CN117305683A - Cold-rolled steel plate with grade of 1300MPa or more and manufacturing method thereof - Google Patents

Cold-rolled steel plate with grade of 1300MPa or more and manufacturing method thereof Download PDF

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Publication number
CN117305683A
CN117305683A CN202210711325.5A CN202210711325A CN117305683A CN 117305683 A CN117305683 A CN 117305683A CN 202210711325 A CN202210711325 A CN 202210711325A CN 117305683 A CN117305683 A CN 117305683A
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cold
rolled steel
1300mpa
less
percent
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薛鹏
朱晓东
李伟
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Baoshan Iron and Steel Co Ltd
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Baoshan Iron and Steel Co Ltd
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Priority to CN202210711325.5A priority Critical patent/CN117305683A/en
Priority to PCT/CN2023/101256 priority patent/WO2023246737A1/en
Publication of CN117305683A publication Critical patent/CN117305683A/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
    • C21D1/26—Methods of annealing
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D6/00—Heat treatment of ferrous alloys
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • 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
    • C21D8/0205—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips of ferrous alloys
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • 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
    • 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
    • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
    • C21D8/0242—Flattening; Dressing; Flexing
    • 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
    • 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
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • 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

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  • Chemical & Material Sciences (AREA)
  • Engineering & Computer Science (AREA)
  • Materials Engineering (AREA)
  • Mechanical Engineering (AREA)
  • Metallurgy (AREA)
  • Organic Chemistry (AREA)
  • Physics & Mathematics (AREA)
  • Thermal Sciences (AREA)
  • Crystallography & Structural Chemistry (AREA)
  • Heat Treatment Of Sheet Steel (AREA)

Abstract

The invention discloses a 1300MPa or more grade cold-rolled steel plate, which contains Fe and unavoidable impurity elements, and also contains the following chemical elements in percentage by mass: c:0.10 to 0.30 percent, si:0.1 to 0.5 percent of Mn:0.8 to 2.5 percent of Al:0.01% -0.03%, B:0.001-0.003%; ti:0 to 0.05%, and the mass percentages of C and Mn satisfy: C+Mn/6 is more than or equal to 0.35 percent. In addition, the invention also discloses a manufacturing method of the cold-rolled steel plate with the grade of more than 1300MPa, which comprises the following steps: (1) smelting and casting; (2) hot rolling; (3) cold rolling; (4) annealing; (5) continuous tempering temperature: tempering at 400-550 deg.c for 10-300s, and cooling to room temperature at 30 deg.c/s; (6) flattening; (7) discontinuous tempering: the tempering temperature is 180-260 ℃ and the tempering time is 0.5-6h.

Description

Cold-rolled steel plate with grade of 1300MPa or more and manufacturing method thereof
Technical Field
The present invention relates to a steel material and a method for manufacturing the same, and more particularly, to a cold-rolled steel sheet and a method for manufacturing the same.
Background
In recent years, with the increase in global energy crisis and environmental problems, "energy conservation" and "safety" have become the main development directions of the automobile manufacturing industry. The light weight design is adopted to reduce the weight of the automobile, so that the automobile is one of important measures for energy conservation and emission reduction.
In recent years, the ultra-high strength steel is very common in the automobile industry, has good mechanical properties and usability, can be used for manufacturing automobile structural parts, and realizes the light weight of the parts, thereby effectively reducing the weight of the automobile. In the current automotive industry, in actually manufacturing vehicles, it is required to use a higher strength steel sheet for weight reduction and safety. The ultra-high strength steel with the tensile strength of 1000Mpa or more has great weight-reducing potential in the aspects of weight reduction and safety performance, can be used for manufacturing safety parts, reinforcing parts and structural parts, and has good popularization prospect.
However, the high-strength steel with the tensile strength of more than 1000MPa has the characteristic of stress corrosion cracking (delayed cracking), the high-strength steel plate is easy to slowly crack under the action of stress and corrosive media, and the delayed cracking causes considerable trouble to the application of the high-strength steel, and greatly limits the application of the ultra-high-strength steel.
The delayed cracking refers to the problem that the part does not crack when being manufactured, but is subjected to stress corrosion cracking under the dual action of stress and corrosive medium along with the time, so that the part is finally invalid and the safety protection effect is lost, and hydrogen plays a role in promoting crack initiation and expansion in the process. In general, in the currently existing high-strength steel, the higher the strength of the steel, the more the tendency to delay cracking, which is the greatest risk for advanced high-strength steel applications.
In the prior art, although some researchers develop ultra-high strength steel materials, the problems of delayed cracking existing in ultra-high strength steel are not well solved by the technical schemes.
For example: chinese patent document with publication No. CN102822375a, publication No. 2012, 12 months and 12 days, entitled "ultra-high strength cold-rolled steel sheet and method for producing same", discloses an ultra-high strength cold-rolled steel sheet and method for producing same, C:0.05-0.4%, si is less than or equal to 2.0%, mn:1.0-3.0%, P is less than or equal to 0.05%, S is less than or equal to 0.02%, al:0.01-0.05%, N is less than or equal to 0.05%. In the continuous annealing of the cold-rolled steel sheet, it is necessary to cool the steel sheet from Ac3 to Ms point-Ms point +200deg.C at a cooling rate of 20 ℃/s or more (gas cooling), hold the steel sheet for 0.1 to 60 seconds, cool the steel sheet at a cooling rate of 100 ℃/s or more (water cooling), and cool the steel sheet to 100 ℃ or less to obtain a high-strength steel having a tensile strength of 1320MPa or more and a flatness of 10mm or less.
Also for example: the Chinese patent literature with publication number of CN102776438A and publication date of 2012 of 11 months of 14 days is entitled "a niobium-lanthanum microalloyed Mn-B series ultra-high strength steel plate and heat treatment process thereof", discloses a niobium-lanthanum microalloyed Mn-B series ultra-high strength steel plate and heat treatment process thereof, wherein the steel plate comprises the following chemical components in percentage by weight: 0.14 to 0.35 percent of C, 1.5 to 2.0 percent of Mn, 0.6 to 1.0 percent of Si, less than or equal to 0.015 percent of P, less than or equal to 0.002 percent of S, 0.01 to 0.06 percent of Nb, 0.0005 to 0.0040 percent of B, 0.001 to 0.5 percent of La, and the balance of Fe and unavoidable impurities. In the technical scheme, the adopted heat treatment process system is as follows: austenitizing at 880-940 deg.c for 0.5-5 hr and water quenching; tempering temperature is 190-250 ℃, and heat preservation time is 1-15 hours. In the technical scheme of the patent, the designed steel plate has excellent mechanical properties, the tensile strength reaches 1200-1400MPa, the yield strength is 1000-1300MPa, the elongation is 6-15%, and the steel plate has the characteristics of low production cost and capability of industrially producing steel plates with thickness of 5-25 mm.
For another example: the Chinese patent document with publication number of CN102321841A and publication date of 2012 of 1 month and 18 days, named as "steel for track shoes with tensile strength reaching 1300MPa and manufacturing method thereof", discloses steel for track shoes with tensile strength reaching 1300MPa and manufacturing method thereof, wherein the steel comprises the following chemical components in percentage by weight: 0.20 to 0.30 percent of Mn:0.80 to 1.40 percent of Si:0.15 to 0.35 percent, P:0 to 0.015 percent, S:0 to 0.016 percent, cr:0 to 0.30 percent of Ni:0 to 0.25 percent, cu:0 to 0.30 percent of Ti:0.01 to 0.02 percent of Al:0.02 to 0.06 percent, B:0.0005 to 0.0035 percent, and the balance of Fe and unavoidable impurity elements. The steel designed by the technical scheme has the tensile strength of more than 1340MPa, the elongation after breaking of less than 12 percent, the U-shaped notch impact absorption power of more than 72J, high strength, few quenching cracks and internal cracks and long service life.
In all three of the above patent documents, although the steel material obtained has an ultra-high strength and good mechanical properties. However, none of the three technical schemes relates to improvement of the delayed cracking resistance of the ultra-high strength steel.
Disclosure of Invention
The invention aims to provide a novel cold-rolled steel sheet with the grade of more than 1300MPa, wherein the cold-rolled steel sheet with the grade of more than 1300MPa adopts reasonable chemical composition design and manufacturing process, has ultrahigh strength and simultaneously has excellent delayed cracking resistance and bending resistance. The cold-rolled steel sheet can be soaked in hydrochloric acid with the concentration of 1mol/L for more than 300 hours without delayed cracking under the condition that the preset stress is more than or equal to 1.05 times of tensile strength, is particularly suitable for manufacturing automobile safety structural members, and has good popularization and application prospects.
In order to achieve the above object, the present invention provides a 1300MPa or more grade cold rolled steel sheet, which contains Fe and unavoidable impurity elements, and further contains the following chemical elements in mass percent:
C：0.10％～0.30％，Si：0.1％～0.5％，Mn：0.8％～2.5％，Al：0.01％～0.03％，B：0.001-0.003％；Ti：0～0.05％；
and the mass percentages of C and Mn are as follows: C+Mn/6 is more than or equal to 0.35 percent.
Further, in the cold-rolled steel sheet with the grade of more than 1300MPa, the mass percentage of each chemical element is as follows:
c:0.10 to 0.30 percent, si:0.1 to 0.5 percent of Mn:0.8 to 2.5 percent of Al:0.01% -0.03%, B:0.001-0.003%; ti:0 to 0.05 percent, and the balance of Fe and other unavoidable impurities;
and the mass percentages of C and Mn are as follows: C+Mn/6 is more than or equal to 0.35 percent.
In the cold-rolled steel sheet with the grade of more than 1300MPa, the design principle of each chemical element is specifically as follows:
c: in the cold-rolled steel sheet with the grade of 1300MPa or more, the addition of the element C not only can improve the strength of steel, but also can improve the hardness of martensite so as to ensure that the martensitic transformation occurs. The inventors have found that when the mass percentage of the C element in the steel is less than 0.1%, the strength of the steel sheet is affected and the formation amount and stability of austenite are adversely affected; when the content of C element in steel is more than 0.30% by mass, the martensite hardness is easily excessively high, the grain size is coarse, and the formability of the steel sheet is not facilitated. Therefore, in the cold-rolled steel sheet with the pressure of more than 1300MPa, the mass percent of the C element is controlled to be between 0.10 and 0.30 percent in consideration of the influence of the C element content on the steel performance.
Si: in the cold-rolled steel sheet with the grade of more than 1300MPa, the hardenability of the steel can be improved by adding Si element. In addition, si dissolved in the steel can influence the interaction of dislocation, increase the work hardening rate, properly improve the elongation and be beneficial to the steel to obtain better formability. Based on this, in order to exert the beneficial effect of the Si element, in the 1300MPa or more grade cold-rolled steel sheet of the present invention, the mass percentage content of the Si element is controlled to be between 0.1% and 0.5%.
Mn: in the cold-rolled steel sheet with the grade of more than 1300MPa, mn element is added, so that the hardenability of the steel can be improved, and the strength of the steel sheet can be effectively improved. The mass percentage of Mn in the selected steel is between 0.8 and 2.5 percent because: when the mass percent of Mn in the steel is lower than 0.8%, the prepared steel is insufficient in hardenability, cannot generate enough martensite in the annealing process, and is insufficient in strength; when the mass percentage of Mn element in the steel is higher than 2.5%, the carbon equivalent is remarkably improved, which has negative effects on the welding performance and delayed cracking resistance of the steel. Therefore, considering the influence of the Mn element content on the steel performance, in the 1300MPa or above grade cold-rolled steel sheet, the mass percent of the Mn element is controlled to be between 0.8 and 2.5 percent.
Al: in the cold-rolled steel sheet with the grade of more than 1300MPa, proper amount of Al element is added into the steel to play a role in deoxidizing and refining grains. Therefore, in order to exert the beneficial effects of the Al element, the mass percentage of the Al element is controlled to be between 0.01 and 0.03 percent in the invention.
B: in the 1300MPa or above grade cold-rolled steel plate, B is an element capable of remarkably improving the hardenability of steel, and the addition of B element can promote the formation of martensite and ensure the strength of martensitic steel. However, it should be noted that the content of B element in the steel is not too high, and if more B is added after the grain boundary defect is filled, the "boron phase" of the grain boundary precipitates, which instead increases the grain boundary energy position, and the "boron phase" will serve as the core of the new phase, thereby promoting the increase of the nucleation speed and the decrease of the hardenability of the steel. Therefore, in the 1300MPa or higher grade cold-rolled steel sheet according to the present invention, the mass percentage of B element is controlled to be 0.001-0.003% in consideration of the influence of B element content on the steel properties.
Ti: in the cold-rolled steel sheet with the grade of 1300MPa or more, the added strong carbide forming element Ti can show a strong effect of inhibiting the growth of austenite grains at high temperature, and meanwhile, the addition of the Ti element in the steel is also beneficial to grain refinement. Therefore, in order to exert the beneficial effects of the Ti element, the mass percentage of the Ti element is controlled to be between 0 and 0.05 percent in the invention.
In order to ensure that the strength of the steel is higher than 1300MPa, in the cold-rolled steel plate with the grade higher than 1300MPa, the inventor controls the mass percentage of a single chemical element and simultaneously further controls the mass percentage of C, mn element in the steel to meet the following conditions: C+Mn/6 is more than or equal to 0.35 percent.
Further, in the cold-rolled steel sheet with the grade of more than 1300MPa, among unavoidable impurities, P is less than or equal to 0.015%, S is less than or equal to 0.003%, and N is less than or equal to 0.006%.
In the 1300MPa or above grade cold-rolled steel plate, the P element, the S element and the N element are all impurity elements in steel, and the content of the impurity elements in the steel is reduced as far as possible under the condition of technical conditions, so as to obtain the steel with better performance and better quality. Therefore, except special requirements, the content of the P element in the steel is reduced as much as possible, and the mass percent of the P element is controlled to be less than or equal to 0.015 percent.
In addition, the forming property of the steel is seriously affected by MnS formed by mixing the impurity element S, so that the mass percent of the S element in the steel is strictly controlled to be less than or equal to 0.003 percent in the invention. In addition, since the impurity element N easily causes cracks or bubbles on the surface of the slab, in the present invention, the mass percentage of the N element is controlled to be 0.006% or less.
Further, in the 1300MPa or higher cold-rolled steel sheet, the microstructure is retained austenite, fine block tempered martensite and bainite.
Further, in the cold-rolled steel sheet of 1300MPa or above, the volume phase proportion of tempered martensite is more than or equal to 55%, and the volume phase proportion of bainite is more than 0 and less than 15%.
Further, in the 1300MPa or higher cold-rolled steel sheet according to the present invention, the tempered martensite has a diameter of not more than 10. Mu.m.
In the invention, the components of the steel design are a component system taking C+Mn+B as a main component, and the volume fraction content of martensite can be ensured to be more than 55% through the matching design of elements C, mn and B. Meanwhile, the bainitic curve is ensured to move leftwards, and the ferrite and pearlite C curve is ensured to move rightwards, so that a certain volume fraction of the bainitic in the finally obtained microstructure is ensured, and the volume phase proportion of the bainitic is less than 15%.
In the invention, according to the prior experience and research results, the microstructure of residual austenite + fine block tempered martensite (the diameter of the block martensite is not more than 10 microns) +bainite can be ensured to be obtained by the cold-rolled steel plate through reasonable design of alloy elements and manufacturing process. Wherein, after martensite tempering, the stress is reduced, the hardness is reduced, and simultaneously, tiny dispersion precipitate which can be used as a hydrogen trap can be generated in the martensite tempering, which are factors favorable for improving the delayed cracking performance; the obtained residual austenite is beneficial to delaying cracking and improving the forming property of the cold-rolled steel plate.
Further, in the cold-rolled steel sheet of 1300MPa or more according to the present invention, the properties thereof satisfy:
when the tensile strength is 1300-1400MPa, the limit value of the 90-degree cold bending performance characterization parameter R/t is less than or equal to 2.5, when the tensile strength is more than 1400MPa and less than or equal to 1500MPa, the limit value of the 90-degree cold bending performance characterization parameter R/t is less than or equal to 3, when the tensile strength is more than 1500MPa and less than or equal to 1650MPa, the limit value of the 90-degree cold bending performance characterization parameter R/t is less than or equal to 3.5, and when the tensile strength is more than 1650MPa, the limit value of the 90-degree cold bending performance characterization parameter R/t is less than or equal to 4, wherein R represents the bending radius and t represents the plate thickness;
under the condition that the preset stress is greater than or equal to 1.05 times of tensile strength, delayed cracking does not occur after soaking in hydrochloric acid with the concentration of 1mol/L for more than 300 hours.
In the present invention, the availability of retained austenite, fine tempered martensite in bulk, and bainite together determine the good formability that the steel sheet of the 1300MPa or higher cold-rolled steel sheet according to the present invention has. In the cold-rolled steel plate with the grade of 1300MPa or more, when the tensile strength is 1300-1400MPa, the limit value of the 90-degree cold-bending performance characterization parameter R/t is less than or equal to 2.5, when the tensile strength is 1401-1500MPa, the limit value of the 90-degree cold-bending performance characterization parameter R/t is less than or equal to 3, when the tensile strength is 1501-1650MPa, the limit value of the 90-degree cold-bending performance characterization parameter R/t is less than or equal to 3.5, and when the tensile strength is 1650MPa or more, the limit value of the 90-degree cold-bending performance characterization parameter R/t is less than or equal to 4.
Accordingly, another object of the present invention is to provide a method for manufacturing the above 1300MPa or higher cold-rolled steel sheet, which optimally designs a manufacturing process, and the cold-rolled steel sheet manufactured by the manufacturing method has an ultra-high strength and also has excellent delayed cracking resistance and bending resistance.
In order to achieve the above object, the present invention provides a method for manufacturing a 1300MPa or higher cold-rolled steel sheet, comprising the steps of:
(1) Smelting and casting;
(2) Hot rolling;
(3) Cold rolling;
(4) Annealing;
(5) Continuous tempering temperature: tempering at 400-550 deg.c for 10-300s, and cooling to room temperature at 30 deg.c/s;
(6) Leveling;
(7) Discontinuous tempering: the tempering temperature is 180-260 ℃ and the tempering time is 0.5-6h.
In the technical scheme designed by the invention, the acquisition of the bainite is one of the characteristics of the invention, and in the cooling process of continuous annealing in the step (4), partial bainite can be acquired firstly by the steel so as to ensure that the martensite which is generated later surrounds fine dispersed nuclei of the bainite and does not grow violently, thereby finally forming fine massive martensite, and finally, fine massive tempered martensite with the diameter not more than 10 microns can be acquired.
In addition, another characteristic of the invention is that the manufacturing method is optimally designed for two tempering, and after the first continuous tempering process is finished, discontinuous secondary tempering is further adopted after the flattening is finished. The purpose of this design is to temper the martensitic structure while enriching the unconverted austenite with carbon so that after cooling the final retained austenite + fine bulk tempered martensite + bainitic structure is obtained.
In the continuous tempering process of step (5), the tempering temperature is specifically controlled to be 400-550 ℃ and the tempering time is controlled to be 10-300s, because the process determines the form and the size of the final martensite. In the manufacturing method designed by the invention, the invention can finally obtain the fine blocky tempered martensite with the diameter not more than 10 micrometers, and the tempering temperature and the tempering time of each specific component of the fine blocky tempered martensite need to be specifically set according to a dynamic CCT curve so as to ensure that bainite with the proportion of less than 15 percent is obtained, and the strength of steel is not greatly influenced.
In addition, in the discontinuous tempering process in the step (7), the tempering temperature is specifically controlled to be 180-260 ℃ and the tempering time is controlled to be 0.5-6h; the process is a discontinuous low-temperature overaging tempering process and can be realized by a bell-type furnace. By utilizing the discontinuous tempering process, the martensitic structure can be tempered, and meanwhile, unconverted austenite is enriched with carbon, so that the final residual austenite, fine tempered martensite and bainite structure can be obtained after cooling. After tempering, the stress is reduced, the hardness is reduced, and simultaneously, tiny dispersion precipitates which can be used as hydrogen traps are generated inside, which are factors favorable for improving the delayed cracking performance. The obtained residual austenite is beneficial to delaying cracking and improving the forming performance of the steel.
It should be noted that the discontinuous tempering process in the step (7) needs to be designed reasonably according to specific components, and when the tempering temperature is too high and/or the tempering time is too long, the tempering temperature is likely to cause the steel to rob, or serious yield stress platform is caused to the material, so that the stamping performance is affected; however, when the tempering temperature is too low and/or the tempering time is too short, martensite cannot be tempered significantly, sufficient retained austenite cannot be obtained, and formability cannot be improved. Therefore, in order to ensure the performance of the steel, in the present invention, the tempering temperature in the discontinuous tempering process is specifically controlled to be 180-260 ℃ and the tempering time is controlled to be 0.5-6h.
Further, in the manufacturing method of the present invention, in the step (2), the temperature is first heated to 1100-1250 ℃, the heat is preserved for more than 0.3 hours, then hot rolling is performed at a temperature above Ar3 (austenite transformation temperature), after the rolling, the hot rolling is rapidly cooled at a speed of 30-80 ℃/s, and the coiling temperature is controlled to be 530-600 ℃.
Further, in the manufacturing method according to the present invention, in the step (3), the cold rolling reduction is controlled to 45 to 65%.
Further, in the manufacturing method of the present invention, in the step (4), the annealing soaking temperature is controlled to be 830-870 ℃, the holding time is controlled to be 30-150s, and then the annealing soaking temperature is cooled to be between 730-780 ℃ at a cooling rate of 5-15 ℃/s; and then cooling to the continuous tempering temperature at a speed of 50-700 ℃/s.
In the above technical solution of the present invention, in the annealing step of step (4), the annealing soaking temperature is limited to 830-870 ℃ and the holding time is 30-150s, because it is to achieve the full austenitizing temperature soaking annealing. When the annealing soaking temperature employed in step (4) is less than 830 ℃ and less than 30s, sufficient tensile strength cannot be obtained; when the annealing soaking temperature is higher than 870 ℃ and is higher than 150s, the formability of the steel is greatly reduced.
Accordingly, in some preferred embodiments, it may be preferable to control the annealing soak temperature between 850-860 ℃ to ensure both complete austenitization and to ensure that the grain size obtained is not coarsened, resulting in better formability.
Further, in the manufacturing method of the present invention, in the step (4), the annealing soaking temperature is controlled to 850 to 860 ℃.
Further, in the manufacturing method according to the present invention, in the step (6), the flatness ratio is controlled to be 0 to 0.3%.
Compared with the prior art, the cold-rolled steel sheet with the grade of more than 1300MPa and the manufacturing method thereof have the following advantages and beneficial effects:
the invention develops a novel cold-rolled steel plate with the grade of more than 1300MPa and a manufacturing method thereof, and the cold-rolled steel plate with the grade of more than 1300MPa with low delayed cracking sensitivity and high bending performance can be obtained through reasonable component matching and process design.
The cold-rolled steel sheet with the grade of above 1300MPa has very excellent delayed cracking resistance, and can be soaked in hydrochloric acid with the concentration of 1mol/L for more than 300 hours without delayed cracking under the condition that the preset stress is more than or equal to 1.05 times of tensile strength. Meanwhile, the microstructure of the retained austenite, the fine block tempered martensite and the bainite of the cold-rolled steel plate directly determines that the cold-rolled steel plate designed by the invention has good forming performance, when the tensile strength of the cold-rolled steel plate is 1300-1400MPa, the limit value of the 90-degree cold-bending performance characterization parameter R/t is less than or equal to 2.5, when the tensile strength is more than 1400MPa and less than or equal to 1500MPa, the limit value of the 90-degree cold-bending performance characterization parameter R/t is less than or equal to 3.5, when the tensile strength is more than or equal to 1500MPa and less than or equal to 1650MPa, the limit value of the 90-degree cold-bending performance characterization parameter R/t is less than or equal to 3.5, and when the tensile strength is more than 1650MPa, the limit value of the 90-degree cold-bending performance characterization parameter R/t is less than or equal to 2.5, wherein R represents the bending radius and t represents the plate thickness.
In conclusion, the cold-rolled steel plate has the advantages of ultra-high strength, excellent delayed cracking resistance and excellent bending forming performance, can be used for effectively preparing automobile parts, and has good popularization prospect and application value.
Detailed Description
The 1300MPa or higher cold rolled steel sheet and the method of manufacturing the same according to the present invention will be further explained and illustrated with reference to specific examples, but the explanation and illustration do not constitute undue limitations on the technical solution of the present invention.
Examples 1 to 18
Table 1 shows the mass percentages of the respective chemical elements designed for the cold rolled steel sheets of 1300MPa or more of examples 1 to 18.
Table 1 (wt.%), the balance Fe and unavoidable impurities other than P, S, N
The cold-rolled steel plates with the pressure of above 1300MPa of the examples 1-18 are prepared by the following steps:
(1) Smelting and casting were performed according to the chemical compositions shown in table 1 to obtain a cast slab.
(2) And (3) hot rolling: for the obtained casting blank, heating to 1100-1250 ℃, preserving heat for more than 0.3 hours, hot rolling at the temperature of more than Ar3, rapidly cooling at the speed of 30-80 ℃/s after rolling, coiling after cooling to the coiling temperature, and controlling the coiling temperature to be 530-600 ℃.
(3) Cold rolling: the cold rolling reduction rate is controlled to be 45-65%.
(4) Annealing: controlling the annealing soaking temperature to be 830-870 ℃, preferably 850-860 ℃ and the heat preservation time to be 30-150s, and then cooling to 730-780 ℃ at the cooling speed of 5-15 ℃/s; and then cooling to the continuous tempering temperature at a speed of 50-700 ℃/s.
(5) Continuous tempering temperature: the tempering temperature is controlled to be 400-550 ℃, the tempering time is controlled to be 10-300s, and then the temperature is cooled to room temperature at a speed of more than 30 ℃/s.
(6) Leveling: the flatness is controlled to be 0-0.3%.
(7) Discontinuous tempering: and carrying out discontinuous tempering on the leveled steel plate, and controlling the tempering temperature to be 180-260 ℃ and the tempering time to be 0.5-6h.
The chemical element components and the related process design of the cold-rolled steel sheet with the grade of more than 1300MPa of the embodiments 1-18 all meet the requirements of the design specification of the invention.
Specific process parameters of the 1300MPa or higher cold-rolled steel sheets of examples 1 to 18 in the above process steps are shown in tables 2 to 1 and 2 to 2.
Table 2-1.
Note that: in the above Table 2-1, examples 1-18 all employed hot rolling temperatures > Ar3, and Ar3 in each example was between 730-850℃within the process of the present invention.
Table 2-2.
In the invention, the cold-rolled steel plates with the grades above 1300MPa of the finished product examples 1-18 obtained through the process steps (1) - (7) are respectively sampled, and the microstructure of the steel plates of each example is observed and analyzed, and the microstructure of the cold-rolled steel plates of each example is observed to be retained austenite, fine blocky tempered martensite and bainite.
Furthermore, the inventors further analyzed the volume phase ratio of each component in the microstructure of 1300MPa or more grade cold rolled steel sheets of final examples 1-18, and examined the diameter of tempered martensite, and the results of the relevant analysis and examination are shown in Table 3 below.
Table 3.
It can be seen from analysis and examination that in the present invention, the tempered martensite volume phase ratio of the 1300MPa or higher cold rolled steel sheet of examples 1 to 18 is between 68 and 91%, the bainite volume phase ratio is between 5 and 14%, and the tempered martensite diameter thereof is between 4.3 and 8.7 μm.
Accordingly, after the above observation and analysis were completed, further prepared cold rolled steel sheets of 1300MPa or more of the final examples 1 to 18 were sampled, respectively, and the cold rolled steel sheet samples of each example were subjected to the relevant mechanical properties test to obtain the mechanical strength, elongation and bending properties thereof, and the obtained mechanical properties test results were listed in table 4.
The related mechanical property testing method is as follows:
tensile test: according to GB/T228 (tensile test of metallic materials, section 1: room temperature test method), a test was conducted to examine the yield strength, tensile strength and elongation of the 1300MPa or higher cold-rolled steel sheets of examples 1 to 18.
In addition, the bending performance of the cold rolled steel sheets of each example is characterized by a 90 degree cold bending performance characterization parameter R/t limit value, wherein the sheet thickness t is fixed, and the bending radius R for ensuring that the bending does not crack is changed. The R/t limit can be obtained when the radius R of the bending angle, which ensures that the bending does not crack, is minimized. The larger the obtained 90-degree cold bending performance characterization parameter R/t limit value is, the worse the bending capability is; the smaller the obtained 90-degree cold bending performance characterization parameter R/t is, the better the bending capability is.
Table 4 shows the mechanical properties of the cold rolled steels of 1300MPa or more of examples 1-18.
Table 4.
As shown in Table 4, the cold-rolled steel sheets of the above 1300MPa grade of examples 1-18 according to the present invention have an ultra-high strength and a good cold bending deformation capability, the yield strength is 1128-1503MPa, the tensile strength is 1321-1738MPa, and the elongation is 6.1-10.1%. Meanwhile, it can be seen from examples 1 to 18 that the limit value of the 90 degree cold bending performance characteristic parameter R/t is not more than 2.5 when the tensile strength is 1300 to 1400MPa, not more than 3 when the tensile strength is 1401 to 1500MPa, not more than 3.5 when the tensile strength is 1501 to 1650MPa, not more than 4 when the tensile strength is 1650 MPa. It can be seen that the cold rolled steel sheets of examples 1 to 18 have very excellent bending deformation properties while having ultra-high strength.
Accordingly, the cold-rolled steel sheets of examples 1 to 18 prepared by the present invention have not only the above-mentioned excellent mechanical properties, but also excellent delayed cracking resistance.
To verify the delayed cracking resistance of the cold rolled steel sheets of examples 1 to 18 prepared, the inventors sampled again the steel sheets of each example and controlled the cold rolled steel sheets of each example to conduct a pickling test, i.e., evaluated by a hydrochloric acid solution soaking test, the wire cut test pieces were respectively loaded to 1.05, 1.1, 1.15, 1.2 times of tensile strength by bending, soaked in 0.1mol/L HCl solution for 300 hours without changing the solution, and surface corrosion products were removed with a brush before each solution change, with a test time of 300 hours being controlled.
In the invention, after the acid soaking experiment is completed, observing the sample plate, and when the sample plate has no crack, the delayed cracking resistance is better under the stress condition, and the mark is OK; if the sample plate is cracked, the delayed cracking resistance under the stress condition is poor, and is marked as NG.
Table 5 shows the test results of the cold rolled steel sheets of examples 1 to 18 after the acid soaking test.
Table 5.
As shown in Table 5 above, the cold rolled steel sheets of examples 1 to 18, which were prepared, had very excellent delayed cracking resistance, and all of the steel sheets of examples were immersed in 1mol/L hydrochloric acid for 300 hours or more under a preset stress of 1.05 times or more of tensile strength, without delayed cracking.
It should be noted that the combination of the technical features in the present invention is not limited to the combination described in the claims or the combination described in the specific embodiments, and all the technical features described in the present invention may be freely combined or combined in any manner unless contradiction occurs between them.
It should also be noted that the above-recited embodiments are merely specific examples of the present invention. It is apparent that the present invention is not limited to the above embodiments, and similar changes or modifications will be apparent to those skilled in the art from the present disclosure, and it is intended to be within the scope of the present invention.

Claims (13)

1. A1300 MPa or more grade cold-rolled steel sheet contains Fe and unavoidable impurity elements, and is characterized by further comprising the following chemical elements in percentage by mass:
C：0.10％～0.30％，Si：0.1％～0.5％，Mn：0.8％～2.5％，Al：0.01％～0.03％，B：0.001-0.003％；Ti：0～0.05％；
and the mass percentages of C and Mn are as follows: C+Mn/6 is more than or equal to 0.35 percent.
2. The cold-rolled steel sheet of 1300MPa or more according to claim 1, wherein the mass percentages of the chemical elements are:
c:0.10 to 0.30 percent, si:0.1 to 0.5 percent of Mn:0.8 to 2.5 percent of Al:0.01% -0.03%, B:0.001-0.003%; ti:0 to 0.05 percent, and the balance of Fe and other unavoidable impurities;
and the mass percentages of C and Mn are as follows: C+Mn/6 is more than or equal to 0.35 percent.
3. The cold-rolled steel sheet of 1300MPa or more as claimed in claim 1 or 2, wherein among the unavoidable impurities, P is 0.015% or less, S is 0.003% or less, and N is 0.006% or less.
4. The 1300MPa or higher cold-rolled steel sheet according to claim 1 or 2, wherein the microstructure is retained austenite+fine tempered martensite in bulk+bainite.
5. The cold-rolled steel sheet of 1300MPa or more according to claim 4, wherein the tempered martensite is not less than 55% by volume and the bainite is not less than 0% by volume and less than 15% by volume.
6. The 1300MPa or higher cold-rolled steel sheet according to claim 4, wherein the tempered martensite has a diameter of not more than 10 μm.
7. The 1300MPa or higher cold-rolled steel sheet according to claim 1 or 2, characterized in that its properties satisfy:
when the tensile strength is 1300-1400MPa, the limit value of the 90-degree cold bending performance characterization parameter R/t is less than or equal to 2.5, when the tensile strength is more than 1400MPa and less than or equal to 1500MPa, the limit value of the 90-degree cold bending performance characterization parameter R/t is less than or equal to 3, when the tensile strength is more than 1500MPa and less than or equal to 1650MPa, the limit value of the 90-degree cold bending performance characterization parameter R/t is less than or equal to 3.5, and when the tensile strength is more than 1650MPa, the limit value of the 90-degree cold bending performance characterization parameter R/t is less than or equal to 4, wherein R represents the bending radius and t represents the plate thickness;
under the condition that the preset stress is greater than or equal to 1.05 times of tensile strength, delayed cracking does not occur after soaking in hydrochloric acid with the concentration of 1mol/L for more than 300 hours.
8. The method for manufacturing a 1300MPa or higher cold-rolled steel sheet according to any one of claims 1 to 7, comprising the steps of:
(1) Smelting and casting;
(2) Hot rolling;
(3) Cold rolling;
(4) Annealing;
(5) Continuous tempering temperature: tempering at 400-550 deg.c for 10-300s, and cooling to room temperature at 30 deg.c/s;
(6) Leveling;
(7) Discontinuous tempering: the tempering temperature is 180-260 ℃ and the tempering time is 0.5-6h.
9. The manufacturing method according to claim 8, wherein in the step (2), the temperature is first raised to 1100 to 1250 ℃, the heat is preserved for more than 0.3 hours, then hot rolling is performed at a temperature of more than Ar3, and after the rolling, the hot rolling is rapidly cooled at a speed of 30 to 80 ℃/s, and the coiling temperature is controlled to 530 to 600 ℃.
10. The method according to claim 8, wherein in the step (3), the cold rolling reduction is controlled to 45 to 65%.
11. The manufacturing method according to claim 8, wherein in the step (4), the annealing soaking temperature is controlled to be 830-870 ℃, the holding time is controlled to be 30-150s, and then the annealing soaking temperature is cooled to be between 730-780 ℃ at a cooling rate of 5-15 ℃/s; and then cooling to the continuous tempering temperature at a speed of 50-700 ℃/s.
12. The method of claim 11, wherein in step (4), the annealing soaking temperature is controlled to 850-860 ℃.
13. The method according to claim 8, wherein in the step (6), the flatness ratio is controlled to be 0 to 0.3%.
CN202210711325.5A 2022-06-22 2022-06-22 Cold-rolled steel plate with grade of 1300MPa or more and manufacturing method thereof Pending CN117305683A (en)

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