ° C.
  • the held temperature is also referred to as a soaking temperature.
  • the heating / soaking temperature is set to 750 ° C. or higher.
  • the heating / soaking temperature is 800 ° C. or higher.
  • the heating / soaking temperature is set to 870 ° C. or less.
  • the heating / soaking temperature is 840 ° C. or lower.
  • the holding time in this temperature range is 10 sec or more.
  • the holding time is also referred to as a soaking time. If the soaking time is less than 10 seconds, austenite is not sufficiently generated, and it is difficult to obtain sufficient strength.
  • the soaking time is 30 sec or more. In order not to impair the productivity, the soaking time is preferably set to 1200 sec or less.
  • the rapid cooling start temperature is set to 550 ° C. or higher.
  • the rapid cooling start temperature is 600 ° C. or higher.
  • the rapid cooling start temperature is set to 750 ° C. or lower.
  • the quench start temperature is 720 ° C. or lower.
  • Cooling from the rapid cooling start temperature to an average cooling rate of more than 100 ° C / sec and cooling to a quenching stop temperature of 300 ° C or less The quenching is insufficient when the cooling rate from the rapid cooling start temperature to the rapid cooling stop temperature is 100 ° C / sec or less. And the strength tends to be insufficient. For this reason, the cooling rate from the rapid cooling start temperature to the rapid cooling stop temperature is over 100 ° C./sec. In order to stabilize the product material, the cooling rate is preferably 500 ° C./sec or more. On the other hand, when the quenching stop temperature exceeds 300 ° C., a bainite phase is generated or austenite remains, and stretch flangeability is deteriorated. For this reason, the rapid cooling stop temperature is set to 300 ° C. or lower. In order to stabilize the product material, the quenching stop temperature is preferably 100 ° C. or lower.
  • the holding temperature after the rapid cooling stop is set to 450 ° C. or less. Further, if the holding time at 150 to 450 ° C. performed after such a rapid cooling stop is less than 100 sec, the above-described effect that martensite is tempered and bending workability is not sufficiently obtained cannot be obtained. Therefore, the holding time at 150 to 450 ° C. is set to 100 sec or more. On the other hand, when the holding time exceeds 1000 sec, the strength is significantly reduced, and it becomes difficult to obtain a tensile strength of 780 MPa or more. Therefore, the holding time at 150 to 450 ° C. is set to 1000 sec or less.
  • temper rolling is preferably performed in the range of 0.1 to 0.7% in order to eliminate yield elongation.
  • the steel sheet of the present invention may be subjected to electroplating or hot dip galvanizing on the surface of the steel sheet, or a solid lubricant may be applied.
  • cold rolling was performed to obtain a sheet thickness of 1.0 mm (cold rolling rate: 61.5%), and further heat treatment was performed simulating continuous annealing.
  • the heat treatment simulating this continuous annealing it was heated at 780 to 830 ° C. at a heating rate of 20 ° C./sec and held for 300 sec.
  • the cooling rate (average cooling rate) at this time was 2000 ° C./sec.
  • the oxide inclusions are investigated and evaluated as shown below, and the metal structure (structure fraction), tensile properties, bending workability, and chemical conversion properties are evaluated. Investigated and evaluated.
  • the ratio of the number corresponding to the composition to the total number of inclusion particles having a particle long diameter of 2 ⁇ m or more ((composition corresponding number) / (total number of inclusion particles having a particle long diameter of 2 ⁇ m or more)) obtained by the above observation was obtained and the composition was determined. Applicable ratio.
  • the amount of retained austenite phase was determined by X-ray diffraction using Mo K ⁇ rays. That is, using a test piece having a surface near a thickness of 1/4 of the steel sheet as a measurement surface, the peaks of the (211) surface and the (220) surface of the austenite phase and the (200) surface and (220) surface of the ferrite phase The volume fraction of the retained austenite phase was calculated from the strength and used as the volume fraction value. Next, the difference between the volume fraction of the retained austenite phase and the volume fraction of the retained austenite phase was determined as the volume fraction of martensite from the volume fraction of the structure regarded as the martensite phase or the retained austenite phase.
  • JIS No. 5 test piece JISZ2201 was sampled with the direction perpendicular to the rolling direction as the longitudinal direction and subjected to a tensile test according to JISZ2241, yield strength (YS), tensile strength (TS), total elongation (El). Asked.
  • Chemical conversion property Chemical conversion treatment was performed by the following method using a chemical conversion treatment liquid (Palbond L3080 (registered trademark)) manufactured by Nihon Parkerizing. After degreasing with a degreasing liquid Fine Cleaner (registered trademark) manufactured by Nihon Parkerizing Co., Ltd., washing with water, and then adjusting the surface for 30 seconds with surface conditioning liquid preparen Z (registered trademark) manufactured by Nihon Parkerizing Co. After immersing in a liquid (Palbond L3080) for 120 seconds, it was washed with water and dried with warm air. The chemical conversion film thus obtained was randomly observed with a scanning electron microscope (SEM) at a magnification of 500 times, and the scale area ratio of the chemical conversion film was measured by image processing. A scale area ratio of 5% or less was regarded as acceptable ( ⁇ ), and the chemical conversion treatment property was considered good. In addition, a scale area ratio of more than 5% was determined to be rejected (x).
  • a chemical conversion treatment liquid Palbond L3080 (
  • Table 2 shows the evaluation results.
  • the examples of the present invention have a tensile strength TS ⁇ 780 MPa, TS ⁇ E1 ⁇ 15000 MPa ⁇ %, and a limit bending radius R / t ⁇ 1.5, which is excellent in mechanical properties and bending workability. Excellent. Moreover, chemical conversion property was also favorable.
  • the comparative example is inferior in any of the characteristics.
  • the steel plate symbol 1C has a C amount that is too low, and a tensile strength of 780 MPa or more is not obtained.
  • Steel plate symbol 1D is inferior in TS ⁇ El and limit bending radius because the amount of C is too high.
  • the steel plate symbol 1E Since the steel plate symbol 1E has a high ratio of the composition of oxide inclusions outside the range of the present invention, the bending workability deteriorated. Since the steel sheet symbol 1F has an excessively large amount of O, the number of oxide inclusions is larger than the range of the present invention, and the bending workability deteriorates. Since the steel sheet symbol 1H has an excessively low Si content, TS ⁇ El decreased, and accordingly, the bending workability deteriorated. Since the steel plate symbol 1I has too much Si, the bendability deteriorated and the chemical conversion property deteriorated.
  • the steel plate symbols 1K, 1N, 1O, and 1S have low ratios in which the composition of oxide inclusions falls within the scope of the present invention, bending workability was deteriorated in all cases. Since the steel sheet symbol 1L has an Mn amount that is too low, a tensile strength of 780 MPa or more is not obtained. Since the steel plate symbol 1M has an excessively high Mn content, TS ⁇ El decreased, and accordingly, the bending workability deteriorated.
  • Steel sheets were manufactured using steel slabs (steel pieces) whose chemical components are shown in Table 1.
  • the average composition of the oxide in the slab was determined by an extraction residue analysis method (Kurabo et al .: Iron and Steel, Vol. 82 (1996), 1017).
  • AlN was solution-treated at 1200 ° C.
  • Extraction of oxide inclusions (oxides in steel in the slab) was performed by a bromine-methanol method to obtain Al, Si, Ca and composite oxides, and the composition ratio of each oxide was calculated.
  • Table 3 shows the steel number of the steel used in Example 2 and the average composition of oxides in the slab.
  • the slabs shown in Table 3 were hot-rolled, cold-rolled, and continuously annealed under the conditions shown in the same table, and further subjected to temper rolling of 0.1 to 0.5% to obtain a cold sheet having a thickness of 1.0 mm. A rolled steel sheet was obtained. The soaking time was 120 to 480 sec.
  • the cold-rolled steel sheet produced in this way was evaluated by investigating oxide inclusions in the same manner as in Example 1, and the metal structure (structure fraction), tensile properties, bending workability, and chemical conversion treatment properties. was also investigated and evaluated in the same manner as in Example 1. The results are shown in Table 4.
  • the steel sheets of the examples of the present invention have a tensile strength TS ⁇ 780 MPa, TS ⁇ E1 ⁇ 15000 MPa ⁇ %, limit bending radius: R / t ⁇ 1.5, and tensile properties. Excellent bending workability. Moreover, chemical conversion processability is also pass and it is excellent also in chemical conversion processability.
  • the steel plate of the comparative example is inferior in either characteristic.
  • steel plate symbols 2A, 2C, and 2F have a low volume ratio of the martensite phase, do not satisfy the metal structure defined in the present invention, and do not have a tensile strength of 780 MPa or more.
  • Steel plate symbol 2D has a high volume ratio of martensite, does not satisfy the metal structure defined in the present invention, TS ⁇ E1 decreases, and bending workability deteriorates accordingly.
  • the composition inclusion ratio of oxide inclusions is out of the scope of the present invention, and the composition ratio of oxide inclusions in the product falls within the scope of the present invention, so the bending workability deteriorates. did.

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  • Chemical & Material Sciences (AREA)
  • Engineering & Computer Science (AREA)
  • Materials Engineering (AREA)
  • Mechanical Engineering (AREA)
  • Metallurgy (AREA)
  • Organic Chemistry (AREA)
  • Heat Treatment Of Sheet Steel (AREA)

Abstract

La présente invention concerne une tôle d'acier à haute résistance et faible coût qui possède une excellente aptitude au pliage et une résistance à la traction d'au moins 780 MPa, et est adaptée à la fabrication d'éléments structurels pour machines, telles que des automobiles. Ladite tôle d'acier à haute résistance possède une composition de composant spécifique. Des inclusions d'oxyde présentes dans la tôle d'acier à haute résistance entre la surface et 100 µm de profondeur, avec un axe majeur de particules présentant une longueur d'au moins 2 µm, satisfont à des conditions spécifiques. La structure métallographique de la tôle d'acier à haute résistance, en rapport volumique, est de 20 à 65 % d'une phase martensitique, de 40 à 80 % d'une phase ferritique, de moins de 5 % (y compris 0 %) d'une phase bainitique, et de moins de 5 % (y compris 0 %) d'une phase austénitique. Le rapport en volume total des structures métallographiques autres que la phase martensitique et la phase ferritique est de 10 % ou moins (y compris 0 %) La résistance à la traction de la tôle d'acier à haute résistance est au moins égale à 780 MPa. Ici, "l'axe majeur de particules" se réfère à l'axe majeur de particules évalué dans une section transversale contenant la direction de laminage de la tôle d'acier à haute résistance.
PCT/JP2015/003114 2014-06-23 2015-06-22 Tôle d'acier à haute résistance WO2015198582A1 (fr)

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WO2018061526A1 (fr) * 2016-09-29 2018-04-05 Ｊｆｅスチール株式会社 Procédé de fabrication de joint soudé au laser, joint soudé au laser et pièce de châssis de véhicule automobile
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US10941471B2 (en) 2015-12-28 2021-03-09 Jfe Steel Corporation High-strength steel sheet, high-strength galvanized steel sheet, method for manufacturing high-strength steel sheet, and method for manufacturing high-strength galvanized steel sheet
KR20200133391A (ko) * 2016-09-29 2020-11-27 제이에프이 스틸 가부시키가이샤 레이저 용접 이음매 및 자동차용 골격 부품
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KR20190029688A (ko) * 2016-09-29 2019-03-20 제이에프이 스틸 가부시키가이샤 레이저 용접 이음매의 제조 방법, 레이저 용접 이음매 및 자동차용 골격 부품
CN112743228A (zh) * 2016-09-29 2021-05-04 杰富意钢铁株式会社 激光焊接接头及汽车用骨架零件
JPWO2018061526A1 (ja) * 2016-09-29 2018-09-27 Ｊｆｅスチール株式会社 レーザ溶接継手の製造方法、レーザ溶接継手および自動車用骨格部品
KR102184912B1 (ko) * 2016-09-29 2020-12-01 제이에프이 스틸 가부시키가이샤 레이저 용접 이음매의 제조 방법, 레이저 용접 이음매 및 자동차용 골격 부품
WO2018061526A1 (fr) * 2016-09-29 2018-04-05 Ｊｆｅスチール株式会社 Procédé de fabrication de joint soudé au laser, joint soudé au laser et pièce de châssis de véhicule automobile
CN107058864B (zh) * 2017-01-10 2019-07-02 北京特冶工贸有限责任公司 铁道车辆车轮
KR20200093002A (ko) 2017-12-27 2020-08-04 제이에프이 스틸 가부시키가이샤 고강도 강판 및 그 제조 방법
WO2019130713A1 (fr) 2017-12-27 2019-07-04 Ｊｆｅスチール株式会社 Tôle d'acier à haute résistance et son procédé de production
US11492677B2 (en) 2017-12-27 2022-11-08 Jfe Steel Corporation High-strength steel sheet and method for producing the same
JP7469670B2 (ja) 2020-10-26 2024-04-17 日本製鉄株式会社 鉄心の製造方法および製造装置

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