Titanium microalloying Q355B low-alloy high-strength structural steel strip production method

Abstract

The invention discloses a production method of a titanium microalloying Q355B low-alloy high-strength structural steel strip. The production method comprises the steps of KR molten iron pretreatment, converter smelting, LF refining, continuous casting, heating, rolling and cooling. The steel strip comprises the following chemical components in percentage by mass: 0.05 to 0.07 percent of C, 0.20 to 0.30 percent of Si, 0.60 to 0.90 percent of Mn, less than or equal to 0.020 percent of P, less than or equal to 0.009 percent of S, 0.17 to 0.27 percent of Als, 0.04 to 0.06 percent of Ti, less than or equal to 0.0050 percent of N and the balance of Fe. And the balance of iron and inevitable impurities. The invention aims to provide a production method of a titanium microalloying Q355B low-alloy high-strength steel strip which is stable in production process, excellent in performance, good in plate shape and free of longitudinal crack defects by adopting a continuous casting and rolling process.

Description

Technical Field

[0001] The invention belongs to the technical field of hot-rolled steel strips in a CSP production line, and in particular relates to a production method of a titanium microalloyed Q355B low-alloy high-strength structural steel strip. Background Art

[0002] Q355B low alloy high strength steel strip has high strength, good cold deformation ability, good plasticity and toughness. It is widely used in engineering structures, mining machinery, steel pipes and other fields. It accounts for more than 60% of the output of low alloy steel plates. Therefore, it is also one of the steel types commonly produced in batches by major domestic steel companies. The national standard specifies the weight content of some elements of Q355B steel, among which the C content is not more than 0.24%, Si is not more than 0.55%, Mn is not more than 1.6%, P is not more than 0.035%, S is not more than 0.035%, Cr is not more than 0.3%, Ni is not more than 0.3%, Cu is not more than 0.4%, and N is not more than 0.012%. Whether it is the Q345B steel before the upgrade or the new national standard Q355B steel currently implemented, the C-Mn-based composition system is basically adopted in the design. Mn is mainly used for solid solution strengthening in steel. On the one hand, a higher manganese content increases the production cost. On the other hand, the main components of Q355B steel produced by conventional casting machines are C, Si, Mn, P, and S, and most of the Mn content is above 1.0%. Due to the design of the continuous casting crystallizer of the CSP production line, when casting ordinary carbon manganese steel with a carbon content between 0.17 and 0.20%, longitudinal cracks on the surface of the ingot and steel strip are prone to occur, which has a great limitation on the increase in production and quality stability. In addition, MnS inclusions are easily generated in the steel, causing defects such as cold bending cracking, affecting the application performance.

[0003] Titanium is one of the important microalloying elements in steel. Under normal circumstances, the presence of titanium in steel can significantly refine the grains, and the precipitation strengthening effect of its carbonitride can significantly improve the strength of steel. Generally speaking, precipitation strengthening is better than solid solution strengthening. The manganese and titanium content levels that achieve the same strengthening effect are basically one order of magnitude or more apart, but the price difference between the two is about 2 to 3 times. Therefore, the use of titanium microalloying and full use of its strengthening effect will greatly reduce the total alloy content added to the molten steel, especially the manganese content, thereby ensuring product performance under the condition of reducing carbon and manganese alloy components.

[0004] The present invention aims to solve the problem of reducing the occurrence of longitudinal cracks on the surface of the ingot steel strip by reasonable component design, developing chemical composition and hot continuous casting and rolling process based on the CSP production line process, reducing the C content, developing and trial-producing Ti microalloying and controlled rolling and controlled cooling process, and carefully studying its mechanical properties, microstructure and inclusion grade. The mechanical properties, impact toughness, microstructure, inclusions, etc. of the low-alloy high-strength steel Q355B trial-produced by Baotou Steel CSP all meet the standard requirements for low-alloy high-strength steel Q355B hot-rolled coils.

[0005] The publication number is: CN117327993A "Low-cost low-alloy high-strength structural steel strip Q355B production method based on CSP production line, the process flow of which is blast furnace molten iron → molten iron desulfurization pretreatment → 120-ton double-blowing converter smelting → LF refining → thin slab continuous casting → roller bottom tunnel furnace soaking → continuous rolling → laminar cooling → coiling → inspection and storage; the components and percentages of the steel are controlled as follows: C: 0.019-0.022, Si: ≤0.25, Mn: 0.15-0.30, P: ≤0.030, S: ≤0.010, N <0.0050, Als: 0.015-0.035, the balance is Fe and inevitable trace elements", this method is consistent with the patent The process flow is basically the same as that of the patent, but its thickness is 1.5mm~12.0mm. In addition, its C content is 0.019~0.022%. For example, the actual C of 1.5mm is 0.205%, the actual C of 5.0mm is 0.21%, and the actual C of 12.0mm is 0.213%. The thickness range of this patent is 7.5~17.75mm, which has a wider application range. In addition, the C content control range is 0.05-0.07%, which avoids the crack sensitive area that causes peritectic steel (W(C)=0.08~0.15%) and subperitectic steel (W(C)=0.16~0.20%). The inherent longitudinal crack defects of the CSP production line are eliminated, and the cold bending performance and impact performance are better than those of ordinary components.

[0006] The publication number is CN11572631A, "A method for producing low alloy high strength structural steel plate Q355B, the process flow is molten iron pretreatment → converter → refining → continuous casting → slab heating → rolling → cooling → hot straightening → finishing → performance inspection, characterized in that: the chemical composition of the steel is C = 0.20 ~ 0.23, Si = 0.25 ~ 0.40, Mn = 0.50 ~ 0.60, P ≤ 0.020, S ≤ 0.015, Als = 0.015 ~ 0.050 , Nb = 0.010 ~ 0.015, the balance is Fe and inevitable impurities", this method adopts thick slab low drawing speed process production, its mechanical properties are good, compared with its Mn content mass percentage of 0.5 ~ 0.6, and the precious alloy element Nb content mass percentage of 0.010 ~ 0.015, its alloy cost is high, on the other hand, this patent is different from the process route of the literature, this patent is based on CSP hot rolling continuous casting and rolling production process, the production process is short, high efficiency, low manufacturing cost.

[0007] Publication No. CN115558840A discloses "a method for producing low-cost low-alloy high-strength structural steel Q355B, whose composition percentage is controlled as follows: C 0.016-0.20%, Si 0.25-0.45%, Mn 1.20-1.45%, P≤0.035%, S≤0.015%, Nb 0, V 0, Ti 0.020-0.040%, Cr≤0.30%, Ni≤0.50%, Cu≤0.40%, N≤0.015%, Mo≤0.10%, Alt 0.020-0.040%", which reduces the refining cost to a certain extent, and the mechanical properties meet the standard requirements, but due to the high Mn content and the addition of titanium, the alloy cost is relatively high, and due to the high Mn content, the quality control is difficult in the CSP thin slab continuous casting and rolling process, and the longitudinal crack rate is high. This patent adopts a low-carbon design (C: 0.05-0.07%) to avoid the crack-sensitive areas that cause peritectic steel (W(C) = 0.08-0.15%) and hypoperitectic steel (W(C) = 0.16-0.20%), eliminating longitudinal crack defects. In addition, the Mn content is 0.6-0.9%, which is 0.30-0.55% lower than that in the literature. Not only is the manufacturing cost lower, but the rolling load is reduced and the rolling stability is improved. Summary of the invention

[0008] The purpose of the present invention is to provide a method for producing a titanium microalloyed Q355B low alloy high strength steel strip with stable production process, excellent performance, good plate shape and no longitudinal crack defects by using continuous casting and rolling process.

[0009] In order to solve the above technical problems, the present invention adopts the following technical solutions:

[0010] The present invention discloses a method for producing a titanium microalloyed Q355B low alloy high-strength structural steel strip, comprising the steps of KR molten iron pretreatment, converter smelting, LF refining, continuous casting, heating, rolling and cooling:

[0011] 1) KR desulfurized hot metal pretreatment steps: after pretreatment, [S] in hot metal is ≤ 0.005%, hot metal temperature is ≮1300℃, and pneumatic double slag pulling is adopted to ensure that the exposed area of ​​hot metal is ≥ 95%;

[0012] 2) Converter smelting steps: the ratio of pre-treated molten iron and clean scrap steel is 5:1-7:1, the temperature of molten iron entering the furnace is 1300-1400℃, the S content in the molten iron is ≤0.04%, and active lime and high-silicon dolomite must be added to make slag to ensure the slag basicity; top and bottom composite blowing, the converter carbon pulling is done once to ensure the CO balance at the end of blowing; during the steel tapping process, the steel outlet does not disperse and the steel tapping time is ≮5 minutes. During the steel tapping process, slag washing is performed to ensure that the inclusions are fully gathered and floated, and the purity of the molten steel is improved. The end temperature of the molten steel shall comply with the current operating standards;

[0013] 3) LF refining steps: white slag desulfurization at the workstation must be fast and maintained for ≥15 minutes, and the final slag composition must be controlled to be CaO / SiO2=4.5:1~7.0:1; the diameter of the "bright circle" during slagging should not exceed 1 / 3 of the diameter of the ladle; the deoxidation and alloying of the molten steel requires that Ca treatment soft blowing should be carried out for ≥5 minutes after the refining is completed, and the slag surface should maintain a slightly fluctuating state during soft blowing. The total [O] content is required to be ≯20ppm and the [S] content is ≯20ppm when leaving the station;

[0014] 4) During continuous casting, low carbon steel protective slag is used, including: SiO226.63%, CaO 30.36%, Al2O35.24%, Na2O 9.64%, MgO 2.36%, F 5.89%, melting speed 29S, melting point 1054℃, viscosity 0.143Pa.S, tundish temperature 1540-1560℃, graphite sealing ring is used to reduce nitrogen increase in tundish, and the average pulling speed is 3.8-4.2m / min;

[0015] 5) Heating process: In order to limit and reduce the precipitation of TiN during the heating stage of the casting, the atmosphere in the heating furnace and the heating rate should be strictly controlled to 12-15℃ / min, and the slab out-of-furnace temperature should be controlled between 1050-1090℃;

[0016] 6) Rolling and cooling process: In order to achieve uniform plate shape and refine austenite grains to avoid mixed crystals, the rolling reduction of the continuous rolling mill F1 is 40-42%, and the rolling reduction of F2 is 38-40%. To ensure the qualified and stable performance of thick specifications, the specifications with thickness > 8.0mm are rolled by the F7 rolling mill, the final rolling temperature is set at 850-880℃, the cooling adopts front concentrated laminar cooling, the cooling water temperature is 20-28℃, and the curling temperature is between 560-590℃;

[0017] The chemical composition of the steel strip by mass percentage is: C: 0.05-0.07, Si: 0.20-0.30, Mn: 0.60-0.90, P: ≤0.020, S: ≤0.009, Als: 0.17-0.27, Ti: 0.04-0.06%, N: ≤0.0050%; the rest is iron and unavoidable impurities.

[0018] The chemical composition of the steel strip in terms of mass percentage is: C: 0.05%, Si: 0.20, Mn: 0.63, P: 0.015, S: 0.005, Als: 0.24%, Ti: 0.048, O: 0.0032%, N: 0.0038%; the remainder is Fe and unavoidable impurities.

[0019] The chemical composition of the steel strip in terms of mass percentage is: C: 0.06, Si: 0.25, Mn: 0.83, P: 0.013, S: 0.004, Als: 0.26, Ti: 0.051, O: 0.0030%, N: 0.0045%; the remainder is Fe and unavoidable impurities.

[0020] The chemical composition of the steel strip in mass percentage is: C: 0.06, Si: 0.22, Mn: 0.84, P: 0.016, S: 0.005, Als: 0.25, Ti: 0.055, O: 0.0028%, N: 0.0037%; the remainder is Fe and unavoidable impurities.

[0021] The chemical composition of the steel strip in terms of mass percentage is: C: 0.06%, Si: 0.24%, Mn: 0.85%, P: 0.015%, S: 0.004%, Als: 0.22, Ti: 0.052, O: 0.0032%, N: 0.0040%; the remainder is Fe and unavoidable impurities.

[0022] The chemical composition of the steel strip in mass percentage is: C: 0.06%, Si: 0.23%, Mn: 0.83%, P: 0.017, S: 0.005, Als: 0.26, Ti: 0.054, O: 0.0036%, N: 0.0043%; the remainder is Fe and unavoidable impurities.

[0023] The chemical composition of the steel strip in terms of mass percentage is: C: 0.06%, Si: 0.26%, Mn: 0.86%, P: 0.017%, S: 0.004%, Als: 0.25%, Ti: 0.055, O: 0.0022%, N: 0.0037%; the remainder is Fe and unavoidable impurities.

[0024] Compared with the prior art, the beneficial technical effects of the present invention are:

[0025] (1) Baosteel CSP hot-rolled low-alloy high-strength steel Q355B is designed with a low-carbon process (C: 0.05-0.07%). By adding a trace amount of titanium alloy, the strength is improved by using its fine grain and precipitation strengthening to reduce the impact of low carbon content on performance. Smelting desulfurization and nitrogen control are adopted to reduce the impact of MnS and TiN on performance and quality, ensure steel cleanliness, uniform structure, and controlled rolling and controlled cooling processes, so as to obtain good mechanical properties and impact toughness, so that all product indicators meet the standard requirements.

[0026] (2) The low alloy high strength steel strip Q355B with thickness of 7.5mm, 9.75mm, 11.5mm, 13.5mm, 15.5mm and 17.75mm has a microstructure of ferrite + pearlite, uniform and fine grains, a grain size of 7 to 11, and the inclusions of type A, type B and type DS are 0.5, 0.5 and 0.5 to 1.0 respectively. The steel is pure and meets the requirements of product and use standards.

[0027] (3) The Q355B trial-production specification steel strip has stable head, tail and transverse mechanical properties, good cold forming and impact toughness, and meets the standard requirements.

[0028] The outstanding advantage of the present invention is that it adopts a low-carbon component design, effectively avoiding the crack-sensitive areas that cause peritectic steel (W(C)=0.08-0.15%) and hypoperitectic steel (W(C)=0.16-0.20%), solving the longitudinal cracking problem that has long plagued the CSP production line. By giving full play to the advantages of large reduction and short process of the CSP rolling mill, taking advantage of the low price and significant strengthening mechanism of titanium alloy, through the adjustment and optimization of the reduction amount, rolling process, cooling mode and process, a lower rolling load and high-efficiency rolling are used to produce products that meet product standards and various user requirements. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] The present invention will be further described below in conjunction with the accompanying drawings.

[0030] Figure 1 It is a typical microstructure of low alloy high strength steel strip Q355B. DETAILED DESCRIPTION

[0031] The present invention is further described below in conjunction with embodiments.

[0032] Example 1

[0033] The mass percentage of the smelting chemical composition is: C: 0.05%, Si: 0.20, Mn: 0.63, P: 0.015, S: 0.005, Als: 0.24%, Ti: 0.048, O: 0.0032%, N: 0.0038%; the balance is Fe and unavoidable impurities. The steel plate with a thickness of 7.5 mm is rolled. The detailed rolling and heating process is shown in Table 1, and its mechanical properties are shown in Table 2.

[0034] Example 2

[0035] The mass percentage of the smelting chemical composition is: C: 0.06, Si: 0.25, Mn: 0.83, P: 0.013, S: 0.004, Als: 0.26, Ti: 0.051, O: 0.0030%, N: 0.0045%; the balance is Fe and unavoidable impurities. The steel plate with a thickness of 9.75 mm is rolled. The detailed rolling and heating process is shown in Table 1, and its mechanical properties are shown in Table 2.

[0036] Example 3

[0037] The mass percentage of the smelting chemical composition is: C: 0.06, Si: 0.22, Mn: 0.84, P: 0.016, S: 0.005, Als: 0.25, Ti: 0.055, O: 0.0028%, N: 0.0037%; the balance is Fe and unavoidable impurities. The steel plate with a thickness of 11.5 mm is rolled. The detailed rolling and heating process is shown in Table 1, and its mechanical properties are shown in Table 2.

[0038] Example 4

[0039] The mass percentage of the smelting chemical composition is: C: 0.06, Si: 0.24, Mn: 0.85, P: 0.015, S: 0.004, Als: 0.22, Ti: 0.052, O: 0.0032%, N: 0.0040%; the balance is Fe and unavoidable impurities. The steel plate with a thickness of 13.5 mm is rolled. The detailed rolling and heating process is shown in Table 1, and its mechanical properties are shown in Table 2.

[0040] Example 5

[0041] The mass percentage of the smelting chemical composition is: C: 0.06, Si: 0.23, Mn: 0.83, P: 0.017, S: 0.005, Als: 0.26, Ti: 0.054, O: 0.0036%, N: 0.0043%; the balance is Fe and unavoidable impurities. The steel plate with a thickness of 15.5 mm is rolled. The detailed rolling and heating process is shown in Table 1, and its mechanical properties are shown in Table 2.

[0042] Example 6

[0043] The mass percentage of the smelting chemical composition is: C: 0.06, Si: 0.26, Mn: 0.86, P: 0.017, S: 0.004, Als: 0.25, Ti: 0.055, O: 0.0022%, N: 0.0037%; the balance is Fe and unavoidable impurities. The steel plate with a thickness of 17.75 mm is rolled. The detailed rolling and heating process is shown in Table 1, and its mechanical properties are shown in Table 2.

[0044] Table 1 Main rolling process parameters of Q355B

[0045]

[0046] Table 2 Mechanical properties of trial-produced Q355B steel strip at various thicknesses

[0047]

[0048]

[0049] The microstructure and inclusion analysis of the trial-produced steel strips with thicknesses of 7.5mm, 9.75mm, 13.5mm and 17.75mm were carried out. Figure 1 It can be seen that the structure of the steel strips of various thicknesses is ferrite + pearlite, with fine and uniform grains. Figure 1 (a) Grain size 8.0-10.0, Figure 1 (b) Grain size 9.0-11.0, Figure 1 (c) Grain size 7.0-9.0, Figure 1 (d) Grain size is 7.0-8.0, and the inclusions of type A, type B, and type DS are 0.5, 0.5, and 0.5-1.0. The steel is pure and meets the product standard requirements.

[0050] The embodiments described above are only descriptions of the preferred modes of the present invention, and are not intended to limit the scope of the present invention. Without departing from the design spirit of the present invention, various modifications and improvements made to the technical solutions of the present invention by ordinary technicians in this field should all fall within the protection scope determined by the claims of the present invention.

Claims

1. A method for producing titanium microalloyed Q355B low alloy high strength structural steel strip, characterized in that: include: KR hot metal pretreatment, converter smelting, LF refining, continuous casting, heating, rolling and cooling steps: 1) KR desulfurized hot metal pretreatment steps: after pretreatment, [S] in hot metal is ≤ 0.005%, hot metal temperature is ≮1300℃, and pneumatic double slag pulling is adopted to ensure that the exposed area of ​​hot metal is ≥ 95%; 2) Converter smelting steps: the ratio of pre-treated molten iron and clean scrap steel is 5:1-7:1, the temperature of molten iron entering the furnace is 1300-1400℃, the S content in the molten iron is ≤0.04%, and active lime and high-silicon dolomite must be added to make slag to ensure the slag basicity; top and bottom composite blowing, the converter carbon pulling is done once to ensure the CO balance at the end of blowing; during the steel tapping process, the steel outlet does not disperse and the steel tapping time is ≮5 minutes. During the steel tapping process, slag washing is performed to ensure that the inclusions are fully gathered and floated, and the purity of the molten steel is improved. The end temperature of the molten steel shall comply with the current operating standards; 3) LF refining steps: white slag desulfurization at the workstation must be fast and maintained for ≥15 minutes, and the final slag composition must be controlled to CaO / SiO2=4.5:1~7.0:1; the diameter of the "bright circle" during slagging should not exceed 1 / 3 of the diameter of the ladle; the deoxidation and alloying of molten steel requires that Ca treatment soft blowing should be carried out for ≥5 minutes after the refining is completed, and the slag surface should maintain a slightly fluctuating state during soft blowing. The total [O] content is required to be ≯20ppm and the [S] content is ≯20ppm when leaving the station; 4) During continuous casting, low carbon steel protective slag is used, including: SiO226.63%, CaO 30.36%, Al2O35.24%, Na2O 9.64%, MgO 2.36%, F 5.89%, melting speed 29S, melting point 1054℃, viscosity 0.143Pa.S, tundish temperature 1540-1560℃, graphite sealing ring is used to reduce nitrogen increase in tundish, and the average pulling speed is 3.8-4.2m / min; 5) Heating process: In order to limit and reduce the precipitation of TiN during the heating stage of the casting, the atmosphere in the heating furnace and the heating rate should be strictly controlled to 12-15℃ / min, and the slab out-of-furnace temperature should be controlled between 1050-1090℃; 6) Rolling and cooling process: In order to achieve uniform plate shape and refine austenite grains to avoid mixed crystals, the rolling reduction of the continuous rolling mill F1 is 40-42%, and the rolling reduction of F2 is 38-40%. To ensure the qualified and stable performance of thick specifications, the specifications with thickness > 8.0mm are rolled by the F7 rolling mill, the final rolling temperature is set at 850-880℃, the cooling adopts front concentrated laminar cooling, the cooling water temperature is 20-28℃, and the curling temperature is between 560-590℃; The chemical composition of the steel strip by mass percentage is: C: 0.05-0.07, Si: 0.20-0.30, Mn: 0.60-0.90, P: ≤0.020, S: ≤0.009, Als: 0.17-0.27, Ti: 0.04-0.06%, N: ≤0.0050%; the rest is iron and unavoidable impurities.

2. The method for producing titanium microalloyed Q355B low alloy high strength structural steel strip according to claim 1, characterized in that: The chemical composition of the steel strip in mass percentage is: C: 0.05%, Si: 0.20, Mn: 0.63, P: 0.015, S: 0.005, Als: 0.24%, Ti: 0.048, O: 0.0032%, N: 0.0038%; the remainder is Fe and unavoidable impurities.

3. The method for producing titanium microalloyed Q355B low alloy high strength structural steel strip according to claim 1, characterized in that: The chemical composition of the steel strip in mass percentage is: C: 0.06%, Si: 0.25%, Mn: 0.83%, P: 0.013, S: 0.004, Als: 0.26, Ti: 0.051, O: 0.0030%, N: 0.0045%; the remainder is Fe and unavoidable impurities.

4. The method for producing titanium microalloyed Q355B low alloy high strength structural steel strip according to claim 1, characterized in that: The chemical composition of the steel strip in mass percentage is: C: 0.06%, Si: 0.22%, Mn: 0.84, P: 0.016, S: 0.005, Als: 0.25, Ti: 0.055, O: 0.0028%, N: 0.0037%; the remainder is Fe and unavoidable impurities.

5. The method for producing titanium microalloyed Q355B low alloy high strength structural steel strip according to claim 1, characterized in that: The chemical composition of the steel strip in mass percentage is: C: 0.06, Si: 0.24, Mn: 0.85, P: 0.015, S: 0.004, Als: 0.22, Ti: 0.052, O: 0.0032%, N: 0.0040%; the remainder is Fe and unavoidable impurities.

6. The method for producing titanium microalloyed Q355B low alloy high strength structural steel strip according to claim 1, characterized in that: The chemical composition of the steel strip in mass percentage is: C: 0.06%, Si: 0.23%, Mn: 0.83%, P: 0.017, S: 0.005, Als: 0.26, Ti: 0.054, O: 0.0036%, N: 0.0043%; the remainder is Fe and unavoidable impurities.

7. The method for producing titanium microalloyed Q355B low alloy high strength structural steel strip according to claim 1, characterized in that: The chemical composition of the steel strip in mass percentage is: C: 0.06%, Si: 0.26, Mn: 0.86, P: 0.017, S: 0.004, Als: 0.25, Ti: 0.055, O: 0.0022%, N: 0.0037%; the remainder is Fe and unavoidable impurities.

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