WO2022063244A1 - 一种700MPa级热成型桥壳钢及其制备方法 - Google Patents

一种700MPa级热成型桥壳钢及其制备方法 Download PDF

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WO2022063244A1
WO2022063244A1 PCT/CN2021/120417 CN2021120417W WO2022063244A1 WO 2022063244 A1 WO2022063244 A1 WO 2022063244A1 CN 2021120417 W CN2021120417 W CN 2021120417W WO 2022063244 A1 WO2022063244 A1 WO 2022063244A1
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Prior art keywords
axle housing
hot
housing steel
700mpa
formed axle
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PCT/CN2021/120417
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English (en)
French (fr)
Inventor
惠亚军
刘锟
朱国森
吴科敏
李秋寒
韩赟
牛涛
田志红
陈斌
肖宝亮
李飞
王松涛
周娜
王全礼
李晓林
黄天华
潘辉
张大伟
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首钢集团有限公司
北京首钢股份有限公司
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Priority to JP2023519297A priority Critical patent/JP7539567B2/ja
Priority to AU2021348595A priority patent/AU2021348595B2/en
Priority to DE112021005074.5T priority patent/DE112021005074T5/de
Publication of WO2022063244A1 publication Critical patent/WO2022063244A1/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
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
    • B21C—MANUFACTURE OF METAL SHEETS, WIRE, RODS, TUBES OR PROFILES, OTHERWISE THAN BY ROLLING; AUXILIARY OPERATIONS USED IN CONNECTION WITH METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL
    • B21C37/00—Manufacture of metal sheets, bars, wire, tubes or like semi-manufactured products, not otherwise provided for; Manufacture of tubes of special shape
    • B21C37/02—Manufacture of metal sheets, bars, wire, tubes or like semi-manufactured products, not otherwise provided for; Manufacture of tubes of special shape of sheets
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C33/00—Making ferrous alloys
    • C22C33/04—Making ferrous alloys by melting
    • 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
    • C22C38/12—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
    • 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
    • C22C38/32—Ferrous alloys, e.g. steel alloys containing chromium with boron
    • 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
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02T—CLIMATE CHANGE MITIGATION TECHNOLOGIES RELATED TO TRANSPORTATION
    • Y02T10/00—Road transport of goods or passengers
    • Y02T10/80—Technologies aiming to reduce greenhouse gasses emissions common to all road transportation technologies
    • Y02T10/86—Optimisation of rolling resistance, e.g. weight reduction 

Definitions

  • the present disclosure relates to the technical field of steel preparation, in particular to a 700MPa-grade hot-formed axle housing steel and a preparation method thereof.
  • Automobile axle housings used to use hot-rolled sheet substitutes such as 610L, Q460, etc.
  • the substitutes have the problem that the material strength is significantly reduced after hot-stamping, which is manifested by the obvious grain size of the sheet after hot-stamping. Coarsening, the fine-grain strengthening effect of the plate disappears, and the automobile axle housing products cannot meet the conditions of use.
  • the special hot-formed axle housing steel designed and developed according to the market demand is mainly based on precipitation strengthening (V(C,N)), but in the on-site production process of the automobile axle housing, the heating process of the axle housing steel sheet (such as: heating temperature and heating and holding time) and the cooling conditions of the axle housing after hot stamping (such as air cooling rate) change greatly, the precipitation amount of V(C,N) precipitates, the precipitation position and the size of the precipitation fluctuate greatly.
  • hot stamping The mechanical properties of the axle housing steel cannot be stably controlled, and the cast slab of the axle housing steel containing V is easy to form transverse cracks on the surface, resulting in abnormal structure at the edge of the plate.
  • the purpose of the present disclosure is to provide a 700MPa-grade hot-formed axle housing steel and a preparation method thereof.
  • the chemical composition is optimized and the process of the present disclosure is adopted.
  • the elongation after fracture is A50 ⁇ 17%, the longitudinal impact energy Akv ⁇ 47 at 0°C, and the plate has no obvious micro-band structure after heat treatment, and has excellent impact toughness.
  • a first aspect of the present disclosure provides a 700MPa-grade hot-formed axle housing steel.
  • the chemical components of the 700-MPa-grade hot-formed axle housing steel are: C: 0.17%-0.27%, Si: 0.30%- 0.50%, Mn: 1.2% to 1.8%, P: ⁇ 0.02%, S: ⁇ 0.009%, Ti: ⁇ 0.1%, Cr: ⁇ 0.2%, the rest are Fe and inevitable impurities; and the Si and Mn The mass fraction of satisfies: 0.23 ⁇ Si/Mn ⁇ 0.27.
  • the chemical composition of the 700MPa-grade hot-formed axle housing steel in terms of mass fraction, is: C: 0.17%-0.27%, Si: 0.30%-0.50%, Mn: 1.2%-1.8%, P: ⁇ 0.02%, S: ⁇ 0.009%, Ti: ⁇ 0.1%, Cr: ⁇ 0.2%,
  • the rest are Fe and inevitable impurities, and the mass fractions of Si and Mn satisfy: 0.23 ⁇ Si/Mn ⁇ 0.27.
  • the chemical composition of the 700MPa-grade hot-formed axle housing steel in terms of mass fraction, is: C: 0.17%-0.26%, Si: 0.30%-0.50%, Mn: 1.2%-1.6%, P: ⁇ 0.02%, S: ⁇ 0.009%, Ti: ⁇ 0.1%, Cr: ⁇ 0.2%, and the rest are Fe and inevitable impurities; and the mass fractions of Si and Mn satisfy: 0.23 ⁇ Si/Mn ⁇ 0.27.
  • the chemical composition of the 700MPa-grade hot-formed axle housing steel in terms of mass fraction, is: C: 0.17%-0.26%, Si: 0.30%-0.50%, Mn: 1.2%-1.6%, P: ⁇ 0.02%, S: ⁇ 0.009%, Ti: ⁇ 0.1%, Cr: ⁇ 0.2%,
  • the rest are Fe and inevitable impurities, and the mass fractions of Si and Mn satisfy: 0.23 ⁇ Si/Mn ⁇ 0.27.
  • At least part of the Nb and/or Mo content is replaced by B element.
  • the metallographic structure of the 700MPa-grade hot-formed axle housing steel in terms of volume percentage, is: 60%-70% ferrite and 30%-40% flake pearlite;
  • the grain size is 5 ⁇ m ⁇ 10 ⁇ m, and the cementite size of the flaky pearlite is 8 ⁇ m ⁇ 15 ⁇ m.
  • the 700MPa-grade hot-formed axle housing steel is heat-treated at the temperature of the hot-forming process, and the microscopic metallographic structure after heat-treatment is calculated as: granular pearlite contains 65%-75% % ferrite and 25%-35% granular cementite; the grain size of the ferrite is 7-12 ⁇ m, and the size of the granular cementite is 3 ⁇ m-10 ⁇ m.
  • a second aspect of the present disclosure provides a method for preparing the 700MPa-grade hot-formed axle housing steel, the preparation method comprising: smelting and continuous casting using the chemical composition of the 700-MPa-grade hot-forming axle casing steel to obtain a plate billet; heating the slab, rough rolling and finish rolling to obtain a hot-rolled sheet; in some embodiments, the heating temperature is 1180°C to 1220°C, and the heating time is 150min to 200min;
  • the hot-rolled sheet is cooled and coiled to obtain a hot-rolled coil; and the hot-rolled coil is processed to obtain a 700 MPa-grade hot-formed axle housing steel.
  • the finishing temperature of rough rolling is 1000°C to 1040°C; during the finish rolling, the finish temperature of finish rolling is 840°C to 870°C.
  • the rolling speed is 4 m/s ⁇ 6 m/s; the thickness of the hot-rolled sheet is 7 mm ⁇ 18 mm.
  • the crimping temperature is 560°C to 620°C.
  • the processing includes uncoiling, leveling and shearing. During the processing, when the length of the steel plate is 10 m, the roughness measured in the diagonal direction is ⁇ 5 mm.
  • the heating temperature of the slab is set at 1180 ° C ⁇ 1220 ° C, and the heating time is 150 min ⁇ 200 min to ensure the homogenization of the C element and prevent the decarburization of the surface layer of the slab, and the mechanical properties of the plate before and after heat treatment.
  • the properties all meet the requirements of yield strength ⁇ 600MPa, tensile strength ⁇ 700MPa, elongation after fracture A50 ⁇ 17%, 0 °C longitudinal impact energy Akv ⁇ 47, the plate has no obvious micro-band structure after heat treatment, with excellent impact toughness and Fatigue performance.
  • Example 1 is a microstructure diagram of a quarter of the thickness of a 700MPa-grade hot-formed axle housing steel plate provided in Example 4 of the present disclosure
  • Example 2 is a microstructure diagram of the 700MPa grade hot-formed axle housing steel plate provided in Example 4 of the present disclosure after heat treatment; wherein (A) is the structure at the edge, (B) is the structure at a quarter, (C) ) is the tissue of the heart;
  • FIG. 3 is a schematic view of the microstructure at a quarter of the thickness of the disclosed comparative example 2;
  • FIG. 6 is a microstructure diagram of the disclosed comparative example 6 hot-pressed axle housing steel after heat treatment
  • FIG. 7 is a flow chart of a method of making a 700 MPa grade hot-formed axle housing steel according to one or more embodiments of the present disclosure.
  • a 700MPa-grade hot-formed axle housing steel The chemical composition of the 700-MPa-grade hot-formed axle housing steel is: C: 0.17%-0.26%, Si: 0.30%-0.50%, Mn: 1.2%- 1.6%, P: ⁇ 0.02%, S: ⁇ 0.009%, Ti: ⁇ 0.1%, Cr: ⁇ 0.2%, and the rest are Fe and inevitable impurities; and the mass fraction of Si and Mn satisfies: 0.23 ⁇ Si /Mn ⁇ 0.27.
  • the present disclosure forms the 700MPa-grade hot-formed axle housing steel with the above chemical composition by optimizing the constituent elements, and is based on the following principles:
  • C 0.17% to 0.26%
  • C is the main element that determines the room temperature microstructure and mechanical properties of carbon steel.
  • the tensile strength increases with the increase of C content.
  • Properly increasing the C content is conducive to obtaining pearlite structure .
  • high C accelerates the formation of austenite structure.
  • cementite precipitates at the points of undissolved carbides and the enriched area of solid solution C elements, forming toughness. Excellent granular pearlite structure.
  • Si 0.30% to 0.50%, the increase of Si content will improve the strength of ferrite matrix and promote the formation of ferrite structure.
  • Mn 1.2% to 1.6%, adding more than 1.2% Mn improves the strength of the matrix, and refines the pearlite lamella spacing. If the content is too high, it will affect the weldability of the material.
  • the Mn element segregates during the continuous casting process, causing the hot-rolled sheet. Produces a micro-banded structure and reduces the impact performance of the sheet.
  • P and S are very easy to segregate during the continuous casting process, which will adversely affect the material properties.
  • the content of P and S in the steel should be strictly controlled.
  • Ti ⁇ 0.1%, adding an appropriate amount of Ti element, TiN inhibits the excessive growth of austenite grains in the slab heating furnace, and the Ti4C2S2 formed with S and C reduces the plate caused by the strip MnS distributed along the rolling direction. Differences in transverse and longitudinal mechanical properties.
  • the mass fractions of Si and Mn satisfy: 0.23 ⁇ Si/Mn ⁇ 0.27. Since Si, Mn, P and S are all elements that are easy to segregate, P and S can be removed during the refining process . During the slab continuous casting process, Si and Mn elements will segregate between the columnar crystals, and it is extremely difficult for the slab to be kept in the heating furnace for a long time to make the element distribution uniform and inherited from the hot-rolled finished plate. In some embodiments Among them, Si is an element that closes the austenite region, which will increase the A3 temperature point of austenite to ferrite transformation.
  • Mn and Si have opposite effects on the A3 temperature point, and the segregation of elements between dendritic dendrites affects the The phase transformation structure distribution of the sheet after hot rolling, resulting in a micro-banded structure.
  • the segregation coefficient and element atomic weight of Si and Mn elements need to be comprehensively considered, so the mass content percentage of Si and Mn elements is designed in the present disclosure to be 0.23 ⁇ Si%/Mn% ⁇ 0.27; slab solidification
  • the segregation of alloying elements in the process forms the original band-like structure (the dendrite branches and the interstitial space are distributed in the form of bands), and the formation of the original band-like structure is unavoidable, and the formation of the micro-band-like structure must be in the original.
  • the specific conditions of the solid-state phase transition determine whether the microscopic band-like structure will be formed.
  • 0.23 ⁇ Si%/Mn% ⁇ 0.27 the effects of Si and Mn segregation in the dendrite gap on the A3 temperature point can cancel each other out, and the A3 temperature difference between the two regions of the dendrite dendrite and the dendrite is small, which can avoid microscopic banding.
  • the formation of microstructure; the value of Si%/Mn% is less than 0.23 or higher than 0.27, it is possible to form micro-banded structure.
  • the chemical composition of the 700MPa-grade hot-formed axle housing steel in terms of mass fraction is: C: 0.17%-0.26%, Si: 0.30%-0.50%, Mn: 1.2%-1.6% , P: ⁇ 0.02%, S: ⁇ 0.009%, Ti: ⁇ 0.1%, Cr: ⁇ 0.2%,
  • the rest are Fe and inevitable impurities, and the mass fractions of Si and Mn satisfy: 0.23 ⁇ Si/Mn ⁇ 0.27.
  • a small amount of Nb element can be selected to be added to play the role of controlled rolling and grain refinement strengthening, and the optimal value of the Nb element content is within 0.03%.
  • an appropriate amount of Mo element can be selected to be added, and the addition of Mo element can play a solid solution strengthening effect, and is beneficial to reduce the segregation of Mn, P and S.
  • an appropriate amount of B element can be added first, and the trace amount of B element segregated at the austenite grain boundary can significantly improve the stability of the supercooled austenite structure and inhibit the segregation of Mn, P, and S elements.
  • the trace amount of B element segregated at the austenite grain boundary can significantly improve the stability of the supercooled austenite structure and inhibit the segregation of Mn, P, and S elements.
  • the optimal value of B element content is controlled within 0.0015% .
  • At least part of the content of Nb and/or Mo is replaced by B element.
  • the metallographic structure of the 700MPa-grade hot-formed axle housing steel is calculated in volume percentage as: 60%-70% ferrite and 30%-40% flake pearlite; the iron
  • the grain size of the element body is 5 ⁇ m ⁇ 10 ⁇ m, and the cementite size of the flaky pearlite is 8 ⁇ m ⁇ 15 ⁇ m.
  • the grain size is refined to ensure that the strength level of the hot-rolled sheet can reach 700MPa.
  • the sheet of the 700MPa-grade hot-formed axle housing steel is heat-treated with reference to the temperature regime of the thermoforming process, and the microscopic metallographic structure of the 700-MPa-grade hot-formed axle housing steel sheet after heat treatment is: granular pearlite.
  • granular pearlite contains 65%-75% ferrite + 25%-35% granular cementite, the grain size of ferrite is 7-12 ⁇ m, and the size of cementite is 3-10 ⁇ m; heat treatment Then, the ferrite structure is distributed with fine granular cementite particles, which ensures that the axle housing steel has a tensile strength of 700MPa and improves the impact toughness of the product; so that the mechanical properties of the plate before and after heat treatment meet the yield strength. ⁇ 600MPa, tensile strength ⁇ 700MPa, elongation after fracture A50 ⁇ 17%, 0 °C longitudinal impact energy Akv ⁇ 47, after heat treatment, the plate has no obvious micro-band structure and has excellent impact toughness.
  • a method for preparing the 700MPa-grade hot-formed axle housing steel is provided. As shown in FIG. 5 , the preparation method includes:
  • the preparation method of the 700MPa-grade hot-formed axle housing steel is based on the following principles:
  • the heating temperature of the slab is set at 1180°C ⁇ 1220°C, and the heating time is 150min ⁇ 200min, so as to ensure the homogenization of the C element and prevent the decarburization of the surface layer of the slab. If the heating temperature is less than 1180°C and the heating time is less than 150min, the C element cannot be guaranteed to be homogenized inside the slab, and the precipitation phase cannot be redissolved; The roughening of the tensite structure affects the properties of the hot-rolled sheet.
  • the above ratio is carried out in terms of components, and the mass fraction of Si and Mn satisfies: 0.23 ⁇ Si/Mn ⁇ 0.27, and the above process is adopted; the microscopic metallographic structure of the hot-rolled sheet is ferrite, and the temperature ( The heating temperature is 1180°C ⁇ 1220°C, and the heating time is 150min ⁇ 200m) to heat treat the plate.
  • the microstructure of the plate after heat treatment is ferrite + granular pearlite.
  • the mechanical properties of the plate meet the yield strength ⁇ 600MPa, tensile strength ⁇ 700MPa, elongation after fracture A50 ⁇ 17%, 0 °C longitudinal impact energy Akv ⁇ 47, the plate has no obvious micro-band structure after heat treatment, and has excellent impact toughness.
  • the rough rolling termination temperature is 1000°C to 1040°C; in the finish rolling, the finish rolling termination temperature is 840°C to 870°C.
  • finishing temperature of rough rolling is controlled to be 1000°C ⁇ 1040°C, and the finishing temperature of finishing rolling is 840°C ⁇ 870°C is: finish rolling at low temperature in the temperature range of austenite unrecrystallized zone, and avoid austenitic iron. Rolling in the two-phase region of the ferrite; make the austenite fully accumulate the deformation to improve the nucleation rate of the ferrite.
  • the rolling speed is both 4 m/s to 6 m/s; the thickness of the hot rolled sheet is 7 mm to 18 mm.
  • the rolling speed is adjusted from 4 m/s to 6 m/s according to the thickness specification, so that the thickness specification variation range of the hot-rolled sheet is controlled within 7 mm to 18 mm.
  • the crimping temperature ranges from 560°C to 620°C. This is beneficial to obtain pearlite structure in the pearlite temperature transition range.
  • the present disclosure adopts the C-Si-Mn-Cr-Ti composition system, which is not seen in the axle housing steel market), uses C as the main alloying element, and controls 0.23 ⁇ Si%/Mn% ⁇ 0.27, combined with the process of the present disclosure, the microscopic metallographic structure of the hot-rolled sheet is ferrite (volume fraction of 60%-71%, grain size of 5-10 ⁇ m) + flaky pearlite (volume fraction of 29-40 %, the grain size is 8-15 ⁇ m), referring to the temperature regime of the hot forming process, heat treatment is performed on the sheet of the 700MPa-grade hot-formed axle housing steel, and the micro-gold of the 700-MPa-grade hot-formed axle housing steel sheet after heat treatment is The phase structure is: granular pearlite.
  • granular pearlite contains 65% ⁇ 75% ferrite + 25% ⁇ 35% granular cementite, the grain size of ferrite is 4 ⁇ 12 ⁇ m, and the size of cementite is 3 ⁇ 10 ⁇ m; heat treatment Before and after, the mechanical properties of the plate meet the yield strength ⁇ 600MPa, the tensile strength ⁇ 700MPa, the elongation after fracture A50 ⁇ 17%, the longitudinal impact energy Akv ⁇ 47 at 0 °C, the plate has no obvious micro-band structure after heat treatment, with excellent impact toughness and fatigue life.
  • a 700MPa-grade hot-formed axle housing steel of the present disclosure and a preparation method thereof will be described in detail below with reference to the embodiments, comparative examples and experimental data.
  • Example 1-Example 8 and Comparative Examples 1-3 respectively adopt the chemical compositions shown in Table 1, according to the alloy composition distribution ratio designed in Table 1, smelt at a temperature of 1630 ° C, and then forge into a billet;
  • the heating temperature of the slab is 1180°C to 1220°C
  • a hot-rolled sheet is obtained through rough rolling and finish rolling, and the thickness specification variation range of the hot-rolled sheet is controlled at 7mm ⁇ 10mm
  • the end temperature of the rough rolling is controlled to be 1000°C to 1040°C
  • 7 passes of finishing rolling are performed, and the end temperature of the finish rolling is controlled to be 840°C to 870°C;
  • the examples and comparative examples were taken for heat treatment experiments, induction heating to 810-830 ° C, holding time 180-240 s, moved into a cold mold, kept for 15 s, and then air-cooled, the three mechanical properties test results were recorded. into Table 3.
  • the phase size and volume fraction data of the microstructure were measured by observation at one quarter of the thickness of the plate.
  • Comparative Example 3 when the heating temperature is less than 1180°C, the homogenization of C element in the slab cannot be guaranteed, and the precipitate phase cannot be redissolved, so there is a disadvantage that the strength level is lower than 700MPa, and the plate has a micro-banded structure.
  • the heating temperature is higher than 1220 °C, which will lead to severe decarburization of the slab, coarsening of the austenite structure, and affect the properties of the hot-rolled sheet.
  • the composition design adopts the process route of medium carbon-medium manganese-high Si, and the Si/Mn ratio is 0.44.
  • the steel for hot stamping axle housing of 800MPa grade is obtained by low-temperature coiling with staged cooling. It is ferrite + fine lower bainite, the proportion of ferrite is 5-10%, the actual yield strength reaches 856MPa, and the tensile strength reaches 981MPa; because the axle housing needs to be sheared before hot stamping.
  • the thickness of the axle housing is generally more than 10mm, even 14mm, 16mm, 18mm and other thickness specifications, the strength is difficult to cut;
  • the Si/Mn ratio is as high as 0.44, there is an obvious banded structure, which is unfavorable to the fatigue performance; in addition, there are many parts that need to be welded in the hot-pressed axle housing, and the lower bainite will appear serious in the heat-affected zone after welding.
  • the softening problem leads to a large difference in the hardness of different positions of the axle housing, which affects the welding fatigue performance.
  • the yield strength of the axle housing after hot stamping is only 500MPa.
  • Comparative Example 6 the C-Mn-Nb-V-Ti-Mo composite microalloying composition system is used, and the mechanical properties of the axle housing before and after hot stamping are improved by precipitation strengthening.
  • the ratio has the following disadvantages: the amount of alloy added is large, and the cost is high; due to the addition of V element, the precipitation temperature is just in the continuous casting bending and straightening zone, which is easy to cause subcutaneous cracks on the surface of the continuous casting billet, which will be damaged in the subsequent hot rolling or axle housing hot stamping.
  • the surface chapped defect occurs in the middle, which affects the fatigue performance of the axle housing; in addition, the heating temperature is high, which is easy to cause the surface decarburization, the surface structure is coarse, and the fatigue performance of the material is affected; in addition, the Si/Mn ratio is only 0.055, and there is a serious banding. shape structure, affecting fatigue properties.
  • Comparative Example 7 the C-Mn-V microalloying composition system is adopted, and the hot stamping axle housing steel with excellent comprehensive performance is obtained through the process of low temperature unheating, low temperature rolling, and low temperature coiling.
  • the 700MPa-grade hot-formed axle housing steel finally prepared in Examples 1-8 of the present disclosure, the mechanical properties of the plates before and after heat treatment satisfy the yield strength ⁇ 600MPa, the tensile strength ⁇ 700MPa, the elongation after fracture A50 ⁇ 17%, the longitudinal direction at 0°C
  • the impact energy Akv ⁇ 47 the plate has no obvious micro-band structure after heat treatment, and has excellent impact toughness and fatigue life.
  • Example 8 of the present disclosure is the best example.
  • elements such as Cr, Mo, Nb, Ti, B and other elements on the basis of C-Si-Mn elements
  • the microstructure content and size can obtain more excellent mechanical properties and fatigue properties. It has the following advantages: (1) In Example 8, due to the compound addition of elements such as Nb-Ti-Mo, the rolling speed is increased by adopting a lower tapping temperature and reducing the temperature drop in the finishing rolling zone. The pinning effect can obtain a uniform and fine original austenite structure, and then obtain a uniform and fine ferrite structure.
  • the austenite Due to the increase of the rolling speed and the addition of Cr, Mo, B and other elements to improve the hardenability, the austenite is improved.
  • the kinetics of transformation to ferrite further promotes the uniform refinement of ferrite and reduces the size of pearlite pellets, and at the same time, by controlling the higher coiling temperature, the low temperature transformation structure is avoided, which is beneficial to obtain excellent thermal Strip steel can be rolled, and good shape quality can be ensured, which is beneficial to cutting and blanking in subsequent production of axle housings and ensuring the quality of cutting and blanking.
  • the hot-rolled steel strip of the axle housing needs to be heated to the austenite region for heat preservation.
  • the corresponding TiNbCN precipitates can hinder the heating process.
  • the recrystallization and coarsening of austenite grains improve the precondition for obtaining the final uniform and fine ferrite structure; during the hot stamping process, due to the occurrence of deformation, the precipitation of TiNbCN, TiNbC and other second phases is promoted, and the structure
  • the recovery of the flattened austenite can further ensure the final uniform and fine ferrite to provide guarantee conditions; generally, the axle housing is air-cooled to room temperature after the stamping is completed. Due to the slow cooling rate, coarse ferrite and pearlescent will be formed.
  • Fig. 1 is the microstructure at a quarter of the thickness of the plate in Example 4 of the disclosure.
  • the microstructure of the plate is: ferrite (volume fraction is 63%, average grain size is 7 ⁇ m) + flaky pearlite structure (volume fraction is 7 ⁇ m) The fraction was 37% and the grain size was 12 ⁇ m).
  • Figure 2 shows the microstructure of Example 4 after heat treatment.
  • the structures at the edge, quarter and core are all granular pearlite.
  • the volume fraction of ferrite is 68% to 73%
  • the grain size is 7 ⁇ m to 10 ⁇ m
  • the volume fraction of granular cementite is 27% to 32%
  • the size is 3 ⁇ m to 8 ⁇ m.
  • FIG 3 is a schematic view of the microstructure at a quarter of the thickness of the plate of Comparative Example 2 of the present disclosure.
  • the microstructure of the plate is: ferrite (volume fraction of 70%, average grain size of 11 ⁇ m) + flaky pearlite structure (volume fraction of 30%, average grain size of 6 ⁇ m).
  • Fig. 4 shows the microstructure of Comparative Example 2 of the present disclosure after heat treatment.
  • the microstructures of the edge, quarter and core are all ferrite (volume fraction is 70%-85%, crystallinity
  • the grain size is 7 ⁇ m ⁇ 13 ⁇ m) + lamellar pearlite (volume fraction is 15% ⁇ 30%, grain size is 6 ⁇ m ⁇ 12 ⁇ m), and there is an obvious microscopic band structure.
  • FIG. 5 is the microstructure of the hot stamped axle housing steel of Comparative Example 5 of the disclosure, the structure is a small amount of ferrite + lower bainite structure, and the banded structure is obvious.
  • FIG 6 shows the microstructure of the hot stamped axle housing steel of Comparative Example 6 of the present disclosure after hot stamping.
  • the 700MPa-grade hot-formed axle housing steel according to one or more embodiments of the present disclosure has a more uniform structure throughout the thickness of the plate after heat treatment, has no microscopic band-like structure, and has more excellent impact Toughness and fatigue properties.

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Abstract

本公开内容公开了一种700MPa级热成型桥壳钢，化学成分质量分数为：C：0.17％～0.27％，Si：0.30％～0.50％，Mn：1.2％～1.8％，P：≤0.02％，S：≤0.009％，Ti：≤0.1％，Cr：≤0.2％，其余为Fe及不可避免的杂质；且所述Si和Mn的质量分数满足：0.23≤Si/Mn≤0.27。本公开内容还公开了制备方法：采用所述的化学成分熔炼、连铸、加热，粗轧和精轧、卷曲和加工处理，加热温度为1180℃～1220℃，加热时间为150min～200min，获得700MPa级热成型桥壳钢，在热处理前后的力学性能均优良，在热处理后板材无明显的显微带状组织，具备优异的冲击韧性。

Description

一种700MPa级热成型桥壳钢及其制备方法
相关申请的交叉引用
本申请要求于2020年9月28日提交、申请号为202011041281.7且名称为“一种700MPa级热成型桥壳钢及其制备方法”的中国专利申请的优先权，其全部内容通过引用合并于此。
技术领域
本公开涉及钢材制备技术领域，特别涉及一种700MPa级热成型桥壳钢及其制备方法。
背景技术
桥壳作为主要支承汽车荷重的结构，其轻量化研究对中重型货车的整车轻量化具有重要意义。目前，我国汽车行业已广泛使用热轧桥壳钢板经过热冲压加上焊接生产桥壳体，取代制作工艺复杂、生产效率偏低、成本较高的铸造桥壳体。
汽车桥壳曾经使用过热轧板材替代品(如610L,Q460等)来进行批量生产，但替代品存在着热冲压成型之后材料强度明显降低的问题，表现为板材在热冲压成型之后晶粒明显粗化，板材细晶强化作用消失，汽车桥壳产品无法满足使用条件。因此，针对市场需求设计开发的专用热成型桥壳钢以析出强化为主(V(C,N))，但在汽车桥壳现场生产过程中，桥壳钢板材的加热工艺(如：加热温度与加热保温时间)以及热冲压后桥壳的冷却条件(如：空冷冷速)变化很大，V(C,N)析出相的析出量，析出位置与析出尺寸产生很大波动，热冲压后桥壳钢的力学性能无法得到稳定控制，并且含V桥壳钢铸坯极易形成表面横裂纹，造成板材边部组织异常。
因此，如何开发一种能够保证热冲压成型后桥壳钢的力学性能，且在热处理后板材无明显的显微带状组织，具备优异的冲击韧性的700MPa级热成型桥壳钢，成为亟待解决的技术问题。
发明内容
本公开目的是提供一种700MPa级热成型桥壳钢及其制备方法，优化化学组分且采用本公开的工艺，板材在热处理前后的力学性能均满足屈服强度≥600MPa，抗拉强度≥700MPa，断后延伸率A50≥17％，0℃纵向冲击功Akv≥47，在热处理后板材无明显的显微带状组织，具备优异的冲击韧性。
本公开的第一方面，提供了一种700MPa级热成型桥壳钢，所述700MPa级热成型桥壳钢的化学成分以质量分数计为：C：0.17％～0.27％，Si：0.30％～0.50％，Mn：1.2％～1.8％，P：≤0.02％，S：≤0.009％，Ti：≤0.1％，Cr：≤ 0.2％，其余为Fe及不可避免的杂质；且所述Si和Mn的质量分数满足：0.23≤Si/Mn≤0.27。
在一些实施方式中，所述700MPa级热成型桥壳钢的化学成分以质量分数计为：C：0.17％～0.27％，Si：0.30％～0.50％，Mn：1.2％～1.8％，P：≤0.02％，S：≤0.009％，Ti：≤0.1％，Cr：≤0.2％，
和Nb≤0.05％，Mo≤0.20％，B≤0.002％中的至少一种；
其余为Fe及不可避免的杂质，且所述Si和Mn的质量分数满足：0.23≤Si/Mn≤0.27。
在一些实施方式中，所述700MPa级热成型桥壳钢的化学成分以质量分数计为：C：0.17％～0.26％，Si：0.30％～0.50％，Mn：1.2％～1.6％，P：≤0.02％，S：≤0.009％，Ti：≤0.1％，Cr：≤0.2％，其余为Fe及不可避免的杂质；且所述Si和Mn的质量分数满足：0.23≤Si/Mn≤0.27。
在一些实施方式中，所述700MPa级热成型桥壳钢的化学成分以质量分数计为：C：0.17％～0.26％，Si：0.30％～0.50％，Mn：1.2％～1.6％，P：≤0.02％，S：≤0.009％，Ti：≤0.1％，Cr：≤0.2％，
和Nb≤0.03％，Mo≤0.20％，B≤0.0015％中的至少一种；
其余为Fe及不可避免的杂质，且所述Si和Mn的质量分数满足：0.23≤Si/Mn≤0.27。
在一些实施方式中，所述Nb和/或Mo的至少部分含量由B元素代替。
在一些实施方式中，所述700MPa级热成型桥壳钢的金相组织以体积百分数计为：60％～70％铁素体和30％～40％片状珠光体；所述铁素体的晶粒尺寸为5μm～10μm，所述片状珠光体的渗碳体尺寸为8μm～15μm。
在一些实施方式中，采用热成型工艺的温度对所述的一种700MPa级热成型桥壳钢进行热处理，热处理后的微观金相组织以体积百分数计为：粒状珠光体中含65％～75％铁素体和25％～35％粒状渗碳体；所述铁素体晶粒尺寸为7～12μm，所述粒状渗碳体尺寸为3μm～10μm。
本公开的第二方面，提供了所述700MPa级热成型桥壳钢的制备方法，所述制备方法包括：采用所述的700MPa级热成型桥壳钢的化学成分进行熔炼、连铸，获得板坯；将所述板坯进行加热，粗轧和精轧，获得热轧板；在一些实施方式中，加热温度为1180℃～1220℃，加热时间为150min～200min；
将所述热轧板进行冷却和卷曲，获得热轧卷；以及将所述热轧卷进行加工处理，获得700MPa级热成型桥壳钢。
在一些实施方式中，所述粗轧时，粗轧终止温度为1000℃～1040℃；所述精轧时，精轧终止温度为840℃～870℃。
在一些实施方式中，所述粗轧和精轧时，轧速均为4m/s～6m/s；所述热轧板的厚度为7mm～18mm。
在一些实施方式中，所述卷曲温度为560℃～620℃。
在一些实施方式中，所述加工处理包括开卷，校平和剪切，所述加工处理时，在钢板长度为10m时，对角线方向测量的不平度≤5mm。
就根据本公开内容的一个或多个实施方式的700MPa级热成型桥壳钢及其制备方法而言，(1)在化学组分上，以Cr，Ti合金元素为主，放弃现有技术中的V，N合金元素，热成型后板材以固溶强化为主，且将Si和Mn的质量分数控制在：0.23≤Si/Mn≤0.27，Si与Mn在枝晶间隙偏析对A3温度点的影响才能相互抵消，树枝晶枝干与枝晶间两区域的A3温度差别小，能避免显微带状组织的形成。(2)在制备方法上，将板坯的加热温度设定在1180℃～1220℃，加热时间为150min～200min，保证C元素均匀化，并防止板坯表层脱碳，板材在热处理前后的力学性能均满足屈服强度≥600MPa，抗拉强度≥700MPa，断后延伸率A50≥17％，0℃纵向冲击功Akv≥47，板材在热处理后无明显的显微带状组织，具备优异的冲击韧性和疲劳性能。
附图说明
为了更清楚地说明本公开实施例中的技术方案，下面将对实施例描述中所需要使用的附图作一简单地介绍，显而易见地，下面描述中的附图是本公开的一些实施例，对于本领域普通技术人员来讲，在不付出创造性劳动的前提下，还可以根据这些附图获得其它的附图。
图1为本公开实施例4提供的700MPa级热成型桥壳钢板厚四分之一处的显微组织图；
图2为本公开实施例4提供的700MPa级热成型桥壳钢板在热处理后的显微组织图；其中(A)为边部的组织，(B)为四分之一处的组织，(C)为心部的组织；
图3为本公开对比例2板厚四分之一处的显微组织示意图；
图4为本公开对比例2在热处理后的显微组织图；
图5为本公开对比例5热压桥壳钢的显微组织图；
图6为本公开对比例6热压桥壳钢热处理后的显微组织图；
图7为根据本公开内容的一个或多个实施方式的一种700MPa级热成型桥壳钢的制备方法的流程图。
具体实施方式
下文将结合具体实施方式和实施例，具体阐述本公开，本公开的优点和各种效果将由此更加清楚地呈现。本领域技术人员应理解，这些具体实施方式和实施例是用于说明本公开，而非限制本公开。
在整个说明书中，除非另有特别说明，本文使用的术语应理解为如本领域中通常所使用的含义。因此，除非另有定义，本文使用的所有技术和科学术语具有与本公开所属领域技术人员的一般理解相同的含义。若存在矛盾，本说明书优先。
除非另有特别说明，本公开中用到的各种原材料、试剂、仪器和设备等，均可通过市场购买获得或者可通过现有方法获得。
根据本公开内容的一个或多个实施方式的技术方案，为了提供一种 700MPa级热成型桥壳钢，总体思路如下：
提供一种700MPa级热成型桥壳钢，所述700MPa级热成型桥壳钢的化学成分以质量分数计为：C：0.17％～0.26％，Si：0.30％～0.50％，Mn：1.2％～1.6％，P：≤0.02％，S：≤0.009％，Ti：≤0.1％，Cr：≤0.2％，其余为Fe及不可避免的杂质；且所述Si和Mn的质量分数满足：0.23≤Si/Mn≤0.27。
本公开通过优化组成元素进而形成以上化学成分的700MPa级热成型桥壳钢，是基于以下原理：
C：0.17％～0.26％，C是决定碳钢室温组织和力学性能的主要元素，在亚共析范围内，抗拉强度随C含量升高而升高，适当提高C含量利于获取珠光体组织。在桥壳钢中频感应加热过程中，高C加速奥氏体组织形成，在冲压成型后的空冷过程中，渗碳体在未溶碳化物质点及固溶C元素的富集区析出，形成韧性优异的粒状珠光体组织。
Si：0.30％～0.50％，Si含量的增加会提高铁素体基体的强度，促进铁素体组织形成。
Mn：1.2％～1.6％，添加1.2％以上的Mn提高基体强度，且细化珠光体片层间距，含量过高会影响材料焊接性，Mn元素在连铸过程中偏析，造成热轧板材中产生显微带状组织，降低板材冲击性能。
P：≤0.02％，S：≤0.009％，P与S在连铸过程中极易偏析，对材料性能产生不利影响，应尽严格控制钢中P与S含量。
Ti：≤0.1％，添加适量的Ti元素，TiN抑制奥氏体晶粒在板坯加热炉中过度生长，与S和C形成的Ti4C2S2，减小沿轧向分布的条状MnS而造成的板材横纵向力学性能的差异。
Cr：≤0.2％，添加适量的Cr元素发挥固溶强化作用，促进珠光体组织形成，因此将Cr元素含量控制在≤0.2％。
本公开内容的一些实施方式中，Si和Mn的质量分数满足：0.23≤Si/Mn≤0.27，由于Si、Mn、P和S元素都是易于偏析的元素，P和S可在精炼过程中去除，板坯连铸过程中，Si与Mn元素会在柱状晶之间偏析，并且板坯在加热炉中长时间保温也极难使元素分布均匀化并遗传于热轧成品板材，在一些实施方式中，Si是封闭奥氏体区元素，会提高奥氏体向铁素体转变的A3温度点，Mn与Si对A3温度点的作用正相反，元素在树状晶枝晶间的偏聚影响了热轧后板材的相变组织分布，从而产生了显微带状组织。为尽可能消除显微带状组织，需要综合考虑Si与Mn元素的偏析系数与元素原子量，因此本公开设计Si与Mn元素的质量含量百分0.23≤Si％/Mn％≤0.27；板坯凝固过程中合金元素的偏析形成了原始带状组织(树枝晶枝干与枝晶间隙呈条带状分布)，并且原始带状组织的形成是无法避免的，显微带状组织的形成必须是在原始带状组织的基础之上，固态相变的具体条件决定显微带状组织是否会形成。0.23≤Si％/Mn％≤0.27时，Si与Mn在枝晶间隙偏析对A3温度点的影响才能相互抵消，树枝晶枝干与枝晶间两区域的A3温度差别小，能避免显微带状组织的形成；Si％/Mn％值小于0.23或高于0.27都有可能形成显微带 状组织。
作为一种可选的实施方式，所述700MPa级热成型桥壳钢的化学成分以质量分数计为：C：0.17％～0.26％，Si：0.30％～0.50％，Mn：1.2％～1.6％，P：≤0.02％，S：≤0.009％，Ti：≤0.1％，Cr：≤0.2％，
和Nb≤0.03％，Mo≤0.20％，B≤0.0015％中的至少一种；
其余为Fe及不可避免的杂质，且所述Si和Mn的质量分数满足：0.23≤Si/Mn≤0.27。
本公开内容的一些实施方式中可以选择添加少量Nb元素，发挥控制轧制，细晶强化的作用，Nb元素含量最优取值为0.03％以内。
本公开内容的一些实施方式中可选择适量添加Mo元素，添加Mo元素发挥固溶强化作用，且利于减轻Mn、P和S偏析。
本公开内容的一些实施方式中可先择适量添加B元素，偏析于奥氏体晶界的微量B元素可以显著提高过冷奥氏体组织的稳定性，且抑制Mn、P、S元素偏析，对于厚规格的桥壳钢，即使不同产品空冷冷速存在差异，板厚各处的冷速不同，也能在板厚方向上获得均匀的组织，B元素含量最优取值控制在0.0015％以内。
作为一种可选的实施方式，所述Nb和/或Mo的至少部分含量由B元素代替。
作为一种可选的实施方式，所述700MPa级热成型桥壳钢的金相组织以体积百分数计为：60％～70％铁素体和30％～40％片状珠光体；所述铁素体的晶粒尺寸为5μm～10μm，所述片状珠光体的渗碳体尺寸为8μm～15μm。细化晶粒尺寸，保证产热轧板材的强度级别可以达到700MPa级。
参考热成型工艺的温度制度对所述700MPa级热成型桥壳钢的板材进行热处理，所述700MPa级热成型桥壳钢的板材在热处理后的微观金相组织为：粒状珠光体。以体积百分数计，粒状珠光体中含65％～75％铁素体+25％～35％粒状渗碳体，铁素体晶粒尺寸为7～12μm，渗碳体尺寸为3～10μm；热处理后，铁素体组织上分布有细小的粒状渗碳体颗粒，保证桥壳钢在抗拉强度为700MPa的基础上，提高产品的冲击韧性；从而使得板材在热处理前后的力学性能均满足屈服强度≥600MPa，抗拉强度≥700MPa，断后延伸率A50≥17％，0℃纵向冲击功Akv≥47，在热处理后板材无明显的显微带状组织，具备优异的冲击韧性。
在本公开内容的第二方面，提供了一种所述700MPa级热成型桥壳钢的制备方法，如图5所示，所述制备方法包括：
S1、采用所述的700MPa级热成型桥壳钢的化学成分进行熔炼、连铸，获得板坯；
S2、将所述板坯进行加热，粗轧和精轧，获得热轧板；其中，加热温度为1180℃～1220℃，加热时间为150min～200min；
S3、将所述热轧板进行冷却和卷曲，获得热轧卷；
S4、将所述热轧卷进行加工处理，获得700MPa级热成型桥壳钢。
根据本公开内容的一个或多个实施方式的所述700MPa级热成型桥壳钢的制备方法，是基于以下原理：
将板坯的加热温度设定在1180℃～1220℃，加热时间为150min～200min，保证C元素均匀化，并防止板坯表层脱碳。若加热温度小于1180℃，加热时间小于150min，无法保证C元素在板坯内部均匀化，析出相无法回溶；若加热温度大于1220℃，加热时间大于200min，会导致板坯严重脱碳，奥氏体组织粗化，影响热轧板材的性能。
在成分上进行上述配比，且Si和Mn的质量分数满足：0.23≤Si/Mn≤0.27，且采用上述工艺；热轧板材的微观金相组织为铁素体，采用热成型工艺的温度(加热温度为1180℃～1220℃，加热时间为150min～200m)对板材进行热处理，板材在热处理后的微观金相组织为铁素体+粒状珠光体，热处理前后，板材力学性能均满足屈服强度≥600MPa，抗拉强度≥700MPa，断后延伸率A50≥17％，0℃纵向冲击功Akv≥47，板材在热处理后无明显的显微带状组织，具备优异的冲击韧性。
作为一种可选的实施方式，所述粗轧时，粗轧终止温度为1000℃～1040℃；所述精轧时，精轧终止温度为840℃～870℃。
所述粗轧终止温度控制为1000℃～1040℃，精轧终止温度为840℃～870℃的原因为：在奥氏体未再结晶区温度范围内低温终轧，且避免在奥氏体铁素体两相区轧制；使奥氏体充分累积变形量以提高铁素体形核率。
作为一种可选的实施方式，所述粗轧和精轧时，轧速均为4m/s～6m/s；所述热轧板的厚度为7mm～18mm。恒速精轧，根据厚度规格调整轧速4m/s～6m/s从而使得所述热轧板厚度规格变化范围控制在7mm～18mm。
作为一种可选的实施方式，所述卷曲温度为560℃～620℃。这样有利于将珠光体温度转变区间获得珠光体组织。
综上可知，本公开采用C-Si-Mn-Cr-Ti成分体系桥壳钢市场上未见到有这种成分体系)，以C为主要的合金元素，控制0.23≤Si％/Mn％≤0.27，结合本公开的工艺，热轧板材的微观金相组织为铁素体(体积分数为60％-71％，晶粒尺寸为5-10μm)+片状珠光体(体积分数为29-40％，晶粒尺寸为8-15μm)，参考热成型工艺的温度制度对所述700MPa级热成型桥壳钢的板材进行热处理，所述700MPa级热成型桥壳钢的板材在热处理后的微观金相组织为：粒状珠光体。以体积百分数计，粒状珠光体中含65％～75％铁素体+25％～35％粒状渗碳体，铁素体晶粒尺寸为4～12μm，渗碳体尺寸为3～10μm；热处理前后，板材力学性能均满足屈服强度≥600MPa，抗拉强度≥700MPa，断后延伸率A50≥17％，0℃纵向冲击功Akv≥47，板材在热处理后无明显的显微带状组织，具备优异的冲击韧性和疲劳寿命。
下面将结合实施例、对比例及实验数据对本公开的一种700MPa级热成型桥壳钢及其制备方法进行详细说明。
S1、实施例1-实施例8以及对比例1-3分别采用如表1所示的化学成分，按表1设计的合金成分配比，在温度为1630℃进行熔炼，后锻造成钢坯；
表1各实施例和对比例的化学成分质量分数
Figure PCTCN2021120417-appb-000001
Figure PCTCN2021120417-appb-000002
S2、将实施例所述板坯进行加热后，所述板坯的加热温度为1180℃～1220℃，经过粗轧、精轧获得热轧板，所述热轧板厚度规格变化范围控制在7mm～10mm，经5道次粗轧轧制，所述粗轧终止温度控制为1000℃～1040℃，再经7道次精轧轧制，所述精轧终止温度控制为840℃～870℃；
S3、将所述热轧板层流冷却后在560℃～620℃卷取温度下卷取获得热轧带钢；
S4、将所述热轧卷经开卷-平整-剪切得到成品板材，在钢板长度为10m时，对角线方向测量的不平度≤5mm。
各实施例与对比例的工艺参数具体见表2所示；
表2
Figure PCTCN2021120417-appb-000003
Figure PCTCN2021120417-appb-000004
对各组别的成品取样进行力学性能测试，结果见表3。显微组织的各相尺寸和体积分数数据由板材厚度四分之一处观察测量得出。
参考热成型工艺的温度制度，取实施例与对比例板材进行热处理实验，感应加热至810-830℃，保温时间180-240s，移入冷模，保持15s，随后空冷，三次力学性能的检测结果记入表3。显微组织的各相尺寸和体积分数数据由板材厚度四分之一处观察测量得出。
表3-热处理前后实施例与对比例的力学性能
Figure PCTCN2021120417-appb-000005
Figure PCTCN2021120417-appb-000006
Figure PCTCN2021120417-appb-000007
Figure PCTCN2021120417-appb-000008
由表3的数据可知：
对比例1中Si/Mn＞0.27％，热处理后有明显的显微带状组织。
对比例2中Si/Mn＜0.23％，热处理后有明显的显微带状组织。
对比例3中，加热温度小于1180℃，无法保证C元素在板坯内部均匀化，析出相无法回溶，故存在强度级别低于700MPa，板材出现显微带状组织的缺点。
对比例4中，加热温度大于1220℃，会导致板坯严重脱碳，奥氏体组织粗化，影响热轧板材的性能，故存在热轧组织晶粒较为粗大，综合力学性能低的缺点。
对比例5中，其成分设计采用中碳-中锰-高Si的工艺路线，Si/Mn比在0.44，通过采用分段冷却的低温卷取获得800MPa级的热冲压桥壳用钢，其组织为铁素体+细密的下贝氏体，铁素体比例在5-10％，实际的屈服强度达到了856MPa、抗拉强度达到了981MPa；由于桥壳在热冲压之前还需要进行剪切下料，同时桥壳厚度一般都在10mm以上，甚至14mm、16mm、18mm等厚规格，该强度很难进行剪切；另一方面，采用低温卷取板形质量较差，不利于桥壳的尺寸精度；同时由于Si/Mn比高达0.44，存在明显的带状组织，对疲劳性能不利；再者热压桥壳需要焊接的部件较多，下贝氏体在焊接后热影响区会出现严重的软化问题，导致桥壳不同位置硬度较大的差异，影响焊接疲劳性能。而该桥壳热冲压后的屈服强度仅有500MPa。
对比例6中，采用C-Mn-Nb-V-Ti-Mo复合微合金化成分体系，利用析出强化提高桥壳热冲压前后的力学性能你，虽然也能达到热压后600MPa，但是该对比例存在如下缺点：合金添加量较大，成本较高；由于添加V元素，析出温度正好处于连铸弯曲矫直区，易导致连铸坯表面皮下裂纹，在后续的热轧或者桥壳热冲压中发生表面皲裂缺陷，影响桥壳的疲劳性能；再者加热 温度较高，容易导致表面脱碳，表层组织粗大，影响材料的疲劳性能；再者Si/Mn比仅有0.055，存在严重的带状组织，影响疲劳性能。
对比例7中，采用C-Mn-V微合金化成分体系，通过低温出炉、低温轧制、低温卷取的工艺获得综合性能优异的热冲压桥壳钢，与本发明专利相比存在如下缺陷：热冲压后屈服强度达不到600MPa，且组织存在明显的带状，由于碳元素添加量较高，对焊接性能不利。
本公开实施例1-8最后制备得到的700MPa级热成型桥壳钢，板材在热处理前后的力学性能均满足屈服强度≥600MPa，抗拉强度≥700MPa，断后延伸率A50≥17％，0℃纵向冲击功Akv≥47，板材在热处理后无明显的显微带状组织，具备优异的冲击韧性和疲劳寿命。
本公开实施例8为最优实施例，通过在C-Si-Mn元素基础上，复合添加Cr、Mo、Nb、Ti、B等元素，严格控制Si/Mn比和热轧工艺参数、显微组织含量和尺寸，获得了更加优异的力学性能和疲劳性能。具有如下的优点：(1)实施例8由于复合添加了Nb-Ti-Mo等元素，通过采用较低的出炉温度，以及缩小精轧区温降提高轧制速度，通过Ti元素析出物的高温钉扎作用可以获得均匀细小的原始奥氏体组织，进而得到均匀细小的铁素体组织，由于轧制速度的提高以及添加了Cr、Mo、B等提高淬透性元素，提高了奥氏体向铁素体转变的动力学，进一步促进铁素体的均匀细化，以及降低珠光体球团尺寸，同时通过控制较高的卷取温度，避免产生低温转变组织，这有利于获得优异的热轧带钢，并可保证良好的板形质量，利于后续生产桥壳时的切割下料和保证切割下料的质量。(2)在桥壳热冲压过程中，需要将桥壳热轧带钢加热至奥氏体区加热保温，由于同时存在Nb、Ti微合金化元素，其符合的TiNbCN析出物可以阻碍加热过程中奥氏体晶粒的再结晶和粗化，为获得最终均匀细小的铁素体组织提高前提条件；在热冲压过程中，由于变形的发生，促进了TiNbCN、TiNbC等第二相的析出，组织压扁奥氏体的回复，又可进一步保证最终均匀细小的铁素体提供保证条件；一般桥壳冲压完成后是空冷至室温的，由于冷却速度较慢，会形成粗大的铁素体和珠光体组织，并且形成严重的珠光体带状，损害桥壳的冲击韧性和疲劳性能，而本专利通过添加Cr、Mo、B等元素，提高试验钢的淬透性，可以在空冷较慢的冷却速度下获得均匀细小的铁素体和珠光体，加上严格控制Si/Mn比，避免了带状组织的发生，保障桥壳热冲压后具有较高的强度和塑性匹配，同时具有优异的低温冲击韧性和疲劳寿命，这对桥壳服役寿命的提升具有重要意义。
附图1-4的说明：
图1为本公开实施例4板厚四分之一处的显微组织，板材微观组织为：铁素体(体积分数为63％，晶粒平均尺寸为7μm)+片状珠光体组织(体积分数为37％，晶粒尺寸为12μm)。
图2为实施例4在热处理后的显微组织，板材厚度方向上，边部、四分之一处和心部的组织均为粒状珠光体。其中，铁素体的体积分数为68％～73％，晶粒尺寸为7μm～10μm，粒状渗碳体体积分数为27％～32％，尺寸 为3μm～8μm。
图3为本公开对比例2板厚四分之一处的显微组织示意图。板材微观组织为：铁素体(体积分数为70％，晶粒平均尺寸为11μm)+片状珠光体组织(体积分数为30％，晶粒平均尺寸为6μm)。
图4为本公开对比例2在热处理后的显微组织，板材厚度方向上，边部、四分之一处和心部的组织均为铁素体(体积分数为70％～85％，晶粒尺寸为7μm～13μm)+片层珠光体(体积分数为15％～30％，晶粒尺寸为6μm～12μm)，且出现了明显的显微带状组织。
图5为本公开对比例5热冲压桥壳钢的显微组织，组织为少量铁素体+下贝氏体组织，带状组织明显。
图6为本公开对比例6热冲压桥壳钢热冲压后的显微组织，组织为铁素体+珠光体组织，铁素体晶粒尺寸约为6.3μm，存在明显的带状组织。
由图1-图6可知，根据本公开内容的一个或多个实施方式的700MPa级热成型桥壳钢热处理后板厚各处组织更为均匀，无显微带状组织，具备更优异的冲击韧性与疲劳性能。
最后，还需要说明的是，术语“包括”、“包含”或者其任何其他变体意在涵盖非排他性的包含，从而使得包括一系列要素的过程、方法、物品或者设备不仅包括那些要素，而且还包括没有明确列出的其他要素，或者是还包括为这种过程、方法、物品或者设备所固有的要素。
尽管已描述了本公开的优选实施例，但本领域内的技术人员一旦得知了基本创造性概念，则可对这些实施例作出另外的变更和修改。所以，所附权利要求意欲解释为包括优选实施例以及落入本公开范围的所有变更和修改。
显然，本领域的技术人员可以对本公开进行各种改动和变型而不脱离本公开的精神和范围。这样，倘若本公开的这些修改和变型属于本公开权利要求及其等同技术的范围之内，则本公开也意图包含这些改动和变型在内。

Claims (10)

  1. 一种700MPa级热成型桥壳钢，所述700MPa级热成型桥壳钢的化学成分以质量分数计为：C：0.17％～0.27％，Si：0.30％～0.50％，Mn：1.2％～1.8％，P：≤0.02％，S：≤0.009％，Ti：≤0.1％，Cr：≤0.2％，其余为Fe及不可避免的杂质；且所述Si和Mn的质量分数满足：0.23≤Si/Mn≤0.27。
  2. 一种700MPa级热成型桥壳钢，其中，所述700MPa级热成型桥壳钢的化学成分以质量分数计为：C：0.17％～0.27％，Si：0.30％～0.50％，Mn：1.2％～1.8％，P：≤0.02％，S：≤0.009％，Ti：≤0.1％，Cr：≤0.2％，
    和Nb≤0.05％，Mo≤0.20％，B≤0.002％中的至少一种；
    其余为Fe及不可避免的杂质，且所述Si和Mn的质量分数满足：0.23≤Si/Mn≤0.27。
  3. 根据权利要求2所述的700MPa级热成型桥壳钢，其中，所述Nb和/或Mo的至少部分含量由B元素代替。
  4. 根据权利要求1-3任一所述的700MPa级热成型桥壳钢，其中，所述700MPa级热成型桥壳钢的金相组织以体积百分数计为：60％～70％铁素体和30％～40％片状珠光体；所述铁素体的晶粒尺寸为5μm～10μm，所述片状珠光体的渗碳体尺寸为8μm～15μm。
  5. 根据权利要求4所述的700MPa级热成型桥壳钢，其中，采用热成型工艺的温度对权利要求1-3任一所述的700MPa级热成型桥壳钢进行热处理，热处理后的微观金相组织以体积百分数计为：粒状珠光体中含65％～75％铁素体和25％～35％粒状渗碳体；所述铁素体晶粒尺寸为7～12μm，所述粒状渗碳体尺寸为3μm～10μm。
  6. 一种权利要求1-5任一所述的700MPa级热成型桥壳钢的制备方法，包括：
    采用权利要求1-5任一所述的700MPa级热成型桥壳钢的化学成分进行熔炼、连铸，获得板坯；
    将所述板坯进行加热，粗轧和精轧，获得热轧板；其中，加热温度为1180℃～1220℃，加热时间为150min～200min；
    将所述热轧板进行冷却和卷曲，获得热轧卷；
    将所述热轧卷进行加工处理，获得700MPa级热成型桥壳钢。
  7. 根据权利要求6所述的700MPa级热成型桥壳钢的制备方法，其中，所述粗轧时，粗轧终止温度为1000℃～1040℃；所述精轧时，精轧终止温度为840℃～870℃。
  8. 根据权利要求6所述的700MPa级热成型桥壳钢的制备方法，其中，所述粗轧和精轧时，轧速均为4m/s～6m/s；所述热轧板的厚度为7mm～18mm。
  9. 根据权利要求6所述的700MPa级热成型桥壳钢的制备方法，其中，所述卷曲温度为560℃～620℃。
  10. 根据权利要求6所述的700MPa级热成型桥壳钢的制备方法，其中，所述加工处理包括开卷，校平和剪切，所述加工处理时，在钢板长度为10m时，对角线方向测量的不平度≤5mm。
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