Production method of steel plate for 440MPa-grade normalized ship body

Abstract

The invention provides a production method of 440MPa-grade normalized high-strength structural steel for a ship body, and belongs to the field of ship body structural steel manufacturing, the production process route of the steel comprises smelting, die casting, rolling, rapid cooling, finishing, normalizing and tempering, the thickness of a produced steel plate is 8-20mm, and the steel comprises the following chemical components in percentage by mass: 0.06-0.10% of C; si: less than or equal to 0.40%; mn is equal to 0.90%-1.20%; less than or equal to 0.015% of P; s: < = 0.005%; the ratio of Ni to Cr is (0.90%-1.30%): (0.50%-0.65%); v is equal to 0.030 percent to 0.050 percent; cu is equal to 0.15%-0.35%; n is less than or equal to 0.0040%, and the balance is Fe and inevitable impurities. The microstructure of the steel is a bainite tempered structure and ferrite, and the average grain size of the ferrite is smaller than or equal to 10 micrometers. The yield strength of the steel is larger than or equal to 440 MPa, the tensile strength is larger than or equal to 600 MPa, the yield ratio is smaller than or equal to 0.75, the impact absorbing energy at the temperature of 60 DEG C below zero is larger than or equal to 200 J, the ductile-brittle transition temperature is not higher than 80 DEG C below zero, and a product has good corrosion resistance.

Description

Technical Field

[0001] The invention belongs to the field of hull steel manufacturing, and relates to a production method for hull structure steel with a strength level of 440MPa, high toughness and low yield strength ratio. Background Art

[0002] With the deepening development of polar regions and deep-sea oil and gas resources, polar ships have long been facing the harsh service environment of ultra-low temperature, and the polar ecological environment is fragile. Therefore, the demand for key materials such as low-temperature steel that adapt to the harsh polar service environment has increased, and high-performance steel with high strength, high and low temperature toughness and corrosion resistance is the basic guarantee for the safe navigation of polar ships. At present, the existing E-series steel plates cannot fully meet the requirements for use in low-temperature environments. As the strength of the material increases, the yield strength ratio of the material will continue to increase, the plasticity will decrease, and the safety will decrease, especially in the construction of oil and gas storage and transportation ship structures, which puts forward more stringent requirements on the yield strength ratio of the material. The existing 440MPa-grade hull steel is generally produced by TMCP process, with good strength and toughness, but the yield strength ratio is relatively high. The normalizing process often produces low yield strength or -40℃ impact toughness, resulting in low product qualification rate. The main reason is that the existing composition and process are difficult to meet the 440MPa-grade strength and toughness matching, and the process window is narrow, which is prone to coarse grains and uneven organization after normalizing, resulting in unstable strength and toughness of the material. Therefore, it is necessary to develop a new production process to improve the uniformity of 440MPa grade normalized steel and thus improve the strength and toughness of the material. Summary of the invention

[0003] The present invention aims to provide a production method of 440MPa grade normalized high-strength structural steel for hull, so as to solve the problem of insufficient strength and low toughness of the current 440MPa grade hull steel under the existing process, further reduce the yield strength ratio of the 440MPa grade hull steel to below 0.75, and the ductile-brittle transition temperature is not higher than -80°C.

[0004] The present invention is achieved through the following technical solutions: A method for producing 440MPa grade normalized high-strength steel for hull structure, wherein the steel production process route is smelting - continuous casting - rolling - rapid cooling - finishing - normalizing - tempering. The chemical composition of the steel is calculated by mass percentage as follows: C=0.06%~0.10%; Si:≤0.40%; Mn=0.90%~1.20%; P:≤0.015%; S:≤0.005%; Ni=0.90%~1.30%: Cr=0.50%~0.65%; V=0.030%~0.050%; Cu=0.15%~0.35%; N ≤0.0040%, and the rest is Fe and unavoidable impurities; the thickness of the produced steel plate is 8~20mm, the microstructure of the steel is bainite tempering structure + ferrite, and the average grain size of the ferrite is ≤10μm; the yield strength of the steel is ≥440MPa, the tensile strength is ≥600MPa, the yield strength ratio is ≤0.75, the impact absorption energy at -60℃ is ≥200J, and the ductile-brittle transition temperature is not higher than -80℃; the following key process steps are included 1) Billet heating: The heating temperature shall not exceed 1180°C, the billet soaking time in the heating furnace shall be controlled to be more than 50 minutes, and the temperature difference of the whole slab shall not exceed 40°C to obtain uniform austenite structure; 2) Rolling: The rolling process adopts a two-stage controlled rolling process, the rough rolling temperature is not less than 980℃, and the average reduction rate of each pass is ≥18%; the final rolling temperature of the finishing rolling is 770~840℃, and air cooling is performed after rolling; 3) Heat treatment: adopt normalizing and tempering process, the normalizing temperature is controlled at 780~820℃, the time is controlled at plate thickness mm× (1.0~1.4) min / mm + (20~30) min / mm; the tempering is controlled at 600~640℃, the time is controlled at plate thickness mm× (1.2~1.6) min / mm + (30~40) min, the temperature accuracy of the whole plate is controlled at ±5℃, and the steel plate is air-cooled after leaving the furnace.

[0005] Compared with the prior art, the present invention has the following beneficial effects: (1) The present invention controls the heating temperature and time so that the billet obtains a uniform austenitized structure, increases the reduction rate and controls the final rolling temperature, so that the rolled structure is fully uniform and refined, reduces the difficulty of structure control in the heat treatment process, and improves the performance stability of the final steel plate.

[0006] (2) The heat treatment of the present invention adopts a low-temperature normalizing + tempering process, and the normalizing temperature is controlled at 780℃~820℃, which is different from the current normalizing process route for complete austenitization (the normalizing temperature is generally controlled to be 30~50℃ above the Ar3 temperature, that is, 860~900℃). By lowering the normalizing temperature to the critical phase transition temperature of austenite, relatively fine initial austenite grains can be formed in the steel; at the same time, air cooling is used after normalizing, so that a part of the undissolved ferrite soft phase dispersed in the form of fine needles or particles is retained in the steel, which is different from the complete austenite structure of conventional normalizing. Through such a heat treatment method, the steel plate finally obtains a two-phase structure of fine tempered bainite + needle-shaped or granular dispersed ferrite, which can make the steel have a good strength-toughness match, effectively reduce the yield strength ratio of the material, and improve the low-temperature toughness of the material.

[0007] (3) The present invention produces a steel plate with a thickness of 8 to 20 mm by controlling the rolling and heat treatment temperatures, especially the normalizing temperature. The mechanical properties of the steel plate are as follows: yield strength ≥ 440 MPa, tensile strength ≥ 600 MPa, yield strength ratio ≤ 0.75, -60°C impact absorption energy ≥ 200 J, and a ductile-brittle transition temperature not higher than -80°C. The product has good corrosion resistance and good performance in use. BRIEF DESCRIPTION OF THE DRAWINGS

[0008] Figure 1 This is the core structure diagram of the normalized steel plate with a capacity of 440MPa for hull structure produced in Example 1.

[0009] Figure 2 This is the core structure diagram of the normalized steel plate with a capacity of 440MPa for hull structure produced in Example 2. DETAILED DESCRIPTION

[0010] The following is a further description of the main contents of the present invention such as the component control range and the best mode of implementation in conjunction with the embodiments: According to the chemical composition design range of the present invention, three examples of 440MPa normalized structural steel for ship hulls are disclosed. The chemical composition of the example is shown in Table 1; the mechanical property test results of the steel are shown in Table 3, where the 8mm thick steel plate uses a 5×10×55 mm non-standard impact specimen. The microstructure of the core of the example steel plate is shown in Table 3. Figure 1 , Figure 2 .

[0011] Embodiment 1~3: The chemical compositions and processes of Examples 1 to 3 are in accordance with the method scheme of the present invention, and the process steps are as follows: (1) Billet heating: The heating temperature is controlled at 1160~1180℃, and the soaking time is 53~60min. (2) Rolling process: The controlled rolling process is adopted, the rough rolling temperature is 980~1050℃, and the reduction rate for two consecutive passes is ≥18%; the final rolling temperature of the finishing rolling is 770~810℃, low temperature and large reduction rolling is adopted, and air cooling is adopted to room temperature after rolling.

[0012] (3) Heat treatment: Normalizing temperature is controlled at 790~810℃, and the time is controlled at plate thickness mm× min / mm + (20~30) min; tempering temperature is controlled at 620~630℃, and the time is controlled at plate thickness mm× min / mm + (30~40) min; Comparative Example 1: The chemical composition range of Comparative Example 1 is not within the scope of the present invention, and the heating temperature and normalizing temperature both exceed the requirements of the present invention. The comparison differences between its process parameters and the process parameters of the present invention are shown in Table 2.

[0013] Table 1 Chemical composition of steel plates produced in the examples and comparative examples (wt%) .

[0014] Table 2 Process parameters for producing steel plates in the examples and comparative examples .

[0015] Table 3 Mechanical properties of steel produced in the examples and comparative examples .

[0016] By comparison, it can be seen that the mass percentage of elemental components and process parameters of the steels produced in Examples 1 to 3 meet the requirements of the technical solution of the present invention, and the mechanical properties of the steels also meet the requirements; the strength, -60°C impact energy and other indicators of the steel in Comparative Example 1 are significantly lower than those of the steel produced in the examples, and its mechanical properties are less abundant, making it difficult to stably meet the demand for 440MPa hull steel.

[0017] By comparison, it can be seen that the present invention achieves excellent mechanical properties of the steel plate based on the special steel element composition and rolling heat treatment process, with yield strength ≥440MPa, tensile strength ≥600MPa, elongation ≥26%, -60℃ impact energy ≥200J, and ductile-brittle transition temperature not higher than -80℃.

Claims

1. A method for producing 440MPa grade normalized high-strength steel for hull structure, wherein the production process of the steel is smelting - die casting - rolling - rapid cooling - finishing - normalizing - tempering, characterized in that: The chemical composition of steel is calculated by mass percentage as follows: C = 0.06% ~ 0.10%; Si: ≤ 0.40%; Mn = 0.90% ~ 1.20%; P: ≤ 0.015%; S: ≤ 0.005%; Ni = 0.90% ~ 1.30%: Cr = 0.50% ~ 0.65%; V = 0.030% ~ 0.050%; Cu = 0.15% ~ 0.35%; N ≤ 0.0040%, and the rest is Fe and unavoidable impurities; The thickness of the steel plate produced is 8~20mm, the microstructure of the steel is bainite tempering structure + ferrite, and the average grain size of ferrite is ≤10μm; the yield strength of the steel is ≥440MPa, the tensile strength is ≥600MPa, the yield strength ratio is ≤0.75, the impact absorption energy at -60℃ is ≥200J, and the ductile-brittle transition temperature is not higher than -80℃; the following key process steps are included 1) Billet heating: The heating temperature is no more than 1180°C, and the time in the furnace is controlled at 200~280min; 2) Rolling: The process adopts a two-stage controlled rolling process, the rough rolling temperature is not less than 980℃, and the average reduction rate of each pass is ≥18%; the finishing rolling temperature is 770~840℃, and air cooling is performed after rolling; 3) Heat treatment: adopt normalizing and tempering process, the normalizing temperature is controlled at 780~820℃, the time is controlled at plate thickness mm×min / mm + (20~30)min / mm; the tempering is controlled at 600~640℃, the time is controlled at plate thickness mm×min / mm + (30~40)min.

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