US20040035500A1 - Dual phase steel sheet with good bake-hardening properties - Google Patents

Dual phase steel sheet with good bake-hardening properties Download PDF

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US20040035500A1
US20040035500A1 US10/639,588 US63958803A US2004035500A1 US 20040035500 A1 US20040035500 A1 US 20040035500A1 US 63958803 A US63958803 A US 63958803A US 2004035500 A1 US2004035500 A1 US 2004035500A1
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steel sheet
less
bake
dual phase
bainite
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Shushi Ikeda
Koichi Makii
Hiroshi Akamizu
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Kobe Steel Ltd
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Kobe Steel Ltd
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Priority to US12/477,299 priority Critical patent/US9194015B2/en
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    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/18—Hardening; Quenching with or without subsequent tempering
    • C21D1/19—Hardening; Quenching with or without subsequent tempering by interrupted quenching
    • C21D1/20—Isothermal quenching, e.g. bainitic hardening
    • 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
    • C21D8/0426—Hot 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
    • C21D8/0421—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
    • C21D8/0473—Final recrystallisation annealing
    • 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
    • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
    • 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
    • C21D2211/001—Austenite
    • 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
    • C21D2211/002—Bainite
    • 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
    • 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

Definitions

  • the present invention relates to a dual phase steel sheet with good bake-hardening properties and, more particularly, to a dual phase steel sheet having well-balanced strength and forming properties.
  • This steel sheet has not only good bake-hardening properties but also good resistance to natural aging.
  • bake-hardening properties implies that the steel sheet improves in strength upon paint baking.
  • resistance to natural aging implies that the steel sheet retains its characteristic properties (such as forming properties) without deterioration after aging at room temperature).
  • the dual phase steel sheet according to the present invention will be widely used in automotive, electric, and machine industries and other industrial fields. The following description is mainly concerned with its use in automotive bodies as a typical example.
  • strain aging occurs also at normal temperature, and in this case, dissolved carbon and nitrogen in the steel migrate to fix dislocations even before paint baking.
  • Any steel sheet with strain aging at normal temperature is poor in ductility due to yield elongation, and poor ductility leads to flaws (such as wrinkles) at the time of press working.
  • automotive steel sheets are required to readily undergo strain aging at high temperatures for paint baking, thereby increasing in strength, and hardly undergo strain aging at normal temperature. In other words, they are required to be good in bake-hardening and also in resistance to natural aging.
  • the BH steel of quasi-IF type mentioned above has a strength of about 440 MPa at most even after bake-hardening on account of its low content of dissolved carbon.
  • DP steel Dual Phase Steel
  • DP steel contains dislocations introduced into the parent phase ferrite by martensitic transformation. It has a low value of yield point as such but has a high value of yield point due to hardening after paint baking which fixes the above-mentioned dislocations and other dislocations introduced by working.
  • TRIP steel is a steel which contains retained austenite of several to tens of percent in the metal structure, so that it exhibits high toughness after plastic forming.
  • Japanese Patent Laid-open No. 11565/2001 discloses a technology for increasing the amount of bake-hardening. This technology aims at developing a steel sheet that absorbs a large amount of collision energy to meet both requirements for safety of passenger cars and weight reduction of car body.
  • Japanese Patent Laid-open No. 297350/2000 proposes an idea that a steel sheet is improved in bake-hardening properties and resistance to natural aging when it has the dual phase structure in which the principal phase is ferrite and the second phase is at least one of pearlite, bainite, martensite, and retained austenite, with dissolved nitrogen controlled in amount and positions where it exists.
  • the principal phase is ferrite
  • the second phase is at least one of pearlite, bainite, martensite, and retained austenite
  • the present invention was completed in view of the foregoing. It is an object of the present invention to provide a dual phase steel sheet having good bake-hardening properties as well as good resistance to natural aging.
  • the gist of the present invention resides in a dual phase steel sheet with good bake-hardening properties which is characterized in containing (in terms of percent by mass):
  • bainite at least 30%
  • ferrite no more than 50% (including 0%),
  • the steel sheet should preferably have a space factor of bainite more than 60%.
  • Mo no less than 0.05% and no more than 1%
  • Ni no less than 0.05% and no more than 0.5%
  • Cr no less than 0.05% and no more than 1%.
  • Nb no less than 0.01% and no more than 0.1%
  • V no less than 0.01% and no more than 0.1%.
  • Ca no less than 3 ppm and no less than % and no more than 30 ppm and/or
  • REM no less than 3 ppm and no more than 30 ppm.
  • the present invention mentioned above provides a steel sheet which has well-balanced strength and workability, exhibits good bake-hardening properties at the time of paint baking, and offers good resistance to natural aging, by virtue of its unique structure in which bainite is the principle constituent and retained austenite and ferrite are present in a specified amount.
  • This steel sheet exhibits outstanding workability at the time of forming and also exhibits high strength after paint baking.
  • FIG. 1 is an SEM microphotograph showing one example of the structure of the steel sheet according to the present invention.
  • FIG. 2 is a diagram illustrating the heat treatment carried out in one Example.
  • FIG. 3 is a diagram illustrating the heat treatment carried out in another Example.
  • FIG. 4 is a diagram illustrating the heat treatment carried out in another Example.
  • FIG. 5 is a diagram illustrating the heat treatment carried out in another Example.
  • FIG. 6 is an SEM microphotograph showing the structure of the steel sheet in experiment No. 3.
  • FIG. 7 is an SEM microphotograph showing the structure of the steel sheet in experiment No. 17.
  • the present invention is based on a finding that the steel sheet is effectively relieved from age hardening at normal temperature if it has the structure of so-called TRIP steel containing retained austenite, with dissolved carbon bonding to dislocations introduced at the time of production.
  • the steel sheet of the present invention has the structure which is composed of at least 3% of retained austenite, at least 30% of bainite, and no more than 50% (including 0%) of ferrite, in terms of space factor. These space factors were established for the reasons given below.
  • Bainite (at Least 30%)
  • the steel sheet of the present invention is characterized most by being composed mainly of bainite. It differs in structure from the conventional TRIP steel as follows. Being composed of ferrite and pearlite as the principal phase, the conventional TRIP steel has the disadvantage of not keeping sufficient dislocations at the time of steel sheet production, and the resulting steel sheet is poor in bake-hardening properties.
  • the steel sheet of the present invention is composed mainly of bainite and it has a high initial dislocation density. Therefore, it exhibits much better bake-hardening properties than any other conventional steel sheets at the time of paint baking, which leads to a greatly improved strength due to strain aging.
  • the steel sheet to produce such an effect it should have the structure in which the amount of bainite is at least 30%, preferably more than 60%, more preferably more than 70%, and most preferably more than 80%. Also, for the steel sheet to exhibit good bake-hardening properties at the time of paint baking and to have good resistance to natural aging, it should substantially have the dual-phase structure composed of retained austenite and bainite.
  • Retained austenite contributes to improvement in total elongation.
  • it should contain retained austenite as much as at least 3%, preferably more than 5%, more preferably more than 7%, and most preferably more than 10%, in terms of space factor.
  • the upper limit should be 30%, preferably 25%; retained austenite with an excess space factor deteriorates stretch flange formability.
  • the steel sheet of the present invention keeps as much retained austenite as necessary to hold therein the dissolved carbon and nitrogen which fix dislocations. In this way the retained austenite prevents dislocations from being fixed by dissolved carbon and nitrogen at normal temperature. Therefore, the steel sheet is relieved from age hardening at normal temperature even in the case where a large number of dislocations are introduced at the time of production.
  • Retained austenite should preferably contain more than 0.8% of carbon for better elongation.
  • the point of the present invention lies in the fact that the steel sheet is composed mainly of bainite so that it has good bake-hardening properties.
  • the present inventors found that the object of the present invention is achieved so long as the steel sheet contains as much bainite and retained austenite as specified above even though its ferrite content is less than 50%.
  • FIG. 1 is an SEM microphotograph ( ⁇ 4000) showing the structure of the steel sheet of the present invention.
  • the black background represents ferrite and the gray parts represent bainite or retained austenite. It was found that the steel sheet has good bake-hardening properties even though its structure is composed mainly of bainite, with the remainder (45%) being ferrite.
  • the steel sheet becomes better in bake-hardening properties as the amount of ferrite decreases and the amount of bainite relatively increases. Therefore, the amount of ferrite should be less than 30%, preferably less than 25%, and more preferably 0%.
  • the steel sheet may contain ferrite in an amount more than 10% and less than the upper limit specified above so that it has good elongation characteristics as well as good workability.
  • the steel sheet of the present invention usually have the mixed structure mentioned above (which consists of retained austenite, ferrite, and bainite, or consists of retained austenite and bainite).
  • the mixed structure may additionally contain pearlite and martensite in an amount not harmful to the desired characteristic properties. These constituents inevitably enter the structure in the manufacturing process; their content should preferably be as little as possible.
  • the steel sheet of the present invention is composed of the basic constituents listed below.
  • the amount of constituents is expressed in terms of mass %.
  • Carbon is an element essential for the steel sheet to exhibit high strength and to contain retained austenite. In other words, carbon sufficiently existing in the austenite phase permits the austenite phase to remain as much as desired at normal temperature.
  • the content of carbon necessary to produce this effect is no less than 0.06%, preferably no less than 0.10%. However, for the steel sheet to have good weldability, the content of carbon should be less than 0.25%, preferably less than 0.20%.
  • Silicon and aluminum are elements to prevent retained austenite from decomposing to give carbides. Silicon plays an important role in solid solution strengthening. The total amount of silicon and aluminum necessary for this effect is no less than 0.5%, preferably no less than 0.7%, and more preferably no less than 1%. However, it should be less than 3%, preferably less than 2.5%, and more preferably less than 2%, because excess silicon and aluminum more than 3% are wasted and lead to high temperature brittleness.
  • Manganese stabilizes austenite to give as much retained austenite as desired.
  • the amount of manganese to produce this effect is no less than 0.5%, preferably no less than 0.7%, and more preferably no less than 1%.
  • its upper limit should be 3%, preferably 2.5%, and more preferably 2%, because excess manganese produces an adverse effect such as ingot cracking.
  • Phosphorus secures as much retained austenite as desired.
  • the amount of phosphorus to produce this effect is no less than 0.03%, preferably no less than 0.05%. However, its upper limit is 0.15%, preferably 0.1%, because excess phosphorus adversely affects secondary workability.
  • Sulfur forms sulfide inclusions such as MnS, which bring about a starting point of cracking, thereby deteriorating workability.
  • the amount of sulfur should be no more than 0.02%, preferably no more than 0.015%.
  • N no more than 60 ppm (excluding 0%)
  • the amount of nitrogen should be no more than 60 ppm, preferably no more than 50 ppm, and more preferably no more than 40 ppm. The less the amount of nitrogen in the steel sheet, the more desirable. However, the lower limit of the amount of nitrogen is about 10 ppm, depending on how much of nitrogen the process employed can reduce.
  • the steel sheet of the present invention is made up of the above-mentioned principal constituents, with the remainder being substantially iron and inevitable impurities. It may additionally contain the following components in an amount not harmful to the effect of the present invention.
  • Mo no less than 0.05% and no more than 1%
  • Ni no less than 0.05% and no more than 0.5%
  • Mo no less than 0.05% (preferably no less than 0.1%); Ni: no less than 0.05% (preferably no less than 0.1%); Cu: no less than 0.05% (preferably no less than 0.1%); and Cr: no less than 0.05% (preferably no less than 0.1%).
  • Mo no more than 0.8%
  • Ni no more than 0.4%
  • Cu no more than 0.4%
  • Cr no more than 0.8%.
  • Nb no less than 0.01% and no more than 0.1%
  • V no less than 0.01% and no more than 0.1%
  • Ti no less than 0.01% (preferably no less than 0.02%); Nb: no less than 0.01% (preferably no less than 0.02%); V: no less than 0.01% (preferably no less than 0.02%).
  • Ca no less than 3 ppm and no more than 30 ppm
  • REM no less than 3 ppm and no more than 30 ppm
  • Ca and REM rare earth elements
  • the rare earth elements include Sc, Y, and lanthanoid.
  • the steel sheet contain each of them in an amount no less than 3 ppm, preferably no less than 5 ppm. When used in excess of 30 ppm, they are wasted without extra effect. Therefore, their desired amount is no more than 25 ppm.
  • the steel sheet of the present invention may be produced by any method without specific restrictions. However, it will have the structure characteristic of the present invention if hot rolling or cold rolling is followed by continuous annealing or plating which is carried out under the following conditions.
  • the steel sheet should be cooled to the bainite transformation temperature (about 500-350° C.) at an average cooling rate larger than 3° C./s, preferably larger than 5° C./s, for avoidance of pearlite transformation.
  • the controlled average cooling rate mentioned above helps introduce a large number of dislocations, thereby imparting the desired bake-hardening properties (defined by as a difference in stress larger than 50 MPa before and after ensuing heat treatment for paint baking at 170° C. for 20 minutes, after application of 2% pre-strain).
  • Better bake-hardening properties with a difference in stress larger than 100 MPa may be attained if cooling is accomplished by using water-cooled rolls, so that the average cooling rate is greater than 5° C./s.
  • the cooling rate should be as great as possible to improve the bake-hardening properties; however, an adequate cooling rate should be established from the practical point of view.
  • the control of the cooling rate specified above should be maintained until the bainite transformation temperature is reached. If the control of the cooling at the above specified rate (rapid cooling) is suspended while the steel sheet is still hotter than the bainite transformation temperature and is followed by slow cooling, the resulting steel sheet is poor in bake-hardening properties due to insufficient dislocations and is also poor in elongation due to insufficient retained austenite. On the other hand, if cooling at the above specified rate is continued until a lower temperature than the bainite transformation temperature, the resulting steel sheet is liable to age hardening at normal temperature and is poor in elongation due to insufficient retained austenite.
  • the steel sheet After cooling, the steel sheet should be kept at the specified temperature for more than one second, so that carbon efficiently concentrates in retained austenite in a short time, giving rise to a large amount of stable retained austenite.
  • the resulting retained austenite greatly contributes to the TRIP effect.
  • an excessively long holding time should be avoided because the resulting steel sheet is poor in bake-hardening properties due to recovery, namely decrease of dislocations formed by cooling.
  • the above-mentioned heat treatment may be accomplished, for example, by heating/cooling using a salt bath or CAL simulator, or by water cooling.
  • the cooling to normal temperature after the keeping at the specified temperature may be accomplished by air cooling or water cooling without any specific restrictions.
  • the steel sheet may undergo plating or alloying to modify the structure as desired to such an extent not harmful to the effect of the present invention.
  • the steel sheet of the present invention may be produced by either of the following steps which include the above-mentioned steps.
  • the hot rolling and cold rolling may be carried out under ordinary conditions without specific restrictions. However, their ensuing steps, namely continuous annealing and plating, under controlled conditions are more influential in formation of the desired structure in the steel sheet of the present invention.
  • the hot rolling step should be completed at a temperature higher than the A r3 point. Then the rolled steel sheet should be cooled at an average cooling rate of about 30° C./s and finally wound up at about 500-600° C. In addition, the cold rolling step may be carried out at a draft of about 30-70%. These conditions are not mandatory, as a matter of course.
  • An experimental slab was prepared from a vacuum-melted steel having the composition shown in Table 1.
  • the slab was made into a steel sheet, 2.4-3.2 mm thick, by hot rolling under the following conditions.
  • Finishing temperature 850° C.
  • Winding temperature 600° C.
  • the hot-rolled steel sheet was cold-rolled (with a draft of 50-75%) for reduction of thickness to 1.0-1.6 mm.
  • the cold-rolled steel sheet subsequently underwent heat treatment as illustrated in FIG. 2 by a continuous annealing line (CAL).
  • CAL continuous annealing line
  • the steel sheet was kept at 900° C. for 2 minutes in a salt bath, quenched in another salt bath at 400° C., kept at 400° C. for 1 minute in the same salt bath, and finally air-cooled to room temperature. After cooling, the steel sheet underwent skin pass rolling, with the reduction of area being 0.5-2%. It was finally wound up.
  • the thus obtained steel sheet was examined for structure by observation under an optical microscope and a scanning electron microscope (SEM) after Lepera etching.
  • the areal ratio of ferrite and bainite was obtained from the microphotographs.
  • the space factor of retained austenite was obtained by X-ray measurement.
  • TS tensile strength
  • El total elongation
  • BH bake-hardening properties
  • the sample used in this example is a steel sheet, 1.0-1.6 mm thick, obtained from an experimental slab having the composition shown in No. 3 of Table 1, by hot rolling and cold rolling under the same conditions as mentioned above.
  • Sample No. 15 underwent heating at about 900° C. for 2 minutes in a salt bath and then water cooling in the continuous annealing as illustrated in FIG. 3, without keeping at about 400° C. as shown in FIG. 2.
  • Sample No. 16 underwent heating at about 900° C. for 2 minutes in a salt bath, quenching in another salt bath at about 400° C., keeping at about 400° C. for 5 minutes, and air cooling to room temperature, as illustrated in FIG. 4.
  • Sample No. 17 underwent heating at about 850° C. for 2 minutes in a salt bath, quenching in another salt bath at about 400° C., keeping at about 400° C. for 1 minute, and air cooling to room temperature, as illustrated in FIG. 5.
  • Sample No. 18 underwent heating at about 900° C. for 2 minutes in a salt bath, cooling to about 400° C. at an average rate of 5° C./sec, keeping at about 400° C. for 1 minute, and air cooling to room temperature.
  • No. 1 has insufficient retained austenite but has excess ferrite on account of low carbon content. Therefore, it is poor in bake-hardening properties and is liable to strain aging at normal temperature.
  • No. 6 has insufficient retained austenite on account of low content of (Si+Al) and low content of Mn. Therefore, it is poor in bake-hardening properties and is liable to strain aging at normal temperature.
  • No. 15 suggests that a prescribed amount of retained austenite can be secured if the sample is quenched in the continuous annealing step and then kept at about 400° C. for a certain period of time.
  • No. 16 suggests that keeping the steel sheet at about 400° C. for a long time after quenching from about 900° C. is not desirable for a large number dislocations necessary for the bake-hardening properties. A probable reason for this is that dislocations which have resulted from quenching from about 900° C. recover, resulting in a low dislocation density, if the steel sheet is kept at about 400° C. for an excessively long time.
  • No. 17 suggests that it is desirable to heat the steel sheet at a temperature higher than the A 3 point at the beginning of the continuous annealing process, if the steel sheet is to have a large number dislocations necessary for the bake-hardening properties.
  • FIG. 6 is an SEM microphotograph ( ⁇ 4000) which shows the structure of No. 3 conforming to the present invention. It is noted that the sample has the bainite structure.
  • FIG. 7 is an SEM microphotograph ( ⁇ 4000) which shows the structure of No. 17 in a comparative example. The black parts represent ferrite and the gray parts represent retained austenite. It is seen that ferrite dominates bainite.

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  • Chemical & Material Sciences (AREA)
  • Engineering & Computer Science (AREA)
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  • Metallurgy (AREA)
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US10/639,588 2002-08-20 2003-08-13 Dual phase steel sheet with good bake-hardening properties Abandoned US20040035500A1 (en)

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US20110162762A1 (en) * 2008-09-10 2011-07-07 Jfe Steel Corporation High strength steel sheet and method for manufacturing the same
US20110146852A1 (en) * 2008-09-10 2011-06-23 Jfe Steel Corporation High strength steel sheet and method for manufacturing the same
US20140334966A1 (en) * 2011-11-21 2014-11-13 Nippon Steel & Sumitomo Metal Corporation Hot-rolled steel sheet for nitriding, cold-rolled steel sheet for nitriding excellent in fatigue strength, manufacturing method thereof, and automobile part excellent in fatigue strength using the same
US9777353B2 (en) * 2011-11-21 2017-10-03 Nippon Steel & Sumitomo Metal Corporation Hot-rolled steel sheet for nitriding, cold-rolled steel sheet for nitriding excellent in fatigue strength, manufacturing method thereof, and automobile part excellent in fatigue strength using the same
US10968502B2 (en) 2016-11-04 2021-04-06 Nucor Corporation Method of manufacture of multiphase, cold-rolled ultra-high strength steel
US11021776B2 (en) 2016-11-04 2021-06-01 Nucor Corporation Method of manufacture of multiphase, hot-rolled ultra-high strength steel
US11965230B2 (en) 2016-11-04 2024-04-23 Nucor Corporation Multiphase ultra-high strength hot rolled steel
WO2022206913A1 (zh) * 2021-04-02 2022-10-06 宝山钢铁股份有限公司 抗拉强度≥980MPa的双相钢和热镀锌双相钢及其快速热处理制造方法

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EP1391526A3 (en) 2004-04-21
JP3764411B2 (ja) 2006-04-05
EP1391526B1 (en) 2010-11-03
JP2004076114A (ja) 2004-03-11
EP1391526B2 (en) 2014-06-04
US20090242085A1 (en) 2009-10-01
DE60334761D1 (de) 2010-12-16
US9194015B2 (en) 2015-11-24

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