WO2023051668A1 - 一种贝氏体钢及其制备方法 - Google Patents

一种贝氏体钢及其制备方法 Download PDF

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WO2023051668A1
WO2023051668A1 PCT/CN2022/122455 CN2022122455W WO2023051668A1 WO 2023051668 A1 WO2023051668 A1 WO 2023051668A1 CN 2022122455 W CN2022122455 W CN 2022122455W WO 2023051668 A1 WO2023051668 A1 WO 2023051668A1
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cooling
bainite
steel
bainitic steel
steel according
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PCT/CN2022/122455
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English (en)
French (fr)
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张瀚龙
陈�光
张玉龙
金鑫焱
柯阳林
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宝山钢铁股份有限公司
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Priority to AU2022355983A priority Critical patent/AU2022355983A1/en
Publication of WO2023051668A1 publication Critical patent/WO2023051668A1/zh

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    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • 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
    • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—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
    • C22C38/32—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

Definitions

  • the invention relates to the technical field of metallurgy, in particular to a bainite steel and a preparation method thereof.
  • the strength of steel used for automobile structural parts is getting higher and higher, and the requirements for materials with different properties in the thickness direction are gradually put forward.
  • the surface layer of the material is required to have high hardness and wear resistance, or the surface layer has a high uniformity of structure to meet the needs of flanging forming, but at the same time, the core must have high plasticity so that the overall steel material will not be stretched during drawing forming.
  • the surface layer is required to have a layered structure with a low hardness to ensure that the material has a certain bending performance, but the subsurface layer must still have a uniform hard phase structure to ensure flanging and strength, and
  • the core has a soft structure to ensure plasticity, toughness, etc., so as to ensure that the material not only has high strength, but also has good comprehensive forming capabilities such as bending, flanging and drawing.
  • the traditional method is to obtain slabs with different compositions or structures through welding, combined rolling, etc. steel material.
  • CN201210368300.6 and CN201310724615.4, etc. use combined rolling of metals to obtain layered composite materials in the thickness direction.
  • this method is complicated in process, slow in production rhythm and extremely high in cost.
  • this method can spontaneously form a microstructure gradient in the thickness direction and obtain a high-strength steel plate with a three-layer composite structure, on the one hand, the strength or hardness difference between the surface layer and the core is too large, and the strength or hardness of the surface layer is too low, which not only greatly limits
  • the scope of application of this type of product results in that although the material has good bending properties, the elongation and The hole expansion rate is not high, that is, the plasticity and flanging performance are poor; on the other hand, this method can only form a 3-layer composite structure, and cannot further obtain more layers of the structure.
  • the present invention provides a bainite steel with yield strength ⁇ 800 MPa, tensile strength ⁇ 1000 MPa, elongation at break ⁇ 12%, the mechanical properties of hole expansion rate ⁇ 40%, in addition, due to the gradient structure of the steel plate or steel strip in the thickness direction, the material has good comprehensive forming properties, that is, the tensile properties and hole expansion and flanging properties are both good , which is reflected in both the elongation at break and the hole expansion rate are relatively high, and the (elongation at break*10+hole expansion rate) of all the examples is greater than or equal to 170%.
  • the bainitic steel of the present invention includes chemical components in terms of mass percentage: C: 0.10-0.19%, Si: 0.05-0.45%, Mn: 1.5-2.2%, B: 0.001-0.0035%, Al: 0.01-0.05% , Cr: 0.05-0.40%, Mo: 0.05-0.40%, Fe ⁇ 90%.
  • the C element In the bainite steel of the present invention, the C element mainly controls the phase transformation of the carbon steel, the size of the carbide and the substructure of the bainite, thereby affecting the mechanical properties of the material.
  • the C element content in the steel is lower than 0.10%, the strength of the steel will not meet the target requirements; and if the C element content in the steel is higher than 0.19%, it is easy to form martensite structure and coarse cementite, Deterioration of the performance of the steel plate.
  • the C element will also affect the sub-morphological structure of bainite, and the higher the C content, the easier it is to form acicular bainite.
  • the mass percentage of C is controlled between 0.10% and 0.19%.
  • the mass percentage of C is 0.13-0.17%.
  • Si In the bainitic steel of the present invention, Si has a certain solid-solution strengthening effect on the one hand, and on the other hand affects the surface quality of the steel plate.
  • the Si element content in the steel is less than 0.05%, it is difficult to obtain a sufficient strengthening effect; and when the Si element content in the steel is higher than 0.45%, it is easy to form oxide scale or tiger stripe stripe color difference, which is not conducive to automotive steel plates surface quality.
  • the Si element will also affect the sub-morphological structure of bainite. The higher the Si content, the easier it is to form polygonal bainite.
  • the mass percentage of Si is controlled between 0.05% and 0.45% in the present invention. between.
  • the mass percentage of Si is 0.05-0.35%. More preferably, the mass percentage of Si is 0.15-0.3%.
  • the Mn element is one of the controlling elements of the structural transformation in the steel, and also affects the submorphic structure of the bainite.
  • the higher the Mn content the easier it is to form polygonal bainite.
  • the content of Mn element in the steel should not be too high.
  • the mass percentage of Mn is controlled between 1.5-2.2%.
  • the mass percentage of Mn is 1.7-2.1%.
  • B element is not only beneficial to the formation of bainite in the steel, but also affects the strength and formability of the steel plate, and also affects the submorphic structure of bainite.
  • the Al element is only added to the steel as a deoxidizing element, which can remove the O element in the steel to ensure the performance and quality of the steel. Therefore, in the bainitic steel of the present invention, the mass percentage of Al is controlled between 0.01-0.05%.
  • Al element is added to the steel in a large amount ( ⁇ 0.1%) as a ferrite forming element and carbide precipitation element, in order to bring about solid solution strengthening, or to change the phase transformation through the addition of Al Temperature (such as A1, A3), bainite formation kinetics and carbide precipitation kinetics to change the phase transformation of the steel, form retained austenite or carbon-free bainite, and ultimately increase the strength of the steel.
  • the existing composition control and process adjustment of the present invention can already obtain bainite steel with good comprehensive formability, and the carbon-free bainite formed by adding a large amount of Al element will destroy the thickness direction to form a bainite structure gradient instead, and also It will cause cost increase and continuous casting production difficulty to be greatly improved. Therefore, in the present invention, the mass percentage of Al is controlled between 0.01% and 0.05%, so as to avoid cost increase or continuous casting production difficulty. A bainite gradient can form.
  • Cr and Mo In the bainite steel of the present invention, Cr and Mo can not only form fine and dispersed carbide precipitates with C, but also can further improve the strength of the steel plate, and also affect the inoculation of pearlite and ferrite in the CCT curve In the long term, the hardenability of the steel plate can be improved, so that it can be designed in conjunction with the cooling rate of the steel plate in the annealing process to control the formation of the microstructure gradient in the thickness direction and different thickness ratios. Based on this, in the present invention, the mass percentages of Cr and Mo are controlled as follows: 0.05% ⁇ Cr ⁇ 0.40%, 0.05% ⁇ Mo ⁇ 0.40%.
  • the steel can spontaneously form a phase with a structural gradient during the preparation process, and at the same time, the hardenability of the steel is improved, thereby Can improve the strength and formability of bainite steel.
  • the above bainitic steel also includes at least one of Ti and Nb, wherein the mass percentages of Ti and Nb need to satisfy: Nb ⁇ 0.1%, Ti ⁇ 0.15%.
  • Ti and Nb are optional alloying elements, which can be added into the steel to form fine and dispersed carbide precipitates and refine the structure grains to further improve the Strength and formability of steel plates. Based on this, in the bainitic steel of the present invention, the mass percentages of Nb and Ti are respectively controlled as follows: Nb ⁇ 0.1%, Ti ⁇ 0.15%. The addition of the above-mentioned alloying elements will increase the cost of the material. In consideration of performance and cost control, in the technical solution of the present invention, at least one of Nb and Ti can be preferably added. In some embodiments, the bainitic steel of the present invention contains Nb and Ti, the content of Nb is 0.001-0.1% by mass, and the content of Ti is 0.001-0.15% by mass.
  • the bainitic steel of the present invention includes chemical components in terms of mass percentage: C: 0.10-0.19%, Si: 0.05-0.45%, Mn: 1.5-2.2%, B: 0.001-0.0035%, Al: 0.01-0.05%, Cr; 0.05-0.40%, Mo: 0.05-0.40%, and the balance is Fe and unavoidable impurities.
  • Both P and S are impurity elements in steel. If technical conditions permit, in order to obtain quenched and tempered steel with better performance and better quality, the content of impurity elements in steel should be reduced as much as possible.
  • R (Mn+Si)/(12*C+160*B) is defined, and it is found through experiments that if this formula is used for calculation, the R value needs to be limited to a certain range, that is, 0.9 ⁇ R ⁇ 1.2, the expected bainitic steel plate/strip structure with microstructure gradient can be obtained.
  • R can be controlled to be 0.9 ⁇ R ⁇ 1.2, so as to ensure that the structure gradient and mechanical properties exist in the steel in the thickness direction.
  • Mn is an order of magnitude higher than that of other elements, and its influence on hardenability is relatively weak.
  • a coefficient of 1/2 is designed for Mn in this formula. Due to the slight difference in the formation temperature of acicular bainite and massive bainite in the annealing process, the formation temperature of acicular bainite is lower, while the formation temperature of massive bainite is higher, so the hardening of the steel plate The higher the hardness, the more conducive to the formation of acicular bainite and not conducive to the formation of massive bainite, and vice versa.
  • the composition design of the steel plate is more conducive to the formation of massive bainite , that is, when the R value is high, it needs to be equipped with higher hardenability to promote the formation of acicular bainite; and when the composition design of the steel plate is more conducive to the formation of acicular bainite, and the R value is low, then It needs to be matched with lower hardenability to promote the formation of massive bainite; therefore, the numerator of the Q value is the alloy content that represents the hardenability of the strip steel, and the higher the hardenability, the stronger the hardenability; and the denominator selection can represent The R value of the formation ability of massive bainite and acicular bainite in the structure, the ratio of its numerator to denominator, that is, the Q value, directly affects the formation ability and final ratio of massive layer and acicular layer
  • the Q value is too small, it means that the formation ability of massive bainite is too strong, and it is difficult to form acicular bainite in the final structure, so it is difficult to form a gradient structure; and if the Q value is too large, it means that acicular bainite is difficult to form. If the body forming ability is too strong, it is difficult to form massive bainite in the final structure, and it is also difficult to form a gradient structure.
  • the above-mentioned bainitic steel has two layers of surface tissue and a layer of core structure, and the core structure is between the two layers of surface structure.
  • the surface structure includes acicular bainite and granular carbide precipitates; the core structure includes massive bainite and granular carbide precipitates.
  • the precipitated phase of acicular bainite and granular carbide accounts for more than 99% of the volume of the surface structure
  • the precipitated phase of massive bainite and granular carbide accounts for more than 99% of the volume of the core structure.
  • FIG. 1 there are three-layer structures in the thickness direction of the steel plate or steel strip, and the structures from one surface to the other surface are respectively:
  • Surface structure 2 Acicular layer, that is, the structure mainly composed of acicular bainite and dispersed precipitated nano-scale, sub-micron-scale or micron-scale granular carbide precipitates, accounting for more than 99% of the total area .
  • the ratio in the thickness direction is 25% to 40%.
  • Core structure 1 It is a massive layer, that is, a structure mainly composed of massive bainite and dispersed precipitated nano-scale, sub-micron-scale or micron-scale granular carbide precipitates, accounting for the total proportion of this area ⁇ 99% %.
  • the ratio in the thickness direction is 20% to 50%.
  • Surface structure 2 Acicular layer, that is, the structure mainly composed of acicular bainite and dispersed precipitated nano-scale, sub-micron-scale or micron-scale granular carbide precipitates, accounting for more than 99% of the total area .
  • the ratio in the thickness direction is 25% to 40%.
  • the sum of the proportions of the three layers in the thickness direction of the bainite steel is 100%.
  • the above-mentioned bainite steel also has two layers of multiphase layers, and the above two layers of surface tissue and one layer of core tissue form an intermediate layer, and the intermediate layer is between the two layers of multiphase layers.
  • the volume of the multiphase layer accounts for 2% to 10% of the volume of the bainitic steel, and the rest is the intermediate layer.
  • the multiphase layer includes polygonal ferrite, acicular bainite and granular carbide precipitates, wherein polygonal ferrite accounts for less than 50% of the volume of the multiphase layer, and polygonal ferrite, acicular bainite and The granular carbide precipitated phase accounts for more than 99% of the volume of the multiphase layer.
  • the structures from one side surface to the other side surface are respectively:
  • Composite layer 3 It is mainly composed of polygonal ferrite, acicular bainite and dispersed precipitated nanoscale, submicron or micron granular carbide precipitates (wherein the polygonal ferrite structure ⁇ 50% ), polygonal ferrite, acicular bainite, and dispersed precipitated nanoscale, submicron or micron-scale granular carbide precipitates account for ⁇ 99% of the total proportion of the region.
  • the proportion in the thickness direction is 1% to 5%.
  • Surface structure 2 Acicular layer, that is, the structure mainly composed of acicular bainite and dispersed precipitated nano-scale, sub-micron-scale or micron-scale granular carbide precipitates, accounting for more than 99% of the total area .
  • the ratio in the thickness direction is 25% to 40%.
  • Core structure 1 It is a massive layer, that is, a structure mainly composed of massive bainite and dispersed precipitated nano-scale, sub-micron-scale or micron-scale granular carbide precipitates, accounting for the total proportion of this area ⁇ 99% %.
  • the ratio in the thickness direction is 25% to 40%.
  • Surface structure 2 Acicular layer, that is, the structure mainly composed of acicular bainite and dispersed precipitated nano-scale, sub-micron-scale or micron-scale granular carbide precipitates, accounting for more than 99% of the total area .
  • the ratio in the thickness direction is 25% to 40%.
  • Composite layer 3 It is mainly composed of polygonal ferrite, acicular bainite and dispersed precipitated nanoscale, submicron or micron granular carbide precipitates (wherein the polygonal ferrite structure ⁇ 50% ), polygonal ferrite, acicular bainite, and dispersed precipitated nanoscale, submicron or micron-scale granular carbide precipitates account for ⁇ 99% of the total proportion of the region.
  • the proportion in the thickness direction is 1% to 5%.
  • the bainite steel of the present invention has excellent ductility and hole expansion flanging properties, that is, elongation at break*10+hole expansion rate ⁇ 170%.
  • the present invention also provides a preparation method for the above-mentioned bainitic steel, comprising the steps of:
  • the bainitic steel with a texture gradient is not prepared by using the common surface decarburization method, the bainitic steel in the present invention does not have the problem that the strength and hardness of the surface layer are significantly lower than those of the core.
  • process parameters for controlling the above-mentioned preparation method meet at least one of the following items:
  • the heating temperature is controlled to be 1100-1230°C, the finish rolling start temperature is 1050-1180°C, and the finish rolling finish temperature is 870-930°C;
  • the cooling rate is controlled to be 30-150°C/s, and the coiling temperature is controlled to be 540-620°C;
  • the cold rolling reduction rate is controlled to be ⁇ 30%.
  • the pre-annealing process step is mainly to obtain a steel plate or steel strip with uniform composition and original structure, so as to ensure that the subsequent annealing process can meet the uniform and stable structure and performance, which plays a key role in the performance of the steel plate
  • the role is the annealing process.
  • the present invention intends to design the gradient structure in the thickness direction of the steel plate/strip, the steel plate or the steel strip will inevitably or intentionally have different temperature ranges in the thickness direction, but due to the limitation of the continuous production mode of the steel plate or the steel strip, the temperature Detection and control can only be aimed at the temperature of the upper and lower surfaces, and cannot detect the temperature of other positions in the thickness direction.
  • the temperature Detection and control can only be aimed at the temperature of the upper and lower surfaces, and cannot detect the temperature of other positions in the thickness direction.
  • the temperature of the upper surface and the lower surface no additional distinction is made, and they are treated according to the same process, and they are both called surface temperature.
  • the temperature and cooling rate mentioned below refer to the surface temperature and the cooling rate calculated from the surface temperature. It should be pointed out that during cooling, the temperature distribution in the thickness direction of the steel plate or steel strip is controlled according to the surface temperature, cooling rate, injection gas pressure during cooling (representing cooling capacity) and hardenability of the steel plate.
  • the annealing step sequentially includes a heating section, a slow cooling section, a fast cooling section, a controlled cooling section, and an air cooling section, and the cooling rate is controlled in the three stages of the slow cooling section, the fast cooling section, and the controlled cooling section to satisfy: the controlled cooling section ⁇ slow cooling section Cold section ⁇ fast cooling section.
  • the heating section heat at a heating rate of ⁇ 50°C/s to a soaking temperature of 840-950°C, and then keep it warm, and control the keep-warm time to 60-180 seconds.
  • the heating section it is necessary to control the heating rate of ⁇ 50°C/s to heat the bainite steel to the soaking temperature of 840-950°C and hold it for 60-180s.
  • the heating rate of the heating section is greater than 50°C/s, or the holding time is less than 60s, the uniformity of the strip steel structure will be poor, which will affect the formation of the subsequent thickness gradient structure.
  • the temperature is lower than the above-mentioned lower limit of the soaking temperature, the strip steel cannot obtain enough bainite structure (whether it is acicular bainite or massive bainite).
  • the heating rate is preferably 5-50°C/s.
  • the holding time is more than 180s, or further, if the soaking temperature is higher than 950°C, the grains of the strip steel will be coarse and the formability of the steel will be deteriorated.
  • the slow cooling rate is controlled to be 5Q-10Q°C/s. In some embodiments, the slow cooling rate is controlled to be 7Q-10Q°C/s.
  • slow cooling is achieved by spraying cooling gas onto the surface of the bainitic steel.
  • the cooling gas is sprayed onto the surface of the bainite steel for cooling, the pressure of the cooling gas injection is controlled to be 0.2*Q ⁇ Q kPa, and the holding time of the cooling gas injection is controlled to be 5 ⁇ 20 seconds.
  • methods such as liquid cooling can also be used to achieve the purpose of slow cooling, as long as it can be cooled to a slow cooling temperature of 720-800°C at a slow cooling rate of Q ⁇ 10*Q°C/s That's it.
  • the main purpose of this stage is to make the temperature of the steel plate or steel strip uniform in the width direction, and the temperature in the thickness direction is less uniform, but no structural transformation occurs in each position.
  • the purpose of controlling the slow cooling rate in this step is to make the steel plate or steel strip reach a uniform temperature in the width direction. Variation and decomposition form ferrite or pearlite. If the temperature is too high, it is not conducive to the high-precision control of the next stage of cooling, and it is not conducive to obtaining a gradient structure in the thickness direction. Controlling the pressure and holding time of the cooling gas sprayed onto the steel plate or strip surface is to control the uneven cooling in the thickness direction of the strip. If the pressure of the cooling gas sprayed onto the steel plate or strip surface is less than 0.2*Q kPa or If the holding time is less than 5 seconds, it means that the cooling capacity is insufficient.
  • the surface of the strip is cold to the set temperature, most of the area below the surface is at a higher temperature, which is not conducive to the formation of a gradient structure in the thickness direction in the next step. , or the acicular bainite area in the gradient structure formed in the next stage is too small; if it is higher than Q kPa or the retention time is longer than 20 seconds, the cooling capacity will be too large, and the temperature of the strip core will approach or even reach the surface temperature , It is also not conducive to the formation of gradient structure in the thickness direction in the next step, or the blocky bainite area in the gradient structure formed in the next stage is too small.
  • the slow cooling rate is controlled to be 5Q-10Q°C/s. In some embodiments, the slow cooling rate is controlled to be 7Q-10Q°C/s.
  • slow cooling is achieved by spraying cooling gas onto the surface of the bainitic steel.
  • the cooling gas is sprayed onto the surface of the bainite steel for cooling, the injection pressure of the cooling gas is controlled to be 0.05*Q-0.15*Q kPa, and the holding time of the cooling gas injection is controlled to be 5-15 seconds.
  • methods such as liquid cooling can also be used to achieve the purpose of slow cooling, as long as the bainite steel can be cooled to slow cooling at a slow cooling rate of Q ⁇ 10*Q°C/s.
  • the technical solutions with a temperature of 620-700°C all belong to the protection scope of the present application.
  • cooling to 620-700°C is to ensure that the surface of the steel plate or strip enters the ferrite transformation temperature range, and through a certain period of heat preservation, the steel plate or steel strip can form a certain amount of ferrite in the surface area , to prepare for the final formation of the multiphase layer on the surface; lower or higher than this temperature cannot guarantee the formation of a certain amount of ferrite on the surface of the strip.
  • the holding time is too short or the cooling rate is too fast, the ferrite on the surface of the strip will not be formed in time, and eventually the surface layer cannot be formed; on the contrary, if the holding time is too long or the cooling rate is too slow, the Too much ferrite formed on the surface of the strip, and the thickness is too thick, not only is not conducive to the formation of the surface multiphase layer, but also leads to the inability to form a sufficient amount of acicular bainite in the shallow surface layer of the fast cooling section, which affects the subsequent Formation of needle-like layers.
  • the pressure of the cooling gas sprayed to the surface of the steel plate or strip is 0.05*Q ⁇ 0.15*Q kPa, in order to control the thickness of the polygonal ferrite formed on the surface of the strip, within this pressure range, and the holding time also meets the set range At this time, only the surface area of the steel plate or strip actually cools to 620-700°C and enters the ferrite phase region, while the temperature in other areas is still higher than 700°C and does not undergo ferrite transformation (due to the formation of ferrite will also release latent heat of phase change).
  • the pressure of the sprayed cooling gas is too high, the temperature of the shallow surface layer or even the core of the steel plate or strip will drop accordingly, which is not conducive to the formation of the subsequent needle-like layer and massive layer.
  • the pressure of the jet cooling gas is too low, it is not conducive to the stable formation of a certain amount of polygonal ferrite on the surface layer, resulting in the inability to form a stable multi-phase layer on the surface layer.
  • rapid cooling is achieved by spraying cooling gas onto the surface of the bainitic steel.
  • the cooling gas needs to be sprayed twice on the surface of the bainite steel during cooling, the first injection pressure of the cooling gas is controlled to be 0.3*Q ⁇ 1.5*Q kPa, and the first holding time of the cooling gas is controlled to be 1 ⁇ 7 seconds; the second injection pressure of the cooling gas is controlled to be 0.08*Q-0.2*Q kPa, and the second holding time of the cooling gas is controlled to be 5-10 seconds.
  • methods such as liquid cooling can also be used to achieve the purpose of slow cooling, as long as the bainite steel can be cooled at a rapid rate of 10*Q ⁇ 20*Q°C/s at this stage.
  • the technical solutions for cooling at a cooling rate to a rapid cooling temperature of 400-540° C. all belong to the protection scope of the present application.
  • the cooling gas used in the annealing step is a mixture of reducing gas and inert gas.
  • the volume fraction of the reducing gas is 1%-8%.
  • the reducing gas in the mixture is hydrogen, and its volume fraction is 1%-8%.
  • the temperature of this cooling gas can be controlled to 5-50 degreeC.
  • the cooling of the steel plate or steel strip is carried out by spraying cooling gas (ie a mixture of reducing gas and inert gas) on its surface, wherein the reducing property can be achieved by hydrogen.
  • cooling gas ie a mixture of reducing gas and inert gas
  • an inert gas refers to a gas that does not chemically react with bainite steel under experimental conditions and affect the structure of the steel. Specifically, in consideration of cost saving, all the inert gas may be nitrogen.
  • the content and temperature of hydrogen in the cooling gas can be further controlled, see Table 2 for details.
  • the cooling capacity or cooling intensity is controlled by controlling the pressure of the injection gas, the content of hydrogen in the cooling gas, and the temperature of the cooling gas.
  • the specific value needs to be determined according to the hardenability of the steel plate or steel strip.
  • the hydrogen content in the cooling gas and the temperature of the cooling gas remain constant in the annealing process, and at this time, the cooling intensity, the cooling rate are positively correlated with the injection gas pressure, as in Example 1, in slow In the cold section, the injection pressure of cooling gas is 0.6kPa, and the cooling rate in the slow cooling section is 12.5°C/s; while in the fast cooling section, the first injection pressure of cooling gas is 1kPa, and the corresponding cooling rate is 19.2°C/s.
  • the cooling capacity and cooling speed are related to the injection pressure of the cooling gas, the hydrogen content in the cooling gas and the temperature of the cooling gas, the higher the hydrogen content in the cooling gas, the lower the temperature of the cooling gas, and the lower the cooling gas temperature.
  • the higher the injection pressure the stronger the cooling capacity and the faster the cooling speed.
  • the temperature of the cooling gas is the same, but the hydrogen content in the cooling gas of Embodiment 9 is higher, the injection pressure of the cooling gas is higher, and the corresponding cooling capacity and cooling speed are also higher. big.
  • controlling the rapid cooling temperature and rapid cooling rate of this stage of reaction is to make the steel plate and steel strip in the bainite phase region at this stage, and the temperature is too high or too low to make the steel plate or steel strip form a sufficient amount.
  • the bainite; and the rapid cooling rate is controlled at 10*Q ⁇ 20*Q°C/s, in order to make the rapid cooling rate as close as possible to the nose temperature region of the CCT curve of the bainite phase region, so that the bainite transformation is more accurate Full, faster rate.
  • the steel plate or steel strip starts from the initial smelting stage, in the long process of production, it is inevitable that there will be inhomogeneity in the composition and structure of the local area, so that some areas will have a low carbon equivalent or austenite excessive If the cooling rate is lower than the set range, there will be areas with low carbon equivalent or austenite undercooling due to excessive cooling rate. Slow enough to enter the pearlite transformation region, or the bainite transformation rate is too slow to cause insufficient transformation; similarly, if the cooling rate is higher than the set range, there will be a region with a lower carbon equivalent or austenite undercooling. The bainite phase region enters the martensite phase region, or the bainite transformation rate is too slow to make the transformation insufficient; these will eventually lead to the inability to form a thickness gradient structure.
  • the pressure of the cooling gas sprayed on the surface of the steel plate or strip is more important. Firstly, the pressure is controlled to be 0.3*Q ⁇ 1.5*Q kPa, and kept for 1 ⁇ 7 seconds, in order to form acicular bainite layer outside the core area in the thickness direction of the steel plate or steel strip, and as these areas are caused by bainite The temperature of the core area in the thickness direction of the strip will be higher than that of the surface layer and subsurface layer, thereby preparing for the formation of massive bainite in the core area.
  • the pressure of the cold injection gas or the holding time is lower than the set range, it is not conducive to the formation of acicular bainite in the surface layer and the subsurface layer, and the injection pressure or the holding time is higher than the set range, which will make the cooling capacity too strong.
  • Acicular bainite is also formed in the core region in the thickness direction of the strip, so the gradient structure in the thickness direction cannot be formed. Then further reduce the injection pressure to 0.08*Q ⁇ 0.2*Q kPa, and maintain it for 5 ⁇ 10 seconds. On the one hand, the surface layer and subsurface layer can still be effectively cooled to continuously form acicular bainite.
  • the temperature of the core area in the strip thickness direction does not continue to decrease or even rises slightly through the reduction of the cooling gas pressure and the latent heat released by the phase transition of the surface layer and the subsurface layer to ensure the formation of massive bainite in the strip core . And finally form a steel plate or steel strip with a structural gradient in the thickness direction.
  • the controlled cooling rate is controlled to be less than or equal to Q°C/s, and the controlled cooling time is kept to be 100 to 200 seconds.
  • the controlled cooling temperature of the bainitic steel is ⁇ 350°C. In some embodiments, the temperature of the bainitic steel at the end of the controlled cooling section is 350-410°C.
  • each bainitic phase transformation is fully completed, and at the set temperature, the structure is relatively slowly and stably formed to ensure the formation of a steel plate or steel strip with a structural gradient in the thickness direction.
  • the controlled cooling rate is higher than the set value or the controlled cooling temperature of the final steel plate or strip is lower than the set value, it will cause the formation of martensite in the structure and deteriorate the formability of the steel plate or strip.
  • the bainitic steel was allowed to air cool to room temperature.
  • a steel plate or steel strip with a structural gradient in the thickness direction is obtained.
  • the air cooling section has no effect on the structure of bainite steel.
  • a bainitic steel with a five-layer gradient structure in order to obtain a bainitic steel with a five-layer gradient structure, it is only necessary to control the cooling parameters of the slow cooling section to be different, and the original three-layer gradient structure can be obtained. , and further form a multi-phase layer on the surface to obtain a steel plate or steel strip with a five-layer gradient structure in the thickness direction. Subsequently, through the rapid cooling section and the controlled cooling section, other regions of the bainite steel will also produce acicular bainite or massive bainite according to the position difference in the thickness direction. Finally, a multiphase layer containing ferrite in the surface layer, acicular layer in the shallow surface layer and massive layer in the core can be formed, and a steel plate or steel strip with a five-layer structure with a structural gradient can be obtained.
  • the present invention uses bainite steel through reasonable element composition design, especially through reasonable control of the content of C, Si, Mn, B elements in the steel, and reasonable control of the content of C, Cr, Mo, Mn elements in the steel Optimizing the hardenability of the steel enables the steel to spontaneously form a phase with a structural gradient during the preparation process, improving the strength and formability of the bainitic steel.
  • the present invention discloses a manufacturing method of bainite steel, through fine annealing step design, especially the control of cooling gas pressure and temperature in the cooling stage, the steel plate/strip with suitable chemical composition can be made in the present invention
  • Three-layer or five-layer tissue gradients are spontaneously formed under the annealing conditions.
  • the tensile strength of the bainite steel obtained by adopting the technical scheme of the invention is ⁇ 1000MPa, the yield strength is ⁇ 800MPa, the hole expansion rate is ⁇ 40%, and the elongation at break is ⁇ 12%.
  • Fig. 1 shows a schematic diagram of a steel strip having a three-layer structure in the thickness direction in an embodiment of the present invention.
  • Fig. 2 shows a schematic diagram of a steel strip having a 5-layer structure in the thickness direction in an embodiment of the present invention.
  • Fig. 3 shows a photo of the metallographic structure at the transition position between the acicular layer (upper part) and the multiphase layer (lower part) in Example 7 of the present invention.
  • Fig. 4 shows a photo of the metallographic structure at the transition position between the needle-like layer (upper part) and the massive layer (lower part) in Example 1 of the present invention.
  • the bainite steel of embodiment 1-14 adopts the following steps to make among the present invention:
  • Step 1 smelting and casting
  • Step 2 hot rolling: control the heating temperature to 1100-1230°C, the start temperature of finish rolling to 1050-1180°C, and the finish rolling temperature to 870-930°C;
  • Step 3 cooling and coiling after rolling: control the cooling rate to 30-150°C/s, and control the coiling temperature to 540-620°C.
  • Step 4 pickling to remove iron oxide scale
  • Step 5 cold rolling: control the reduction rate of cold rolling to ⁇ 30%, so as to achieve the required target thickness.
  • the thickness of the steel plate or strip after cold rolling is ⁇ 2.2 mm;
  • Step 6 annealing.
  • the bainitic steels of Comparative Examples 1-6 are also prepared through smelting, continuous casting, hot rolling, cooling and coiling after rolling, pickling and cold rolling, and annealing steps.
  • the chemical composition of the steel and the process parameters of the preparation process are specific See Table 1-2.
  • Table 1 lists the mass percentages of each chemical element in the bainitic steels of Examples 1-14 and Comparative Examples 1-3.
  • Table 2 lists the specific process parameters of the bainitic steels of Examples 1-14 and the comparative steels of Comparative Examples 1-6.
  • Example 1 A 0.155 0.22 175 0.001 0.01 0.18 0.18 0.002 0.003 0.01 0.001 0.98 143
  • Example 2 B 0.165 0.35 2.15 0.001 0.03 0.05 0.40 0.004 0.002 0.008 0.001 117 145
  • Example 3 C 0.125 0.05 185 0.002 0.02 0.31 0.13 0.003 0.11 0.008 0.001 104 143
  • Example 4 D. 0.10 01 165 0.003 0.02 0.12 0.35 0.08 0.02 0.006 0.001 104 134
  • Example 5 E.
  • Examples 1-5, 8, and 10-11 have all obtained a three-layer tissue structure in the thickness direction, the upper and lower surface layers are needle-like layers, and the core is a massive layer; Examples 6-7, 9, and 12-14 have all obtained There are 5 layers in the thickness direction, the upper surface and the lower surface are complex layers, the upper surface and the lower surface are acicular layers, and the core is a massive layer.
  • the hardness of the acicular layer is the highest, the hardness of the multiphase layer is the smallest, and the hardness of the massive layer is between the acicular layer and the multiphase layer.
  • the acicular layers on the upper and lower surfaces can ensure that the material has high surface hardness and surface yield strength, while the massive layer in the middle ensures that the material has relatively high toughness and plasticity, so it can be used for Auto parts that have high requirements on the surface hardness or fatigue limit of the material, and at the same time have high requirements on the toughness and plasticity of the material as a whole, such as car seat slide rails, chassis torsion beams and other structural parts; and for 5-layer composite materials,
  • the relatively soft multiphase layer on the upper and lower surfaces can make the surface layer have better local forming ability, while the subsequent harder acicular layer and the massive layer in the core endow the material with higher strength and better toughness. Therefore, it can be used to prepare parts that require high strength and comprehensive forming capabilities, such as control arms and triangle arms of automobile chassis.
  • Comparative Examples 1-3 because the composition design does not meet the requirements of the invention, no steel plate or steel strip with a gradient structure in the thickness direction can be obtained.
  • Comparative Example 1 only pure massive laminar tissue was obtained because the R value was too high, and in Comparative Examples 2-3, only pure needle-like lamellar tissue was obtained because the R value was too low.
  • Comparative example 4-6 uses steel type A. Although the composition design meets the requirements, the annealing process in the manufacturing process does not meet the requirements of the invention, and no steel plate or steel strip with a thickness direction gradient structure can be obtained.
  • Comparative Example 4 because the cooling gas pressure in the slow cooling section is higher than the design value, a large proportion of ferrite is formed in the direction of the entire thickness of the steel plate or strip, and in the fast cooling section, because the cooling gas pressure is higher than
  • the design value leads to the formation of acicular bainite in the direction of the entire thickness of the steel plate or strip instead of massive bainite, and because a certain large proportion of ferrite has been preferentially formed in the steel plate or strip, resulting in Partially supercooled austenite is rich in carbon and does not undergo bainite transformation. Instead, it will transform into fresh martensite in the final air-cooled section.
  • the steel plate or steel strip not only cannot form a gradient structure in the thickness direction, but also has poor formability.
  • Comparative Example 5 only pure acicular bainite can be obtained because the pressure of the cooling gas in the rapid cooling section is higher than the design value, and correspondingly, in Comparative Example 6, the pressure of the cooling gas in the rapid cooling section is lower than the design value. to pure massive laminar tissue.
  • Fig. 3 is a photo of the metallographic structure of the lower surface layer region of Example 7 of the present invention, specifically the transition position between the acicular layer (upper part) and the multiphase layer (lower part) (photographed by a scanning electron microscope).
  • the organization In the upper part of the picture, that is, the area closer to the core, the organization is typical acicular bainite, representing the beginning of the acicular layer in this area; while in the lower part of the picture, that is, the area closer to the lower surface, contains polygonal ferrite, Acicular bainite and dispersed precipitated nanoscale, submicron or micron-scale granular carbide precipitates represent the multi-phase layer that this region begins to enter the surface layer.
  • Fig. 4 is the area near the upper surface layer of the core in Example 1 of the present invention, specifically the metallographic structure photo (photographed by scanning electron microscope) of the transition position between the acicular layer (upper part) and the massive layer (lower part); in the upper part of the picture , that is, the area closer to the upper surface, the tissue contains a large amount of typical acicular bainite, which means that this area begins to enter the acicular layer; while in the lower part of the picture, that is, the area closer to the core, a large amount of bainite transforms into The morphology of blocky polygons, that is, the formation of a large number of blocky bainite in this region, represents the beginning of entry into the blocky layer in this region.
  • Table 3 has listed the mechanical performance test result of the bainite steel of embodiment 1-14 and comparative example 1-6, gets along transverse JIS 5# tensile sample and measures the yield strength, tensile strength and elongation at break of steel , using GB/T 228.1-2010 "Metallic Materials Tensile Test Part 1: Room Temperature Test Method" for testing. Take the middle area of the plate to measure the hole expansion rate of the steel. The hole expansion rate is measured by the hole expansion test. By using the punch to press the test piece with a hole in the center into the die, the center hole of the test piece is enlarged until the edge of the plate hole shrinks or penetrates cracks.
  • the test and test method are carried out according to the hole expansion rate test method specified in the ISO/DIS 16630 standard , the original hole in the center core adopts the form of one-time punching and blanking, which corresponds to the worst processing method for the edge of the original hole.
  • the corresponding hole expansion rate will increase by 20% on the basis of the values in the table; if the original hole in the center core If the hole is made by wire cutting, the corresponding hole expansion rate will be increased by 50% on the basis of the value in the table; if the core original hole is made by laser blanking, the corresponding hole expansion rate will be the value in the table An increase of 80% on the basis.

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Abstract

本发明公开了一种贝氏体钢，包括以质量百分比计的化学成分：C：0.10～0.19％，Si：0.05～0.45％，Mn：1.5～2.2％，B：0.001～0.0035％，Al：0.01～0.05％，Cr；0.05～0.40％，Mo：0.05～0.40％，Fe≥90％。通过合理控制钢中C、Si、Mn、B、Al、Cr、Mo等元素的含量使得钢在制备过程中能自发形成具有组织梯度的相，同时也提升了钢的淬透性，从而可以提高贝氏体钢的强度和成形性能。本发明还公开了上述贝氏体钢的制备方法：包括步骤冶炼和铸造；热轧；轧后冷却和卷取；酸洗和冷轧；退火。采用本发明的制备方法可以制得具有厚度方向上组织梯度的贝氏体钢，这种贝氏体钢具有良好的成形能力。

Description

一种贝氏体钢及其制备方法 技术领域
本发明涉及冶金技术领域，尤其涉及一种贝氏体钢及其制备方法。
背景技术
随着新一代汽车“绿色、安全”的发展理念，汽车结构件用钢的强度越来越高，且逐渐对材料提出了在厚度方向具有不同性能的要求。比如要求材料表层硬度较大，耐磨，或者表层具有较高的组织均匀性，以满足翻边成形需要，但同时又要使得芯部具有较高的塑性，使得钢材整体在拉延成形时不发生颈缩和断裂；又或者要求表层具有一层硬度较低的层状结构，来保证材料具有一定的弯曲性能，但次表层仍要具有均匀的硬质相组织来保证翻边和强度，以及芯部具有较软的组织来保证塑性、韧性等，以保证材料既具有较高的强度，还能同时具有较好的弯曲、翻边和拉延等综合成形能力。
面对汽车工业越来越多对钢材在厚度方向具有不同组织或性能的要求，传统的方法是通过将不同成分或组织的板坯通过焊接、组合轧制等方法，获得具有厚度方向梯度组织的钢铁材料。如CN201210368300.6和CN201310724615.4等，就是利用金属的组合轧制获得厚度方向的层状复合材料。但是这种方法工艺复杂，生产节奏缓慢，成本极高。
也有专利尝试通过采用表层脱碳的方法，获得表层和芯部具有不同组织结构的钢板或钢带，如利用带钢表面脱碳，形成数微米至数十微米的脱碳层，使得上、下两个表层组织为纯铁素体或铁素体占比＞50％的组织，而芯部则为其他单相或复相组织，如马氏体、回火马氏体或贝氏体组织。这种方法虽然可以自发形成厚度方向组织梯度，获得3层复合组织的高强度钢板，但是一方面，表层和芯部的强度或硬度差异过大，表层的强度或硬度过低，不仅大幅度限制了该类产品的应用范围(如对表层有高硬度要求、或耐疲劳要求的应用领域，如汽车座椅滑轨、底盘扭力梁等)造成材料虽然具有较好的弯曲性能，但是延伸率和扩孔率都不高，即塑性和翻边性能较差；而另一方面，这种方法只能形成3层复合组织，无法进一步获得更多层的组织。
发明内容
鉴于现有技术中获得具有组织梯度的钢板成本高、无法获得超过3层复合组织的问题，本发明提供了一种贝氏体钢，其屈服强度≥800MPa，抗拉强度≥1000MPa，断裂延伸率≥12％，扩孔率≥40％的力学性能，此外由于钢板或钢带在厚度方向成梯度组织，使材料兼具较好的综合成形性能，即拉延性和扩孔翻边性均较好，体现为断裂延伸率和扩孔率均比较高，所有实施例的(断裂延伸率*10+扩孔率)≥170％。
本发明的贝氏体钢包括以质量百分比计的化学成分：C：0.10～0.19％，Si：0.05～0.45％，Mn：1.5～2.2％，B：0.001～0.0035％，Al：0.01～0.05％，Cr；0.05～0.40％，Mo：0.05～0.40％，Fe≥90％。
其中，各元素的设计思路如下：
C：在本发明的贝氏体钢中，C元素主要控制碳钢中组织相变、碳化物尺寸和贝氏体亚结构形态，从而影响材料的力学性能。当钢中C元素含量低于0.10％，则会导致钢的强度达不到目标要求；而若钢中C元素含量高于0.19％时，则容易生成马氏体组织和粗大的渗碳体，恶化钢板的性能。此外，在本发明中，C元素还会影响贝氏体亚形态结构，C含量越高，越容易形成针状贝氏体。基于此，在本发明中，将C的质量百分比控制在0.10～0.19％之间。优选地，C的质量百分比为0.13～0.17％。
Si：在本发明的贝氏体钢中，Si一方面具有一定的固溶强化作用，另一方面还会影响钢板表面质量。当钢中Si元素含量低于0.05％时，难以获得充分的强化效果；而当钢中Si元素含量高于0.45％时，则容易形成氧化铁皮或虎皮纹条状色差，不利于汽车用钢板的表面质量。此外，在本发明中，Si元素还会影响贝氏体亚形态结构，Si含量越高，越容易形成多边形贝氏体，基于此，本发明中将Si的质量百分比控制在0.05～0.45％之间。优选地，Si的质量百分比为0.05～0.35％。更优选地，Si的质量百分比为0.15～0.3％。
Mn：在本发明的贝氏体钢中，Mn元素为钢中组织相变的控制元素之一，还会影响贝氏体亚形态结构，Mn含量越高，越容易形成多边形贝氏体。但需要注意的是，钢中Mn元素含量不宜过高，当钢中Mn元素含量过高时，会恶化耐腐蚀性能和焊接性能。基于此，在本发明将Mn的质量百分比控制在1.5～2.2％之间。优选地，Mn的质量百分比为1.7～2.1％。
B：在本发明的贝氏体钢中，B元素不仅有利于钢中贝氏体的形成，还会影响钢板的强度与成形性能，同时也会影响贝氏体亚形态结构。当B含量越高，越容易获得针状贝氏体，钢板强度越高，也越容易形成脆性硼化物，影响钢板的扩孔率。基于此，在本发明中，将B的质量百分比控制在0.001～0.0035％之间。
Al：在本发明的贝氏体钢中，Al元素仅作为脱氧元素加入钢中，其可以除去钢中的O元素，以保证钢的性能和质量。因此，在本发明的贝氏体钢中，将Al的质量百分比控制在0.01～0.05％之间。虽然在一些现有技术中Al元素作为铁素体形成元素和抑制碳化物析出元素而大量(≥0.1％)加入钢中，以期望带来固溶强化作用，或通过Al的添加来改变相变温度(如A1，A3)、贝氏体形成动力学和碳化物析出动力学来改变钢材的相变，形成残余奥氏体或无碳贝氏体，并最终提高钢材强度。但本发明现有的成分控制和工艺调节已可以获得综合成形性能良好的贝氏体钢，添加大量Al元素所诱发形成的无碳贝氏体反而会破坏厚度方向形成贝氏体组织梯度，还会造成成本上升和连铸生产难度的大幅提升，因此在本发明中，将Al的质量百分比控制在0.01～0.05％之间，以避免成本升高或连铸制造难度大幅增加，保证厚度方向的贝氏体组织梯度能够形成。
Cr和Mo：在本发明的贝氏体钢中，Cr和Mo不仅可以与C形成细小弥散的碳化物析出相，能进一步提升钢板的强度，更影响CCT曲线中珠光体和铁素体的孕育期，可以提高钢板的淬透性，从而可以与退火工艺中的钢板冷却速率配合设计，以控制厚度方向的组织梯度形成和不同的厚度比例。基于此，在本发明中，将Cr和Mo的质量百分比控制为：0.05％≤Cr≤0.40％，0.05％≤Mo≤0.40％。
本发明通过合理控制钢中C、Si、Mn、B、Al、Cr、Mo等元素的含量使得钢在制备过程中能自发形成具有组织梯度的相，同时也提升了钢的淬透性，从而可以提高贝氏体钢的强度和成形性能。
进一步地，在上述贝氏体钢中，还包括Ti和Nb中的至少一种，其中，Ti和Nb的质量百分比需要满足：Nb≤0.1％，Ti≤0.15％。
Ti和Nb：在本发明的贝氏体钢中，Ti和Nb作为可选合金元素，可以将其添加进钢中，从而形成细小弥散的碳化物析出相，并细化组织晶粒，进一步提升钢板的强度和成形性能。基于此，在本发明的贝氏体钢中，将Nb和Ti的质量百分比分别控制为：Nb≤0.1％，Ti≤0.15％。上述合金元素的加入会增加材料的成本，综合考虑到性能与成本控制，在本发明的技术方案中，可以优选地添加Nb和Ti中的至少一种。在一些实施方案中，本发明的贝氏体钢含有Nb和Ti，Nb的质量百分比含量为0.001～0.1％，Ti的质量百分比含量为0.001～0.15％。
在一些实施方案中，本发明的贝氏体钢包括以质量百分比计的化学成分：C：0.10～0.19％，Si：0.05～0.45％，Mn：1.5～2.2％，B：0.001～0.0035％，Al：0.01～0.05％，Cr；0.05～0.40％，Mo：0.05～0.40％，余量为Fe及不可避免的杂质。
进一步地，在上述不可避免的杂质中，P≤0.015％，S≤0.004％。
P和S均为钢中的杂质元素，在技术条件允许情况下，为了获得性能更好且质量更优的调质钢，应尽可能降低钢中杂质元素的含量。
进一步地，在本发明的贝氏体钢中，化学元素的质量百分比需要满足以下关系：R＝(Mn+Si)/(12*C+160*B)，其中，0.9≤R≤1.2，式中的各化学元素均代入该化学元素的质量百分含量的百分号前面的数值。
在本发明中，定义了R＝(Mn+Si)/(12*C+160*B)，经过实验发现，若用此公式进行计算，需要将R值限定在一定的范围，即0.9≤R≤1.2，才能得到期望的具有组织梯度的贝氏体钢板/钢带结构。其中，C和B元素越高，越有利于形成针状贝氏体；而Mn和Si元素越高，越有利于形成块状贝氏体。因此，通过合理设计C和B、与Mn和Si的含量，就能使钢板和钢带在成分设计上处于既有利于形成针状贝氏体又有利于形成块状贝氏体的临界状态，即本公式中0.9≤R≤1.2的状态。继而通过与优化的退火工艺相匹配，就能够最终在钢板中形成厚度方向的组织梯度。此外，由于C、B元素含量虽低，但对贝氏体形成及其形态的影响更强，因此在公式中需要配以较大的系数，来平衡Mn和Si的高含量，毕竟Mn、Si元素对贝氏体形成及其形态的影响要显著弱于C、B元素。在本设计中，0.9≤R≤1.2的水平为最适合形成梯度组织的临界水平，若R过高，则导致梯度组织中的块状层厚度过大而针状层厚度过小，甚至无针状层而导致组织在厚度方向不存在梯度；而若R过低，则导致梯度组织中的针状层厚度过大而块状层厚度过小，甚至无块状层而导致组织在厚度方向不存在梯度。因此在本发明中可以将R控制为0.9≤R≤1.2，从而保证钢中在厚度方向存在组织梯度和力学性能。
进一步地，在本发明的贝氏体钢中，化学元素的质量百分比需要满足以下关系：Q＝(C+Cr+Mo+Mn/2)/R，其中，1.15≤Q≤1.5，计算时代入元素质量百分比百分号前的数值。
在本发明中，定义了Q＝(C+Cr+Mo+Mn/2)/R，可以进一步指导钢的成分设计，经过实验发现，当1.15≤Q≤1.5时，钢具有适宜的淬透性以及组织梯度形成能力。因组织的梯度或其层状结构分布于钢板和带钢的厚度方向，因此，钢板和钢带的淬透性同样是厚度方向梯度组织形成的最重要影响因素，在本申请中，C、Cr、Mo、Mn都影响钢板和钢带的淬透性，这些元素含量越高，淬透性越强。但是，由于Mn含量比其他元素高一个数量级，且对淬透性的影响相对较弱，故在本公式中，对Mn设计了1/2的系数。由于针状贝氏体和块状贝氏体在退火工艺中的生成温度略有差异，其中针状贝氏体生成温度更低，而块状 贝氏体生成温度更高，因此钢板的淬透性越高，越有利于形成针状贝氏体而不利于形成块状贝氏体，反之亦然。因此，为保证钢板和钢带中针状贝氏体与块状贝氏体的在厚度方向上的占比能形成一个合适的三明治比例，当钢板的成分设计更有利于形成块状贝氏体，即R值较高时，需要配以更高的淬透性来促进针状贝氏体形成；而当钢板的成分设计更有利于形成针状贝氏体，及R值较低时，则需要配以较低的淬透性了促进块状贝氏体形成；因此，Q值的分子即为代表带钢淬透性的合金含量，越高代表淬透性越强；而分母选取能代表组织中块状贝氏体与针状贝氏体形成能力的R值，其分子与分母的比值，即Q值则直接影响退火过程中块状层与针状层的形成能力与最终比例。若Q值过小，则代表块状贝氏体形成能力过强，最终组织中很难形成针状贝氏体，也就难以形成梯度组织；而若Q值过大，则代表针状贝氏体形成能力过强，最终组织中很难形成块状贝氏体，同样难以形成梯度组织。
进一步地，上述贝氏体钢具有两层表层组织和一层芯部组织，芯部组织在两层表层组织之间。
进一步地，贝氏体钢中，芯部组织的体积占贝氏体钢体积的20％～50％，剩余为表层组织。
进一步地，表层组织包括针状贝氏体和粒状碳化物析出相；芯部组织包括块状贝氏体和粒状碳化物析出相。
进一步地，针状贝氏体和粒状碳化物析出相占表层组织体积的99％以上，块状贝氏体和粒状碳化物析出相占芯部组织体积的99％以上。
具体地，在本发明一些实施例的贝氏体钢中，参考图1，在钢板或钢带的厚度方向存在3层结构，自一侧表面至另一侧表面的组织分别为：
表层组织2：为针状层，即以针状贝氏体和弥散析出的纳米级、亚微米级或微米级的粒状碳化物析出相为主的组织，占该区域相比例总量≥99％。在厚度方向的占比为25％～40％。
芯部组织1：为块状层，即以块状贝氏体和弥散析出的纳米级、亚微米级或微米级的粒状碳化物析出相为主的组织，占该区域相比例总量≥99％。在厚度方向的占比为20％～50％。
表层组织2：为针状层，即以针状贝氏体和弥散析出的纳米级、亚微米级或微米级的粒状碳化物析出相为主的组织，占该区域相比例总量≥99％。在厚度方向的占比为25％～40％。
3层区域在贝氏体钢厚度方向上的占比总和为100％。
进一步地，上述贝氏体钢还具有两层复相层，上述两层表层组织和一层芯部组织组成中间层，中间层在两层复相层之间。
进一步地，贝氏体钢中，复相层的体积占贝氏体钢体积的2％～10％，剩余为中间层。
进一步地，复相层包括多边形铁素体、针状贝氏体和粒状碳化物析出相，其中多边形铁素体占复相层体积的50％以下，多边形铁素体、针状贝氏体和粒状碳化物析出相占复相层体积的99％以上。
具体地，在本发明一些实施例的贝氏体钢中，参考图2，若在钢板或钢带的厚度方向存在5层结构，自一侧表面至另一侧表面的组织分别为：
复相层3：为以多边形铁素体、针状贝氏体和弥散析出的纳米级、亚微米级或微米级的粒状碳化物析出相为主的组织(其中多边形铁素体组织＜50％)，多边形铁素体、针状贝氏体和弥散析出的纳米级、亚微米级或微米级的粒状碳化物析出相占该区域相比例总量≥99％。在厚度方向的占比为1％～5％。
表层组织2：为针状层，即以针状贝氏体和弥散析出的纳米级、亚微米级或微米级的粒状碳化物析出相为主的组织，占该区域相比例总量≥99％。在厚度方向的占比为25％～40％。
芯部组织1：为块状层，即以块状贝氏体和弥散析出的纳米级、亚微米级或微米级的粒状碳化物析出相为主的组织，占该区域相比例总量≥99％。在厚度方向的占比为25％～40％。
表层组织2：为针状层，即以针状贝氏体和弥散析出的纳米级、亚微米级或微米级的粒状碳化物析出相为主的组织，占该区域相比例总量≥99％。在厚度方向的占比为25％～40％。
复相层3：为以多边形铁素体、针状贝氏体和弥散析出的纳米级、亚微米级或微米级的粒状碳化物析出相为主的组织(其中多边形铁素体组织＜50％)，多边形铁素体、针状贝氏体和弥散析出的纳米级、亚微米级或微米级的粒状碳化物析出相占该区域相比例总量≥99％。在厚度方向的占比为1％～5％。
5层区域占比总和为100％。其中，针状层硬度最大，复相层硬度最小。
在本发明的贝氏体钢中，粒状碳化物析出相的直径≤5μm。限制碳化物析出相的原因是为了避免扩孔率恶化。当碳化物析出相尺寸＞5μm时，在钢板承受扩孔翻边变形时或检测扩孔率时，容易在碳化物与基体的结合处发生开裂，造成钢板扩孔率降低及扩孔翻边性 能的恶化。
进一步地，在本发明的贝氏体钢中，贝氏体钢的抗拉强度≥1000MPa，屈服强度≥800MPa，扩孔率≥40％，断裂延伸率≥12％。
进一步地，在本发明的贝氏体钢中具有优异的拉延性和扩孔翻边性，即断裂延伸率*10+扩孔率≥170％。
本发明还提供了上述贝氏体钢的制备方法，包括步骤：
冶炼和铸造；
热轧；
轧后冷却和卷取；
酸洗和冷轧；
退火。
由于不采用常见的表层脱碳的方式来制备具有组织梯度的贝氏体钢，本发明中贝氏体钢不存在表层强度和硬度大幅低于芯部的问题。
进一步地，控制上述制备方法的工艺参数满足下述各项中的至少一项：
在热轧步骤中，控制加热温度为1100～1230℃，精轧开轧温度为1050～1180℃，精轧终轧温度为870～930℃；
在轧后冷却和卷取步骤中，控制冷却速率为30～150℃/s，控制卷取温度为540～620℃；
在冷轧步骤中，控制冷轧压下率≥30％。
在上述制备方法中，退火前工艺步骤主要是为了获得成分和原始组织均匀的钢板或钢带，以保证后续的退火工艺实施时能够满足组织和性能的均匀稳定，而对钢板的性能起到关键作用的是退火工艺。
在介绍退火工艺前，需先介绍以下概念：
由于本发明意在设计钢板/钢带厚度方向的梯度组织，故钢板或钢带不可避免或故意地会在厚度方向具有不同的温度范围，但由于钢板或钢带的连续生产模式的限制，温度检测与控制只能针对上下表面温度，无法检测厚度方向的其他位置温度。而对于上表面和下表面的温度，不做额外区分，按相同工艺处理，均称为表面温度。下文中提及的温度及冷速均指表面温度和由表面温度计算得到的冷速。需要指出的是，冷却时，根据表面温度、冷速、冷却时的喷射气体压强(代表冷却能力)和钢板的淬透性来控制钢板或钢带的厚度方向的温度分布。
进一步地，退火步骤依次包括加热段，缓冷段，快冷段，控冷段和空冷段，控制冷却 速率在缓冷段，快冷段，控冷段三个阶段满足：控冷段＜缓冷段＜快冷段。
进一步地，在加热段，以≤50℃/s的加热速率加热至均热温度840～950℃，然后保温，控制保温时间为60～180秒。
在加热段，需要控制以≤50℃/s的加热速率加热贝氏体钢至均热温度840～950℃并保温60～180s。其中，若加热段的加热速率＞50℃/s，或保温时间＜60s，都会造成带钢组织的均匀性较差，影响后续厚度方向梯度组织的形成。此外，若温度低于上述均热温度下限，则带钢无法获得足够多的贝氏体组织(无论是针状贝氏体还是块状贝氏体)。进一步地，加热速率优选为5～50℃/s。而若保温时间＞180s，或进一步地，若均热温度高于950℃，都会造成带钢晶粒粗大，导致钢的成形性能恶化。
本发明中，为了形成厚度方向上具有3层组织梯度的贝氏体钢，在缓冷段，以Q～10*Q℃/s的缓冷速率冷却至缓冷温度720～800℃；其中，化学元素的质量百分比满足关系：Q＝(C+Cr+Mo+Mn/2)/R，1.15≤Q≤1.5，R＝(Mn+Si)/(12*C+160*B)，0.9≤R≤1.2，式中的各化学元素均代入该化学元素的质量百分含量的百分号前面的数值。在一些实施方案中，控制该缓冷速率为5Q～10Q℃/s。在一些实施方案中，控制该缓冷速率为7Q～10Q℃/s。
具体地，在本发明的实施例中，采用向贝氏体钢表面喷射冷却气体的方法来实现缓冷。示例性地，冷却时，通过向贝氏体钢的表面喷射冷却气体进行冷却，控制冷却气体喷射压强为0.2*Q～Q kPa，控制冷却气体喷射的保持时间为5～20秒。当然，在其他可能的实施例方式中，也可以采用如液体冷却等方式来达到缓冷的目的，只要能够以Q～10*Q℃/s的缓冷速率冷却至缓冷温度720～800℃即可。这一阶段的主要目的是使钢板或钢带在宽度方向达到温度均匀，厚度方向温度较不均匀，但各位置都不发生组织转变。
此步骤控制缓冷速率，是为了使钢板或钢带在宽度方向达到均匀的温度，控制温度则是为了带钢各位置都不发生相变，若温度过低，则有可能使奥氏体相变分解形成铁素体或珠光体，温度过高则不利于下一段冷却的高精度控制，进而不利于获得厚度方向的梯度组织。而控制冷却气体喷射至钢板或带钢表面的压强和控制保持时间，都是为了控制在带钢厚度方向的冷却不均匀，若冷却气体喷射至钢板或带钢表面的压强小于0.2*Q kPa或保持时间低于5秒，都意味着冷却能力的不足，虽然带钢表面冷到设定温度，但大部分的表层以下的区域都处于较高的温度，不利于下一步在厚度方向形成梯度组织，或下一阶段形成的梯度组织中针状贝氏体区域过小；而高于Q kPa或保持时间大于20秒，则会造成冷却能力过大，使带钢芯部温度接近甚至达到表面温度，也不利于下一步在厚度方向形成梯度 组织，或下一阶段形成的梯度组织中块状贝氏体区域过小。
本发明中，为了形成厚度方向上具有5层组织梯度的贝氏体钢，在缓冷段，以Q～10*Q℃/s的缓冷速率冷却至缓冷温度620～700℃；其中，化学元素的质量百分比满足关系：Q＝(C+Cr+Mo+Mn/2)/R，1.15≤Q≤1.5，R＝(Mn+Si)/(12*C+160*B)，0.9≤R≤1.2，式中的各化学元素均代入该化学元素的质量百分含量的百分号前面的数值。在一些实施方案中，控制该缓冷速率为5Q～10Q℃/s。在一些实施方案中，控制该缓冷速率为7Q～10Q℃/s。
具体地，在本发明的实施例中，采用向贝氏体钢表面喷射冷却气体的方法来实现缓冷。示例性地，冷却时，通过向贝氏体钢的表面喷射冷却气体进行冷却，控制冷却气体的喷射压强为0.05*Q～0.15*Q kPa，控制冷却气体喷射的保持时间为5～15秒。当然，在其他可能的实施例方式中，也可以采用如液体冷却等方式来达到缓冷的目的，只要能够使贝氏体钢以Q～10*Q℃/s的缓冷速率冷却至缓冷温度620～700℃的技术方案均属于本申请的保护范围。
此步骤中，冷却至620～700℃是为了保证钢板或钢带的表面进入铁素体转变温度区间，并通过一定时间的保温，使得钢板或钢带能够在表面区域形成一定含量的铁素体，为最终形成表层的复相层做准备；低于或高于该温度都不能保证带钢表面形成一定含量的铁素体。同样，保持时间过短，或冷速过快，将造成带钢表面铁素体来不及生成，而最终导致表层复相层无法形成；而相反，若保持时间过长，或者冷速过慢，将导致带钢表面形成的铁素体含量过多，厚度过厚，不仅不利于表层复相层的形成，更会导致在快冷段浅表层无法形成足量的针状贝氏体，即影响后续针状层的形成。
而冷却气体喷射至钢板或带钢表面的压强为0.05*Q～0.15*Q kPa，是为了控制带钢表面形成多边形铁素体的厚度，在该压强范围内，并且保持时间也符合设定范围时，钢板或钢带实际上只有表层区域冷却到620～700℃并进入铁素体相区，而其他区域温度仍然高于700℃并不发生铁素体转变(由于铁素体形成同样会释放相变潜热)。但若喷射冷却气体的压强过高，将导致钢板或钢带的浅表层甚至芯部温度也随之下降，不利于后续针状层和块状层的形成。而若喷射冷却气体的压强过低，则不利于表层稳定的形成一定量的多边形铁素体，导致表层无法形成稳定的形成复相层。
缓冷结束之后，在快冷段，不论是为了形成厚度方向上具有3层还是5层梯度组织的贝氏体钢，均需要控制以10*Q～20*Q℃/s的快冷速率冷却至快冷温度400～540℃。
具体地，在本发明的实施例中，采用向贝氏体钢表面喷射冷却气体的方法来实现快冷。 在此阶段，冷却时需要向贝氏体钢的表面喷射两次冷却气体，控制冷却气体的第一喷射压强为0.3*Q～1.5*Q kPa，控制冷却气体的第一保持时间为1～7秒；控制冷却气体的第二喷射压强为0.08*Q～0.2*Q kPa，控制冷却气体的第二保持时间为5～10秒。同样地，在其他可能的实施例方式中，也可以采用如液体冷却等方式来达到缓冷的目的，只要在此阶段能够使贝氏体钢以10*Q～20*Q℃/s的快冷速率冷却至快冷温度400～540℃的技术方案均属于本申请的保护范围。
进一步地，退火步骤所用的冷却气体为还原性气体和惰性气体的混合物。优选地，该混合物中，还原性气体的体积分数为1％～8％。在一些实施方案中，该混合物中的还原性气体为氢气，其体积分数为1％～8％。可将该冷却气体的温度控制为5～50℃。
在本发明的一些实施例中，钢板或钢带的冷却是通过向其表面喷射冷却气体(即还原性气体和惰性气体的混合物)来进行的，其中还原性可由氢气实现。本发明中，惰性气体是指不会在实验条件下与贝氏体钢发生化学反应影响钢的组织的气体。具体地，出于节省成本考虑，惰性气体可全部采用氮气。冷却气体中氢气的含量与温度可进一步控制，具体见表2。在贝氏体钢的冷却过程中通过控制喷射气体的压强、冷却气体中氢气的含量和冷却气体的温度等来控制冷却能力或冷却强度，具体数值需要根据钢板或钢带的淬透性来确定。对同一实施例来说，通常情况下，冷却气体中的氢气含量和冷却气体温度在退火工艺中保持不变，此时冷却强度、冷速与喷射气体压强成正相关，如实施例1，在缓冷段，冷却气体喷射压强为0.6kPa，缓冷段冷却速率为12.5℃/s；而在快冷段，冷却气体第一喷射压强为1kPa，对应的冷速为19.2℃/s。而对于不同实施例来说，冷却能力和冷却速度则与冷却气体喷射压强，冷却气体中的氢气含量和冷却气体温度都相关，冷却气体中的氢气含量越高，冷却气体温度越低，冷却气体喷射压强越大，则冷却能力越强，冷却速度越快。示例性地，如实施例7和实施例9，冷却气体温度相同，但其中实施例9的冷却气体中的氢气含量更高，冷却气体喷射压强更大，对应的冷却能力和冷却速度也就越大。
具体地，控制此阶段反应的快冷温度和快冷速率，都是为了使该阶段钢板和钢带处于贝氏体相区，温度过高或过低，都不能使钢板或钢带形成足量的贝氏体；而快冷速率控制在10*Q～20*Q℃/s，是为了使快冷速率尽可能的贴近贝氏体相区CCT曲线的鼻温区域，使贝氏体转变更充分，速率更快。因为钢板或钢带从最初的冶炼阶段开始，在长流程的生产过程中，不可避免地会出现局部区域成分和组织的不均匀性，从而会出现有些区域碳当量较低，或奥氏体过冷较小，而有些区域碳当量较高，或奥氏体过冷程度较大，若冷速小于设定范围，会出现碳当量较低或奥氏体过冷较小的区域因冷速过慢而进入珠光体转变区 域，或贝氏体转变速率过慢而导致转变不充分；同样若冷速高于设定范围，会出现碳当量较低或奥氏体过冷较大的区域绕过贝氏体相区进入马氏体相区，或贝氏体转变速率过慢而使转变不充分；这些都会最终导致无法形成厚度方向梯度组织。
在影响快冷段进行的所有因素中，冷却气体喷射至钢板或带钢表面的压强更为重要。首先控制压强为0.3*Q～1.5*Q kPa，并保持1～7秒，是为了在钢板或钢带的厚度方向芯部区域以外形成针状贝氏体层，而随着这些区域因发生贝氏体相变而释放相变潜热，带钢厚度方向芯部区域的温度会高于表层和次表层，从而为芯部区域形成块状贝氏体而做准备。此时，若冷喷射气体压强或保持时间低于设定范围，都不利于表层和次表层形成针状贝氏体，而喷射压强或保持时间高于设定范围，会使得冷却能力过强而导致带钢厚度方向的芯部区域也形成针状贝氏体，故都无法形成厚度方向的梯度组织。之后进一步地将喷射压力降低至降低为0.08*Q～0.2*Q kPa，并维持5～10秒，一方面使得表层和次表层仍然能得到有效的冷却而不断形成针状贝氏体，另一方面则通过冷却气体压强的降低，配合表层和次表层相变释放的潜热，使得带钢厚度方向的芯部区域温度不继续降低甚至小幅度升高，保证带钢芯部形成块状贝氏体。并最终形成厚度方向具有组织梯度的钢板或钢带。
快冷段结束之后，不论是为了得到厚度方向上具有三层或者五层组织梯度的贝氏体钢，都需要再进行控冷步骤。在控冷段，控制控冷速率≤Q℃/s，保持控冷时间为100～200秒，在控冷段结束时贝氏体钢的控冷温度≥350℃。在一些实施方案中，控冷段结束时贝氏体钢的温度为350～410℃。
通过对钢板或带钢的长时间控冷，使各贝氏体相变充分结束，在设定温度下，让组织比较缓慢地稳定生成，保证形成厚度方向具有组织梯度的钢板或钢带。在此阶段，若控冷速率高于设定值或最终钢板或钢带的控冷温度低于设定值，都会造成组织中马氏体的形成，恶化钢板或钢带的成形性能。
在控冷段结束之后，使贝氏体钢空冷至室温。就得到了厚度方向具有组织梯度的钢板或钢带。空冷段对贝氏体钢的组织不产生影响。
综上所述，在本发明的一些实施例中，为了得到具有五层组织梯度的贝氏体钢，只需控制缓冷段的冷却参数不同，就可在原有的三层梯度组织的基础上，进一步在表层形成复相层而得到厚度方向上具有五层梯度组织的钢板或钢带。随后，再经快冷段、控冷段，则贝氏体钢的其他区域同样也会按厚度方向位置的差异而发生针状贝氏体或块状贝氏体。最终，可以形成表层含有铁素体的复相层，浅表层的针状层和芯部的块状层，获得具有组织梯度的5层结构的钢板或钢带。
本发明的有益效果在于：
1.本发明通过贝氏体钢通过合理的元素成分设计，特别是通过合理控制钢中C、Si、Mn、B元素的含量，并合理控制钢中C、Cr、Mo、Mn元素的含量来优化钢的淬透性，使得钢在制备过程中能自发形成具有组织梯度的相，提高了贝氏体钢的强度和成形性能。
2.本发明公开了一种贝氏体钢的制造方法，通过精细的退火步骤设计，特别是冷却阶段的冷却气体压强和温度的控制，能够使具有合适化学成分的钢板/钢带在本发明的退火条件下自发形成三层或五层的组织梯度。采用本发明的技术方案得到的贝氏体钢的抗拉强度≥1000MPa，屈服强度≥800MPa，扩孔率≥40％，断裂延伸率≥12％。
附图说明
图1示出本发明实施例中具有厚度方向上3层结构的钢带示意图。
图2示出本发明实施例中具有厚度方向上5层结构的钢带示意图。
图3示出本发明实施例7中针状层(上部)与复相层(下部)之间过渡位置的金相组织照片。
图4示出本发明实施例1中针状层(上部)与块状层(下部)之间过渡位置的金相组织照片。
具体实施方式
以下由特定的具体实施例说明本发明的实施方式，本领域技术人员可由本说明书所揭示的内容轻易地了解本发明的其他优点及功效。虽然本发明的描述将结合较佳实施例一起介绍，但这并不代表此发明的特征仅限于该实施方式。恰恰相反，结合实施方式作发明介绍的目的是为了覆盖基于本发明的权利要求而有可能延伸出的其它选择或改造。为了提供对本发明的深度了解，以下描述中将包含许多具体的细节。本发明也可以不使用这些细节实施。此外，为了避免混乱或模糊本发明的重点，有些具体细节将在描述中被省略。需要说明的是，在不冲突的情况下，本发明中的实施例及实施例中的特征可以相互组合。
实施例1-14以及对比例1-6
本发明中实施例1-14的贝氏体钢采用以下步骤制得：
步骤1、冶炼和铸造；
步骤2、热轧：控制加热温度为1100～1230℃，精轧开轧温度为1050～1180℃，精轧终轧温度为870～930℃；
步骤3、轧后冷却和卷取：控制冷却速率为30～150℃/s，控制卷取温度为540～620℃。
步骤4、酸洗去除氧化铁皮；
步骤5、冷轧：控制冷轧压下率≥30％，以达到要求目标厚度。具体地，在本发明的实施例中冷轧后钢板或钢带的厚度≤2.2mm；
步骤6、退火。
对比例1-6的贝氏体钢也是通过冶炼、连铸，热轧、轧后冷却和卷取，酸洗和冷轧，退火步骤制备得到的，钢的化学成分和制备过程的工艺参数具体参见表1-2。
表1列出了实施例1-14和对比例1-3的贝氏体钢的各化学元素的质量百分比。
表2列出了实施例1-14的贝氏体钢及对比例1-6的对比钢的具体工艺参数。
表1：(％，余量为Fe和除了P、S以外的其他不可避免的杂质)
  钢种 C Si Mn B Al Cr Mo Nb Ti P S R值 Q值
实施例1 A 0.155 0.22 175 0.001 0.01 0.18 0.18 0.002 0.003 0.01 0.001 0.98 143
实施例2 B 0.165 0.35 2.15 0.001 0.03 0.05 0.40 0.004 0.002 0.008 0.001 117 145
实施例3 C 0.125 0.05 185 0.002 0.02 0.31 0.13 0.003 0.11 0.008 0.001 104 143
实施例4 D 0.10 01 165 0.003 0.02 0.12 0.35 0.08 0.02 0.006 0.001 104 134
实施例5 E 0.15 0.29 22 0.0025 0.03 0.21 0.11 0.001 0.15 0.009 0.001 113 139
实施例6 F 0.13 03 19 0.0035 0.02 0.27 0.15 0.002 0.004 0.012 0.001 104 145
实施例7 G 0.135 0.35 16 0.002 0.04 0.25 01 0.04 0.002 0.015 0.003 101 128
实施例8 H 0.145 02 2 0.002 0.01 0.15 0.12 0.003 0.004 0.013 0.002 107 132
实施例9 1 0.19 0.42 21 0.0015 0.05 0.13 0.08 0.002 0.004 0.011 0.001 100 145
实施例10 J 0.18 0.37 2.05 0.001 0.03 0.08 0.19 0.004 0.005 0.007 0.001 104 141
实施例11 K 0.14 0.25 195 0.003 0.02 0.07 0.23 0.02 0.08 0.009 0.001 102 139
实施例12 L 0.145 0.12 17 0.001 0.02 0.07 0.22 0.002 0.003 0.008 0.002 0.96 134
实施例13 M 0.12 0.15 19 0.0025 0.03 0.40 0.05 0.08 0.04 0.005 0.001 111 136
实施例14 N 011 0.07 175 0.0015 0.01 0.11 0.27 01 0.01 0.002 0.001 117 117
对比例1 O 0.08 03 1.3 0.002 0.02 0.25 0.22 0.003 0.02 0.01 0.002 1.25 0.96
对比例2 P 0.21 0.22 18 0.003 0.03 03 02 0.004 0.002 0.012 0.003 0.67 2.39
对比例3 Q 0.17 0.15 171 0.002 0.02 0.42 0.05 0.02 0.002 0.008 0.003 0.79 1.90
Figure PCTCN2022122455-appb-000001
实施例1-5、8、10-11都得到了厚度方向3层组织结构，上下表层均为针状层，芯部为块状层；实施例6-7、9、12-14都得到了厚度方向5层组织结构，上表下表均为复相层，上次表和下次表为针状层，芯部为块状层。在本申请贝氏体钢的组织中，针状层的硬度最大，复相层的硬度最小，块状层的硬度处于针状层和复相层之间。因此，对于3层复合的材料，上下表层的针状层可以保证材料具有较高的表面硬度和表面屈服强度，而中部的块状层又保证了材料具有比较高的韧塑性，因此可以用于对材料表面硬度或疲劳极限有高要求，同时又对材料整体的韧塑性有较高要求的汽车零部件，如汽车座椅滑轨，底盘扭力梁等结构件；而对于5层复合的材料，上下表层相对较软的复相层可以使得表层具有较好的局部成形能力，而紧接着的较硬的针状层和芯部的块状层又赋予材料较高的强度和较好的韧性，因此可以用于制备对强度和综合成形能力要求较高的零部件，如汽车底盘的控制臂、三角臂等。
对比例1-3因为成分设计不符合发明要求，未能得到具有厚度方向梯度组织的钢板或钢带。其中对比例1因为R值过高只得到了纯块状层组织，对比例2-3因R值过低只得到了纯针状层组织。对比例4-6采用钢种A，虽然成分设计符合要求，但是制造工艺中退火工艺不符合发明要求，未能得到具有厚度方向梯度组织的钢板或钢带。其中，对比例4因为在缓冷段的冷却气体压强大于设计值，导致钢板或钢带的全厚度方向都形成了较大比例的铁素体，并且在快冷段，同样因为冷却气体压强大于设计值，导致钢板或钢带的全厚度方向都形成了针状贝氏体而无法形成块状贝氏体，且由于钢板或钢带中已优先形成了一定较大比例的铁素体，导致部分过冷奥氏体中富碳而不发生贝氏体转变，反而会在最终空冷段转变为新鲜马氏体，故钢板或钢带不仅无法形成厚度方向的梯度组织，成形性也较差。而对比例5由于在快冷段的冷却气体压强大于设计值，故只能得到纯针状贝氏体组织，而相应地，对比例6由于在快冷段的冷却气体压强小于设计值故只得到了纯块状层组织。
图3为本发明实施例7的下表层区域，具体为针状层(上部)与复相层(下部)之间过渡位置的金相组织照片(扫描电镜拍摄)。在图片上部，即更靠近芯部的区域，组织为典型的针状贝氏体，代表该区域开始进入针状层；而在图片下部，即更靠近下表面的区域，包含多边形铁素体，针状贝氏体和弥散析出的纳米级、亚微米级或微米级的粒状碳化物析出相，代表该区域开始进入表层的复相层。
图4为本发明实施例1的芯部靠上表层的区域，具体为针状层(上部)与块状层(下部)之间过渡位置的金相组织照片(扫描电镜拍摄)；在图片上部，即更靠近上表面的区域，组织中包含大量的典型的针状贝氏体，代表该区域开始进入针状层；而在图片下部， 即更靠近芯部的区域，大量贝氏体转变为块状多边形的形貌，即该区域内形成大量块状贝氏体，代表该区域开始进入块状层。
表3列出了实施例1-14和对比例1-6的贝氏体钢的力学性能测试结果，取沿横向JIS 5#拉伸试样测定钢的屈服强度、抗拉强度和断裂延伸率，采用GB/T 228.1-2010《金属材料拉伸试验第1部分：室温试验方法》的方式进行测试。取板中部区域测定钢的扩孔率。扩孔率采用扩孔试验测定，通过使用凸模把中芯带孔的试件压入凹模，使试件中芯孔扩大，直到板孔边缘出现颈缩或贯穿裂纹为止。由于试件中芯原始孔的制备方式及对应的原始孔边部质量对扩孔率测试结果存在较大影响，因此，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，中芯原始孔采用一次冲压落料制孔形式，对应为原始孔边部质量最差的加工方式。相应地，若中芯原始孔采用二次冲压落料制孔，或者采用钻孔或铰孔的形式，则相应的扩孔率会在表中的数值基础上增加20％；若中芯原始孔采用线切割的方式制孔，相应的扩孔率会在表中的数值基础上增加50％；若中芯原始孔采用激光落料的方式制孔，相应的扩孔率会在表中的数值基础上增加80％。
表3：实施例1-14及对比例1-6的贝氏体钢的力学性能测试结果
Figure PCTCN2022122455-appb-000002
Figure PCTCN2022122455-appb-000003
由表3可以看出，当钢板或钢带的成分与工艺满足设计要求时，所有的实施例都能得到屈服强度≥800MPa，抗拉强度≥1000MPa，断裂延伸率≥12％，扩孔率≥40％的力学性能，此外由于钢板或钢带在厚度方向成梯度组织，使材料兼具较好的综合成形性能，即拉延性和扩孔翻边性均较好，体现为断裂延伸率和扩孔率均比较高，所有实施例的(断裂延伸率*10+扩孔率)≥170％。
而当成分或工艺不符合设计要求时，则无法获得理想的力学性能。如对比例1，因为C、Mn含量低于下限，而导致材料的强度偏低；对比例2，由于C含量高于上限，导致材料强度过大而成形性极差；对比例3，由于R值低于设计下限，导致钢板或钢带无法形成芯部的块状层，组织全部为针状贝氏体，虽然扩孔率极高，但是断裂延伸率较差；而类似的对比例5，由于工艺不符合设计要求(见上文)，导致组织同样全部为针状贝氏体，因此也是扩孔率极高，但是断裂延伸率较差；这两组对比例都因为成形能力过于“偏科”而导致综合成形能力较差(断裂延伸率*10+扩孔率)＜170％。而对比例4和6也是因为工艺不符合设计要求(见上文)，无法形成厚度方向的梯度组织，故也是表现为成形能力过于“偏科”而导致综合成形能力较差(断裂延伸率*10+扩孔率)＜170％。
本案中各技术特征的组合方式并不限本案权利要求中所记载的组合方式或是具体实施例所记载的组合方式，本案记载的所有技术特征可以以任何方式进行自由组合或结合，除非相互之间产生矛盾。
还需要注意的是，以上所列举的实施例仅为本发明的具体实施例。显然本发明不局限于以上实施例，随之做出的类似变化或变形是本领域技术人员能从本发明公开的内容直接得出或者很容易便联想到的，均应属于本发明的保护范围。

Claims (26)

  1. 一种贝氏体钢，其特征在于，包括以质量百分比计的化学成分：C：0.10～0.19％，Si：0.05～0.45％，Mn：1.5～2.2％，B：0.001～0.0035％，Al：0.01～0.05％，Cr；0.05～0.40％，Mo：0.05～0.40％，Fe≥90％。
  2. 根据权利要求1所述的贝氏体钢，其特征在于，还包括Ti和Nb中的至少一种，其中，Nb≤0.1％，Ti≤0.15％。
  3. 一种贝氏体钢，其特征在于，包括以质量百分比计的化学成分：C：0.10～0.19％，Si：0.05～0.45％，Mn：1.5～2.2％，B：0.001～0.0035％，Al：0.01～0.05％，Cr；0.05～0.40％，Mo：0.05～0.40％，余量为Fe及不可避免的杂质。
  4. 根据权利要求3所述的贝氏体钢，其特征在于，在所述不可避免的杂质中，P≤0.015％，S≤0.004％。
  5. 根据权利要求1或3所述的贝氏体钢，其特征在于，所述贝氏体钢的化学元素的质量百分比满足以下关系：R＝(Mn+Si)/(12*C+160*B)，其中，0.9≤R≤1.2，计算时代入元素质量百分比百分号前的数值。
  6. 根据权利要求5所述的贝氏体钢，其特征在于，所述贝氏体钢的化学元素的质量百分比需要满足以下关系：Q＝(C+Cr+Mo+Mn/2)/R，其中，1.15≤Q≤1.5，计算时代入元素质量百分比百分号前的数值。
  7. 根据权利要求1或3所述的贝氏体钢，其特征在于，所述贝氏体钢具有两层表层组织和一层芯部组织，所述芯部组织在所述两层表层组织之间。
  8. 根据权利要求7所述的贝氏体钢，其特征在于，所述贝氏体钢中，所述芯部组织的体积占所述贝氏体钢体积的20％～50％，剩余为所述表层组织。
  9. 根据权利要求7所述的贝氏体钢，其特征在于，所述表层组织包括针状贝氏体和粒状碳化物析出相；所述芯部组织包括块状贝氏体和粒状碳化物析出相。
  10. 根据权利要求9所述的贝氏体钢，其特征在于，所述针状贝氏体和粒状碳化物析出相占所述表层组织体积的99％以上，所述块状贝氏体和粒状碳化物析出相占所述芯部组织体积的99％以上。
  11. 根据权利要求7所述的贝氏体钢，其特征在于，所述贝氏体钢还具有两层复相层，所述两层表层组织和一层芯部组织组成中间层，所述中间层在所述两层复相层之间。
  12. 根据权利要求11所述的贝氏体钢，其特征在于，所述贝氏体钢中，所述复相层的 体积占所述贝氏体钢体积的2％～10％，剩余为所述中间层。
  13. 根据权利要求11所述的贝氏体钢，其特征在于，所述复相层包括多边形铁素体、针状贝氏体和粒状碳化物析出相，其中多边形铁素体占所述复相层体积的50％以下，所述多边形铁素体、针状贝氏体和粒状碳化物析出相共占所述复相层体积的99％以上。
  14. 根据权利要求1-13中任一项所述的贝氏体钢，其特征在于，所述贝氏体钢的抗拉强度≥1000MPa，屈服强度≥800MPa，扩孔率≥40％，断裂延伸率≥12％。
  15. 一种制备如权利要求1-14中任一项所述的贝氏体钢的制备方法，其特征在于，包括步骤：
    冶炼和铸造；
    热轧；
    轧后冷却和卷取；
    酸洗和冷轧；
    退火。
  16. 根据权利要求15所述的贝氏体钢的制备方法，其特征在于，所述退火步骤依次包括加热段，缓冷段，快冷段，控冷段和空冷段，控制冷却速率为控冷段的冷却速率＜缓冷段的冷却速率＜快冷段的冷却速率。
  17. 根据权利要求16所述的贝氏体钢的制备方法，其特征在于，在所述缓冷段，以Q～10*Q℃/s的缓冷速率冷却至缓冷温度720～800℃；其中，化学元素的质量百分比满足关系：Q＝(C+Cr+Mo+Mn/2)/R，1.15≤Q≤1.5，R＝(Mn+Si)/(12*C+160*B)，0.9≤R≤1.2，式中的各化学元素均代入该化学元素的质量百分含量的百分号前面的数值。
  18. 根据权利要求17所述的贝氏体钢的制备方法，其特征在于，通过向所述贝氏体钢的表面喷射冷却气体进行冷却，控制所述冷却气体的喷射压强为0.2*Q～Q kPa，控制所述冷却气体喷射的保持时间为5～20秒。
  19. 根据权利要求16所述的贝氏体钢的制备方法，其特征在于，在所述缓冷段，以Q～10*Q℃/s的缓冷速率冷却至缓冷温度620～700℃；其中，化学元素的质量百分比满足关系：Q＝(C+Cr+Mo+Mn/2)/R，1.15≤Q≤1.5，R＝(Mn+Si)/(12*C+160*B)，0.9≤R≤1.2，式中的各化学元素均代入该化学元素的质量百分含量的百分号前面的数值。
  20. 根据权利要求19所述的贝氏体钢的制备方法，其特征在于，通过向所述贝氏体钢的表面喷射冷却气体进行冷却，控制所述冷却气体的喷射压强为0.05*Q～0.15*Q kPa，控制所述冷却气体喷射的保持时间为5～15秒。
  21. 根据权利要求17或19所述的贝氏体钢的制备方法，其特征在于，在所述快冷段，以10*Q～20*Q℃/s的快冷速率冷却至快冷温度400～540℃。
  22. 根据权利要求21所述的贝氏体钢的制备方法，其特征在于，通过向所述贝氏体钢的表面喷射两次冷却气体进行冷却，控制所述冷却气体的第一喷射压强为0.3*Q～1.5*Q kPa，控制所述冷却气体的第一保持时间为1～7秒；控制所述冷却气体的第二喷射压强为0.08*Q～0.2*Q kPa，控制所述冷却气体的第二保持时间为5～10秒。
  23. 根据权利要求18、20或22所述的贝氏体钢的制备方法，其特征在于，所述冷却气体为还原性气体和惰性气体的混合物，其中，所述还原性气体为氢气，其体积分数为1％～8％，控制所述冷却气体的温度为5～50℃。
  24. 根据权利要求16所述的贝氏体钢的制备方法，其特征在于，在所述控冷段，控制控冷速率≤Q℃/s，保持控冷时间为100～200秒，在所述控冷段结束时所述贝氏体钢的控冷温度≥350℃。
  25. 根据权利要求16所述的贝氏体钢的制备方法，其特征在于，在所述加热段，以≤50℃/s的加热速率加热至均热温度840～950℃，然后保温，控制保温时间为60～180秒。
  26. 根据权利要求15所述的贝氏体钢的制备方法，其特征在于，控制所述制备方法的工艺参数满足下述各项中的至少一项：
    在所述热轧步骤中，控制加热温度为1100～1230℃，精轧开轧温度为1050～1180℃，精轧终轧温度为870～930℃；
    在所述轧后冷却和卷取步骤中，控制冷却速率为30～150℃/s，控制卷取温度为540～620℃；
    在所述冷轧步骤中，控制冷轧压下率≥30％。
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