Warm rolling cryogenic critical annealing synergistic process for inhibiting plastic instability of low-carbon medium manganese steel PLC (Programmable Logic Controller) belt

Abstract

The invention discloses a warm rolling cryogenic critical annealing synergistic process for inhibiting low-carbon medium manganese steel PLC (programmable logic controller) belt plasticity instability, which comprises the following steps: S1) warm rolling deformation: heating a low-carbon medium manganese steel blank to a warm rolling temperature interval of 600-800 DEG C, and then carrying out double-pass compression deformation operation; (S2) subzero treatment is conducted, specifically, the low-carbon medium-manganese steel workpiece subjected to warm rolling deformation and water quenching is placed in a liquid nitrogen environment with the temperature being-196 DEG C for soaking treatment; and S3) critical annealing: performing critical annealing on the workpiece subjected to subzero treatment, putting the workpiece into a heating furnace, heating to a critical annealing temperature range of 580-600 DEG C, preserving heat for 1-8 hours at the temperature, and then cooling to room temperature along with the furnace to finally form a composite structure with superfine austenite grains and Mn element in gradient distribution. According to the method, cooperative regulation and control of austenite ultra-refining (200-800 nm) and Mn gradient distribution (5-12 wt%) are achieved for the first time, the technical problem that the austenite ultra-refining (200-800 nm) and the Mn gradient distribution (5-12 wt%) are difficult to consider in a traditional process is solved, and an ideal Mn element diffusion source is provided for subsequent inverse phase change.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of low-carbon medium-manganese steel plate preparation, in particular to a warm rolling and deep cryogenic critical annealing synergistic process for inhibiting plastic instability of low-carbon medium-manganese steel PLC. BACKGROUND

[0002] Medium-manganese steel (Mn content 4-13wt%) as the third generation of advanced high-strength steel (AHSS) is concerned in the field of automobile lightweight and marine engineering due to its excellent product of strength and ductility (≥30GPa·%) However, the existing technology still has the following key problems:

[0003] (1) The contradiction between yield strength and plastic instability:

[0004] 1) The yield strength of existing commercial medium-manganese steel is usually 500-650MPa, which is difficult to meet the high-end demand (such as ≥740MPa required for automobile crash structure);

[0005] 2) The traditional method of increasing strength (such as precipitation strengthening and increasing martensite content) will significantly reduce plasticity, and the product of strength and ductility is insufficient (such as <30GPa·%);

[0006] (2) The universality of PLC effect (plastic instability):

[0007] 1) The dynamic interaction between solute atoms such as Mn and C and dislocations (dynamic strain aging, DSA) leads to local deformation bands, causing uneven extension and even early fracture. The uniform distribution of Mn in austenite leads to the consistency of austenite stability, which makes the TRIP effect occur quickly and does not achieve good phase change induced strength and ductility effect, resulting in insufficient work hardening;

[0008] 2) The existing methods to inhibit PLC (such as pre-deformation and composition adjustment) mostly sacrifice plasticity, and lack a comprehensive solution.

[0009] In the prior art, in the patent with application number CN202210135100: a gradient structure medium-manganese steel is prepared by torsional shear + critical annealing, the yield strength reaches 740MPa, and the product of strength and ductility is >45GPa·%, but the problem of PLC plastic instability is not solved. In the patent with application number CN118957222A: the heterogeneity of austenite Mn is regulated by variable temperature treatment, but the relevance between ultra-fine austenite and PLC effect is not clear. In the patent with application number CN202411048632.5: PLC effect is eliminated by adding Al, but Al high alloying greatly increases the cost, and the strength is limited (tensile strength <1300MPa).

[0010] Therefore, the present application proposes a new warm rolling and deep cryogenic critical annealing synergistic process, which aims to effectively inhibit PLC effect and improve the comprehensive mechanical properties of low-carbon medium-manganese steel. SUMMARY

[0011] The purpose of the present application is to provide a warm rolling and deep cryogenic critical annealing synergistic process for inhibiting plastic instability of low-carbon medium-manganese steel PLC strips, to solve the problems existing in the prior art proposed in the background art.

[0012] To achieve the above purpose, the present application provides the following technical solution, a warm rolling and deep cryogenic critical annealing synergistic process for inhibiting plastic instability of low-carbon medium-manganese steel PLC strips, comprising the following steps:

[0013] S1) Warm rolling deformation

[0014] The low-carbon medium-manganese steel blank is heated to a warm rolling temperature range of 600-800℃, and then subjected to double-pass compression deformation operation;

[0015] S2) Deep cryogenic treatment

[0016] The low-carbon medium-manganese steel workpiece after warm rolling deformation and water quenching is placed in a liquid nitrogen environment at a temperature of-196℃ for immersion treatment;

[0017] S3) Critical annealing

[0018] The workpiece after deep cryogenic treatment is subjected to critical annealing, and is placed in a heating furnace, heated to a critical annealing temperature range of 580-600℃, and held at this temperature for 1-8h, and then cooled to room temperature with the furnace, finally forming a composite structure with ultra-fine austenite grains and gradient distribution of Mn elements.

[0019] Preferably, in step S1, the deformation amount of each pass is controlled to be 35%, and the total deformation amount is controlled to be 70%.

[0020] Preferably, in step S1, the strain rate of each pass in the double-pass compression deformation operation is set to 0.1s -1 .

[0021] Preferably, in step S1, the blank needs to stay for 100s between the two passes.

[0022] Preferably, in step S1, the blank is immediately subjected to water quenching treatment after deformation to fix the organizational structure formed during warm rolling.

[0023] Preferably, in step S2, the immersion time is controlled within the range of 10min-16h to promote the transformation of the internal microstructure of the workpiece and the precipitation of Mn-rich carbides.

[0024] Compared with the prior art, the present application has the following beneficial effects:

[0025] 1) Breakthrough in microstructure control: for the first time, synergistic control of ultra-fine austenite (200-800 nm) and Mn gradient distribution (5-12 wt%) is achieved, overcoming the technical difficulty of traditional processes that are difficult to balance both, by precisely controlling the precipitation behavior of Mn-rich carbides (size < 50 nm, spacing < 200 nm), providing an ideal Mn diffusion source for subsequent reverse transformation;

[0026] 2) Significant improvement in mechanical properties: the product of strength and ductility breaks through 40 GPa·%, an increase of more than 30% compared to conventional medium manganese steel (usually < 30 GPa·%), yield strength reaches 740-800 MPa, tensile strength 1000 MPa, elongation 30-40%, achieving the best match of high strength and high ductility, uniform elongation increased by more than 20%, and material forming performance improved significantly;

[0027] 3) Fundamental solution to plastic instability problem: the formation of ultra-fine austenite grains reduces dislocation density, thereby suppressing PLC band formation, and the Mn gradient distribution in austenite after deformation induces incomplete martensitic transformation, dividing and refining austenite grains, improving deformation uniformity, and fundamentally solving the problem of local deformation bands;

[0028] In summary, the present application not only solves the long-standing problems of strength-ductility inversion and PLC effect in medium manganese steel, but also through innovative microstructure control methods, the comprehensive performance of the material reaches the international leading level, and has important industrial application value. BRIEF DESCRIPTION OF DRAWINGS

[0029] Figure 1 A schematic diagram of PLC bands with different critical sawtooth strains and density in the present application test example 1;

[0030] Figure 2 Engineering stress-strain curve of the cryogenic treated sample annealed at 600℃ for 4h in test example 1 of the present application;

[0031] Figure 3 Schematic diagram of the geometry of the compression sample in test example 1 of the present application;

[0032] Figure 4 Schematic diagram of the double-pass hot compression test in test example 1 of the present application;

[0033] Figure 5 Process flow chart of the present application. DETAILED DESCRIPTION

[0034] The technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, rather than all the embodiments of the present application. Based on the embodiments in the present application, all other embodiments obtained by a person of ordinary skill in the art without creative work are within the protection scope of the present application.

[0035] In the description of the application, it should be noted that the execution order of the steps is not limited by the serial number, and the order of some steps can be changed, the steps can be executed synchronously, or the steps can be executed separately, all of which are within the protection scope of the present application.

[0036] Please refer to Figures 1-5 The present application provides a technical solution: a warm rolling and deep cryogenic critical annealing synergistic process for inhibiting plastic instability of low-carbon medium-manganese steel PLC, comprising the following steps:

[0037] S1) Warm rolling deformation

[0038] The low-carbon medium-manganese steel blank is heated to a warm rolling temperature range of 600-800℃, and then a double-pass compression deformation operation is performed;

[0039] S2) Deep cryogenic treatment

[0040] The low-carbon medium-manganese steel workpiece after warm rolling deformation and water quenching is placed in a liquid nitrogen environment with a temperature of -196℃ for immersion treatment;

[0041] S3) Critical annealing

[0042] The workpiece after deep cryogenic treatment is subjected to critical annealing, is placed in a heating furnace, is heated to a critical annealing temperature range of 580-600℃, and is kept at this temperature for 1-8h, and then is cooled to room temperature with the furnace, and finally a composite structure with ultra-fine austenite grains and gradient distribution of Mn elements is formed.

[0043] Specifically, the Portevin-Le Chatelier (PLC) effect is a local plastic instability phenomenon, which is manifested as sawtooth-shaped deformation during tensile. The existing medium-manganese steel is limited in application due to problems such as insufficient regulation of austenite stability and significant PLC effect, and the present application solves the core contradiction between strong plasticity inversion and plastic instability through microstructure innovation and process optimization.

[0044] Specifically, in the prior art, there are problems in the synergistic optimization of austenite stability and strong plasticity: the traditional process cannot consider both austenite ultra-fining and Mn gradient distribution, resulting in uncoordinated release of TRIP effect, and the present application adopts a warm rolling + deep cryogenic + critical annealing synergistic process, uses carbide as a nucleation point to regulate Mn / C distribution, and realizes 200-800nm ultra-fine austenite and gradient stabilization.

[0045] Specifically, in the prior art, the PLC effect suppression problem lies in that the dynamic interaction of Mn / C atoms and dislocations leads to sawtooth flow, reducing the forming stability. The present application reduces the dislocation density through the formation of ultra-fine-grained austenite grains, thereby suppressing the formation of PLC bands. After deformation, the gradient distribution of Mn in austenite induces incomplete martensitic transformation, divides and refines and stabilizes austenite grains, eliminates sawtooth fluctuations, and improves the uniform elongation by more than 20%.

[0046] Specifically, in the prior art, the problem of precise regulation of Mn-rich carbides lies in that it is difficult to obtain ultra-fine dispersed carbides through conventional cryogenic treatment, which restricts the improvement of mechanical properties. The present application uses an innovative cryogenic-annealing coupling process to make the carbides have the functions of nucleation points and element storage, achieving excellent comprehensive performance of yield ≥ 740 MPa, tensile strength ≥ 1000 MPa, and elongation ≥ 30%.

[0047] Specifically, the present application uses austenite ultra-fining + Mn gradient design: “warm rolling deformation + cryogenic treatment + critical annealing” synergistic process to form gradient Mn distribution nanometer austenite, taking into account high strength and sustained TRIP effect.

[0048] Specifically, the present application has the effect of suppressing PLC: using carbides to provide additional nucleation points and C, Mn element storage, constructing ultra-fine Mn gradient distribution austenite through austenite reverse transformation, and realizing segmented martensitic transformation induced by deformation, achieving the effect of inhibiting the plastic instability of medium manganese steel PLC.

[0049] Specifically, the process innovation of the present application: combining micro-alloying and short-time annealing to realize the synergistic improvement of strength (> 740 MPa) and plasticity (elongation > 30%).

[0050] Specifically, through the synergistic effect of “warm rolling deformation-cryogenic treatment-critical annealing” in three steps, the low-carbon medium manganese steel structure is realized. The deformation energy introduced by warm rolling provides a dislocation basis for subsequent cryogenic treatment, cryogenic treatment promotes the dispersion of Mn-rich carbides, and critical annealing forms a composite structure of ultra-fine austenite grains and Mn element gradient distribution. The three synergistically suppress dynamic strain aging (DSA), fundamentally eliminate the plastic instability of PLC bands, and realize the good matching of strength (yield strength ≥ 740 MPa) and plasticity (elongation ≥ 30%).

[0051] In step S1, the deformation amount in each pass is controlled at 35%, and the total deformation amount is controlled at 70%. Specifically, this deformation amount ensures sufficient strain accumulation to refine the substructure while avoiding crack defects caused by excessive deformation. The 35% deformation in two passes can make the dislocation density uniformly distributed, providing uniform nucleation sites for carbide precipitation during cryogenic treatment, reducing local stress concentration, and improving the plastic stability of the steel.

[0052] In step S1, during the two-pass compression deformation operation, the strain rate for each pass is set to 0.1 s⁻¹. -1 Specifically, 0.1s -1 The appropriate strain rate can balance deformation efficiency and dislocation evolution law, avoiding excessive dynamic recovery (reduced dislocation density) caused by low strain rate, and preventing local temperature rise caused by high strain rate (leading to coarsening of austenite grains), ensuring the formation of a uniform dislocation network after warm rolling, creating conditions for the directional precipitation of carbides in subsequent cryogenic treatment.

[0053] In step S1, the billet needs to be held for 100 seconds between two passes. Specifically, the 100-second inter-pass hold can release internal stress through partial dynamic recovery, preventing the billet from cracking due to excessive accumulated stress. At the same time, the short hold can promote short-range diffusion of carbon atoms, laying the foundation for dislocation-carbon interaction in subsequent warm rolling deformation and reducing the local manifestation of PLC bands caused by uneven deformation.

[0054] In step S1, the billet is immediately subjected to water quenching after deformation to fix the microstructure formed during warm rolling. Specifically, water quenching can quickly fix the ultrafine subgrain structure after warm rolling (preventing grain growth) and retain a large number of unrecovered dislocations, providing high-density nucleation sites for the precipitation of Mn-rich carbides during cryogenic treatment. If cooling is slow, it will lead to subgrain coarsening and reduce the strengthening effect of subsequent cryogenic treatment.

[0055] Specifically, in step S1, the deformation at 600-800℃ is considered warm deformation. Warm deformation, due to its moderate driving force, results in high-density dislocations (>10). 14 m -2 The deformation zone provides nucleation sites for austenite in reverse phase transformation. Annealing involves partial recrystallization, which retains more nucleation sites while ensuring that martensite recovery and austenite inverse phase transformation dominate during annealing, contributing to the acquisition of fine and abundant austenite after annealing. Lower deformation temperatures result in higher deformation energy, making complete recrystallization during annealing more likely, thus becoming a competing mechanism for inverse phase transformation and indirectly affecting the inverse phase transformation of austenite. Higher deformation temperatures result in lower deformation energy, smaller original austenite grain size, lower dislocation density, lower grain boundary distribution, and fewer austenite inverse phase transformation nucleation sites.

[0056] In the S2 step, the soaking time is controlled in the range of 10 min to 16 h, which promotes the transformation of the internal microstructure of the workpiece and the precipitation of Mn-rich carbides. Specifically, in the liquid nitrogen environment at -196℃, soaking for 10 min to 16 h can promote the combination of supersaturated carbon atoms with Mn to form Mn-rich carbides (size < 50 nm, spacing < 200 nm). The carbides can pin dislocation movement and inhibit the periodic process of "dislocation unlocking-slipping-repinning" in the DSA effect, thereby eliminating the sawtooth fluctuations of the PLC band.

[0057] Specifically, in the S2 step, the mechanism of action is to promote the dispersion precipitation of Mn-rich carbides such as Fe3C, with a size < 50 nm and a spacing < 200 nm, which provides a basis for refining austenite grains and regulating the Mn gradient distribution. If the deep cooling time is too short, the amount of carbide precipitation is small; if the deep cooling time is too long, the amount of carbide precipitation does not change, affecting the efficiency.

[0058] Specifically, in the S3 step, the organization formation law is: Mn-rich carbide dissolution → Mn element gradient diffusion → reverse phase austenite nucleation (grain size 100-300 nm). The distribution of manganese elements in the crystal fluctuates, forming an austenite Mn gradient. 600-750℃ belongs to the two-phase zone annealing temperature, and a two-phase structure of ferrite + austenite can be obtained in this temperature range. Below this temperature, all ferrite structure will be obtained, and above this temperature, complete austenite structure will be obtained.

[0059] The final material mechanical properties of the finished product prepared by the present application are: yield strength ≥ 740 MPa, tensile strength ≥ 980 MPa, elongation ≥ 30%, and product ≥ 30 GPa·%. PLC effect inhibition: the tensile curve has no sawtooth fluctuation, and the uniform elongation is increased by more than 20%.

[0060] Example 1: Low-carbon medium-manganese steel treatment using the process of the present application

[0061] Raw materials: low-carbon medium-manganese steel chemical composition (wt%): C 0.25%, Mn 7%, Si 0.2%, P 0.004%, S 0.001%, and the balance Fe.

[0062] Process scheme:

[0063] S1 warm rolling: 700℃ warm deformation, pass deformation amount 35%, interval between passes 100 s, total deformation amount 70%, strain rate 0.1 s -1 , water quenching after deformation;

[0064] S2 deep cooling: soaking in liquid nitrogen at -196℃ for 8 h;

[0065] S3 critical annealing: 600℃ for 4 h, furnace cooling.

[0066] Performance test results:

[0067] Mechanical properties: yield strength = 848 MPa, tensile strength = 986 MPa, elongation = 37%, product of strength and ductility = 36 GPa-%;

[0068] PLC effect inhibition: tensile curve without sawtooth fluctuation, uniform elongation increased by more than 20% compared with untreated steel.

[0069] Test Example 1: Test of part of finished product during treatment of low-carbon medium-manganese steel in Example 1

[0070] In Example 1, the steel used was smelted in a vacuum furnace, and the ingot was free-forged into a 240 x 120 mm x 40 mm billet.

[0071] 1) After warm deformation, the medium-manganese steel was subjected to double-pass hot compression at different temperatures using a Gleeble-3500 thermal-mechanical simulation testing machine, and the sample was as shown in Figure 3 ; the double-pass compression deformation process was as shown in Figure 4 , the deformation temperature was 600, 700, and 800°C, the strain rate was 0.1 s-1, and the inter-pass dwell time was 100 s. The deformation amount of the two passes was both 35%, and the total compression deformation amount was 70%, after which the sample was immediately water-quenched to preserve the high-temperature deformation structure.

[0072] Figure 1 In the figure, PLC bands with different critical sawtooth strains (i.e., sawtooth initiation strain, green dot representing the position of critical sawtooth strain) and density are shown.

[0073] Figure 3 In the figure, the compression sample geometry (unit: mm) is as follows: (a) sample before compression; (b) sample after compression; (c) tensile sample.

[0074] 2) Cryogenic treatment

[0075] The deformed sample was immediately placed in liquid nitrogen and soaked for 10 min, 30 min, 1 h, 8 h, and 16 h before being transferred to room temperature.

[0076] 3) Critical annealing process

[0077] The cryogenically treated sample was vacuum-sealed and then annealed at 600-700°C for 1-8 h.

[0078] Comparative Example 1: Process with deviated critical annealing temperature (660°C)

[0079] Raw material: same as Example 1.

[0080] Process scheme:

[0081] S1 warm rolling, S2 cryogenic treatment parameters same as Example 1;

[0082] S3 critical annealing: 660℃ for 4h (out of the range of 580-600℃ in the present application), furnace cooling.

[0083] Performance test results:

[0084] Mechanical properties: yield strength = 686 MPa, tensile strength = 1285 MPa, elongation = 19.45%, strength-plasticity product = 25 GPa·%.

[0085] PLC effect suppression: obvious sawtooth fluctuation in tensile curve, lower uniform elongation.

[0086] Comparative Example 2: Process with deviated critical annealing temperature (630℃)

[0087] Raw materials: same as Example 1.

[0088] Process scheme:

[0089] S1 warm rolling, S2 deep cooling parameters same as Example 1;

[0090] S3 critical annealing: 630℃ for 4h (out of the range of 580-600℃ in the present application), furnace cooling.

[0091] Performance test results:

[0092] Mechanical properties: yield strength = 832 MPa, tensile strength = 1200 MPa, elongation = 32%, strength-plasticity product = 39 GPa·%.

[0093] PLC effect suppression: obvious sawtooth fluctuation in tensile curve, higher uniform elongation but still with plastic instability.

[0094] From Example 1, Test Example 1, Comparative Example 1 and Comparative Example 2, it can be seen that:

[0095] 1) Example 1 with 600℃ critical annealing (within the range defined in the present application) completely eliminates PLC sawtooth fluctuation while ensuring high strength (yield strength 848 MPa) and high elongation (37%), with uniform elongation increased by more than 20%, proving the suppression effect of the process on plastic instability and the comprehensive performance advantage.

[0096] 2) Comparative Example 1 (annealing at 660℃) has a too high temperature, leading to austenite grain coarsening and disappearance of Mn gradient distribution, although the tensile strength is improved, the elongation is sharply reduced (19.45%), and the PLC effect is significant, indicating that annealing temperature exceeding the critical range will destroy the organizational regulation effect.

[0097] 3) Comparative Example 2 (annealed at 630℃) has a slightly higher product of strength and ductility (39 GPa·%) than Example 1, but still has a clear sawtooth fluctuation, proving that even if the temperature is close to the critical zone (not falling into 580-600℃), the PLC effect cannot be effectively suppressed, further verifying the necessity of the annealing temperature range of the present application.

[0098] In summary, the "600℃ critical annealing" defined in the present application is the optimal parameter for balancing mechanical properties and PLC effect suppression, and the stable production of low-carbon medium-manganese steel can be realized through synergistic warm rolling and cryogenic treatment.

[0099] Although the present application has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions recorded in the foregoing embodiments, or make equivalent replacements for part of the technical features, and any modification, equivalent replacement, improvement, etc. made within the spirit and principles of the present application shall be included in the protection scope of the present application.

Claims

1. A synergistic process of warm rolling and deep cryogenic critical annealing for suppressing plastic instability in low-carbon medium-manganese steel PLC belts, characterized in that, Includes the following steps: S1) Warm rolling deformation The low-carbon medium-manganese steel billet is heated to the warm rolling temperature range of 600-800℃, and then subjected to a two-pass compression deformation operation. S2) Cryogenic treatment The low-carbon medium-manganese steel workpieces that have undergone warm rolling deformation and water quenching are immersed in a liquid nitrogen environment at a temperature of -196℃. S3) Critical Annealing The workpiece after cryogenic treatment is subjected to critical annealing. It is placed in a heating furnace and heated to the critical annealing temperature range of 580-600℃. It is held at this temperature for 1-8 hours and then cooled to room temperature with the furnace. Finally, a composite structure with ultrafine austenite grains and Mn element gradient distribution is formed.

2. The warm rolling and deep cryogenic critical annealing synergistic process for suppressing plastic instability in low-carbon medium-manganese steel PLC belts according to claim 1, characterized in that, In step S1, the deformation amount for each pass is controlled at 35%, and the total deformation amount is controlled at 70%.

3. The warm rolling and deep cryogenic critical annealing synergistic process for suppressing plastic instability in low-carbon medium-manganese steel PLC belts according to claim 1, characterized in that, In step S1, during the double-pass compression deformation operation, the strain rate for each pass is set to 0.1 s⁻¹.

4. The warm rolling and deep cryogenic critical annealing synergistic process for suppressing plastic instability in low-carbon medium-manganese steel PLC belts according to claim 1, characterized in that, In step S1, the billet needs to remain for 100 seconds between two passes.

5. The warm rolling and deep cryogenic critical annealing synergistic process for suppressing plastic instability in low-carbon medium-manganese steel PLC belts according to claim 1, characterized in that, In step S1, the billet is immediately subjected to water quenching after deformation.

6. The warm rolling and deep cryogenic critical annealing synergistic process for suppressing plastic instability in low-carbon medium-manganese steel PLC belts according to claim 1, characterized in that, In step S2, the soaking time is controlled within the range of 10 min to 16 h to promote the transformation of the internal microstructure of the workpiece and the precipitation of Mn-rich carbides.

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