Low-cost quenched and tempered high-strength steel with yield strength of 800mpa and method for manufacturing the same

Abstract

This application discloses a low-cost, tempered high-strength steel with a yield strength of 800 MPa and its preparation method. The method includes: providing refined molten steel, which, by mass percentage, comprises the following components: C: 0.14 wt.%~0.16 wt.%, Si: 0.05 wt.%~0.15 wt.%, Mn: 1.2 wt.%~1.4 wt.%, P≤0.015 wt.%, S≤0.0025 wt.%, Ti: 0.01 wt.%~0.03 wt.%, Cr: 0.40 wt.%~0.50 wt.%, Nb: 0.01 wt.%~0.03 wt.%, Mo: 0.10 wt.%~0.20 wt.%, V: 0.01 wt.%~0.03 wt.%, B: 0.0008 wt.%~0.0015 wt.%, N≤0.005 wt.% The remaining amount contains Fe and other unavoidable impurities. The refined molten steel is continuously cast to obtain a continuously cast slab. The continuously cast slab is then heated in a furnace at a holding temperature of 1180℃~1260℃, resulting in a furnace exit temperature of 1180℃~1260℃, thus obtaining a heated slab. The heated slab is then rolled to obtain a hot-rolled steel coil. The hot-rolled steel coil is then leveled to obtain a leveled steel strip. The leveled steel strip undergoes single-plate quenching and tempering treatment. The quenching temperature is 870℃~910℃, and the holding time is 20min~60min. The tempering temperature is 540℃~600℃, and the tempering time is 20min~100min, resulting in a low-cost, quenched and tempered high-strength steel with a yield strength of 800MPa.

Description

Technical Field

[0001] This application belongs to the field of steel manufacturing technology, and in particular relates to a low-cost quenched and tempered high-strength steel with a yield strength of 800MPa and its preparation method. Background Technology

[0002] 800MPa grade quenched and tempered steel is widely used in construction machinery, mainly in bulldozers, construction cranes, coal mining machinery, excavators, loaders, and other equipment. These machines operate in harsh environments and under demanding stress conditions, requiring high mechanical properties from the steel. In addition to meeting the tensile properties of the steel plate (yield strength ≥ 800MPa, tensile strength 840~1000MPa, elongation after fracture ≥ 13%), it must also meet additional requirements. The impact toughness at 40℃ is ≥27J. Currently, to ensure strength and impact toughness, 800MPa grade quenched and tempered steel plates typically employ a hot-rolling + quenching and tempering process, which involves numerous steps, high energy consumption, and the addition of alloying elements such as Cu, Ni, Mo, V, Cr, Ti, and B. With increasingly fierce competition in the steel market and significant cost pressures on steel companies, reducing production costs and improving market competitiveness while ensuring the comprehensive performance of 800MPa grade quenched and tempered high-strength steel has become an urgent issue. Summary of the Invention

[0003] This application provides a low-cost quenched and tempered high-strength steel with a yield strength of 800MPa and its preparation method. It can significantly reduce the cost of steel while maintaining the mechanical properties and impact toughness of 800MPa-grade quenched and tempered high-strength steel, and even further improve the mechanical properties and impact resistance of quenched and tempered high-strength steel with a yield strength of 800MPa.

[0004] In a first aspect, this application provides a method for preparing a low-cost, tempered high-strength steel with a yield strength of 800 MPa, comprising: providing refined molten steel, wherein the refined molten steel comprises, by mass percentage, the following components: C: 0.14 wt.%~0.16 wt.%, Si: 0.05 wt.%~0.15 wt.%, Mn: 1.2 wt.%~1.4 wt.%, P≤0.015 wt.%, S≤0.0025 wt.%, Ti: 0.01 wt.%~0.03 wt.%, Cr: 0.40 wt.%~0.50 wt.%, Nb: 0.01 wt.%~0.03 wt.%, Mo: 0.10 wt.%~0.20 wt.%, V: 0.01 wt.%~0.03 wt.%, B: 0.0008 wt.%~0.0 The steel is composed of 0.15 wt.%, N ≤ 0.005 wt.%, with the balance including Fe and other unavoidable impurities. Refined molten steel is continuously cast to obtain a continuously cast slab. The continuously cast slab is then heated in a furnace at a holding temperature of 1180℃~1260℃, resulting in a tapping temperature of 1180℃~1260℃, thus obtaining a heated slab. The heated slab is rolled to obtain a hot-rolled steel coil. The hot-rolled steel coil is leveled to obtain a leveled steel strip. The leveled steel strip undergoes single-plate quenching and tempering treatment. The quenching temperature is 870℃~910℃, and the holding time is 20min~60min. The tempering temperature is 540℃~600℃, and the tempering time is 20min~100min, resulting in a low-cost, quenched and tempered high-strength steel with a yield strength of 800MPa.

[0005] According to an embodiment of the first aspect of this application, the thickness of the continuously cast slab is 230 mm to 240 mm.

[0006] According to the embodiment of the first aspect of this application, the continuous casting slab is heated in a heating furnace for a heating time of 20 min to 100 min, and the total time the continuous casting slab is in the heating furnace is 170 min to 400 min.

[0007] According to an embodiment of the first aspect of this application, before the continuous casting slab is heated in a heating furnace, the method further includes: performing centralized stacking cooling treatment on the continuous casting slab, with a stacking cooling temperature of 500°C to 800°C, and stacking the continuous casting slab to a temperature of ≤500°C.

[0008] According to an embodiment of the first aspect of this application, the heating slab is subjected to rolling treatment, including: roughing the heating slab at a roughing temperature of 1120℃~1220℃ and a roughing temperature of 1040℃~1140℃ to obtain an intermediate slab; finishing the intermediate slab at a finishing temperature of 940℃~1080℃ and a finishing temperature of 860℃~930℃, with a reduction rate of ≥10% in the last pass of the finishing rolling treatment to obtain a finished strip; and then subjecting the finished strip to ultra-rapid cooling treatment, laminar flow cooling treatment, and coiling treatment to obtain a hot-rolled steel coil.

[0009] According to an embodiment of the first aspect of this application, before the heated slab is subjected to rough rolling, the process further includes: descaling the heated slab.

[0010] According to an embodiment of the first aspect of this application, in the step of rough rolling the heated slab, 5 to 7 passes of rolling are used to obtain an intermediate slab.

[0011] According to an embodiment of the first aspect of this application, the thickness of the intermediate billet is 36 mm to 54 mm.

[0012] According to the embodiment of the first aspect of this application, in the step of finishing the intermediate billet, a 7-stand rolling mill is used for continuous finishing rolling.

[0013] According to an embodiment of the first aspect of this application, in the step of finishing rolling the intermediate billet, the descaling water pressure at the inlet of the finishing mill is ≥18MPa.

[0014] According to the embodiment of the first aspect of this application, in the steps of performing ultra-fast cooling treatment and laminar flow cooling treatment on the precision rolled strip, the first stage of cooling adopts ultra-fast cooling with a cooling rate of 80℃ / s~200℃ / s; the second stage adopts cold flow cooling with a cooling rate of ≥15℃ / s, so as to reduce the strip temperature to 500℃~680℃ for coiling treatment.

[0015] According to an embodiment of the first aspect of this application, after obtaining a hot-rolled steel coil, the hot-rolled steel coil can be leveled to obtain a strip of the required length.

[0016] Secondly, this application provides a low-cost quenched and tempered high-strength steel with a yield strength of 800 MPa, comprising the following components by weight percentage: C: 0.14 wt.%–0.16 wt.%, Si: 0.05 wt.%–0.15 wt.%, Mn: 1.2 wt.%–1.4 wt.%, P ≤ 0.015 wt.%, S ≤ 0.0025 wt.%, Ti: 0.01 wt.%–0.03 wt.%, Cr: 0.40 wt.%–0.50 wt.%, Nb: 0.01 wt.%–0.03 wt.%, Mo: 0.10 wt.%–0.20 wt.%, V: 0.01 wt.%–0.03 wt.%, B: 0.0008 wt.%–0.0015 wt.%, N ≤ 0.005 wt.%, with the balance being Fe and other unavoidable impurities.

[0017] According to an embodiment of the second aspect of this application, the mechanical properties of the low-cost quenched and tempered high-strength steel with a yield strength of 800MPa meet the following requirements: yield strength ≥ 800MPa, tensile strength ≥ 840MPa, elongation after fracture ≥ 14%, and impact toughness KV2 ≥ 120J at -40℃.

[0018] The low-cost 800MPa grade quenched and tempered high-strength steel and its preparation method of this application, by adopting a production process of controlled rolling and offline quenching and tempering heat treatment, can significantly reduce the cost of steel while maintaining the mechanical properties and impact toughness of 800MPa grade quenched and tempered high-strength steel, and even further improve the mechanical properties and impact resistance of 800MPa grade quenched and tempered high-strength steel. Attached Figure Description

[0019] To more clearly illustrate the technical solutions of the embodiments of this application, the accompanying drawings used in the embodiments of this application will be briefly introduced below. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0020] Figure 1 This is a schematic flowchart of the preparation method of low-cost quenched and tempered high-strength steel with a yield strength of 800MPa provided in the embodiments of this application.

[0021] Figure 2 This is a metallographic image of a low-cost, tempered high-strength steel with a yield strength of 800 MPa provided in Example 1 of this application, wherein the unit length is 20 μm.

[0022] Figure 3 This is a metallographic image of a low-cost tempered high-strength steel with a yield strength of 800MPa provided in Example 2 of this application, wherein the unit length is 20μm.

[0023] Figure 4This is a metallographic image of a low-cost, tempered high-strength steel with a yield strength of 800 MPa provided in Example 3 of this application, wherein the unit length is 20 μm. Detailed Implementation

[0024] The features and exemplary embodiments of various aspects of this application will be described in detail below. To make the objectives, technical solutions, and advantages of this application clearer, the application will be further described in detail below with reference to the accompanying drawings and specific embodiments. It should be understood that the specific embodiments described herein are only intended to explain this application and not to limit it. For those skilled in the art, this application can be implemented without some of these specific details. The following description of the embodiments is merely to provide a better understanding of this application by illustrating examples.

[0025] It should be noted that, in this document, relational terms such as "first" and "second" are used merely to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Without further limitations, an element defined by the phrase "comprising..." does not exclude the presence of additional identical elements in the process, method, article, or apparatus that includes said element.

[0026] To address the problems of the prior art, this application provides a low-cost method for preparing quenched and tempered high-strength steel with a yield strength of 800 MPa. The method for preparing the low-cost quenched and tempered high-strength steel with a yield strength of 800 MPa provided in this application will be described below.

[0027] Figure 1 A schematic flowchart illustrating the preparation method of low-cost quenched and tempered high-strength steel with a yield strength of 800 MPa provided in this application embodiment is shown. Figure 1The method for preparing low-cost quenched and tempered high-strength steel with a yield strength of 800 MPa, as shown, includes: providing refined molten steel, which, by mass percentage, comprises the following components: C: 0.14 wt.%~0.16 wt.%, Si: 0.05 wt.%~0.15 wt.%, Mn: 1.2 wt.%~1.4 wt.%, P≤0.015 wt.%, S≤0.0025 wt.%, Ti: 0.01 wt.%~0.03 wt.%, Cr: 0.40 wt.%~0.50 wt.%, Nb: 0.01 wt.%~0.03 wt.%, Mo: 0.10 wt.%~0.20 wt.%, V: 0.01 wt.%~0.03 wt.%, B: 0.0008 wt.%~0.0015 wt.%. The steel contains t.%, N≤0.005wt.%, with the balance including Fe and other unavoidable impurities; the refined molten steel is continuously cast to obtain a continuously cast slab; the continuously cast slab is heated in a furnace at a holding temperature of 1180℃~1260℃, resulting in a tapping temperature of 1180℃~1260℃, thus obtaining a heated slab; the heated slab is rolled to obtain a hot-rolled steel coil; the hot-rolled steel coil is leveled to obtain a leveled steel strip; the leveled steel strip is subjected to single-plate quenching and tempering treatment, with a quenching temperature of 870℃~910℃ and a holding time of 20min~60min; the tempering temperature is 540℃~600℃ and the tempering time is 20min~100min, resulting in a low-cost quenched and tempered high-strength steel with a yield strength of 800MPa.

[0028] The method for preparing low-cost quenched and tempered high-strength steel with a yield strength of 800MPa provided in this application adopts a production process of controlled rolling and offline quenching and tempering heat treatment, which makes the produced quenched and tempered high-strength steel with a yield strength of 800MPa have excellent performance and good uniformity, and can meet the increasingly stringent requirements for steel used in engineering machinery.

[0029] The method for preparing low-cost quenched and tempered high-strength steel with a yield strength of 800MPa provided in this application produces quenched and tempered high-strength steel with a yield strength of 800MPa. The resulting steel has a low carbon equivalent, excellent weldability, and a plate thickness of 4mm to 25.4mm. It also exhibits excellent mechanical properties, with a yield strength ≥800MPa, tensile strength ≥840MPa, elongation after fracture ≥14%, and an impact toughness (KV2) ≥120J at -40℃, indicating excellent low-temperature toughness. This method has strong adaptability and promotional value, while also offering high cost-effectiveness and a promising market prospect.

[0030] In some embodiments, the thickness of the continuously cast slab is 230 mm to 240 mm.

[0031] In some embodiments, the continuous casting slab is heated in a heating furnace for a heating time of 20 min to 100 min, and the total time the continuous casting slab spends in the heating furnace is 170 min to 400 min.

[0032] In some embodiments, before the continuous casting slab is heated in a heating furnace, the method further includes: performing centralized stacking cooling on the continuous casting slab, with a stacking cooling temperature of 500°C to 800°C, and stacking the continuous casting slab to ≤500°C.

[0033] In some embodiments, the stack cooling time is ≥8h.

[0034] The method for preparing low-cost quenched and tempered high-strength steel with a yield strength of 800MPa provided in this application embodiment eliminates the internal stress of the continuous casting slab during the manufacturing process and promotes the full diffusion of dissolved hydrogen inside the continuous casting slab before heat treatment, thereby reducing the risk of hydrogen embrittlement of the steel made from the continuous casting slab.

[0035] In some embodiments, the heating slab is subjected to rolling treatment, including: roughing the heating slab at a roughing rolling temperature of 1120℃~1220℃ and a roughing rolling finishing temperature of 1040℃~1140℃ to obtain an intermediate slab; finishing the intermediate slab at a finishing rolling entry temperature of 940℃~1080℃ and a finishing rolling finishing cooling temperature of 860℃~930℃, with a reduction rate of ≥10% in the last pass of the finishing rolling treatment to obtain a finished strip; and subjecting the finished strip to ultra-rapid cooling treatment, laminar flow cooling treatment, and coiling treatment to obtain a hot-rolled steel coil.

[0036] In some embodiments, prior to rough rolling the heated slab, the process further includes descaling the heated slab.

[0037] In some embodiments, the rough rolling process of the heated slab is performed in 5 to 7 passes to obtain an intermediate slab.

[0038] In some embodiments, the thickness of the intermediate blank is 36 mm to 54 mm.

[0039] In some embodiments, the step of finishing the intermediate billet is performed using a 7-stand mill for continuous finishing rolling.

[0040] In some embodiments, during the finishing rolling process of the intermediate billet, the descaling water pressure at the inlet of the finishing mill is ≥18MPa.

[0041] In some embodiments, in the steps of performing ultra-fast cooling treatment and laminar flow cooling treatment on the precision rolled strip, the first stage of cooling adopts ultra-fast cooling with a cooling rate of 80℃ / s~200℃ / s; the second stage adopts cold flow cooling with a cooling rate of ≥15℃ / s, so as to reduce the strip temperature to 500℃~680℃ for coiling treatment.

[0042] In some embodiments, after obtaining the hot-rolled steel coil, the hot-rolled steel coil can be leveled to obtain strip steel of the required length.

[0043] Secondly, this application provides a low-cost quenched and tempered high-strength steel with a yield strength of 800 MPa, comprising the following components by weight percentage: C: 0.14 wt.%–0.16 wt.%, Si: 0.05 wt.%–0.15 wt.%, Mn: 1.2 wt.%–1.4 wt.%, P ≤ 0.015 wt.%, S ≤ 0.0025 wt.%, Ti: 0.01 wt.%–0.03 wt.%, Cr: 0.40 wt.%–0.50 wt.%, Nb: 0.01 wt.%–0.03 wt.%, Mo: 0.10 wt.%–0.20 wt.%, V: 0.01 wt.%–0.03 wt.%, B: 0.0008 wt.%–0.0015 wt.%, N ≤ 0.005 wt.%, with the balance being Fe and other unavoidable impurities.

[0044] The low-cost 800MPa grade quenched and tempered high-strength steel provided in this application embodiment significantly reduces the content of precious metals in its chemical composition. While maintaining the mechanical properties and impact toughness of 800MPa grade quenched and tempered high-strength steel, it can significantly reduce the cost of steel and even further improve the mechanical properties and impact resistance of 800MPa grade quenched and tempered high-strength steel.

[0045] The low-cost quenched and tempered high-strength steel with a yield strength of 800MPa provided in this application has a low C content and a low crack sensitivity index, which meets the welding requirements of Volkswagen.

[0046] In the composition design of the low-cost quenched and tempered high-strength steel with a yield strength of 800MPa provided in the embodiments of this application: Carbon: Carbon is the most economical strengthening element and plays a very important role in improving the strength of steel. However, if the carbon content is too high, it will affect the weldability and impact toughness of the steel. Therefore, this application limits the carbon content to the range of 0.14 wt.% to 0.16 wt.%.

[0047] Silicon: Silicon plays a role in solid solution strengthening in steel. Adding a certain amount of Si to steel can effectively improve its strength. However, silicon can also easily form Fe2SiO4 and form eutectoid products with FeO on the surface of the steel billet, which solidify into an anchor-like structure, making FeO difficult to remove and affecting the final surface quality. Therefore, the silicon content is limited to the range of 0.05wt.% to 0.15wt.%.

[0048] Manganese: Manganese is the most effective element for improving strength and toughness. It can expand the austenite phase region, reduce the critical quenching rate of steel, stabilize austenite, refine grains, and effectively delay the pearlite transformation. If the manganese content is too high, it is detrimental to welding and toughness. In this application, the manganese content is limited to the range of 1.2 wt.% to 1.4 wt.%.

[0049] Phosphorus: Phosphorus is prone to center segregation, which affects molding performance. In this application, the phosphorus content is controlled at ≤0.015 wt.%.

[0050] Sulfur and nitrogen: They easily combine with Ti in steel, affecting the strengthening effect of Ti and greatly affecting the plasticity of steel. In this application, sulfur is controlled at ≤0.0025wt.% and nitrogen is controlled at ≤0.005wt.%.

[0051] Titanium: Titanium is a strong carbide and nitride forming element. Because titanium can refine grains, it can also improve the toughness of steel. Appropriate amounts of titanium form second-phase particles that can inhibit the growth of coarse-grained regions during welding, thus improving the toughness of the weld metal; however, excessive titanium will reduce this. Therefore, this application adds 0.01 wt.% to 0.03 wt.% titanium to improve the toughness of the steel and the weld.

[0052] Chromium: Chromium is a carbide-forming element with a strong affinity for carbon, which hinders the diffusion of carbon atoms. Combined with the effects of manganese, this significantly delays the transformation of pearlite and bainite. In this application, the chromium content is controlled at 0.40 wt.% to 0.50 wt.%.

[0053] Niobium: Niobium has a strong grain refinement and precipitation strengthening effect. A small amount of niobium contributes greatly to the strength, but it is expensive. Therefore, the niobium content in this application is set to 0.01 wt.% to 0.03 wt.%.

[0054] Molybdenum: Molybdenum is an element that narrows the austenite phase region. In steel, it can dissolve in ferrite, austenite, and carbides, improving the hardenability and hot strength of the steel. Molybdenum can form molybdenum carbide with carbon, which has a secondary hardening effect, improving the hardness and wear resistance of steel. Molybdenum has a solid solution strengthening effect on ferrite, improving the strength of steel. Molybdenum can improve the corrosion resistance of steel in acid and alkali solutions and liquid metals, making it suitable for applications with severe corrosion, such as chemical equipment. When present as a single alloying element, molybdenum increases the temper brittleness of steel, but when coexisting with chromium, manganese, etc., it can reduce or inhibit the temper brittleness caused by other elements. In this application, the chromium content is controlled at 0.10 wt.%~0.20 wt.%.

[0055] Vanadium combines with carbon, nitrogen, and oxygen in steel to form refractory metallic carbides, nitrides, and oxides. These compounds help refine the microstructure of steel, improve its tempering stability, and produce a secondary hardening effect. The addition of vanadium can reduce the overheating sensitivity of steel and improve its wear resistance and corrosion resistance, especially at high temperatures. Furthermore, vanadium can improve the physical, chemical, and processing properties of steel. Therefore, this application adds 0.01 wt.% to 0.03 wt.% vanadium.

[0056] Boron: The addition of boron to steel improves the hardenability of the steel plate and forms bainitic or martensitic structures. When the boron content is high, boron atoms will accumulate at the grain boundaries, reducing the grain boundary binding energy, thus causing intergranular dissociation fracture under impact. Therefore, the amount of boron added in this application is 0.0008 wt.% to 0.0015 wt.%.

[0057] In addition to limiting the range of the above chemical components, from the point of view of improving the formability and economy of materials, this application does not add precious alloying elements such as Ni.

[0058] In some embodiments, the mechanical properties of low-cost quenched and tempered high-strength steel with a yield strength of 800MPa meet the following requirements: yield strength ≥ 800MPa, tensile strength ≥ 840MPa, elongation after fracture ≥ 14%, and impact toughness KV2 ≥ 120J at -40℃.

[0059] The technical solutions and effects of this application will be further explained below through specific embodiments and comparative examples.

[0060] Examples 1-3 and Comparative Example 1 Examples 1-3 and Comparative Example 1 all provide a quenched and tempered high-strength steel with a yield strength of 800 MPa. The preparation method of the quenched and tempered high-strength steel with a yield strength of 800 MPa in Examples 1-3 includes: Refined molten steel is provided. The composition and content of the refined molten steel in Examples 1-3 are shown in Table 1 below. Table 1. Composition control of molten steel in Examples 1-3 and Comparative Example 1 Refined molten steel is continuously cast to obtain continuously cast slabs; The continuous casting slab is subjected to centralized stacking cooling treatment, which gradually cools the slab from 800℃ to 500℃. The stacking cooling time is ≥8h, and the temperature of the continuous casting slab is ≤500℃.

[0061] The continuously cast slab is heated in a heating furnace at a holding temperature of 1180℃~1260℃, so that the slab exits the furnace at a temperature of 1180℃~1260℃, thus obtaining a heated slab. The rolling process for the heated slab includes: descaling before rough rolling with a descaling water pressure of 18 MPa; rough rolling at 1120℃~1220℃ for 5-7 passes, with a final rolling temperature of 1040℃~1140℃ to obtain an intermediate slab; finishing rolling on a 7-stand finishing mill at 940℃~1080℃ for 7 passes, with a final cooling temperature of 860℃~930℃, and a reduction rate of ≥10% in the final pass to obtain a finished strip; and finally, ultra-rapid cooling, laminar flow cooling, and coiling to obtain a hot-rolled steel coil. Hot-rolled steel coils are leveled to obtain leveled steel strips; Single-plate quenching and tempering treatments were performed on the leveled steel strip. The quenching temperature was 870℃~910℃, and the quenching holding time was 20min~60min. The tempering temperature was 540℃~600℃, and the tempering time was 20min~100min, resulting in low-cost quenched and tempered high-strength steel with a yield strength of 800MPa. The main process parameters for each stage from heating the continuously cast slab to quenching and tempering are shown in Tables 2 and 3.

[0062] Table 2. Main process parameters of the preparation methods of Examples 1-3 and Comparative Example 1 Table 3. Main process parameters of the preparation methods of Examples 1-3 and Comparative Example 1 The 800MPa yield strength grade quenched and tempered high-strength steel of Comparative Example 1 was prepared according to the following steps: Comparative Example 1 provides a 800MPa yield strength grade quenched and tempered high-strength steel, wherein the preparation method of the 800MPa yield strength grade quenched and tempered high-strength steel of Comparative Example 1 adopts the existing technology, including: providing refined molten steel, the composition and content of the refined molten steel of Comparative Example 1 are shown in Table 1. Refined molten steel is continuously cast to obtain continuously cast slabs; The continuous casting slab is subjected to centralized stacking cooling treatment, which gradually cools the slab from 800℃ to 500℃. The stacking cooling time is ≥8h, and the temperature of the continuous casting slab is ≤500℃.

[0063] The continuously cast slab is heated in a heating furnace at a holding temperature of 1200℃~1260℃, so that the exit temperature of the continuously cast slab is 1200℃~1260℃, thus obtaining a heated slab. The rolling process for the heated slab includes: before rough rolling, descaling is performed on the heated slab at a water pressure of 18 MPa; rough rolling is performed on the heated slab at an initial rolling temperature of 1120℃~1220℃, with 5-7 passes, and a final rolling temperature of 1040℃~1140℃ to obtain an intermediate slab; the intermediate slab is then finished using a 7-stand finishing mill at an initial rolling temperature of 940℃~1080℃, with 7 consecutive passes, and a final cooling temperature of 860℃~930℃, with a reduction rate of ≥10% in the final pass to obtain a finished strip; the finished strip is then subjected to ultra-rapid cooling and laminar flow cooling, with a coiling temperature of 500~580℃, and after cooling, the strip is coiled to obtain a hot-rolled steel coil. Hot-rolled steel coils are leveled to obtain leveled steel strips; Single-plate quenching and tempering treatments were performed on the leveled steel strip. The quenching temperature was 870℃~910℃, and the quenching holding time was 20min~60min. The tempering temperature was 540℃~600℃, and the tempering time was 20min~100min, resulting in low-cost quenched and tempered high-strength steel with a yield strength of 800MPa. The main process parameters for each stage from heating the continuously cast slab to quenching and tempering are shown in Tables 2 and 3.

[0064] The quenched and tempered high-strength steels with a yield strength of 800 MPa in Examples 1-3 and Comparative Example 1 were tested for tensile and impact properties according to GB / T 228.1 Metallic materials - Tensile testing - Part 1: Test method at room temperature and GB / T 229 Metallic materials - Charpy pendulum impact test method. The test results are shown in Table 4 below.

[0065] Table 4 The microstructure of the tempered high-strength steel with a yield strength of 800 MPa in Comparative Example 1 is tempered sorbite. Figure 2 , Figure 3 , Figure 4The metallographic images of the low-cost yield strength 800MPa grade quenched and tempered high-strength steel corresponding to Examples 1-3 show that the metallographic structure of the low-cost yield strength 800MPa grade quenched and tempered high-strength steel is tempered sorbite. Furthermore, the method for preparing the low-cost yield strength 800MPa grade quenched and tempered high-strength steel of this application can effectively reduce costs by more than 33% for steel of the same specifications and grade. As shown in Table 4 above, the method for preparing the yield strength 800MPa grade quenched and tempered high-strength steel provided in this application produces a yield strength ≥ 800MPa, a tensile strength ≥ 840MPa, and an elongation after fracture ≥ 14%, meeting the impact energy requirement of KV2 ≥ 100J at -40℃. It exhibits high low-temperature impact toughness and high mechanical strength, and has high application value.

[0066] The above description is merely a specific implementation of this application. Those skilled in the art will clearly understand that, for the sake of convenience and brevity, the specific working process described above can be referred to the corresponding process in the foregoing method embodiments, and will not be repeated here. It should be understood that the protection scope of this application is not limited thereto. Any person skilled in the art can easily conceive of various equivalent modifications or substitutions within the technical scope disclosed in this application, and these modifications or substitutions should all be covered within the protection scope of this application.

Claims

1. A method for preparing low-cost quenched and tempered high-strength steel with a yield strength of 800 MPa, characterized in that, include: We supply refined steel, which, by mass percentage, comprises the following components: C: 0.14wt.%~0.16wt.%, Si: 0.05wt.%~0.15wt.%, Mn: 1.2wt.%~1.4wt.%, P≤0.015wt.%, S≤0.0025wt.%, Ti: 0.01wt.%~0.03wt.%, Cr: 0.40wt.%~0.50wt.%, Nb: 0.01wt.%~0.03wt.%, Mo: 0.10wt.%~0.20wt.%, V: 0.01wt.%~0.03wt.%, B: 0.0008wt.%~0.0015wt.%, N≤0.005wt.%, balance includes Fe and other unavoidable impurities; Refined molten steel is continuously cast to obtain continuously cast slabs; The continuously cast slab is heated in a heating furnace at a holding temperature of 1180℃~1260℃, so that the slab exits the furnace at a temperature of 1180℃~1260℃, thus obtaining a heated slab. The heated slab is rolled to obtain hot-rolled steel coils; Hot-rolled steel coils are leveled to obtain leveled steel strips; The flattened steel strip is subjected to single-plate quenching and tempering treatment. The quenching temperature is 870℃~910℃ and the quenching holding time is 20min~60min. The tempering temperature is 540℃~600℃ and the tempering time is 20min~100min, resulting in low-cost tempered high-strength steel with a yield strength of 800MPa.

2. The preparation method according to claim 1, characterized in that, The thickness of the continuously cast slab is 230mm to 240mm; The heating time for the continuous casting slab in the heating furnace is 20 min to 100 min, and the total time the continuous casting slab is in the heating furnace is 170 min to 400 min. Optionally, before the continuously cast slab undergoes heat treatment in a heating furnace, the process further includes: The continuously cast slabs are subjected to centralized stacking cooling treatment at a temperature of 500℃~800℃, and the temperature of the continuously cast slabs is stacked to ≤500℃.

3. The preparation method according to claim 1, characterized in that, The rolling process of the heated slab includes: The heated slab is subjected to rough rolling treatment. The rough rolling start temperature is 1120℃~1220℃, and the rough rolling finish temperature is 1040℃~1140℃ to obtain an intermediate slab. The intermediate billet is subjected to finish rolling. The entry temperature of finish rolling is 940℃~1080℃, the final cooling temperature of finish rolling is 860℃~930℃, and the reduction rate of the last pass of finish rolling is ≥10% to obtain finish rolled strip. The finished strip steel is subjected to ultra-rapid cooling treatment, laminar flow cooling treatment, and coiling treatment to obtain hot-rolled steel coils.

4. The preparation method according to claim 3, characterized in that, Before the heating slab is subjected to rough rolling, the process also includes descaling the heating slab.

5. The preparation method according to claim 3, characterized in that, In the step of rough rolling the heated slab, 5 to 7 rolling passes are used to obtain an intermediate slab; Optionally, the thickness of the intermediate blank is 36mm to 54mm.

6. The preparation method according to claim 3, characterized in that, In the step of finishing the intermediate billet, a 7-stand rolling mill is used for continuous finishing rolling.

7. The preparation method according to claim 3, characterized in that, In the step of finishing rolling the intermediate billet, the descaling water pressure at the inlet of the finishing mill is ≥18MPa.

8. The preparation method according to claim 3, characterized in that, In the steps of performing ultra-fast cooling and laminar flow cooling on the precision rolled strip, the first stage of cooling adopts ultra-fast cooling with a cooling rate of 80℃ / s to 200℃ / s; the second stage adopts cold flow cooling with a cooling rate of ≥15℃ / s, so as to reduce the strip temperature to 500℃ to 680℃ for coiling.

9. A low-cost, tempered high-strength steel with a yield strength of 800 MPa, characterized in that, The low-cost quenched and tempered high-strength steel with a yield strength of 800 MPa, prepared according to any one of claims 1-9, comprises, by mass percentage, the following components: C: 0.14 wt.%–0.16 wt.%, Si: 0.05 wt.%–0.15 wt.%, Mn: 1.2 wt.%–1.4 wt.%, P ≤ 0.015 wt.%, S ≤ 0.0025%. wt.%, Ti: 0.01wt.%~0.03wt.%, Cr: 0.40wt.%~0.50wt.%, Nb: 0.01wt.%~0.03wt.%, Mo: 0.10wt.%~0.20wt.%, V: 0.01wt.%~0.03wt.%, B: 0.0008wt.%~0.0015wt.%, N≤0.005wt.%, balance includes Fe and other unavoidable impurities.

10. The low-cost, tempered high-strength steel with a yield strength of 800 MPa according to claim 9, characterized in that, The mechanical properties of the low-cost quenched and tempered high-strength steel with a yield strength of 800MPa meet the following requirements: yield strength ≥ 800MPa, tensile strength ≥ 840MPa, elongation after fracture ≥ 14%, and impact toughness KV2 ≥ 120J at -40℃.

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