US20130087257A1 - Ultra high strength cold rolled steel sheet having excellent ductility and delayed fracture resistance and method for manufacturing the same - Google Patents

Ultra high strength cold rolled steel sheet having excellent ductility and delayed fracture resistance and method for manufacturing the same Download PDF

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US20130087257A1
US20130087257A1 US13/805,144 US201113805144A US2013087257A1 US 20130087257 A1 US20130087257 A1 US 20130087257A1 US 201113805144 A US201113805144 A US 201113805144A US 2013087257 A1 US2013087257 A1 US 2013087257A1
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temperature
steel
steel sheet
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Masataka Yoshino
Kohei Hasegawa
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JFE Steel Corp
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JFE Steel Corp
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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
    • C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/005—Modifying the physical properties by deformation combined with, or followed by, heat treatment of ferrous alloys
    • 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
    • C21D6/00—Heat treatment of ferrous alloys
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/04—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing
    • C21D8/0421—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing characterised by the working steps
    • C21D8/0436—Cold rolling
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/04—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing
    • C21D8/0447—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing characterised by the heat treatment
    • 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
    • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
    • 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
    • C22C38/06—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
    • C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
    • 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
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/005—Ferrite
    • 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
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/008—Martensite

Definitions

  • This disclosure relates to an ultra-high-strength cold-rolled steel sheet which has an excellent strength-ductility balance and excellent delayed fracture resistance and which is a material suitable for use principally in ultra-high-strength automobile structural parts such as center pillars and door impact beams for automobiles and also relates to a method for manufacturing the same.
  • Japanese Unexamined Patent Application Publication No. 2005-163055 discloses an example in which a steel sheet assumed to have a tensile strength of 1350 MPa and a tempered martensite single-phase microstructure is obtained by quench and tempering although the percentages of phases are not described therein.
  • the total elongation of the steel sheet is small, 7%. Therefore, it is extremely difficult to manufacture automobile safety parts from the steel sheet by pressing.
  • the martensite single-phase microstructure is probably obtained by quenching and therefore the steel sheet probably has a seriously had shape. This case needs a step of correcting the shape thereof after annealing and therefore is not preferable in terms of manufacture.
  • Japanese Unexamined Patent Application Publication No. 2006-307325 discloses a TRIP (Transformation-induced Plasticity) steel sheet which has high strength and ductility and which is obtained by making use of strain-induced transformation, that is, the transformation of retained austenite into martensite by strain during deformation. That steel sheet contains 0.3% to 2% Al on a mass basis to ensure the amount of retained austenite necessary to develop a TRIP effect. A large amount of Al causes a problem that casting defects are likely to he caused. To allow retained austenite to remain in a microstructure, isothermal holding needs to be performed at a temperature not lower than the its transformation temperature in the course of cooling from the annealing temperature, which results in an increased number of manufacturing steps. Since the change in rate of cooling to the temperature of isothermal holding during operation causes a significant change in material quality, operating conditions needs to be strictly controlled to stably manufacture steel sheets with a certain level of quality, which is not preferable in terms of manufacture.
  • TRIP Transformation-induced Plasticity
  • a microstructure needs to be transformed into a martensite single-phase microstructure by quenching.
  • a microstructure is a martensite single-phase
  • sufficient ductility cannot be achieved.
  • hydrogen-trapping sites which cause delayed fracture, are preferably diminished as much as possible. Martensite phases are preferably diminished as much as possible because a large number of dislocations serving as hydrogen-trapping sites are introduced into the martensite phases during crystallographic transformation from austenitic phases.
  • Retained austenite which contributes to an increase in ductility, is known to serve as a hydrogen-trapping site like a dislocation and is present on a grain boundary in the form of a film. Therefore, the penetration of hydrogen into retained austenite may possibly cause grain boundary fracture to reduce delayed fracture resistance.
  • a metal microstructure contains retained austenite
  • annealing temperature and the course of cooling are appropriately controlled during annealing and cooling subsequent to cold rolling and tempering heat treatment is performed at a temperature of 100° C. to 300° C.
  • Our cold-rolled steel sheet has extremely high tensile strength, high ductility, and therefore excellent workability. Parts fortned from the cold-rolled steel sheet have resistance to delayed fracture due to hydrogen coming from surroundings, that is excellent delayed fracture resistance. For example, a tensile strength of 1320 MPa or more, a total elongation of 12% or more, and such delayed fracture resistance that fracture does not occur for 100 hours in a 25° C. hydrochloric acid environment with a pH of 3 can be readily achieved, Furthermore, a cold-rolled steel sheet having such excellent properties as described above can be stably manufactured by our method.
  • the following sheet can be stably manufactured: an ultra-high-strength cold-rolled steel sheet which has a tensile strength of 1320 MPa or more and which exhibits excellent workability during forming.
  • Parts formed from the cold-rolled steel sheet by press molding have resistance to delayed fracture due to hydrogen coming from surroundings, that is, excellent delayed fracture resistance.
  • Ultra-high-strength parts such as automobile safety parts including center pillars and impact beams, resistant to delayed fracture can be provided.
  • FIG. 1 is a schematic view of a 180-degree bent specimen subjected to stress by bolting.
  • An ultra-high-strength cold-rolled steel sheet has a specific chemical composition and a microstructure as described below. The chemical composition of the cold-rolled steel sheet is first described.
  • C is an element which stabilizes austenite and is necessary to ensure the strength of the steel sheet.
  • the content of C is less than 0.15% by mass, it is difficult for a microstructure having a tempered martensite phase and a ferrite phase to stably obtain a tensile strength of 1320 MPa or more.
  • the content of C is more than 0.25% by mass, welded portions and heat-affected zones affected by welding are significantly hardened and therefore weldability is reduced. Therefore, the content of C is preferably 0.15% to 0.25% by mass and more preferably 0.18% to 0.22% by mass.
  • Si is a substitutional solid solution hardening element effective in hardening the steel sheet.
  • the content of Si needs to be 1.0% by mass or more to develop this effect.
  • the content of Si is more than 3.0% by mass, scales are significantly formed during hot rolling and the failure rate of final products is increased, which is not economically preferred. Therefore, the content of Si is 1.0% to 3.0% by mass.
  • Mn is an element which stabilizes austertite and is effective in hardening steel.
  • the content of Mn is less than 1.5% by mass, it is difficult to stably manufacture the steel sheet having a target strength because the hardenability of steel is insufficient, the production of a ferrite phase during cooling from the annealing temperature and the formation of pearlite and bainite begin early, and the strength is significantly reduced.
  • the content thereof is more than 2.5% by mass, segregation is serious, workability is deteriorated in some cases, and delayed fracture resistance is reduced. Therefore, the content of Mn is preferably 1.5% to 2.5% by mass and more preferably 1.5% to 2.0% by mass.
  • P is an element conductive to grain boundary fracture due to grain boundary segregation and Therefore is preferably low.
  • the upper limit thereof is 0.05% by mass and is preferably 0.010% by mass. In view of an increase in weldability. the upper limit thereof is more preferably 0.008% by mass or less.
  • S forms an inclusion such as MnS, causing a reduction in impact resistance and/or delayed fracture resistance and is preferably minimized.
  • the upper limit thereof is 0.02% by mass and preferably 0.002% by mass.
  • Al is an element effective in deoxidization.
  • the content thereof needs to be 0,01% by mass or more to achieve an effective deoxidizing effect.
  • the content thereof is excessive, more than 0.05% by mass, the steel sheet contains increased amounts of inclusions and has reduced ductility. Therefore, the content of Al is 0.01% to 0.05% by mass.
  • the content of N is 0.005% by mass or more, the formation of nitrides causes a reduction in ductility at high temperature and low temperature. Therefore, the content of N is less than 0.005% by mass,
  • the steel sheet may further contain one or more of Nb, Ti, and B as required. The effect of the addition of these three elements and the preferred content thereof are described below.
  • Nb and Ti are elements which have a grain-re:Fining effect and are effective in increasing the strength of the steel sheet.
  • the content of is preferably 0.015% by mass or more.
  • the content of each of Nb and Ti is more than 0.1% by mass, the effect thereof is saturated, which is not economically preferred. Therefore, the content of each of Nb and Ti is 0.1% by mass or less.
  • B is an element effective in increasing the strength of the steel sheet.
  • the strength-in-creasing effect of B cannot be expected when the content of B is less than 5 ppm by mass.
  • the content of B is more than 30 ppm by mass, hot workability is reduced, which is not preferable in terms of manufacture, Therefore, the content of B is 5 ppm to 30 ppm by mass.
  • the remainders other than the above components are Fe and unavoidable impurities.
  • microstructure of the cold-rolled steel sheet is described below.
  • microstructure contains 40% or more of a tempered martensite phase on a volume fraction basis after continuous annealing, the remainder being a ferrite phase.
  • the microstructure is obtained by quenching from the annealing temperature and tempering subsequent to quenching. According to this method, an ultra-high-strength cold-rolled steel sheet with high ductility can be obtained without excessively using a transition metal element such as V or Mo, causing an increase in cost or an alloying element such as Al, possibly causing casting defects.
  • the microstructure contains an appropriate amount of the ferrite phase, the number of the dislocations, which serve as hydrogen-trapping sites causing delayed fracture, can be more significantly reduced as compared with a tempered martensite single-phase microstructure and therefore the amount of hydrogen permeating through can be reduced.
  • the tensile strength of steel with a microstructure containing a tempered martensite phase and a ferrite phase increases with an increase in volume fraction of the tempered martensite phase. This is because the hardness of the tempered martensite phase is higher than the hardness of the ferrite phase, the tempered martensite phase, which is a hard phase, exhibits resistance to deformation during tensile deformation, and the larger the volume fraction of the tempered martensite phase is, the more the tensile strength of the steel is close to the tensile strength of the tempered martensite single-phase microstructure.
  • a tensile strength of 1320 MPa or more is not achieved when the volume fraction of the tempered martensite phase is less than 40%. Since ductility decreases with at increase in volume fraction of the tempered martensite phase, a microstructure containing more than 85% of the tempered martensite phase on a volume fraction basis cannot ensure the volume fraction of the ferrite phase that is necessary to achieve a high ductility of 12% or more in terms of total elongation and necessary to increase the delayed fracture resistance. When the volume fraction of the ferrite phase is less than 15%, a high ductility of 12% or more in terms of total elongation is not achieved or an increase in delayed fracture resistance not sufficient. However, when the volume fraction thereof is more than 60%, the volume fraction of the tempered martensite phase that is necessary to achieve a predetermined strength cannot be ensured.
  • the volume fraction of the tempered martensite phase and that of the ferrite phase are 40% to 85% and 15% to 60%, respectively, and more preferably 60% to 85% and 15% to 40%, respectively.
  • the microstructure of the cold-rolled steel sheet may be a two-phase microstructure containing a tempered martensite phase and ferrite phase each having a desired volume fraction and may contain a constituent phase such as a retained austenite phase, a bainite phase, or a pearlite phase, other than these two phases.
  • large amounts of the bainite and Pearlite phases are present, the bainite phase and the pearlite phase cause a reduction in ductility and a reduction in strength, respectively.
  • the microstructure contains large amounts of the bainite and pearlite phases.
  • the retained austenite phase is principally present at a grain boundary in the form of a film, serves as a hydrogen-trapping site, and therefore may possibly act as an origin of fracture due to hydrogen embrittlement. Hence, the content thereof is preferably minimized. Therefore, the volume fraction of the constituent phase (the retained austenite phase, the bainite phase, or the pearlite phase) other than the tempered martensite phase and the ferrite phase is preferably 1% or less in total.
  • the tensile strength and ductility are 1320 MPa or more and 12% or more, respectively.
  • the total elongation corresponds to the minimum elongation capable of pressing automobile structural parts such as impact beams.
  • Such a strength level and elongation level can be readily achieved in our steel sheets.
  • the delayed fracture resistance is such a performance that fracture does not occur for 100 hours in a 25° C. hydrochloric acid environment with a pH of 3. Such a performance can be readily achieved in our steel sheets.
  • the cold-rolled steel sheet is particularly suitable for ultra-high-strength automobile safety parts such as automobile door impact beams and center pillars.
  • Steel sheets include steel strips.
  • the cold-rolled steel sheet may be subjected to surface treatment such as plating (electroplating or the like) or chemical conversion to be used as a surface-treated steel sheet.
  • the heating temperature of the slab is 1200° C. or higher.
  • An increase in oxidation causes an increase in scale loss when the heating temperature thereof is excessively high.
  • the heating temperature of the slab is preferably 1300° C. or lower.
  • a uniform hot-rolled microstructure can be obtained when the finish rolling end temperature is 800° C. or higher.
  • the finish rolling end temperature is lower than 800° C., the microstructure of the steel sheet is nonuniform, the ductility thereof is, and the risk of causing various failures during molding is increased.
  • the finish rolling end temperature is 800° C. or higher.
  • the upper limit of the finish rolling end temperature is not particularly limited and is preferably 1000° C. or lower because rolling at excessively high temperature causes scale defects.
  • the hot-rolled steel sheet is coiled.
  • the coiling temperature thereof is not particularly limited. When the coiling temperature thereof is excessively high, the microstructure of the steel sheet is nonuniform and the ductility thereof is low, due to formation of coarse grains. When the coiling temperature thereof is excessively low, a detbrmed microstructure caused by hot rolling remains to increase the rolling load in cold rolling subsequent to hot rolling. Therefore, the coiling temperature thereof is preferably 600° C. to 700° C. In particular, the coiling temperature thereof is preferably 600° C. to 650° C.
  • the hot-rolled steel sheet is pickled, cold-rolled, continuously annealed, and then tempered.
  • Pickling and cold rolling conditions are not particularly limited.
  • steel sheet is continuously annealed such that the steel sheet is held at a temperature ranging from the Ac 1 transformation temperature to Ac 3 transformation temperature thereof for 30 s to 1200 s, cooled to a temperature of 600° C. to 800° C. at an average cooling rate of 100° C./s or less, and then cooled to 100° C. or lower at an average cooling rate of 100° C./s to 1000° C./s.
  • the steel sheet is subsequently tempered such that the steel sheet is reheated and held at a temperature of 100° C. to 300° C. for 120 s to 1800 s.
  • Reasons for limiting continuous annealing and tempering conditions are described below.
  • the annealing temperature When the annealing temperature is lower than the Ac i transformation temperature, an austenite phase (transformed into a martensite phase after quenching) necessary to ensure a predetermined strength is not produced during annealing and therefore such a predetermined strength cannot be achieved even if quenching is performed subsequently to annealing. Even if the annealing temperature is higher than the Ac 3 transformation temperature, 40% or more of the martensite phase can be obtained on a volume fraction basis by controlling a ferrite phase precipitated during cooling from the annealing temperature. In the ease of performing annealing at a temperature higher than the Ac 3 transformation temperature, a desired microstructure is unlikely to be obtained. Therefore, the annealing temperature ranges from the Ac 1 transformation temperature to the Ac 3 transformation temperature.
  • the holding time (annealing time) at the annealing temperature is excessively short, a microstructure is not sufficiently annealed, a nonuniform microstructure in which a deformed microstructure caused by hot rolling is present is caused, and the ductility is reduced.
  • the holding time is 30 seconds to 1200 seconds. In particular, the holding time is preferably 250 seconds to 600 seconds.
  • the steel sheet is cooled (the term “cool” is hereinafter referred to as “anneal” in some cases) to a temperature (annealing end temperature) of 600° C. to 800° C. from the annealing temperature at an average cooling rate of 100° C./s or less.
  • annealing end temperature 600° C. to 800° C. from the annealing temperature at an average cooling rate of 100° C./s or less.
  • the ferrite phase is precipitated during annealing from the annealing temperature and the strength-ductility balance ran be thereby controlled.
  • the annealing end temperature is lower than 600° C., a large amount of pearlite is formed in the microstructure to cause a significant reduction in strength and therefore a tensile strength of 1320 MPa cannot be achieved.
  • the annealing end temperature is 600° C. to 800° C.
  • the annealing end temperature is preferably 700° C. to 750° C. to suppress a change in material quality due to an operational change in annealing end temperature.
  • the average annealing rate during annealing is 100° C./s or less.
  • the average annealing rate is preferably 5° C./s or less to sufficiently concentrate carbon in the austenitic phase.
  • the steel sheet is cooled (the term “cool” is hereinafter referred to as “quench” in some cases) to a temperature (cooling end temperature) of 100° C. or lower at an average cooling rate of 100° C./s to 1000° C./s. Quenching subsequent to annealing is performed for the purpose of transforming the austenite phase into the martensite phase.
  • the average cooling rate is less than 100° C./s, the austenite phase is transformed into the ferrite phase, a bainite phase, or a pearlite phase during cooling and therefore a predetermined strength cannot be achieved.
  • the average cooling rate during quenching is 100° C./s to 1000° C./s.
  • the steel sheet is preferably cooled by water quenching.
  • the cooling end temperature is preferably 100° C. or lower, When the cooling end temperature is higher than 100° C., the volume fraction of the martensite phase is reduced because of the insufficient transformation of austenite phase into martensite phase during quenching and a reduction in material strength is caused by the self-tempering of the martensite phase produced by quenching, which is not preferable in terms of manufacture.
  • the steel sheet is tempered for the purpose of tempering the martensite phase such that the steel sheet is reheated and then held at a temperature of 100° C. to 100° C. for 120 seconds to 1800 seconds.
  • the tempering thereof softens the martensite phase to increase the workability.
  • the softening of martensite is insufficient and therefore the effect of increasing the workability cannot he expected when performing tempering at lower than 100° C.
  • Performing tempering at higher than 300° C. increases manufacturing costs for reheating causes a significant reduction in strength, and is incapable of achieving a useful effect.
  • the ultra-high-strength cold-rolled steel sheet can he manufactured through the above manufacturing steps. Since the ultra-high-strength cold-rolled steel sheet has excellent shapeahility (flatness) after annealing, a step of correcting the shape of the steel sheet by roiling, leveling, or the like is not necessarily needed. In view of adjusting the quality and/or surface roughness thereof, the annealed steel sheet may he rolled with an elongation of several percent.
  • Specimens were taken from the obtained cold-rolled steel sheets. A surface of each specimen that was parallel to the rolling direction was mirror-polished and was etched with nitai. The microstructure thereof was observed and photographed with an optical microscope or a scanning electron microscope, whereby the type of a constituent phase such as a tempered martensite phase or a ferrite phase was identify. A photograph of the microstructure was binarized, whereby the volume fraction of each of the tempered martensite phase the ferrite phase was determined. Since there was a possibility that a retained austenite phase was present in the obtained cold-rolled steel sheets, attempts were made to measure our examples for retained austenite phase by X-ray (Mo-Ka) determination. However, the amount of the retained austenite phase present therein was substantially zero and therefore was not included in the remainder shown in Table 3.
  • JIS No. 5 tensile specimens were taken from the obtained cold-rolled steel sheets in a direction perpendicular to the rolling direction and were subjected to a tensile test according to SIS Z 2241, whereby the specimens were determined for tensile property (0.2% proof stress (YS)), tensile strength (TS), and total elongation (EL).
  • YS proof stress
  • TS tensile strength
  • EL total elongation
  • a specimen with a size of 30 mm ⁇ 100 mm was cut out of each of the obtained cold-roiled steel sheets such that the longitudinal direction of the specimen corresponded to the rolling direction of the cold-rolled steel sheets.
  • An end surface of the specimen was ground.
  • the specimen was bent to 180 degrees using a punch having a tip with a radius of curvature of 10 mm.
  • the springback caused in the bent specimen was retained with a bolt 2 such that the distance between inner portions of the specimen I was 20 mm.
  • the specimen 1 was immersed in hydrochloric acid with a pH of 3 at 25° C. and was measured for up to 100 hours until the specimen I was broken. A specimen that was not broken within 100 hours was judged to be acceptable.
  • Tables 1 to 3 confirm that our Examples meet requirements specified herein and have a tensile strength of 1320 MPa or more, a total elongation of 12% or more, a high strength-ductility balance, and excellent delayed fracture resistance because the Examples were not broken for 100 hours in the delayed fracture characterization test
  • Example Nos. 1 to 23 which meet the requirements specified herein, were not broken for 100 hours in the delayed fracture characterization test. This confirms that our cold-rolled steel sheets have sufficient delayed fracture resistance, However, Comparative Example Nos. 25 and 29, each of which the microstructure is a tempered martensite single-phase which is outside our range, were broken within 100 hours and therefore failed in the delayed fracture characterization test.
  • the thin steel sheet for quenching or tempering, the thin steel sheet being suitable for use principally in ultra-high-strength automobile structural parts such as door impact beams and center pillars for automobiles.
  • the composition, rolling conditions, and annealing conditions are appropriately controlled.
  • This allows the stud sheet to have a microstructure containing 40% to 85% of a tempered martensite phase and 15% to 60% of a ferrite phase on a volume fraction basis, a tensile strength of 1320 NIPa or more, a total elongation of 12% or more, an excellent strength ductility balance, and excellent delayed fracture resistance.
  • the use of an ultra-high-strength cold-rolled steel sheet enables the pressing of automobile safety parts such as impact beams, The automobile safety parts exhibit excellent delayed fracture resistance,

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US13/805,144 2010-06-30 2011-06-24 Ultra high strength cold rolled steel sheet having excellent ductility and delayed fracture resistance and method for manufacturing the same Abandoned US20130087257A1 (en)

Applications Claiming Priority (3)

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JP2010148531A JP5668337B2 (ja) 2010-06-30 2010-06-30 延性及び耐遅れ破壊特性に優れる超高強度冷延鋼板およびその製造方法
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US20160136712A1 (en) * 2013-06-05 2016-05-19 Neturen Co., Ltd. Heating method, heating apparatus, and hot press molding method for plate workpiece
US20180037969A1 (en) * 2015-03-18 2018-02-08 Jfe Steel Corporation High-strength cold-rolled steel sheet and method of producing the same
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US10590504B2 (en) 2014-12-12 2020-03-17 Jfe Steel Corporation High-strength cold-rolled steel sheet and method for manufacturing the same
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CN103805840B (zh) 2012-11-15 2016-12-21 宝山钢铁股份有限公司 一种高成形性热镀锌超高强度钢板及其制造方法
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US20160136712A1 (en) * 2013-06-05 2016-05-19 Neturen Co., Ltd. Heating method, heating apparatus, and hot press molding method for plate workpiece
US20190030584A1 (en) * 2013-06-05 2019-01-31 Neturen Co., Ltd. Heating method, heating apparatus, and hot press molding method for plate workpiece
US10435762B2 (en) * 2014-03-31 2019-10-08 Jfe Steel Corporation High-yield-ratio high-strength cold-rolled steel sheet and method of producing the same
US10253389B2 (en) * 2014-03-31 2019-04-09 Jfe Steel Corporation High-yield-ratio, high-strength cold-rolled steel sheet and production method therefor
US10590504B2 (en) 2014-12-12 2020-03-17 Jfe Steel Corporation High-strength cold-rolled steel sheet and method for manufacturing the same
US20180037969A1 (en) * 2015-03-18 2018-02-08 Jfe Steel Corporation High-strength cold-rolled steel sheet and method of producing the same
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CN108474080A (zh) * 2015-11-16 2018-08-31 本特勒尔钢管有限公司 具有高能量吸收能力的钢合金和钢管产品
US11384415B2 (en) * 2015-11-16 2022-07-12 Benteler Steel/Tube Gmbh Steel alloy with high energy absorption capacity and tubular steel product
US11008635B2 (en) 2016-02-18 2021-05-18 Jfe Steel Corporation High-strength cold-rolled steel sheet
US11293103B2 (en) * 2017-01-05 2022-04-05 Jfe Steel Corporation High-strength cold-rolled steel sheet

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JP5668337B2 (ja) 2015-02-12
WO2012002520A1 (fr) 2012-01-05
JP2012012642A (ja) 2012-01-19
KR101540507B1 (ko) 2015-07-29
EP2589674A1 (fr) 2013-05-08

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