Microalloyed high-strength steel and corner-cleaning-free hot charging rolling method thereof

Abstract

The microalloyed high-strength steel comprises the following chemical components in percentage by mass: 0.055 to 0.075 percent of C, 0.40 to 1.00 percent of Si, 1.90 to 2.40 percent of Mn, 0.20 to 0.60 percent of Cr, 0.010 to 0.025 percent of Nb, 0.020 to 0.040 percent of Ti, less than or equal to 0.040 percent of P, 0.01 to 0.15 percent of Als, less than or equal to 0.0005 percent of B, less than or equal to 0.006 percent of S, less than or equal to 0.0055 percent of N, less than or equal to 0.006 percent of Ca and the balance of Fe and inevitable impurity elements. The method comprises the following steps: controlling the carbon equivalent CElt; and carrying out smelting, continuous casting and hot rolling on 0.07% of the raw materials. Aiming at the industrial pain and difficulty that corner-cleaning-free hot charging rolling of the microalloyed high-strength steel is not easy to realize, restricting factors are deeply excavated from the sources of component design, a continuous casting process and a hot rolling process, and industrial practice of corner-cleaning-free hot charging rolling of the Nb-Ti microalloyed high-strength steel is realized through integrated design; key indexes including the hot charging rate, the charging temperature, the yield and the yield are very bright.

Description

Technical Field

[0001] The invention relates to the technical field of metallurgical plates, and in particular to a microalloyed high-strength steel and a corner-clearing-free hot-charging rolling method thereof. Background Art

[0002] Hot delivery and hot charging is a revolutionary technology in the steel industry that emerged in the 1980s. Compared with cold charging and rolling, hot charging and rolling can reduce gas consumption, increase machine hour output, speed up billet turnover, reduce corner iron loss and reduce oxidation and burning loss, which has very significant advantages. Under the guidance of extreme efficiency, extreme cost and extreme benefit, advanced steel companies represented by Baosteel attach great importance to the improvement of technical indicators of hot charging and rolling.

[0003] With the continuous development of automobile lightweighting, automobile plates represented by high-strength steel have become the first choice for body structural materials. With the increase in strength level, it is difficult to achieve the target strength by solid solution strengthening alone. It is also necessary to increase fine grain strengthening, precipitation strengthening, phase transformation strengthening, and even dislocation strengthening. Whether it is traditional high-strength steel or advanced high-strength steel, micro-alloying elements such as Nb, Ti, V, and B play an important role. Although the micro-alloying composition design can improve the product's formability while achieving high strength, the accompanying strength fluctuations, more difficult acid rolling, and a series of problems such as corner cracking have long plagued the industry and academia. At present, the cooling technology used by continuous annealing units and galvanizing units is generally air cooling, and a very small number of them are equipped with roller cooling and water quenching. For high-strength steel continuous annealing bare plates or hot-dip galvanized plates with a tensile strength of 780MPa or above, there are many types of alloy elements and high content. In order to achieve corner-free hot rolling, the requirements for casting equipment, casting process, hot rolling equipment, hot rolling process and surface inspection level are extremely high. In the existing technology, industry and academia have also conducted a lot of exploration and research.

[0004] The Chinese invention patent with application number CN200720185022.5 proposes an online corner cleaning device for continuous casting of slabs, the goal of which is to clean the corner crack defects of the ingots online and improve production efficiency, but it does not mention the ability to control edge defects. Chinese patent CN105414502B discloses a continuous casting fan-shaped positioning and locking device and an assembly method. By adding a fan-shaped positioning and locking device, the self-weight positioning of the fan-shaped segment is avoided, and the fan-shaped segment is prevented from shifting under production conditions, which greatly reduces the mechanical stress of the ingot, thereby stabilizing the arc connection accuracy and improving the quality of the ingot. In order to eliminate the surface cracks of the ingot caused by ferrite, Chinese patent CN110653352A adds a row of cooling water nozzles with small spray angles in front of the straightening section, providing a narrow area strong cooling zone to eliminate the surface proeutectoid ferrite to control the surface cracks of the ingot. Chinese patent CN107695313A also uses rapid cooling and quenching to eliminate the precipitation of ferrite and carbonitride on the surface of the ingot to avoid embrittlement and improve the plasticity of the ingot to reduce the surface cracks of the ingot. Chinese patent CN105478704B has established a secondary cooling water spray rack lifting device, which can realize the dynamic control of the cooling area of ​​the secondary cooling water and automatic adjustment of the water volume. This method avoids the secondary cooling water directly spraying on the corners of the ingot, resulting in corner cracks caused by low temperature or temperature changes. Chinese patent CN113843403B improves the plasticity of the ingot and reduces the surface cracks of the ingot by controlling the ferrite on the surface of the ingot before the straightening point of the casting machine to more than 35%. The Iron and Steel Research Institute proposed and developed and applied the patented technology of chamfered crystallizer and chamfered foot roller to improve the corner crack defects of continuous casting ingots. Whether it is to add a positioning locking device or to adjust the surface temperature and structure of the ingot by controlling the secondary cooling process, it is very dependent on equipment technical transformation, and the difficulty of industrial implementation is still very large. When it comes to specific micro-alloyed high-strength steels, there is no direct solution for hot-rolling without corner cleaning that can be used to reduce cracks in the corners of ingots and cracks on the edges of hot-rolled steels. Summary of the invention

[0005] In order to overcome the deficiencies of the prior art, the purpose of the present invention is to provide a micro-alloyed high-strength steel and a corner-free hot-rolling method thereof to solve the problems of corner cracks, edge warping, edge cracks, etc. of the ingot.

[0006] To achieve the above purpose, the technical ideas of the present invention are as follows:

[0007] Usually, at high temperatures, there are three brittle intervals in which the thermoplasticity of the ingot changes significantly, namely the first brittle zone in the solidification temperature range, the second brittle zone in the 1200-950℃ range, and the third brittle zone in the 900-600℃ range. It is generally believed that surface cracks are rarely produced in the ingot in the second brittle interval. For high-strength steel continuous annealing bare plate or hot-dip galvanized plate with a tensile strength of 780MPa or above, major steel mills have long adopted the composition design of sub-peritectic steel. In industrial practice, the C content generally falls within the range of 0.08-0.12%. The transformation of sub-peritectic steel from δ phase to γ ​​phase during solidification is accompanied by a large volume shrinkage, which significantly reduces the high-temperature plasticity. In order to achieve hot rolling without corner clearing, the requirements for casting equipment, casting process, hot rolling equipment, hot rolling process and surface inspection level are extremely high. However, due to technical confidentiality, many key parameters and control points have not been published. For specific microalloyed high-strength steel, there is no corner-free hot-rolling solution that can be directly referenced to reduce corner cracks and edge defects in ingots. Under the condition of insufficient technical capabilities, avoiding the peritectic / subperitectic region in composition design is the key.

[0008] Carbon equivalent calculation model: CE = 0.1C / (0.1+0.08Si-0.02Mn-0.04P-0.25S+0.03Cr+0.03Ti); where C, Si, Mn, P, S, Cr, and Ti are the mass percentages of the corresponding elements. According to the carbon equivalent calculation model, controlling the carbon equivalent CE within 0.07% can avoid the peritectic and subperitectic regions.

[0009] The heat transfer, stress and phase change processes during continuous casting solidification are very complex, and the elements themselves also have an impact on corner cracks. C is a sensitive element for surface cracks on the ingot. If the C content is too high, it is not only easy to cause peritectic reaction, but also may increase the size of austenite grains, and even increase the hardness difference between the soft and hard phases of the banded structure and the finished product, which seriously affects the forming performance of microalloyed high-strength steel; if the C content is too little, more alloy content needs to be added, which not only increases the cost, but also increases the difficulty of controlling corner cracks. In the present invention, the C content is 0.055-0.075%. Secondly, the precipitation of niobium carbonitrides will significantly deteriorate the high-temperature plasticity of the ingot in the brittle zone II. Practice shows that when Nb≤0.025%, the difficulty of controlling corner cracks can be reduced. Although Ti will also precipitate carbonitrides, the precipitation temperature is higher, which can weaken the influence of Nb and N. An appropriate Ti content is very necessary, but it is also necessary to consider the influence of Ti on the yield strength ratio of the product. In the present invention, the content of Nb is 0.010-0.025%, and the content of Ti is 0.020-0.040%. Therefore, the chemical composition and mass percentage of the high-strength automobile sheet of the present invention meet the following conditions: C: 0.055-0.075%, Si: 0.40-1.00%, Mn: 1.90-2.40%, Cr: 0.20-0.60%, Nb: 0.010-0.025%, Ti: 0.020-0.040, P≤0.040%, Als: 0.01-0.15%, B≤0.0005%, S≤0.006%, N≤0.0055%, Ca≤0.006%, and the balance is Fe and unavoidable impurity elements.

[0010] Compared with high-strength steel without Nb, it is more difficult to control the corner cracks of the ingot containing Nb high-strength steel, and the requirements for casting machine equipment, casting process, hot rolling equipment, and hot rolling process are also more stringent. With the improvement of strength grade, the types and contents of alloy elements further increase, and the difficulty of controlling corner cracks of the ingot containing Nb high-strength steel also increases accordingly. Any unreasonable design or control of arc accuracy, liquid level fluctuation, protective slag parameters, vibration process, temperature control, and ingot pulling speed will cause corner cracks. Therefore, in the present invention, the continuous casting process adopts the process measures of short crystallizer vibration negative slip time + high basicity and low viscosity crystallizer protective slag + increasing the target temperature of the ingot surface in the straightening zone.

[0011] During hot rolling production, cracks on the corners of the ingot will evolve into edge warping defects. In addition, if the hot rolling process is improper, the ingot will be turned up and flattened under the alternating action of side pressure and flat pressure, which will lead to edge crack defects. Edge warping and edge crack defects are usually within 35mm of the edge of the strip. To reduce and mitigate hot rolling edge crack defects, it is key to increase the edge temperature and reduce the turning up of the edge metal. Figure 1 The concave hammer head shown and the controlled width reduction are important means.

[0012] Based on the above ideas, in order to achieve the purpose of the present invention, the technical solution adopted by the present invention is as follows:

[0013] A microalloyed high-strength steel, the chemical composition and mass percentage of which meet the following conditions: C: 0.055-0.075%, Si: 0.40-1.00%, Mn: 1.90-2.40%, Cr: 0.20-0.60%, Nb: 0.010-0.025%, Ti: 0.020-0.040, P≤0.040%, Als: 0.01-0.15%, B≤0.0005%, S≤0.006%, N≤0.0055%, Ca≤0.006%, the remainder being Fe and unavoidable impurity elements; the carbon equivalent CE of the microalloyed high-strength steel is less than 0.07%, and the calculation model of the carbon equivalent CE is as follows:

[0014] CE=0.1C / (0.1+0.08Si-0.02Mn-0.04P-0.25S+0.03Cr+0.03Ti), the unit is %, where C, Si, Mn, P, S, Cr and Ti are the mass percentages of the corresponding elements.

[0015] Furthermore, in the microalloyed high-strength steel, N≤0.0050%, Als: 0.010-0.040%, and P≤0.015%.

[0016] Furthermore, the tensile strength of the microalloyed high-strength steel is ≥780MPa.

[0017] The above-mentioned corner-clearing-free hot-charging rolling method of microalloyed high-strength steel comprises the following steps:

[0018] 1) Smelting: Controlled according to the chemical composition of microalloyed high-strength steel, refining adopts LF+RH double process;

[0019] 2) Continuous casting, using a straight mold arc continuous casting machine, the arc accuracy is controlled at ±0.5mm, the liquid level fluctuation of the mold is controlled at ±5mm, the basicity of the mold protection slag is 1.25±0.25, the viscosity of the mold protection slag is 0.15±0.06Pa·s, and the negative slip time of the mold vibration device is 0.12~0.16s; the target temperature of the billet surface in the straightening zone is ≥920℃, and the billet pulling speed is 1.1~1.6m / min;

[0020] 3) Hot rolling, the ingot is not clear, the charging temperature is 400-900℃, the total width reduction is ≤200mm, the heating temperature is 1230±40℃, the side press uses a concave hammer head with a concave depth of 20-40mm; and a heat preservation cover is used.

[0021] Furthermore, the corner-clearing-free hot-rolling method also includes: 4) pickling and cold rolling; 5) continuous annealing or hot-dip galvanizing.

[0022] Furthermore, in the step 1), the chemical composition of the microalloyed high-strength steel is controlled, the converter end point C is ≤ 0.050%, and the refining adopts the LF+RH dual process.

[0023] Furthermore, in the step 2), the crystallizer outlet temperature is ≥1050°C, the tundish superheat is 15-35°C; and electromagnetic stirring and dynamic soft pressure are used.

[0024] Furthermore, in the step 2), the target thickness of the ingot is 230 mm, and the corner shape of the ingot is a right angle.

[0025] Furthermore, in the step 2), the casting billet drawing speed is 1.2-1.4 m / min.

[0026] Furthermore, in the step 3), a U-shaped cooling process is adopted for coiling, and the temperature within a range of at least 50m from the head to the tail is compensated by 30 to 80°C, and the target coiling temperature of the remaining middle parts is set to 500 to 600°C.

[0027] Furthermore, in the step 3), the final rolling temperature is 900±30°C, and an edge heater is used.

[0028] Furthermore, in the step 3), the total width reduction is ≤100 mm.

[0029] Compared with the prior art, the present invention has the following beneficial effects:

[0030] 1) The microalloyed high-strength steel and the corner-clearing-free hot-charging rolling method provided by the present invention effectively avoid the crack-sensitive sub-peritectic zone through the composition design of carbon equivalent CE<0.07%, 0.055%≤C≤0.075%, and 0.010%≤Nb≤0.025%, thereby eliminating the tendency of corner cracks in the ingot from the source of composition design.

[0031] 2) The present invention reduces and alleviates the defects of corner cracks in the billet from the source of the continuous casting process through reasonable designs of arc accuracy, liquid level fluctuation, protective slag parameters, vibration process, temperature control, and billet drawing speed.

[0032] 3) The present invention reduces, alleviates or even eliminates edge defects (edge ​​warping + edge cracks) from the source of the hot rolling process through the reasonable design of the hammer head shape of the side press, the side press width reduction, the heating temperature, the furnace loading temperature, and the edge insulation. The edge defects can be controlled within 10 mm of the edge, which is convenient for removal in the subsequent process.

[0033] 4) The present invention improves the through-coil stability of the hot-rolled microstructure and performance of the microalloyed high-strength steel through the precise design of the B content and the layer cooling method, and effectively reduces the thickness fluctuation defect during the subsequent acid rolling production.

[0034] In summary, the present invention adopts an integrated design of chemical composition, continuous casting process, and hot rolling process to realize the industrialized corner-free hot rolling of Nb-Ti microalloyed high-strength dual-phase steel. The yield rate of edge defects (edge ​​warping + edge cracks) within 10 mm of the hot rolled edge can reach more than 90%, the hot charging rate can reach more than 90%, the furnace time of the heating furnace can be reduced by more than 20 minutes compared with cold charging rolling, and the hot rolling yield rate is increased by more than 0.3%. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Figure 1 A physical picture of a concave hammer head used in the corner-clearing-free hot-loading rolling method of the present invention;

[0036] Figure 2 This is the pickling diagram of the corner of the ingot of Comparative Example 1;

[0037] Figure 3 This is a diagram of the acid rolling thickness fluctuation process of Comparative Example 4. DETAILED DESCRIPTION

[0038] In order to better explain the present invention, the main contents of the present invention are further explained below in conjunction with the drawings and specific embodiments, but the contents of the present invention are not limited to the following embodiments.

[0039] The chemical compositions of the microalloyed high strength steels in Examples 1 to 3 (S1 to S3) and Comparative Examples 1 to 4 (D1 to D4) are shown in Table 1. The target thickness of the ingot is 230 mm, and the shape of the ingot corner is a right angle.

[0040] The preparation of microalloyed high-strength steel in Examples 1 to 3 and Comparative Example 3 adopts the corner-free hot-rolling method of the present invention; the preparation of microalloyed high-strength steel in Comparative Examples 1, 2, and 4 does not adopt the method of the present invention. The relevant parameters of each process are shown in Tables 2 and 3.

[0041] Table 1: Main chemical composition (wt%, the balance is Fe and unavoidable impurity elements)

[0042] serial number C Si Mn Cr Als P S Ca N Nb Ti B CE S1 0.068 0.45 2.10 0.45 0.07 0.016 0.004 0.0040 0.0040 0.015 0.025 / <0.07 S2 0.060 0.70 2.00 0.30 0.02 0.008 0.003 0.0025 0.0046 0.012 0.032 / <0.07 S3 0.062 0.50 2.30 0.25 0.04 0.012 0.005 0.0038 0.0038 0.020 0.039 / <0.07 D1 0.087 0.30 2.10 0.35 0.04 0.010 0.004 0.0027 0.0042 0.040 0.020 / >0.07 D2 0.087 0.30 2.10 0.35 0.04 0.010 0.004 0.0027 0.0042 0.040 0.020 / >0.07 D3 0.068 0.45 2.10 0.45 0.07 0.016 0.004 0.0040 0.0040 0.015 0.025 / <0.07 D4 0.068 0.45 2.10 0.45 0.07 0.016 0.004 0.0040 0.0040 0.015 0.025 0.0015 <0.07

[0043] Table 2: Main smelting and continuous casting process parameters

[0044] serial number S1 S2 S3 D1 D2 D3 D4 End point C content / % 0.038 0.042 0.040 0.048 0.048 0.038 0.038 Arc accuracy / mm ±0.3 ±0.3 ±0.4 ±0.3 ±0.3 ±0.7 ±0.3 Liquid level fluctuation / mm ±4 ±5 ±5 ±4 ±4 ±5 ±4 Target basicity of mold slag 1.20 1.20 1.20 1.20 1.20 1.00 1.20 Target viscosity of mold slag / (Pa·s) 0.15 0.15 0.15 0.15 0.15 0.25 0.15 Negative slip time / s 0.15 0.13 0.15 0.15 0.15 0.14 0.15 Crystallizer outlet temperature / ℃ ≥1080 ≥1050 ≥1100 ≥1080 ≥1080 ≥1080 ≥1080 Target temperature of the billet surface in the straightening zone / ℃ 950 950 925 950 950 900 950 Tundish superheat / ℃ 25 20 18 25 25 25 25 Billet drawing speed / (m / min) 1.2 1.5 1.2 1.2 1.2 1.2 1.2 Electromagnetic stirring and dynamic soft reduction Commissioning Commissioning Commissioning Commissioning Commissioning Commissioning Commissioning

[0045] Table 3: Main hot rolling process parameters

[0046]

[0047] Table 4: Comparison of program effects

[0048]

[0049] Note: The hot coil yield rate refers to the proportion of steel coils with edge defects within 10mm of the hot coil edge.

[0050] The corner quality of the ingots of Examples 1 to 3 produced by the method of the present invention is good, and no corner cracks are found after low-multiple pickling of the corners. The scheme effects of Examples 1 to 3 and Comparative Examples 1 to 4 are shown in Table 4. The C content of Comparative Examples 1 to 2 is relatively high, and the carbon equivalent CE>0.07%, and the hypoperitectic region is not avoided. Although the continuous casting process of the present invention is used, the ingots still have serious corner cracks, such as Figure 2 As shown. When the corner cracks of the ingot are serious, no matter what hot rolling process is used, the yield rate of edge defects within 10mm of the hot rolled edge is very low. The chemical composition of Comparative Example 3 is the same as that of Example 1, but the continuous casting process of Comparative Example 3 is different from that of Example 1. The difference in composition between Comparative Example 4 and Example 1 is that 0.0015% B is added. The continuous casting process is the same, but the hot rolling process is different. The yield rate of edge defects within 10mm of the hot rolled edge of Comparative Examples 3 to 4 is also very low, and edge defects that affect use will still exist after trimming in the subsequent production process. As shown in Tables 4 and Figure 3 As shown, the acid rolling thickness of comparative example 4 fluctuates greatly and cannot meet production requirements. The hot-rolled coil yield rate of each embodiment is very high, and the hot rolling yield rate is not less than 90%. Compared with the cold-rolled comparative example 2, the hot rolling yield rate is increased by more than 0.4%, and the furnace time is reduced by more than 20 minutes. After heat treatment, the tensile strength of each embodiment reaches 780MPa.

[0051] In summary, the method of the present invention adopts an integrated design of chemical composition, continuous casting process, and hot rolling process, which can realize the industrial hot rolling of micro-alloyed high-strength steel without corner cleaning, and has the advantages of low cost, good product surface quality, and simple process flow, and reduces corner cleaning iron loss and oxidation burning loss.

Claims

1. A microalloyed high-strength steel, characterized in that: The microalloyed high-strength steel comprises the following chemical compositions in percentage by mass: C: 0.055-0.075%, Si: 0.40-1.00%, Mn: 1.90-2.40%, Cr: 0.20-0.60%, Nb: 0.010-0.025%, Ti: 0.020-0.040%, P≤0.040%, Als: 0.01-0.15%, B≤0.0005%, S≤0.006%, N≤0.0055%, Ca≤0.006%, and the remainder is Fe and unavoidable impurity elements; the carbon equivalent CE of the microalloyed high-strength steel is less than 0.07%, and the calculation model of the carbon equivalent CE is as follows: CE=0.1C / (0.1+0.08Si-0.02Mn-0.04P-0.25S+0.03Cr+0.03Ti), the unit is %, where C, Si, Mn, P, S, Cr and Ti are the mass percentages of the corresponding elements.

2. The microalloyed high-strength steel according to claim 1, characterized in that: In the microalloyed high-strength steel, N≤0.0050%, Als: 0.010-0.040%, and P≤0.015%.

3. The microalloyed high-strength steel according to claim 1, characterized in that: The tensile strength of the microalloyed high-strength steel is ≥780MPa.

4. The corner-clearing-free hot-charging rolling method of microalloyed high-strength steel according to claim 1, characterized in that: The following steps are involved: 1) Smelting: Control the chemical composition of microalloyed high-strength steel, and use LF+RH double process for refining; 2) Continuous casting, using a straight mold arc continuous casting machine, the arc accuracy is controlled at ±0.5mm, the liquid level fluctuation of the mold is controlled at ±5mm, the basicity of the mold protection slag is 1.25±0.25, the viscosity of the mold protection slag is 0.15±0.06Pa·s, and the negative slip time of the mold vibration device is 0.12~0.16s; the target temperature of the billet surface in the straightening zone is ≥920℃, and the billet pulling speed is 1.1~1.6m / min; 3) For hot rolling, the ingot is not cleared, the charging temperature is 400-900℃, the total width reduction is ≤200mm, the heating temperature is 1230±40℃, the side press uses a concave hammer head with a concave depth of 20-40mm; and a heat preservation cover is used.

5. The corner-clearing-free hot-rolling method according to claim 4, characterized in that: In the step 2), the crystallizer outlet temperature is ≥1050°C, the tundish superheat is 15-35°C; electromagnetic stirring and dynamic soft pressure are used.

6. The corner-clearing-free hot-charging rolling method according to claim 4, characterized in that: In the step 2), the casting billet drawing speed is 1.2-1.4 m / min.

7. The corner-clearing-free hot-charging rolling method according to claim 4, characterized in that: In the step 2), the target thickness of the ingot is 230 mm, and the corner shape of the ingot is a right angle.

8. The corner-clearing-free hot-charging rolling method according to claim 4, characterized in that: In the step 3), a U-shaped cooling process is adopted for coiling, and the temperature compensation within a range of at least 50m from the head to the tail is 30-80°C, and the target coiling temperature of the remaining middle parts is set to 500-600°C.

9. The corner-clearing-free hot-charging rolling method according to claim 4, characterized in that: In the step 3), the final rolling temperature is 900±30° C., and an edge heater is used.

10. The corner-clearing-free hot-rolling method according to any one of claims 4 to 9, characterized in that: In the step 3), the total width reduction is ≤100 mm.

Images