Bainite steel and its manufacturing method

Abstract

The present invention discloses a bainite steel containing the following chemical components in mass percent: 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%. By rationally controlling the contents of elements such as C, Si, Mn, B, Al, Cr, and Mo in the steel, a phase having a structure gradient is spontaneously formed in the steel during the preparation process, and at the same time, the hardenability of the steel is improved, thereby improving the strength and formability of the bainite steel. The present invention further discloses a method for preparing the above bainite steel, which includes the following steps: smelting and casting; hot rolling; cooling after rolling and coiling; pickling and cold rolling; annealing. The manufacturing method of the present invention provides a bainite steel having a structure gradient through the thickness direction, and such a bainite steel has good formability.

Description

Technical Field

[0004] ,

[0001] The present invention relates to the field of metallurgical technology, and particularly to bainite steel and a method for producing the same.

Background Art

[0002] With the development concept of "Green & Safety" for new-generation automobiles, the strength of steel used in automotive structural parts is becoming increasingly higher, and materials with different characteristics in the thickness direction are gradually required. For example, the surface layer of the material is required to have high hardness and wear resistance, or high tissue uniformity for flange processing and forming. On the other hand, in order for no necking and fracture to occur during tensile elongation forming of the entire steel material, the core part must have high plasticity. Or, when a low-hardness layer structure is required as the surface layer so that the material has bending characteristics to a certain extent, the subsurface layer needs to have a uniform hard-phase structure for flange processing and maintaining strength, and the core part needs to have a softer structure to ensure plasticity, toughness, etc. Thereby, while the material has high strength, it also has good comprehensive formability such as bending, flange processing, and tensile elongation.

[0003] In the automotive industry, while the requirements for different structures and performances of steel materials in the thickness direction are increasing, in the conventional method, slabs with different components or structures are used by methods such as welding and combined rolling to obtain a structure with a gradient in the thickness direction of the steel material. For example, CN201210368300.6 and CN201310724615.4, etc., obtained a laminated composite material in the thickness direction using combined rolling of metals. However, this method is complex, slow-paced, and very costly.

[0004] There are also patents that attempt to obtain steel sheets or strips with different microstructures in the surface layer and core by utilizing a method of surface layer decarburization. For example, by using surface decarburization of steel strip to form a decarburized layer of several to tens of microns, the upper and lower surface layers may have a pure ferrite or ferrite >50% structure, while the core has other single-phase or multi-phase structures, such as martensite, tempered martensite, or bainite. This method can spontaneously form a microstructure gradient in the thickness direction and produce high-strength steel sheets with a three-layer composite structure. However, the difference in strength or hardness between the surface layer and the core is too large, and the strength or hardness of the surface layer is too low. As a result, the range of application for such products (e.g., automotive seat rails, drive shafts, etc., where high hardness of the surface layer is required, or where fatigue resistance is required) is greatly limited. Consequently, the material has good bending properties, but its elongation and hole expansion rates are not high, meaning its plasticity and flange workability are poor. On the other hand, this method can only form a three-layer composite structure, and it is not possible to obtain structures with more layers. [Overview of the project] [Means for solving the problem]

[0005] In response to the problems of high cost and inability to obtain composite structures with more than three layers in conventional technologies, the present invention provides bainite steel that exhibits mechanical properties of yield strength ≥ 800 MPa, tensile strength ≥ 1000 MPa, elongation at break ≥ 12%, and hole expansion ratio ≥ 40%. Furthermore, because a structure with a gradient in the thickness direction of the steel sheet or strip is formed, the material has good overall formability, good tensile ductility and hole expansion flange workability, and high elongation at break and hole expansion ratio, with (elongation at break * 10 + hole expansion ratio) ≥ 170% in all examples.

[0006] The bainite steel of the present invention contains the following chemical components by mass percent: 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%.

[0007] However, the design principles for each element are as follows: C: In the bainite steel of the present invention, element C primarily controls the microstructure phase transition, carbide size, and bainite substructure morphology of the carbon steel, influencing the mechanical properties of the material. If the element C content in the steel is less than 0.10%, the strength of the steel will not meet the target requirements; however, if the element C content in the steel exceeds 0.19%, martensitic structure and coarse cementite are more likely to form, degrading the performance of the steel sheet. Furthermore, in the present invention, C further influences the bainite substructure morphology, with acicular bainite more likely to form the higher the C content. Therefore, in the present invention, the mass percentage of C is controlled to 0.10-0.19%. Preferably, the mass percentage of C is 0.13-0.17%.

[0008] Si: In the bainite steel of the present invention, Si has a certain solid solution strengthening effect and also affects the surface quality of the steel sheet. If the Si content in the steel is less than 0.05%, it is difficult to obtain a sufficient strengthening effect; however, if the Si content in the steel exceeds 0.45%, oxide scale or tiger stripe pattern color differences are easily formed, which is unfavorable for the surface quality of automotive steel sheets. Furthermore, in the present invention, Si affects the substructure morphology of bainite, and the higher the Si content, the easier it is to form polygonal bainite. Therefore, in the present invention, the mass percentage of Si is set to 0.05 to 0.45%. Preferably, the mass percentage of Si is set to 0.05 to 0.35%. More preferably, the mass percentage of Si is set to 0.15 to 0.3%.

[0009] Mn: In the bainite steel of the present invention, Mn is one of the controlling elements for the phase transition of the steel's structure and also influences the substructure morphology of the bainite. The higher the Mn content, the easier it is to form polygonal bainite. However, it is important to note that the Mn content of the steel should not be too high, as excessively high Mn content leads to poor corrosion resistance and weldability. Therefore, in the present invention, the mass percentage of Mn is controlled to 1.5 to 2.2%. Preferably, the mass percentage of Mn is 1.7 to 2.1%.

[0010] B: In the bainite steel of the present invention, B not only contributes to the formation of bainite in the steel, but also affects the strength and formability of the steel sheet, and simultaneously influences the substructure morphology of the bainite. The higher the B content, the easier it is to obtain acicular bainite, resulting in higher steel sheet strength, easier acquisition of brittle borides, and affecting the hole expansion rate of the steel sheet. Therefore, in the present invention, the mass percentage of B is controlled to 0.001 to 0.0035%.

[0011] Al: In the bainite steel according to the present invention, Al is added to the steel simply as a deoxygenating element, which removes the oxygen element from the steel and ensures the performance and quality of the steel. Therefore, in the bainite steel of the present invention, the mass percentage of Al is set to 0.01 to 0.05%. In the prior art, Al is added to steel in large quantities (≧0.1%) for the purpose of solid solution strengthening effect as a ferrite-forming element and a carbide precipitation-inhibiting element, or the addition of Al changes the phase transition of the steel material by altering the phase transition temperature (e.g., A1, A3), bainite formation dynamics and carbide precipitation dynamics, forming retained austonite or carbonless bainite, and ultimately increasing the strength of the steel material. However, the composition control and process adjustment of the present invention yield bainite steel with good overall formability. Adding a large amount of Al to form carbonless bainite destroys the gradient of the bainite structure formed in the thickness direction, leading to increased costs and difficulties in continuous casting production. Therefore, in the present invention, the mass percentage of Al is controlled between 0.01 and 0.05% to avoid increased costs or difficulties in continuous casting production, while forming a gradient of the bainite structure in the thickness direction.

[0012] Cr and Mo: In the bainite steel of the present invention, Cr and Mo can form a fine dispersed carbide precipitate phase with C, further increasing the strength of the steel sheet and influencing the timing of pearlite and ferrite appearance in the CCT curve, thereby improving the hardenability of the steel sheet. This can be designed in conjunction with the cooling rate of the steel sheet during the annealing process to form a microstructure gradient in the thickness direction and control different thickness proportions. Therefore, in the present invention, the mass percentages of Cr and Mo are as follows: 0.05% ≤ Cr ≤ 0.40%, 0.05% ≤ Mo ≤ 0.40%.

[0013] In this invention, by rationally controlling the content of elements such as C, Si, Mn, B, Al, Cr, and Mo in the steel, phases with a microstructure gradient spontaneously form in the steel during the manufacturing process, and at the same time the hardenability of the steel is increased, thereby improving the strength and formability of bainite steel.

[0014] Furthermore, the above bainite steel further contains at least one of Ti and Nb, provided that the mass percentages of Ti and Nb satisfy the following: Nb ≤ 0.1%, Ti ≤ 0.15%.

[0015] Ti and Nb: In the bainite steel of the present invention, Ti and Nb are optional alloying elements. By adding them to the steel, a finely dispersed carbide precipitate phase is formed, refining the microstructure and crystal grains, further improving the strength and formability of the steel sheet. Therefore, in the bainite steel of the present invention, the mass percentages of Nb and Ti are as follows: Nb ≤ 0.1%, Ti ≤ 0.15%. Since the addition of the above alloying elements increases the cost of the material, considering performance and cost management comprehensively, the technical proposal of the present invention may preferably contain at least one of Nb and Ti in the steel. In one embodiment, the bainite steel of the present invention contains Nb and Ti in a mass percentage content of 0.001 to 0.1%, and Ti in a mass percentage content of 0.001 to 0.15%.

[0016] In one embodiment, the bainite steel of the present invention contains, by mass percent, the following chemical components: 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%, with the remainder being Fe and unavoidable impurities.

[0017] Furthermore, for the unavoidable impurities mentioned above, P ≤ 0.015% and S ≤ 0.004%. Both P and S are impurity elements in steel, and to obtain tempered steel with better performance and superior quality, the content of these impurity elements in the steel should be reduced as much as possible, to the extent technically possible.

[0018] Furthermore, in the bainite steel of the present invention, the mass percentage of chemical elements satisfies the following relationship: R = (Mn + Si) / (12 * C + 160 * B), where 0.9 ≤ R ≤ 1.2, and each chemical element in the formula is replaced with the numerical value before the percentage sign of the mass percentage content of that chemical element.

[0019] In this invention, the formula R = (Mn + Si) / (12 * C + 160 * B) is defined. Experiments show that, when calculated using this formula, in order to obtain a bainite sheet / strip structure with an ideal microstructure gradient, the R value must be limited to a certain range, specifically 0.9 ≤ R ≤ 1.2. However, the higher the C and B content, the easier it is for acicular bainite to form, and the higher the Mn and Si content, the easier it is for massive bainite to form. Therefore, by rationally designing the C and B content, and the Mn and Si content, the sheet and strip can be brought to a critical state that is favorable for both the formation of acicular and massive bainite from a component design perspective, i.e., the state of 0.9 ≤ R ≤ 1.2 in the formula. Combined with an optimized annealing process, a microstructure gradient can ultimately be formed in the thickness direction of the sheet. Furthermore, although the C and B content is low, they have a stronger influence on bainite formation and its morphology, so a large coefficient is used in the formula to balance this with the high content of Mn and Si. This is because the influence of Mn and Si on bainite formation and its morphology is significantly weaker than that of C and B. In this design, the level 0.9 ≤ R ≤ 1.2 is the critical level most suitable for the formation of a gradient structure. If R is too high, the thickness of the massive layer in the gradient structure is too large and the thickness of the acicular layer is too small, and in some cases there is no acicular layer, resulting in no gradient in the thickness direction of the structure. If R is too low, the thickness of the acicular layer in the gradient structure is too large and the thickness of the massive layer is too small, and in some cases there is no massive layer, resulting in no gradient in the thickness direction of the structure. Therefore, in this invention, by setting R to 0.9 ≤ R ≤ 1.2, a microstructure gradient and mechanical properties in the thickness direction of the steel are guaranteed.

[0020] Furthermore, in the bainite steel of the present invention, the mass percentage of chemical elements must satisfy the following relationship: Q = (C + Cr + Mo + Mn / 2) / R, where 1.15 ≤ Q ≤ 1.5. When calculating, the numerical value before the percentage sign of the element's mass percentage is used.

[0021] In this invention, Q = (C + Cr + Mo + Mn / 2) / R is defined, which further guides the compositional design of the steel. Experiments show that when 1.15 ≤ Q ≤ 1.5, the steel has appropriate hardenability and the ability to form a microstructure gradient. Since the microstructure gradient or its layered structure is distributed in the thickness direction of steel sheets and strips, the hardenability of steel sheets and strips is also the most important influencing factor in the formation of a gradient structure in the thickness direction. In this application, C, Cr, Mo, and Mn all affect the hardenability of steel sheets and strips, and the higher the content of these elements, the stronger the hardenability. However, the Mn content is an order of magnitude higher than the other elements, and its influence on hardenability is relatively weak, so in this formula, the design of Mn is given a coefficient of 1 / 2. Acicular bainite and massive bainite form at slightly different temperatures during the annealing process. Acicular bainite forms at a lower temperature, while massive bainite forms at a higher temperature. Therefore, the higher the hardenability of the steel sheet, the more favorable it is for the formation of acicular bainite and the less favorable it is for the formation of massive bainite, and vice versa. To ensure that the ratio of acicular bainite to massive bainite in the thickness direction of the steel sheet and strip forms an appropriate sandwich, the composition design of the steel sheet should be favorable for massive bainite formation (i.e., a high R value), requiring the addition of higher hardenability to promote acicular bainite formation. Conversely, when the composition design is favorable for acicular bainite formation (i.e., a low R value), requiring the addition of lower hardenability to promote massive bainite formation. Thus, the numerator of the Q value represents the alloy content, which indicates the hardenability of the strip; a higher Q value indicates stronger hardenability. The denominator is the R value, which represents the ability of the tissue to form massive and acicular bainite. The ratio of the numerator to the denominator, i.e., the Q value, directly affects the ability to form massive and acicular layers during the annealing process and the final proportion. If the Q value is too small, the ability to form massive bainite is too strong, making it difficult for acicular bainite to form in the final tissue and for gradient tissue to form. If the Q value is too large, the ability to form acicular bainite is too strong, making it difficult for massive bainite to form in the final tissue and for gradient tissue to form.

[0022] Furthermore, the bainite steel described above has a two-layer surface structure and a one-layer core structure, with the core structure located between the two-layer surface structure.

[0023] Furthermore, in the bainite steel, the volume of the core structure occupies 20% to 50% of the volume of the bainite steel, and the remainder is the surface layer structure.

[0024] Furthermore, the surface layer structure contains acicular bainite and granular carbide precipitation phases, and the core structure contains massive bainite and granular carbide precipitation phases.

[0025] Furthermore, the acicular bainite and granular carbide precipitation phases occupy more than 99% of the volume of the surface layer structure, and the massive bainite and granular carbide precipitation phases occupy more than 99% of the volume of the core structure.

[0026] Specifically, in the bainite steel of an embodiment of the present invention, as shown in FIG. 1, there is a three-layer structure in the thickness direction of the steel plate or strip steel, and the structures from one surface to the other surface are as follows: Surface layer structure 2: Acicular layer, that is, a structure mainly composed of acicular bainite and granular carbide precipitation phases dispersed and precipitated at the nanometer level, sub-micrometer level or micrometer level, and the proportion of their total amount in the region phase is ≧99%. The proportion occupied in the thickness direction is 25% to 40%.

[0027] Core structure 1: Massive layer, that is, a structure mainly composed of massive bainite and granular carbide precipitation phases dispersed and precipitated at the nanometer level, sub-micrometer level or micrometer level, and the proportion of their total amount in the region phase is ≧99%. The proportion occupied in the thickness direction is 20% to 50%.

[0028] Surface layer structure 2: Acicular layer, that is, a structure mainly composed of acicular bainite and granular carbide precipitation phases dispersed and precipitated at the nanometer level, sub-micrometer level or micrometer level, and the proportion of their total amount in the region phase is ≧99%. The proportion occupied in the thickness direction is 25% to 40%.

[0029] The proportions occupied in the thickness direction of the three-layer region of the bainite steel are all 100%. Furthermore, the bainite steel has two multiphase layers, and the two surface layers and the single core layer form an intermediate layer, which lies between the two multiphase layers.

[0030] Furthermore, in bainite steel, the volume of the multiphase layer accounts for 2% to 10% of the bainite steel volume, with the remainder being the intermediate layer.

[0031] Furthermore, the multiphase layer contains polygonal ferrite, acicular bainite, and granular carbide precipitate phases. Polygonal ferrite accounts for less than 50% of the multiphase layer volume, while polygonal ferrite, acicular bainite, and granular carbide precipitate phases account for more than 99% of the multiphase layer volume.

[0032] Specifically, in one embodiment of the present invention, the bainite steel has a five-layer structure in the thickness direction of the steel sheet or strip, as shown in Figure 2, and the microstructure from one surface to the other is as follows: Multiphase layer 3: The structure mainly consists of polygonal ferrite, acicular bainite, and dispersed nanometer-, submicrometer-, or micrometer-sized granular carbide precipitates (however, polygonal ferrite structure <50%), and the total amount of polygonal ferrite, acicular bainite, and dispersed nanometer-, submicrometer-, or micrometer-sized granular carbide precipitates accounts for ≥99% of the region phase. The proportion in the thickness direction is 1% to 5%.

[0033] Surface layer structure 2: Needle-like layer, i.e., a structure mainly consisting of needle-like bainite and dispersed nanometer-, submicrometer-, or micrometer-sized granular carbide precipitates, with the total amount of these components accounting for ≥99% of the entire region's phase. The proportion in the thickness direction is 25% to 40%.

[0034] Core structure 1: The structure consists mainly of a massive layer, i.e., massive bainite and dispersed nanometer-, submicrometer-, or micrometer-sized granular carbide precipitates, with the total amount of these components accounting for ≥99% of the region's phase. The proportion of these components in the thickness direction is 25% to 40%.

[0035] Surface layer structure 2: Acicular layer, i.e., a structure mainly consisting of acicular bainite and dispersed nanometer-, submicrometer-, or micrometer-sized granular carbide precipitates, with the total amount of these components accounting for ≥99% of the region's phase. The proportion in the thickness direction is 25% to 40%.

[0036] Multiphase layer 3: The structure mainly consists of polygonal ferrite, acicular bainite, and dispersed nanometer-, submicrometer-, or micrometer-sized granular carbide precipitates (however, polygonal ferrite structure <50%), and the total amount of polygonal ferrite, acicular bainite, and dispersed nanometer-, submicrometer-, or micrometer-sized granular carbide precipitates accounts for ≥99% of the region phase. The proportion in the thickness direction is 1% to 5%.

[0037] The proportion of the 5-layer region is 100% in total. However, the hardness of the acicular layer is the highest, and the hardness of the multiphase layer is the lowest.

[0038] In the bainite steel of the present invention, the diameter of the granular carbide precipitate phase is ≤ 5 μm. The reason for limiting the carbide precipitate phase is to avoid deterioration of the hole expansion ratio. When the carbide precipitate phase size is > 5 μm, cracks are likely to occur at the bonding sites between the carbides and the base material when the steel sheet undergoes deformation during hole expansion flange processing or when measuring the hole expansion ratio, leading to a decrease in the hole expansion ratio of the steel sheet and deterioration of the hole expansion flange processing performance.

[0039] Furthermore, the bainite steel of the present invention has a tensile strength of ≥1000 MPa, a yield strength of ≥800 MPa, a hole expansion ratio of ≥40%, and a fracture elongation ratio of ≥12%.

[0040] Furthermore, the bainite steel of the present invention has excellent tensile ductility and hole-expanding flange workability, with a fracture elongation*10 + hole expansion ratio ≥ 170%.

[0041] The present invention further provides a method for producing the bainite steel described above, comprising the following steps: Smelting and casting; Hot rolling; Cooling and winding after rolling; Pickling and cold rolling; Annealing.

[0042] Since the bainite steel of the present invention does not produce bainite steel with a microstructure gradient using the conventional surface layer decarbonization method, the bainite steel of the present invention does not have the problem of the surface layer strength and hardness being lower than that of the core.

[0043] Furthermore, the parameters of the above manufacturing method satisfy any one of the following: In the hot rolling process, the heating temperature shall be 1100-1230°C, the start temperature for finish rolling shall be 1050-1180°C, and the end temperature for finish rolling shall be 870-930°C. In the post-rolling cooling and winding process, the cooling rate shall be 30-150°C / s, and the winding temperature shall be 540-620°C. In the cold rolling process, the cold rolling reduction ratio is ≥ 30%.

[0044] In the above manufacturing method, the pre-annealing process is mainly for obtaining a steel sheet or strip with a uniform composition and original structure, and the subsequent annealing process is to ensure that a uniform and stable structure and properties can be achieved. However, the annealing process plays a crucial role in the properties of the steel sheet.

[0045] Before introducing the annealing process, it is necessary to introduce the following concepts: Since this invention aims to design a gradient structure in the thickness direction of steel sheets / strips, it is inevitable or intentional that the steel sheets or strips have different temperature ranges in the thickness direction. However, due to limitations in the continuous production mode of steel sheets or strips, temperature detection and control can only be directed to the upper and lower surface temperatures, and it is not possible to detect temperatures at other locations in the thickness direction. The temperatures of the upper and lower surfaces are not additionally distinguished and are referred to as surface temperatures according to the same process. The temperatures and cooling rates described below all refer to the surface temperature and the cooling rate calculated from the surface temperature. It is important to note that during cooling, the temperature distribution in the thickness direction of the steel sheet or strip is controlled by the surface temperature, cooling rate, injection gas pressure during cooling (indicating cooling capacity), and the hardenability of the steel sheet.

[0046] Furthermore, the annealing process includes, in order, a heating stage, a slow cooling stage, a rapid cooling stage, a controlled cooling stage, and an air cooling stage, and the controlled cooling rate satisfies the following conditions for the three stages: slow cooling stage, rapid cooling stage, and controlled cooling stage: controlled cooling stage < slow cooling stage < rapid cooling stage.

[0047] Furthermore, during the heating stage, the mixture is heated to a soaking temperature of 840-950°C at a heating rate of ≤50°C / s, and then kept warm for a period of 60-180 seconds.

[0048] In the heating stage, the bainite steel is heated to a soaking temperature of 840-950°C at a heating rate of ≤50°C / s, and then held for 60-180 seconds. However, if the heating rate in the heating stage is >50°C / s, or the holding time is <60 seconds, the uniformity of the steel strip structure will be poor, affecting the subsequent formation of the gradient structure in the thickness direction. Also, if the temperature is below the lower limit of the soaking temperature, the steel strip will not obtain a sufficient bainite structure (whether acicular bainite or massive bainite). Furthermore, the heating rate is preferably 5-50°C / s. If the holding time is >180 seconds, or even further, if the soaking temperature exceeds 950°C, the grain size of the steel strip will coarseen, and the formability of the steel will deteriorate.

[0049] In this invention, in order to form bainite steel having a three-layer structure gradient in the thickness direction, the slow cooling stage is performed by cooling to a slow cooling temperature of 720 to 800°C at a slow cooling rate of Q ~ 10*Q°C / s, and the mass percentage of chemical elements satisfies Q = (C + Cr + Mo + Mn / 2) / R, 1.15 ≤ Q ≤ 1.5, and R = (Mn + Si) / (12*C + 160*B), 0.9 ≤ R ≤ 1.2, where each chemical element in the formula is replaced with the numerical value before the percentage sign of the mass percentage content of that chemical element. In one embodiment, the slow cooling rate is 5Q ~ 10Q°C / s. In another embodiment, the slow cooling rate is 7Q ~ 10Q°C / s.

[0050] Specifically, in the embodiments of the present invention, slow cooling is performed by injecting a cooling gas onto the surface of the bainite steel. For example, during cooling, cooling is performed by injecting a cooling gas onto the surface of the bainite steel, with a cooling gas injection pressure of 0.2*Q to QkPa and a holding time of cooling gas injection of 5 to 20 seconds. Of course, in other possible embodiments, the purpose of slow cooling may be achieved by methods such as liquid cooling, as long as the material can be cooled to a slow cooling temperature of 720 to 800°C at a slow cooling rate of Q to 10*Q°C / s. The main objective of this stage is to make the steel plate or strip have a uniform temperature in the width direction and a non-uniform temperature in the thickness direction, and to prevent microstructural changes at each location.

[0051] The reason for controlling the slow cooling rate in this process is to achieve a uniform temperature in the width direction of the steel sheet or strip, and the temperature control is to prevent phase transitions at any point in the strip. If the temperature is too low, austonite may decompose through a phase transition, potentially forming ferrite or pearlite. If the temperature is too high, it is detrimental to the high-precision control of the next cooling stage and to obtaining a gradient structure in the thickness direction. The control of the injection pressure and holding time of the cooling gas onto the surface of the steel sheet or strip is intended to control uneven cooling in the thickness direction of the strip. If the injection pressure of the cooling gas onto the surface of the steel sheet or strip is less than 0.2*QkPa, or the holding time is less than 5 seconds, the cooling capacity is insufficient. Although the surface of the strip can be cooled to the set temperature, most of the area below the surface layer remains at a high temperature, which is unfavorable for forming a gradient structure in the thickness direction in the next step, or the acicular bainite region in the gradient structure formed in the next step will be too small. If the pressure exceeds QkPa, or the holding time exceeds 20 seconds, the cooling capacity is too high, the core temperature of the strip approaches or reaches the surface temperature, which is unfavorable for forming a gradient structure in the thickness direction in the next step, or the massive bainite region formed in the next step will be too small.

[0052] In this invention, in order to form bainite steel having a five-layer microstructure gradient in the thickness direction, the slow cooling stage is performed by cooling to a slow cooling temperature of 620 to 700°C at a slow cooling rate of Q ~ 10*Q°C / s, and the mass percentage of chemical elements satisfies Q = (C + Cr + Mo + Mn / 2) / R, 1.15 ≤ Q ≤ 1.5, and R = (Mn + Si) / (12*C + 160*B), 0.9 ≤ R ≤ 1.2, where each chemical element in the formula is replaced with the numerical value before the percentage sign of the mass percentage content of that chemical element. In one embodiment, the slow cooling rate is 5Q ~ 10Q°C / s. In another embodiment, the slow cooling rate is 7Q ~ 10Q°C / s.

[0053] Specifically, in the embodiments of the present invention, slow cooling is performed by injecting a cooling gas onto the surface of bainite steel. For example, during cooling, cooling is performed by injecting a cooling gas onto the surface of bainite steel, with a cooling gas injection pressure of 0.05*Q to 0.15*Q kPa and a holding time of cooling gas injection of 5 to 15 seconds. Of course, in other possible embodiments, the purpose of slow cooling can be achieved by methods such as liquid cooling, and any technical solution that can cool bainite steel to a slow cooling temperature of 620 to 700°C at a slow cooling rate of Q to 10*Q°C / s is all within the scope of this application.

[0054] In this process, cooling to 620-700°C is necessary to bring the surface of the steel sheet or strip into the ferrite transition temperature range. Holding the temperature for a certain period of time allows a certain amount of ferrite to form on the surface region of the steel sheet or strip, preparing the multiphase layer of the final surface layer. If the temperature is below or above this range, the formation of a certain amount of ferrite on the surface of the steel strip cannot be guaranteed. Similarly, if the holding time is too short or the cooling rate is too fast, the ferrite on the surface of the steel strip will not be formed in time, and ultimately the multiphase layer of the surface layer cannot be formed. Conversely, if the holding time is too long or the cooling rate is too slow, the ferrite content formed on the surface of the steel strip will be too high, resulting in an excessive thickness, which is unfavorable for the formation of the multiphase layer of the surface layer. Furthermore, in the rapid cooling stage, sufficient acicular bainite will not form in the shallow surface layer, affecting the formation of the subsequent acicular layer.

[0055] The reason for setting the cooling gas injection pressure to 0.05*Q~0.15*QkPa on the surface of the steel plate or strip is to control the thickness of the polygonal ferrite formed on the surface of the strip. When the holding time also follows the set range within this pressure range, the steel plate or strip actually cools only the surface layer region to 620~700°C and enters the ferrite phase region, while the temperature in other regions remains above 700°C, and ferrite transformation does not occur (because ferrite formation also releases latent heat of transformation). However, if the pressure of the injected cooling gas is too high, the temperature of the shallow surface layer of the steel plate or strip, and consequently the core, decreases, which is unfavorable for the subsequent formation of needle-like and massive layers. If the pressure of the injected cooling gas is too low, it is not possible to stably form a certain amount of polygonal ferrite in the surface layer, and as a result, it is not possible to stably form a multiphase layer in the surface layer.

[0056] After slow cooling is complete, in the rapid cooling stage, to form bainite steel with a gradient structure of 3 or 5 layers in the thickness direction, it is necessary to cool to a rapid cooling temperature of 400 to 540°C at a rapid cooling rate of 10*Q to 20*Q°C / s.

[0057] Specifically, in the embodiments of the present invention, rapid cooling is performed by injecting a cooling gas onto the surface of bainite steel. In this stage, it is necessary to inject the cooling gas onto the surface of the bainite steel twice during cooling, with the first injection pressure of the cooling gas being 0.3*Q to 1.5*Q kPa and the first holding time of the cooling gas being 1 to 7 seconds, and the second injection pressure of the cooling gas being 0.08*Q to 0.2*Q kPa and the second holding time of the cooling gas being 5 to 10 seconds. Similarly, in other possible embodiments, the purpose of slow cooling can be achieved by methods such as liquid cooling, and any technical method that can cool the bainite steel to a rapid cooling temperature of 400 to 540 at a rapid cooling rate of 10*Q to 20*Q°C / s at this stage is all within the scope of this application.

[0058] Furthermore, the cooling gas used in the annealing process is a mixture of a reducing gas and an inert gas. Preferably, the volume fraction of the reducing gas in this mixture is 1% to 8%. In one embodiment, the reducing gas in this mixture is hydrogen gas, and its volume fraction is 1% to 8%. The temperature of the cooling gas may be controlled to 5 to 50°C.

[0059] In one embodiment of the present invention, cooling of a steel plate or strip is performed by injecting a cooling gas (i.e., a mixture of a reducing gas and an inert gas) toward its surface, with the reducing property being hydrogen gas. In this invention, the inert gas is a gas that does not chemically react with bainite steel under experimental conditions and affect the steel's structure. Specifically, for cost savings, the inert gas may be entirely nitrogen gas. The hydrogen gas content and temperature of the cooling gas can be further controlled, as shown in Table 2. During the cooling process of bainite steel, the cooling capacity or cooling intensity is controlled by controlling the pressure of the injected gas, the hydrogen gas content of the cooling gas, and the temperature of the cooling gas, and the specific values ​​are determined by the hardenability of the steel plate or strip. In the same embodiment, the hydrogen gas content and cooling gas temperature in the cooling gas typically remain constant throughout the annealing process. In this case, the cooling intensity, cooling rate, and injection gas pressure are positively correlated. For example, in Example 1, during the slow cooling stage, the cooling gas injection pressure is 0.6 kPa, and the slow cooling rate is 12.5 °C / s. During the rapid cooling stage, the first cooling gas injection pressure is 1 kPa, and the cooling rate is 19.2 °C / s. In different embodiments, the cooling capacity and cooling rate are related to the cooling gas injection pressure, the hydrogen gas content in the cooling gas, and the cooling gas temperature. A higher hydrogen gas content in the cooling gas results in a lower cooling gas temperature, and a higher cooling gas injection pressure results in a stronger cooling capacity and a faster cooling rate. For example, in Examples 7 and 9, although the cooling gas temperatures are the same, Example 9 has a higher hydrogen gas content and a higher cooling gas injection pressure, resulting in a greater cooling capacity and cooling rate.

[0060] Specifically, the control of the quenching temperature and quenching rate in this stage of the reaction is intended to bring the steel sheet and strip into the bainite phase region. If the temperature is too high or too low, sufficient bainite cannot be formed in the steel sheet or strip. Controlling the quenching rate to 10*Q~20*Q°C / s is intended to bring the quenching rate as close as possible to the nose region of the CCT curve in the bainite phase region, resulting in a more sufficient bainite transition and a faster rate. In steel sheets and strips, from the initial smelting stage through the long production process, heterogeneity in composition and structure inevitably appears in some areas. In some areas, the carbon equivalent is too low or the austonite undercooking is insufficient, while in other areas, the carbon equivalent is too high or the austonite undercooking is excessive. If the cooling rate is below the set range, regions with low carbon equivalent or minimal austonite overcooling will enter the pearlite transition region due to the slow cooling rate, or the bainite transition rate will be too slow, resulting in insufficient transition. Similarly, if the cooling rate is above the set range, regions with low carbon equivalent or significant austonite overcooling will enter the martensite phase region, bypassing the bainite phase region, or the bainite transition rate will be too slow, resulting in insufficient transition. Both of these lead to failure in the formation of a gradient structure in the thickness direction.

[0061] Of all the factors influencing the progress of the rapid cooling phase, the injection pressure of the cooling gas onto the surface of the steel sheet or strip is the most important. First, the pressure is controlled to 0.3*Q~1.5*QkPa and held for 1~7 seconds in order to form an acicular bainite layer in areas other than the core region in the thickness direction of the steel sheet or strip. These regions release latent heat of transformation through the bainite phase transition, and since the temperature of the core region in the thickness direction of the strip is higher than that of the surface layer and subsequent surface layers, it prepares the core region for the formation of massive bainite. At this time, if the cold injection gas pressure or holding time is lower than the set range, it is unfavorable for the formation of acicular bainite in the surface layer and subsequent surface layers. If the injection pressure or holding time is higher than the set range, the cooling capacity is too strong, and acicular bainite will also form in the core region in the thickness direction of the strip, preventing the formation of a gradient structure in the thickness direction. Subsequently, by further reducing the injection pressure to 0.08*Q~0.2*QkPa and maintaining it for 5~10 seconds, effective cooling is still obtained for the surface and subsurface layers, allowing for continuous formation of acicular bainite. Furthermore, due to the decrease in cooling gas pressure and the release of latent heat of transformation in the surface and subsurface layers, the temperature of the core region in the thickness direction of the steel strip does not decrease further or even increases slightly, causing the formation of massive bainite in the core of the steel strip. Ultimately, a steel sheet or strip with a microstructure gradient in the thickness direction is formed.

[0062] After the rapid cooling stage is complete, a controlled cooling step is required to obtain bainite steel having a 3- or 5-layer microstructure gradient in the thickness direction. In the controlled cooling step, the controlled cooling rate is ≤ Q°C / s, the controlled cooling time is 100 to 200 seconds, and the controlled cooling temperature of the bainite steel is ≥ 350°C when the controlled cooling step is completed. In one embodiment, the temperature of the bainite steel is 350 to 410°C when the controlled cooling step is completed.

[0063] Through long-term controlled cooling of steel sheets or strips, the phase transition of each bainite is sufficiently completed, and at the set temperature, the microstructure slowly and stably develops, forming a steel sheet or strip with a microstructure gradient in the thickness direction. At this stage, if the controlled cooling rate exceeds the set value, or if the final controlled cooling temperature of the steel sheet or strip falls below the set value, martensite will form in the microstructure, degrading the formability of the steel sheet or strip.

[0064] After the controlled cooling stage is complete, the bainite steel is air-cooled to room temperature. A steel sheet or strip with a microstructure gradient in the thickness direction is obtained. The air-cooling stage does not affect the microstructure of the bainite steel.

[0065] As described above, in one embodiment of the present invention, by varying the cooling parameters in the controlled slow cooling stage to obtain bainite steel having a five-layer microstructure gradient, a multiphase layer is formed on the surface layer in addition to the conventional three-layer gradient structure, and a steel sheet or strip having a five-layer gradient structure in the thickness direction can be obtained. Subsequently, after further rapid cooling and controlled cooling stages, acicular bainite or massive bainite is generated in other areas of the bainite steel, similarly at different positions in the thickness direction. Finally, a multiphase layer containing ferrite in the surface layer, an acicular layer in the shallow surface layer, and a massive layer in the core can be formed, resulting in a steel sheet or strip with a five-layer structure having a microstructure gradient.

[0066] The beneficial effects of the present invention are as follows: 1. The present invention provides a rational elemental composition design for bainite steel, particularly by rationally controlling the content of C, Si, Mn, and B in the steel, and by rationally controlling the content of C, Cr, Mo, and Mn in the steel, thereby optimizing the hardenability of the steel. The steel spontaneously forms phases with a microstructure gradient during the manufacturing process, increasing the strength and formability of the bainite steel.

[0067] 2. The present invention discloses a method for manufacturing bainite steel, and by designing a fine annealing process, particularly by controlling the cooling gas pressure and temperature during the cooling stage, steel sheets / strips with appropriate chemical composition spontaneously form a 3- or 5-layer microstructure gradient under the annealing conditions of the present invention. The bainite steel obtained by the technical method of the present invention has a tensile strength of ≥1000 MPa, a yield strength of ≥800 MPa, a hole expansion ratio of ≥40%, and a fracture elongation ratio of ≥12%. [Brief explanation of the drawing]

[0068] [Figure 1] Figure 1 is a schematic diagram of a steel strip having a three-layer structure in the thickness direction in an embodiment of the present invention. [Figure 2]Figure 2 is a schematic diagram of a steel strip having a five-layer structure in the thickness direction according to an embodiment of the present invention. [Figure 3] Figure 3 is a photograph of the metallurgical structure at the transition site between the needle-like layer (upper part) and the diphase layer (lower part) in Example 7 of the present invention. [Figure 4] Figure 4 is a photograph of the metallurgical structure at the transition site between the needle-like layer (upper part) and the massive layer (lower part) in Example 1 of the present invention. [Modes for carrying out the invention]

[0069] The embodiments of the present invention will be described below with reference to specific examples, but those skilled in the art will readily understand the other advantages and effects of the present invention from what is disclosed herein. The present invention will be described in relation to preferred embodiments, but this does not mean that the features of the present invention are limited to these embodiments. On the contrary, the purpose of describing the present invention in conjunction with embodiments is to cover other choices and modifications that can be extended based on the claims of the present invention. In fact, the purpose of introducing the invention with embodiments is to cover other choices and modifications that can be obtained based on the claims of the present invention. In order to provide a deeper view of the present invention, the following description includes numerous specific details. The present invention can be carried out without these details. Also, in order to avoid ambiguity of the emphasis of the present invention, some specific details are omitted from the description. To the extent that they do not contradict each other, embodiments and features in embodiments of the present invention can be combined with each other.

[0070] Examples 1-14 and Comparative Examples 1-6 In this invention, the bainite steels of Examples 1 to 14 were produced by the following process: Process 1: Smelting and casting; Process 2, Hot Rolling: The heating temperature shall be 1100-1230°C, the start temperature for finish rolling shall be 1050-1180°C, and the end temperature for finish rolling shall be 870-930°C. Step 3, Post-rolling cooling and winding: The cooling rate shall be 30-150°C / s, and the winding temperature shall be 540-620°C; Step 4: Remove oxide scale by pickling; Step 5, Cold Rolling: To achieve the required target thickness, the cold rolling reduction ratio shall be ≥ 30%. Specifically, in the embodiment of the present invention, the thickness of the cold-rolled steel sheet or strip is ≤ 2.2 mm; Step 6: Annealing.

[0071] The bainite steels of Comparative Examples 1 to 6 were also produced by smelting, continuous casting, hot rolling, post-rolling cooling and coiling, pickling and cold rolling, and annealing processes. The chemical composition of the steel and the parameters of the production process are shown in Tables 1 and 2.

[0072] Table 1 shows the mass percentages of each chemical element in the bainite steels of Examples 1-14 and Comparative Examples 1-3.

[0073] Table 2 shows the specific parameters of the bainite steels of Examples 1-14 and the comparative steels of Comparative Examples 1-6.

[0074] [Table 1]

[0075] [Table 2-1]

[0076] [Table 2-2]

[0077] Examples 1-5, 8, and 10-11 each yield a microstructure with three layers in the thickness direction, where the upper and lower surface layers are acicular layers and the core is a massive layer. Examples 6-7, 9, and 12-14 each yield a microstructure with five layers in the thickness direction, where the upper and lower surface layers are multiphase layers, the upper and lower surface layers are acicular layers and the core is a massive layer. In the microstructure of the bainite steel of this application, the acicular layer has the highest hardness, the multiphase layer has the lowest hardness, and the massive layer's hardness is between that of the acicular and multiphase layers. Therefore, in a three-layer composite material, the acicular layers of the upper and lower surface layers provide high surface hardness and surface yield strength, while the massive layer in the middle provides high ductility and plasticity. This makes it suitable for automotive parts such as car seat rails and drive shafts, where surface hardness or fatigue limit is required, and the overall ductility and plasticity of the material are also required. Furthermore, in a five-layer composite material, the relatively soft multi-phase layers of the upper and lower surface layers provide good local moldability to the surface layers, while the adjacent hard needle-like layers and the massive core layer also provide high strength and good toughness to the material. Therefore, it can be used to manufacture parts that require strength and overall moldability, such as control arms and triangular arms for automobile chassis.

[0078] In Comparative Examples 1-3, the component design deviated from the invention, and steel sheets or strips with a structure that slopes in the thickness direction could not be obtained. In Comparative Example 1, the R value was too high, and only a pure massive layer structure was obtained. In Comparative Examples 2-3, the R value was too low, and only a pure acicular layer structure was obtained. In Comparative Examples 4-6, steel type A was used, and although the component design met the conditions, the annealing process in the manufacturing process deviated from the invention, and steel sheets or strips with a structure that slopes in the thickness direction could not be obtained. However, in Comparative Example 4, the cooling gas pressure during the slow cooling stage exceeded the design value, resulting in the formation of a large proportion of ferrite throughout the thickness direction of the steel sheet or strip. Also, during the rapid cooling stage, the cooling gas pressure exceeded the design value, resulting in the formation of acicular bainite throughout the thickness direction of the steel sheet or strip, and massive bainite could not be formed. Furthermore, because a large proportion of ferrite preferentially formed in the steel sheet or strip, carbon accumulated in the partially supercooled austonite, preventing the bainite transition. Ultimately, the transition to new martensite occurred during the air-cooling stage, resulting in the inability to form a structure with a gradient in the thickness direction of the steel sheet or strip, and poor formability. In Comparative Example 5, the cooling gas pressure during the rapid cooling stage exceeded the design value, so only a pure needle-like bainite structure was obtained. In Comparative Example 6, the cooling gas pressure during the rapid cooling stage did not reach the design value, so only a pure massive layer structure was obtained.

[0079] Figure 3 shows the lower surface layer region of Example 7 of the present invention, specifically a photograph (scanning electric mirror) of the metallurgical structure at the transition site between the acicular layer (upper) and the biphase layer (lower). In the upper part of the figure, i.e., the region close to the core, the structure is typical acicular bainite, meaning that the acicular layer begins from this region. In the lower part of the figure, i.e., the region close to the lower surface, polygonal ferrite, acicular bainite, and dispersed granular carbide precipitates of the nano-order, sub-micro-order, or micro-order are included, meaning that the biphase layer of the surface layer begins from this region.

[0080] Figure 4 shows a region of the core of Embodiment 1 of the present invention that is close to the upper surface layer, specifically a photograph (scanning electric mirror) of the metallurgical structure at the transition site between the acicular layer (upper) and the massive layer (lower). In the upper part of the figure, i.e., the region close to the upper surface, the structure contains a large amount of typical acicular bainite, meaning that the acicular layer begins from this area. In the lower part of the figure, i.e., the region close to the core, a large amount of bainite is transformed into a massive polygonal morphology, meaning that a large amount of massive bainite is formed in this region, meaning that the massive layer begins from this area.

[0081] Table 3 shows the results of the mechanical property measurements of bainite steel for Examples 1-14 and Comparative Examples 1-6. Transverse JIS 5# tensile test specimens were taken to measure the yield strength, tensile strength, and elongation at break of the steel, and GB / T 228.1-2010 "Tensile tests for metallic materials, Part 1: Room temperature test methods" was adopted as the test method. To measure the hole expansion ratio of the steel, the central part of the steel plate was taken. The hole expansion ratio was measured by a hole expansion test, in which a sample with a hole in the center made of a male die was pressed into a female die, and the central hole of the sample was expanded until necking or through cracks appeared at the edge of the hole in the plate. Since the method of creating the initial hole in the center of the sample and the quality of the corresponding initial hole edge have a significant impact on the measurement result of the hole expansion ratio, the test and measurement method was carried out according to the hole expansion ratio measurement method specified in ISO / DIS16630, and the initial central hole was made using the primary punch blanking method (corresponding to the processing method with the worst quality of the initial hole edge). Therefore, if the central initial hole is formed by secondary punch blanking, drilling, or reaming, the corresponding hole expansion ratio increases by 20% based on the values ​​shown in the table. If the central initial hole is formed by wire cutting, the corresponding hole expansion ratio increases by 50% based on the values ​​shown in the table. If the central initial hole is formed by laser blanking, the corresponding hole expansion ratio increases by 80% based on the values ​​shown in the table.

[0082] [Table 3]

[0083] As can be seen from Table 3, when the composition and process of the steel plate or strip satisfy the design, all examples obtain mechanical properties of yield strength ≥ 800 MPa, tensile strength ≥ 1000 MPa, elongation at break ≥ 12%, and hole expansion ratio ≥ 40%. Furthermore, because a structure with a gradient in the thickness direction of the steel plate or strip is formed, the material has good overall formability, good tensile ductility and hole expansion flange workability, and high elongation at break and hole expansion ratio, with (elongation at break * 10 + hole expansion ratio) ≥ 170% in all examples.

[0084] When the composition or process does not meet the design requirements, ideal mechanical properties cannot be obtained. For example, in Comparative Example 1, the C and Mn content is below the lower limit, resulting in low material strength. In Comparative Example 2, the C content exceeds the upper limit, resulting in excessively high material strength and extremely poor formability. In Comparative Example 3, the R value is below the design lower limit, so a massive core layer cannot be formed in the steel plate or strip, and the entire structure becomes acicular bainite, resulting in an extremely high hole expansion rate but poor elongation at break. Comparative Example 5 is similar; the process does not meet the design requirements (as described above), so the structure is similarly entirely acicular bainite, resulting in an extremely high hole expansion rate but poor elongation at break. Both of these sets of comparative examples have "uneven" formability, resulting in poor overall formability (elongation at break * 10 + hole expansion rate) < 170%. In Comparative Examples 4 and 6, the process does not meet the design requirements (as described above), resulting in the inability to form a structure with a gradient in the thickness direction. Consequently, the moldability is "uneven," leading to poor overall moldability (elongation at break * 10 + hole expansion) < 170%.

[0085] The combination of each technical feature in this application is not limited to the combination described in the claims or the combination described in the specific embodiments. All technical features described in this application may be freely combined or combined in any way, as long as they do not contradict each other.

[0086] Furthermore, it should be noted that the embodiments listed above are merely specific examples of the present invention. The present invention is not limited to these embodiments, and similar variations and modifications can be directly obtained or easily conceived by those skilled in the art from the disclosure of the present invention, and therefore, it goes without saying that they fall within the scope of protection of the present invention.

Claims

1. Bainite steel sheet or strip, in mass percent, consisting of the following chemical components: 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%, the remainder being Fe and unavoidable impurities. The mass percentages of chemical elements are related by the following: R = (Mn + Si) / (12 * C + 160 * B), where 0.9 ≤ R ≤ 1.2 Satisfying the conditions, The mass percentages of chemical elements are related by the following: Q = (C + Cr + Mo + Mn / 2) / R, where 1.15 ≤ Q ≤ 1.

5. During calculations, the numerical value before the percentage sign of the element's mass percentage is used. Satisfying the conditions, The bainite steel sheet or strip has two layers of surface structure and one layer of core structure, the core structure is located between the two layers of surface structure. In the bainite steel sheet or strip, the core structure accounts for 20% to 50% of the volume of the bainite steel sheet or strip, with the remainder being the surface layer structure. The surface layer structure includes needle-like bainite and granular carbide precipitate phases, and the core structure includes massive bainite and granular carbide precipitate phases. The acicular bainite and granular carbide precipitate phases account for 99% or more of the surface layer microstructure volume, and the massive bainite and granular carbide precipitate phases account for 99% or more of the core microstructure volume. The diameter of the granular carbide precipitate phase is ≤ 5 μm. The bainite steel sheet or strip has a tensile strength of ≥ 1000 MPa, a yield strength of ≥ 800 MPa, a hole expansion ratio of ≥ 40%, and a fracture elongation ratio of ≥ 12%.

2. The bainite steel sheet or strip according to claim 1, further comprising at least one of Ti and Nb, wherein Nb ≤ 0.1% and Ti ≤ 0.15%.

3. The aforementioned unavoidable impurities are P ≤ 0.015% and S ≤ 0.004%, and the bainite steel sheet or strip according to claim 1.

4. The bainite steel sheet or strip further has two multiphase layers, the two surface layer structures and the single core structure constitute an intermediate layer, and the intermediate layer lies between the two multiphase layers. In the bainite steel sheet or strip, the volume of the multiphase layer accounts for 2% to 10% of the volume of the bainite steel sheet or strip, and the remainder is the intermediate layer. The bainite steel sheet or strip according to claim 1, wherein the multiphase layer comprises polygonal ferrite, acicular bainite, and granular carbide precipitate phase, the polygonal ferrite occupies 50% or less of the volume of the multiphase layer, and the polygonal ferrite, acicular bainite, and granular carbide precipitate phase together occupy 99% or more of the volume of the multiphase layer.

5. A method for producing a bainite steel sheet or strip according to any one of claims 1 to 4, comprising the following steps: Smelting and casting; Hot rolling; Cooling and winding after rolling; Pickling and cold rolling; Annealing.

6. The annealing process includes, in order, a heating stage, a slow cooling stage, a rapid cooling stage, a controlled cooling stage, and an air cooling stage, wherein the controlled cooling rate is such that the cooling rate in the controlled cooling stage < the cooling rate in the slow cooling stage < the cooling rate in the rapid cooling stage, the method for producing a bainite steel sheet or strip according to claim 5.

7. A method for producing a bainite steel sheet or strip according to claim 6, which satisfies any one of the following conditions: (1) In the slow cooling stage, the mixture is cooled to a slow cooling temperature of 720 to 800°C at a slow cooling rate of Q to 10*Q°C / s, and the mass percentage of the chemical elements satisfies Q = (C + Cr + Mo + Mn / 2) / R, 1.15 ≤ Q ≤ 1.5, and R = (Mn + Si) / (12*C + 160*B), 0.9 ≤ R ≤ 1.2, where each chemical element in the formula is replaced with the numerical value before the percentage sign of the mass percentage content of that chemical element; or (2) In the slow cooling stage, the mixture is cooled to a slow cooling temperature of 620 to 700°C at a slow cooling rate of Q to 10*Q°C / s, and the mass percentage of the chemical elements satisfies Q = (C + Cr + Mo + Mn / 2) / R, 1.15 ≤ Q ≤ 1.5, and R = (Mn + Si) / (12*C + 160*B), 0.9 ≤ R ≤ 1.2, where each chemical element in the formula is replaced with the numerical value before the percentage sign of the mass percentage content of that chemical element.

8. A method for producing a bainite steel sheet or strip according to claim 7, which satisfies any one of the following conditions: (1) Cooling is performed by spraying a cooling gas onto the surface of the bainite steel plate or strip, with the spray pressure of the cooling gas being 0.2*Q to QkPa, and the holding time of the cooling gas spray being 5 to 20 seconds. (2) Cooling is performed by spraying a cooling gas onto the surface of the bainite steel plate or strip, with the spray pressure of the cooling gas being 0.05*Q to 0.15*Q kPa, and the holding time for the cooling gas spray being 5 to 15 seconds.

9. In the aforementioned rapid cooling stage, the rapid cooling rate is 10*Q to 20*Q°C / s, and the rapid cooling temperature is 400 to 540°C. A method for producing a bainite steel sheet or strip according to claim 7, wherein the sheet is cooled to the specified temperature.

10. The method for manufacturing a bainite steel sheet or strip according to claim 9, wherein in the rapid cooling stage, cooling is performed by spraying a cooling gas twice onto the surface of the bainite steel sheet or strip, the first injection pressure of the cooling gas being 0.3*Q to 1.5*Q kPa, the first holding time of the cooling gas being 1 to 7 seconds, the second injection pressure of the cooling gas being 0.08*Q to 0.2*Q kPa, and the second holding time of the cooling gas being 5 to 10 seconds.

11. The method for producing a bainite steel sheet or strip according to claim 8, wherein the cooling gas is a mixture of a reducing gas and an inert gas, the reducing gas is hydrogen gas, its volume fraction is 1% to 8%, and the temperature of the cooling gas is 5 to 50°C.

12. The method for producing a bainite steel sheet or strip according to claim 10, wherein in the rapid cooling stage, the cooling gas is a mixture of a reducing gas and an inert gas, the reducing gas is hydrogen gas, its volume fraction is 1% to 8%, and the temperature of the cooling gas is 5 to 50°C.

13. In the controlled cooling stage, the controlled cooling rate is ≤ Q°C / s, the controlled cooling time is 100 to 200 seconds, and at the end of the controlled cooling stage, the controlled cooling temperature of the bainite steel sheet or strip is ≥ 350°C, the method for manufacturing a bainite steel sheet or strip according to claim 6.

14. The method for producing a bainite steel sheet or strip according to claim 6, wherein in the heating step, the material is heated to a soaking temperature of 840 to 950°C at a heating rate of ≤50°C / s, and then kept warm for a period of 60 to 180 seconds.

15. The parameters of the manufacturing method described above satisfy any one of the following conditions for the method of manufacturing a bainite steel sheet or strip according to claim 5: In the hot rolling process, the heating temperature is set to 1100 to 1230°C, the finish rolling start temperature is set to 1050 to 1180°C, and the finish rolling end temperature is set to 870 to 930°C. In the post-rolling cooling and winding process, the cooling rate shall be 30 to 150°C / s, and the winding temperature shall be 540 to 620°C; In the aforementioned cold rolling process, the cold rolling reduction ratio is ≥ 30%.

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