US20110162762A1 - High strength steel sheet and method for manufacturing the same - Google Patents

High strength steel sheet and method for manufacturing the same Download PDF

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US20110162762A1
US20110162762A1 US13/062,574 US200913062574A US2011162762A1 US 20110162762 A1 US20110162762 A1 US 20110162762A1 US 200913062574 A US200913062574 A US 200913062574A US 2011162762 A1 US2011162762 A1 US 2011162762A1
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steel sheet
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strength steel
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Hiroshi Matsuda
Yoshimasa Funakawa
Yasushi Tanaka
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JFE Steel Corp
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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
    • C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
    • 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
    • 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
    • C21D9/48—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals deep-drawing sheets
    • 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
    • C22C38/005—Ferrous alloys, e.g. steel alloys containing rare earths, i.e. Sc, Y, Lanthanides
    • 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
    • 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
    • C21D2211/008—Martensite

Definitions

  • This disclosure relates to a high strength steel sheet used in industrial fields of automobile, electric apparatus, and the like and which has excellent workability, especially elongation and stretch-flangeability, and a tensile strength (TS) of 980 MPa or more, and a method for manufacturing the same.
  • TS tensile strength
  • a hard phase e.g., martensite or bainite
  • the increase in strength of the steel sheet through the increase in proportion of the hard phase causes a reduction in workability. Therefore, development of a steel sheet having high strength and excellent workability in combination has been desired.
  • Various complex microstructure steel sheets e.g., a ferrite-martensite double phase steel (DP steel) and a TRIP steel taking the advantage of the transformation induced plasticity of retained austenite, have been developed.
  • DP steel ferrite-martensite double phase steel
  • TRIP steel taking the advantage of the transformation induced plasticity of retained austenite
  • the workability of the steel sheet is affected by the workability of the hard phase significantly. This is because in the case where the proportion of the hard phase is small and that of soft polygonal ferrite is large, the deformability of polygonal ferrite is predominant over the workability of the steel sheet, and even in the case where the workability of the hard phase is inadequate, the workability, e.g., elongation, is ensured, whereas in the case where the proportion of the hard phase is large, deformability of the hard phase itself rather than deformation of polygonal ferrite exerts an influence directly on formability of the steel sheet. Therefore, if the workability of the hard phase itself is inadequate, deterioration of the workability of the steel sheet becomes significant.
  • a steel sheet in which the hard phase is other than martensite there is a steel sheet in which a primary phase is polygonal ferrite, a hard phase is bainite and pearlite, and carbides are generated in such bainite and pearlite serving as the hard phase.
  • This steel sheet exhibits improved workability not only by polygonal ferrite, but also by generating carbides in the hard phase to improve the workability of the hard phase in itself and, in particular, an improvement of the stretch-flangeability is intended.
  • the primary phase is polygonal ferrite, it is difficult to allow an increase in strength to 980 MPa or more in terms of tensile strength (TS) and the workability to become mutually compatible.
  • Japanese Unexamined Patent Application Publication No. 4-235253 proposes a high strength steel sheet having excellent bendability and impact characteristic, wherein alloy components are specified and the steel microstructure is fine uniform bainite including retained austenite.
  • Japanese Unexamined Patent Application Publication No. 2004-76114 proposes a complex microstructure steel sheet having excellent bake hardenability, wherein predetermined alloy components are specified, the steel microstructure is bainite including retained austenite, and the amount of retained austenite in the bainite is specified.
  • Japanese Unexamined Patent Application Publication No. 11-256273 proposes a complex microstructure steel sheet having excellent impact resistance, wherein predetermined alloy components are specified, the steel microstructure is specified in such a way that bainite including retained austenite is 90% or more in terms of area percentage and the amount of austenite in the bainite is 1% or more, and 15% or less, and the hardness (HV) of the bainite is specified.
  • the steel sheet described in JP '273 is for the purpose of improving the impact resistance and the microstructure contains bainite having a hardness of HV 250 or less as a primary phase, specifically at a content exceeding 90%. Therefore, it is difficult to make the tensile strength (TS) 980 MPa or more.
  • Our high strength steel sheets include a steel sheet in which galvanizing or galvannealing is applied to a surface of the steel sheet.
  • Excellent workability refers to a value of TS ⁇ T.EL satisfying 20,000 MPa ⁇ % or more and a value of TS ⁇ satisfying 25,000 MPa ⁇ % or more.
  • TS represents tensile strength (MPa)
  • T.EL represents total elongation (%)
  • ⁇ represents hole-expansion limit (%).
  • a high strength steel sheet having excellent workability, especially the elongation and the stretch-flangeability, and a tensile strength (TS) of 980 MPa or more, as well as an advantageous method for manufacturing the same can be provided. Therefore, the utility value in the industrial fields of automobiles, electric, and the like is very large, and in particular, the usefulness in weight reduction of an automobile body is significant.
  • FIG. 1 is a diagram showing a temperature pattern of a heat treatment in our manufacturing method.
  • area percentage refers to an area percentage relative to the whole steel sheet microstructure.
  • Lower bainite and martensite are microstructures necessary to increase the strength of the steel sheet. If the area percentage of a total amount of lower bainite and whole martensite is less than 10%, the steel sheet does not satisfy the tensile strength (TS) of 980 MPa or more. On the other hand, if the total amount of lower bainite and whole martensite exceeds 90%, the upper bainite is reduced and, as a result, stable retained austenite, in which C is concentrated, cannot be ensured. Consequently, a problem occurs in that the workability, e.g., elongation, deteriorates. Therefore, the area percentage of the total amount of lower bainite and whole martensite is 10% or more, and 90% or less. A preferable range is 20% or more, and 80% or less. A more preferable range is 30% or more, and 70% or less.
  • the tensile strength becomes 980 MPa or more, but the stretch-flangeability is poor.
  • the as-quenched martensite is very hard, and deformability of the as-quenched martensite in itself is very low. Therefore, workability, especially stretch-flangeability, of the steel sheet deteriorates significantly. Furthermore, since the difference in hardness between the as-quenched martensite and the upper bainite is significantly large, if the amount of as-quenched martensite is large, the interface between the as-quenched martensite and upper bainite increases.
  • the proportion of as-quenched martensite in the martensite is 75% or less relative to the total amount of lower bainite and whole martensite present in the steel sheet.
  • the proportion is 50% or less.
  • the as-quenched martensite is a microstructure in which no carbide is detected in the martensite and can be observed with SEM. Amount of retained austenite: 5% or more, and 50% or less
  • the retained austenite undergoes martensitic transformation through a TRIP effect during working and, thereby, strain dispersive power is enhanced to improve elongation.
  • Retained austenite in which the amount of concentrated C is increased, is formed in the upper bainite through the use of upper bainite transformation.
  • retained austenite capable of making the TRIP effect apparent even in a high strain region during working can be obtained.
  • TS tensile strength
  • the value of TS ⁇ T.El can be 20,000 MPa ⁇ % or more, and a steel sheet having an excellent balance between strength and elongation can be obtained.
  • the retained austenite in the upper bainite is formed between laths of bainitic ferrite in the upper bainite and distributes finely, large amounts of measurement at high magnification is necessary for determination of the amount (area percentage) thereof through microstructure observation, and it is difficult to quantify accurately.
  • the amount of retained austenite formed between laths of the bainitic ferrite is an amount corresponding to the amount of formed bainitic ferrite to some extent.
  • the amount of retained austenite is within the range of 5% or more, and 50% or less.
  • the range is preferably more than 5%, and more preferably within the range of 10% or more, and 45% or less.
  • the range is further preferably within the range of 15% or more, and 40% or less.
  • the amount of C in the retained austenite is important. C is concentrated into the retained austenite formed between laths of bainitic ferrite in the upper bainite. It is difficult to accurately evaluate the amount of C concentrated into the retained austenite between the laths.
  • the average amount of C in the retained austenite is 0.70% or more.
  • the amount is preferably 0.90% or more.
  • the average amount of C in the retained austenite exceeds 2.00%, the retained austenite becomes excessively stable, martensitic transformation does not occur during working, and the TRIP effect is not apparent so that elongation deteriorates. Therefore, it is preferable that the average amount of C in the retained austenite is 2.00% or less. More preferably, the average amount is 1.50% or less.
  • Upper bainite is characterized in that lath-shaped bainitic ferrite and retained austenite and/or carbides present between bainitic ferrite are included and fine carbides regularly arranged in the lath-shaped bainitic ferrite are not present.
  • lower bainite is characterized in that lath-shaped bainitic ferrite and retained austenite and/or carbides present between bainitic ferrite are included, as is common to upper bainite, and in the lower bainite, fine carbides regularly arranged in the lath-shaped bainitic ferrite are present.
  • the upper bainite and the lower bainite are distinguished on the basis of presence or absence of fine carbides regularly arranged in the bainitic ferrite.
  • the above-described difference in the generation state of carbides in the bainitic ferrite exerts a significant influence on concentration of C into the retained austenite. That is, in the case where the area percentage of bainitic ferrite in the upper bainite is less than 5%, even when bainite transformation proceeds, the amount of C formed into carbides in the bainitic ferrite increases. As a result, the amount of concentration of C into the retained austenite present between laths decreases, and a problem occurs in that the amount of retained austenite, which exerts the TRIP effect in a high strain region during working, decreases.
  • the area percentage of bainitic ferrite in the upper bainite is 5% or more in terms of area percentage relative to the whole steel sheet microstructure.
  • the area percentage of bainitic ferrite in the upper bainite exceeds 85% relative to the whole steel sheet microstructure, it may become difficult to ensure the strength. Consequently, it is preferable that the area percentage is 85% or less.
  • the area percentage of polygonal ferrite exceeds 10%, it becomes difficult to satisfy the tensile strength (TS): 980 MPa or more and, at the same time, strain is concentrated on soft polygonal ferrite present together in the hard microstructure during working so that cracking occurs easily during working. As a result, the desired workability is not obtained. If the area percentage of the polygonal ferrite is 10% or less, even when the polygonal ferrite is present, a state in which a small amount of polygonal ferrite is discretely dispersed in a hard phase is brought about, concentration of strain can be suppressed, and deterioration of workability can be avoided. Therefore, the area percentage of the polygonal ferrite is 10% or less. The area percentage is preferably 5% or less, further preferably 3% or less, and may be 0%.
  • the hardness of the hardest microstructure in the steel sheet microstructure is HV ⁇ 800. That is, in the case where as-quenched martensite is not present in the steel sheet, any one of tempered martensite, lower bainite, and upper bainite becomes the hardest phase. All of these microstructures are phases which become HV ⁇ 800. Alternatively, in the case where as-quenched martensite is present, the as-quenched martensite becomes the hardest microstructure. Regarding the as-quenched martensite, the hardness becomes HV ⁇ 800, a significantly hard martensite exhibiting HV >800 is not present, and good stretch-flangeability can be ensured.
  • the steel sheet may include pearlite, Widmanstaetten ferrite, and lower bainite as the remainder microstructure.
  • the allowable content of the remainder microstructure is 20% or less in terms of area percentage. More preferably, the allowable content is 10% or less.
  • the basic configuration of the steel sheet microstructure of the high strength steel sheet is as described above, and the following configuration may be added as necessary.
  • % hereafter representing the following component composition refers to percent by mass.
  • the element C is an indispensable element to increase the strength of the steel sheet and ensure the amount of stable retained austenite, and an element necessary to ensure the amount of martensite and retain austenite at room temperature. If the amount of C is less than 0.17%, it is difficult to ensure the strength and workability of the steel sheet. On the other hand, if the amount of C exceeds 0.73%, hardening of welded and heat-affected zones is significant so that weldability deteriorates. Therefore, the amount of C is within the range of 0.17% or more, and 0.73% or less. The range is preferably more than 0.20% and 0.48% or less, and further preferably 0.25% or more.
  • the element Si contributes to an improvement in the strength of steel by strengthening through solid solution.
  • the amount of Si exceeds 3.0%, an increase in the amount of solid solution into the polygonal ferrite and the bainitic ferrite causes deterioration of workability and tenacity, and causes deterioration of surface characteristics due to occurrence of red scale and the like and deterioration of wettability and adhesion of the coating in the case where hot dipping is applied. Therefore, the amount of Si is 3.0% or less.
  • the amount is preferably 2.6% or less.
  • the amount is further preferably 2.2% or less.
  • Si is an element useful for suppressing generation of carbides and facilitating generation of retained austenite. Therefore, it is preferable that the amount of Si is 0.5% or more. However, in the case where generation of carbides is suppressed by merely Al, Si is not necessarily added, and amount of Si may be 0%.
  • Mn 0.5% or More, and 3.0% or Less
  • the element Mn is useful for strengthening steel. If the amount of Mn is less than 0.5%, carbides are deposited in a temperature range higher than the temperature, at which bainite and martensite are generated, during cooling after annealing. Consequently, it is not possible to ensure the amount of hard phase, which contributes to strengthening of steel. On the other hand, the amount of Mn exceeding 3.0% causes deterioration of castability and the like. Therefore, the amount of Mn is 0.5% or more and 3.0% or less. The range is preferably 1.5% or more and 2.5% or less.
  • the element P is useful for strengthening steel. If the amount of P exceeds 0.1%, the impact resistance deteriorates due to embrittlement based on grain boundary segregation, and in the case where galvannealing is applied to a steel sheet, the alloying rate is reduced significantly. Therefore, the amount of P is 0.1% or less.
  • the amount is preferably 0.05% or less. In this connection, it is preferable that the amount of P is reduced. However, reduction to less than 0.005% causes a significant increase in cost. Therefore, it is preferable that the lower limit thereof is about 0.005%.
  • the element S generates MnS to become an inclusion and causes deterioration of the impact resistance and cracking along a metal flow of welded zones. Therefore, it is preferable that the amount of S is minimized.
  • the amount of S is 0.07% or less.
  • the amount is 0.05% or less, and more preferably 0.01% or less. In this connection, reduction of S to less than 0.0005% is attended with a significant increase in production cost. Therefore, the lower limit thereof is about 0.0005% from the viewpoint of the production cost.
  • the element Al is useful for strengthening steel and, in addition, is a useful element which is added as a deoxidizing agent in a steel making process. If the amount of Al exceeds 3.0%, inclusion in a steel sheet increases and elongation deteriorates. Therefore, the amount of Al is 3.0% or less. The amount is preferably 2.0% or less.
  • Al is an element useful for suppressing generation of carbides and facilitating generation of retained austenite. Furthermore, it is preferable that the amount of Al is 0.001% or more to obtain a deoxidation effect, and more preferably 0.005% or more. In this regard, the amount of Al is the amount of Al contained in the steel sheet after deoxidation.
  • the element N causes maximum deterioration of the aging resistance of steel and is preferably minimized. If the amount of N exceeds 0.010%, deterioration of the aging resistance becomes significant and, therefore, the amount of N is 0.010% or less. In this connection, reduction of N to less than 0.001% causes a significant increase in production cost so that the lower limit thereof is about 0.001% from the viewpoint of production cost.
  • both Si and Al are elements useful for suppressing generation of carbides and facilitating generation of retained austenite.
  • an effect is exerted by containing Si or Al alone, but it is necessary to satisfy that a total of the amount of Si and Al is 0.7% or more.
  • the amount of Al in the above-described formula is the amount of Al contained in the steel sheet after deoxidation.
  • the elements Cr, V, and Mo function to suppress generation of pearlite during cooling from an annealing temperature.
  • the effect thereof is obtained at Cr: 0.05% or more, V: 0.005% or more, and Mo: 0.005% or more.
  • Cr: 5.0%, V: 1.0%, and Mo: 0.5% are exceeded, the amount of hard martensite becomes too large, and the strength becomes high more than necessary. Therefore, in the case where Cr, V, and Mo are contained, the ranges are Cr: 0.05% or more and 5.0% or less, V: 0.005% or more and 1.0% or less, and Mo: 0.005% or more and 0.5% or less.
  • the elements Ti and Nb are useful for strengthening steel through deposition, and the effect thereof is obtained when the individual contents are 0.01% or more. On the other hand, if the individual contents exceed 0.1%, workability and shape fixability deteriorate. Therefore, in the case where Ti and Nb are contained, the ranges are Ti: 0.01% or more and 0.1% or less and Nb: 0.01% or more and 0.1% or less.
  • the element B is useful for suppressing generation•growth of ferrite from austenite grain boundaries. The effect thereof is obtained when the content is 0.0003% or more. On the other hand, if the content exceeds 0.0050%, workability deteriorates. Therefore, in the case where B is contained, the range is B: 0.0003% or more and 0.0050% or less.
  • Ni and Cu are useful for strengthening steel. Furthermore, in the case where galvanizing or galvannealing is applied to a steel sheet, internal oxidation of a steel sheet surface layer portion is facilitated and, thereby, adhesion of the coating is improved. These effects are obtained when individual contents are 0.05% or more. On the other hand, if the individual contents exceed 2.0%, the workability of the steel sheet deteriorates. Therefore, in the case where Ni and Cu are contained, the ranges are Ni: 0.05% or more and 2.0% or less and Cu: 0.05% or more and 2.0% or less.
  • the elements Ca and REM are useful for spheroidizing the shape of sulfides and improve the adverse effect of sulfides on stretch-flangeability.
  • the effects thereof are obtained when individual contents are 0.001% or more.
  • the individual contents exceed 0.005%, increases of inclusion and the like are invited to cause surface defects, internal defects, and the like. Therefore, in the case where Ca and REM are contained, the ranges are Ca: 0.001% or more and 0.005% or less and REM: 0.001% or more and 0.005% or less.
  • components other than those described above are Fe and incidental impurities. However, components other than those described above may be contained within the bounds of not impairing the effects of our steel sheets.
  • the hot rolling is terminated in a temperature range of 870° C. or higher and 950° C. or lower.
  • the resulting hot-rolled steel sheet is taken up in a temperature range of 350° C. or higher and 720° C. or lower.
  • the hot-rolled steel sheet is pickled and, thereafter, cold-rolling is conducted at a reduction ratio within the range of 40% or more and 90% or less to produce a cold-rolled steel sheet.
  • the steel sheet is produced through usual individual steps of steel making, casting, hot rolling, pickling, and cold rolling.
  • production may be conducted through thin slab casting or strip casting while a part of or an entire hot rolling step is omitted.
  • FIG. 1 A heat treatment shown in FIG. 1 is applied to the resulting cold-rolled steel sheet. The explanation will be conducted below with reference to FIG. 1 .
  • Annealing is conducted for 15 seconds or more and 600 seconds or less in an austenite single phase region.
  • the steel sheet contains upper bainite, lower bainite, and martensite, which are transformed from untransformed austenite in a relatively low temperature range of 350° C. or higher and 490° C. or lower, as primary phases. Therefore, it is preferable that polygonal ferrite is minimized and annealing in an austenite single phase region is required.
  • the annealing temperature is not specifically limited insofar as it is in the austenite single phase region. If the annealing temperature exceeds 1,000° C., growth of austenite grains is significant, coarser configuration phases are generated by downstream cooling, and tenacity and the like deteriorate.
  • the annealing temperature is lower than A 3 point (austenite transformation point)
  • a 3 point austenite transformation point
  • polygonal ferrite has already been generated in an annealing stage, and it becomes necessary that a temperature range of 500° C. or more is cooled very rapidly to suppress growth of polygonal ferrite during cooling. Therefore, it is necessary that the annealing temperature is the A 3 point or higher and, preferably, 1,000° C. or lower.
  • the annealing time is less than 15 seconds, in some cases, reverse transformation to austenite does not proceed adequately or carbides in the steel sheet are not dissolved adequately.
  • the annealing time exceeds 600 seconds, an increase in cost is invited along with high energy consumption. Therefore, the annealing time is within the range of 15 seconds or more, and 600 seconds or less. Preferably, the annealing time is within the range of 60 seconds or more, and 500 seconds or less.
  • the A 3 point can be calculated on the basis of
  • a 3 point (° C.) 910 ⁇ 203 ⁇ [C %]1 ⁇ 2+44.7 ⁇ [Si %] ⁇ 30 ⁇ [Mn %]+700 ⁇ [P %]+130 ⁇ [Al %] ⁇ 15.2 ⁇ [Ni %] ⁇ 11 ⁇ [Cr %] ⁇ 20 ⁇ [Cu %]+31.5 ⁇ [Mo %]+104 ⁇ [V %]+400 ⁇ [Ti %].
  • [X %] represents percent by mass of component element X of the steel sheet.
  • the cold-rolled steel sheet after annealing is cooled to a cooling termination temperature: T° C. determined in a first temperature range of 350° C. or higher and 490° C. or lower, wherein cooling to at least 550° C. is conducted while the average cooling rate is controlled at 5° C./s or more.
  • the average cooling rate from the annealing temperature to the first temperature range is 5° C./s or more.
  • the average cooling rate is 10° C./s or more.
  • the upper limit of the average cooling rate is not specifically limited insofar as variations do not occur in the cooling termination temperature. If the average cooling rate exceeds 100° C./s, variations in microstructure in a longitudinal direction and a sheet width direction of a steel sheet becomes large significantly. Therefore, 100° C./s or less is preferable.
  • the steel sheet cooled to 550° C. is cooled succeedingly to the cooling termination temperature: T° C.
  • the rate of cooling of the steel sheet in the temperature range of T° C. or higher and 550° C. or lower is not specifically limited except that a maintenance time in the first maintenance temperature range is 15 seconds or more and 1,000 seconds or less.
  • a maintenance time in the first maintenance temperature range is 15 seconds or more and 1,000 seconds or less.
  • carbides are generated from untransformed austenite and, thereby, there is a high probability that a desired microstructure is not obtained. Therefore, it is preferable that the steel sheet is cooled at an average rate of 1° C./s or more in a temperature range of T° C. or higher and 550° C. or lower.
  • the steel sheet cooled to the cooling termination temperature: T° C. is kept in the first temperature range of 350° C. or higher and 490° C. or lower for a period of 15 seconds or more, and 1,000 seconds or less. If the upper limit of the first temperature range exceeds 490° C., carbides are deposited from the untransformed austenite and, thereby, a desired microstructure is not obtained. On the other hand, in the case where the lower limit of the first temperature range is lower than 350° C., a problem occurs in that lower bainite is generated rather than upper bainite and the amount of C concentrated into austenite is reduced. Therefore, the first temperature range is 350° C. or higher and 490° C. or lower. Preferably, the range is 370° C. or higher and 460° C. or lower.
  • the maintenance time in the first temperature range is less than 15 seconds, a problem occurs in that the amount of upper bainite transformation is reduced and the amount of C concentrated into untransformed austenite is reduced.
  • the maintenance time in the first temperature range exceeds 1,000 seconds, carbides are deposited from untransformed austenite which serves as retained austenite in the final microstructure of the steel sheet, stable retained austenite, into which C has been concentrated, is not obtained and, as a result, a desired workability is not obtained. Therefore, the maintenance time is 15 seconds or more and 1,000 seconds or less. Preferably, the range is 30 seconds or more and 600 seconds or less.
  • the resulting steel sheet is cooled to a second temperature range of 200° C. or higher and 350° C. or lower at any rate and is kept in the second temperature range for a period of 15 seconds or more and 1,000 seconds or less.
  • the upper limit of the second temperature range exceeds 350° C., a problem occurs in that lower bainite transformation does not proceed and, as a result, the amount of as-quenched martensite increases.
  • the lower limit of the second temperature range is lower than 200° C. as well, a problem occurs in that lower bainite transformation does not proceed and the amount of as-quenched martensite increases. Therefore, the second temperature range is 200° C. or higher and 350° C. or lower. Preferably, the range is 250° C. or higher and 340° C. or lower.
  • the maintenance time is 15 seconds or more and 1,000 seconds or less.
  • the range is 30 seconds or more and 600 seconds or less.
  • the maintenance temperature is not necessarily a constant insofar as the maintenance temperature is within the above-described predetermined temperature range, and fluctuation within a predetermined temperature range does not impair the steel sheets.
  • the steel sheet may be heat-treated with any facility insofar as only the thermal history is satisfied.
  • temper rolling may be applied to the surface of the steel sheet or a surface treatment, e.g., electroplating, may be applied after the heat treatment to correct the shape.
  • the method for manufacturing a high strength steel sheet can further include a galvanizing treatment or a galvannealing treatment in which an alloying treatment is further added to the galvanizing treatment.
  • the galvanizing treatment or, furthermore, the galvannealing treatment may be conducted during the above-described cooling to the first temperature range or in the first temperature range.
  • the maintenance time in the first temperature range is 15 seconds or more and 1,000 seconds or less, in which a treatment time of the galvanizing treatment or the galvannealing treatment in the first temperature range is included.
  • the galvanizing treatment or the galvannealing treatment is conducted with a continuous galvanizing and galvannealing line.
  • the method for manufacturing a high strength steel sheet can include that the high strength steel sheet is produced following the above-described manufacturing method where steps up to the heat treatment have been completed and, thereafter, the galvanizing treatment or, furthermore, the galvannealing treatment is conducted.
  • the galvanizing treatment or the galvannealing treatment can be conducted succeedingly.
  • a method for applying a galvanizing treatment or a galvannealing treatment to a steel sheet is as described below.
  • the steel sheet is immersed into a plating bath, and the amount of adhesion is adjusted through gas wiping or the like. It is preferable that the amount of Al dissolved in the plating bath is 0.12% or more and 0.22% or less in the case of the galvanizing treatment and 0.08% or more and 0.18% or less in the case of the galvannealing treatment.
  • the temperature of the plating bath may be 450° C. or higher and 500° C. or lower and, furthermore, in the case where the galvannealing treatment is applied, it is preferable that the temperature during alloying is 550° C. or lower. In the case where the alloying temperature exceeds 550° C., carbides are deposited from untransformed austenite and in some cases, pearlite is generated. Consequently, the strength or the workability, or the two are not obtained. In addition, the powdering property of the coating layer deteriorates. On the other hand, if the temperature during alloying is lower than 450° C., in some cases, alloying does not proceed. Therefore, it is preferable that the alloying temperature is 450° C. or higher.
  • the coating mass is 20 g/m 2 or more and 150 g/m 2 or less per surface. If the coating mass is less than 20 g/m 2 , the corrosion resistance becomes inadequate. On the other hand, even when 150 g/m 2 is exceeded, the corrosion-resisting effect is saturated and an increase in the cost is likely.
  • the degree of alloying of the coating layer (Fe percent by mass (Fe content)) is 7 percent by mass or more and 15 percent by mass or less. If the degree of alloying of the coating layer is less than 7 percent by mass, alloying variations occur so that the quality of outward appearance deteriorates, or a so-called a ⁇ phase is generated in the coating layer so that the sliding property of the steel sheet deteriorates. On the other hand, if the degree of alloying of the coating layer exceeds 15 percent by mass, large amounts of hard brittle ⁇ phase is formed so that the adhesion of the coating deteriorates.
  • T in Table 2 refers to a temperature at which cooling of a steel sheet is terminated in cooling of the steel sheet from the annealing temperature.
  • a part of cold-rolled steel sheets were subjected to a galvanizing treatment or a galvannealing treatment.
  • the galvanizing treatment plating was conducted on both surfaces at a plating bath temperature: 463° C. in such a way that a mass per unit area (per surface): 50 g/m 2 was ensured.
  • the galvannealing treatment plating was conducted on both surfaces while the alloying condition was adjusted in such a way that a mass per unit area (per surface): 50 g/m 2 was ensured and the degree of alloying (Fe percent by mass (Fe content)) became 9 percent by mass.
  • the galvanizing treatment and the galvannealing treatment were conducted after cooling was once conducted to T° C. shown in Table 2.
  • the resulting steel sheet was subjected to temper rolling at a reduction ratio (elongation percentage): 0.3 after a heat treatment in the case where a plating treatment is not conducted, or after a galvanizing treatment or a galvannealing treatment in the case where these treatments were conducted.
  • a sample was cut from each steel sheet and was polished. Microstructures of ten fields of view of a surface parallel to the rolling direction were observed with a scanning electron microscope (SEM) at 3,000-fold magnification, the area percentage of each phase was measured, and a phase structure of each crystal grain was identified.
  • SEM scanning electron microscope
  • the steel sheet was ground•polished up to one-quarter of a sheet thickness in the sheet thickness direction and the amount of retained austenite was determined by X-ray diffractometry.
  • Co—K ⁇ was used and the amount of retained austenite were calculated from the average value of the intensity ratio of each of (200), (220), and (311) faces of austenite to the diffraction intensity of each of (200), (211), and (220) faces of ferrite.
  • a0 represents a lattice constant (nm)
  • [X %] represents percent by mass of an element X.
  • the percent by mass of an element other than C was percent by mass relative to whole steel sheet.
  • the tensile test was conducted based on JIS Z2241 by using a test piece of JIS No. 5 size taken in a direction perpendicular to the rolling direction of the steel sheet.
  • the TS tensile strength
  • T.E total elongation
  • TS ⁇ T.El total elongation
  • the stretch-flangeability was evaluated on the basis of the Japan Iron and Steel Federation Standard JFST 1001.
  • Each of the resulting steel sheets was cut into 100 mm ⁇ 100 mm, a hole having a diameter: 10 mm was punched with a clearance of 12% of sheet thickness. Thereafter, a dice having an inside diameter: 75 mm was used, a 60° circular cone punch was pushed into the hole while holding was conducted with a holddown force: 88.2 kN, a hole diameter at crack occurrence limit was measured, and a hole-expansion limit ⁇ (%) was determined from the formula (1):
  • Df represents a hole diameter (mm) at occurrence of crack and D0 represents an initial hole diameter (mm).
  • Stretch-flangeability was evaluated as “good” in the case where TS ⁇ 25,000 MPa ⁇ %.
  • the hardness of the hardest microstructure in the steel sheet micro-structure was determined by a method described below. That is, as a result of microstructure observation, in the case where as-quenched martensite was observed, 10 points of the as-quenched martensite were measured with an ultramicro-Vickers at a load: 0.02 N, and an average value thereof was assumed to be the hardness of the hardest microstructure in the steel sheet microstructure. In this connection, in the case where as-quenched martensite is not observed, as described above, the microstructure of any one of the tempered martensite, the upper bainite, and the lower bainite becomes the hardest phase in our steel sheets. In the case of our steel sheets, the hardest phase was a phase showing HV ⁇ 800.
  • a desired steel sheet microstructure was not obtained. Although the tensile strength (TS) ⁇ 980 MPa was satisfied, any one of TS ⁇ T.El ⁇ 20,000 MPa ⁇ % and TS ⁇ 25,000 MPa ⁇ % was not satisfied. Regarding Sample Nos. 30 to 34, the component compositions were out of the appropriate range. Therefore, a desired steel sheet microstructure was not obtained, and at least one of the tensile strength (TS) ⁇ 980 MPa, TS ⁇ T.El ⁇ 20,000 MPa ⁇ %, and TS ⁇ 25,000 MPa ⁇ % was not satisfied.

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CA2734978A1 (en) 2010-03-18
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CA2734978C (en) 2016-03-29
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TWI412609B (zh) 2013-10-21
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