A method for manufacturing a low yield ratio high toughness thin gauge TMCP state Q420GJE

Abstract

The application discloses a kind of low yield ratio high toughness thin specification TMCP state Q420GJE manufacturing method, belong to high construction steel plate production field.The application optimizes steel chemical composition, limit C, Si, Mn, Cr, Nb, Ti, Al and the content of other elements, strictly control P, S, N impurities, and satisfy Ti / N≥3, V+Nb+Ti≤0.15% Matching requirement.Product process is KR desulphurization, converter blowing, LF+RH double refining, continuous casting in turn and obtains slab;Slab is heated after 1110~1140 ℃ long time, using subsection temperature control rolling process;After rolling, using double segment variable speed cooling mode, open cooling temperature 700~800 ℃, final cooling temperature 300~480 ℃, regulate and control ferrite and bainite structure ratio.The application can stably output 10~35mm thin specification steel plate, product yield strength 420~540MPa, tensile strength 530~680MPa, yield ratio≤0.83,‑40 ℃ full-size impact energy≥47J.Effectively solve TMCP state thin specification steel plate yield ratio is high, undercooling is excessive, head and tail black head and other problems, product comprehensive mechanical property is excellent, suitable for large-scale production.

Description

Technical Field

[0001] This invention relates to the field of high-strength steel plate production, and more particularly to a manufacturing method for thin-gauge TMCP-state Q420GJE steel with low yield strength ratio and high toughness. Background Technology

[0002] High-strength structural steel is among the most challenging products in the thick plate category. Both tensile and yield strength have upper and lower limits; achieving the lower or upper limit is relatively easy, but maintaining a range within a certain range is difficult. Furthermore, the yield-to-tensile strength ratio requirement is very strict. The new standard for high-strength structural steel came into effect in 2024, changing the lower yield strength to an upper yield strength, further increasing production difficulty. Low yield-to-tensile strength ratios are generally produced using dry rolling, but with improved welding efficiency, orders for TMCP (Medium-Temperature Concentrate) delivery are increasing. Thin-gauge steel plates in TMCP are prone to overcooling, making yield-to-tensile strength ratio very difficult to control, and strength variations are significant, making it difficult to maintain a specific range. Thin-gauge steel plates are more sensitive to water and experience faster temperature changes, and are also more prone to blackheads, making it even more difficult to control the strength range and yield-to-tensile strength ratio of thin-gauge steel plates. Summary of the Invention

[0003] Purpose of the invention: The invention provides a manufacturing method for thin-gauge, high-toughness, low-yield-strength-ratio TMCP state Q420GJE steel, which solves the problem of uncontrollable low yield-strength ratio of steel plates when ensuring the strength range of thin-gauge high-strength steel.

[0004] Technical Solution: To solve the above-mentioned technical problems, according to one aspect of the present invention, more specifically, is a manufacturing method for thin-gauge, high-toughness, low-yield-strength-ratio TMCP state Q420GJE steel. The chemical composition (by weight) of this high-strength steel is as follows: C: C is the most basic strengthening element. C dissolves in steel to form interstitial solid solutions, playing a role in solid solution strengthening. It also reacts with strong carbide-forming elements to form carbide precipitation, thus playing a role in precipitation strengthening. However, too high a C content is detrimental to the toughness and weldability of the steel; too low a C content reduces the strength of the steel. Therefore, the present invention controls the C content to be 0.04~0.1%.

[0005] Si: Si is a solid solution strengthening element and also a deoxidizing element in steel. However, excessive content will deteriorate the weldability of steel and is not conducive to the removal of hot-rolled iron oxide scale during rolling. Therefore, the present invention controls the Si content to be 0.2~0.3%.

[0006] Mn: Mn enhances the strength of steel through solid solution strengthening and is the most important and economical strengthening element to compensate for the strength loss caused by the reduction of carbon content. Mn also expands the γ phase region, which can lower the γ→α phase transformation temperature of steel and help obtain fine phase transformation products, thus improving the toughness of steel. However, Mn is a segregating element. When the Mn content is high, Mn tends to segregate in the center of the plate thickness during casting, forming a hard martensitic structure after rolling, which reduces the low-temperature toughness and dynamic tear resistance of the material. Therefore, this invention controls the Mn content to be 1.5-1.7%.

[0007] Cr: Cr is an important element for improving the hardenability of steel, ensuring the uniformity of microstructure and properties of steel plates throughout their thickness. Moreover, when the Cr content is above 0.10%, it can effectively improve the corrosion resistance of steel. However, the addition of too high levels of chromium and manganese to steel at the same time will lead to the formation of low-melting-point Cr-Mn composite oxides, which will cause surface cracks during hot working and severely deteriorate the weldability. Therefore, the Cr content in this invention is limited to 0.1-0.2%.

[0008] Nb: Nb is one of the important elements in low-carbon microalloyed steel. During hot rolling, the Nb dissolved in the steel undergoes strain-induced precipitation to form Nb(N,C) particles, which pin grain boundaries, inhibit the growth of deformed austenite, and suppress recrystallization. Through controlled rolling and controlled cooling, the deformed austenite transforms into fine products with high dislocation density. Too low an Nb content results in insignificant dispersion precipitation and fails to refine grains or strengthen the matrix; too high an Nb content, by inhibiting recrystallization in the steel core, is detrimental to grain refinement. Furthermore, the solid solution of Nb is related to the C content. Too high a C content results in insufficient Nb solid solution, failing to achieve precipitation strengthening and grain sliding effects; too low a C content leads to grain boundary weakening, and too low an Nb content results in insignificant precipitation strengthening. Therefore, in this invention, the Nb content should be limited to 0.03-0.045%, and V+Nb+Ti ≤ 0.15%.

[0009] Ti: Ti is a strong carbonitride forming element. Undissolved Ti carbonitrides can inhibit austenite grain growth when steel is heated. TiN precipitated during rough rolling in the high-temperature austenite region can effectively suppress austenite grain growth and reduce the grain size of rough-rolled austenite. Therefore, in this invention, the Ti content is controlled at 0.011-0.017%, and the Ti / N ratio is ≥3.

[0010] S and P: S and P are unavoidable impurity elements in steel, and their levels should be as low as possible. By controlling the morphology of sulfide inclusions through ultra-low sulfur (KR stirring desulfurization process, reducing it to less than 20 ppm) and Ca treatment, while controlling the P content to below 0.012%, the invented steel can be guaranteed to have good low-temperature impact toughness.

[0011] The manufacturing method of the high-strength structural steel is as follows:

[0012] S1. Smelting and casting: The designed chemical composition is cast. First, it is desulfurized by KR stirring, then blown in a converter, then desulfurized and dehydrogenated by LF+RH double refining, and finally cast in a continuous casting machine to obtain slabs.

[0013] S2. Heating: The slab is heated to a temperature range of 1110~1140℃;

[0014] S3. Rolling: The rolling process includes roughing and finishing. The roughing temperature is greater than 930℃ and the cumulative reduction rate is less than 60%. After the roughing is completed, the rollers are allowed to cool down in the intermediate roller table. When the temperature drops to the specified temperature, the finishing mill is used for rolling. The reduction rate of the first two passes of finishing is less than 10%, and the finishing temperature is less than 830℃.

[0015] S4. Cooling: The steel plate enters the cooling equipment and is cooled by cooling water with different pressures, volumes, and opening angles. At the same time, the speed of the steel plate and the cooling rate of the steel plate are controlled. The initial cooling temperature is controlled at 700℃~800℃, and the final cooling temperature range is 350~450℃.

[0016] Furthermore, the Q420GJE finally obtained in step S4 has the following characteristics: yield strength ReH: 420-540MPa; tensile strength Rm: 530-680MPa; yield-to-tensile ratio requirement ≤0.83 (upper yield strength / tensile strength); and full-size Charpy impact energy AKv ≥47J at -40℃.

[0017] Furthermore, in step S4, the cooling mode of the steel plate is a combination of high and low cooling rates to adjust the ratio of bainite and ferrite. First, a portion of bainite is obtained through a high cooling rate, and then more ferrite is obtained through a low cooling rate. This cooling mode can adjust the ratio of soft and hard phases to control the yield strength ratio, achieving a lower yield strength ratio, while simultaneously controlling the water outlet angle to avoid blackheads at the beginning and end of the flow.

[0018] Beneficial effects:

[0019] (1) This invention uses precise component matching combined with a two-stage controlled cooling process to regulate the ratio of ferrite and bainite structures, resulting in a yield strength ratio of ≤0.83 for the finished steel plate, which completely solves the industry problem of the high yield strength ratio of traditional TMCP thin-specification Q420GJE high-strength steel; the yield strength and tensile strength are stably within the standard range, the Charpy impact energy at -40℃ is significantly up to standard, and the low-temperature toughness is outstanding.

[0020] (2) The present invention optimizes the cooling water pressure, water volume, spray angle and cooling rate, and combines them with a reasonable rolling process to effectively improve common appearance and process defects such as excessive cooling and black head and tail of thin TMCP steel plates, thereby improving the overall quality uniformity of the steel plate.

[0021] (3) The present invention rationally combines alloying elements such as C, Si, Mn, Cr, Nb, and Ti, strictly controls the content of harmful impurities P and S, and follows the ratio requirements such as Ti / N≥3 and V+Nb+Ti≤0.15%. This not only plays the role of solid solution strengthening, precipitation strengthening and grain refinement, but also avoids the segregation of alloying elements and the formation of harmful inclusions, ensuring that the microstructure and mechanical properties of the steel plate are uniform and consistent throughout the thickness. Attached Figure Description

[0022] Figure 1 This is a schematic diagram of the method flow of the present invention. Detailed Implementation

[0023] To make the technical solution of the present invention clearer, the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0024] Example 1

[0025] Reference Figure 1 A manufacturing method for thin-gauge TMCP state Q420GJE with low yield strength ratio and high toughness, the implementation steps are as follows:

[0026] Chemical composition design: The chemical composition is designed according to the following mass percentages: C: 0.062%, Si: 0.25%, Mn: 1.54%, P: 0.0097%, S: 0.001%, Cr: 0.173%, Nb: 0.039%, Ti: 0.015%, Al: 0.0314%, N: 0.0025%, with the remainder being Fe and unavoidable impurities.

[0027] Step 1, Smelting and Casting: The designed chemical composition is cast. First, it is desulfurized by KR stirring, then blown in a converter, then desulfurized and dehydrogenated by LF+RH double refining, and finally cast in a continuous casting machine to obtain a slab; the slab thickness is 227mm (see Table 1 and Table 2).

[0028] Step 2, heating: Heat the slab at a temperature of 1110℃ for a time of more than 160 minutes.

[0029] Step 3, Rolling: The rolling includes roughing and finishing. The roughing temperature is greater than 930℃ and the cumulative reduction rate is 58%. After the roughing is completed, the rollers are allowed to cool down in the intermediate roller table. When the temperature drops to the specified temperature, the finishing mill is used for rolling. The reduction rates of the first two passes of finishing are 8% and 7% respectively, and the finishing temperature is 798℃.

[0030] Step 4, Cooling: The steel plate enters the cooling equipment and is cooled by cooling water with different pressures, volumes, and opening angles. At the same time, the speed of the steel plate and the cooling rate of the steel plate are controlled. The initial cooling temperature is set to 772℃, the final cooling temperature is set to 480℃, the maximum cooling rate is 34℃ / s, and the minimum cooling rate is 22℃ / s, resulting in a finished steel plate with a thickness of 10mm.

[0031] Example 2:

[0032] Chemical composition design: The chemical composition is designed according to the following mass percentages: C: 0.057%, Si: 0.24%, Mn: 1.57%, P: 0.0104%, S: 0.0009%, Cr: 0.169%, Nb: 0.04%, Ti: 0.013%, Al: 0.0296%, N: 0.0028%, with the remainder being Fe and unavoidable impurities.

[0033] Step 1, Smelting and Casting: The designed chemical composition is cast, first by KR stirring and desulfurization treatment, then by blowing in a converter, followed by LF+RH double refining for desulfurization and dehydrogenation, and finally by continuous casting to obtain a slab; the slab thickness is 227mm. (Refer to Tables 1 and 2)

[0034] Step 2, heating: Heat the slab at a temperature of 1110℃ for a time of more than 160 minutes.

[0035] Step 3, Rolling: The rolling includes roughing and finishing. The roughing temperature is greater than 930℃ and the cumulative reduction rate is 50%. After the roughing is completed, the rollers are allowed to cool down in the intermediate roller table. When the temperature drops to the specified temperature, the finishing mill is used for rolling. The reduction rates of the first two passes of finishing are 7% and 6% respectively, and the finishing temperature is 780℃.

[0036] Step 4, Cooling: The steel plate enters the cooling equipment and is cooled by cooling water with different pressures, volumes, and opening angles. At the same time, the speed of the steel plate and the cooling rate of the steel plate are controlled. The initial cooling temperature is set to 772℃, the final cooling temperature is set to 350℃, the maximum cooling rate is 22℃ / s, and the minimum cooling rate is 14℃ / s. Finally, a finished steel plate with a thickness of 35mm is obtained.

[0037] Table 1: Comparison of specific components between Example 1 and Example 2

[0038]

[0039] Table 2: Comparison of specific processes between Example 1 and Example 2

[0040]

[0041] Table 3: Comparison of experimental performance between Example 1 and Example 2

[0042]

[0043] Different processes in Comparative Example 1 and Example 1 resulted in different properties, and different compositions in Comparative Example 2 and Example 2 resulted in different properties.

[0044] The embodiments described above are merely illustrative of several implementations of the present invention, and while the descriptions are specific and detailed, they should not be construed as limiting the scope of the present invention. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these modifications and improvements all fall within the scope of protection of the present invention. Therefore, the scope of protection of this patent should be determined by the appended claims.

Claims

1. A method for manufacturing a thin-gauge TMCP state Q420GJE with low yield strength ratio and high toughness, characterized in that, The chemical composition (by weight) of this high-strength steel is as follows: C: 0.04~0.1%, Si: 0.2~0.3%, Mn: 1.5~1.7%, P≤0.012%, S≤0.002%, Cr: 0.1~0.2%, Nb: 0.03~0.045%, Ti: 0.011~0.017%, Al: 0.021~0.04%, N≤0.004%, with the balance being Fe and unavoidable impurities, and satisfying the following relationships: Ti / N≥3, V+Nb+Ti≤0.15%; The manufacturing method of the high-strength structural steel is as follows: S1. Smelting and casting: The designed chemical composition is cast. First, it is desulfurized by KR stirring, then blown in a converter, then desulfurized and dehydrogenated by LF+RH double refining, and finally cast in a continuous casting machine to obtain slabs. S2. Heating: The slab is heated at a temperature range of 1110~1140℃ for a duration of more than 160 minutes. S3. Rolling: The rolling process includes roughing and finishing. The roughing temperature is greater than 930℃ and the cumulative reduction rate is less than 60%. After the roughing is completed, the rollers are allowed to cool down in the intermediate roller table. When the temperature drops to the specified temperature, the finishing mill is used for rolling. The reduction rate of the first two passes of finishing is less than 10%, and the finishing temperature is less than 830℃. S4. Cooling and straightening: After rolling, the steel plate enters the cooling equipment and is cooled by cooling water with different pressures, water volumes and opening angles. The initial cooling temperature is controlled at 700℃~800℃ and the final cooling temperature range is 300~480℃. To obtain high structural steel with low yield strength ratio and a steel plate thickness of 10~35mm.

2. The manufacturing method of a thin-gauge TMCP state Q420GJE with low yield strength ratio and high toughness according to claim 1, characterized in that: The final high-strength steel has the following properties: yield strength ReH: 420-540MPa; tensile strength Rm: 530-680MPa; yield-to-tensile ratio requirement ≤0.83 (upper yield strength / tensile strength); and full-size Charpy impact energy AKv ≥47J at -40℃.

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