750MPa-grade high-strength steel and production method thereof

Abstract

The invention relates to the technical field of steel and iron materials, in particular to 750MPa-grade high-strength steel and a production method thereof.The 750MPa-grade high-strength steel comprises, by weight, 0.06%-0.08% of C, smaller than or equal to 0.20% of Si, 1.70%-1.90% of Mn, smaller than or equal to 0.050% of Als, 0.050%-0.070% of Nb, 0.30%-0.40% of Cu, 0.40%-0.50% of Cr, smaller than or equal to 0.020% of P, smaller than or equal to 0.003% of S, smaller than or equal to 0.0060% of N and the balance Fe and inevitable inclusions. The production method of the high-strength steel mainly comprises the steps of steelmaking and hot rolling. Steelmaking comprises a raw material treatment procedure, a converter smelting procedure, a refining procedure and a continuous casting procedure; the hot rolling comprises a heating process of a heating furnace, a rough rolling process, a finish rolling process and a cooling and coiling process. The high-strength steel with high strength, high corrosion resistance and high formability can be obtained by adopting the composition of low C, low Si and medium-high Mn and combining an optimized hot rolling process.

Description

Technical Field

[0001] This invention relates to the field of steel materials technology, and in particular to a 750MPa high-strength steel and its production method. Background Technology

[0002] With increasingly stringent carbon emission requirements, lightweighting has become a core trend in the automotive industry. Related research shows that using lightweight materials in automobiles can reduce weight by 25%, thereby reducing fuel consumption by 18%; in the construction sector, reducing steel usage by 30% can reduce carbon emissions over the entire lifecycle by 38%. This demonstrates the significant effectiveness of lightweighting in reducing energy consumption and carbon emissions.

[0003] Furthermore, in recent years, the demand for infrastructure projects such as large-scale engineering, bridges, and basic construction in my country has remained robust, and transportation media such as concrete mixer trucks and sand and gravel transport vehicle bodies continue to maintain a large demand. However, existing steel used for mixing tanks suffers from problems such as poor wear resistance, low strength, difficulty in forming, springback, and poor shape control.

[0004] Therefore, there is an urgent need to design a 750MPa grade high-strength steel and its production method to meet the application requirements of lightweight, high strength, wear and corrosion resistance and high formability. Summary of the Invention

[0005] To overcome the shortcomings of existing technologies, the technical problem solved by this invention is to provide a 750MPa grade high-strength steel and its production method. This invention adopts a combination of low C, low Si, and medium-high Mn composition, combined with an optimized hot rolling process, to obtain high-strength steel with high strength, high wear and corrosion resistance, and high formability.

[0006] To achieve the above objectives, the present invention employs the following technical solution: A 750MPa grade high-strength steel, wherein the chemical composition and weight percentage of the high-strength steel are as follows: C: 0.06~0.08%, Si≤0.20%, Mn1.70~1.90%, Als≤0.050%, Nb: 0.050~0.070%, Cu: 0.30~0.40%, Cr: 0.40~0.50%, P≤0.020%, S≤0.003%, N≤0.0060%, with the balance being Fe and unavoidable inclusions.

[0007] The main elements function as follows: C: 0.06~0.08wt%. When the carbon mass fraction is <0.09%, the material has better formability and weldability. When the carbon mass fraction is 0.09%~0.16%, the peritectic reaction L+δ→γ during the solidification process of molten steel is more intense, and the maximum volume shrinkage rate can reach 0.38%, resulting in large changes in the linear shrinkage of the solidified billet shell. When the linear shrinkage reaches a certain level, it will cause the air gap between the continuously cast billet shell and the crystallizer to increase, leading to the occurrence of depressions. At the same time, the heat flow decreases, the initial billet shell becomes thinner, and under the action of thermal stress and other stresses, cracks form at the troughs of the depressions. Therefore, the C content is 0.06~0.08wt%. Si: ≤0.20wt%. Si has a strong affinity for O and is a strong deoxidizing element. It exists in steel in solid solution form. Si can improve the strength, fatigue limit, corrosion resistance, and wear resistance of steel. Adding a certain amount of ferrosilicon to molten steel for deoxidation and alloying often yields better results. At the same time, Si is a non-carbide-forming element, which promotes the diffusion of carbon from ferrite into austenite during phase transformation, increases the phase transformation temperature, accelerates the ferrite phase transformation, and is beneficial for increasing the ferrite content in the microstructure and improving formability.

[0008] Mn: 1.70~1.90 wt%. It exists in steel in a solid solution state and is a solid solution strengthening element, which can improve the strength of ferrite. Mn in low-carbon steel has a significant effect on improving strength. However, Mn and S easily form MnS plastic inclusions, which elongate along the rolling direction during hot rolling, deteriorating the formability of the steel. Increasing Mn content can also cause segregation defects; therefore, the Mn content should not be too high.

[0009] Als: Not greater than 0.0050 wt%. During high-temperature processes, Al often precipitates as AlN. The lower the content of Al and N, the lower the precipitation temperature. Since grain boundaries have high interfacial energy, which is conducive to nucleation, AlN usually precipitates at the grain boundaries of austenite, reducing the grain boundary strength. This leads to tensile stress in the straightening section, causing corner cracks in the cast billet, which is not conducive to hot running. Therefore, the content of Als is controlled at a low level.

[0010] Nb: 0.050~0.070wt%. The most effective grain-refining element in controlled-rolled steel. Adding Nb expands the temperature range of the non-recrystallized austenite region, allowing more rolling deformation to occur within this region. The deformed austenite can increase the ferrite nucleation rate during the γ-α phase transformation, and Nb also has a certain precipitation strengthening effect.

[0011] Cr: 0.40~0.50wt%. Cr can increase the hardenability of steel and has a secondary hardening effect, which can increase the hardness of steel without making it brittle. A lower Cr content in steel can, to some extent, expand the bainite region, making it easier to obtain bainite, which is a relatively ideal microstructure.

[0012] Cu: 0.30~0.40wt%. When copper is added to steel as an alloy, it mainly improves corrosion resistance. In some steels, it can also cause precipitation strengthening, increasing strength. However, it is prone to copper cracking, so care must be taken during production.

[0013] N: Not more than 0.0060 wt%. Excessive N content will react with Al and Ti, easily forming AlN and sharp-cornered inclusions TiN, thus affecting the surface quality and performance of the product.

[0014] P: Not greater than 0.020 wt%. Generally speaking, phosphorus is a harmful element in steel, increasing its cold brittleness, deteriorating its weldability, reducing its plasticity, and worsening its cold bending performance.

[0015] S: Not more than 0.003 wt%. As a harmful element, it causes hot brittleness in steel, reduces the ductility and toughness of steel, makes it prone to cracking during rolling, and is also detrimental to weldability.

[0016] A method for producing 750MPa high-strength steel mainly includes steelmaking and hot rolling; the steelmaking process includes raw material processing, converter smelting, refining, and continuous casting; the hot rolling process includes furnace heating, rough rolling, finish rolling, and cooling and coiling. The heating process of the heating furnace is as follows: the upper temperature of the preheating section is 700-1000℃, the lower temperature of the preheating section is 700-1000℃, and the heating time of the preheating section is 30-40 minutes. The upper part of the first heating section has a temperature of 1130-1170℃, the lower part of the first heating section has a temperature of 1100-1150℃, the heating time of the first heating section is 50-60 minutes, and the weak oxidizing atmosphere of the first heating section is 1:2.2-2.4 (air-fuel ratio, i.e., the volume ratio of coal gas to air). The upper temperature of the second heating section is 1200-1250℃, the lower temperature of the second heating section is 1180-1230℃, the heating time of the second heating section is 60-70 min, and the weak oxidizing atmosphere of the second heating section is 1:1.9-2.1. The upper temperature of the soaking zone is 1180–1230℃, the lower temperature is 1150–1200℃, the heating time in the soaking zone is greater than or equal to 30 min, and the weak oxidizing atmosphere in the soaking zone is 1:2.1–2.5; the average heating time in the furnace is greater than 130 min but not more than 200 min, and the furnace exit temperature is 1180℃–1220℃; the furnace pressure is controlled at a slightly positive pressure, and the excess air coefficient is 2.0.

[0017] Furthermore, the raw material processing steps include: pre-treatment of the furnace feed with S≤0.0030%, and removal of slag; and the use of refined scrap steel.

[0018] Furthermore, the converter smelting process is as follows: the converter smelting adopts deep decarburization operation and constant oxygen pressure variable lance position operation, and adopts single slag method smelting and blowing, with oxygen supply time of 14~18min. Before tapping, the ladle is purged with argon gas, and slag is blocked in the early stage and slag is blocked in the later stage of tapping to ensure that the slag thickness in the ladle is less than 100mm; the tapping time is guaranteed to be 4~7min, the shape of the tapping spout is controlled to avoid tapping spillage, deoxidation and alloying are started when 1 / 5 of the steel is tapped, and alloying is completed when 4 / 5 of the steel is tapped.

[0019] Furthermore, the refining process: refining adopts a dual-path LF+RH furnace; soft argon blowing for ≥10 minutes before the end of the process, and the LF departure temperature of a normal furnace is controlled at 1540~1560℃.

[0020] Furthermore, in the continuous casting process: the arc connection qualification rate between the casting machine crystallizer and the zero section, and between the zero section and the first sector section is ≥99%; the arc connection qualification rate between the first and second sections, between the 7th and 8th sections, within the section, and the main body is ≥98%, with a measurement standard of ±0.3mm; protective pouring is carried out throughout the process; the constant casting speed is controlled at 1.0~1.5m / min, and the target for continuous casting superheat control is ≤25℃.

[0021] Furthermore, the roughing process adopts a 3+3 mode, controlling the final rolling temperature of R1 to be 950~1030℃, the final rolling temperature of R2 to be 1020~1100℃, the thickness of the intermediate billet to be 30~80mm, and the cumulative reduction rate in the roughing stage to be greater than 70%; the descaling water pressure in the roughing stage is above 18MPa.

[0022] Furthermore, the finishing rolling process includes: a finishing rolling inlet temperature of 1000~1090℃ for F1, a finishing rolling temperature of 830~880℃ for F7, a finishing rolling time of 60~100s, and a finishing rolling descaling water pressure of over 23MPa.

[0023] Furthermore, the cooling winding process employs intermittent cooling, with a winding temperature of 450~490℃.

[0024] Compared with the prior art, the beneficial effects of the present invention are: 1) This invention significantly improves formability and weldability while ensuring strength through a combination of low C, low Si, and medium-high Mn composition. Low C control avoids surface cracks and depressions on the continuously cast billet caused by the intense shrinkage of the peritectic reaction; low Si can improve the surface quality of the steel plate and promote ferrite phase transformation; solid solution strengthening of Mn contributes to the main strength basis.

[0025] 2) The heating regime of this invention is optimized: low-temperature steelmaking combined with a weak oxidizing atmosphere, and control of heating and holding time and furnace exit temperature, to achieve austenite homogenization and suppress copper embrittlement (Cu enrichment leads to surface cracking).

[0026] 3) In the finishing rolling process of this invention, the F1 entry temperature is 1000~1090℃ and the F7 final rolling temperature is 830~880℃, and the deformation is completed in the non-recrystallization zone; combined with 23MPa high-pressure descaling, the surface quality is ensured; intermittent cooling + coiling at 450~490℃ promotes the transformation of bainite / acicular ferrite, taking into account both strength and formability.

[0027] 4) The high-strength steel obtained by this invention has a tensile strength ≥750MPa, an elongation ≥20%, and a low corrosion rate, which can meet the application requirements of lightweight, high strength, wear and corrosion resistance and high formability in the fields of automotive structural parts and mechanical equipment. Attached Figure Description

[0028] The accompanying drawings, which form part of this invention, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an undue limitation of the invention. In the drawings: Figure 1 This is a metallographic diagram of Embodiment 2 of the present invention.

[0029] Figure 2 This is a metallographic diagram of Comparative Example 1 of the present invention. Detailed Implementation

[0030] The specific embodiments of the present invention will be further described below with reference to the accompanying drawings: Example: A 750MPa grade high-strength steel, wherein the chemical composition and weight percentage of the high-strength steel are as follows: C: 0.06~0.08%, Si≤0.20%, Mn1.70~1.90%, Als≤0.050%, Nb: 0.050~0.070%, Cu: 0.30~0.40%, Cr: 0.40~0.50%, P≤0.020%, S≤0.003%, N≤0.0060%, with the balance being Fe and unavoidable inclusions.

[0031] A method for producing 750MPa high-strength steel mainly includes steelmaking and hot rolling.

[0032] I. Steelmaking: (1) Raw material processing steps: Pre-treated furnace feed S≤0.0030%, slag removed; refined scrap steel is used.

[0033] (2) Converter smelting process: The converter smelting adopts deep decarburization operation and constant oxygen pressure variable lance position operation, and adopts single slag smelting and blowing method. The oxygen supply time is 14~18min. Argon gas is used to purge the ladle before tapping. Slag is blocked in the early stage and blocked with slag darts in the later stage of tapping to ensure that the slag thickness in the ladle is less than 100mm. The tapping time is guaranteed to be 4~7min. The shape of the tapping spout is controlled to avoid slag spillage. The above measures can effectively reduce the oxygen content in the steel, thereby indirectly controlling the aluminum content. Deoxidation and alloying are started when 1 / 5 of the steel is tapped, and the alloying is completed when 4 / 5 of the steel is tapped.

[0034] (3) Refining process: The refining process uses a dual-path LF+RH furnace; the LF furnace uses active lime and fluorite to create a fluid reducing slag, and avoids exposing the molten steel as much as possible; before the end of the process, soft argon blowing is performed for ≥10 minutes, and the LF temperature at the station for normal furnace cycles is controlled at 1540~1560℃.

[0035] (4) Continuous casting process: The arc connection qualification rate between the casting machine crystallizer and the zero section, and between the zero section and the first sector section, is ≥99%. The arc connection qualification rate between the first and second sections, between sections 7 and 8, within sections, and on the main body is ≥98%, with a measurement standard of ±0.3mm. Protective casting is performed throughout the entire process. The constant casting speed is controlled at 1.0~1.5m / min, and the target superheat of continuous casting is ≤25℃.

[0036] II. Hot Rolled: (1) Heating process of heating furnace: strictly control the heating time and heating temperature of each section in the furnace, as shown in Table 1. The furnace outlet temperature is controlled at the target temperature of 1200℃. The average heating time in the furnace is greater than 130min but not more than 180min. The furnace pressure is controlled at a slightly positive pressure. The excess air coefficient is 2.0, which can effectively reduce copper cracking and gas consumption. The gas consumption per unit is reduced to 1.1GJ / t.

[0037] Table 1 Heating time and temperature for each section of the furnace (2) Rough rolling process: A 3+3 rolling mode is adopted, controlling the final rolling temperature of R1 at 950~1030℃ and R2 at 1020~1100℃. High-temperature water spray descaling can better remove iron oxide scale from the steel surface. The intermediate slab thickness is 30~80mm, and the cumulative reduction rate in the roughing stage is greater than 70%. The water pressure for roughing descaling is above 18MPa to ensure the surface quality of the slab.

[0038] (3) Finishing rolling process: The inlet temperature of finishing mill F1 is 1000~1090℃, the final rolling temperature of F7 is 830~880℃, the finishing mill rolling time is 60~100s, and the descaling water pressure of finishing mill is above 23MPa.

[0039] (4) Cooling coiling: The cooling mode is intermittent cooling, and the winding temperature is 450~490℃, which is conducive to bainite precipitation and good forming performance.

[0040] Following the above preparation method, 750MPa grade high-strength steel was prepared according to the composition in Table 2 and the processes in Tables 3 and 4, resulting in Examples 1-4 and Comparative Example 1.

[0041] Table 2 Chemical Composition / % Table 3 Hot Rolling Process (I) Table 4 Hot Rolling Process (II) Table 5 Mechanical Properties Table 6 Comparison (Components / %) Table 7 Comparison (Performance) The composition and performance comparison of Example 2 and Comparative Examples 1 and 2 are shown in Tables 6 and 7. As can be seen from Table 7, the strength, wear resistance, and other properties of the high-strength steel involved in this invention are significantly improved. The corrosion test method was as follows: 0.01 mol / L sodium bisulfite solution at 20°C for 168 hours.

[0042] Figure 1 The metallographic structure of Example 2 is F+B, with a grain size of 12.5.

[0043] Figure 2 The metallographic structure of Comparative Example 1 is F+P, with a grain size of grade 13.

[0044] Example 2 shows a ferrite + bainite microstructure obtained through low-temperature winding. This is suitable for comparative applications of products with complex molding processes or requiring high formability. The structural components used in this patent, such as mixing tanks, meet these requirements, thus exhibiting better formability compared to the conventional ferrite + pearlite microstructure in the comparative example, as evidenced by the elongation. Furthermore, bainite has higher hardness than pearlite, which is also reflected in the higher HV10. Simultaneously, the bainite microstructure is more uniform, with carbides consisting mainly of finely dispersed particles, resulting in smaller electrode potential differences and superior corrosion resistance. In summary, ferrite + bainite not only provides higher strength and hardness but also better formability and corrosion resistance.

[0045] The preferred embodiments of the present invention have been described in detail above with reference to the accompanying drawings. However, the present invention is not limited to the specific details of the above embodiments. Within the scope of the technical concept of the present invention, various simple modifications can be made to the technical solutions of the present invention, and these simple modifications all fall within the protection scope of the present invention. Furthermore, it should be noted that the various specific technical features described in the above specific embodiments can be combined in any suitable manner without contradiction. To avoid unnecessary repetition, the present invention will not describe the various possible combinations separately. In addition, various different embodiments of the present invention can also be arbitrarily combined, as long as they do not violate the spirit of the present invention, and should also be considered as the content disclosed by the present invention.

Claims

1. A 750MPa grade high-strength steel, characterized in that, The chemical composition and weight percentage of the high-strength steel are as follows: C: 0.06~0.08%, Si≤0.20%, Mn1.70~1.90%, Als≤0.050%, Nb: 0.050~0.070%, Cu: 0.30~0.40%, Cr: 0.40~0.50%, P≤0.020%, S≤0.003%, N≤0.0060%, with the balance being Fe and unavoidable inclusions.

2. The method for producing 750MPa grade high-strength steel according to claim 1, characterized in that, It mainly includes steelmaking and hot rolling; the steelmaking process includes raw material processing, converter smelting, refining and continuous casting; the hot rolling process includes furnace heating, roughing, finishing and cooling coiling. The heating process of the heating furnace is as follows: the upper temperature of the preheating section is 700-1000℃, the lower temperature of the preheating section is 700-1000℃, and the heating time of the preheating section is 30-40 minutes. The upper part of the first heating section has a temperature of 1130-1170℃, the lower part of the first heating section has a temperature of 1100-1150℃, the heating time of the first heating section is 50-60 minutes, and the weak oxidizing atmosphere of the first heating section is 1:2.2-2.4 (air-fuel ratio, i.e., the volume ratio of coal gas to air). The upper temperature of the second heating section is 1200-1250℃, the lower temperature of the second heating section is 1180-1230℃, the heating time of the second heating section is 60-70 min, and the weak oxidizing atmosphere of the second heating section is 1:1.9-2.

1. The upper temperature of the soaking zone is 1180–1230℃, the lower temperature is 1150–1200℃, the heating time in the soaking zone is greater than or equal to 30 min, and the weak oxidizing atmosphere in the soaking zone is 1:2.1–2.5; the average heating time in the furnace is greater than 130 min but not more than 200 min, and the furnace exit temperature is 1180℃–1220℃; the furnace pressure is controlled at a slightly positive pressure, and the excess air coefficient is 2.

0.

3. The method for producing 750MPa grade high-strength steel according to claim 2, characterized in that, The raw material processing steps are as follows: pre-treatment of the furnace feed S≤0.0030%, slag removal; and the use of refined scrap steel.

4. The method for producing 750MPa grade high-strength steel according to claim 2, characterized in that, The converter smelting process is as follows: the converter smelting adopts deep decarburization operation and constant oxygen pressure variable lance position operation, and adopts single slag method smelting and blowing. The oxygen supply time is 14~18min. The ladle is purged with argon before tapping. Slag blocking is carried out in the early stage and slag blocking darts are used in the later stage of tapping to ensure that the slag thickness in the ladle is less than 100mm. The tapping time is guaranteed to be 4~7min. The shape of the tapping spout is controlled to avoid tapping spillage. Deoxidation and alloying are started when 1 / 5 of the steel is tapped and the alloying is completed when 4 / 5 of the steel is tapped.

5. The method for producing 750MPa grade high-strength steel according to claim 2, characterized in that, The refining process: refining adopts a dual-path LF+RH furnace; soft argon blowing for ≥10min before the end of the process, and the LF departure temperature of a normal furnace is controlled at 1540~1560℃.

6. The method for producing 750MPa grade high-strength steel according to claim 2, characterized in that, The continuous casting process is as follows: the arc connection qualification rate between the casting machine crystallizer and the zero section, and between the zero section and the first sector section is ≥99%; the arc connection qualification rate between the first and second sections, between the 7th and 8th sections, within the section, and the main body is ≥98%, with a measurement standard of ±0.3mm; protective pouring is carried out throughout the process; the constant casting speed is controlled at 1.0~1.5m / min, and the target for continuous casting superheat control is ≤25℃.

7. The method for producing 750MPa grade high-strength steel according to claim 2, characterized in that, The roughing process adopts a 3+3 mode, controlling the final rolling temperature of R1 to be 950~1030℃, the final rolling temperature of R2 to be 1020~1100℃, the thickness of the intermediate billet to be 30~80mm, and the cumulative reduction rate in the roughing stage to be greater than 70%; the descaling water pressure in the roughing stage is above 18MPa.

8. The method for producing 750MPa grade high-strength steel according to claim 2, characterized in that, The finishing rolling process is as follows: the inlet temperature of finishing rolling F1 is 1000~1090℃, the final rolling temperature of F7 is 830~880℃, the finishing rolling time is 60~100s, and the water pressure for descaling in finishing rolling is above 23MPa.

9. A method for producing 750MPa grade high-strength steel according to claim 2, characterized in that, The cooling winding process uses intermittent cooling and a winding temperature of 450~490℃.

Images