EP2314730B1 - Aciers haute résistance ayant d'excellentes propriétés d'absorption d'énergie aux chocs. - Google Patents

Aciers haute résistance ayant d'excellentes propriétés d'absorption d'énergie aux chocs. Download PDF

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Publication number
EP2314730B1
EP2314730B1 EP10181458A EP10181458A EP2314730B1 EP 2314730 B1 EP2314730 B1 EP 2314730B1 EP 10181458 A EP10181458 A EP 10181458A EP 10181458 A EP10181458 A EP 10181458A EP 2314730 B1 EP2314730 B1 EP 2314730B1
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Prior art keywords
deformation
retained austenite
rolled
steel sheet
strain
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German (de)
English (en)
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EP2314730A1 (fr
Inventor
Osamu Kawano
Yuzo Takahashi
Junichi Wakita
Hidesato Mabuchi
Manabu Takahashi
Akihiro Uenishi
Riki Okamoto
Yukihisa Kuriyama
Yasuharu Sakuma
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Nippon Steel Corp
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Nippon Steel Corp
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Priority claimed from JP33138096A external-priority patent/JPH10158735A/ja
Priority claimed from JP22300597A external-priority patent/JPH1161326A/ja
Priority claimed from JP25888797A external-priority patent/JP3530355B2/ja
Priority claimed from JP25883497A external-priority patent/JP3530353B2/ja
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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
    • 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
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C—CHEMISTRY; METALLURGY
    • 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
    • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
    • C21D8/0226—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
    • 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
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/008—Martensite
    • 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
    • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
    • C21D8/0236—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
    • C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
    • C21D8/0273—Final recrystallisation annealing

Definitions

  • the present invention relates to high strength hot rolled and high strength cold rolled steel sheets having high flow stress during dynamic deformation, which can be used for automotive members and the like to provide assurance of safety for passengers by efficiently absorbing the impact energy of a collision.
  • JP-A-7-18372 which provides retained austenite-containing high strength steel sheets with excellent impact resistance and a method for their production, discloses a solution for impact absorption simply by increasing the yield stress brought about by a higher deformation rate; however, it has not been demonstrated what other aspects of the retained austenite should be controlled, apart from the amount of retained austenite, in order to improve impact absorption.
  • Collision impact absorbing members such as front side members in automobiles and the like are produced by subjecting steel sheets to a bending or press forming step. After being worked in this manner they are usually subjected to impact by automobile collision following painting and baking. The steel sheets, therefore, are required to exhibit high impact energy absorption properties after their working into members, painting and baking.
  • the ideal microstructure is a composite structure including ferrite and/or bainite which are readily solid-solution strengthened by various substitutional elements, either of which as the dominant phase, and a third phase containing a 3 ⁇ 50% volume fraction of retained austenite which is transformed into hard martensite during deformation, while it has further been found that high-strength steel sheets with high flow stress during dynamic deformation can also be obtained with a composite structure wherein martensite is present in the third phase of the initial microstructure, provided that specific conditions are satisfied.
  • the present inventors discovered that the amount of pre-deformation corresponding to shape forming of impact absorbing members such as front side members sometimes reaches a maximum of over 20% depending on the section, but that the majority of the sections undergo deformation of greater than 0% and less than or equal to 10% with equivalent strain.
  • deformation of greater than 0% and less than or equal to 10% of equivalent strain was selected as the amount of pre-deformation to be applied to members during their working.
  • Fig. 1 is a graph showing the relationship between the average value ⁇ dyn of the flow stress in the range of 3 ⁇ 10% of equivalent strain when deformed in a strain rate range of 5 x 10 2 ⁇ 5 x 10 3 (1/s), and the static material strength TS (i.e., the maximum stress TS (MPa) in the static tensile test as measured in a strain rate range of 5 x 10 -4 ⁇ 5 x 10 -3 (1/s)), as an indicator of the collision impact energy absorption property according to the invention.
  • TS maximum stress TS
  • Impact absorbing members such as front side members have a hat-shaped cross-section, and as a result of analysis of deformation of such members upon being crushed by high-speed collision, the present inventors have found that despite deformation proceeding up to a high maximum strain of over 40%, at least 70% of the total absorption energy is absorbed in a strain range of 10% or lower in a high-speed stress-strain diagram. Therefore, the flow stress during dynamic deformation with high-speed deformation at 10% or lower was used as the index of the high-speed collision energy absorption property.
  • the index used for the impact energy absorption property was the average stress ⁇ dyn in the range of 3 ⁇ 10% of equivalent strain when deformed in a strain rate range of 5 x 10 2 ⁇ 5 x 10 3 (1/s) high-speed tensile deformation.
  • the average stress ⁇ dyn of 3 ⁇ 10% upon high-speed deformation generally increases with increasing static tensile strength ⁇ maximum stress: TS (MPa) in a static tensile test measured in a stress rate range of 5 x 10 -4 ⁇ 5 x 10 -3 (1/s) ⁇ of the steel material without pre-deformation or baking treatment. Consequently, increasing the static tensile strength (synonymous with the static material strength) of the steel material directly contributes to improved impact energy absorption property of the member. However, increased strength of the steel material results in poorer formability into members, making it difficult to obtain members with the necessary shapes. Consequently, steel materials having a high ⁇ dyn with the same tensile strength (TS) are preferred.
  • TS tensile strength
  • steel materials wherein the average value ⁇ dyn (MPa) of the flow stress in the range of 3-10% of equivalent strain, when deformed in a strain rate range of 5 x 10 2 ⁇ 5 x 10 3 (1/s) after pre-deformation at greater than 0% and less than or equal to 10%, satisfy the inequality ⁇ dyn - TS ⁇ -0.234 x TS + 250 as expressed in terms of the maximum stress TS (MPa) in the static tensile test as measured in a strain rate range of 5 x 10 -4 ⁇ 5 x 10 -3 (1/s) without pre-deformation, have higher impact energy absorption properties as actual members compared to other steel materials, and that the impact energy absorption property is improved without increasing the overall weight of the member, making it possible to provide high-strength steel sheets with high flow stress during dynamic deformation.
  • MPa maximum stress TS
  • the present inventors have also discovered that for improved anti-collision safety, an increased work hardening during pre-working as represented by the work hardening coefficient between 1% and 5% of strain is necessary for greater initial deformation resistance at the initial point of collision, as well as for higher work hardening during collision deformation by the presence of martensite transformed during pre-deformation, and for an increased ⁇ dyn. That is to say, the anti-collision safety may be increased by controlling the microstructure of the steel material as explained above so that, as shown in Fig. 2 and Fig.
  • the work hardening coefficient of the steel is at least 0.080, and preferably at least 0.108, and so that the work hardening coefficient between 1% and 5% of at yield strain x yield strength is at least 40, and preferably at least 54.
  • the dynamic energy absorption which is an indicator of the anti-collision safety of automobile members
  • the work hardening coefficient and yield strength x work hardening coefficient of the steel sheets it can be seen that the dynamic energy absorption improves as the values increase, suggesting that a proper evaluation can be made based on the work hardening coefficient of the steel sheets as an indicator of anti-collision safety of automobile members, so long as the yield strength level is the same, or based on the yield strength x work hardening coefficient if the yield strength differs.
  • the dynamic energy absorption was determined in the following manner by the impact crush test method as shown in Fig. 4a , Fig. 4b and Fig. 4c .
  • the work hardening coefficient of the steel sheet at a 1 ⁇ 5% strain and the work hardening coefficient between 1% and 5% of at yield strain x yield strength were calculated in the following manner. Specifically, the steel sheet was worked into a JIS-5 test piece (gauge length: 50 mm, parallel part width: 25 mm) and a tensile test at a strain rate of 0.001/sec was carried out to determine the yield strength and work hardening coefficient (n value for strain of 1 ⁇ 5%).
  • a suitable amount of retained austenite is preferably 3% to 50%. Specifically, if the volume fraction of the retained austenite is less than 3%, the shaped member cannot exhibit its excellent work hardening property upon undergoing collision deformation, the deformation load remains at a low level resulting in a low deformation work and therefore the dynamic energy absorption is lower making it impossible to achieve improved anti-collision safety, and the anti-necking effect is also insufficient, making it impossible to obtain a high tensile strength x total elongation.
  • the volume fraction of the retained austenite is greater than 50%, working-induced martensite transformation occurs in a concatenated fashion with only slight shape working strain, and no improvement in the tensile strength x total elongation can be expected since the hollow extension ratio instead deteriorates as a result of notable hardening which occurs during punching, while even if shaping of the member is possible, the shaped member cannot exhibit its excellent work hardening property upon undergoing collision deformation; the above-mentioned range for the retained austenite content is determined from this viewpoint.
  • the mean grain diameter of the retained austenite should be no greater than 5 ⁇ m, and preferably no greater than 3 ⁇ m. Even if the retained austenite volume fraction of 3 ⁇ 50% is satisfied, a mean grain diameter of greater than 5 ⁇ m is not preferred because this will prevent fine dispersion of the retained austenite in the steel, resulting in only local inhibition of the improving effect by the characteristics of the retained austenite.
  • the microstructure is such that the ratio of the aforementioned mean grain diameter of the retained austenite to the average grain diameter of the ferrite or bainite of the dominant phase is no greater than 0.6, and the average grain diameter of the dominant phase is no greater than 10 ⁇ m, and preferably no greater than 6 ⁇ m.
  • the carbon concentration in the retained austenite can be experimentally determined by X-ray diffraction and Mossbauer spectrometry, and for example, it can be calculated by the method indicated in the Journal of The Iron and Steel Institute, 206(1968), p60 , utilizing the integrated reflection intensity of the (200) plane, (211) plane of the ferrite and the (200) plane, (220) plane and (311) plane of the austenite, with X-ray diffraction using Mo K ⁇ rays.
  • the retained ⁇ (austenite) which is not adjacent to ferrite tends to escape the strain and thus fails to be transformed into martensite with deformation of about 1 ⁇ 5%; because of this lessened effect, it is preferred for the retained austenite to be adjacent to the ferrite.
  • the volume fraction of the ferrite is desired to be at least 40%, and preferably at least 60%, and the mean grain diameter (corresponding to the mean circle-equivalent diameter) is desired to be no greater than 10 ⁇ m, and preferably no greater than 6 ⁇ m.
  • ferrite is the softest substance in the constituent composition, it is an important factor in determining the work hardening coefficient between 1% and 5% of strain x yield strength and the yield ratio.
  • the volume fraction should preferably be within the prescribed values.
  • increasing the volume fraction and fineness of the ferrite is effective for raising the carbon concentration of the untransformed austenite and finely dispersing it, thus resulting in greater fineness of the martensite produced from the untransformed austenite as well as of the remaining composition, and increasing the volume fraction and fineness of the retained austenite, which will contribute to improved anti-collision safety effects and formability.
  • the martensite is at a volume fraction of 3 ⁇ 30% and it is desired to have a mean grain diameter (corresponding to the mean circle-equivalent diameter) of no greater than 10 ⁇ m, and preferably no greater than 6 ⁇ m.
  • the martensite primarily creates mobile transfer in the surrounding ferrite, contributing to a lower yield rate and improved work hardening coefficient, and therefore results in further improvement in the anti-collision safety effect and formability by satisfying the designated values mentioned above, allowing a more desired level of properties to be achieved, specifically a work hardening coefficient between 1% and 5% of strain more than 54 x yield strength more than 75%.
  • the relationship between the volume fraction and the mean grain diameter of the martensite is such that even with a low volume fraction and a large mean grain diameter the effect is limited to local influence, making it impossible to satisfy the aforementioned properties.
  • the location of the martensite when the martensite is not adjacent to ferrite, the influence of the mobile transfer, etc. of the martensite barely reaches the ferrite, thus lessening its effect. Consequently, the martensite is preferred to be adjacent to the ferrite.
  • the high-strength steel sheets used according to the invention are high-strength steel sheets containing, in terms of weight percentage, C at from 0.03% to 0.3%, either or both Si and Al at a total of from 0.5% to 3.0% and if necessary one or more from among Mn, Ni, Cr, Cu and Mo at a total of from 0,5% to 3.5%, with the remainder Fe as the primary component, or they are high-strength steel sheets with high dynamic deformation resistance obtained by further addition if necessary to the aforementioned high-strength steel sheets, one or more from among Nb, Ti, V, P, B, Ca and REM, with one or more from among Nb, Ti and V at a total of no greater than 0.3%, P at no greater than 0.3%, B at no greater than 0.01%, Ca at from 0.0005% to 0.01% and REM at from 0.005% to 0.05%, with the
  • C is the most inexpensive element for stabilizing austenite at room temperature and thus contributing to the necessary stabilization of austenite for its retention, and therefore it may be considered the most essential element according to the invention.
  • the average carbon content in the steel sheet not only affects the retained austenite volume fraction which can be ensured at room temperature but, by increasing the concentration in the untransformed austenite during the working at the heat treatment of production, it is possible to improve the stability of the retained austenite for working. If the C content is less than 0.03%, however, a final retained austenite volume fraction of at least 3% cannot be ensured, and therefore 0.03% is the lower limit.
  • the ensurable retained austenite volume fraction also increases, allowing the stability of the retained austenite to be ensured by ensuring the retained austenite volume fraction.
  • the C content of the steel sheet is too great, not only does the strength of the steel sheet exceed the necessary level thus impairing the formability for press working and the like, but the dynamic stress increase is also inhibited with respect to the static strength increase, while the reduced weldability limits the use of the steel sheet as a member; the upper limit for the C content was therefore determined to be 0.3%.
  • Si, Al: Si and Al are both ferrite-stabilizing elements, and they serve to increase the ferrite volume fraction for improved workability of the steel sheet.
  • Si and Al both inhibit production of cementite, allowing C to be effectively concentrated in the austenite, and therefore addition of these elements is essential for retention of austenite at a suitable volume fraction at room temperature.
  • Other elements whose addition has this effect of suppressing production of cementite include, in addition to Si and Al, also P, Cu, Cr, Mo, etc. A similar effect can be expected by appropriate addition of these elements as well.
  • the cementite production-inhibiting effect will be insufficient, thus wasting as carbides most of the added C which is the most effective component for stabilizing the austenite, and this will either render it impossible to ensure the retained austenite volume fraction required for the invention, or else the production conditions necessary for ensuring the retained austenite will fail to satisfy the conditions for volume production processes; the lower limit was therefore determined to be 0.5%.
  • Si scaling may be avoided by having Si ⁇ 0.1% or conversely Si scaling may be generated over the entire surface, to be rendered less conspicuous, by having Si ⁇ 1.0%.
  • Mn, Ni, Cr, Cu, Mo are all austenite-stabilizing elements, and are effective elements for stabilizing austenite at room temperature.
  • these austenite-stabilizing elements can effectively promote retention of austenite.
  • These elements also have an effect of inhibiting production of cementite, although to a lesser degree than Al and Si, and act as aids for concentration of C in the austenite.
  • these elements cause solid-solution strengthening of the ferrite and bainite matrix together with Al and Si, thus also acting to increase the flow stress during dynamic deformation at high speeds.
  • the lower limit was therefore determined to be 0.5%.
  • the total amount of those elements exceeds 3.5%, the primary phase of ferrite or bainite will tend to be hardened, not only inhibiting increased deformation resistance from the increased strain rate, but also leading to lower workability and lower toughness of the steel sheet, and increased cost of the steel material; the upper limit was therefore determined to be 3.5%.
  • Nb, Ti or V which are added as necessary can promote higher strength of the steel sheet by forming carbides, nitrides or carbonitrides, but if their total exceeds 0.3%, excess amounts of the nitrides, carbides or carbonitrides will precipitate in the crystal grains or at the grain boundaries of the ferrite or bainite primary phase, becoming a source of mobile transfer during high-speed deformation and making it impossible to achieve high flow stress during dynamic deformation.
  • production of carbides inhibits concentration of C in the retained austenite which is the most essential aspect of the present invention, thus wasting the C content; the upper limit was therefore determined to be 0.3%.
  • B or P are also added as necessary.
  • B is effective for strengthening of the grain boundaries and high strengthening of the steel sheet, but if it is added at greater than 0.01% its effect will be saturated and the steel sheet will be strengthened to a greater degree than necessary, thus inhibiting increased deformation resistance against high-speed deformation and lowering its workability into parts; the upper limit was therefore determined to be 0.01%.
  • P is effective for ensuring high strength and retained austenite for the steel sheet, but if it is added at greater than 0.2% the cost of the steel sheet will tend to increase, while the deformation resistance of the dominant phase of ferrite or bainite will be increased to a higher degree than necessary, thus inhibiting increased deformation resistance against high-speed deformation and resulting in poorer season cracking resistance and poorer fatigue characteristics and tenacity; the upper limit was therefore determined to be 0.2%. From the standpoint of preventing reduction in the secondary workability, tenacity, spot weldability and recyclability, the upper limit is more desirably 0.02%.
  • the upper limit is more desirably 0.01% from the standpoint of preventing reduction in formability (especially the hollow extension ratio) and spot weldability due to sulfide-based inclusions.
  • Ca is added to at least 0.0005% for improved formability (especially hollow extension ratio) by shape control (spheroidization) of sulfide-based inclusions, and its upper limit was determined to be 0.01% in consideration of effect saturation and the adverse effect due to increase in the aforementioned inclusions (reduced hollow extension ratio).
  • REM has a similar effect as Ca, its added content was also determined to be from 0.005% to 0.05%.
  • a continuous cast slab having the component composition described above is fed directly from casting to a hot rolling step, or is hot rolled after reheating.
  • Continuous casting for thin gause strip and hot rolling by the continuous hot rolling techniques may be applied for the hot rolling in addition to normal continuous casting, but in order to avoid a lower ferrite volume fraction and a coarser mean grain diameter of the thin steel sheet microstructure, the steel sheet thickness at the hot rolling approach side (the initial steel billet thickness) is preferred to be at least 25 mm.
  • the final pass rolling speed for the hot rolling is preferred to be at least 500 mpm and more preferably at least 600 mpm, in light of the problems described above.
  • the finishing temperature for the hot rolling during production of the high-strength hot-rolled steel sheets is preferably in a temperature range of Ar 3 - 50°C to Ar 3 ⁇ 120°C as determined by the chemical components of the steel sheet.
  • Ar 3 - 50°C deformed ferrite is produced, with an inferior flow stress during dynamic deformation ⁇ dyn, 1 ⁇ 5% work hardening property and formability.
  • Ar 3 + 120°C the flow stress during dynamic deformation ⁇ dyn, the 1 ⁇ 5% work hardening property, etc. are inferior because of a coarser steel sheet microstructure, while it is also not preferred from the viewpoint of scale defects.
  • the steel sheets which have been hot-rolled in the manner described above are subjected to a coiling step after being cooled on a run-out table.
  • the average cooling rate here is at least 5°C/sec.
  • the cooling rate is decided from the standpoint of ensuring the volume fraction of the retained austenite.
  • the cooling method may be carried out at a constant cooling rate, or with a combination of different cooling'rates which include a low cooling rate range during the procedure.
  • the hot-rolled steel sheets are then subjected to a coiling step, where they are preferably coiled at a coiling temperature of 500°C or below.
  • a coiling temperature of higher than 500°C will result in a lower retained austenite volume fraction.
  • the coiling temperature is set to 350°C or below.
  • the aforementioned coiling conditions are for steel sheets to be directly provided as hot-rolled steel sheets after coiling, and these restricting conditions are unnecessary for cold-rolled steel sheets which have been further cold rolled and subjected to annealing, as such coiling may be carried out under common production conditions.
  • the hot-rolling is carried out so that when the finishing temperature for hot rolling is in the range of Ar 3 - 50°C to Ar 3 ⁇ 120°C, the metallurgy parameter: A satisfies inequalities (1) and (2).
  • the average cooling rate on the run-out table is 5°C/sec, and the coiling is preferably carried out under conditions such that the relationship between the metallurgy parameter: A and the coiling temperature (CT) satisfies inequality (3).
  • the upper limit for ⁇ T is preferred to be 300°C from the viewpoint of increasing size of facility, lower retained austenite volume fraction and coarseness of the microstructure. Furthermore, if the relationship with the coiling temperature in inequality (3) is not satisfied, there will be an adverse effect on ensuring the amount of retained ⁇ , while the retained ⁇ will be excessively stable even if retained ⁇ can be obtained, and although transformation of the retained ⁇ will proceed during deformation it will not occur to a sufficient degree in the low strain region, and will result in inferior flow stress during dynamic deformation ⁇ dyn and 1 ⁇ 5% work hardening property, etc.
  • the lower limit for the coiling temperature (CT) is more flexible with a higher logA.
  • the CT when the initial martensite volume fraction is greater than 3%, the CT may be higher than 350°C. However, it is preferred from CT to be higher than 250°C in order to prevent overproduction of martensite.
  • the cold-rolled steel sheets according to the invention are then subjected to the different steps following hot-rolling and coiling and are cold-rolled at a reduction ratio of 40% or greater, after which the cold-rolled steel sheets are subjected to annealing.
  • the annealing is ideally continuous annealing through an annealing cycle such as shown in Fig.
  • annealing for 10 seconds to 3 minutes at temperature To of from 0.1 x (Ac 3 - Ac 1 ) + Ac 1 °C to Ac 3 + 50°C is followed by cooling to a primary cooling stop temperature Tq in the range of 550 ⁇ 720°C at a primary cooling rate of 1 ⁇ 10°C/sec and then by cooling to a secondary cooling stop temperature Te at a secondary cooling rate of 10 ⁇ 200°C/sec, after which temperature Toa is held for 15 seconds to 20 minutes prior to cooling to room temperature.
  • the amount of austenite obtained at the annealing temperature will be too low, making it impossible to leave stably retained austenite in the final steel sheets; the lower limit was therefore determined to be 0.1 x (Ac 3 - Ac 1 ) + Ac 1 °C.
  • the upper limit for the annealing temperature was determined to be Ac 3 + 50°C.
  • the required annealing time at this temperature is a minimum of 10 seconds in order to ensure a uniform temperature and an appropriate amount of austenite for the steel sheets, but if the time exceeds 3 minutes the effect described above becomes saturated and costs will thus be increased.
  • the rapid cooling of the subsequent secondary cooling must be carried out at a cooling rate of at least 10°C/sec so as not to cause pearlite transformation or precipitation of iron carbides during the cooling, but cooling carried out at greater than 200°C/sec will create a burden on the equipment. Also, if the cooling stop temperature in the secondary cooling is lower than 150°C, virtually all of the remaining austenite prior to cooling will be transformed into martensite, making it impossible to ensure the final necessary amount of retained austenite. Conversely, if the cooling stop temperature is higher than 450°C the final flow stress during dynamic deformation ⁇ dyn will be lowered.
  • a portion thereof is preferably transformed to bainite to further increase the carbon concentration in the austenite.
  • the secondary cooling stop temperature is lower than the temperature maintained for bainite transformation, it is increased to the maintained temperature.
  • the final characteristics of the steel sheets will not be impaired so long as this heating rate is from 5°C/sec to 50°C/sec.
  • the secondary cooling stop temperature is higher than the bainite processing temperature, the final characteristics of the steel sheets will not be impaired even with forced cooling to the bainite processing temperature at a cooling rate of 5°C/sec to 200°C/sec and with direct conveyance to a heating zone preset to the desired temperature.
  • the range for the holding temperature was determined to be 150°C to 500°C. If the temperature is held at 150°C to 500°C for less than 15 seconds, the bainite transformation does not proceed to a sufficient degree, making it impossible to obtain the final necessary amount of retained austenite, while if it is held in that range for more than 20 minutes, precipitation of iron carbides or pearlite transformation will result after bainite transformation, resulting in waste of the carbon which is indispensable for production of the retained austenite and making it impossible to obtain the necessary amount of retained austenite; the holding time range was therefore determined to be from 15 seconds to 20 minutes.
  • the holding at 150°C to 500°C in order to promote bainite, transformation may be at a constant temperature throughout, or the temperature may be deliberately varied within this temperature range without impairing the characteristics of the final steel sheets.
  • annealing for 10 seconds to 3 minutes at a temperature of from 0.1 x (Ac 3 - Ac 1 ) + Ac 1 °C to Ac 3 ⁇ 50°C is followed by cooling to a secondary cooling start temperature Tq in the range of 550 ⁇ 720°C at the primary cooling rate of 1 ⁇ 10°C/sec and then by cooling to a secondary cooling stop temperature Te in the range from the temperature Tem - 100°C to Tem determined by the steel component and annealing temperature To at the secondary cooling rate of 10-200°C/sec, after which the temperature Toa is held in a range of Te - 50°C to 500°C for 15 seconds to 20 minutes prior to cooling to room temperature.
  • T1 is the temperature calculated from the solid solution element concentration excluding carbon
  • T2 is the temperature calculated from the carbon concentration in the retained austenite at Ac 1 and Ac 3 determined by the components of the steel sheets and Tq determined by the annealing temperature To.
  • Ceq * represents the carbon equivalents in the retained austenite at the annealing temperature To.
  • Te when Te is less than (Tem - 100)°C, almost all of the austenite is transformed into martensite, making it impossible to obtain the necessary amount of retained austenite. If Te is higher than Tem the steel sheets will be softened, making it impossible to achieve the dynamic strength expected from the static strength (TS); the upper limit for Te was therefore determined to be Tem. Also, if Te is higher than 500°C, pearlite or iron carbides are produced resulting in waste of the carbon which is indispensable for production of the retained austenite and making it impossible to obtain the necessary amount of retained austenite.
  • the microstructure of the steel sheets in their final form is a composite microstructure of a mixture of ferrite and/or bainite, either of which is the dominant phase, and a third phase including retained austenite at a volume fraction between 3% and 50%, wherein the average value ⁇ dyn (MPa) of the flow stress in the range of 3 ⁇ 10% of equivalent strain when deformed in a strain rate range of 5 x 10 2 ⁇ 5 x 10 3 (1/s) after pre-deformation of greater than 0% and less than or equal to 10% of equivalent strain, satisfies the inequality: ⁇ dyn ⁇ 0.766 x TS + 250 as expressed in terms of the maximum stress TS (MPa) in the static tensile test as measured in a strain rate range of 5 x 10 -4 ⁇ 5 x 10 -3 (1/s) without deformation, and the average value ⁇ dyn (MPa) of the flow stress in the range of 3 ⁇ 10% of equivalent strain when deformed in a strain rate range of 5
  • the 15 steel materials listed in Table 1 were heated to 1050 ⁇ 1250°C and subjected to hot rolling, cooling and coiling under the production conditions listed in Table 2, to produce hot-rolled steel sheets.
  • the steel sheets satisfying the component conditions and production conditions according to the invention contain from 3% to 50% of initial retained austenite in terms of volume fraction and had an M value of at least 70 and less than or equal to 250 as determined by the solid solution [C] in the retained austenite and the average Mn eq in the steel sheets, while having suitable stability as represented by a ratio more than 0.3 between the (initial retained austenite volume fraction - retained austenite volume fraction after 5% deformation)/initial retained austenite volume fraction, exhibiting excellent anti-collision safety as represented by ⁇ dyn ⁇ 0.766 x TS + 250, 1 ⁇ 5% work hardening coefficient more than 0.080 and 1 ⁇ 5% work hardening coefficient x yield strength more than 40, as well as suitable formability and spot weldability.
  • the 25 steel materials listed in Table 5 were subjected to a complete hot-rolling process at Ar3 or greater, and after cooling they were coiled and then cold-rolled following acid picking.
  • the Ac1 and Ac3 temperatures were then determined from each steel component, and after heating, cooling and holding under the annealing conditions listed in Table 6, they were cooled to room temperature. As shown in Figs.
  • the steel sheets satisfying the production conditions and component conditions according to the invention have an M value of at least 70 and no greater than 250 as determined by the solid solution [C] in the retained austenite and the average Mn eq in the steel sheets, and all clearly exhibit excellent anti-collision safety as represented by ⁇ dyn ⁇ 0.076 x TS + 250 and a 1 ⁇ 5% strain work hardening coefficient value of at least 40.
  • the microstructure was evaluated by the following methods.
  • the mean circle equivalent diameter of the retained austenite was determined from a 1000 magnification optical micrograph, with the rolling direction cross-section etched with the reagent disclosed in Japanese Patent Application No. 3-351209 . The position was also observed from the same photograph.
  • V ⁇ volume fraction of the retained austenite (V ⁇ : percentage unit) was calculated according to the following equation, upon Mo-K ⁇ X-ray analysis.
  • V ⁇ 2 / 3 100 / 0.7 x ⁇ 211 / ⁇ 220 + 1 + 1 / 3 100 / 0.78 x ⁇ 211 / ⁇ 311 + 1 where ⁇ (211), ⁇ (220), ⁇ (211) and ⁇ (311) represent pole intensities.
  • C concentration of the retained ⁇ (C ⁇ : percentage unit) was calculated according to the following equation, upon determining the lattice constant (unit: Angstroms) from the reflection angle on the (200) plane, (220) plane and (311) plane of the austenite using Cu-K ⁇ X-ray analysis.
  • C ⁇ lattice constant - 3.572 / 0.033
  • TS tensile strength
  • YS yield strength
  • T.E1 total elongation
  • work hardening coefficient n value for strain of 1 ⁇ 5%
  • the stretch flanging property was measured by expanding a 20 mm punched hole from the burrless side with a 30° cone punch, and determining the hollow extension ratio (d/do) between the hollow diameter at the moment at which the crack penetrated the sheet thickness and (d) the original hollow diameter (do, 20 mm).
  • the spot weldability was judged to be unsuitable if a spot welding test piece bonded at a current of 0.9 times the expulsion current using an electrode with a tip radius of 5 times the square root of the steel sheet thickness underwent peel fracture when ruptured with a chisel.
  • the present invention makes it possible to provide in an economical and stable manner high-strength hot-rolled steel sheets and cold-rolled steel sheets for automobiles which provide previously unobtainable excellent anti-collision safety and formability, and thus offers a markedly wider range of objects and conditions for uses of high-strength steel sheets.

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Claims (5)

  1. Tôle d'acier laminée à chaud ou laminée à froid à haute résistance avec une contrainte d'écoulement élevée pendant la déformation dynamique, caractérisée en ce que la tôle d'acier laminée à chaud ou laminée à froid contient, en termes de pourcentage en poids, C : de 0,03 % à 0,3 %, l'un ou les deux parmi Si et Al pour un total de l'ordre de 0,5 % à 3,0 % et facultativement un ou plusieurs éléments parmi Mn, Ni, Cr, Cu et Mo pour un total de l'ordre de 0,5 % à 3,5 %, facultativement un ou plusieurs éléments parmi Nb, Ti, V, P et B, avec un ou plusieurs éléments parmi Nb, Ti et V pour un total non supérieur à 0,3 %, P : non supérieur à 0,3 % et B : non supérieur à 0,01 %, en outre facultativement Ca : de l'ordre de 0,0005 % à 0,01 % et REM : de l'ordre de 0,005 % à 0,05 %, dont le reste est représenté par Fe et les impuretés inévitables, la microstructure de la tôle d'acier avant la déformation de la tôle laminée à chaud ou laminée à froid est une microstructure composite composée d'un mélange de ferrite et/ou de bainite, dont l'une ou l'autre est la phase dominante, et une troisième phase comprenant l'austénite résiduelle selon une fraction de volume comprise entre 3 % et 50 %, dans laquelle la valeur moyenne σdyn (MPa) de la contrainte d'écoulement est de l'ordre de 3-10 % de déformation équivalente lorsqu'elle est déformée dans une plage de vitesses de déformation de 5 x 102 - 5 x 103 (1/s) après la pré-déformation supérieure à 0 % et inférieure ou égale à 10 % de déformation équivalente, satisfait l'inégalité : σdyn ≥ 0,766 x TS + 250, comme exprimé en termes de contrainte maximum TS (MPa) dans le test de traction statique, comme mesuré dans une plage de vitesses de déformation de 5 x 10-4 - 5 x 10-3 (1/s) sans déformation, la valeur (M) déterminée par la solution solide [C] dans l'austénite résiduelle et les équivalents en Mn moyens de l'acier {Mn eq = Mn + (Ni + Cr + Cu + Mo) / 2}, définie par l'équation : M = 678 - 428 x [C] - 33 Mn eq est au moins de 70 et non supérieure à 250, la différence entre la fraction de volume d'austénite résiduelle sans pré-déformation et la fraction de volume d'austénite résiduelle après l'application d'une pré-déformation de 5 % de déformation équivalente représente au moins 30 % de la fraction de volume d'austénite résiduelle sans pré-déformation, et le coefficient d'écrouissage entre 1 % et 5 % d'une déformation est d'au moins 0,080, dans lequel le diamètre de grain moyen de ladite austénite résiduelle n'est pas supérieur à 5 µm, le rapport de diamètre de grain moyen de ladite austénite résiduelle et le diamètre de grain moyen de la ferrite ou bainite dans la phase dominante n'est pas supérieur à 0,6, et le diamètre de grain moyen de la phase dominante n'est pas supérieur à 10 µm et de préférence non supérieur à 6 µm.
  2. Tôle d'acier laminée à chaud ou laminée à froid à haute résistance avec une contrainte d'écoulement élevée pendant la déformation dynamique selon la revendication 1, dans laquelle la valeur du coefficient d'écrouissage entre 1 % et 5 % d'une contrainte x résistance à l'élasticité est d'au moins 40.
  3. Tôle d'acier laminée à chaud ou laminée à froid à haute résistance selon la revendication 1 ou 2, dans laquelle la fraction de volume de martensite est de 3-30 % et le diamètre de grain moyen de ladite martensite est non supérieur à 10 µm et de préférence non supérieur à 5 µm.
  4. Tôle d'acier laminée à chaud ou laminée à froid à haute résistance selon l'une quelconque des revendications 1 à 3, dans laquelle la fraction de volume de la ferrite est d'au moins 40 %.
  5. Tôle d'acier laminée à chaud ou laminée à froid à haute résistance selon l'une quelconque des revendications 1 à 4, dans laquelle le rapport d'élasticité n'est pas supérieur à 85 % et la valeur de la résistance à la traction x allongement total est d'au moins 20 000.
EP10181458A 1996-11-28 1997-11-28 Aciers haute résistance ayant d'excellentes propriétés d'absorption d'énergie aux chocs. Expired - Lifetime EP2314730B1 (fr)

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JP33138096A JPH10158735A (ja) 1996-11-28 1996-11-28 耐衝突安全性及び成形性に優れた自動車用熱延高強度薄鋼板とその製造方法
JP2829697 1997-01-29
JP22300597A JPH1161326A (ja) 1997-08-06 1997-08-06 耐衝突安全性及び成形性に優れた自動車用高強度鋼板とその製造方法
JP25888797A JP3530355B2 (ja) 1997-09-24 1997-09-24 高い動的変形抵抗を有する衝突時衝撃吸収用高強度熱延鋼板とその製造方法
JP25883497A JP3530353B2 (ja) 1997-09-24 1997-09-24 高い動的変形抵抗を有する衝突時衝撃吸収用高強度冷延鋼板とその製造方法
EP97913471.5A EP0952235B2 (fr) 1996-11-28 1997-11-28 Procede de fabrication d'aciers à haute resistance mècanique ayant une haute capacite d'absorption d'energie de chock

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EP97913471.5 Division 1998-06-04

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CN1078623C (zh) 2002-01-30
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CA2273334A1 (fr) 1998-06-04
EP0952235B1 (fr) 2011-10-12
EP0952235B2 (fr) 2015-09-30
CN1241219A (zh) 2000-01-12
EP2314730A1 (fr) 2011-04-27
KR20000057266A (ko) 2000-09-15
AU5067998A (en) 1998-06-22
AU711873B2 (en) 1999-10-21
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KR100318213B1 (ko) 2001-12-22
TW384313B (en) 2000-03-11

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