A reinforced and toughened ferritic-martensitic dual phase hot-rolled steel sheet and a method of manufacturing the same

Abstract

The application discloses a kind of enhanced toughening ferrite martensite dual-phase hot-rolled steel plate and its manufacturing method, and its component mass percentage is as follows: C: 0.10~0.25%, Si: 0.6~2.0%, Mn: 0.8~3.0%, Al: 0.3~1.0%, Nb: 0.025~0.1%, V: 0.025~0.1%, P≤0.01%, S≤0.005%, the balance includes Fe and other inevitable impurities;Its microstructure is not more than 5 microns of ultrafine equiaxed ferrite and martensite dual-phase structure.The application obtains less than 5 microns of ultrafine equiaxed ferrite and martensite dual-phase structure by refining ferrite and metastable austenite structure, realizes that dual-phase steel is promoted to ultra-high strength level, and keeps excellent plasticity and toughness, and its yield strength is ≥1800MPa, tensile strength 2000~2500MPa, and fracture elongation 4.0~12.0%.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of high-strength metal material processing and manufacturing, and in particular to a reinforced and toughened ferrite-martensite dual-phase hot-rolled steel plate and a manufacturing method thereof. BACKGROUND

[0002] Ferrite-martensite dual-phase DP steel is generally obtained by heat treatment or controlled rolling of low-carbon steel or low-alloy high-strength steel in the critical zone, and can achieve a good combination of high strength and high ductility. The typical dual-phase steel has a yield and tensile strength of less than 1000 MPa. The dual-phase steel has good comprehensive performance of strength and toughness, and also has good wear resistance, fatigue resistance, and good hole expansion and forming performance, and can be used to manufacture cold punching and deep drawing complex components, and can also be used as pipeline steel, chain, cold-drawn steel wire, prestressed steel bar, etc., and is widely used in the fields of automobile industry, rail transportation, engineering machinery, aerospace and military industry.

[0003] The improvement of the strength of metal alloy materials is generally accompanied by the decrease of plasticity and toughness, that is, the so-called strength and toughness "inversion" problem. Grain refinement is one of the effective methods to improve the strength and toughness of materials, and can also improve the processing and forming performance of materials. Therefore, how to obtain ultra-fine grain or even nano-crystal structure on the basis of the existing technology is an important research direction to improve the comprehensive mechanical properties of materials. Severe plastic deformation (SPD) is a new plastic deformation method, which can introduce a large strain during deformation, thereby effectively refining the grain, and obtaining a complete large-size bulk sample. Through the control of microstructure during deformation, a bulk ultra-fine grain / nano-crystal material with high strength and large plasticity can be obtained. The most representative processes include high pressure and torsion (HPT), equal channel angular pressing (ECAP), and accumulative roll bonding (ARB). The phase transformation control method combined with rolling and cooling process can effectively refine the organizational state of steel and iron, and then improve the strength and toughness of the material. This method is simple in operation, and can realize large-size sample and large-scale production. For example:

[0004] Chinese patent CN108018493A prepares a 1180MPa grade hot-rolled dual-phase steel suitable for industrial production by the method of hot rolling + segmented cooling + coiling.

[0005] Chinese patent CN107058869A produces a kind of ultra-low yield ratio 980MPa grade cold rolled dual phase steel suitable for industrial production by cold rolling+continuous annealing process.And using deformation induced phase transition technology can realize the ultra-fining of organization and the substantial promotion of comprehensive mechanical properties, but its preparation process conditions such as deformation rate, cooling rate are strictly required, it is difficult to realize large-scale mass production.

[0006] Chinese patent CN108018503A utilizes the organizational inheritance phenomenon of martensite during heating in two-phase region, realizes grain ultra-fining by large deformation rolling after martensite two-phase region heat preservation, and prepares a kind of lamellar ultra-fine grain dual phase ferrite-martensite steel.

[0007] Chinese patent CN107338393A discloses a production method of high-strength steel plate, the carbon mass content of the steel plate is 0.22%-0.30%, it is a kind of low-carbon low-alloy high-strength steel with martensite structure as the main part, the patent produces the steel plate with yield strength greater than 1400MPa by rolling combined with low-temperature tempering method.

[0008] Chinese patent CN105463329A introduces a production method of ferrite-based ultra-high strength steel, the method prepares a kind of ferrite high-strength steel with yield strength ≥900MPa, tensile strength ≥980MPa and elongation ≥15% by controlling the component content and proportion of steel and preparing a kind of ferrite high-strength steel by hot rolling.The method combines heat treatment to control organization by controlled rolling and controlled cooling, obtains refined ferrite and metastable austenite organization, and then obtains ultra-fine ferrite martensite organization, and improves the strength and plasticity and toughness of the material.Ferrite martensite dual phase steel can obtain good plasticity and toughness due to the existence of relatively soft phase ferrite, but it is difficult to obtain ultra-high strength, especially low-carbon low-alloy steel, it is difficult to make the yield strength increase to more than 1000MPa and the tensile strength more than 1380MPa by conventional rolling and heat treatment method.Therefore, it is of important engineering application significance to develop new process method and improve the strength-plasticity product of dual phase steel material. SUMMARY

[0009] The purpose of the present application is to provide a kind of enhanced toughening ferrite martensite dual phase hot-rolled steel plate and its manufacturing method, by refined ferrite and metastable austenite organization, and then obtain ultra-fine ferrite martensite organization, i.e. ultra-fine equiaxed ferrite and martensite dual phase organization less than 5 microns, realize the improvement of dual phase steel to ultra-high strength level and the maintenance of excellent plasticity and toughness, its yield strength is ≥1800MPa, tensile strength 2000-2500MPa, and fracture elongation 4.0-12.0%.

[0010] To achieve the above purpose, the technical scheme of the present application is:

[0011] The application discloses a kind of enhanced toughening ferrite martensite dual-phase hot-rolled steel plate, and the component mass percentage is as follows: C: 0.10~0.25%, Si: 0.6~2.0%, Mn: 0.8~3.0%, Al: 0.3~1.0%, Nb: 0.025~0.1%, V: 0.025~0.1%, P≤0.01%, S≤0.005%, the balance contains Fe and other inevitable impurities; its microstructure is not more than 5 microns of ultra-fine equiaxed ferrite and martensite dual-phase structure.

[0012] Preferably, the component mass percentage of the dual-phase hot-rolled steel plate is as follows: C: 0.15~0.20%, Si: 0.8~1.2%, Mn: 1.0~2.0%, and the total content of other alloy contents except Fe element is not more than 4.5%.

[0013] Preferably, the microstructure of the dual-phase hot-rolled steel plate contains 3.0~8.0% of residual austenite.

[0014] Preferably, the grain size of the microstructure of the dual-phase hot-rolled steel plate is less than 1 micron, and the residual austenite content is 5.0~8.0%.

[0015] The yield strength of the dual-phase hot-rolled steel plate is greater than or equal to 1800 MPa, the tensile strength is 2000~2500 MPa, and the fracture elongation is 4.0~12.0%.

[0016] In the component design of the dual-phase hot-rolled steel plate,

[0017] The dual-phase steel is a low-carbon low-alloy common carbon manganese steel, the carbon content is controlled to be 0.10~0.25%wt., the alloy content is less than 5.0%wt., and only Si and Mn conventional inexpensive alloy elements are contained in the component, and trace Nb and V elements are added. The low-carbon low-alloy steel has good ferrite and martensite phase dual-phase steel organization forming capacity, and through the cooperation of Si and Mn, the stability of low-temperature austenite can be effectively controlled, and then in the low-temperature cooling and coiling process, the stability of residual austenite can be controlled through carbon partition, and the plasticity and toughness of the material are improved.

[0018] However, the conventional controlled rolling and controlled cooling process is difficult to improve the strength to the super-high strength level, the plasticity and toughness of the material will be significantly reduced through the usual rolling work hardening means, and a high strength and plasticity product cannot be obtained. According to the component characteristics of the alloy, the integrated process means of hot rolling and subsequent segmented cooling+medium-temperature rolling+short-time heat treatment+coiling partition is used in the application, the dual-phase steel organization is refined, and the residual austenite is controlled to achieve the purpose of strengthening and toughening.

[0019] This method uses hot rolling to initially refine the original austenite structure. Then, segmented warm deformation rolling is used to refine the ferrite-martensite grains, adjusting the ratio, morphology, and distribution of the soft and hard phases in the ferrite-martensite dual-phase steel while introducing a large number of dislocation defects. Rapid heating to the dual-phase temperature and a short-term heat treatment further refine the grains through phase transformation. This achieves a strong and tough combination between the soft ferrite phase and the martensite phase, and the ultra-fine grains significantly improve the overall mechanical properties of the dual-phase steel.

[0020] The method for manufacturing the reinforced and toughened ferrite-martensite dual-phase hot-rolled steel plate of the present invention comprises the following steps:

[0021] 1) Smelting and casting

[0022] Smelting, refining and casting into ingots according to the above ingredients;

[0023] 2) Homogenization + blanking

[0024] The ingot is heated to 1150-1280°C, kept at this temperature for 1.0-2.0 hours, and then hot forged or hot rolled to form an intermediate billet, the billet temperature being not less than 1100°C, and the final rolling or final forging temperature being not less than 950°C, to form an intermediate billet;

[0025] 3)Austenitizing + hot rolling

[0026] The intermediate billet is heated to 1100-1200°C, kept at this temperature for 1.0-2.0 hours, and hot rolled, with the starting rolling temperature at 1000-1050°C and the final rolling temperature at not less than 900°C, to obtain a hot rolled plate with a final thickness of 10-30 mm;

[0027] 4) Segmented cooling + segmented warm rolling

[0028] The hot rolled plate is cooled at 750-800°C at ≥50°C / s for 2-3 passes, and then cooled at 600-650°C at ≥60°C / s for 1-2 passes with a total deformation of ≥20% of the deformation amount, to obtain a steel plate with a final thickness of 1.5-10 mm;

[0029] 5) Heating heat treatment + rapid cooling + coiling

[0030] Heat the steel plate to 850-950℃ and keep it warm for 5-15 minutes. Immediately water-cool the steel plate to 200-300℃ at a cooling rate of ≥50℃ / s. Then coil it. After coiling, cool it to below 180℃ at a rate of ≤20℃ / s. Then air-cool it to room temperature.

[0031] Steps 3), 4) and 5) are completed by continuous integration of the rolling and heat treatment production lines.

[0032] Preferably, in step 2), the heating temperature of the intermediate blank is 1150-1200℃, the holding time is 1.5-2.0 hours, the hot rolling starting temperature is 1100-1150℃, the final rolling or forging temperature is not lower than 950℃, and the thickness of the intermediate blank is not lower than 60mm.

[0033] Preferably, in step 3), austenitizing + hot rolling

[0034] The heating temperature of the intermediate blank is 1100-1150℃, the holding time is 1.5-2.0 hours, the hot rolling starting temperature is 1020-1050℃, the final rolling temperature is 900-930℃, and the final rolling thickness is 20-25mm.

[0035] Preferably, in step 4), the warm rolling is performed by means of stepwise cooling + stepwise warm rolling

[0036] The steel plate is cooled at 50℃ / s to 800℃ and then continuously rolled for 3 passes, and then cooled at 60℃ / s to 600-630℃ and then warm rolled for a deformation amount of more than 20%;

[0037] Preferably, in step 5), the heating treatment is performed by means of induction heating.

[0038] The steel plate is heated to 900-930℃ and held for 5.0-15.0min, and then immediately water cooled to 250-300℃ at a cooling rate of 50-80℃ / s, and then coiled, and then cooled to below 180℃ at a cooling rate of 10-20℃ / s, and then air cooled to room temperature.

[0039] Preferably, in step 5), the heating treatment is performed by means of induction heating.

[0040] Preferably, in step 4), the warm deformation rolling is performed by means of asynchronous rolling, and the asynchronous ratio is 1.2-2.0; the asynchronous rolling includes any one of the following modes: the working rolls have different diameters but the same rotating speed, the working rolls have the same diameter but different rotating speeds, or the working rolls have different diameters and different rotating speeds. Compared with synchronous rolling, the asynchronous rolling mode can introduce shear strain, and the greater the asynchronous ratio, the greater the introduced shear strain. It is found that the introduction of shear strain can effectively increase the total equivalent strain of the material, and thus under the same reduction amount, the microstructure of the prepared material is finer.

[0041] In the method for manufacturing the dual-phase hot-rolled steel plate according to the present application:

[0042] The present invention is completed by continuous integration of rolling and heat treatment production lines, that is, austenitization + hot rolling, segmented cooling + segmented warm rolling are continuously integrated in the production line, which can achieve that part of the deformed metastable austenite is maintained and does not undergo phase change, and directly enters the heating heat treatment + rapid cooling + coiling step, and continuously undergoes direct and rapid high-temperature recovery of metastable austenite. If it is not integrated, it will lead to further martensitic phase transformation or ferrite phase transformation. Then, in the subsequent heating treatment process, it is not the recovery and recrystallization of metastable austenite, but the phase transformation is carried out again. On the one hand, it causes an increase in the transformation kinetics process, and on the other hand, it causes the consistency of material stability and mechanical properties to deteriorate. Rapid heating of metastable austenite can achieve full refinement of the organization and significantly improve the comprehensive performance of strength and toughness of the material. Therefore, continuous integration is the core method to improve the strength and toughness of the material of the present invention.

[0043] In the method of the present invention, the combination of medium-temperature (large) deformation and heat treatment can significantly refine the microstructure of austenite, and obtain fine duplex ferrite and martensite structures through phase transformation regulation, thereby improving the strength of duplex steel, providing a new approach for the preparation and development of ultra-high-strength and high-toughness duplex steel.

[0044] Since the dual-phase steel described in the present invention is a low-carbon, low-alloy ordinary carbon-manganese steel, the carbon content is controlled at 0.10-0.25wt.%, the alloy content is lower than 5.0wt.%, and the composition only contains conventional and cheap alloying elements Si and Mn, and trace amounts of Nb and V elements are added. This type of low-carbon, low-alloy steel has a good ability to form a dual-phase steel structure of ferrite and martensite phases, and through the combination of Si and Mn, it is possible to effectively control the stability of low-temperature austenite, and then during low-temperature cooling and coiling, the stability of residual austenite can be controlled by carbon partitioning, thereby improving the plastic toughness of the material. However, due to the alloy system adopted in the present invention, it is difficult for conventional controlled rolling and controlled cooling processes to actually increase the strength to an ultra-high strength level. Through the usual means of rolling work hardening, the plastic toughness of the material will be significantly reduced, and a higher strength-to-plasticity product cannot be obtained.

[0045] In view of the composition characteristics of this type of alloy, the present invention achieves the purpose of strengthening and toughening at the same time by refining the dual-phase steel structure and controlling the retained austenite through an integrated process of hot rolling and subsequent segmented cooling + medium-temperature rolling + short-time heat treatment + coiling and partitioning.

[0046] This method uses hot rolling to initially refine the original austenite structure. Then, segmented warm deformation rolling is used to refine the ferrite-martensite grains, adjusting the ratio, morphology, and distribution of the soft and hard phases in the ferrite-martensite dual-phase steel while introducing a large number of dislocation defects. Rapid heating to the dual-phase temperature and a short-term heat treatment further refine the grains through phase transformation. This achieves a strong and tough combination between the soft ferrite phase and the martensite phase, and the ultra-fine grains significantly improve the overall mechanical properties of the dual-phase steel.

[0047] In addition, the process of adopting sectional cooling and warm rolling + short-time heat treatment + coiling after hot rolling is adopted.

[0048] On the one hand, compared with high-temperature deformation rolling, medium-temperature rolling can significantly reduce energy consumption, save cost, and more easily introduce high-density defects in steel, including dislocations, dislocation cells, subgrain boundaries and dynamic recovery recrystallized ultra-fine high-angle grain boundaries. Compared with low-temperature and cold rolling, the deformation resistance can be effectively reduced, the material is easy to deform, the steel deformation efficiency is improved, the rolling effect is ensured, and the damage to the equipment is also smaller compared with low-temperature large deformation rolling.

[0049] On the other hand, the present application can effectively refine ferrite, pro-eutectoid austenite grains and transformed martensite structure by using sectional cooling and sectional medium-temperature deformation rolling, mainly for three reasons:

[0050] Firstly, the regulation of ferrite and austenite dual-phase structure can be realized, the transformation of ferrite is regulated in the process of sectional cooling and rolling, and more different states of fine ferrite can be induced in different stages of strain, the ferrite will present gradient distribution and control of the structure, and at the same time, high-density dislocation state will be reserved inside, providing high-density nucleation sites for subsequent rapid heating and short-time heat treatment. When the rolling deformation is carried out in the undercooled austenite region, the austenite grains are difficult to undergo dynamic recovery and dynamic recrystallization, so the phenomenon of grain growth caused by recovery and recrystallization is avoided;

[0051] Secondly, since the deformation of austenite is carried out at a lower temperature, the temperature reduction can effectively slow down the growth of austenite grains;

[0052] Finally, since the dynamic recovery process of austenite is inhibited, a large number of dislocations, deformation bands and dislocation cells are formed in the austenite grains, providing a large number of nucleation sites for martensite transformation, thereby refining the martensite structure after phase transformation; the ultra-fining of the structure of the martensite steel is realized, and the strength and plasticity and toughness of the material are improved.

[0053] The rapid short-time heat treatment after sectional warm rolling and the subsequent phase transformation regulation can further realize the grain refinement. Further recovery and recrystallization occur in the rapid heating and heat treatment process, the structure is further refined, and then relative to the original structure material, the synergistic effect of strengthening and toughening is achieved.

[0054] In the process of warm deformation, the stable austenite in the dual-phase structure is broken and flattened under the action of mechanical external stress, the equiaxed grains in the original austenite structure are broken and flattened, a strip-shaped structure is formed, a large number of dislocations and large-angle grain boundaries and other defects are introduced, a large number of nucleation sites are provided for austenite grains in the subsequent rapid heating and short-time heat treatment process, and the nucleation rate is improved; after the large deformation rolling is completed, the strip-shaped structure after rolling is subjected to austenite phase transformation through short-time heat treatment, the austenite grain growth is controlled by controlling the heating temperature and the holding time, and thus fine martensite structure is obtained after quenching. Finally, the content and form of residual austenite can be adjusted through low-temperature tempering and coiling distribution, the dual-phase steel can further introduce the transformation induced plasticity (TRIP) effect, and through the treatment, internal stress can be further eliminated, and finally the super-high strength and high toughness alloy steel material with excellent comprehensive mechanical properties is obtained.

[0055] Compared with the prior art, the application has the following advantages:

[0056] 1. The dual-phase steel reinforcing and toughening method can make the hot-rolled steel obtain excellent comprehensive mechanical properties, that is, the product of strength and plasticity is significantly improved. The fine-grained and ultra-fine-grained ferrite and martensite structure combined with a certain amount of residual austenite makes the alloy steel have super-high strength and excellent plasticity and toughness.

[0057] 2. The application is aimed at low-carbon and low-alloy plain carbon steel which is cheap. The low-cost alloy can obtain high strength and high toughness after being treated by the technical means, can replace some medium and high steel service environment, especially some load-bearing structure applications, and significantly reduces the cost of materials. In addition, the application adopts an integrated manufacturing technology, improves the production efficiency of products, saves energy and reduces production cost.

[0058] 3. The production process of the application is simple, compared with other SPD methods, the large deformation rolling does not need to design special molds, has low requirements for the size of the material, and adopts conventional industrial production equipment, which is beneficial to industrial production; compared with general rolling process, the application obtains fine-grained or ultra-fine-grained dual-phase structure and uniform equiaxed grain structure, and to some extent, overcomes the mechanical anisotropy caused by the rolling process, and the use range of the material is more extensive.

[0059] 4. The application is aimed at conventional ferrite and martensite dual-phase hot-rolled steel, adopts an integrated continuous preparation process of hot rolling, segmented medium-temperature rolling, controlled cooling, subsequent short-time heat treatment and coiling distribution, controls the metastable austenite structure in the process of hot rolling and further warm deformation, obtains refined ferrite and metastable austenite structure, and then obtains ultra-fine ferrite and martensite structure. The method can significantly refine the structure of the dual-phase steel, and further improve the dual-phase steel to the super-high strength level and maintain excellent plasticity and toughness. BRIEF DESCRIPTION OF DRAWINGS

[0060] Figure 1 Process route map of the manufacturing method of the reinforced and toughened ferrite and martensite dual-phase hot-rolled steel plate according to the present application.

[0061] Figure 2 SEM ultra-fine structure map of the reinforced and toughened ferrite and martensite dual-phase hot-rolled steel plate according to the present application. DETAILED DESCRIPTION

[0062] The present application will be further described below in conjunction with examples and drawings.

[0063] It should be noted that the manufacturing process and method and alloy composition according to the present application can float within a certain range, and the mechanism of each component and process method is basically the same within the optional range. Except for carbon, the floating of each component and process has no significant effect on the microstructure and mechanical properties of the alloy steel. For those skilled in the art, without departing from the concept of the present application, some improvements and adjustments can be made, which are within the protection scope of the present application.

[0064] Example 1

[0065] The chemical composition of the reinforced and toughened ferrite and martensite dual-phase hot-rolled steel plate is as follows: C: 0.20%, Si: 1.2%, Mn: 3.0%, Al: 0.3%, Nb: 0.1%, V: 0.1%, P: 0.01%, S: 0.005%, and the balance containing Fe and inevitable impurities; and the manufacturing method is as follows:

[0066] 1) Smelting and casting

[0067] After the ingredients are prepared according to the above composition, smelting, refining and casting are performed to form a casting blank;

[0068] 2) Homogenization treatment + breakdown

[0069] The casting blank is heated to 1280℃ for 1.0 hours, and then hot forging or hot rolling breakdown is performed at a breakdown temperature of 1180℃ and a final forging or rolling temperature of 950℃ to form an intermediate blank with a thickness of 240mm;

[0070] 3) Austenitizing + hot rolling

[0071] The intermediate blank is reheated to 1100℃ for 2.0 hours, and then hot rolling is performed at a starting rolling temperature of 1050℃ and a final rolling temperature of 900℃ to obtain a hot-rolled plate with a final rolling thickness of 20mm;

[0072] 4) Step cooling + step warm rolling

[0073] The hot-rolled steel plate is cooled at 50°C / s to 750°C, 3 passes of continuous rolling are performed, then cooled at 60°C / s to 600°C, 1-2 passes of warm rolling with deformation of >20% are performed, and a plate with a final rolling thickness of 5.0 mm is obtained. The warm deformation rolling adopts an asynchronous rolling mode, and an asynchronous rolling ratio of 1.2 is adopted. The asynchronous rolling mode is that the roll diameters are different but the rotating speeds are the same;

[0074] 5) rapid heating short-time heat treatment + rapid cooling + coiling

[0075] After the steel plate is reheated to 850°C and short-time holding for 15 min, the heating mode of the rapid heating short-time treatment is induction heating, and the steel plate is immediately water-cooled to 250°C at a cooling rate of 50°C / s; then coiling is performed, the steel plate is cooled to 180°C at a cooling rate of 10°C / s after coiling, and then air-cooled to room temperature.

[0076] The hot rolling, the segmented cooling + segmented warm rolling, the rapid heating short-time heat treatment + rapid cooling + coiling of the above steps are integrated on a continuous production line. See Figure 1 .

[0077] Taking a steel plate with a typical thickness specification of 2.0 mm as an example, the effective grain size of the dual-phase structure can be 1.0-5.0 microns, as shown in Figure 2 The residual austenite content after coiling is up to 8.0%, the yield strength is 2100 MPa, the tensile strength is 2500 MPa, and the uniform elongation is 4.0-10.0%.

[0078] Example 2

[0079] The chemical composition of the reinforced and toughened ferrite-martensite dual-phase hot-rolled steel plate is as follows: C: 0.25%, Si: 0.6%, Mn: 0.8%, Al: 1.0%, Nb: 0.025%, V: 0.05%, P: 0.005%, S: 0.003%, and the balance contains Fe and inevitable impurities; and the manufacturing method is as follows:

[0080] 1) smelting and casting

[0081] After the ingredients are prepared according to the above composition, smelting, refining and casting into a casting blank are performed;

[0082] 2) homogenization treatment + breakdown

[0083] The casting blank is heated to 1150°C, and the holding time is 2.0 hours, then hot forging or hot rolling is performed for breakdown, the breakdown temperature is 1300°C, the final rolling or forging temperature is 950°C, and an intermediate blank with a thickness of 180 mm is formed;

[0084] 3) austenitizing + hot rolling

[0085] The intermediate blank is reheated to 1200°C, the holding time is 1.0 hour, hot rolling is carried out, the starting rolling temperature is 1100°C, the final rolling temperature is 920°C, and a hot-rolled plate with a final rolling thickness of 25 mm is obtained;

[0086] 4) Staged cooling + staged warm rolling

[0087] After hot rolling, the steel plate is cooled at 60°C / s to 800°C, then 2-pass continuous rolling is carried out, and then the steel plate is cooled at 60°C / s to 630°C, and warm rolling with a deformation of ≥20% is carried out in 2 passes, and a plate with a final rolling thickness of 1.5 mm is obtained;

[0088] 5) Rapid heating + short-time heat treatment + rapid cooling + coiling

[0089] After the steel plate is reheated to 950°C and held for 5 minutes, the steel plate is immediately water-cooled to 200°C at a cooling rate of 80°C / s; then coiling is carried out, the steel plate is cooled to 170°C at a cooling rate of 20°C / s after coiling, and then air-cooled to room temperature; the warm deformation rolling adopts an asynchronous rolling mode, the rolling diameters are the same but the rotating speeds are different, or the rolling diameters and the rotating speeds are both different, and the asynchronous speed ratio is 1.5.

[0090] Taking a steel plate with a typical thickness specification of 1.5 mm as an example, the grain size of the dual-phase structure can be 0.8-3.0 microns, the residual austenite content after coiling is 3.0-5.0%, the yield strength is 1600-1800 MPa, the tensile strength is 2000-2450 MPa, and the fracture elongation is 5.0-10.0%.

[0091] Example 3

[0092] The chemical composition of the reinforced and toughened ferrite-martensite dual-phase hot-rolled steel plate is as follows: C: 0.10%, Si: 2.0%, Mn: 1.0%, Al: 0.7%, Nb: 0.05%, V: 0.025%, P: 0.007%, S: 0.001%, and the balance comprising Fe and unavoidable impurities; and the manufacturing method is as follows:

[0093] 1) Smelting and casting

[0094] After the ingredients are prepared according to the above composition, smelting, refining and casting are carried out to form a cast blank;

[0095] 2) Homogenization treatment + breakdown

[0096] The cast blank is heated to 1200°C, the holding time is 1.5 hours, and then hot forging or hot rolling is carried out to break down, the breakdown temperature is 1180°C, and the final rolling or forging temperature is 950°C, to form an intermediate blank with a thickness of 120 mm;

[0097] 3) Austenitizing + hot rolling

[0098] The intermediate blank is reheated to 1150°C for 1.5 hours, hot rolling is performed, the starting rolling temperature is 1050°C, the final rolling temperature is 910°C, and a hot-rolled plate with a final rolling thickness of 21 mm is obtained;

[0099] 4) Staged cooling + staged warm rolling

[0100] After hot rolling, the steel plate is cooled at 50°C / s to 780°C, 2 passes of continuous rolling are performed, then cooled at 60°C / s to 625°C, 1-2 passes of asynchronous warm rolling are performed, the asynchronous ratio is 1.3, and a plate with a final rolling thickness of 1.5 mm and 2.0 mm is obtained;

[0101] 5) Rapid heating + short-time heat treatment + rapid cooling + coiling

[0102] The steel plate is reheated to 900°C for short-time holding of 10 minutes, the heating mode is induction heating, then the steel plate is immediately water-cooled to 300°C at a cooling rate of 60°C / s; then coiling is performed, the steel plate is cooled to 175°C at a cooling rate of 15°C / s after coiling, and then air-cooled to room temperature.

[0103] Taking a steel plate with a typical thickness specification of 2.0 mm as an example, the yield strength is 1300-1500 MPa, the tensile strength is 1800-2200 MPa, and the fracture elongation is 8.0-12.0%.

[0104] Example 4

[0105] The chemical composition of the reinforced and toughened ferrite-martensite dual-phase hot-rolled steel plate is as follows: C: 0.15%, Si: 0.8%, Mn: 2.0%, Al: 0.4%, Nb: 0.07%, V: 0.06%, P: 0.006%, S: 0.004%, and the balance containing Fe and inevitable impurities; and the manufacturing method is as follows:

[0106] 1) Smelting and casting

[0107] After batching according to the above composition, smelting, refining and casting are performed to form a cast blank;

[0108] 2) Homogenization treatment + breakdown

[0109] The cast blank is heated to 1250°C for 1.6 hours, then hot forging or hot rolling is performed to form an intermediate blank with a thickness of 200 mm;

[0110] 3) Austenitizing + hot rolling

[0111] The intermediate blank is reheated to 1180°C for 1.7 hours, hot rolling is performed, the starting rolling temperature is 1025°C, the final rolling temperature is 905°C, and a hot-rolled plate with a final rolling thickness of 23 mm is obtained;

[0112] 4) Staged cooling + Staged warm rolling

[0113] The hot-rolled steel plate is cooled at 50°C / s to 760°C, then is continuously rolled for 2 passes, and then is cooled at 60°C / s to 610°C, and is warm-rolled for 1-2 passes with a deformation of 20%, to obtain a plate with a final rolling thickness of 1.5-5.0 mm;

[0114] 5) Rapid heating + Short-time heat treatment + Rapid cooling + Coiling

[0115] The steel plate is reheated to 860°C, and is short-time heat treated for 13 min, and then is immediately water-cooled to 275°C at a cooling rate of 70°C / s; then is coiled, and is cooled to 165°C at a cooling rate of 20°C / s, and then is air-cooled to room temperature.

[0116] The hot-rolling, the staged cooling + staged warm rolling, and the rapid heating + short-time heat treatment + rapid cooling + coiling of the above steps are integrated on a continuous production line.

[0117] Taking a steel plate with a typical thickness of 5.0 mm as an example, the residual austenite content after coiling is about 5.0%, the yield strength is 1500-1800 MPa, the tensile strength is 2000-2200 MPa, and the fracture elongation is 8.0-12.0%.

[0118] Example 5

[0119] The chemical composition of the reinforced and toughened ferrite and martensite dual-phase hot-rolled steel plate is as follows: C: 0.18%, Si: 1.0%, Mn: 1.5%, Al: 0.6%, Nb: 0.035%, V: 0.035%, P: 0.008%, S: 0.002%, and the balance comprising Fe and inevitable impurities; and the manufacturing method is as follows:

[0120] 1) Smelting and casting

[0121] After the ingredients are prepared according to the above composition, smelting, refining, and casting are performed to form a cast blank;

[0122] 2) Homogenization treatment + breakdown

[0123] The cast blank is heated to 1210°C for 1.4 hours, and then is hot forged or hot rolled for breakdown, the breakdown temperature is 1130°C, the final rolling or forging temperature is 950°C, and an intermediate blank with a thickness of 120 mm is formed;

[0124] 3) Austenitizing + hot rolling

[0125] The intermediate blank is reheated to 1120 °C, the holding time is 1.4 hours, hot rolling is performed, the starting rolling temperature is 1020 °C, the final rolling temperature is 930 °C, and a hot-rolled plate with a final rolling thickness of 24 mm is obtained;

[0126] 4) Staged cooling + staged warm rolling

[0127] After hot rolling, the steel plate is cooled at 80 °C / s to 755 °C, 2-3 passes of continuous rolling are performed, then cooled at 60 °C / s to 625 °C, 1-2 passes of warm rolling are performed, and a plate with a final rolling thickness of 3.0-6.0 mm is obtained.

[0128] 5) Rapid heating + short-time heat treatment + rapid cooling + coiling

[0129] The steel plate is reheated to 880 °C and held for 11 minutes, the heating is performed by induction heating, then the steel plate is immediately water-cooled to 270 °C at a cooling rate of 75 °C / s, then coiling is performed, after coiling, the steel plate is cooled to 170 °C at a cooling rate of 18 °C / s, and then air-cooled to room temperature.

[0130] Taking a steel plate with a typical thickness of 3.0 mm as an example, the yield strength is 1400-1600 MPa, the tensile strength is 1800-2100 MPa, and the elongation at break is 6.0-10.0%.

[0131] Example 6

[0132] The chemical composition of the reinforced and toughened ferrite-martensite dual-phase hot-rolled steel plate is as follows: C: 0.21%, Si: 1.3%, Mn: 1.8%, Al: 0.8%, Nb: 0.03%, V: 0.08%, P: 0.009%, S: 0.0035%, and the balance contains Fe and inevitable impurities; and the manufacturing method is as follows:

[0133] 1) Smelting and casting

[0134] After the ingredients are prepared according to the above composition, smelting, refining, and casting are performed to form a cast blank;

[0135] 2) Homogenization treatment + breakdown

[0136] The cast blank is heated to 1220 °C, the holding time is 1.2 hours, then hot forging or hot rolling is performed to break down, the breakdown temperature is 1200 °C, the final rolling or forging temperature is 950 °C, and an intermediate blank with a thickness of 60 mm is formed;

[0137] 3) Austenitizing + hot rolling

[0138] The intermediate blank is reheated to 1160 °C, the holding time is 1.3 hours, hot rolling is performed, the starting rolling temperature is 1050 °C, the final rolling temperature is 900 °C, and a hot-rolled plate with a final rolling thickness of 25 mm is obtained;

[0139] 4) Staged cooling + Staged warm rolling

[0140] The hot rolled steel plate is cooled at 70°C / s to 790°C, then is continuously rolled for 3 passes, and then is cooled at 60°C / s to 630°C, and is warm rolled for 1-2 passes with a deformation of 25%, the warm rolling is in an asynchronous rolling mode, the asynchronous ratio is 2.0, then is synchronously rolled and straightened, the synchronous rolling depression is not higher than 5%, and the final rolling thickness is 1.0 and 2.0 mm plate;

[0141] 5) Rapid heating + Short time heat treatment + Rapid cooling + Coiling

[0142] The steel plate is re-induction heated to 890°C, and is short time heat treated for 9 min, then is immediately water cooled to 260°C at a cooling rate of 65°C / s, and then is coiled, and after coiling is cooled to 180°C at a cooling rate of 15°C / s, and then is air cooled to room temperature.

[0143] Taking the final material sample plate thickness of 1.0 mm as an example, the average grain size of the dual phase structure is less than 1.0 micron, the retained austenite content after coiling is 6.0%, the yield strength is 1800-2000 MPa, the tensile strength is 2200-2400 MPa, and the fracture elongation is 6.0-12.0%.

[0144] Example 7

[0145] The chemical composition of the reinforced and toughened ferrite and martensite dual phase hot rolled steel plate is: C: 0.12%, Si: 1.0%, Mn: 2.5%, Al: 0.55%, Nb: 0.06%, V: 0.09%, P: 0.002%, S: 0.0015%, and the balance contains Fe and inevitable impurities; and the manufacturing method is as follows:

[0146] 1) Smelting and casting

[0147] After the ingredients are prepared according to the above composition, smelting, refining and casting are performed to form a cast blank;

[0148] 2) Homogenization treatment + breakdown

[0149] The cast blank is heated to 1170°C for 1.3 hours, and then is hot forged or hot rolled to form an intermediate blank with a thickness of 100 mm;

[0150] 3) Austenitizing + Hot rolling

[0151] The intermediate blank is re-heated to 1160°C for 1.3 hours, and is hot rolled, the rolling temperature is 1050°C, and the final rolling temperature is 930°C, and a hot rolled plate with a final rolling thickness of 20.0 mm is obtained;

[0152] 4) Staged cooling + Staged warm rolling

[0153] The hot rolled steel plate is cooled at 50°C / s to 785°C, then is continuously rolled for 2-3 passes, and then is cooled at 60°C / s to 620°C for 2 passes of 30% deformation, to obtain a plate with a final rolling thickness of 8.0-10.0 mm;

[0154] 5) Rapid heating + Short time heat treatment + Rapid cooling + Coiling

[0155] The steel plate is re-induction heated to 910°C, then is short-time heat treated for 6 min, immediately cooled to 250°C at a cooling rate of 55°C / s, and then is coiled, cooled to 170°C at 16°C / s, and then air cooled to room temperature.

[0156] The hot rolling, the staged cooling + staged warm rolling, and the rapid heating + short time heat treatment + rapid cooling + coiling of the above steps are integrated on a continuous production line.

[0157] The plate obtained in this embodiment has a yield strength of 1500-1700 MPa, a tensile strength of 1800-1900 MPa, and an elongation at break of 10.0-12.0%.

[0158] Example 8

[0159] The chemical composition of the reinforced and toughened ferrite-martensite dual-phase hot-rolled steel plate is as follows: C: 0.22%, Si: 1.25%, Mn: 1.7%, Al: 0.85%, Nb: 0.08%, V: 0.055%, P: 0.0055%, S: 0.0025%, and the balance comprising Fe and unavoidable impurities; and the manufacturing method is as follows:

[0160] 1) Smelting and casting

[0161] After the ingredients are prepared according to the above composition, smelting, refining, and casting are performed to form a cast blank;

[0162] 2) Homogenization treatment + breakdown

[0163] The cast blank is heated to 1180°C for 1.9 hours, and then is hot forged or hot rolled to form an intermediate blank with a thickness of 100 mm;

[0164] 3) Austenitizing + hot rolling

[0165] The intermediate blank is reheated to 1185°C for 1.6 hours, and then is hot rolled at a breakdown temperature of 1030°C and a final rolling temperature of 920°C to obtain a hot-rolled plate with a final rolling thickness of 21.0 mm;

[0166] 4) Staged cooling + Staged warm rolling

[0167] The hot rolled steel plate is cooled at 50°C / s to 765°C, then is continuously rolled for 2-3 passes, and then is cooled at 60°C / s to 630°C for 2 passes of warm rolling with 30% deformation per pass to obtain a plate with a final rolling thickness of 1.5-3.0 mm;

[0168] 5) Rapid heating + Short time heat treatment + Rapid cooling + Coiling

[0169] The steel plate is re-induction heated to 870°C, then is short-time heat treated for 12 min, immediately cooled to 260°C at a cooling rate of 60°C / s, and then is coiled, cooled to 180°C at a cooling rate of 18°C / s, and then is air cooled to room temperature.

[0170] The hot rolling, the staged cooling + staged warm rolling, and the rapid heating + short time heat treatment + rapid cooling + coiling of the above steps are integrated on a continuous production line.

[0171] The plate obtained in this embodiment has a yield strength of 1700-1900 MPa, a tensile strength of 2000-2300 MPa, and a fracture elongation of 6.0-10.0%.

[0172] Example 9

[0173] The chemical composition of the reinforced and toughened ferrite-martensite dual-phase hot-rolled steel plate is as follows: C: 0.17%, Si: 1.4%, Mn: 1.2%, Al: 0.65%, Nb: 0.02%, V: 0.07%, P: 0.004%, S: 0.002%, and the balance comprising Fe and inevitable impurities; and the manufacturing method is as follows:

[0174] 1) Smelting and casting

[0175] After the ingredients are prepared according to the above composition, smelting, refining, and casting are performed to form a cast blank;

[0176] 2) Homogenization treatment + breakdown

[0177] The cast blank is heated to 1270°C for 1.8 hours, and then is hot forged or hot rolled to form an intermediate blank with a thickness of 100 mm;

[0178] 3) Austenitizing + hot rolling

[0179] The intermediate blank is reheated to 1175°C for 1.8 hours, and then is hot rolled at a breakdown temperature of 1030°C and a final rolling temperature of 920°C to obtain a hot-rolled plate with a final rolling thickness of 23.0 mm;

[0180] 4) Segmented cooling + segmented warm rolling

[0181] After hot rolling, the steel plate is cooled to 755°C at 50°C / s for 2-3 passes, and then cooled to 625°C at 60°C / s for 2 passes with a deformation of 30% in each pass to obtain a plate with a final thickness of 1.5-3.0 mm.

[0182] 5) Rapid heating, short-time heat treatment + rapid cooling + coiling

[0183] The steel plate was re-induction heated to 930°C and then kept warm for 8 minutes. The steel plate was immediately water-cooled to 270°C at a cooling rate of 70°C / s. The steel plate was then coiled and cooled to 175°C at a rate of 17°C / s after coiling, and then air-cooled to room temperature.

[0184] The hot rolling, segmented cooling + segmented warm rolling, rapid heating short-time heat treatment + rapid cooling + coiling in the above steps are completed in an integrated manner on a continuous production line.

[0185] The plate obtained in this embodiment has a yield strength of 1500-1700 MPa, a tensile strength of 1900-2200 MPa, and an elongation at break of 8.0-12.0%.

[0186] Example 10

[0187] The chemical composition of the reinforced and toughened ferrite-martensite dual-phase hot-rolled steel plate is as follows by weight: C: 0.16%, Si: 1.55%, Mn: 2.6%, Al: 0.50%, Nb: 0.055%, V: 0.065%, P: 0.085%, S: 0.0045%, and the balance includes Fe and unavoidable impurities. The manufacturing method thereof is as follows:

[0188] 1) Smelting and casting

[0189] After the ingredients are prepared according to the above ingredients, they are smelted, refined and cast into ingots;

[0190] 2) Homogenization + blanking

[0191] The ingot is heated to 1230°C and held at this temperature for 1.7 hours, and then hot forged or hot rolled at a temperature of 1200°C. The final rolling or forging temperature is 950°C to form an intermediate billet with a thickness of 100 mm.

[0192] 3)Austenitizing + hot rolling

[0193] The intermediate billet was reheated to 1170°C and held at this temperature for 1.9 hours, and then hot rolled with the starting rolling temperature at 1040°C and the final rolling temperature at 910°C to obtain a hot rolled plate with a final thickness of 22.0 mm.

[0194] 4) Staged cooling + Staged warm rolling

[0195] After hot rolling, the steel plate is cooled at 50°C / s to 795°C, then is continuously rolled for 2-3 passes, and then is cooled at 60°C / s to 630°C, then is warm rolled for 2 passes with 30% deformation per pass, to obtain a plate with a final rolling thickness of 1.5-3.0 mm.

[0196] 5) Rapid heating + short-time heat treatment + rapid cooling + coiling

[0197] After the steel plate is re-induction heated to 885°C and short-time heat treated for 7 min, the steel plate is immediately water cooled to 240°C at a cooling rate of 75°C / s, and then is coiled, and after coiling, is cooled to 170°C at a cooling rate of 19°C / s, and then is air cooled to room temperature.

[0198] The hot rolling, the staged cooling + staged warm rolling, and the rapid heating + short-time heat treatment + rapid cooling + coiling of the above steps are integrated on a continuous production line.

[0199] The plate obtained in the embodiment has a yield strength of 1400-1700 MPa, a tensile strength of 1800-2100 MPa, and a fracture elongation of 8.0-12.0%.

[0200] The specific chemical compositions and processes of the above embodiments are shown in Tables 1-3.

[0201] Table 1

[0202] C Si Mn Al Nb V P S Example 1 0.20 1.20 3.00 0.30 0.100 0.100 0.010 0.005 Example 2 0.25 0.60 0.80 1.00 0.025 0.050 0.005 0.003 Example 3 0.10 2.00 1.00 0.70 0.050 0.025 0.007 0.001 Example 4 0.15 0.80 2.00 0.40 0.070 0.060 0.006 0.004 Example 5 0.18 1.00 1.50 0.60 0.035 0.035 0.008 0.002 Example 6 0.21 1.30 1.80 0.80 0.030 0.080 0.009 0.0035 Example 7 0.12 1.00 2.50 0.55 0.060 0.090 0.002 0.0015 Example 8 0.22 1.25 1.70 0.85 0.080 0.055 0.0055 0.0025 Example 9 0.17 1.40 1.20 0.65 0.020 0.070 0.004 0.002 Example 10 0.16 1.55 2.60 0.50 0.055 0.065 0.085 0.0045

[0203] Table 2

[0204]

[0205] Table 3

[0206] Yield strength MPa Tensile strength MPa Elongation % Example 1 2100 2500 7.0 Example 2 1800 2450 7.3 Example 3 1500 2200 12.0 Example 4 1800 2200 11.2 Example 5 1600 2100 10.0 Example 6 1800 2400 6.6 Example 7 1700 1900 12.0 Example 8 1900 2300 8.5 Example 9 1700 1900 11.8 Example 10 1700 2100 10.8

Claims

1. A reinforced and toughened ferrite-martensite dual-phase hot-rolled steel plate, comprising the following components by mass: C: 0.10-0.25%, Si: 0.6-2.0%, Mn: 0.8-3.0%, Al: 0.3-1.0%, Nb: 0.025-0.1%, V: 0.025-0.1%, P≤0.01%, S≤0.005%, with the remainder being Fe and other unavoidable impurities; and having a microstructure of an ultrafine equiaxed ferrite and martensite dual-phase structure not exceeding 5 microns.

2. The reinforced and toughened ferrite-martensite dual-phase hot-rolled steel plate according to claim 1, characterized in that: The mass percentages of the components of the dual-phase hot-rolled steel plate are: C: 0.15-0.20%, Si: 0.8-1.2%, Mn: 1.0-2.0%, and the total mass percentage of other alloy contents except Fe element does not exceed 4.5%.

3. The reinforced and toughened ferrite-martensite dual-phase hot-rolled steel plate according to claim 1 or 2, characterized in that: The microstructure of the dual-phase hot-rolled steel plate contains 3.0-8.0% of retained austenite.

4. The reinforced and toughened ferrite-martensite dual-phase hot-rolled steel plate according to claim 1 or 2, characterized in that: The microstructure of the dual-phase hot-rolled steel plate has a grain size of less than 1 micron and a retained austenite content of 5.0-8.0%.

5. The reinforced and toughened ferrite-martensite dual-phase hot-rolled steel sheet according to any one of claims 1 to 4, characterized in that: The yield strength of the dual-phase hot-rolled steel plate is ≥1800 MPa, the tensile strength is 2000-2500 MPa, and the elongation at break is 4.0-12.0%.

6. The method for manufacturing a reinforced and toughened ferrite-martensite dual-phase hot-rolled steel sheet according to any one of claims 1 to 5, wherein: The steps include: 1) Smelting and casting Smelting, refining and casting into ingots according to the composition of claim 1 or 2; 2) Homogenization + blanking The ingot is heated to 1150-1280°C, kept at this temperature for 1.0-2.0 hours, and then hot forged or hot rolled to form an intermediate billet, the billet temperature being not less than 1100°C, and the final rolling or final forging temperature being not less than 950°C, to form an intermediate billet; 3)Austenitizing + hot rolling The intermediate billet is heated to 1100-1200°C, kept at this temperature for 1.0-2.0 hours, and hot rolled, with the starting rolling temperature at 1000-1050°C and the final rolling temperature at not less than 900°C, to obtain a hot rolled plate with a final thickness of 10-30 mm; 4) Segmented cooling + segmented warm rolling The hot rolled plate is cooled at 750-800°C at ≥50°C / s for 2-3 passes, and then cooled at 600-650°C at ≥60°C / s for 1-2 passes with a total deformation of ≥20% of the deformation amount, to obtain a steel plate with a final thickness of 1.5-10 mm; 5) Heating heat treatment + rapid cooling + coiling Heat the steel plate to 850-950℃ and keep it warm for 5-15 minutes. Immediately water-cool the steel plate to 200-300℃ at a cooling rate of ≥50℃ / s. Then coil it. After coiling, cool it to below 180℃ at a rate of ≤20℃ / s. Then air-cool it to room temperature. Steps 3), 4) and 5) are completed by continuous integration of the rolling and heat treatment production lines.

7. The method for manufacturing the reinforced and toughened ferrite-martensite dual-phase hot-rolled steel sheet according to claim 6, wherein: In the step 3), the heating temperature of the intermediate billet is 1150-1200° C., the holding time is 1.5-2.0 hours, the hot rolling start temperature is 1100-1150° C., the final rolling or final forging temperature is not less than 950° C., and the thickness of the intermediate billet is not less than 60 mm.

8. The method for manufacturing the reinforced and toughened ferrite-martensite dual-phase hot-rolled steel sheet according to claim 6, wherein: Step 3) Austenitization + hot rolling The intermediate billet heating temperature is 1100-1150℃, the holding time is 1.5-2.0 hours, the hot rolling start temperature is 1020-1050℃, the final rolling temperature is 900-930℃, and the final rolling thickness is 20-25mm; Step 4) Segmented cooling + segmented warm rolling The steel plate is rolled continuously for three passes by cooling to 800°C at 50°C / s, and then warm rolled by cooling to 600-630°C at 60°C / s with a deformation of more than 20%; Step 5) Heating heat treatment + rapid cooling + coiling After the steel plate is heated to 900-930°C, it is kept warm for 5.0-15.0 minutes, and immediately water-cooled to 250-300°C at a cooling rate of 50-80°C / s; then it is coiled, and after coiling, it is cooled to below 180°C at a rate of 10-20°C / s, and air-cooled to room temperature.

9. The method for manufacturing the reinforced and toughened ferrite-martensite dual-phase hot-rolled steel sheet according to claim 6 or 8, wherein: The heating treatment in step 5) is carried out by induction heating.

10. The method for manufacturing the reinforced and toughened ferrite-martensite dual-phase hot-rolled steel sheet according to claim 6, wherein: The warm rolling in step 4) adopts an asynchronous rolling method with a speed ratio of 1.2 to 2.0; the asynchronous rolling includes any of the following methods: the two working rolls have different diameters but the same rotational speed, the working rolls have the same diameter but different rotational speeds, or the working rolls have different diameters and rotational speeds.

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