Accurate control method for titanium content of high-strengthening bake-hardening steel component

Abstract

The invention discloses a method for accurately controlling the content of titanium in high-strength bake-hardened steel, which comprises the following steps of: optimizing the processes of steelmaking and continuous casting, optimizing the recarburization and nitrogen increase rules in the steelmaking and continuous casting process, stably controlling the content of Ti in molten steel in the steelmaking process, controlling the content of free carbon in the product component to be between 10ppm and 20ppm, and controlling the content of titanium in the product component to be between 10ppm and 20ppm; the precise control method for the titanium content of the phosphorus-added strengthened bake-hardened steel is achieved, the yield of phosphorus-added strengthened bake-hardened steel products is increased, and the production cost of the high-strengthened bake-hardened steel is reduced.

Description

Technical Field

[0001] The present invention belongs to the technical field of iron and steel metallurgy steelmaking process, and in particular relates to a method for accurately controlling the titanium content of a high-strength bake-hardened steel component. Background Art

[0002] High-strength bake-hardening steel is made by adding an appropriate amount of Nb or Ti to ultra-low carbon steel, so that most of the C and N atoms in the steel are fixed into carbonitrides Ti(CN) and Nb(CN) to ensure its deep drawability, and then adding solid solution strengthening elements such as Mn and P to obtain a material with a certain strength level. The C, N, and S atoms in the steel are fixed into carbonitrides to ensure its deep drawability. Bake-hardening steel is softer before stamping and is easy to form. The strength increases during the baking process after forming, which improves the anti-sag performance of the parts. Therefore, high-strength bake-hardening steel is widely used in the production of automotive outer panels.

[0003] High-strength bake-hardening steel can well solve the contradiction between the strength, stamping performance and local dent resistance of automobile steel plates, and also reduce the thickness of automobile panels. It has been widely used in automobile manufacturing and other fields, and is one of the current research and development focuses of automobile steel plates. The bake hardening (BH) value of high-strength bake-hardening steel can be obtained through experimental measurement. A certain amount of pre-strain is given to the bake-hardening steel, and it is heated to a certain temperature and then kept warm. The yield strength is measured. It is found that the yield strength after pre-strain is improved compared with the original yield strength. This strength increase value is the bake hardening (BH) value. Major domestic and foreign automobile manufacturers require steel properties to develop in the direction of light weight and high strength. High-strength bake-hardening steel, with its good formability before baking and increased strength after baking, will surely become one of the research and development focuses of major domestic steel companies in my country. At present, the research and development and production of high-strength bake-hardening steel in China is still in the development stage, and a variety of new technologies and new processes for bake-hardening steel plates still need to be vigorously developed.

[0004] The carbon and nitrogen contents of high-strength bake-hardening steel are not stably controlled during the steelmaking process. The carbon content is controlled at about 20ppm and the nitrogen content is controlled at about 20ppm during the RH vacuum treatment process. The carbon and nitrogen elements in the molten steel will increase slightly during the casting process of the casting machine. The Ti element is added during the RH vacuum process. Since the carbon and nitrogen contents are not stably controlled, the difficulty of controlling the Ti content in the RH vacuum treatment process is increased. Therefore, developing a method for accurately controlling the titanium content of high-strength bake-hardening steel is a key control process for achieving mass production of high-strength bake-hardening steel. Summary of the invention

[0005] The purpose of the present invention is to provide a method for accurately controlling the titanium content of high-strength bake-hardened steel. The present invention realizes the method for accurately controlling the titanium content of high-strength bake-hardened steel and achieves stable control of the free carbon content. By analyzing the influencing factors of unstable control of the carbon and nitrogen contents in the steelmaking and continuous casting process, stabilizing the carbon and nitrogen contents of molten steel, and proposing a method for accurately controlling the Ti content in steel, the free carbon of high-strength bake-hardened steel can be accurately controlled.

[0006] In order to solve the above technical problems, the present invention adopts the following technical solutions:

[0007] The present invention provides a method for accurately controlling the titanium content of a high-strength bake-hardened steel component, comprising:

[0008] (1) Use carbon-free steel ladles, which have good thermal stability and a ladle turnover time of less than 90 minutes.

[0009] (2) Ensure the accuracy of the weighing equipment of the RH furnace unloading system and the normal operation of the RH vacuum system;

[0010] (3) Using KR desulfurization, the Si content of the molten iron entering the converter is 0.30-0.60%, the sulfur content is required to be less than 0.002%, and the slag cleaning area of ​​the molten iron is greater than 95%;

[0011] (4) The converter is successfully carbonized once, the converter terminal tapping temperature is controlled to be ≥1670°C, the terminal oxygen content is controlled to be between 650ppm and 850ppm, and the tapping time is between 5min and 7min;

[0012] (5) After the RH furnace vacuum decarburization is completed, the oxygen content at the decarburization endpoint is controlled within 300 ppm, the number of Al2O3 inclusions in the steel is reduced, the inclusions in the molten steel are controlled within level 1.0, and the castability of the molten steel is improved;

[0013] (6) The casting machine uses a hollow stopper rod and adopts protective casting throughout the whole process to prevent secondary oxidation of the molten steel. The carbon and nitrogen content is controlled within 5ppm.

[0014] Furthermore, the temperature of the molten iron entering the furnace is 1353°C.

[0015] Furthermore, the converter successfully pulled carbon in one go.

[0016] Furthermore, the final temperature of the converter is controlled at 1709°C and the oxygen content of the steel out of the converter is controlled at 786ppm.

[0017] Furthermore, steel was tapped using a turnover bag, and the temperature measured after the converter was 1631°C. Lime and modifiers were added to the converter steel to modify the top slag.

[0018] Furthermore, 1000 kg of small-grain lime is added during the steel-making process, and 400 kg of modifier is added at the end of steel-making.

[0019] Furthermore, after RH re-pressing, 50 kg of top slag modifier and 100 kg of aluminum particles were added to perform top slag modification.

[0020] Further, titanium iron is added to the RH later, according to the formula %Ti = 4×(%Ctotal-%Cfree-%Nb / 7.75)+3.42×%(N+N 增氮 )+1.5×%S to calculate the required titanium content in molten steel.

[0021] The special features of the technical solution of the present invention are:

[0022] (1) The oxidizability of the converter slag for high-strength bake-hardening steel is reduced to less than 15%, and the RH is reduced to less than 8% through secondary top slag modification;

[0023] (2) Stable high-strength bake-hardening steel product composition control C≤30ppm, N≤30ppm, S≤80ppm, TO≤25ppm;

[0024] (3) Improve the cleanliness of molten steel for enhanced bake hardening, ensure the castability of continuous casting, and control all kinds of inclusions within 1.0;

[0025] (4) During the RH vacuum treatment, the formula %Ti = 4×(%Ctotal-%Cfree-%Nb / 7.75)+3.42×%(N+N 增氮 )+1.5×%S calculates the required titanium content in molten steel and accurately controls the Ti content in molten steel.

[0026] Compared with the prior art, the beneficial technical effects of the present invention are:

[0027] The present invention optimizes the process of each steelmaking continuous casting process, optimizes the law of carbon addition and nitrogen addition in the steelmaking continuous casting process, stably controls the Ti content of the component in the molten steel during the steelmaking process, controls the free carbon content of the product component between 10ppm and 20ppm, realizes a method for accurately controlling the titanium content of phosphorus-enhanced bake-hardening steel, improves the product yield of phosphorus-enhanced bake-hardening steel, and reduces the production cost of high-strength bake-hardening steel. DETAILED DESCRIPTION

[0028] The following examples are used to specifically illustrate the present invention. These examples are only general descriptions of the present invention and do not limit the present invention.

[0029] Embodiment 1

[0030] In order to explore the carbon and nitrogen addition during the casting process, the last cast of the 9th cast was made of high-strength bake-hardening steel HC180B.

[0031] First, we confirmed the factors that affect the smelting process, such as test time and test accuracy. We organized the production of the first five heats of IF steel and explored the changes in the composition of nitrogen and carbon during the casting process of the smelting casting machine, so as to better and more accurately control the free carbon.

[0032] In order to ensure the control of low sulfur content in the finished bake-hardened steel product, the influence of sulfur content in molten steel and slag in the previous batch on this batch, as well as the influence of turnover time on temperature drop, should be considered when selecting the ladle; the previous batch of 5# ladle was DC03, the sulfur content of the finished product of the casting machine was 0.005%, the ladle turnover time was 70 minutes, the ladle was in good condition, and it fully met the ladle turnover control requirements. The specific ladle turnover situation is shown in Table 1.

[0033] Table 1 Ladle turnover status

[0034]

[0035] The Si content of the molten iron entering the furnace in this batch is 0.52%, which basically meets the smelting requirements. This batch KR adopts deep desulfurization process, with the amount of desulfurizer added being 2.8 tons. The S content of the molten iron entering the furnace is 0.001%, the P content of the molten iron entering the furnace is relatively high at 0.132%, and the temperature of the molten iron entering the furnace is 1353℃. The molten iron conditions are shown in Table 2.

[0036] Table 2: Molten iron entering the furnace (%)

[0037]

[0038] The converter was successfully carbonized in one step, the final temperature of the converter was controlled at 1709℃, the oxygen content of the steel tapped from the converter was controlled at 786ppm, a turnover bag was used to tap the steel, the temperature measured after the converter was 1631℃, lime and modifier were added to the steel tapped from the converter for top slag modification, and the process parameters of the converter are shown in Table 3.

[0039] Table 3 Converter process parameters

[0040]

[0041] The converter endpoint composition control meets the requirements, the sulfur content of converter steel is 0.003%, and the converter endpoint composition control meets the requirements (see Table 4).

[0042] Table 4 Converter end point composition (%)

[0043] serial number C Mn P S 1# 0.035 0.08 0.031 0.003

[0044] RH is subjected to vacuum decarburization treatment. In order to prevent the addition of ferrophosphorus to the molten steel to increase carbon, ferrophosphorus is added during the decarburization stage to ensure the phosphorus content in the molten steel. After 15 minutes of decarburization, the CO content of the tail gas reaches below 1%, and a sample is taken. The oxygen content at the end of 20 minutes of decarburization is 262ppm, and deoxidation operation is carried out. According to the results obtained in the laboratory, titanium iron and manganese iron are added for alloying operation. The process parameters of the vacuum treatment process are shown in Table 5.

[0045] Table 5 Vacuum treatment process parameters

[0046]

[0047] The oxidizability of the converter final slag is slightly higher, with a total iron content of 21.44%. 1000kg of small-grained lime is added during the steel-making process, and 400kg of modifier is added at the end of steel-making. The iron oxide TFe content in the slag is reduced from 21.44% to 14.04%. After RH re-pressing, 50Kg of top slag modifier and 100Kg of aluminum particles are added to modify the top slag. After modification, the iron oxide TFe content of the steel slag is 5.70%. The slag results of the smelting process are shown in Table 6.

[0048] Table 6 Slag sample test results (%)

[0049]

[0050] The inclusion test of this heat found that except for inclusion D which was level 1.0, the test results of other inclusions were all level 0, which met the design requirements (as shown in Table 7).

[0051] Table 7 Inspection of inclusions (%)

[0052]

[0053] The RH furnace is vacuum decarburized. From the carbon content of G1 in RH position, it can be known that the carbon content in the molten steel reaches 10ppm after 15 minutes of RH decarburization. After 20 minutes, aluminum particles are added for deoxidation. After 3 minutes, manganese is added for alloying. Titanium iron is added in the later stage of RH. According to the formula %Ti=4×(%Ctotal-%Cfree-%Nb / 7.75)+3.42×%(N+N 增氮)+1.5×%S calculates the required titanium content in the molten steel, and sets the target free carbon in the steel to 16ppm. According to the previous furnace analysis, the carbon and nitrogen addition of the tundish sampling is 5ppm, and the titanium content added is 0.014% in combination with G3. The Ti content of the component added after the titanium alloy is added to RH is 0.014%. The casting machine uses a hollow stopper rod and adopts protective casting throughout the process to prevent the secondary oxidation of the molten steel. The carbon addition of the tundish sampling is 0.004%, and the nitrogen addition is 0.005%. According to the composition of the casting machine tundish according to the above company formula, the free carbon is calculated to be 16ppm, and the free carbon Cfree is controlled between 10ppm and 20ppm. By accurately controlling the Ti content of the molten steel, the free carbon content in the molten steel is successfully controlled. The composition of the molten steel during the baking hardening steel smelting process is shown in Table 8.

[0054] Table 8 Molten Steel Composition in Bake Hardening Process (%)

[0055]

[0056] The embodiments described above are only descriptions of the preferred embodiments 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 accurately controlling the titanium content of a high-strength bake-hardened steel component, characterized in that: include: (1) Use carbon-free steel ladles, which have good thermal stability and a ladle turnover time of less than 90 minutes. (2) Ensure the accuracy of the weighing equipment of the RH furnace unloading system and the normal operation of the RH vacuum system; (3) Using KR desulfurization, the Si content of the molten iron entering the converter is 0.30-0.60%, the sulfur content is required to be less than 0.002%, and the slag cleaning area of ​​the molten iron is greater than 95%; (4) The converter is successfully carbonized once, the converter terminal tapping temperature is controlled to be ≥1670°C, the terminal oxygen content is controlled to be between 650ppm and 850ppm, and the tapping time is between 5min and 7min; (5) After the RH furnace vacuum decarburization is completed, the oxygen content at the decarburization endpoint is controlled within 300 ppm, the number of Al2O3 inclusions in the steel is reduced, the inclusions in the molten steel are controlled within level 1.0, and the castability of the molten steel is improved; (6) The casting machine uses a hollow stopper rod and adopts protective casting throughout the whole process to prevent secondary oxidation of the molten steel. The carbon and nitrogen content is controlled within 5ppm.

2. The method for accurately controlling the titanium content of high-strength bake-hardening steel according to claim 1, characterized in that: The temperature of molten iron entering the furnace is 1353℃.

3. The method for accurately controlling the titanium content of high-strength bake-hardening steel according to claim 1, characterized in that: The converter successfully pulled carbon in one go.

4. The method for accurately controlling the titanium content of high-strength bake-hardening steel according to claim 1, characterized in that: The final temperature of the converter is controlled at 1709°C and the oxygen content of the steel out of the converter is controlled at 786ppm.

5. The method for accurately controlling the titanium content of high-strength bake-hardening steel according to claim 1, characterized in that: Steel was tapped using a turnover bag, and the temperature measured after the converter was 1631°C. Lime and modifiers were added to the steel tapped from the converter to modify the top slag.

6. The method for accurately controlling the titanium content of high-strength bake-hardening steel according to claim 1, characterized in that: 1000kg of small-grain lime is added during the steel-making process, and 400kg of modifier is added at the end of steel-making.

7. The method for accurately controlling the titanium content of high-strength bake-hardening steel according to claim 1, characterized in that: After RH re-pressing, 50 kg of top slag modifier and 100 kg of aluminum particles were added to modify the top slag.

8. The method for accurately controlling the titanium content of high-strength bake-hardening steel according to claim 1, characterized in that: Titanium iron is added in the later stage of RH, according to the formula %Ti=4×(%Ctotal-%Cfree-%Nb / 7.75)+3.42×%(N+N 增氮 )+1.5×%S to calculate the required titanium content in molten steel.

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