WO2020001430A1 - 具有良好的疲劳及扩孔性能的超高强热轧钢板和钢带及其制造方法 - Google Patents

具有良好的疲劳及扩孔性能的超高强热轧钢板和钢带及其制造方法 Download PDF

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WO2020001430A1
WO2020001430A1 PCT/CN2019/092766 CN2019092766W WO2020001430A1 WO 2020001430 A1 WO2020001430 A1 WO 2020001430A1 CN 2019092766 W CN2019092766 W CN 2019092766W WO 2020001430 A1 WO2020001430 A1 WO 2020001430A1
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ultra
steel sheet
strength hot
rolled steel
steel strip
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PCT/CN2019/092766
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English (en)
French (fr)
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张瀚龙
张玉龙
王利
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宝山钢铁股份有限公司
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Priority to US17/256,080 priority Critical patent/US11578380B2/en
Priority to CA3104189A priority patent/CA3104189A1/en
Priority to JP2020571425A priority patent/JP7119135B2/ja
Priority to EP19825033.4A priority patent/EP3816316A4/en
Priority to AU2019296099A priority patent/AU2019296099A1/en
Priority to KR1020217001004A priority patent/KR20210028189A/ko
Publication of WO2020001430A1 publication Critical patent/WO2020001430A1/zh

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    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
    • 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
    • C21D6/002—Heat treatment of ferrous alloys containing Cr
    • 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
    • C21D6/005—Heat treatment of ferrous alloys containing Mn
    • 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
    • C21D6/008—Heat treatment of ferrous alloys containing Si
    • 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
    • C21D6/02—Hardening by precipitation
    • 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
    • C21D8/021—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips involving a particular fabrication or treatment of ingot or slab
    • 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
    • C21D8/0247—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
    • 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
    • C21D8/0263—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment following 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
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/002—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
    • C22C38/06—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
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/22—Ferrous alloys, e.g. steel alloys containing chromium with molybdenum or tungsten
    • 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
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/24—Ferrous alloys, e.g. steel alloys containing chromium with vanadium
    • 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
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/26—Ferrous alloys, e.g. steel alloys containing chromium with niobium or tantalum
    • 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
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/28—Ferrous alloys, e.g. steel alloys containing chromium with titanium or zirconium
    • 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
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/38—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of manganese
    • 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
    • C21D2211/004—Dispersions; Precipitations

Definitions

  • the invention belongs to the field of metal materials, and particularly relates to an ultra-high-strength hot-rolled steel plate and a steel strip with good fatigue and hole expansion properties, and a manufacturing method thereof.
  • the invention is mainly used for manufacturing automobile chassis, suspension parts and other products.
  • Lightweight cars can directly reduce emissions and reduce fuel consumption, which is the goal of the development of today's automobile manufacturing industry.
  • An important measure to reduce the weight of automobiles is to use high-strength and ultra-high-strength steel plates instead of mild steel. After a large amount of high-strength steel is used, a weight reduction effect of 20 to 25% can be achieved.
  • body-in-white structural parts have been widely adopted to adopt advanced high-strength steels with both high strength and high elongation to achieve "light weight", which has achieved excellent energy saving and emission reduction effects.
  • the concept of "light weight” is further applied to automobile chassis and suspension systems. Increasingly stringent environmental protection requirements and market demands also require the use of high-strength steel to achieve "light weight”.
  • Chinese patent application CN102612569A discloses a high-strength hot-rolled steel sheet with a tensile strength greater than 780 MPa, a bending fatigue limit ratio of 10 million cycles greater than 0.45, and a hole expansion ratio (the original hole is punched) of 30-50%. Although the steel plate has high strength and a certain bending fatigue limit, the hole expansion ratio is relatively low.
  • Chinese patent application CN103108971A discloses a high-strength hot-rolled steel sheet with excellent fatigue resistance.
  • the steel sheet has a tensile strength greater than 780 MPa and a tensile fatigue limit of 0.66 to 0.78 million.
  • the fatigue limit is only the fatigue limit under 2 million loads. According to common knowledge, the fatigue limit is inversely proportional to the number of cycles. Therefore, if the number of fatigue test loadings is further increased, the fatigue limit will be further reduced, and the patent The application did not consider the material's reaming performance.
  • Chinese patent application CN101906567A discloses a high-strength hot-rolled steel sheet with excellent hole expandability.
  • the steel sheet has a tensile strength greater than 780 MPa and a hole expansion ratio (the original hole is punched) of 43-89%.
  • Chinese patent application CN104136643A discloses a high-strength hot-rolled steel sheet with a tensile strength greater than 780 MPa and a hole expansion ratio (the original hole is a reamed hole) of 37-103%.
  • neither of the above patent applications considers the fatigue properties of the material.
  • the Ti element is an optional or mandatory beneficial element, in order to improve the strength of the material or inhibit the growth of the original austenite grains.
  • Ti element and the common impurity N element in steel will form square (or triangle) large, brittle, sharp TiN particles with sharp corners at high temperature, which will have a detrimental effect on the forming properties such as bending and hole expansion of steel, and Will greatly reduce the fatigue limit of steel. These adverse effects caused by the Ti element have not been considered in the prior art.
  • the strength, fatigue limit and hole expansion performance belong to the three categories.
  • Mutually restricted performance the strength of the material is usually inversely proportional to the hole expansion performance.
  • the precipitation strengthening effect of carbides is urgently needed.
  • the large amount of precipitation and coarsening of carbides will greatly impair the hole expanding performance of the material.
  • the purpose of the present invention is to provide an ultra-high-strength hot-rolled steel plate and steel strip with good fatigue and hole expansion properties, and a method for manufacturing the steel plate.
  • the steel plate has a tensile strength of ⁇ 780 MPa, a yield strength of ⁇ 660 MPa, and a performance index of hole expansion: If the original hole is a punched hole: the hole expansion rate is> 85%; if the original hole is a reamed hole: the hole expansion rate is> 120%; anti-fatigue performance indicators: high frequency fatigue limit (10 million cycles) FL ⁇ 570MPa, or fatigue limit Specific tensile strength FL / Rm ⁇ 0.72; more preferably, the steel plate has a tensile strength ⁇ 780 MPa, a yield strength ⁇ 660 MPa, a tensile fatigue limit (10 million cycles) FL ⁇ 600 MPa, or a fatigue limit specific tensile strength FL / Rm ⁇ 0.75, the hole expansion rate is satisfied: if the original hole
  • An ultra-high-strength hot-rolled steel plate and steel strip with good fatigue and hole expansion properties is: C: 0.07 to 0.14%, Si: 0.1 to 0.4%, Mn: 1.55 to 2.00%, and P ⁇ 0.015.
  • C 0.07 to 0.09%, in terms of weight percentage.
  • Si 0.1 to 0.3%, based on weight percent.
  • Mn is 1.70 to 1.90%, in terms of weight percentage.
  • Cr 0.35 to 0.50%, in terms of weight percentage.
  • V 0.12 to 0.22%, in terms of weight percentage.
  • Mo 0.15 to 0.3%, in terms of weight percentage.
  • Nb 0.02 to 0.05%, in terms of weight percentage.
  • Al 0.02 to 0.04%, based on weight percentage.
  • the tensile strength of the ultra-high-strength hot-rolled steel plate and steel strip is greater than or equal to 780 MPa, and the yield strength is greater than or equal to 660 MPa.
  • Anti-fatigue performance index High frequency fatigue limit (10 million cycles) FL ⁇ 570MPa, or fatigue limit specific tensile strength FL / Rm ⁇ 0.72.
  • the anti-fatigue performance index of the ultra-high-strength hot-rolled steel sheet and steel strip is: high-frequency fatigue limit (10 million cycles) FL ⁇ 600 MPa, or fatigue limit specific tensile strength FL / Rm ⁇ 0.75.
  • the anti-fatigue performance index of the ultra-high-strength hot-rolled steel sheet and steel strip high-frequency fatigue limit (10 million cycles) FL ⁇ 640 MPa, or fatigue limit specific tensile strength FL / Rm ⁇ 0.8.
  • the A50 of the ultra-high-strength hot-rolled steel sheet and steel strip is ⁇ 15.0%, more preferably ⁇ 16.0%.
  • the performance index of the hole expansion rate of the ultra-high-strength hot-rolled steel plate and steel strip is: if the original hole is a punched hole: the hole expansion rate is ⁇ 90%; if the original hole is a reamed hole: the hole expansion rate is ⁇ 125%.
  • the microstructure of the ultra-high-strength hot-rolled steel sheet and steel strip of the present invention is a bainite microstructure mainly composed of lower bainite.
  • Silicon is an essential element for deoxidation in steelmaking, and it also has a certain solid solution strengthening effect. When it is less than 0.1%, it is difficult to obtain a sufficient deoxidation effect; when the silicon content is higher than 0.5%, it is easy to generate polygonal iron. The body structure is not conducive to the improvement of the hole expansion rate, and at the same time, it deteriorates the plateability, which is not conducive to the production of hot-dip galvanized steel. Therefore, the present invention limits the silicon content to the range of 0.1 to 0.4%. In a preferred embodiment, the Si content ranges from 0.1 to 0.3%.
  • Manganese is an effective element for improving strength and has low cost. Therefore, the present invention uses manganese as a main additive element. However, when the manganese content is higher than 2.00%, a large amount of martensite is formed, which is disadvantageous to the hole expansion performance; when the manganese content is less than 1.55%, the strength of the steel sheet is insufficient. Therefore, the present invention limits the manganese content to 1.55 to 2.00%. In a preferred embodiment, the Mn content ranges from 1.7 to 1.9%.
  • Aluminum has a deoxidizing effect in the steel making process, and is an element added to improve the purity of molten steel.
  • Aluminum can also fix nitrogen in steel to form a stable compound and effectively refine the grains, but the effect is less when the aluminum content is less than 0.01%: when the aluminum content exceeds 0.05%, the deoxidation reaches saturation, and the higher the content is Has a negative impact on the base metal and welding heat affected zone. Therefore, the present invention limits the aluminum content to 0.01 to 0.05%. In a preferred embodiment, the Al content ranges from 0.02 to 0.04%.
  • Niobium can effectively delay the recrystallization of deformed austenite, prevent the growth of austenite grains, increase the austenite recrystallization temperature, refine the grains, and increase strength and elongation.
  • the niobium content is higher than 0.06%, the cost is increased, and the effect is no longer significant. Therefore, the niobium content is limited to 0.06% or less in the present invention. In a preferred embodiment, the Nb content ranges from 0.02-0.05%.
  • Vanadium (V) the role of vanadium is to form carbide precipitation and solid solution strengthening to improve the strength of steel, but after the vanadium content is greater than 0.35%, the effect of increasing its content is not significant. When the V content is less than 0.10%, the precipitation strengthening effect is not significant. Significantly. Therefore, the vanadium content in the present invention is limited to 0.1 to 0.35%. In a preferred embodiment, the V content ranges from 0.12 to 0.22%.
  • Chromium and molybdenum increase the incubation period of pearlite and ferrite in the CCT curve, inhibit the formation of pearlite ferrite, and make it easy to obtain a bainite structure during cooling, which is beneficial to improve Reaming rate.
  • chromium and molybdenum contribute to the refinement of austenite grains and the formation of fine bainite during rolling, and increase the strength of the steel by solid solution strengthening and carbide precipitation, but the addition amount exceeds 0.5%
  • the content of Cr and Mo is less than 0.15%, the effect on the CCT curve is not significant. Therefore, the present invention limits the chromium and molybdenum contents to 0.15 to 0.5%.
  • the Cr content ranges from 0.35 to 0.50%.
  • the Mo content ranges from 0.15-0.30%.
  • the measurement relationship between the above alloy elements and carbon elements should also satisfy the following formula: 1.0 ⁇ [(Cr / 52) / (C / 4) + (Nb / 93 + Ti / 48 + V / 51 + Mo / 96) / (C / 12)] ⁇ 1.6:
  • the addition of alloying elements can enhance the strength of the material through the solid solution strengthening effect and carbide precipitation effect.
  • the carbide precipitation effect has a greater negative impact on hole expansion performance and fatigue limit.
  • the more alloying elements the easier it is to combine with a large amount of carbon elements in the steel to form coarse carbide precipitation phases. Therefore, the proportion of alloying elements and carbon elements needs to reach the range set by the above formula, so as to ensure that the material can simultaneously achieve the strength and hole expansion performance that meet the design standards.
  • Titanium is a harmful element that reduces the fatigue limit in the present invention.
  • Ti element can improve the strength of this type of steel, it will produce large, brittle, and sharp edges of TiN particles, becoming a potential
  • the source of fatigue cracks greatly reduces the fatigue performance of steel, and the higher the Ti element content, the larger the size of the TiN particles formed, and the more serious the adverse effect on the fatigue performance.
  • adding a large amount of Ti element will also cause a large amount of coarse TiC to precipitate, which will damage the hole expansion performance. Therefore, a strict upper limit of the Ti content is required. In the case where no additional Ti element is added, Ti ⁇ 0.02% is required, and preferably, Ti ⁇ 0.005%.
  • the upper limit of the impurity elements in the steel is controlled at P: ⁇ 0.015%, S: ⁇ 0.004%, N: ⁇ 0.005%, and the more pure the steel, the better the effect. Furthermore, in order to obtain the highest fatigue limit, when the Ti element content is guaranteed to be less than 0.003%, the N element content is required to be 0.003% or less.
  • the manufacturing method of the ultra-high-strength hot-rolled steel sheet and steel strip with good fatigue and hole expansion performance according to the present invention includes the following steps:
  • the slab is heated at a heating temperature of 1100 to 1250 ° C; the finish rolling start temperature is 950 to 1000 ° C, and the finish rolling finish rolling temperature is 900 to 950 ° C;
  • the billet after the upper rolling is water-cooled, the cooling rate is ⁇ 30 ° C / s, and the coiling temperature is 450-580 ° C;
  • step 3 heat preservation and slow cooling are performed after the coiling and coiling, and then pickling is performed.
  • the heat preservation and slow cooling step is controlled to be maintained at 450 ° C or higher for 2 to 4 hours.
  • the heat preservation and slow cooling can put the hot-rolled coil in a non-heating type heat preservation device and keep it at 450 ° C or higher for 2 to 4 hours.
  • the slab heating temperature affects the austenite grain size.
  • added alloying elements such as V and Nb will form carbides to increase the strength of the steel sheet.
  • these alloy elements must be dissolved into the austenite to form a complete solid solution, and small carbides or nitrides can be formed in the subsequent cooling process, which plays a strengthening role. Therefore, the present invention heats the slab The temperature is limited to 1100 to 1250 ° C.
  • the finishing rolling temperature when the finishing rolling temperature is not less than 900 ° C, a fine and uniform structure can be obtained.
  • the finishing rolling temperature is below 900 ° C, the band structure formed during hot working will be retained, which will improve the Hole expansion performance is unfavorable. Therefore, the finishing rolling temperature is limited to not less than 900 ° C.
  • the upper limit of the final rolling temperature does not need to be specified, but in consideration of the slab heating temperature, the finishing rolling temperature does not exceed 950 ° C.
  • the cooling rate is limited to not less than 30 ° C / s in order to prevent the supercooled austenite from transforming into polygonal ferrite or pearlite and the precipitation of carbides at higher temperatures, forming the following bainite mainly Microstructure.
  • the coiling temperature is one of the most critical process parameters for obtaining high strength, high hole expansion ratio and high fatigue limit.
  • the coiling temperature is greater than 580 ° C, due to the strong precipitation and coarsening of the alloy carbides, the strength of the ferrite is reduced, which has a negative effect on the hole expansion ratio and fatigue limit of the steel plate.
  • the coiling temperature is less than At 450 °C, a lot of martensite structure will be formed. Although it can enhance the strength of the material, it will adversely affect the hole expansion rate.
  • the winding temperature is limited to 450 to 580 ° C.
  • the tensile strength of this type of steel can be further improved by the hot rolling insulation method, specifically: after rolling, the hot coil is placed in a heat preservation pit, and the heat of the hot coil is used to heat and slowly cool, and the temperature is maintained above 450 ° C. In 2 to 4 hours, fine dispersion precipitation of vanadium carbide can be promoted, thereby significantly improving the strength of the material of the present invention, and at the same time, it will not cause a significant decrease in the hole expansion ratio and fatigue limit. In the hot-roll insulation process, the minimum holding temperature and holding time have an effect on the performance of the final product.
  • the holding temperature is lower than 450 ° C, the precipitation power of vanadium carbide (molybdenum) is insufficient, and fine dispersed vanadium carbide (molybdenum) cannot be formed.
  • the holding time is less than 2h, the precipitation of vanadium carbide (molybdenum) is limited, and the strength of this type of steel cannot be improved; while if the holding time is more than 4h, the vanadium (molybdenum) carbide will grow and coarsen after precipitation, which will significantly reduce this type. Reaming rate and fatigue limit of steel.
  • the primary requirements for materials for automotive chassis and suspension system components are high strength and high hole expansion performance.
  • ferrite or Ferrite plus bainite structure in which the bainite structure content is greater than 50%
  • Ti element is used as a required or optional beneficial element to improve the strength of this type of steel.
  • Ti element and N element in steel will form large, brittle, and sharp at high temperatures. Corner TiN particles are not conducive to the hole expansion performance of this type of steel.
  • the present invention adopts a composition design idea without Ti element, does not add Ti element, and strictly controls the Ti content in the steel to reduce the generation of TiN particles and obtain a high fatigue limit; and through the optimization of Mo-V compounding and manufacturing processes to Obtain a high-strength hot-rolled steel plate with high strength, high hole expansion ratio, and high fatigue limit.
  • the microstructure of the steel plate adopts the microstructure of bainite mainly composed of lower bainite to ensure the strength and toughness of the steel plate.
  • the structure content (volume ratio) of lower bainite is in the range of 30% to 70%.
  • the microstructure of the steel sheet of the present invention has a structure content of lower bainite of 40% to 70%. By adding the alloying elements Cr and Mo to the ferrite transformation zone to the right, the critical cooling rate can be reduced, and the lower bainite structure can be easily obtained.
  • the microstructure of the steel sheet of the present invention may further include ferrite, carbide precipitates, and optionally tempered martensite.
  • Mo, V, Nb alloy elements are added to refine the crystal grains to generate fine dispersed carbides to further improve the strength of the steel.
  • excessive carbide precipitation will further coarsen, which is not only detrimental to the further improvement of strength, but will also reduce the hole expansion performance and fatigue limit of the steel. Therefore, it is necessary to optimize the hot rolling process to obtain finely dispersed alloy carbides and to achieve the purpose of improving the hole expanding performance.
  • the sum of the structure content of the lower bainite and the ferrite is ⁇ 80%, and the structure content of the lower bainite is ⁇ 40%.
  • the properties of the ultra-high-strength hot-rolled steel sheet and steel strip provided by the present invention satisfy the following indicators:
  • the hole expansion rate is greater than 85%
  • the enlargement rate is greater than 120%.
  • the ultra-high-strength hot-rolled steel sheet and steel strip manufactured by the present invention have both high strength, high hole expandability and high fatigue limit.
  • the ultra-high-strength hot-rolled steel sheet and steel strip product is hot-dip galvanized to obtain a hot-rolled hot-dip galvanized steel product.
  • the ultra-high-strength hot-rolled steel sheet products, steel strip products, and hot-dip galvanized steel sheet products can be used to prepare automobile chassis and suspension system components to achieve "lightweight" of automobiles.
  • FIG. 1 is a photograph of a microstructure of a G-1 steel according to an example of the present invention (magnified 1000 times).
  • Fig. 2 is a photograph of the morphology of TiN particles in the microstructure of Comparative Steel P (1000 times magnification).
  • the steels of different compositions shown in Table 1 are smelted, and then the heating + hot rolling process is performed as shown in Table 2 to obtain steel plates having a thickness of less than 4 mm.
  • the fatigue limit is measured using the transverse sample.
  • sample size and test method refer to GB 3075-2008 Metal.
  • Axial fatigue test method; test data are shown in Table 2.
  • the hole expansion rate is measured by a hole expansion test, and the test piece with a hole in the center is pressed into the concave mold with a male die, so that the center hole of the test piece is enlarged until necking or penetration cracks occur on the edge of the plate hole. Because the preparation method of the original hole in the center of the test piece has a great impact on the test result of the hole expansion rate, punching and reaming are used to prepare the original hole in the center of the test piece. The subsequent tests and test methods are in accordance with ISO / DIS16630 The hole expansion test method is performed. The fatigue limit is determined by the axial high-frequency tensile fatigue test. The GB 3075-2008 metal axial fatigue test method is used. The test frequency is 85 Hz. The maximum strength that does not fail after the sample is cyclically loaded 10 million times is taken as its fatigue limit RL. .
  • Examples A to H are steels of the present invention, and the contents of carbon or manganese or other alloying elements in Comparative Examples J to P exceed the range of the ingredients of the present invention.
  • M (all) in the table refers to the calculated value of (Cr / 52) / (C / 4) + (Nb / 93 + Ti / 48 + V / 51 + Mo / 96) / (C / 12) .
  • Comparative Examples M, N, O, P When the content of Ti element deviates from the scope of the present invention, it will negatively affect the fatigue limit of the steel.
  • Comparative Examples M, N, O, P Such as Comparative Examples M, N, O, P.
  • the comparative examples M and P have a relatively high Ti content, so that although the steel reaches the strength standard designed in the present invention, the fatigue limit is far below 570 MPa, and the fatigue limit ratio is also far below the minimum design standard of 0.72.
  • the Ti content is low, but still exceeds the minimum upper limit of the present invention, so that the fatigue limit and the fatigue limit ratio do not meet the requirements of the present invention.
  • the ratio of the alloying element to the carbon element that is, M (all) did not reach the range of the range designed by the present invention, which caused the hole expansion performance of the two groups of materials to fail.
  • the present invention controls a reasonable composition range on the basis of carbon-manganese steel, adds microalloying elements, limits the content of Ti element, and further controls the coiling temperature on the basis of a conventional automotive steel production line, and It can further produce ultra-high-strength hot-rolled steel plates and strips with good hole expansion and fatigue performance through the use of heat preservation ring cooling technology.
  • Example A 0.09 0.35 1.75 0.011 0.005 0.031 0.003 0.055 0.018 0.10 0.45 0.16 1.00
  • Example B 0.07 0.24 1.87 0.011 0.004 0.027 0.003 0.030 0.015 0.20 0.35 0.21 1.54
  • Example C 0.14 0.40 1.57 0.010 0.004 0.036 0.004 0.045 0.016 0.33 0.42 0.18 1.02
  • Example D 0.07 0.28 1.59 0.010 0.005 0.034 0.003 0.025 0.009 0.15 0.44 0.19 1.41
  • Example E 0.11 0.40 1.63 0.010 0.005 0.031 0.003 0.030 0.005 0.13 0.50 0.41 1.14
  • Example F 0.09 0.15 1.55 0.010 0.003 0.036 0.003 0.025 0.004 0.27 0.46 0.27 1.52
  • Example G 0.07 0.20 1.62 0.010 0.002 0.024 0.002 0.020 0.003 0.21 0.37 0.15 1.43
  • Example H 0.09 0.29 1.55 0.011 0.004 0.026 0.002 0.015 0.005 0.16 0.39 0.20 1.06
  • Comparative Example I 0.15 0.25 1.82 0.012 0.005 0.030 0.004 0.048 0.020 0.10 0.50 0.17 0.63 Comparative

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Abstract

具有良好的疲劳及扩孔性能的超高强热轧钢板和钢带及其制造方法，该钢板和钢带成分重量百分比为：C：0.07～0.14％，Si：0.1～0.4％，Mn：1.55～2.00％，P≤0.015％，S≤0.004％，Al：0.01～0.05％，N≤0.005％，Cr：0.15～0.50％，V：0.1～0.35％，Nb：0.01％～0.06％，Mo：0.15～0.50％，且Ti≤0.02％，其余为Fe和不可避免的杂质；且同时需满足： 1.0≤[(Cr/52)/(C/4)+(Nb/93+Ti/48+V/51+Mo/96)/(C/12)]≤1.6。该超高强热轧钢板和钢带的抗拉强度≥780MPa、屈服强度≥660MPa，拉伸疲劳极限(循环1000万次)FL≥570MPa，或疲劳极限比抗拉强度FL/Rm≥0.72，扩孔率满足：若原始孔为冲压孔：扩孔率＞85％；若原始孔为铰孔：扩孔率＞120％。

Description

具有良好的疲劳及扩孔性能的超高强热轧钢板和钢带及其制造方法 技术领域
本发明属于金属材料领域，具体涉及一种具有良好的疲劳及扩孔性能的超高强热轧钢板和钢带及其制造方法，主要应用于制造汽车底盘、悬挂件等产品。
背景技术
汽车“轻量化”可直接减少排放，降低油耗，是当今汽车制造业发展的目标。汽车“轻量化”的一个重要的措施就是采用高强度和超高强度的钢板来代替软钢。大量采用高强钢后可实现20～25％的减重效果。在过去的十年中，白车身结构件已广泛采用兼具高强度与高延伸率的先进高强钢实现“轻量化”，取得了优异的节能减排效果。目前，“轻量化”概念进一步沿用至汽车底盘及悬挂系统，日益严苛的环保要求和市场需求也要求汽车底盘材料采用高强钢实现“轻量化”。
然而，对于汽车底盘及悬挂系统的结构件，其成形过程要求材料具有很高的扩孔率性能。此外，底盘及悬挂系统的结构件的服役特点亦要求材料具有较高的疲劳性能。尽管以贝氏体组织为主的高强钢因具有高强度和良好的扩孔性能而成为目前汽车底盘及悬挂系统零件的常用钢种，但由于贝氏体钢成分及组织复杂，且材料的高强度、高扩孔率与高疲劳极限三项性能之间相互制约，设计及制造兼具高强度、良好扩孔性与良好疲劳性能的钢材，具有极大的难度。
中国专利申请CN102612569A公开了一种高强度热轧钢板，其抗拉强度大于780MPa，1000万次弯曲疲劳极限比大于0.45，扩孔率(原始孔为冲孔)为30～50％。尽管该钢板具有较高的强度和一定的弯曲疲劳极限，但是扩孔率相对较低。
中国专利申请CN103108971A公开了一种耐疲劳特性优良的高强度热轧钢板，该钢板抗拉强度大于780MPa，200万次拉伸疲劳极限0.66～0.78。但是，该疲劳极限仅是200万次加载下的疲劳极限，根据公知常识，疲劳极限与循环次数成反比，故若进一步增加该材料的疲劳试验加载次数，其疲劳极限会进一步降低，且该专利申请未考虑材料的扩孔性能。
中国专利申请CN101906567A公开了一种扩孔加工性优异的高强度热轧钢板，该钢板抗拉强度大于780MPa，扩孔率(原始孔为冲孔)介于43～89％。中国专利申请CN104136643A公开了一种抗拉强度大于780MPa，扩孔率(原始孔为铰孔)介于37～103％的高强度热轧钢板。但是，上述两个专利申请均未考虑材料的疲劳性能。
而在上述四个专利申请中，Ti元素均为可选或必选的有益元素，是为了提高材料强度或抑制原始奥氏体晶粒长大。但是Ti元素与钢中常见杂质N元素在高温下会形成方形(或三角形)的大块、脆性、具有尖锐边角的TiN颗粒，会对钢材的弯曲、扩孔等成形性能产生有害影响，并会大幅降低钢材的疲劳极限。现有技术中均未考虑Ti元素会导致的这些不利影响。
此外，对于抗拉强度达到800MPa级别的，以贝氏体为主且以碳化物析出为增强相的本类材料(以下简称本类材料)，强度、疲劳极限和扩孔性能三者之间属于相互制约的性能。首先，材料的强度通常与扩孔性能成反比，本类钢种为了获得较高的强度，尤其是屈服强度，亟需碳化物的析出强化效应。但是，碳化物的大量析出与粗化更会极大的损害材料的扩孔性能。此外，通常来说，材料的屈服强度越高，材料的疲劳极限越高；但是，对于本类材料，屈服强度的提升极大地依赖于碳化物的大量析出，但是碳化物大量析出与粗化同样会大幅降低本类材料的疲劳极限。因此，本类材料要想获得同时兼备高强度、高扩孔性与高疲劳极限，具有极大的设计与制造难度。
发明内容
本发明的目的在于提供一种具有良好的疲劳及扩孔性能的超高强热轧钢板和钢带及其制造方法，该钢板的抗拉强度≥780MPa、屈服强度≥660MPa，扩孔率性能指标：若原始孔为冲压孔：扩孔率＞85％；若原始孔为铰孔：扩孔率＞120％；抗疲劳性能指标：高频疲劳极限(循环1000万次)FL≥570MPa，或疲劳极限比抗拉强度FL/Rm≥0.72；更优选地，该钢板的抗拉强度≥780MPa、屈服强度≥660MPa，拉伸疲劳极限(循环1000万次)FL≥600MPa，或疲劳极限比抗拉强度FL/Rm≥0.75，扩孔率满足：若原始孔为冲压孔：扩孔率＞85％；若原始孔为铰孔：扩孔率＞120％。本发明的超高强热轧钢板和钢带主要用于汽车底盘、悬挂系统零部件的制备。
为达到上述目的，本发明的技术方案如下：
一种具有良好的疲劳及扩孔性能的超高强热轧钢板和钢带，其成分重量百分比为：C：0.07～0.14％，Si：0.1～0.4％，Mn：1.55～2.00％，P≤0.015％，S≤0.004％，Al：0.01～0.05％，N≤0.005％，Cr：0.15～0.50％，V：0.1～0.35％，Nb：0.01％～0.06％，Mo：0.15～0.50％，且Ti≤0.02％，其余为Fe和不可避免的杂质；且以其重量百分比计，上述元素同时需满足如下关系：1.0≤[(Cr/52)/(C/4)+(Nb/93+Ti/48+V/51+Mo/96)/(C/12)]≤1.6。
优选的，所述超高强热轧钢板和钢带的化学成分中C：0.07～0.09％，以重量百分比计。
优选的，所述超高强热轧钢板和钢带的化学成分中Si：0.1～0.3％，以重量百分比计。
优选的，所述超高强热轧钢板和钢带的化学成分中Mn：1.70～1.90％，以重量百分比计。
优选的，所述超高强热轧钢板和钢带的化学成分中Cr：0.35～0.50％，以重量百分比计。
优选的，所述超高强热轧钢板和钢带的化学成分中V：0.12～0.22％，以重量百分比计。
优选的，所述超高强热轧钢板和钢带的化学成分中Mo：0.15～0.3％，以重量百分比计。
优选的，所述超高强热轧钢板和钢带的化学成分中Nb：0.02～0.05％，以重量百分比计。
优选的，所述超高强热轧钢板和钢带的化学成分中Al：0.02～0.04％，以重量百分比计。
优选的，所述超高强热轧钢板和钢带的化学成分中Ti≤0.005％，以重量百分比计。
更优选的，所述超高强热轧钢板和钢带的化学成分中Ti≤0.003％，N≤0.003％，以重量百分比计。
进一步，所述超高强热轧钢板和钢带的抗拉强度≥780MPa、屈服强度≥660MPa，扩孔率性能指标：若原始孔为冲压孔：扩孔率＞85％；若原始孔为铰孔：扩孔率＞120％；抗疲劳性能指标：高频疲劳极限(循环1000万次)FL≥570MPa， 或疲劳极限比抗拉强度FL/Rm≥0.72。
更优选的，所述超高强热轧钢板和钢带的抗疲劳性能指标：高频疲劳极限(循环1000万次)FL≥600MPa，或疲劳极限比抗拉强度FL/Rm≥0.75。
优选的，所述超高强热轧钢板和钢带的抗疲劳性能指标：高频疲劳极限(循环1000万次)FL≥640MPa，或疲劳极限比抗拉强度FL/Rm≥0.8。
优选的，所述超高强热轧钢板和钢带的A50≥15.0％，更优选≥16.0％。
优选的，所述超高强热轧钢板和钢带的扩孔率性能指标为：若原始孔为冲压孔：扩孔率≥90％；若原始孔为铰孔：扩孔率≥125％。
本发明所述超高强热轧钢板和钢带的微观组织为下贝氏体为主的贝氏体微观组织。
在本发明钢的成分设计中：
碳(C)：碳对钢板的强度、成形性能和焊接性能影响很大。碳和其他合金元素形成合金碳化物提高钢板的强度，若碳含量低于0.07％，钢的强度达不到目标要求；碳含量高于0.14％，则易生成马氏体组织和粗大的渗碳体，使延伸率和扩孔率下降，因此，本发明控制碳含量的范围为0.07～0.14％。在优选的实施方案中，C含量范围为0.07-0.09％。
硅(Si)：硅是炼钢脱氧的必要元素，也具有一定的固溶强化作用，当低于0.1％时，难以获得充分的脱氧效果；当硅含量高于0.5％时，易于生成多边形铁素体组织，对扩孔率的提高不利，同时使得可镀性恶化，不利于生产热镀锌钢板。因此，本发明将硅含量限定在0.1～0.4％的范围内。在优选的实施方案中，Si含量范围为0.1-0.3％。
锰(Mn)：锰是提高强度的有效元素，而且成本低廉，因此本发明将锰作为主要添加元素。但是当锰含量高于2.00％时，有大量马氏体生成，对扩孔性能不利；当锰含量低于1.55％时，钢板的强度不足。因此本发明将锰含量限定在1.55～2.00％。在优选的实施方案中，Mn含量范围为1.7-1.9％。
铝(Al)：铝在炼钢过程中具有脱氧的作用，是为了提高钢水纯净度而添加的元素。铝还能固定钢中的氮使之形成稳定的化合物，有效的细化晶粒，但铝含量小于0.01％时，效果较小：铝含量超过0.05％时，脱氧作用达到饱和，含量再高则对母材及焊接热影响区有负面影响。因此，本发明将铝含量限定在0.01～0.05％。在优 选的实施方案中，Al含量范围为0.02-0.04％。
铌(Nb)：铌能够有效的延迟变形奥氏体的再结晶，阻止奥氏体晶粒的长大，提高奥氏体再结晶温度，细化晶粒，同时提高强度和延伸率。但铌含量高于0.06％时，使成本增加，且效果不再显著，因此，本发明中铌含量限定在0.06％以下。在优选的实施方案中，Nb含量范围为0.02-0.05％。
钒(V)，钒的作用是形成碳化物析出与固溶强化来提高钢的强度，但钒含量大于0.35％后，再增加其含量效果并不显著，V含量小于0.10％时沉淀强化效果不显著。因此，本发明中钒含量限定在0.1～0.35％。在优选的实施方案中，V含量范围为0.12-0.22％。
铬和钼(Cr、Mo)：铬和钼使CCT曲线中珠光体和铁素体的孕育期增长，抑制珠光体的铁素体的形成，使冷却时易于得到贝氏体组织，有利于提高扩孔率。同时，铬和钼有助于轧制时奥氏体晶粒的细化和细小的贝氏体的生成，并通过固溶强化和形成碳化物析出来提高钢的强度，但添加量超过0.5％时，成本提高，可焊性明显降低。Cr和Mo含量小于0.15％时对CCT曲线影响不显著。因此，本发明将铬和钼含量都限定在0.15～0.5％。在优选的实施方案中，Cr含量范围为0.35-0.50％。在优选的实施方案中，Mo含量范围为0.15-0.30％。
应理解的是，本文所述的各元素的各含量范围可相互组合，构成本发明的一个或多个优选的技术方案。
此外，上述合金元素与碳元素的计量关系还应满足如下公式：1.0≤[(Cr/52)/(C/4)+(Nb/93+Ti/48+V/51+Mo/96)/(C/12)]≤1.6：合金元素的添加可以通过固溶强化效应与碳化物析出效应来提升材料的强度。但是，相比固溶强化，碳化物析出效应对扩孔性能和疲劳极限的负面影响更大。合金元素越多，越容易与钢中的碳元素大量结合，形成粗大的碳化物析出相。因此，合金元素与碳元素的配比需要达到上述公式设定的范围，以保证材料可以同时获得达到设计标准的强度与扩孔性能。
钛(Ti)：钛在本发明中属于降低疲劳极限的有害元素，虽然Ti元素的添加能提高该类钢种的强度，但是会生产大块、脆性、具有尖锐边角的TiN颗粒，成为潜在的疲劳裂纹源，大幅降低钢材的疲劳性能，且Ti元素含量越高，形成的TiN颗粒尺寸越大，对疲劳性能的不利影响越严重。此外，Ti元素大量添加亦会导致粗大的TiC大量析出，损害扩孔性能。因此，需要Ti元素含量严控上限。在不额外 添加Ti元素的情况下，要求Ti≤0.02％，优选的，要求Ti≤0.005％。
钢中的杂质元素的上限控制在P：≤0.015％，S：≤0.004％，N：≤0.005％，钢质越纯净效果更佳。更进一步地，为获得最高的疲劳极限，在保证Ti元素含量低于0.003％时，要求N元素含量≤0.003％。
本发明所述具有良好的疲劳及扩孔性能的超高强热轧钢板和钢带的制造方法，包括如下步骤：
1)冶炼、铸造
按上述化学成分冶炼和铸造成铸坯；
2)轧制
对铸坯加热，加热温度为1100～1250℃；精轧开轧温度为950～1000℃，精轧终轧温度为900～950℃；
3)冷却、卷取
将上轧制后坯料进行水冷却，冷却速度≥30℃/s，卷取温度为450～580℃；
4)酸洗。
进一步，步骤3)冷却、卷取后进行保温缓冷，再进行酸洗，其中保温缓冷步骤中控制在450℃以上保温2～4h。该保温缓冷可将热轧卷放于非加热型保温装置中于450℃以上保温2～4h。
上述步骤2)中，板坯加热温度影响奥氏体晶粒尺寸。制造超高强度的复相钢时，加入的合金元素如V和Nb会形成碳化物，以提高钢板的强度。板坯加热时，这些合金元素须溶入奥氏体中形成完全的固溶体，在后续的冷却过程中才能形成细小的碳化物或氮化物，起到强化的作用，因此，本发明将板坯加热温度限定在1100～1250℃。
上述步骤2)中，当精轧终轧温度不小于900℃时，可获得细小均匀的组织，精轧终轧温度低于900℃时，热加工时形成的带状组织将会保留，对提高扩孔性能不利，因此，精轧终轧温度限定为不小于900℃。通常情况下终轧温度的上限并不需要特别规定，但考虑到板坯加热温度，精轧终轧温度不超过950℃。
上述步骤3)中，冷却速率限定为不小于30℃/s是为了阻止过冷奥氏体转变为多边形铁素体或珠光体以及在较高温度碳化物的析出，形成以下贝氏体为主的微观组织。
上述步骤3)中，卷取温度是获得高强度、高扩孔率和高疲劳极限的最为关键的工艺参数之一。当卷取温度大于580℃时，由于合金碳化物的强烈析出和粗化，降低了铁素体的强度，对钢板扩孔率和疲劳极限均有负面作用，另一方面，当卷取温度小于450℃时会形成较多的马氏体组织，虽然能增强材料的强度，但会对扩孔率产生不利影响，因此。本发明将卷取温度限定为450～580℃。
进一步地，可以通过热轧保温的方法，进一步提升该类钢种的抗拉强度，具体是：卷曲后将热卷放入保温坑中，利用热卷自身热量保温缓冷，在450℃以上保温2～4h，可以促进碳化钒的细小弥散析出，从而显著提高本发明材料的强度，同时并不会造成扩孔率和疲劳极限的明显下降。在热卷保温工艺中，最低保温温度和保温时间对最终产品性能有影响。其中保温温度若低于450℃，则碳化钒(钼)析出动力不足，无法形成细小弥散的碳化钒(钼)析出。若保温时间小于2h，则碳化钒(钼)析出有限，无法提高该类钢种的强度；而若保温时间大于4h，则碳化钒(钼)析出后长大、粗化，会显著降低该类钢种的扩孔率和疲劳极限。
汽车底盘、悬挂系统零部件对材料的首要需求是高强度及高扩孔性能，为了达到780MPa以上强度以及至少60％以上的扩孔性能(原始孔为冲孔)，目前通常采用铁素体或铁素体加贝氏体组织(其中贝氏体组织含量大于50％)的钢种。由于铁素体基体较软，通常需要加入较多的合金元素，形成固溶强化和细小的合金碳化物强化铁素体基体以得到较高的强度。在现有技术中，Ti元素均作为必选或可选的有益元素，用于提高这类钢种的强度，但是，Ti元素与钢中N元素会在高温下形成大块、脆性、具有尖锐边角的TiN颗粒，不利于该类钢种的扩孔性能。此外，随着汽车用底盘零件对钢材疲劳性能的要求愈来愈高，本发明研究证明：大块、脆性、具有尖锐边角的TiN颗粒更会成为潜在的疲劳裂纹源而大幅降低该类钢种的疲劳极限。而且研究发现由于TiN颗粒是在炼钢、连铸(或模铸)过程中生成，后续工艺几乎无法改变TiN颗粒的尺寸及形貌，更无法消除TiN颗粒，因而，为获得更高的扩孔性能和疲劳性能，该类钢种应尽可能降低Ti元素的含量。
因此，本发明采用无Ti元素的成分设计思路，不添加Ti元素并严格控制钢中的Ti含量，以减少TiN颗粒的生成，获得高疲劳极限；并通过Mo-V复合和制造工艺的优化来获得兼具高强度、高扩孔率和高疲劳极限的高强热轧钢板。该钢板的组织采用下贝氏体为主的贝氏体微观组织，以保证钢板的强度和韧性。本发明的钢 板的微观组织中，下贝氏体的组织含量(体积比)在30％-70％的范围内。当下贝氏体组织含量低于30％时，钢板强度无法达到设计要求；当下贝氏体组织含量高于70％时，将损害钢板的塑性和扩孔性能。在一些实施方案中，本发明钢板的微观组织中下贝氏体的组织含量为40％-70％。而通过添加合金元素Cr、Mo使得铁素体相变区右移，可以降低临界冷却速率，也易于得到下贝氏体组织。除贝氏体外，本发明钢板的微观组织中还可包括铁素体、碳化物析出物以及任选的回火马氏体。通过加入Mo、V、Nb合金元素细化晶粒，生成弥散细小的碳化物，以进一步提高钢种的强度。但碳化物析出过多后会进一步粗化，不仅不利于强度的进一步提高，更会降低钢材的扩孔性能和疲劳极限。因此，需要优化热轧工艺，以获得细小弥散分布的合金碳化物，达到提高扩孔性能的目的。在一些实施方案中，本发明钢板的微观组织中，下贝氏体与铁素体的组织含量之和≥80％，其中，下贝氏体的组织含量≥40％。
经检测，本发明提供的超高强热轧钢板和钢带性能满足如下指标：
常温力学性能：
抗拉强度≥780MPa；屈服强度≥660MPa。
扩孔率性能：
若原始孔为冲压孔：则扩孔率大于85％；
若原始孔为铰孔：则扩孔率大于120％。
抗疲劳性能：
高频疲劳极限(循环1000万次)FL≥570MPa；
或疲劳极限比抗拉强度FL/Rm≥0.72。
当钢成分中Ti≤0.005％，抗疲劳性能满足如下指标：
高频疲劳极限(循环1000万次)FL≥600MPa；
或疲劳极限比抗拉强度FL/Rm≥0.75。
当钢成分中Ti≤0.003％且N≤0.003％，抗疲劳性能满足如下指标：
高频疲劳极限(循环1000万次)FL≥640MPa；或
疲劳极限比抗拉强度FL/Rm≥0.8。
本发明制造的超高强热轧钢板和钢带同时兼备高强度、高扩孔性与高疲劳极限，所述超高强热轧钢板和钢带产品经热镀锌获得热轧热镀锌钢板成品，该超高强 热轧钢板产品和钢带产品及热镀锌钢板成品可用于制备汽车底盘、悬挂系统零部件，实现汽车“轻量化”。
附图说明
图1为本发明实施例G-1钢微观组织照片(放大1000倍)。
图2为对比例P钢微观组织中TiN颗粒形貌照片(放大1000倍)。
具体实施方式
下面结合实施例对本发明做进一步说明。
将表1中所示的不同成分的钢经冶炼，然后按表2所示进行加热+热轧工艺，得到厚度小于4mm的钢板。取沿横向JIS 5#拉伸试样测定屈服及抗拉强度，取板中部区域测定扩孔率和疲劳极限，疲劳极限的测定采用横向试样，试样尺寸与实验方法参照GB 3075-2008金属轴向疲劳试验方法；试验数据如表2所示。其中，扩孔率采用扩孔试验测定，用凸模把中心带孔的试件压入凹模，使试件中心孔扩大，直到板孔边缘出现颈缩或贯穿裂纹为止。由于试件中心原始孔的制备方式对扩孔率测试结果存在较大影响，因此，分别采用冲孔和铰孔制备试件中心原始孔，后续试验及测试方法按ISO/DIS 16630标准中规定的扩孔率测试方法执行。疲劳极限测定采用轴向高频拉伸疲劳试验测定，采用GB 3075-2008金属轴向疲劳试验方法，试验频率85Hz，取试样循环加载1000万次后未发生失效的最大强度作为其疲劳极限RL。
表1中，实施例A～H为本发明的钢，对比例J～P中碳或锰或其他合金元素含量超出本发明成分的范围。备注：表中M(all)指成分中(Cr/52)/(C/4)+(Nb/93+Ti/48+V/51+Mo/96)/(C/12)项的计算值。
由表1～3可知，当C、Mn等合金成分偏离本发明范围时，如C和Mn含量较低时，对比例J和对比例K钢的屈服强度小于660MPa，抗拉强度小于780MPa；而当C和Mn含量高出本发明的成分范围时，热轧态组织中含有大量的马氏体，会对钢的成形性能产生负面影响，扩孔性能变差，不符合本发明的目的，如对比例I和L的扩孔率均小于本发明。
而当Ti元素含量偏离本发明范围时，会对钢材的疲劳极限产生负面影响。如 比较例M、N、O、P。其中比较例M和P中Ti含量较高，使得钢材尽管达到了本发明设计的强度标准，但是疲劳极限远低于570MPa，疲劳极限比也远低于0.72的最低设计标准；而比较例N和O中，Ti含量较低，但仍然超过本发明的最低上限值，使得疲劳极限和疲劳极限比未达到本发明的要求。同时又由于这两组成分设计中，合金元素与碳元素的配比即M(all)未达到本发明设计的区间范围，导致两组材料扩孔性能未达标。
由表2～3可知，当卷终轧温度较低时，如表2中的对比钢A-2和F-1，扩孔率不满足本发明设计标准；当卷取温度大于550℃时则会产生珠光体组织和大量的碳化物析出，恶化了扩孔性能，如比较例F-2。另外，若采用保温缓冷技术，保温温度过低会抑制碳化物的析出，导致钢材强度不足，而若保温时间过长，则会生成大量粗大的碳化物，对扩孔率产生比例影响，如比较例F-3，G-3和H-3。
由图1可知，G-1钢中由于Ti元素含量被控制的极低，组织中无大块方形的TiN颗粒，碳化物析出主要是细小弥散的(Mo,V)C。而如图2所示，对比例P钢，由于采用Ti元素增强的设计思路，组织中常见大块方形的TiN颗粒，且颗粒边界具有尖锐边角。此外，本发明钢中存在Mo、V复合的碳化物析出相所形成的细小弥散析出分布(如图1所示)，而对比例P钢中基体中的TiC析出相(基体中的黑灰色团状、圆形析出)尺寸更粗大，分布也不够均匀弥散(如图2所示)，因此降低材料的扩孔性能。
综上所述，本发明在碳锰钢的基础上，控制合理的成分范围，并添加微合金元素，限制Ti元素的含量，在常规的汽车用钢生产线基础上，进一步控制卷取温度，并可进一步通过采用保温环冷技术，生产出兼具良好扩孔性能与疲劳性能的超高强热轧钢板和钢带，其屈服强度Rp0.2≥660MPa，抗拉强度Rm≥780MPa，扩孔率≥85％(原始孔为冲孔)，扩孔率≥120％(原始孔为铰孔)，高频疲劳极限强度RL≥570MPa，或拉伸疲劳极限比RL/Rm≥0.72，适合用于制造汽车底盘、悬挂件等产品。
表1(单位：重量百分比)
  C Si Mn P N Al S Nb Ti V Cr Mo M(all)
实施例A 0.09 0.35 1.75 0.011 0.005 0.031 0.003 0.055 0.018 0.10 0.45 0.16 1.00
实施例B 0.07 0.24 1.87 0.011 0.004 0.027 0.003 0.030 0.015 0.20 0.35 0.21 1.54
实施例C 0.14 0.40 1.57 0.010 0.004 0.036 0.004 0.045 0.016 0.33 0.42 0.18 1.02
实施例D 0.07 0.28 1.59 0.010 0.005 0.034 0.003 0.025 0.009 0.15 0.44 0.19 1.41
实施例E 0.11 0.40 1.63 0.010 0.005 0.031 0.003 0.030 0.005 0.13 0.50 0.41 1.14
实施例F 0.09 0.15 1.55 0.010 0.003 0.036 0.003 0.025 0.004 0.27 0.46 0.27 1.52
实施例G 0.07 0.20 1.62 0.010 0.002 0.024 0.002 0.020 0.003 0.21 0.37 0.15 1.43
实施例H 0.09 0.29 1.55 0.011 0.004 0.026 0.002 0.015 0.005 0.16 0.39 0.20 1.06
比较例I 0.15 0.25 1.82 0.012 0.005 0.030 0.004 0.048 0.020 0.10 0.50 0.17 0.63
比较例J 0.057 0.39 1.64 0.014 0.004 0.018 0.004 0.034 0.014 0.11 0.34 0.16 1.40
比较例K 0.08 0.40 1.47 0.012 0.005 0.021 0.003 0.014 0.018 0.10 0.37 0.17 0.99
比较例L 0.08 0.38 2.20 0.016 0.004 0.014 0.002 0.026 0.019 0.16 0.50 0.16 1.30
比较例M 0.07 0.24 1.87 0.011 0.004 0.027 0.003 0.030 0.075   0.35   0.71
比较例N 0.08 0.30 1.57 0.010 0.005 0.036 0.003 0.046 0.027 0.25 0.45 0.30 1.80
比较例O 0.14 0.40 1.57 0.010 0.005 0.036 0.004 0.025 0.025 0.15 0.42 0.18 0.71
比较例P 0.10 0.35 1.90 0.010 0.004 0.038 0.004 0.030 0.12 0.15 0.44 0.24 1.33
表2
Figure PCTCN2019092766-appb-000001
Figure PCTCN2019092766-appb-000002
表3
Figure PCTCN2019092766-appb-000003
Figure PCTCN2019092766-appb-000004

Claims (15)

  1. 具有良好的疲劳及扩孔性能的超高强热轧钢板和钢带，其成分及其重量百分比为：C：0.07～0.14％，Si：0.1～0.4％，Mn：1.55～2.00％，P≤0.015％，S≤0.004％，Al：0.01～0.05％，N≤0.005％，Cr：0.15～0.50％，V：0.1～0.35％，Nb：0.01％～0.06％，Mo：0.15～0.50％，且Ti≤0.02％，其余为Fe和不可避免的杂质；且上述元素同时需满足如下关系：1.0≤[(Cr/52)/(C/4)+(Nb/93+Ti/48+V/51+Mo/96)/(C/12)]≤1.6。
  2. 如权利要求1所述的具有良好的疲劳及扩孔性能的超高强热轧钢板和钢带，其特征在于，所述超高强热轧钢板和钢带的化学成分中C：0.07～0.09％，以重量百分比计。
  3. 如权利要求1所述的具有良好的疲劳及扩孔性能的超高强热轧钢板和钢带，其特征在于，所述超高强热轧钢板和钢带的化学成分中Si：0.1～0.3％，以重量百分比计。
  4. 如权利要求1所述的具有良好的疲劳及扩孔性能的超高强热轧钢板和钢带，其特征在于，所述超高强热轧钢板和钢带的化学成分中Mn：1.70～1.90％，以重量百分比计。
  5. 如权利要求1所述的具有良好的疲劳及扩孔性能的超高强热轧钢板和钢带，其特征在于，所述超高强热轧钢板和钢带的化学成分中Cr：0.35～0.50％，以重量百分比计。
  6. 如权利要求1所述的具有良好的疲劳及扩孔性能的超高强热轧钢板和钢带，其特征在于，所述超高强热轧钢板和钢带的化学成分中V：0.12～0.22％，以重量百分比计。
  7. 如权利要求1所述的具有良好的疲劳及扩孔性能的超高强热轧钢板和钢带，其特征在于，所述超高强热轧钢板和钢带的化学成分中Mo：0.15～0.3％，以重量百分比计。
  8. 如权利要求1所述的具有良好的疲劳及扩孔性能的超高强热轧钢板和钢带，其特征在于，所述超高强热轧钢板和钢带的化学成分中Ti≤0.005％，以重量百分比计。
  9. 如权利要求1所述的具有良好的疲劳及扩孔性能的超高强热轧钢板和钢带， 其特征在于，所述超高强热轧钢板和钢带的化学成分中Ti≤0.003％，N≤0.003％，以重量百分比计。
  10. 如权利要求1-9任一项所述的具有良好的疲劳及扩孔性能的超高强热轧钢板和钢带，其特征在于，所述超高强热轧钢板和钢带的微观组织为中，下贝氏体含量占30％～70％。
  11. 如权利要求1-10任一项所述的具有良好的疲劳及扩孔性能的超高强热轧钢板和钢带，其特征在于，所述超高强热轧钢板和钢带的抗拉强度≥780MPa、屈服强度≥660MPa，扩孔率性能指标：若原始孔为冲压孔：扩孔率＞85％；若原始孔为铰孔：扩孔率＞120％；抗疲劳性能指标：高频疲劳极限(循环1000万次)FL≥570MPa，或疲劳极限比抗拉强度FL/Rm≥0.72。
  12. 如权利要求1或8或10所述的具有良好的疲劳及扩孔性能的超高强热轧钢板和钢带，其特征在于，所述超高强热轧钢板和钢带的抗拉强度≥780MPa、屈服强度≥660MPa，扩孔率性能指标：若原始孔为冲压孔：扩孔率＞85％；若原始孔为铰孔：扩孔率＞120％；抗疲劳性能指标：高频疲劳极限(循环1000万次)FL≥600MPa，或疲劳极限比抗拉强度FL/Rm≥0.75。
  13. 如权利要求1或9或10任一项所述的具有良好的疲劳及扩孔性能的超高强热轧钢板和钢带，其特征在于，所述超高强热轧钢板和钢带的抗疲劳性能指标：高频疲劳极限(循环1000万次)FL≥640MPa，或疲劳极限比抗拉强度FL/Rm≥0.8。
  14. 如权利要求1-13任一项所述的具有良好的疲劳及扩孔性能的超高强热轧钢板和钢带的制造方法，包括如下步骤：
    1)冶炼、铸造
    按权利要求1-9任一项所述的化学成分冶炼和铸造；
    2)轧制
    加热温度为1100～1250℃；精轧开轧温度为950～1000℃，精轧终轧温度为900～950℃；
    3)冷却、卷取
    冷却速度≥30℃/s，卷取温度为450～580℃；
    4)酸洗。
  15. 如权利要求14所述的具有良好的疲劳及扩孔性能的超高强热轧钢板和钢带 的制造方法，其特征在于，在步骤3)轧后冷却、卷取后，还包括保温缓冷：控制在450℃以上保温2～4h。
PCT/CN2019/092766 2018-06-27 2019-06-25 具有良好的疲劳及扩孔性能的超高强热轧钢板和钢带及其制造方法 WO2020001430A1 (zh)

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