A method for controlling residual stress in offline-quenched high-strength martensitic steel based on strain accumulation in the non-recrystallization zone

Abstract

This invention discloses a method for controlling residual stress in offline quenched high-strength martensitic steel based on strain accumulation in the non-recrystallized region, relating to the field of high-strength martensitic steel strip production technology. This invention utilizes the strain accumulation and microstructure inheritance effect in the non-recrystallized austenite region to transform the energy stored in hot rolling deformation into an endogenous driving force for grain refinement during offline heat treatment. While ensuring high-temperature solid solution of microalloying elements, transverse microstructure uniformity, and solid solution strengthening effects, it modulates the original austenite grains to change the thermodynamic and kinetic path of martensitic phase transformation, improving the coordination of phase transformation sequence between the strip surface and core, enhancing the synchronicity of three-dimensional phase transformation, fundamentally reducing residual stress and improving dimensional stability.

Description

Technical Field

[0001] This invention relates to the field of high-strength martensitic steel strip production technology, and in particular to a method for controlling the residual stress of offline quenched high-strength martensitic steel based on strain accumulation in the non-recrystallization zone. Background Technology

[0002] With the increasing demand for lightweight and high-safety modern equipment, ultra-high-strength martensitic steels (such as hot-formed steel, bulletproof steel, and wear-resistant steel) with tensile strengths reaching 1000 MPa or even 1500 MPa have been widely used. These steels typically employ a "hot rolling + offline quenching" process, where high-temperature austenitization followed by rapid cooling and quenching yields a high-strength microstructure primarily composed of lath martensite.

[0003] However, during the high-intensity rapid cooling process of offline quenching, the strip steel faces extremely difficult-to-overcome residual stress and shape distortion. Due to the geometric characteristics of the steel plate, its edges exhibit two-dimensional heat dissipation, resulting in a significantly higher cooling rate at the edges than at the core. This non-uniformity in cooling rate directly triggers asynchronous phase transformation processes on the strip steel's cross-section. Specifically, the strip steel edges cool first to the martensitic transformation initiation temperature (Ms), undergoing crystallographic shear from face-centered cubic to body-centered cubic, accompanied by severe volume expansion. At this point, the edge material rapidly loses plasticity due to phase transformation hardening, while the strip steel core remains in the high-temperature austenitic phase. Subsequently, when the core temperature drops to the Ms point and martensitic transformation expansion occurs, it is inevitably constrained by the rigid mechanical forces of the hardened edges. This temporal and spatial phase transformation volumetric strain misalignment irreversibly evolves into a high-order residual stress field within the strip steel. When the strip steel is processed downstream, the mechanical equilibrium is disrupted, and the residual torque is released instantaneously, leading to severe distortion of the strip steel and even inducing edge cracking.

[0004] To suppress residual stress and plate distortion during quenching, current research mainly focuses on improving quenching cooling processes and equipment, primarily including the following three technical approaches:

[0005] 1. Edge shielding and differentiated cooling technology. This type of technology attempts to slow down the edge cooling rate and forcibly suppress the transverse temperature difference by adding mechanical baffles to both sides of the quenching equipment or artificially reducing the nozzle flow rate of the edge manifold. However, for ultra-high strength martensitic steel, the critical cooling rate required to obtain a fully martensitic structure is extremely high. Any operation that artificially reduces the edge cooling rate carries the possibility of causing the strip edge cooling curve to pass through the ferrite or bainite transformation zone; this can lead to softening of the strip edge structure, disrupting the transverse uniformity of the material, and ultimately causing the material to fail during subsequent service.

[0006] 2. Enhanced Heat Transfer and Vapor Film Breaking Technology. Some studies indicate that instability in the film boiling (vapor film stage) during the initial stage of quenching is a significant cause of uneven cooling. Therefore, existing technologies often employ methods such as increasing water pressure or flow rate to attempt to quickly break the vapor film on the steel plate surface, allowing heat transfer to rapidly enter the nucleation boiling stage. However, while globally enhanced heat transfer shortens the vapor film stage, it significantly increases the overall cooling rate. According to phase transformation kinetics, extremely high cooling rates lead to a sharp increase in the martensitic phase transformation rate (i.e., the α value in the KM equation), resulting in an explosive phase transformation. This instantaneous and violent volume expansion amplifies the mechanical constraint misalignment caused by the time difference between the phase transformation at the edges and the core, failing to fundamentally reduce the residual stress peak.

[0007] 3. Roller Quenching. Roller quenching is currently the mainstream equipment technology in the industry for controlling the shape of high-strength steel plates. This technology applies high-intensity normal forced displacement constraint to the steel plate during quenching and cooling using closely spaced rollers. At the macroscopic mechanical level, this constraint forcibly resists the bending moment generated by thermal stress in the steel plate, suppressing out-of-plane deformation (warping, swaying). At the microscopic physical metallurgical level, the external normal stress triggers phase transformation-induced plasticity (TRIP effect, i.e., Greenwood-Johnson effect) and martensitic variant selection (Magee effect), causing local micro-plastic rheology at the moment of phase transformation, thereby dynamically relaxing the peak stress in the thickness direction.

[0008] Although roll forming effectively ensures the apparent flatness of the steel sheet, its external mechanical constraint is essentially one-dimensional (Z-axis normal). It is mainly used to eliminate stress gradients in the thickness direction, but has extremely limited improvement on the in-plane residual stress gradient in the width direction (X-axis) that leads to longitudinal shearing. Roll forming constraint essentially "locks" the transverse thermal stress and structural stress in the elastic / plastic strain field of the steel sheet to maintain macroscopic equilibrium. Once the downstream longitudinal shearing is performed, the strip, having lost its overall constraint, will still experience severe residual stress release and lateral bending.

[0009] In summary, existing technologies are all limited to the single process of "offline quenching," attempting to address the internal stresses inevitably generated by thermodynamics and phase transformation kinetics by altering external physical boundary conditions (local cooling medium distribution, enhanced overall heat transfer, and application of normal mechanical constraints). To date, no technology in this field has proposed how to improve the residual stress of high-strength martensitic steel by starting from the kinetic properties of the material's endogenous microstructure, while maintaining transverse microstructure homogeneity and without sacrificing solid solution strengthening. Summary of the Invention

[0010] To address the problems of high-level in-plane / out-of-plane residual stress, edge bending, and internal stress concentration in existing ultra-high-strength martensitic steels during offline quenching, which are caused by asynchronous martensitic phase transformation in three-dimensional space due to differences in cooling rates in the strip width direction (edge ​​and center) and thickness direction (surface and core), this invention provides a method for controlling residual stress in offline quenched high-strength martensitic steel based on strain accumulation in the non-recrystallization zone.

[0011] This invention provides a method for controlling residual stress in offline-quenched high-strength martensitic steel based on strain accumulation in the non-recrystallization zone, comprising the following steps:

[0012] At the austenite non-recrystallization temperature of the strip steel (T nr The following steps involve precision rolling to control the cumulative reduction (R) in the non-recrystallized austenite region. acc This is done to introduce deformation defects within the austenite grains and induce strain-induced precipitation of microalloying elements.

[0013] The strip steel after precision rolling is cooled so that the deformation defects are transformed into a room temperature structure with a high interfacial area, and the strain-induced precipitates are retained in the strip steel matrix.

[0014] The room temperature microstructure is heated to austenitize it; the high interfacial area is used as the preferred nucleation point for the austenite inverse phase transformation, and the refined original austenite grains are reconstructed based on the pinning effect of strain-induced precipitation relative grain boundary migration (Zener pinning force).

[0015] The austenitized strip is quenched and cooled; relying on the mechanical resistance of the grain boundaries of the refined original austenite grains to the martensitic shear process, the martensitic transformation initiation temperature and the transformation kinetic rate (α) are reduced; so that during the quenching and cooling process, when the core temperature of the strip drops to the martensitic transformation initiation temperature (M... s When the martensitic phase transformation begins, the amount of martensitic transformation on the surface of the strip is suppressed and does not exceed the preset transformation threshold, thereby reducing the time difference of phase transformation expansion between the strip surface and the core in the thickness direction and reducing the macroscopic residual stress difference.

[0016] The purpose of this invention is to overcome the limitations of conventional quenching, which relies solely on cooling rate control, and to transform the deformation energy stored in the hot rolling process into an endogenous driving force for controlling grain size in the offline heat treatment stage through a "microstructure inheritance" mechanism. While ensuring high-temperature solid solution of microalloying elements, by controlling ultrafine proto-austenite grains, the thermodynamic and kinetic paths of martensitic phase transformation are altered, improving the phase transformation synchronicity of the strip in three-dimensional space, fundamentally reducing residual stress and enhancing dimensional stability.

[0017] Furthermore, the finishing rolling method is as follows: continuous rolling is carried out in the last N stands controlled below the austenite non-recrystallization temperature, wherein N is not less than 2.

[0018] Furthermore, the cumulative reduction rate in the non-recrystallized austenite region should be controlled to be no less than 40%.

[0019] Further, the room temperature tissue is heated to A c3 +50℃ to A c3 Austenitization was carried out at +150℃, A c3 Critical temperature.

[0020] Furthermore, this invention can achieve the refinement and reconstruction of various original austenite grain sizes for martensitic steels with different design requirements. Optionally, the average grain size of the refined original austenite grains obtained by the reconstruction does not exceed 15 μm. More optionally, the average grain size of the refined original austenite grains obtained by the reconstruction does not exceed 8 μm.

[0021] Furthermore, when quenching and cooling the austenitized strip, the preset transformation threshold is 85%.

[0022] Furthermore, the preset threshold for the transformation variable is 80%.

[0023] Furthermore, the quenching and cooling process of austenitized strip steel also includes a micro-precipitation plastic relaxation mechanism: at the end of the quenching and cooling process, when the strip steel temperature drops to the transition carbide precipitation temperature range, the self-tempering precipitation of ε-transition carbides is induced. The precipitation plastic strain generated under the macroscopic residual stress field by the self-tempering precipitation process is used to further offset and relax the quenching residual stress inside the strip steel.

[0024] The transition carbide precipitation temperatures of different martensitic steels vary, and the surface cooling rate of the austenitized strip during the quenching process also varies accordingly; optionally, the transition carbide precipitation temperature range is ≤250℃; the surface cooling rate of the austenitized strip during the quenching process is not less than 40℃ / s.

[0025] Furthermore, the strip steel is an ultra-high strength steel and may contain at least one alloying element such as Nb, V, and Ti, but the types of alloying elements are not limited thereto; in some examples, the volume fraction of martensite in the microstructure after quenching is not less than 90%.

[0026] The present invention also provides a high-strength martensitic steel strip manufactured by the above method, which exhibits a "U"-shaped distribution of residual stress along the thickness direction, with the surface layer under tension and the core under compression, and the extreme difference of rolling residual stress along the entire thickness direction does not exceed 500 MPa.

[0027] Compared with the prior art, the advantages of the present invention are:

[0028] 1. Resolving the contradiction between solid solution strengthening and grain refinement. This invention utilizes a strain accumulation-driven microstructure inheritance mechanism to replace the traditional cooling grain refinement method, allowing austenitization at higher offline quenching temperatures. This ensures both sufficient solid solution of microalloying elements (guaranteeing ultra-high strength) and the acquisition of an ultra-fine grain structure.

[0029] 2. Full three-dimensional stress field reconstruction and plate shape control. Through the endogenous regulation of phase change thermodynamics and kinetics, residual stress in both the thickness and width directions is simultaneously resolved. This completely overcomes the technical blind spot of traditional forced roll quenching, which "only presses the plate into shape without eliminating internal stress".

[0030] 3. Improve the overall service performance of materials. Eliminate residual tensile stress on the surface and stress concentration in the macrostructure, significantly improving the resistance to hydrogen-induced delayed cracking, fatigue life, and complex stamping and fine blanking performance of ultra-high strength martensitic steel.

[0031] 4. No new hardware investment required, strong industrial applicability. This solution relies on the collaborative optimization of software models of thermodynamic parameters throughout the entire process, without requiring hardware modifications to existing hot rolling production lines or quenching equipment (such as adding edge shielding or expensive roll forming systems), thus possessing extremely high engineering feasibility and economic benefits. Attached Figure Description

[0032] Figure 1 The effect of cumulative downward pressure rate in the non-recrystallized region on grain size.

[0033] Figure 2 Comparison of quenching phase transformation rates with different cumulative downforce rates. Wherein, (a): 30% cumulative downforce rate; (b): 42% cumulative downforce rate.

[0034] Figure 3 The effect of different compression ratios on residual stress is shown. Among them, (a): 30% cumulative downforce rate in the non-recrystallized region; (b): 42% cumulative downforce rate in the non-recrystallized region. Detailed Implementation

[0035] The technical solution of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0036] This invention provides a method for controlling residual stress in offline quenched martensitic steel based on strain accumulation in the non-recrystallization zone, specifically a method for cross-process collaborative control, which includes the following steps:

[0037] (1) Strain accumulation and defect structure during hot rolling and finishing stages: Obtain the austenite non-recrystallization temperature of the target steel grade. During the finishing rolling stage, control the cumulative reduction rate of the strip in the non-recrystallization region. By applying a large reduction rate, the austenite grains are drastically elongated along the rolling direction, introducing a high density of deformation bands and dislocation networks within the grains. This process produces two key metallurgical effects: firstly, it significantly increases the effective phase interface and total grain boundary area per unit volume (i.e., The first is the high-density strain-induced precipitation of microalloyed carbonitrides such as Nb, V, and Ti at dislocation nodes and subgrain boundaries.

[0038] (2) Solidification and energy conversion during the cooling stage: Hot-rolled plates undergo laminar flow cooling. During the cooling phase transformation after hot rolling (… During the process, most of the original dislocation distortion energy introduced in step (1) is dissipated, but the high The physical characteristics of the values ​​serve as non-uniform nucleation points, which are transformed into extremely fine grain sizes and large phase boundary areas in room temperature microstructures (i.e., mechanical energy is converted into interfacial energy). At the same time, strain-induced nanoscale carbonitrides are stably retained in the matrix, achieving cross-stage "inheritance" of microstructure characteristics.

[0039] (3) Reverse phase transformation and grain length limitation during offline heating stage: The hot-rolled plate is sent into the offline quenching furnace and the heating temperature is set. for Offline heating induces reverse austenite phase transformation ( When the high-density phase boundary retained in step (2) becomes the preferred nucleation point, it induces a high-density explosive nucleation of austenite. As the grains attempt to grow, they are hindered by three factors: ① steric hindrance caused by adjacent high-density nucleation points; ② strong Zener pinning force generated by the relative migration of nanoscale precipitates; ③ low interface mobility caused by the inheritance of hot-rolled deformation texture. This allows for the "reverse" reconstruction of the average size even under high-temperature conditions that ensure sufficient solid solution of alloy carbides. The ultrafine austenite grains.

[0040] (4) Phase transformation kinetic hysteresis and three-dimensional stress improvement during offline quenching: The austenitized strip is subjected to uniform and rapid cooling across the entire cross section (cooling rate) Based on the strong mechanical resistance of the ultrafine austenite grain boundaries obtained in step (3) to the martensitic shear process, the martensitic phase transformation initiation temperature can be reduced. and reduce the phase transition kinetic rate ( Synchronization in the thickness direction (surface and core): The ultrafine grains retard the phase transformation dynamics in the extremely cold surface region, forcing the surface phase transformation to be delayed to a lower temperature; during this delay period, heat is continuously transferred from the core, and the core temperature approaches the surface temperature, ultimately improving the synchronicity of the phase transformation between the surface and the core, and reducing the residual tensile and compressive stresses on the upper and lower surfaces; Synchronization in the width direction (edge ​​and center): Phase transformation The decrease in the temperature at the edge leads to a lower temperature entering the phase transition region, while the temperature in the middle also decreases significantly, resulting in a convergence of the lateral temperature difference. Simultaneously, this reduces the phase transition rate. The phase transition period at the edge is lengthened, causing it to overlap with the subsequent phase transition at the center on the time axis, thereby eliminating the in-plane lateral residual moment that causes the tangential edge to bend.

[0041] The present invention will be illustrated below with specific implementation examples. It should be noted that the specific implementation examples below are for illustrative purposes only and do not limit the scope of the present invention in any way.

[0042] Implementation Cases

[0043] To further illustrate the operability of the technical solution of this invention and the effect of cross-process multi-field coupling control, the following specific industrial application cases based on Q1100 grade ultra-high strength martensitic steel are provided:

[0044] Target material and initial parameters:

[0045] Steel type and specifications: The target cross-sectional specifications for Q1100 grade ultra-high strength martensitic steel are 8 mm thickness × 3000 mm width.

[0046] Chemical composition (wt.%): C 0.17, Si 0.21, Mn 1.14, Nb 0.022, V 0.058, Ti 0.019, Cr 0.22, balance Fe and unavoidable impurities.

[0047] Thermophysical parameters: The coefficient of thermal expansion of Q1100 grade ultra-high strength martensitic steel in the range of 20-800℃ was determined, among which the average coefficient of thermal expansion of austenite α A =2.18×10 -5 / ℃, Martensitic thermal expansion coefficient α M =1.24×10 -5 / ℃.

[0048] This embodiment takes the aforementioned Q1100 grade ultra-high strength martensitic steel as the research object and provides a method for controlling the residual stress of offline quenched high-strength martensitic steel based on strain accumulation in the non-recrystallization zone. The specific steps are as follows:

[0049] Step 1: Hot rolling strain accumulation and microstructure solidification

[0050] 1. Rolling execution

[0051] After austenitizing heating, Q1100 grade ultra-high strength martensitic steel strip is rough rolled at 1050℃ (reduction of approximately 30%); then it enters the non-recrystallization zone (T nr The final pass of the finishing mill is performed at approximately 900℃, controlling the cumulative reduction rate (R) in this region. acc The figure reached over 42.8%.

[0052] By using a large reduction rate, the austenite grains are drastically elongated along the rolling direction, and a high density of deformation bands and dislocation networks are introduced into the grains.

[0053] 2. Cooling

[0054] After finishing rolling, the temperature is lowered to room temperature using laminar flow cooling mode, transforming the high-density crystal defects and dislocation networks generated in the non-recrystallized region into a large phase interface (high S) with a fine room-temperature structure. v (value), and induce strain-induced precipitation of Nb, V, and Ti carbonitrides.

[0055] During the cooling phase transformation (γ→α) after hot rolling, most of the introduced original dislocation distortion energy is dissipated, but high S v The physical characteristics of the values ​​serve as non-uniform nucleation points, which are transformed into extremely fine grain sizes and large phase boundary areas in room temperature microstructures (i.e., mechanical energy is converted into interfacial energy). At the same time, strain-induced nanoscale carbonitrides are stably retained in the matrix, achieving cross-stage "inheritance" of microstructure characteristics.

[0056] Step 2: Offline heating and reverse phase transformation grain refinement reconstruction

[0057] 1. Heating before quenching

[0058] The steel plate is fed into an annealing furnace and heated to the quenching temperature of 870℃ at a rapid heating rate of 100℃ / s, and held at that temperature for 15 minutes.

[0059] When the inverse austenite transformation (α→γ) is induced by offline heating, the high-density phase boundary retained in the cooling transformation stage after hot rolling becomes the preferred nucleation point, inducing high-density explosive nucleation of austenite.

[0060] 2. Tissue genetic verification

[0061] When grains attempt to grow, they are hindered by three factors: (1) the steric hindrance effect caused by the spatial collision of adjacent high-density nucleation sites; (2) the strong Zener pinning force generated by the relative migration of nanoscale precipitates; and (3) the low interface mobility caused by the inheritance of hot-rolled deformation texture. This allows for the "reverse" reconstruction of ultrafine proto-austenite grains with an average size ≤15 μm under high-temperature conditions that ensure sufficient solid solution of alloy carbides.

[0062] Figure 1 This invention demonstrates that, through cross-process control, the grain size in the non-recrystallized region was reduced to 9-14 μm after increasing the compression ratio to 42%, significantly better than >20 μm at a 30% compression ratio. This proves the significant effect of the "organic heredity" mechanism in inhibiting grain growth.

[0063] By leveraging the high-density phase boundary nucleation and Zener pinning effect of nanocarbides retained after cooling in step one, the original austenite grain size was successfully controlled at an ultrafine equiaxed grain level of 9-14 μm after the reverse phase transformation, which is significantly finer than the >20 μm of conventional online quenching.

[0064] Step 3: Offline quenching phase transformation hysteresis and precipitation plasticity coupling control

[0065] 1. Quenching process

[0066] The austenitized strip at 870℃ is subjected to uniform and rapid water cooling across the entire cross section at a cooling rate of not less than 50℃ / s.

[0067] The strong mechanical resistance of the grain boundaries of the ultrafine austenite grains obtained in step two to the martensitic shear process can reduce the martensitic transformation initiation temperature (M). s This reduces the phase transition kinetic rate (α).

[0068] 2. Phase transition kinetic hysteresis (stress reduction mechanism A)

[0069] Based on the hindering effect of ultrafine austenite grain boundaries on martensitic shear, the martensitic phase transformation initiation temperature (M) was measured. s The temperature was lowered to 427.8℃. The phase transition kinetics exhibited a "fast at the beginning and slow at the end" characteristic, with the 10%-90% transition time being extended to 2.2 s.

[0070] Synchronization in the thickness direction (surface and core): The ultrafine grains retard the phase transformation dynamics of the extremely cold region on the surface, forcing the surface phase transformation to be delayed to a lower temperature; during this delay period, heat is continuously transferred out of the core, and the core temperature approaches the surface temperature, which ultimately improves the synchronicity of the phase transformation between the surface and the core and reduces the residual tensile and compressive stress on the upper and lower surfaces.

[0071] Synchronization in the width direction (edge ​​and center): Martensitic phase transformation initiation temperature (M) s The decrease in the phase transition rate (α) lowers the temperature at the edge entering the phase transition region. At this time, the temperature in the middle also decreases significantly, and the lateral temperature difference converges. At the same time, reducing the phase transition kinetic rate (α) lengthens the phase transition period at the edge, causing it to overlap with the subsequent phase transition in the middle on the time axis, thereby eliminating the in-plane lateral residual moment that causes the tangential edge to bend.

[0072] Figure 2 It demonstrates the technical characteristic of "phase transition kinetic hysteresis". Figure 2 The curves clearly show that, within the same cooling rate range, the martensitic phase transformation initiation temperature (M) of the embodiment of the present invention (42% compression ratio) is... s The compression ratio is generally lower than that of the conventional mode (30% compression ratio), and the total phase transition time is extended. This provides a dynamic basis for the physical control logic of the subsequent "surface-center synchronous phase transition".

[0073] 3. Key mechanical control points

[0074] When the strip core begins to undergo martensitic transformation during cooling, the amount of martensite transformation in the surface pre-transformation zone is reduced to 79% due to kinetic hysteresis (compared to 87% in conventional unoptimized processes). This significantly weakens the resistance of the rigid outer shell to core expansion, resulting in an initial decrease in the extreme difference of residual stress in the thickness direction.

[0075] 4. Precipitation of plastic relaxation (stress reduction mechanism B)

[0076] In the final region, cooled to below 250°C, martensite lath formation is induced. - Self-tempering precipitation of transition carbides. This precipitation process produces significant "precipitation plastic strain" under macroscopic stress field (measured precipitation plasticity coefficient k). ε =2.15×10 -5 The residual stress from quenching was further relaxed and offset by microscopic plastic deformation.

[0077] Step 4: Final Product Performance and Stress Relief Results

[0078] After the above-mentioned full-process coupled control, Q1100 strip steel achieves the following comprehensive indicators:

[0079] 1. Mechanical properties

[0080] With a yield strength of 1204 MPa, a tensile strength of 1381 MPa, and an elongation after fracture of 11%, it achieves a perfect combination of fine grain strengthening and solid solution strengthening.

[0081] 2. Residual stress parameters (based on actual measurement and verification using the crack compliance method)

[0082] The residual stress in the thickness direction of the strip exhibits a convergent "U"-shaped distribution. Specifically, the residual tensile stress on the surface of the steel plate is controlled at +462 MPa, and the residual compressive stress in the core is 23 MPa.

[0083] Figure 3 The figure shows the effect of changing the compression ratio in the non-recrystallized region on residual stress. As can be seen from the figure, when the compression ratio in the non-recrystallized region is increased from 30% to 42%, the extreme difference of residual stress between the steel strip surface and the core in the full thickness direction decreases from 883 MPa to only 439 MPa; compared with the conventional process that does not consider the microstructure inheritance and precipitation plasticity control, the difference decreases by nearly 440 MPa.

[0084] The above detailed embodiments describe the implementation of the present invention; however, the present invention is not limited to the specific details described in the above embodiments. Within the scope of the claims and technical concept of the present invention, various simple modifications and changes can be made to the technical solution of the present invention, and these simple modifications all fall within the protection scope of the present invention.

Claims

1. A method for controlling residual stress in offline quenched high-strength martensitic steel based on strain accumulation in the non-recrystallization zone, characterized in that, Includes the following steps: Fine rolling is performed on the strip below the non-recrystallization temperature of austenite to control the cumulative reduction rate in the non-recrystallization region of austenite, so as to introduce deformation defects in the austenite grains and induce strain-induced precipitation of microalloying elements. The strip steel after precision rolling is cooled so that the deformation defects are transformed into a room temperature structure with a high interfacial area, and the strain-induced precipitates are retained in the strip steel matrix. The room temperature microstructure is heated to austenitize it; the high interfacial area is used as the preferred nucleation point for the austenite inverse transformation, and the refined original austenite grains are reconstructed based on the pinning effect of strain-induced precipitation relative to grain boundary migration. The austenitized strip is quenched and cooled; relying on the grain boundaries of the refined original austenite grains to mechanically hinder the martensitic shear process, the martensitic transformation initiation temperature and the transformation kinetic rate are reduced; so that during the quenching and cooling process, when the core temperature of the strip drops to the martensitic transformation initiation temperature and martensitic transformation begins, the martensitic transformation amount on the surface of the strip is suppressed and does not exceed a preset transformation amount threshold, thereby reducing the phase transformation expansion time difference between the surface and the core of the strip in the thickness direction and reducing the macroscopic residual stress difference.

2. The method for controlling residual stress in offline quenched high-strength martensitic steel based on strain accumulation in the non-recrystallization zone according to claim 1, characterized in that, The finishing rolling method is as follows: continuous rolling is carried out in the last N stands controlled below the austenite non-recrystallization temperature, where N is not less than 2.

3. The method for controlling residual stress in offline quenched high-strength martensitic steel based on strain accumulation in the non-recrystallization zone according to claim 1, characterized in that, The cumulative reduction rate in the non-recrystallized austenite region should be no less than 40%.

4. The method for controlling residual stress in offline quenched high-strength martensitic steel based on strain accumulation in the non-recrystallization zone according to claim 1, characterized in that, The room temperature tissue was heated to A. c3 +50℃ to A c3 Austenitization was carried out at +150℃.

5. The method for controlling residual stress in offline quenched high-strength martensitic steel based on strain accumulation in the non-recrystallization zone according to claim 1, characterized in that, The average grain size of the refined original austenite grains obtained by reconstruction does not exceed 15 μm; Furthermore, the average grain size of the refined original austenite grains obtained by reconstruction does not exceed 8 μm.

6. The method for controlling residual stress in offline quenched high-strength martensitic steel based on strain accumulation in the non-recrystallization zone according to claim 1, characterized in that, When quenching and cooling austenitized strip steel, the preset transformation threshold is 85%; Furthermore, the preset threshold for the transformation variable is 80%.

7. The method for controlling residual stress in offline quenched high-strength martensitic steel based on strain accumulation in the non-recrystallization zone according to claim 1, characterized in that, The quenching and cooling process of austenitized strip steel also includes a micro-precipitation plastic relaxation mechanism: at the end of the quenching and cooling process, when the strip steel temperature drops to the transition carbide precipitation temperature range, the self-tempering precipitation of ε-transition carbides is induced. The precipitation plastic strain generated under the macroscopic residual stress field by the self-tempering precipitation process is used to further offset and relax the quenching residual stress inside the strip steel.

8. The method for controlling residual stress in offline quenched high-strength martensitic steel based on strain accumulation in the non-recrystallization zone according to claim 7, characterized in that, The transition carbide precipitation temperature range is ≤250℃; the surface cooling rate of the austenitized strip during the quenching process is not less than 40℃ / s.

9. The method for controlling residual stress in offline quenched high-strength martensitic steel based on strain accumulation in the non-recrystallization zone according to claim 1, characterized in that, The strip steel is an ultra-high strength steel containing at least one of Nb, V, and Ti, and the volume fraction of martensite in the microstructure after quenching is not less than 90%.

10. A high-strength martensitic steel strip, characterized in that, It is manufactured by any one of claims 1-9; optionally, the high-strength martensitic steel strip has a "U"-shaped distribution of residual stress along the thickness direction, with the surface layer under tension and the core under compression, and the extreme difference of rolling residual stress in the full thickness direction does not exceed 700 MPa.

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