A low yield ratio 1300MPa grade hot-rolled coil and its manufacturing method

Abstract

This invention belongs to the field of iron and steel metallurgy technology, specifically relating to a low yield strength ratio (1300MPa) hot-rolled coil and its manufacturing method. The low yield strength ratio (1300MPa) hot-rolled coil provided by this invention comprises, by weight percentage: C 0.20-0.30%, Si 1.2-1.8%, Mn 0.8-1.2%, P≤0.020%, S≤0.040%, Nb 0.02-0.10%, Al 0.02-0.06%, with the remainder being Fe and unavoidable impurities. The low yield strength ratio (1300MPa) hot-rolled coil of this invention exhibits a microstructure containing both high-strength martensite and soft ferrite, resulting in advantages such as high strength, low yield strength ratio, and good machinability.

Description

Technical Field

[0001] This invention belongs to the field of iron and steel metallurgy technology, specifically relating to a low yield strength ratio 1300MPa grade hot-rolled coil and its manufacturing method. Background Technology

[0002] High strength is one of the development directions of steel for engineering machinery. 1300MPa grade hot-rolled high-strength steel is mainly strengthened by martensitic structure. Since the traditional hot continuous rolling process cannot solve the matching problem of the performance and plate shape of single-phase martensitic structure, it is not possible to produce hot-rolled coils with tensile strength of 800MPa or above. Generally, quenching + tempering heat treatment process is used for production, but it has disadvantages such as adding more alloying elements, long process flow and low efficiency.

[0003] In recent years, by developing ferrite + martensite dual-phase hot-rolled coils, the martensitic structure is used to ensure the strength of the material, while the soft ferrite structure absorbs part of the martensitic phase transformation stress and reduces the yield point, thus improving the straightening plate shape and solving the problem of matching performance and plate shape.

[0004] Chinese invention patent CN110592347A discloses a method for producing 1200MPa grade multiphase steel coils with online heat treatment, which requires a slow cooling process to control the cooling rate, resulting in low efficiency. Chinese invention patent CN115198178A discloses a 1200MPa grade hot-rolled ultra-high-strength steel plate and its manufacturing method, which incorporates a significant amount of Mn and Cr in its composition, leading to higher alloy costs. Summary of the Invention

[0005] The purpose of this invention is to address the shortcomings of existing technologies by providing a low yield strength ratio of 1300MPa hot-rolled coil and its manufacturing method.

[0006] The low yield strength ratio 1300MPa grade hot-rolled coil provided by the present invention comprises, by weight percentage: C 0.20-0.30%, Si 1.2-1.8%, Mn 0.8-1.2%, P≤0.020%, S≤0.040%, Nb 0.02-0.10%, Al 0.02-0.06%, with the remainder being Fe and unavoidable impurities.

[0007] The aforementioned low yield strength ratio 1300MPa grade hot-rolled coil, by weight percentage, comprises: C 0.22%–0.27%, Si 1.2%–1.5%, Mn 0.8%–1.0%, P≤0.020%, S≤0.040%, Nb 0.03%–0.06%, Al 0.02%–0.04%, with the remainder being Fe and unavoidable impurities.

[0008] On the other hand, the present invention also provides a method for manufacturing the above-mentioned low yield strength ratio 1300MPa grade hot-rolled coil, comprising:

[0009] (1) Pretreated molten steel is smelted in a converter and refined outside the ladle before being continuously cast into slabs;

[0010] (2) The slab is subjected to slab heating, hot rolling, post-rolling cooling, coiling, leveling and slitting processes in sequence to obtain a hot-rolled coil with a low yield strength ratio of 1300MPa.

[0011] In the above-mentioned method for manufacturing hot-rolled coils with a low yield strength ratio of 1300MPa, the slab is heated at a temperature of 1260–1350°C.

[0012] The manufacturing method of the above-mentioned low yield strength ratio 1300MPa grade hot-rolled coil, wherein the final rolling temperature of hot rolling is 810~900℃ and the finishing rolling speed is 5~6m / s.

[0013] The above-mentioned manufacturing method for hot-rolled coils with a low yield strength ratio of 1300MPa uses laminar flow cooling after rolling. In the laminar flow cooling, the first stage is not water-cooled and the air cooling time is guaranteed to be 10-15s. The second stage is strong cooling with a cooling rate of ≥80℃ / s to cool the steel strip to below 100℃.

[0014] In the above-mentioned method for manufacturing hot-rolled coils with a low yield strength ratio of 1300MPa, the coiling temperature is <100℃.

[0015] The manufacturing method of the above-mentioned low yield strength ratio 1300MPa grade hot-rolled coil, wherein the leveling tension is ≥5Kg / mm 2 Leveling pressure ≥1000 tons.

[0016] In another aspect, the present invention also provides a hot-rolled coil with a low yield strength ratio of 1300MPa, which is manufactured using the above-described manufacturing method.

[0017] The aforementioned low yield strength ratio 1300MPa grade hot-rolled coil has a yield strength ReL≥800MPa, tensile strength Rm≥1300MPa, elongation A≥9%, yield strength ratio≤0.7, and metallographic structure of 10~20% ferrite + 80~90% martensite.

[0018] The technical solution of the present invention has the following beneficial effects:

[0019] (1) This invention relies on a hot rolling mill unit, and by designing new composition and rolling process, and using laminar flow cooling online quenching, it obtains a 1300MPa grade hot-rolled coil with a metallographic structure of 10-20% ferrite + 80-90% martensite. The martensite structure ensures the material strength, while the soft ferrite structure absorbs part of the martensitic phase transformation stress and reduces the yield point, thus improving the straightening plate shape and solving the matching problem of performance and plate shape.

[0020] (2) The low yield strength ratio 1300MPa grade hot-rolled coil of the present invention has a yield strength ReL≥800MPa, tensile strength Rm≥1300MPa, elongation A≥9%, yield strength ratio≤0.7, and flatness of steel plate≤5mm / m;

[0021] (3) The low yield strength ratio 1300MPa grade hot-rolled coil of the present invention has the characteristics of low alloy cost, short production process and good plate shape;

[0022] (4) The low yield strength ratio 1300MPa grade hot-rolled coil of the present invention has a metallographic structure that contains both high-strength martensite and soft ferrite, and has the advantages of high strength, low yield strength ratio and good processing performance.

[0023] (5) The method for manufacturing hot-rolled coils with a low yield strength ratio of 1300MPa of the present invention uses a hot continuous rolling mill for production. Compared with the quenching + tempering heat treatment process, the process flow is shorter, the efficiency is higher, and the energy consumption is lower. Attached Figure Description

[0024] Various other advantages and benefits will become apparent to those skilled in the art upon reading the following detailed description of preferred embodiments. The accompanying drawings are for illustrative purposes only and are not intended to limit the invention.

[0025] Figure 1 This is a metallographic diagram of the 1300MPa grade hot-rolled coil with low yield strength ratio of the present invention. Detailed Implementation

[0026] To fully understand the purpose, features, and effects of this invention, the following detailed embodiments are provided. Except as described below, the process methods of this invention employ conventional methods or apparatus in the art. Unless otherwise specified, the terms and expressions used below have the meanings commonly understood by those skilled in the art.

[0027] When a range of values ​​is disclosed herein, the range is considered continuous and includes the minimum and maximum values ​​of the range, as well as every value between the minimum and maximum values. Furthermore, when the range refers to integers, it includes every integer between the minimum and maximum values ​​of the range. Additionally, when multiple ranges are provided to describe a feature or characteristic, the ranges may be combined. In other words, unless otherwise specified, all ranges disclosed herein should be understood to include any and all subranges to which they are incorporated.

[0028] The low yield strength ratio 1300MPa grade hot-rolled coil of this invention uses C, Si, Mn, and Nb as the main elements, without adding alloys such as Cr and Mo, resulting in low cost. Through ladle refining control, vacuum degassing and continuous casting, as well as control of heating temperature, final rolling temperature, coiling temperature, and post-coiling leveling and unleveling, a high-strength, well-shaped, and high-performance low yield strength ratio 1300MPa grade hot-rolled coil is obtained. Its microstructure is 10-20% ferrite + 80-90% martensite. The martensite microstructure ensures the material strength, while the soft ferrite microstructure absorbs some of the martensitic phase transformation stress and lowers the yield point, improving the straightened plate shape, thereby solving the problem of matching performance and plate shape.

[0029] The low yield strength ratio 1300MPa grade hot-rolled coil provided by the present invention comprises, by weight percentage: C 0.20-0.30%, Si 1.2-1.8%, Mn 0.8-1.2%, P≤0.020%, S≤0.040%, Nb 0.02-0.10%, Al 0.02-0.06%, with the remainder being Fe and unavoidable impurities.

[0030] This invention, in hot-rolled coils with a low yield strength ratio of 1300 MPa, incorporates the ferrite-forming element Si. This ensures the smooth formation of 10-20% ferrite during the initial laminar cooling stage while preventing carbide precipitation and maintaining a stable austenite concentration of carbon, thus guaranteeing the formation of high-strength martensite during the subsequent rapid cooling process. Simultaneously, solid solution strengthening element Mn and grain refinement strengthening element Nb are added to compensate for the overall strength loss caused by the ferrite structure. Therefore, through the synergistic effect of these specific elements in specific amounts, the material achieves a balance between good strength and toughness.

[0031] The specific roles and contents of each component in the low yield strength ratio 1300MPa grade hot-rolled coil of this invention are described below:

[0032] The carbon (C) content of this invention is 0.20%–0.30%. C is the most economical strengthening element in steel and also a highly hardenable element. It directly affects the volume fraction of martensite and the carbon content of martensite in duplex steel. Increasing the C content shifts the CCT diagram to the right, reducing the ferrite transformation amount and relatively increasing the martensite transformation amount, thus increasing the strength of the steel and decreasing its plasticity. However, too low a C content will reduce the total amount of martensite, resulting in insufficient strength of the steel. The optimal C content is 0.22%–0.27%.

[0033] The silicon (Si) content of this invention is 1.2% to 1.8%. Silicon is an element that expands the two-phase region, improves the stability of austenite during cooling after hot rolling, accelerates the precipitation of ferrite, promotes carbon enrichment in austenite, and increases the hardness of martensite. Too low a Si content will not achieve the above effects, while too high a Si content will deteriorate the weldability and surface quality of the steel. The optimal Si content is 1.2% to 1.5%.

[0034] The manganese (Mn) content of this invention is 0.8% to 1.2%. Mn can improve the yield strength and tensile strength of steel, playing a role in solid solution strengthening. However, excessive Mn content will reduce the plasticity of steel. The most preferred Mn content is 0.8% to 1.0%.

[0035] The niobium (Nb) content of this invention is 0.02–0.10%, mainly used for grain refinement and precipitation strengthening to improve material strength. Nb is more stable to recover than Ti, and the optimal Nb content is 0.03%–0.06%.

[0036] The aluminum (Al) content of this invention is 0.02% to 0.06%. The main function of aluminum is to remove oxygen from molten steel. When the aluminum content is greater than the maximum value of this invention, it will reduce the fluidity of the molten steel; when the aluminum content is less than the minimum value of this invention, the deoxidation in the steel is insufficient, and subcutaneous bubbles are easily formed in the cast billet. The most preferred Al content is 0.02% to 0.04%.

[0037] The phosphorus (P) content of this invention is ≤0.020% and the sulfur (S) content is ≤0.040%. As harmful elements in steel, the lower the content of P and S, the better.

[0038] On the other hand, the present invention also provides a method for manufacturing a hot-rolled coil with a low yield strength ratio of 1300MPa, comprising the following steps: hot metal pretreatment → converter smelting → ladle refining (LF+RH) → continuous casting → slab heating → hot rolling → post-rolling cooling → coiling → leveling → slitting → packaging and marking → warehousing.

[0039] In some preferred embodiments, the slab is heated to a temperature of 1260–1350°C. By controlling the heating temperature to 1260–1350°C, and by rationally setting the heating temperature, it is possible to ensure that the precipitates are fully dissolved and that the austenite grains formed after rolling recrystallization are uniform and fine, thereby effectively avoiding insufficient strength caused by coarse grains after rolling.

[0040] In some preferred embodiments, a multi-stand hot rolling mill is used to roll the slab to the target thickness (4–12 mm) at a final rolling temperature of 810–900°C. A suitable final rolling temperature not only ensures a good slab shape but also avoids coarse grains. In practice, a lower final rolling temperature results in a higher rolling load, which is detrimental to slab shape quality; while a higher final rolling temperature leads to coarse grains, which is detrimental to strength improvement.

[0041] In some preferred embodiments, the finishing rolling speed is controlled at 5–6 m / s to obtain a ferrite + martensite microstructure with a reasonable ratio. In practice, rolling speeds that are too fast result in insufficient ferrite formation time, leading to a low ferrite ratio, excessively high yield strength, and significant difficulty in subsequent shape improvement. Conversely, rolling speeds that are too slow result in excessive ferrite formation time, leading to a low martensite ratio and insufficient strength.

[0042] In some preferred embodiments, laminar cooling employs a non-water-cooled front section and a strong water-cooled rear section. Specifically, the non-water-cooled front section, due to the steel strip finishing rolling speed of 5-6 m / s, ensures that the air cooling time of the steel strip is 10-15 s by arranging the starting position of the laminar cooling in the rear section, ensuring that 10-20% of the ferrite transformation is completed at 700-900℃. If the air cooling time is too short, the ferrite formation time is insufficient, resulting in a low ferrite content; conversely, if it is too long, there will be an excess of ferrite. The strong water cooling in the rear section cools the steel strip to below 100℃ at a cooling rate ≥80℃ / s, i.e., the coiling temperature <100℃, so that the remaining austenite is rapidly and completely transformed into martensite. In practice, if the coiling temperature is too high, it will lead to insufficient or no martensite transformation, resulting in insufficient strength.

[0043] After the steel coil is cooled and wound, it is leveled. Due to the thinness and high strength of this steel grade, controlling the sheet shape is difficult. Simply controlling the leveling pressure during the leveling process cannot effectively improve the sheet shape. Therefore, a high-tension, high-pressure leveling process was designed based on the product characteristics. In some preferred embodiments, the leveling tension is ≥5 kg / mm. 2 The leveling pressure is ≥1000 tons, which makes the shape of the steel strip stable and controllable after leveling.

[0044] The low yield strength ratio 1300MPa grade hot-rolled coil manufactured according to the method of the present invention has a yield strength ReL≥800MPa, tensile strength Rm≥1300MPa, elongation A≥9%, yield strength ratio≤0.7, and its metallographic structure is 10-20% ferrite + 80-90% martensite. Figure 1 ).

[0045] Example

[0046] The present invention is further illustrated below by way of embodiments, but the invention is not limited to the scope of the embodiments described herein. Experimental methods in the following embodiments, unless otherwise specified, are performed according to conventional methods and conditions.

[0047] Example 1

[0048] This embodiment uses a hot-rolled coil with a low yield strength ratio of 1300MPa. The steel plate is 5mm thick and 1600mm wide, and its chemical composition by mass percentage is as follows:

[0049] C: 0.25%, Si: 1.30%, Mn: 1.02%, P: 0.012%, S: 0.015%, Nb: 0.04%, Al 0.03%, with the remainder being Fe and unavoidable impurities.

[0050] The low-cost HB400 grade duplex wear-resistant hot-rolled coil manufacturing method of this embodiment includes the following sequential steps:

[0051] (1) Smelting and continuous casting processes

[0052] After being smelted in a converter and refined in a ladle (LF+RH), it is continuously cast into slabs.

[0053] (2) Hot rolling process

[0054] The slab is heated to 1280℃ and rolled to the target thickness of 5mm using a multi-stand hot continuous rolling mill. The final rolling temperature is 810℃. Laminar flow cooling is achieved by using no water cooling in the front section and strong water cooling in the back section. The coiling temperature is <100℃.

[0055] (3) Leveling process

[0056] After the steel coil is cooled and coiled, it is leveled using a high-tension, high-pressure leveling process, with a leveling tension of 5 kg / mm. 2 The leveling pressure is 1050 tons.

[0057] (4) Leveling process

[0058] After the steel strip passes through a coarse straightening machine and a fine straightening machine to eliminate internal stress, it is flattened into a steel plate with a length of 6200mm.

[0059] Performance test results: The yield strength ReL of the steel plate is 893MPa, the tensile strength Rm is 1355MPa, the elongation A is 11%, the yield strength ratio is 0.66, and the flatness of the steel plate is 3mm / m.

[0060] Examples 2-5

[0061] Low yield strength ratio 1300MPa grade hot-rolled coils were prepared according to the method of Example 1. The steel plate composition of each example is shown in Table 1, the process parameters used in the preparation process of each example are shown in Table 2, and the performance test results of the prepared steel plates are shown in Table 3.

[0062] Table 1 Summary of steel plate composition in Examples 2-5 (unit: wt%)

[0063] C Si Mn P S Nb Al Example 2 0.23 1.27 1.10 0.008 0.013 0.027 0.03 Example 3 0.25 1.30 1.05 0.009 0.011 0.025 0.04 Example 4 0.25 1.30 1.05 0.009 0.011 0.025 0.04 Example 5 0.24 1.33 0.98 0.011 0.013 0.023 0.04

[0064] Table 2. Process parameters used in Examples 2-5

[0065]

[0066] Table 3. Test results of steel plate performance in Examples 2-5

[0067]

[0068]

[0069] The present invention has been disclosed above with reference to preferred embodiments. However, those skilled in the art should understand that these embodiments are merely illustrative of the invention and should not be construed as limiting its scope. It should be noted that any variations and substitutions equivalent to these embodiments should be considered to be covered within the scope of the claims. Therefore, the scope of protection of the present invention should be determined by the scope defined in the claims.

Claims

1. A hot-rolled coil with a low yield strength ratio of 1300 MPa, characterized in that, By weight percentage, it includes: C 0.20-0.30%, Si 1.2-1.8%, Mn 0.8-1.2%, P≤0.020%, S≤0.040%, Nb 0.02-0.10%, Al 0.02-0.06%, with the remainder being Fe and unavoidable impurities; The method for preparing the low yield strength ratio 1300MPa grade hot-rolled coil includes: (1) Pretreated molten steel is smelted in a converter and refined outside the ladle before being continuously cast into slabs; (2) The slab is subjected to slab heating, hot rolling, post-rolling cooling, coiling, leveling and slitting processes in sequence to obtain a hot-rolled coil with a low yield strength ratio of 1300MPa. The slab is heated to a temperature of 1260–1350℃; the hot rolling temperature is 810–900℃ and the finishing rolling speed is 5–6 m / s; the post-rolling cooling adopts laminar flow cooling, wherein the first stage of laminar flow cooling is not water-cooled and the air cooling time is guaranteed to be 10–15 s, and the latter stage is strongly cooled to below 100℃ at a cooling rate of ≥80℃ / s; The low yield strength ratio 1300MPa grade hot-rolled coil has a yield strength ReL≥800MPa, tensile strength Rm≥1300MPa, elongation A≥9%, yield strength ratio≤0.7, and metallographic structure of 10~20% ferrite + 80~90% martensite.

2. The low yield strength ratio 1300MPa grade hot-rolled coil according to claim 1, characterized in that, By weight percentage, it includes: C 0.22%–0.27%, Si 1.2%–1.5%, Mn 0.8%–1.0%, P≤0.020%, S≤0.040%, Nb 0.03%–0.06%, Al 0.02%–0.04%, with the remainder being Fe and unavoidable impurities.

3. The method for manufacturing the low yield strength ratio 1300MPa grade hot-rolled coil as described in claim 1 or 2, characterized in that, include: (1) Pretreated molten steel is smelted in a converter and refined outside the ladle before being continuously cast into slabs; (2) The slab is subjected to slab heating, hot rolling, post-rolling cooling, coiling, leveling and slitting processes in sequence to obtain a hot-rolled coil with a low yield strength ratio of 1300MPa.

4. The method for manufacturing a low yield strength ratio (1300 MPa) grade hot-rolled coil according to claim 3, characterized in that, The winding temperature is <100℃.

5. The method for manufacturing a low yield strength ratio (1300 MPa) grade hot-rolled coil according to claim 3, characterized in that, The flatness tension is ≥5Kg / mm. 2 Leveling pressure ≥1000 tons.

6. A hot-rolled coil with a low yield strength ratio of 1300 MPa, characterized in that, It is manufactured using the manufacturing method described in any one of claims 3-5.

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