Classifications

• • 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
• • 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
  • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
• • 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
  • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
• • 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
  • C22C38/42—Ferrous alloys, e.g. steel alloys containing chromium with nickel with copper
• • 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
  • C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
• • 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
  • C22C38/46—Ferrous alloys, e.g. steel alloys containing chromium with nickel with vanadium
• • 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
  • C22C38/50—Ferrous alloys, e.g. steel alloys containing chromium with nickel with titanium or zirconium

Definitions

• the present invention relates to a high-strength steel plate member produced by hot press forming, having excellent toughness and having a tensile strength of 1.8 GPa or more, and a method for producing the same.
• This steel plate member is suitable as a machine structural part such as an automobile body structural part and underbody part.
• the present invention also relates to a hot press forming steel plate used for manufacturing the steel plate member and a manufacturing method thereof.
• roll forming is used instead of press forming, high-strength steel sheets can be easily processed.
• roll forming can only be applied to the manufacture of parts having a uniform cross section in the longitudinal direction, so it cannot be used to manufacture members with complex shapes.
• the above-described hot press forming method or pre-formed press turn method can simultaneously achieve press forming of a steel plate and high strength of a press-formed product.
• the present invention provides a technique that makes it possible to produce a hot-pressed product that is excellent in toughness and has a tensile strength of 1.8 GPa or more without performing tempering after quenching. .
• the chemical composition of the steel sheet is appropriately selected, and the heat rolling conditions such as cold rolling, annealing, and galvannealing after the hot rolling conditions at the time of steel sheet production, and further,
• the above objective can be achieved by appropriately controlling the quenching conditions after hot press forming.
• the present invention is a mass 0/0, C: 0.26 ⁇ 0.45% , Mn + Cr: 0.5 ⁇ 3.0%, Nb: 0.02 ⁇ 1.0%, of the amount satisfying the following formula (1) Ti , Si: 0-0.5%, Ni: 0-2%, Cu: 0-1%, V: 0-1%, A1: 0-1%, B: 0-0.01%, Mo: 0-1.0%, Ca: 0 to 0.005%, chemical composition consisting essentially of the balance Fe and impurities, a prior austenite average grain size of 10 m or less, a microstructure containing self-tempered martensite, and tensile strength A hot-pressed steel sheet member that is greater than 1.8 GPa: 3.42N + 0.001 ⁇ Ti ⁇ 3.42N + 0.5... (1)
• ⁇ and ⁇ in the formula means the content (mass%) of the element in steel, and ⁇ is included as an impurity in the steel.
• the hot press forming is performed in addition to the hot press forming method in a narrow sense in which the steel sheet is preheated to a temperature in the austenite region (Ac point or higher) and then press forming is performed. It also includes a pre-formed press quench method in which after the press molding at a lower temperature (eg, room temperature), the molded product is heated to the temperature of the austenite region and quenched in the mold used for press molding.
• a lower temperature eg, room temperature
• One or two or more selected from 5% may be contained.
• one or more of P, S, and N contained as impurities in the steel are P: 0.005% or less, S: 0.005% or less, and N: 0.002% by mass%.
• the content should satisfy the following requirements! /.
• the present invention is a hot press-formed steel plate that can produce a hot press-formed steel plate member having the above chemical composition and having a tensile strength of 1.8 GPa or more.
• the present invention also holds a steel plate having the above chemical composition in a temperature range of Ac point or higher and (Ac point + 100 ° C) or lower for 5 minutes or less, and then hot press-molded the steel plate, Hot press Molded high-temperature molded product is cooled so that the cooling rate to the Ms point is higher than the upper critical cooling rate and the average cooling rate from the Ms point to 150 ° C is 10 to 500 ° C / sec.
• the manufacturing method of the hot press-formed steel plate member is also provided.
• the hot-pressed steel plate member can be manufactured by the above-described pre-forming press quench method.
• a method for producing a steel plate member according to this aspect is the method of press-forming a steel plate having the above chemical composition using a mold at a temperature lower than the Ac point, and keeping the press-formed steel plate in the mold, the Ac point or higher, Hold for 5 minutes or less in the temperature range below (Ac point + 100 ° C), then the cooling rate to the Ms point is higher than the upper critical cooling rate, and the average cooling rate from the Ms point to 150 ° C is 10 Including cooling to ⁇ 500 ° C / sec.
• the press molding temperature in this embodiment is typically room temperature.
• the present invention further provides a method for producing a steel sheet for hot press forming excellent in press formability, which is particularly suitable for use in a pre-formed press taenti method.
• This hot press forming steel sheet is produced by subjecting the steel ingot or steel slab having the above-described chemical yarn formation to a temperature of 1050 to 1300 ° C, followed by hot rolling, and a temperature of 800 to 950 ° C. The hot rolling is completed, and the steel strip obtained by hot rolling is wound at a temperature of 500 to 700 ° C.
• the method for producing a hot press-formed steel sheet may further include the following steps:
• the temperature is 500 ° C or less at an average cooling rate of 1 to 60 ° C / sec. Cooling to this region and subjecting this steel strip to molten zinc plating, followed by alloying heat treatment in the temperature range of 500 ° C to Ac point; or
• the hot press-formed steel sheet of the present invention is a hot-rolled steel sheet, a cold-rolled steel sheet, a heat-treated cold-rolled steel sheet, and a hot-rolled steel sheet or a cold-rolled steel sheet as a base material. Including alloyed hot-dip galvanized steel sheets.
• a high-strength hot press-formed steel plate member having excellent toughness and tensile strength of 1.8 GPa or more is obtained by hot press forming and quenching at that time without performing tempering. Manufacturing It becomes possible to do. As a result, the manufacturing cost of the high-strength steel plate member using hot press forming can be significantly reduced.
• FIG. 1 is an explanatory diagram of the shape of a test piece for measuring a critical cooling rate.
• FIG. 2 is a TEM photograph showing the microstructure of the hot pressed steel sheet member of the present invention.
• FIG. 3 is a schematic explanatory view of a hat forming method.
• the chemical composition of the steel sheet is as follows.
• C is a very important element that enhances the hardenability of the steel sheet and mainly determines the post-quenching strength.
• the C content is at least 0.26%.
• the desirable C content is 0.28 to 0.33%.
• Mn and Cr are elements that are very effective in improving the hardenability of the steel sheet and stably obtaining high strength after quenching. If the total content of Mn and Cr (hereinafter referred to as “(Mn + Cr) content”) is less than 0.5%, the effect is not sufficient. On the other hand, when the (Mn + Cr) content exceeds 3.0%, the effect is saturated, and on the contrary, it is difficult to secure a stable strength.
• the desirable (Mn + 0>) content is 0.8-2.0%.
• Nb is a fine carbide that suppresses recrystallization when the steel sheet is heated above the Ac point.
• the Ti content (%) is set to (3.42N + 0.001) or more.
• N is contained as an impurity in the steel. N may be substantially 0%.
• the Ti content exceeds (3.42N + 0.5), the above effect of Ti will be saturated, leading to cost increase.
• the desired Ti content is 3.42N + 0.02 ⁇ Ti ⁇ 3.42N + 0.08.
• Si 0 to 0.5%
• Ni 0 to 2%
• Cu 0 to 1%
• V 0 to 1%
• A1 0 to 1%
• any of these elements which are optional additive elements, are effective in enhancing the hardenability of the steel sheet and stably achieving high post-quenching strength. Therefore, one or more of these elements should be contained. It is preferable. This effect of these elements becomes significant when Si: 0.01% or more, Ni: 0.01% or more, Cu: 0.01% or more, V: 0.01% or more, A1: 0.01% or more. However, even if each element is contained in the upper limit value or more, the above effect is small and the cost is unnecessarily increased. Therefore, the content of each element is within the above range.
• the preferred contents when adding one or more of these elements are: Si: 0.02 to 0.4%, Ni: 0.02 to l%, Cu: 0.02 to 0.8%, V: 0.02 to 0.5%, Al : 0.01 to 0.1%.
• B is an optional additive element, and is effective in improving the hardenability of the steel sheet and stably obtaining high strength after quenching.
• B segregates at the grain boundaries to increase the grain boundary strength and improve the toughness of the steel sheet after quenching, and also has a high effect of suppressing austenite grain growth during heating. These effects become significant when the B content is 0.001% or more. However, if the B content exceeds 0.01%, these effects are saturated and cost increases.
• the B content is preferably 0.001 to 0.01%, more preferably 0.001 to 0.0030%.
• Mo is an optional additive element, and when the steel sheet is heated to the Ac point or higher, it forms fine carbides and makes the austenite grains fine, greatly improving the toughness of the steel sheet after quenching. Has an effect. These effects become significant when the Mo content is 0.01% or more. However, when the Mo content power exceeds 1.0%, the effect is saturated and the cost is unnecessarily increased. When Mo is contained, the desirable Mo content is 0.01 to 1.0%, and more desirably 0.04 to 0.20%.
• Ca is an optional additive element, and has the effect of reducing the inclusions in the steel and improving the toughness of the steel sheet after quenching. These effects become significant when the Ca content is 0.001% or more. However, when the Ca content exceeds 3 ⁇ 4.005%, the effect is saturated. Therefore, the desired content when Ca is contained, the Ca content is 0.001 to 0.005%, more preferably 0.002 to 0.004%.
• the balance of the chemical composition is essentially Fe and impurity power.
• Impurities can include non-metallic elements such as P, S, N, and other metallic elements. Of these, the contents of P, S, and N are preferably as follows.
• P is an element that greatly deteriorates the toughness of the steel sheet after quenching, so it is preferably 0.005% or less. More desirably, it is 0.003% or less.
• S is an element that greatly deteriorates the toughness of the steel sheet after quenching, it is preferably 0.005% or less. More desirably, it is 0.003% or less.
• N 0.002% or less
• N is an element that forms inclusions in the steel and deteriorates the toughness of the steel sheet after quenching, and therefore is preferably 0.002% or less. More desirably, it is 0.001% or less.
• the content of at least one of P, S, and N is preferably as described above.
• the content of the remaining impurity elements may exceed the above upper limit, but it is particularly preferable that all the contents of p, S, and N are less than the upper limit.
• the hot-press formed steel sheet member of the present invention has a high strength with a tensile strength of 1.8 GPa or more. This tensile strength is achieved by quenching following press forming in the hot press forming process. Quenching is usually performed in the mold used for hot press molding, It is not limited to that.
• the crystal grains having a prior austenite average grain size of 10 ⁇ or less are refined. It has a fine structure.
• the prior austenite average particle diameter is desirably 8 ⁇ or less, more desirably 4 ⁇ or less.
• the prior austenite average particle diameter changes depending on the heating conditions (holding temperature and holding time) before hot press forming, as will be described below.
• hot press forming is performed on a steel sheet having the above chemical composition.
• the heating conditions (holding temperature and holding time) before hot press forming at that time are as follows.
• the steel plate to be subjected to hot press forming is not less than 5 minutes in the temperature range from Ac point to (Ac point + 100 ° C). Hold for hours.
• the holding temperature is set to the Ac point or higher in order to obtain the desired strength by quenching once the steel structure is an austenite single phase.
• the upper limit of holding temperature and upper limit of holding time are to suppress the prior austenite grain size after quenching to 10 ⁇ m or less, and to achieve good toughness even when the tensile strength of steel sheet is 1.8 GPa or more. . If the holding temperature exceeds (Ac point + 100 ° C) or the holding time exceeds 5 minutes, the old austenite grain size becomes 10 ⁇ m or more, and good toughness can be obtained after quenching. There are things that cannot be done. A more desirable holding temperature is not less than Ac point and not more than (Ac point + 50 ° C), and more desirable holding time is not more than 2 minutes. Since the prior austenite grain size is preferably finer, the lower limit of the holding time is not particularly specified.
• the hot press molding in the present invention is not particularly limited per se, including the mold used.
• the hot press forming is preferably performed in advance by heating the steel plate under the above-mentioned conditions, but it can also be performed according to the pre-formed pressing method described above. In that case, the molded article may be heated under the above conditions.
• the cooling conditions and cooling method for quenching in the hot press forming process are as follows.
• the microstructure after quenching of the steel sheet member obtained by hot press forming is completely composed of only martensite. It is important not to use a martensite structure, but to use a structure that includes automatic tempered martensite.
• Automatic tempered martensite refers to tempered martensite generated during cooling during quenching without performing heat treatment for tempering.
• Tempered martensite can be distinguished from fully martensite by the precipitation of fine cementite inside the lath.
• the microstructure including the auto-tempered martensite has a cooling rate at the time of quenching that is equal to or higher than the upper critical cooling rate so that diffusion transformation does not occur up to the Ms point. It can be obtained by setting the average cooling rate in the temperature range from the Ms point to 150 ° C to 10 to 500 ° CZ seconds.
• the preferred average cooling rate from the Ms point to 150 ° C is 15 to 200 ° C / sec.
• the average cooling rate in the temperature range from the Ms point to 150 ° C is the force that makes the cooling rate slower than the cooling rate to the Ms point. If cooling below the Ms point is performed with the same cooling method as reaching the Ms point, the Ms The required cooling rate may not be achieved due to the large transformation heat generation at the point. In that case, it is necessary to perform the cooling from the Ms point to 150 ° C more strongly than the cooling to the Ms point. Specifically, it is preferable to perform the following.
• a high-temperature steel sheet is used at room temperature or several tens.
• C grade steel By press molding with a mold, cooling is achieved by the mold.
• the cooling rate can be changed by changing the heat capacity of the mold by changing the mold dimensions (eg, thickness).
• the cooling rate can also be changed by changing the mold material to a different metal (such as copper). If the dimensions and materials of the mold cannot be changed, the cooling rate can also be changed by changing the amount of cooling water using a water-cooled mold.
• the cooling rate is changed by passing water through the groove during press molding, or the press molding machine is raised during press molding, while water is poured into the mold.
• the cooling rate can be changed by flowing it.
• the cooling rate can also be changed by changing the mold clearance and changing the contact area with the steel plate.
• the cooling rate is changed by flowing water between the mold and the molded product and changing the amount of water.
• the form of molding in the hot press molding method in the present invention is not particularly limited, and examples thereof include bending, drawing, stretch forming, hole expansion molding, and flange molding.
• the press forming method can be appropriately selected depending on the type of the target steel plate member. Typical examples of hot-pressed steel sheet members include door guard bars and bumper reinforcements, which are compelling parts for automobiles.
• the method for producing a steel plate member of the present invention can be applied to a forming method other than press forming, for example, roll forming, as long as it has means for cooling the steel plate simultaneously with forming or immediately after forming.
• the steel sheet member according to the present invention retains good toughness.
• As a toughness level that can withstand practical use it is desirable that the Charpy impact value at 120 ° C is 30 Jm 2 or more.
• Steel plates that have been hot-pressed are usually processed by shot blasting for scale removal purposes. This shot blasting has the advantage of suppressing delayed fracture and improving fatigue strength because it has the effect of introducing compressive stress on the surface. Five
• a steel sheet is heated to an austenite temperature range and undergoes austenite transformation. Therefore, apart from the pre-formed press Taenti method, which has a low press forming temperature, the mechanical properties of the steel sheet at room temperature before heating are not important, so there are no particular restrictions on the type of steel sheet or the microstructure before heating. . That is, as the hot press forming steel plate, any of a hot rolled steel plate, a cold rolled steel plate (full hard material, annealed material), and a plated steel plate may be used. Also, its production method is not particularly limited.
• Plated steel sheets include aluminum-based plated steel sheets (eg, hot-dip aluminum-plated steel sheets, molten 55% A1-Zn alloy-plated steel sheets), zinc-based plated steel sheets (eg, electric or hot-dip galvanized steel sheets, molten 5% A1) -Zn-plated steel sheets, steel sheets with alloyed molten steel, steel sheets with electric Ni-Zn alloy).
• aluminum-based plated steel sheets eg, hot-dip aluminum-plated steel sheets, molten 55% A1-Zn alloy-plated steel sheets
• zinc-based plated steel sheets eg, electric or hot-dip galvanized steel sheets, molten 5% A1
• -Zn-plated steel sheets eg, steel sheets with alloyed molten steel, steel sheets with electric Ni-Zn alloy.
• a hot press forming method such as a pre-formed press quench method in which press forming is performed in advance at room temperature or at a temperature lower than the austenite region
• the steel sheet to be subjected to hot press forming is as soft as possible.
• hot-rolled steel sheets and cold-rolled steel sheets that have been subjected to continuous annealing have a tensile strength of 780 MPa or less
• steel sheets that have been cold-rolled have a tensile strength of 780 to 1180 MPa.
• the tensile strength be 590 MPa or less.
• a steel ingot or steel slab having the above-described chemical composition is heated to 1050 to 1300 ° C and then hot-rolled to form a steel strip. Hot rolling is completed at a temperature of 800-950 ° C, and the resulting steel strip is scraped at a temperature of 500-700 ° C.
• the reason why the steel ingot or steel slab is set to 1050 to 1300 ° C is to sufficiently dissolve nonmetallic inclusions that deteriorate the workability. Such an effect is recognized by setting the temperature to 1050 ° C. or higher with respect to the steel sheet having the above composition. Even at temperatures above 1300 ° C, the scale loss increases as the effect saturates. This temperature is more desirably 1050 to 1250 ° C, and further desirably 1050 to 1200 ° C.
• the method of setting the temperature of the steel ingot or steel slab to be subjected to hot rolling to 1050 to 1300 ° C is to heat the steel ingot or steel slab that is less than 1050 ° C to 1050 to 1300 ° C. Not only in case but continuous forging Five
• the hot rolling completion temperature should be lower than the Ar point. When the rolling is performed at a temperature lower than the Ar point, the processed ferrite remains and the ductility deteriorates significantly. For steel plates with the chemical composition described above, these problems do not occur if the hot rolling completion temperature is 800 ° C or higher. On the other hand, if the hot rolling completion temperature is higher than 950 ° C, surface defects such as scale penetration may occur. Therefore, the hot rolling completion temperature is set to 800 to 950 ° C.
• the brazing temperature is too low, many low-temperature transformation structures such as pearlite, bainite, and martensite are generated, and the ferrite structure is reduced, so that the steel sheet strength becomes too high. For this reason, the lower limit of the coiling temperature is set to 500 ° C. On the other hand, if the scraping temperature is too high, the oxide scale becomes thick and the descaling process becomes difficult, so the upper limit of the scraping temperature is set to 700 ° C.
• the scissoring temperature is more preferably 550 to 650 ° C.
• the hot-rolled steel strip produced in this way is not suitable for press forming at room temperature in the pre-formed press-taench method, and in order to obtain good formability as hot rolled, Therefore, it is preferable that the structure contains 50% or more of ferrite and the tensile strength is 780 MPa or less.
• the remainder of the tissue can include one or more of perlite, bainite, martensite, and residual austenite.
• Ferrite may contain Fe-based carbides such as cementite, Ti-based, Nb-based, Mo-based, Cr-based, V-based, and Mn-based carbides. From the viewpoint of formability, it is desirable that the strength of the steel strip is low, but it is desirable that the strength is 590 MPa or more, and more desirably 690 MPa or more, in terms of cost and ease of strength adjustment. It is.
• Steel strips that have been scraped and allowed to cool after hot rolling are usually uncoiled and then formed on the surface by one or more of pickling, shot blasting, grinding, etc. Remove scale (deschedule).
• the tensile strength is preferably 1180 MPa or less.
• the strength of the cold-rolled steel strip is preferably low from the viewpoint of formability, but is preferably 780 MPa or more from the viewpoint of cost and strength adjustment.
• the tensile strength of the cold-rolled steel strip is more preferably in the range of 780 to 1100 MPa, and even more preferably in the range of 780 to 1050 MPa.
• the rolling reduction during cold rolling is preferably 30 to 80%, more preferably 40 to 70%.
• the steel strip that has been cold-rolled as described above can be annealed by continuous annealing in an uncoiled state or box annealing by scraping it into a coil!
• the heating temperature was lower than (Ac point + 10 ° C), and recrystallization did not proceed sufficiently.
• the holding time after heating is shorter than 10 seconds, segregation of substitutional elements such as Mn remains and the microstructure after annealing becomes non-uniform. Since heating for a long time unnecessarily increases costs, the holding time after heating is preferably 300 seconds or less.
• the annealing atmosphere is preferably a non-oxidizing atmosphere (for example, 98 volume% N +2 volume% H).
• the preferred average cooling rate is 1-20 ° C / second, more preferably 1-10 ° CZ second.
• the reason why the cooling stop temperature range is set to 300 to 500 ° C. is to suppress the generation of the low temperature transformation phase as much as possible.
• the cooling stop temperature range is preferably 350 to 500 ° C, more preferably 400 to 450 ° C.
• the reason for holding for 30 seconds to 10 minutes in the cooling stop temperature range is to promote the ferrite transformation of untransformed austenite. This holding time is preferably 30 seconds to 5 minutes, more preferably 30 seconds to 3 minutes.
• the steel strip is cooled at an average cooling rate of 1-50 ° CZ seconds. Allow to cool to room temperature. If the average cooling rate at this time is faster than 50 ° C / sec, many low-temperature transformation phases are generated and the steel strip strength is increased. On the other hand, when the average cooling rate is slower than ° C / sec, the production efficiency decreases.
• the preferred average cooling rate is 1-10 ° C / sec.
• the tensile strength is not sufficiently reduced.
• the holding temperature is higher than (Ac point + 30 ° C)
• the re-solution and reversion of cementite proceeds too much, and the low temperature transformation phase is generated in the subsequent cooling process, and the tensile strength of the steel strip is reduced. Too high.
• the holding time is less than 1 hour, the effect will be saturated even if the holding time exceeds 24 hours, where the strength of the steel strip is not sufficiently reduced, leading to wasted energy.
• the cooling rate is high, a low-temperature transformation phase is generated. However, if it is too slow, the processing efficiency will be reduced, so the cooling rate is 1-100 ° CZ, preferably 1-50 ° C / hour.
• the furnace atmosphere in the box annealing is preferably a gas containing 95% by volume or more of hydrogen, in which nitrogen gas is not mixed in and the dew point is as low as possible.
• the cold-rolled steel strip that has been annealed after the cold rolling thus obtained has a volume ratio of 50% or more in order to obtain good formability in comparison with hot press forming by the pre-forming press tent method.
• the tensile strength of the steel strip is preferably 780 MPa or less in the case of continuous annealing and 590 MPa or less in the case of box annealing.
• the tensile strength of the steel strip is preferably low, but it is desirable that the tensile strength of the steel strip be 440 MPa or higher for any annealing method because of cost and strength adjustment.
• Hot-dip galvanizing can be applied to any of hot-rolled steel strip, cold-rolled steel strip, and steel strip annealed after cold rolling. It is preferable to perform the hot dip galvanizing with a continuous hot dip galvanizing line from the viewpoint of manufacturing cost.
• a normal continuous hot dip galvanizing line consists of a heating furnace, cooling zone, hot dip galvanizing bath and alloy furnace.
• the annealing temperature is set to 700 to 900 ° C. It is preferable. At temperatures below 700 ° C, recrystallization does not proceed sufficiently and the strength of the steel strip tends to increase. On the other hand, at a temperature higher than 900 ° C, due to the austenite single phase, a low temperature transformation phase is formed during cooling, and the strength of the steel strip tends to increase immediately. It is not necessary to anneal a hot-rolled steel strip or a cold-rolled steel strip that has been annealed. However, it is difficult to make the heating temperature extremely low, so it is preferable to perform heating within the normal operating range. In that case, it is preferable to set the maximum heating temperature to 900 ° C. or less for the reason described above.
• Annealing temperature or maximum heating temperature force The steel strip is cooled in order to apply hot-dip zinc plating.
• the average cooling rate to the temperature range below 500 ° C during this cooling is preferably 1 to 60 ° CZ seconds. If the cooling is too fast, a lot of low-temperature transformation phase is generated, ferrite decreases, and the strength of the steel strip becomes too high. On the other hand, if the cooling is too slow, the production efficiency falls.
• Hot-dip zinc plating can be performed by immersing the steel strip in a plating bath containing molten zinc or a zinc alloy (eg, a zinc alloy containing up to 5% A1) by a conventional method. ,.
• the plating deposition amount is controlled by adjusting the pulling speed and the flow rate of the wiping gas blown from the nozzle.
• the steel strip to which the hot dip galvanization has been applied is removed from the plating bath, and then sent to an alloying furnace such as a gas furnace or an induction heating furnace to be heated.
• alloying by metal diffusion proceeds between the plating layer and the base steel strip, and the plating layer becomes a zinc-iron alloy.
• the heating temperature (alloying temperature) is preferably 500 ° C or higher.
• the alloying temperature is lower than 500 ° C, the alloying speed is slow, and therefore, it is necessary to take measures such as a force that hinders productivity due to a decrease in the line speed or a longer alloying furnace.
• the higher the alloying temperature the faster the alloying speed.
• the alloying temperature is higher than the Ac point, the steel strip becomes stronger for the same reason as the upper limit of the annealing temperature and the maximum heating temperature.
• the preferred range of alloying temperature is 550-650 ° C
• temper rolling may be performed on the hot press-formed steel sheets manufactured by various manufacturing methods in order to correct flatness and adjust the surface roughness.
• the hot press forming steel plate may be other plated steel plate, for example, an aluminum-based steel plate such as a 55% A1-Zn alloy steel plate. 15
• a cold-rolled steel plate (thickness: 1.6 mm) having the chemical composition shown in Table 1 was used as the base steel plate.
• These steel plates are steel plates produced by hot rolling and cold rolling of slabs melted in the laboratory. '
• A1 plating was applied to the steel sheet No. 1 (the adhesion amount per side was 120 g / m 2 ), and the hot galvanizing was applied to the No. 2 steel sheet (single side The amount of plating per unit was 60 g / m 2 ). Furthermore, the No. 2 steel plate was subjected to alloying heat treatment (Fe content in the plating film was 15% by mass). The annealing temperature in the plating simulator was 800 ° C, and the average cooling rate from 800 ° C to Ms point was 5 ° CZ seconds.
• the obtained hot press-formed steel plate member was subjected to old austenite particle size measurement by a cutting method and a tensile test (JIS No. 5 test piece).
• a cylindrical specimen (Fig. 1) with a diameter of 3.0 mm and a length of 10 mm was cut out from the hot-rolled steel sheet, and this specimen was heated in air to 900 a C at a heating rate of 10 ° CZ seconds. After holding for a minute, it was cooled to room temperature at various cooling rates. The Ac point and Ms point were measured by measuring the thermal expansion change of the test piece during heating and cooling. The obtained specimens were measured for Vickers hardness (load 49N, number of measurements: 3) and microstructure observation, and the upper critical cooling rate of the steel sheet was estimated from the results. [0075] These results are shown in Table 3.
• [] 00783 Is as fine as ⁇ ⁇ ⁇ or less, has a tensile strength of 1.8 GPa or more, and has a good toughness value.
• the prior austenite grain size exceeds 10 ⁇ and the toughness value is unsatisfactory.
• Nos. 13 and 14 are examples in which the chemical composition of the steel is outside the scope of the present invention, and No. 15 is a force in which the chemical composition of the steel falls within the scope of the present invention. This is an example in which the average cooling rate from the Ms point to 150 ° C. is outside the scope of the present invention.
• the steel plate members No. 1 to No. 14 had a structure containing automatically tempered martensite. It was confirmed.
• the steel plate member No. 15 had a complete martensite structure. Even if the hot-pressed steel sheet member has a structure containing tempered martensite, good toughness cannot be obtained if the prior austenite grain size exceeds 10 ⁇ m.
• Fig. 2 shows TEM photographs at different magnifications of the hot-pressed steel sheet member of No. 2 which is an example of the invention.
• the upper row is 10,000 times and the lower row is 40,000 times.
• Most of the structure is lath martensite, but in the part where the lath width is large, fine acicular cementite is precipitated inside the lath, and it is an auto-tempered martensite. It can be confirmed from the double photo.
• the portion indicated by the arrow in the figure is fine cementite.
• FIG. 3 is a schematic explanatory diagram of the hat forming method.
• the hot press forming conditions were: forming height 70 mm, Rd (die shoulder R) force 3 ⁇ 4 mm, Rp (punch shoulder R) 8 mm, clearance 1.0 mm, and wrinkle holding force 12.7 kN. It was.
• a low temperature impact test was conducted on the hat molded product. After cooling the member to 140 ° C., a weight body with a weight of 2450 N (250 kg £) from a height of 1000 mm was made to collide with the member, and the presence of cracks was investigated. As a result, it has been found that it has sufficient toughness without cracking.

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