Super-thick stainless steel clad plate and method for manufacturing the same

Abstract

This invention discloses an extra-thick stainless steel composite plate and its preparation method. The composite plate is an extra-thick plate with a thickness of 35 mm or more, and the corresponding composite billet has a thickness of 490 mm or more. In the roughing stage, the final rolling temperature is ≥1000℃, and in the finishing stage, the final rolling temperature is ≥780℃. After rolling, cooling is performed. The large composite plate enters an ultra-fast cooling system. Every 2 to 4 sets of cooling manifolds are opened, and then 2 to 4 sets of cooling manifolds are not opened. The cooling water pressure is 0.2 MPa, the cooling rate is 3 to 15℃ / s, the final cooling temperature is 380 to 450℃, and the roller speed is 0.6 to 0.9 m / s. The large composite plate first enters the ultra-fast cooling system in the forward direction. When its head reaches the 24th set of cooling manifolds, the rollers reverse, and the large composite plate passes through the ultra-fast cooling system in the reverse direction and leaves from the ultra-fast cooling system inlet, completing intermittent cooling.

Description

Technical Field

[0001] This invention belongs to the field of steel material preparation technology, and relates to an extra-thick stainless steel composite plate and its preparation method. Background Technology

[0002] With the continuous development of science and industry, ordinary alloys or single metals are no longer sufficient to meet the comprehensive performance requirements of materials in industrial development, leading to the emergence of composite plates. Stainless steel composite plates use carbon steel or low-alloy steel as the base layer and stainless steel as the cladding layer, achieving metallurgical bonding at the composite interface through methods such as explosive bonding and rolling bonding. This achieves resource conservation and cost reduction without compromising performance (mechanical strength, corrosion resistance, etc.). Stainless steel composite plates are widely used in industries such as petrochemicals, pressure vessels, power equipment, medical equipment, water conservancy, papermaking, and bridges.

[0003] The implementation of national highway and railway network construction and the large-scale investment in infrastructure development have spurred the rapid development of the bridge manufacturing industry. The construction of cross-river and cross-sea passages urgently requires the construction of long-span steel bridges. As the span of steel bridges increases, the requirements for stainless steel composite plates used in bridge structures are also becoming increasingly stringent, such as large thickness, good plate shape, excellent bonding performance, high strength, and high toughness.

[0004] Existing stainless steel composite plates suffer from problems such as poor plate shape and uniformity, and for extra-thick steel plates, the problems in plate shape and uniformity are even more severe. Summary of the Invention

[0005] The purpose of this invention is to provide an extra-thick stainless steel composite plate and its preparation method.

[0006] To achieve the above-mentioned objective, one embodiment of the present invention provides a method for preparing an extra-thick stainless steel composite plate, which includes three steps performed sequentially: composite billet preparation, composite billet rolling, and composite plate separation and straightening.

[0007] In the composite preform preparation step, the composite preform is prepared in the following stacking order: substrate, composite, composite, substrate;

[0008] The composite billet rolling step includes:

[0009] The resulting composite billet is heated to a homogenization temperature of 1170–1220℃, and the total heating time is ≥1.2×tmin / mm, where t is the thickness of the composite billet.

[0010] The rolling process is controlled by two stages: roughing and finishing. In the roughing stage, the final rolling temperature is ≥1000℃, and the roughing stage ends when the thickness of the intermediate billet is 2.5 to 3.5 times the target thickness of the composite plate. After that, the plate is allowed to cool down by water. When the surface temperature of the intermediate billet drops below 840℃, the finishing stage begins. The final rolling temperature of the finishing stage is ≥780℃, resulting in a composite plate with a thickness ≥70mm.

[0011] After rolling, the composite plate is cooled and enters the ultra-fast cooling system for intermittent cooling. The opening and closing status of all 24 cooling manifolds in the ultra-fast cooling system is controlled by opening N sets of cooling manifolds and then closing M sets of cooling manifolds. The cooling water pressure is 0.2MPa, the cooling rate is 3-15℃ / s, the final cooling temperature is 380-450℃, and the roller speed is 0.6-0.9m / s. N takes the value of 2, 3, or 4, and M takes the value of 2, 3, or 4. The composite plate first enters the ultra-fast cooling system in the forward direction. When its head reaches the 24th set of cooling manifolds, the roller reverses, and the composite plate passes through the ultra-fast cooling system in the reverse direction and leaves the ultra-fast cooling system from the inlet, completing the intermittent cooling.

[0012] Preferably, the cooling distance of each group of cooling manifolds is 1m.

[0013] Preferably, in the step of "heating the obtained composite blank", a five-stage heating process is adopted, consisting of preheating, first heating, second heating, third heating, and homogenization. The preheating temperature is ≤850℃, and the residence time is (0.45~0.55) tmin / mm; the first heating temperature is 1030~1090℃, and the residence time is (0.35~0.45) tmin / mm; the second heating temperature is 1100~1160℃, and the residence time is (0.25~0.35) tmin / mm; the third heating temperature is 1140~1180℃, and the residence time is (0.15~0.25) tmin / mm; and the homogenization temperature is 1170~1210℃, and the residence time is (0.10~0.20) tmin / mm.

[0014] Preferably, in the step of "two-stage controlled rolling of roughing and finishing", during the roughing stage, the first pass uses longitudinal rolling with a rolling reduction of ≥46mm; the second pass begins transverse rolling until the composite billet is rolled to the target width of the final composite plate in the nth pass, with a rolling reduction of ≥25mm in the second pass; the (n+1)th pass begins longitudinal rolling, and the roughing stage ends when the thickness of the intermediate billet is 2.5 to 3.5 times the target thickness of the composite plate, with a rolling reduction of ≥30mm in the (n+1)th pass.

[0015] Preferably, during the entire roughing stage, the rolling temperature of the first pass is ≥1060℃, the initial rolling temperature of the remaining passes is ≤1050℃, and the final rolling temperature is ≥1000℃. After the roughing stage, the billet is allowed to cool down by water. When the surface temperature of the intermediate billet drops below 840℃, the finishing stage begins. The initial rolling temperature of the finishing stage is 810℃~840℃, and the final rolling temperature is 780~810℃.

[0016] Preferably, the composite billet preparation steps include billet preparation, billet surface treatment, application of release agent, billet assembly, sealing and wrapping, gas shielded welding, overlay welding, vacuuming and sealing;

[0017] Among them, the vacuum degree of the vacuum pumping is ≤10. -1 Pa, maintain pressure for 4 hours.

[0018] Preferably, the composition of the release agent used is as follows by weight: 25-35% silicon nitride, 5-10% thermosetting amino resin, and 55-70% water;

[0019] The thickness of the release agent coating should be 0.2–0.5 mm;

[0020] Before assembly, the composite material coated with release agent is heated and dried at a temperature of 100-250℃ for 20-40 minutes.

[0021] Preferably, the composition of the release agent used is as follows by weight: 25-35% silicon nitride, 5-10% thermosetting amino resin, and 55-70% water;

[0022] The thickness of the release agent coating should be 0.2–0.5 mm;

[0023] Before assembly, the composite material coated with release agent is heated and dried at a temperature of 100-250℃ for 20-40 minutes.

[0024] Preferably, during gas shielded welding, the welding speed is 300-360 mm / min, and the interpass temperature is controlled at 135-165°C.

[0025] Preferably, submerged arc welding is used in the welding process;

[0026] Before welding, the flux is baked at 350℃ for 2 hours, and then kept at 150℃ for 1 hour.

[0027] During the welding process, the interpass temperature is controlled at 135–165℃, and the welding speed is 420–480 mm / min.

[0028] Preferably, when the composite panel passes through the ultra-fast cooling system, the cooling manifolds of groups 1 to 4 are turned on, the cooling manifolds of groups 5 to 8 are not turned on, the cooling manifolds of groups 9 to 12 are turned on, the cooling manifolds of groups 13 to 16 are not turned on, the cooling manifolds of groups 17 to 20 are turned on, the cooling manifolds of groups 21 to 22 are not turned on, and the cooling manifolds of groups 23 to 24 are turned on.

[0029] After the large composite plate leaves the inlet of the ultra-fast cooling system, it is naturally cooled to room temperature on the cooling bed. To achieve the above-mentioned objective, one embodiment of the present invention provides an extra-thick stainless steel composite plate, wherein the chemical composition of the base layer, by mass percentage, is: C: 0.03-0.07%, Si: 0.11-0.19%, Mn:

[0030] The composition is as follows: 1.46–1.54%, P ≤ 0.010%, S ≤ 0.0015%, Cr: 0.21–0.29%, Ni: 0.16–0.24%, Cu: 0.16–0.24%, Mo: 0.16–0.24%, Nb: 0.026–0.034%, Ti: 0.011–0.019%, Al: 0.030–0.040%, with the remainder being Fe and unavoidable impurities; the composite plate is prepared using the aforementioned preparation method.

[0031] Preferably, the total thickness of the composite board is ≥35mm, the thickness of the base layer is ≥32mm, the thickness of the cladding layer is 1~10mm, the Vickers hardness difference in the thickness direction of the base layer of the composite board is ≤10, the strength difference at the head, middle and tail is ≤40MPa, the strength difference at all points of the board is ≤40MPa, and the unevenness is ≤2mm / m.

[0032] Preferably, the composite board has a yield strength ≥500MPa, tensile strength ≥630MPa, elongation after fracture ≥18%, yield strength ratio ≤0.86; impact energy at 0℃ ≥240J, impact energy at -20℃ ≥200J, and impact energy at -40℃ ≥150J.

[0033] Compared with the prior art, the beneficial effects of the present invention are as follows: On the one hand, through specific process control, the corrosion resistance and mechanical strength of the stainless steel composite plate are guaranteed in the entire preparation method, and the corrosion resistance and mechanical properties are prevented from deteriorating during the rolling of the composite billet; on the other hand, through intermittent cooling, especially forward water quenching, and reverse water quenching when the head reaches the 24th group of cooling manifolds, combined with the control of the roller speed, the plate shape and uniformity are greatly improved, and the mechanical properties and toughness are guaranteed, realizing the plate shape control and uniformity control of extra-thick plates, overcoming the production difficulties of existing extra-thick stainless steel composite plates; and on the other hand, the yield rate and production efficiency are high during the production process. Attached Figure Description

[0034] For clarity of illustration and explanation, certain dimensions of structures or parts in the various figures of this invention are enlarged relative to other structures or parts. Therefore, only the basic structure of the subject matter of this invention is used to illustrate the invention.

[0035] Figure 1 This is a cross-sectional schematic diagram of a steel billet according to the first embodiment of the billet surface treatment step in this invention;

[0036] Figure 2 It corresponds to Figure 1 A schematic diagram of the cross-section of the composite blank;

[0037] Figure 3 This is a flowchart of the composite billet rolling steps according to an embodiment of the present invention;

[0038] Figure 4 yes Figure 2 A schematic diagram of the cross-section of two single-sided stainless steel composite plates rolled from a composite billet. Detailed Implementation

[0039] This invention provides a method for preparing a single-sided stainless steel composite plate with excellent plate shape, as well as the composite plate prepared based on this method. Specifically, the preparation method includes three main steps: composite billet preparation, composite billet rolling, and composite plate separation and straightening.

[0040] The overall steps for preparing the composite preform include the following sub-steps:

[0041] Prepare two carbon steel billets with thickness T1, length L1, and width W1 as base materials; and prepare two stainless steel billets with thickness T2, length L2, and width W2 as composite materials.

[0042] At least one surface of each of the two substrates and the two composites is surface treated;

[0043] Apply a release agent to one surface of a composite material;

[0044] The blanks are assembled in the following order: substrate, composite material, composite material, substrate.

[0045] Prepare four sealing strips with a width of W3, where W3 = 2T2-1~2mm. Attach the sealing strips to the four sides of the two composite materials. Perform gas shielded welding between adjacent sealing strips and between the sealing strips and the substrate, so that the two substrates and the sealing strips form a whole, thus obtaining the composite blank base.

[0046] A round hole is machined on the seal at the groove on the side of the composite billet base, and a seamless steel pipe is welded at the round hole;

[0047] The grooves on the four sides of the composite billet base are welded together.

[0048] A vacuum pump is used to evacuate the composite billet through the seamless steel pipe, with a vacuum level ≤10. -1 The pressure is increased to Pa, then maintained for more than 4 hours; finally, the seamless steel pipe is sealed.

[0049] Furthermore, the above steps are explained in detail below.

[0050] The steps “Prepare two carbon steel billets with length L1 and width W1 as base materials; and prepare two stainless steel billets with length L2 and width W2 as composite materials”, which are the billet preparation steps.

[0051] Among them, the carbon steel billet used as the base material has a thickness of T1, a length of L1, and a width of W1, which is a rectangular billet; similarly, the stainless steel billet used as the composite material has a thickness of T2, a length of L2, and a width of W2, which is also a rectangular billet.

[0052] Furthermore, L2 < L1, W2 < W1, and the length and width dimensions of the composite are both smaller than those of the substrate. Preferably, L2 = L1 - L0, W2 = W1 - W0, and the preferred values ​​for L0 and W0 are 90–150 mm, respectively.

[0053] As a preferred embodiment, the surface oxide scale indentation depth and surface pit depth of the carbon steel billet are both ≤0.3mm, and the flatness is ≤3mm / m; the surface of the stainless steel billet is free of scratches, and the flatness is ≤2mm / m. This avoids billets with obvious surface defects or shape defects from entering the composite plate production line.

[0054] Next, regarding the step "surface treatment of at least one surface of each of the two substrates and the two composites", that is, the blank surface treatment step.

[0055] In this embodiment, one surface of each substrate and each composite material is polished to remove the surface oxide layer and expose the metallic luster. (See reference...) Figure 1 As shown, for example, the surface p1a of the substrate 11a is polished using a grinding wheel, belt sander, or milling machine to remove the surface oxide scale and expose the metallic luster; similarly, the surface p2a of the prepared substrate 12a is polished using a grinding wheel, belt sander, or milling machine to remove the surface oxide scale and expose the metallic luster.

[0056] For the surface p3a of the prepared composite material 21a, a steel wire wheel is used for polishing to remove the surface oxide scale and expose the metallic luster; similarly, for the surface p4a of the prepared composite material 22a, a steel wire wheel is used for polishing to remove the surface oxide scale and expose the metallic luster.

[0057] As will be seen later, during the assembly process, the surface that has undergone surface treatment (polishing in this embodiment) is used as the surface where the substrate and composite material come into contact with each other. For example, surface p1a and surface p3a come into contact with each other, and surface p4a and surface p2a come into contact with each other. This ensures the quality of the interface bonding.

[0058] The other steps in the composite preform preparation process will be described below.

[0059] The step "apply release agent to one surface of a composite material" is also known as the release agent application step.

[0060] As mentioned above, the surface treatment of the composite material in the previous billet surface treatment steps, such as grinding and polishing, is performed on the surface that will be in contact with the substrate during billet assembly in order to ensure the interface bonding quality of the composite plate. The purpose of this step of applying the release agent is to prevent the surfaces of the composite material that are in contact with each other during billet assembly from bonding together in the subsequent composite billet rolling step, which would make them difficult to separate in the end.

[0061] Based on this, a release agent is applied to either of the two composite materials. If, in the previous blank surface treatment step, one surface of the selected composite material was treated while the other was not, then in this release agent application step, the release agent is applied to the "untreated" surface. However, if, as mentioned above, both surfaces of the selected composite material were treated in the previous blank surface treatment step, then in this release agent application step, the release agent is applied to the surface that is planned to face the other composite material during blank assembly.

[0062] For example, with Figure 1 For example, a release agent can be applied to the surface p6a of composite 22a or the surface p5a of composite 21a.

[0063] Regarding the separating agent, two preferred embodiments are provided here, which are described below.

[0064] <First Implementation Method of the Separating Agent>

[0065] In this embodiment, the separating agent is a coating liquid containing silicon oxide and magnesium oxide, wherein the mass ratio of silicon oxide to magnesium oxide is 3:1. The separating agent in this embodiment can achieve a good separating effect, ensuring the separation of the two subsequent composite board panels.

[0066] When applying the release agent to the surface of the composite material, the amount of release agent applied is 20 μmg / m³. 2That is, the weight of the release agent per unit area on the surface of the composite material is 20 μmg. Wherein, y is the ratio of the thickness of the composite billet obtained in the composite billet preparation step to the thickness of the composite plate subsequently rolled, which is also called the composite billet rolling compression ratio.

[0067] Furthermore, based on this embodiment, after the release agent is applied and before subsequent preform assembly, the composite material coated with the release agent is placed in a bogie furnace for heating and drying at a temperature of 340–360°C for 35–45 minutes.

[0068] <Second Implementation Method of the Separating Agent>

[0069] In this embodiment, the composition of the release agent by weight is: 25-35% silicon nitride, 5-10% thermosetting amino resin, and 55-70% water. Compared with existing release agents, and even compared with the first embodiment of the aforementioned release agent, the release agent of this embodiment not only achieves a good release effect, ensuring the separation of the two subsequent composite board panels, but also features strong chemical stability and high temperature and thermal shock resistance of the active ingredient silicon nitride. The thermosetting amino resin, as the adhesive, can be cured at low temperatures, is non-toxic, and achieves strong adhesion with a small dosage. Therefore, the overall cost is low, the operation is simple, and the release and adhesion effects are good.

[0070] Hereinafter, a preferred preparation method for the release agent is provided, comprising: first, placing 5-10% silicon nitride (by weight) in a beaker or other container, then adding 15-25% water and stirring; after the silicon nitride is free of particles and bubbles, adding 2-3% thermosetting amino resin and continuing to stir; when it becomes viscous, adding the remaining silicon nitride and water, stirring for 3-5 minutes, and then adding the remaining thermosetting amino resin; when it becomes viscous, the release agent is prepared.

[0071] When the release agent is applied to the surface of the composite material, the thickness of the release agent is 0.2 to 0.5 mm.

[0072] Furthermore, based on this embodiment, after the release agent is applied and before subsequent preform assembly, the composite material coated with the release agent is heated and dried at a temperature of 100–250°C for 20–40 minutes.

[0073] Next, after completing the step of applying the release agent, the step of "assembling the blank according to the stacking order of substrate, composite material, composite material, and substrate" will be introduced.

[0074] This step, "assembling the blank according to the stacking order of substrate, composite, composite, and substrate," is also known as the blank assembly step. In addition to following the stacking order of substrate, composite, composite, and substrate, the following conditions must also be met:

[0075] 1) The surfaces of the substrate and composite material that come into contact with each other are all surfaces that have undergone the aforementioned surface treatment; for example, refer to Figure 2 The surfaces p2a of the substrate 12a and p4a of the composite 22a are in contact with each other, and the surfaces p1a of the substrate 11a and p3a of the composite 21a are in contact with each other.

[0076] 2) The surface coated with the release agent faces the other composite material; for example, refer to Figure 2 One of surfaces p6a and p5a is coated with release agent 30a;

[0077] 3) The composite material is placed centered relative to the substrate. Since the composite preform is roughly symmetrically arranged vertically, this will be explained using only one set of substrate + composite material within the composite preform as an example. For instance, the upper set... Figure 2 In the preform state, the distance from the side edge of the composite material 21a in the transverse direction (corresponding to the long side edge of surface p3a) to the side edge of the substrate 11a in the transverse direction (corresponding to the long side edge of surface p1a) is half of W0, and the distance from the side edge of the composite material 21a in the longitudinal direction (corresponding to the short side edge of surface p3a) to the side edge of the substrate 11a in the longitudinal direction (corresponding to the short side edge of surface p1a) is half of L0. As mentioned above, L0 and W0 are 90 to 150 mm respectively, that is, the distance from the side edge of the composite material 21a to the side edge of the substrate 11a is 45 to 75 mm.

[0078] The billet assembly steps have been described above. In a preferred embodiment, after the billet assembly step, the four stacked billets are placed under a four-column hydraulic press to apply pressure to the opposing surfaces of the two substrates (i.e., the upper surface of the upper substrate and the lower surface of the lower substrate), with a pressure ≥ 500 tons. This allows for a closer contact between adjacent billets.

[0079] Furthermore, in the step "prepare four sealing strips of width W3, attach the sealing strips to the four sides of the two composite materials, and perform gas shielded welding between adjacent sealing strips and between the sealing strips and the substrate, so that the two substrates and the sealing strips form a whole, obtaining a composite billet base blank," the sealing strips enable the four steel billets stacked together to form a composite billet base blank as a whole. Specifically, see... Figure 2 The composite blank is composed of two base materials 11a and 12a forming the upper and lower surfaces, two composite materials 21a and 22a located in the middle, and four sealing strips 40a forming a four-sided frame surrounding the two composite materials 21a and 22a and connecting the two base materials 11a and 12a.

[0080] The width of the seal, W3, is 2T2 - 1 to 2 mm, meaning the width of the seal is slightly smaller than the sum of the thicknesses of the two composite materials by 1 to 2 mm. Using a seal of this width to wrap both the upper and lower composite materials simultaneously improves the wrapping effect.

[0081] Furthermore, of the four seals, two seals are respectively attached to the two sides of the two composite materials in the horizontal direction, with a length L31 = L2-1 to 2mm; the other two seals are respectively attached to the two sides of the two composite materials in the vertical direction, with a length L32 = W2-1 to 2mm.

[0082] Preferably, the thickness T3 of the seal is 12-15 mm.

[0083] Regarding the forming method of each seal, it can be cut directly from a steel plate according to thickness T3, width W3, and length L31 or L32 without welding, or it can be spliced ​​together by welding multiple seals of different lengths.

[0084] As a preferred embodiment, in this step, before performing gas shielded welding between adjacent seals and between the seal and the substrate, the two ends and two sides of each seal can be ground and polished to remove surface oxide scale and improve the welding effect; and / or, the two ends and two sides of each seal can be beveled.

[0085] Furthermore, as a preferred embodiment, in the step of "performing gas shielded welding between adjacent seals and between the seal and the substrate", the welding speed is 300-360 mm / min, and the interpass temperature is controlled at 135-165°C during the welding process.

[0086] Next, for the step "machine a round hole on the seal at the groove on the side of the composite billet base, and weld a seamless steel pipe at the round hole", the groove is the groove formed between the two base materials and outside the seal; in this step, the round hole is machined to weld the seamless steel pipe so as to facilitate the subsequent vacuuming of the interior of the composite billet.

[0087] As a preferred embodiment, the circular hole is machined in the middle of the short side (i.e., the side in the longitudinal direction) of the composite billet base, but it is not limited to this.

[0088] As a preferred embodiment, the diameter of the circular hole is 8-12 mm; correspondingly, the outer diameter of the seamless steel pipe is the same as the diameter of the circular hole, which is 8-12 mm, the wall thickness is 1.2-2 mm, and the length is 200-400 mm.

[0089] Next, for the step "surfacing the grooves on the four sides of the composite billet base," submerged arc welding is specifically employed. It can be understood that, outside the four-sided frame formed by the sealing strip, a four-sided frame-shaped filling layer 50a is formed by welding in this step, see [link to documentation]. Figure 2 .

[0090] As a preferred method, before welding, the flux is baked at 350℃ for 2 hours, followed by a holding time at 150℃ for 1 hour. During welding, the interpass temperature is controlled at 135–165℃, and the welding speed is 420–480 mm / min. Thus, this submerged arc welding technology, combined with the preceding sealing and gas-shielded welding, achieves a stable connection between the four steel billets, ensuring connection strength and preventing cracking abnormalities in the subsequent composite billet rolling process. Furthermore, in addition to achieving the quality advantages of the composite plate mentioned above, it can further improve the interface bonding effect.

[0091] In addition, during the welding process, before each welding operation, it is necessary to clean the weld bead to keep it clean; after welding, insulation cotton should be used to cover it for insulation.

[0092] Next, the step involves using a vacuum pump to evacuate the composite billet through the seamless steel pipe, achieving a vacuum level of [missing information].

[0093] ≤10 -1 In the process of "holding pressure for more than 4 hours after reaching Pa, and finally sealing the seamless steel pipe", the suction port of the vacuum pump is connected to the seamless steel pipe, and the seamless steel pipe is connected to the space inside the composite billet (such as the surface gap between the composite and the base material, the surface gap between the composites, the end face gap between the composite and the seal, etc.) to expel the air in the space until the vacuum degree is ≤10. -1 The pressure is maintained at 4 Pa, and a vacuum level can be guaranteed by holding the pressure for more than 4 hours. This avoids air in the space from causing surface oxidation at the composite interface during subsequent composite billet rolling, thus ensuring the bonding quality of the composite interface.

[0094] Furthermore, in this step, the seamless steel pipe is sealed, which can be carried out in a manner that is currently feasible in the steel industry, such as heating and flattening the seamless steel pipe with a flame gun to achieve sealing.

[0095] The above details the overall steps for preparing the composite billet. Specific steps such as surface treatment, vacuuming, welding, and sealing lay the foundation for excellent interfacial bonding and surface quality in subsequent rolling of the composite billet. The use of a release agent further facilitates the smooth separation of the two composite plates.

[0096] As mentioned above, the preparation method of the present invention further includes a composite billet rolling step following the overall composite billet preparation step. The present invention provides four embodiments of the composite billet rolling step, which are described in detail below.

[0097] <First Implementation Method of Composite Billet Rolling Step>

[0098] In this embodiment, the reference Figure 3The composite billet rolling process includes the following sub-steps:

[0099] The obtained composite billet is heated to a homogenization temperature of 1170~1220℃, preferably 1200~1220℃, and the total heating time is ≥1.2×t min / mm, where t is the thickness of the composite billet. The holding time in the homogenization section is 30min~50min.

[0100] A two-stage controlled rolling process of roughing and finishing is adopted. In the roughing stage, the initial rolling temperature is ≤1050℃ and the final rolling temperature is ≥1000℃. First, transverse rolling is performed, followed by longitudinal rolling. During longitudinal rolling, at least one pass has a reduction of ≥35mm. The total reduction in roughing is 40-60%. The roughing stage ends when the thickness of the intermediate billet is 2.5-3.5 times the target thickness of the composite plate. After that, the plate is allowed to cool down by water. When the surface temperature of the intermediate billet drops below 840℃, preferably below 830℃, the finishing stage begins. The final rolling temperature in the finishing stage is ≥780℃, preferably ≥800℃, and the total reduction in finishing is 55-75%, resulting in a composite plate with a thickness of ≥70mm.

[0101] After rolling, the composite plate is cooled and then enters the ultra-fast cooling system for intermittent cooling. The opening and closing status of all 24 cooling manifolds in the ultra-fast cooling system is controlled by opening N sets of cooling manifolds and then closing M sets of cooling manifolds. The cooling water pressure is 0.2MPa, the cooling rate is 3-15℃ / s, the final cooling temperature is 380-450℃, and the roller speed is 0.6-0.9m / s. N is 2, 3, or 4, and M is 2, 3, or 4. The composite plate first enters the ultra-fast cooling system in the forward direction. When its head reaches the 24th cooling manifold, the roller reverses, and the composite plate passes through the ultra-fast cooling system in the reverse direction and leaves the ultra-fast cooling system from the inlet, completing the intermittent cooling.

[0102] After intermittent cooling is completed, the composite plate is naturally cooled to room temperature on the cooling bed, thus completing step 2) of composite billet rolling and proceeding to step 3) of composite plate separation and straightening.

[0103] Compared to existing technologies, this embodiment employs an intermittent cooling method. As the composite panel passes through the ultra-fast cooling system, it alternates between opening and closing the cooling manifolds. This causes each part of the composite panel to cool, then glow red, then cool again, then glow red again… in a continuous cycle until the composite panel leaves the ultra-fast cooling system. During this cooling-glow-reddening cycle, the carbon steel substrate continuously undergoes phase transformation and self-tempering effects, with the phase transformation reaction gradually penetrating towards the core until the entire carbon steel substrate has completed the phase transformation. This intermittent cooling process differs from conventional reciprocating cooling. In reciprocating cooling, the re-heating and self-tempering occur after the surface or near-surface layer has completed its phase transformation, resulting in a significant temperature difference or cooling rate between the surface and the core, leading to substantial differences in microstructure and mechanical properties. In contrast, the intermittent cooling process in this embodiment involves some parts of the composite board being cooled while others are undergoing re-heating / self-tempering simultaneously. Furthermore, each part of the composite board alternates between cooling and re-heating / self-tempering over time, minimizing differences in temperature, cooling rate, microstructure, and properties between the surface and core. For example, the resulting composite board exhibits a Vickers hardness difference ≤10 in the thickness direction of the base layer, a strength difference ≤40 MPa between the head, middle, and tail, and a strength difference ≤40 MPa across the entire board. Moreover, intermittent cooling further improves the shape of the composite board, resulting in low unevenness. Even without straightening after cooling and directly cooled on a cooling bed, an excellent shape can still be achieved.

[0104] Preferably, the cooling distance of each set of cooling manifolds is 1m. That is, when one set of cooling manifolds is turned on, it can cool a roughly 1m length of the composite panel on the roller conveyor.

[0105] When the composite panel passes through the ultra-fast cooling system, the cooling manifolds of groups 1 to 4 are turned on, groups 5 to 8 are turned off, groups 9 to 12 are turned on, groups 13 to 16 are turned off, groups 17 to 20 are turned on, groups 21 to 22 are turned off, and groups 23 to 24 are turned on.

[0106] For example, in one experimental case, Q500q steel was selected as the base material; 316L stainless steel was selected as the cladding material. A composite billet with a thickness of 490 mm and a constant base material thickness was prepared according to this embodiment. The composite billet was then rolled into a large composite plate with a thickness of 70 mm according to this embodiment. The large composite plate was then subjected to the composite plate separation and straightening steps described later, resulting in a single-sided stainless steel composite plate with a total thickness of 35 mm, a base layer thickness of 32 mm, and a cladding layer thickness of 3 mm.

[0107] The composite panel of this test example was sampled and tested. The interface bonding rate was 100%, the inner 180° bend was qualified (no cracks), the outer 180° bend was qualified (no cracks), and after boiling in sulfuric acid-copper sulfate solution for 20 hours and bending at 180°, there were no intergranular corrosion cracks in the strata. The yield strength was 545 MPa, the tensile strength was 672 MPa, the elongation after fracture was 24%, the yield strength ratio was 0.81, and the impact energy at -40℃ was 263 J. The Vickers hardness difference in the thickness direction of the base layer was 9, the strength difference between the head, middle and tail was ≤37 MPa, the strength difference throughout the panel was ≤37 MPa, the unevenness was ≤2 mm / m, and the shear strength was 404 MPa.

[0108] <Second Implementation Method of Composite Billet Rolling Step>

[0109] In this embodiment, the composite billet rolling step includes the following sub-steps:

[0110] A five-stage heating system is adopted, consisting of preheating, primary heating, secondary heating, tertiary heating, and homogenization. The preheating temperature is ≤850℃ with a residence time of (0.45~0.55) t min / mm; the primary heating temperature is 1030~1090℃ with a residence time of (0.35~0.45) t min / mm; the secondary heating temperature is 1100~1160℃ with a residence time of (0.25~0.35) t min / mm; the tertiary heating temperature is 1140~1180℃ with a residence time of (0.15~0.25) t min / mm; and the homogenization temperature is 1170~1210℃ with a residence time of (0.10~0.20) t min / mm.

[0111] A two-stage controlled rolling process of roughing and finishing is adopted. In the roughing stage, the first pass uses longitudinal rolling with a reduction of ≥46mm; the second pass and subsequent passes use transverse rolling until the nth pass rolls the composite billet to the target width of the final composite plate, with a reduction of ≥25mm in the second pass; the (n+1)th pass begins longitudinal rolling, ending when the intermediate billet thickness is 2.5–3.5 times the target thickness of the final composite plate. The reduction is ≥30mm; during the entire roughing stage, the rolling temperature of the first pass is ≥1060℃, the initial rolling temperature of the remaining passes is ≤1050℃, and the final rolling temperature is ≥1000℃; after the roughing stage, the plate is allowed to cool down by water. When the surface temperature of the intermediate billet drops below 840℃, the finishing stage begins. The initial rolling temperature of the finishing stage is 810℃~840℃, and the final rolling temperature is 780~810℃, resulting in a composite plate with a thickness ≥70mm.

[0112] After rolling, the composite plate is cooled and then enters the ultra-fast cooling system for intermittent cooling. The opening and closing status of all 24 cooling manifolds in the ultra-fast cooling system is controlled by opening N sets of cooling manifolds and then closing M sets of cooling manifolds. The cooling water pressure is 0.2MPa, the cooling rate is 3-15℃ / s, the final cooling temperature is 380-450℃, and the roller speed is 0.6-0.9m / s. N is 2, 3, or 4, and M is 2, 3, or 4. The composite plate first enters the ultra-fast cooling system in the forward direction. When its head reaches the 24th cooling manifold, the roller reverses, and the composite plate passes through the ultra-fast cooling system in the reverse direction and leaves the ultra-fast cooling system from the inlet, completing the intermittent cooling.

[0113] After intermittent cooling is completed, the composite plate is naturally cooled to room temperature on the cooling bed, thus completing step 2) of composite billet rolling and proceeding to step 3) of composite plate separation and straightening.

[0114] That is, the difference between this embodiment and the first embodiment of the composite billet rolling process mentioned above lies in the two processes of heating and rolling.

[0115] Compared with existing technologies, this embodiment also employs intermittent cooling, and correspondingly possesses the beneficial effects of intermittent cooling processes, as described in the first embodiment of the composite billet rolling process above. Furthermore, compared to existing technologies, the heating process in this embodiment can better control the heating rate of the composite billet in each section, ensuring uniform heating of the billet and preventing cracking and air leakage in the composite billet due to differences in the thermal properties of the base material and composite material, thereby ensuring the interface bonding effect. Moreover, in the rolling process of this embodiment, the rough rolling adopts a sequence of longitudinal rolling, then transverse rolling, and then longitudinal rolling again, which ensures the realization of high-reduction rolling, allowing effective penetration into the core of the composite billet, promoting core deformation, and ensuring the bonding rate of the composite interface. During the waiting period, an instant cooling device is used to reduce the waiting time, improve rolling efficiency, and avoid excessively long waiting times that could lead to grain growth in the carbon steel base material. Temperature control during the finish rolling stage can refine the grains, ensuring that the thick composite plate has good low-temperature impact toughness.

[0116] For example, in one experimental case, Q500q steel was selected as the base material; 316L stainless steel was selected as the cladding material. A composite billet with a thickness of 490 mm and a constant base material thickness was prepared according to this embodiment. The composite billet was then rolled into a large composite plate with a thickness of 70 mm according to this embodiment. The large composite plate was then subjected to the composite plate separation and straightening steps described later, resulting in a single-sided stainless steel composite plate with a total thickness of 35 mm, a base layer thickness of 32 mm, and a cladding layer thickness of 3 mm.

[0117] The composite panel of this test example was sampled and tested. The interface bonding rate was 100%, the 180° inner bend was qualified (no cracks), the 180° outer bend was qualified (no cracks), and after boiling in sulfuric acid-copper sulfate solution for 20 hours and bending at 180°, there were no intergranular corrosion cracks in the strata. The yield strength was 546 MPa, the tensile strength was 669 MPa, the elongation after fracture was 24%, the yield strength ratio was 0.82, and the impact energy at -40℃ was 268 J. The Vickers hardness difference in the thickness direction of the base layer was 9, the strength difference between the head, middle and tail was ≤37 MPa, the strength difference throughout the panel was ≤37 MPa, the unevenness was ≤2 mm / m, and the shear strength was 437 MPa.

[0118] <Third Implementation Method of Composite Billet Rolling Step>

[0119] This implementation method is the same as the first and second implementation methods of the aforementioned composite billet rolling steps in terms of heating, two-stage controlled rolling, and cooling steps, except that it is the step after intermittent cooling is completed.

[0120] In the first and second embodiments of the aforementioned composite billet rolling process, after the composite plate leaves the ultra-rapid cooling system, it is naturally cooled to room temperature on the cooling bed. In contrast, in this embodiment, after leaving the ultra-rapid cooling system, the composite plate directly enters a straightening machine for straightening. After straightening, the composite plate is naturally cooled on the cooling bed, and when the surface temperature drops below 200°C, it is cold-straightened using a cold straightening machine. This further improves the plate shape.

[0121] <Fourth Implementation Method of Composite Billet Rolling Step>

[0122] This implementation method is the same as the first and second implementation methods of the aforementioned composite billet rolling steps in terms of heating, two-stage controlled rolling, and cooling steps. The only difference is the steps after the composite plate leaves the ultra-fast cooling system.

[0123] In the first and second embodiments of the aforementioned composite billet rolling process, after the composite plate leaves the ultra-rapid cooling system, it is naturally cooled to room temperature on the cooling bed. In contrast, in this embodiment, after the composite plate leaves the ultra-rapid cooling system, it directly enters the straightening machine for 1-3 straightening passes. Unlike the first embodiment of the aforementioned composite billet rolling process, in this embodiment, the composite plate is then placed at a temperature of T. f Two steel plates with a temperature of +100~150℃ are stacked for cooling for 0.4min / mm×t±5min. This allows the composite plate to cool slowly during this cooling time while maintaining uniform core and surface temperature by being held between the steel plates. After stacking, the composite plate is allowed to cool naturally on the cooling bed.

[0124] T f = 550 + 30[Si] - 20[Mn] + 15[Cr] - 15[Ni] + 10[Mo], where [Si], [Mn], [Mo], [Cr], and [Ni] are 100 times the mass percentage of each element in the base material. In this preferred embodiment, the stacking cooling, especially the temperature and cooling time of the two steel plates, can further improve the microstructure, properties, and shape of the final composite plate compared to the first embodiment described above. For example, in the experimental example, stacking cooling can reduce the unevenness to below 1 mm / m.

[0125] The overall steps of the composite billet rolling process have been described in detail above. As mentioned earlier, the preparation method of the present invention also includes a composite plate separation and straightening step. Specifically, the composite plate separation and straightening step includes the following sub-steps:

[0126] For the composite plate obtained from the previous composite billet rolling process, a plasma cutting machine is used to cut its four sides to remove the part other than the seal strip, and the composite plate is separated into two smaller composite plates.

[0127] The small composite panels are laterally flattened and cold-straightened to obtain the finished stainless steel composite panel.

[0128] Specifically, the part outside the seal in the step "cutting its four sides to remove the portion other than the seal" refers to the edge portion of the large composite plate formed by the seal and filler layer in the composite billet mentioned earlier, after the preceding composite billet rolling step. Removing this portion exposes the stainless steel cladding layer, and without its connecting function, the large composite plate separates into two smaller composite plates. See also... Figure 4 This corresponds to the five implementation methods of the blank surface treatment steps described above. Figure 4 The cross-sectional shapes of the two corresponding composite panels (i.e., the final single-sided stainless steel composite panels) are shown respectively.

[0129] Each composite panel consists of a lamellae and a base layer. The lamellae are obtained from the original composite material through rolling, and the base layer is obtained from the original substrate through rolling. Therefore, in Figure 4 The original composite material number is still marked for the multilayer layer, and the original substrate number is still marked for the base layer.

[0130] Furthermore, the single-sided stainless steel composite plate obtained by the preparation method of the present invention.

[0131] The cladding layer is preferably austenitic stainless steel, with the following chemical composition by mass percentage: C≤0.15%, Si≤1.00%, Mn≤2.00%, P≤0.045%, S≤0.030%, Ni: 6.0~22.0%, Cr: 16.0~26.0%, Mo≤3.0%, with the balance being Fe and unavoidable impurities. Using this chemical composition further ensures the performance of the composite plate, especially the corrosion resistance of the cladding layer, while maintaining the aforementioned technical effects. For example, after boiling the cladding layer in a sulfuric acid-copper sulfate solution for 20 hours and bending it 180°, no intergranular corrosion cracks are observed.

[0132] The base layer is made of carbon steel, and its chemical composition by mass percentage is as follows: C: 0.03-0.16%, Si: 0.11-0.29%, Mn: 1.31-1.54%, P≤0.018%, S≤0.0030%, Cr: 0.06-0.29%, Ni≤0.24%, Cu≤0.24%, Mo≤0.24%, Nb: 0.011-0.034%, Ti: 0.011-0.019%, Al: 0.030-0.040%, with the balance being Fe and unavoidable impurities.

[0133] Preferably, the chemical composition of the base layer, by mass percentage, is: C: 0.03–0.07%, Si: 0.11–0.19%, Mn: 1.46–1.54%, P ≤ 0.010%, S ≤ 0.0015%, Cr: 0.21–0.29%, Ni: 0.16–0.24%, Cu: 0.16–0.24%, Mo: 0.16–0.24%, Nb: 0.026–0.034%, Ti: 0.011–0.019%, Al: 0.030–0.040%, with the remainder being Fe and unavoidable impurities. Using this chemical composition, combined with the control of temperature, time, reduction, and cooling rate in the composite billet rolling process, the mechanical properties of the composite plate can be further improved while maintaining the aforementioned technical effects, and toughness can be guaranteed.

[0134] Furthermore, the microstructure of the base layer of the composite plate is bainite with a small amount of ferrite. The composite plate has excellent mechanical properties, strong impact toughness, and excellent corrosion resistance. In addition, it has good interfacial bonding quality, plate shape and surface quality, and high uniformity.

[0135] Specifically, the total thickness of the composite panel is ≥35mm, the thickness of the base layer is ≥32mm, and the thickness of the cladding layer is 1~10mm.

[0136] The unevenness of the composite board is ≤3mm / m, or even ≤2mm / m.

[0137] The composite plate has a yield strength ≥345MPa, tensile strength ≥490MPa, elongation after fracture ≥18%, and yield strength ratio ≤0.86. Even further, the composite plate has a yield strength ≥500MPa and tensile strength ≥630MPa.

[0138] The composite plate has a 100% bonding rate at the composite interface, a shear strength ≥300MPa, an impact energy ≥120J at 0℃, an impact energy ≥120J at -20℃, and an impact energy ≥120J at -40℃. Even further, the composite plate has an impact energy ≥240J at 0℃, an impact energy ≥200J at -20℃, and an impact energy ≥150J at -40℃.

[0139] The Vickers hardness difference of the base layer of the composite board is ≤10 in the thickness direction, the strength difference between the head, middle and tail is ≤40MPa, and the strength difference at all parts of the whole board is ≤40MPa.

[0140] Furthermore, the composite plate showed no cracks when bent outwards at 180° and no cracks when bent inwards at 180°. In the corrosion resistance test, the cladding layer was sensitized at 650°C for 2 hours, then boiled in a sulfuric acid-copper sulfate solution for 20 hours, and after a 180° cold bend, no intergranular corrosion cracks were observed.

[0141] In summary, compared with the prior art, the present invention, on the one hand, ensures the corrosion resistance and mechanical strength of the stainless steel composite plate through specific process control in the entire preparation method, and avoids the deterioration of corrosion resistance and mechanical properties during the rolling of the composite billet; on the other hand, it achieves shape control and uniformity control of extra-thick plates, overcoming the production difficulties of existing extra-thick stainless steel composite plates; and on the other hand, it has a high yield and high production efficiency in the production process.

[0142] The detailed descriptions listed above are merely specific descriptions of feasible embodiments of the present invention, and are not intended to limit the scope of protection of the present invention. All equivalent embodiments or modifications made without departing from the spirit of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for preparing an extra-thick stainless steel composite plate, characterized in that, It includes three steps carried out in sequence: composite billet preparation, composite billet rolling, and composite plate separation and straightening; In the composite preform preparation step, the composite preform is prepared in the following stacking order: substrate, composite, composite, substrate; The composite billet rolling step includes: The resulting composite billet is heated to a homogenization temperature of 1170~1220℃, and the total heating time is ≥1.2×t min / mm, where t is the thickness of the composite billet; A two-stage controlled rolling process of roughing and finishing is adopted. In the roughing stage, the first pass uses longitudinal rolling with a reduction of ≥46mm; from the second pass onwards, transverse rolling is adopted until the nth pass when the composite billet is rolled to the target width of the final composite plate, with a reduction of ≥25mm in the second pass. The rolling temperature of the first pass in the roughing stage is ≥1060℃, the initial rolling temperature of the remaining passes is ≤1050℃, and the final rolling temperature is ≥1000℃. The roughing stage begins with longitudinal rolling, ending when the intermediate billet thickness is 2.5 to 3.5 times the target thickness of the composite plate. The reduction in the (n+1)th pass is ≥30mm. After that, the plate is allowed to cool down by water. When the surface temperature of the intermediate billet drops below 840℃, the finishing stage begins. The finishing stage has a final rolling temperature of 810℃ to 840℃ and a final rolling temperature of 780℃ to 810℃, resulting in a composite plate with a thickness ≥70mm. After rolling, the composite plate is cooled and enters the ultra-fast cooling system for intermittent cooling. The opening and closing status of all 24 cooling manifolds in the ultra-fast cooling system is controlled by opening N sets of cooling manifolds and then closing M sets of cooling manifolds. The cooling water pressure is 0.2MPa, the cooling rate is 3~15℃ / s, the final cooling temperature is 380~450℃, and the roller speed is 0.6~0.9m / s. N takes the value of 2, 3, or 4, and M takes the value of 2, 3, or 4. The composite plate first enters the ultra-fast cooling system in the forward direction. When its head reaches the 24th set of cooling manifolds, the roller reverses, and the composite plate passes through the ultra-fast cooling system in the reverse direction and leaves the ultra-fast cooling system from the inlet, completing the intermittent cooling.

2. The method for preparing the extra-thick stainless steel composite plate according to claim 1, characterized in that, The cooling distance for each set of cooling manifolds is 1m.

3. The method for preparing the extra-thick stainless steel composite plate according to claim 1, characterized in that, In the step of "heating the obtained composite blank", a five-stage heating process is adopted, consisting of preheating, first heating, second heating, third heating, and homogenization. The preheating temperature is ≤850℃, and the residence time is (0.45~0.55)t min / mm. The first heating temperature is 1030~1090℃, and the residence time is (0.35~0.45)t min / mm. The second heating temperature is 1100~1160℃, and the residence time is (0.25~0.35)t min / mm. The third heating temperature is 1140~1180℃, and the residence time is (0.15~0.25)t min / mm. The homogenization temperature is 1170~1210℃, and the residence time is (0.10~0.20)t min / mm.

4. The method for preparing the extra-thick stainless steel composite plate according to claim 1, characterized in that, The composite billet preparation steps include billet preparation, billet surface treatment, application of release agent, billet assembly, sealing and wrapping, gas shielded welding, overlay welding, vacuuming and sealing; Among them, the vacuum degree of the vacuum pumping is ≤10. -1 Pa, maintain pressure for 4 hours.

5. The method for preparing the extra-thick stainless steel composite plate according to claim 4, characterized in that, The composition of the separating agent used is as follows by weight: 25-35% silicon nitride, 5-10% thermosetting amino resin, and 55-70% water; The thickness of the release agent coating should be 0.2~0.5mm; Before assembly, the composite material coated with release agent is heated and dried at a temperature of 100~250℃ for 20~40 minutes.

6. The method for preparing the extra-thick stainless steel composite plate according to claim 4, characterized in that, The composition of the separating agent used is as follows by weight: 25-35% silicon nitride, 5-10% thermosetting amino resin, and 55-70% water; The thickness of the release agent coating should be 0.2~0.5mm; Before assembly, the composite material coated with release agent is heated and dried at a temperature of 100~250℃ for 20~40 minutes.

7. The method for preparing the extra-thick stainless steel composite plate according to claim 4, characterized in that, During gas shielded welding, the welding speed is 300~360mm / min, and the interpass temperature is controlled at 135~165℃.

8. The method for preparing the extra-thick stainless steel composite plate according to claim 4, characterized in that, Submerged arc welding is used in the welding process; Before welding, the flux is baked at 350℃ for 2 hours, and then kept at 150℃ for 1 hour. During the welding process, the interpass temperature is controlled at 135~165℃, and the welding speed is 420~480mm / min.

9. The method for preparing the extra-thick stainless steel composite plate according to claim 1, characterized in that, When the composite panel passes through the ultra-fast cooling system, the cooling manifolds of groups 1-4 are turned on, groups 5-8 are turned off, groups 9-12 are turned on, groups 13-16 are turned off, groups 17-20 are turned on, groups 21-22 are turned off, and groups 23-24 are turned on. After the large composite panel leaves the inlet of the ultra-fast cooling system, it is naturally cooled to room temperature on the cooling bed.

10. An extra-thick stainless steel composite plate, characterized in that, The chemical composition of the base layer, by mass percentage, is as follows: C: 0.03~0.07%, Si: 0.11~0.19%, Mn: 1.46~1.54%, P≤0.010%, S≤0.0015%, Cr: 0.21~0.29%, Ni: 0.16~0.24%, Cu: 0.16~0.24%, Mo: 0.16~0.24%, Nb: 0.026~0.034%, Ti: 0.011~0.019%, Al: 0.030~0.040%, with the remainder being Fe and unavoidable impurities. The composite board is prepared by the preparation method according to any one of claims 1 to 9.

11. The extra-thick stainless steel composite plate according to claim 10, characterized in that, The total thickness of the composite board is ≥35mm, the thickness of the base layer is ≥32mm, the thickness of the cladding layer is 1~10mm, the Vickers hardness difference in the thickness direction of the base layer of the composite board is ≤10, the strength difference at the head, middle and tail is ≤40MPa, the strength difference at all points of the board is ≤40MPa, and the unevenness is ≤2mm / m.

12. The extra-thick stainless steel composite plate according to claim 10, characterized in that, The composite plate has a yield strength ≥500MPa, tensile strength ≥630MPa, elongation after fracture ≥18%, and yield strength ratio ≤0.86; impact energy at 0℃ ≥240J, impact energy at -20℃ ≥200J, and impact energy at -40℃ ≥150J.

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