A method for producing a 440mpa grade normalized steel plate for a ship body

Abstract

The application provides a production method of a 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 is smelting-mold casting-rolling-rapid cooling-finishing-normalizing-tempering. The thickness of the produced steel plate is 8-20mm. The chemical components of the steel are as follows in terms of mass percentage: 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 inevitable impurities. The microstructure of the steel is a bainite tempered structure+ferrite, the average grain size of the ferrite is ≤10μm, the yield strength of the steel is ≥440MPa grade, the tensile strength is ≥600MPa, the yield strength ratio is ≤0.75, the impact absorption energy at-60℃ is ≥200J, the ductile-brittle transition temperature is not higher than-80℃, and the product has good corrosion resistance.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of ship hull steel manufacturing, and relates to a production method of a 440MPa high-toughness low-yield ratio ship hull structural steel. BACKGROUND

[0002] With the increasing development of polar regions and deep-sea oil and gas resources, polar ships are long-term exposed to the harsh service environment of ultra-low temperature, and the polar ecological environment is fragile. Therefore, the demand for low-temperature steels and other key materials suitable for the harsh service environment of the polar region is increasing, and high-performance steels with high strength, high low-temperature toughness and corrosion resistance are the basic guarantee for the safe navigation of polar ships. At present, the existing E series steel plates cannot fully meet the use requirements in low-temperature environments, and with the increase of material strength, the yield ratio of the material will continue to rise, the plasticity will decrease, and the safety will decrease, especially in the construction of oil and gas storage and transportation ships, which puts forward more stringent requirements on the yield ratio of the material. The existing 440MPa grade ship hull steel is generally produced by TMCP process, with good strength and toughness, but the yield ratio is relatively high, and the yield strength or-40℃ impact toughness often appears low after normalizing, resulting in low product qualification rate. The reason is that the existing composition and process are difficult to meet the matching of 440MPa grade strength and toughness, and the process window is narrow, which easily leads to coarse grains and uneven structure after normalizing, resulting in unstable strength and toughness of the material. Therefore, a new production process needs to be developed to improve the uniformity of 440MPa grade normalized steel and further improve the strength and toughness of the material. SUMMARY

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

[0004] The present application is achieved by the following technical solutions:

[0005] The application discloses a production method of a normalized high-strength steel for a 440MPa-grade ship structure, and relates to the technical field of steel production.

[0006] 1) blank heating: the heating temperature is not more than 1180 DEG C, the soaking time of the blank in the heating furnace is controlled to be more than 50 min, and the temperature difference of the whole slab is not more than 40 DEG C, so that uniform austenite organization is obtained;

[0007] 2) rolling: the rolling process adopts a two-stage controlled rolling process, the rough rolling temperature is not less than 980 DEG C, and the average reduction rate of each pass is greater than or equal to 18%; the final rolling temperature of the finishing rolling is 770 DEG C to 840 DEG C, and the slab is air-cooled after rolling;

[0008] 3) heat treatment: the normalized tempering process is adopted, the normalizing temperature is controlled to be 780 DEG C to 820 DEG C, the time is controlled to be mm of the plate thickness x (1.0-1.4) min / mm + (20-30) min / mm, the tempering is controlled to be 600 DEG C to 640 DEG C, the time is controlled to be mm of the plate thickness x (1.2-1.6) min / mm + (30-40) min, the temperature precision of the whole slab is controlled to be ± 5 DEG C, and the slab is air-cooled after being discharged.

[0009] Compared with the prior art, the application has the beneficial effects:

[0010] (1) The application controls the heating temperature and time, so that the blank obtains uniform austenite organization, the reduction rate is improved, the final rolling temperature is controlled, the as-rolled organization is fully and uniformly refined, the organization regulation difficulty in the heat treatment process is reduced, and the performance stability of the final steel plate is improved.

[0011] (2) The heat treatment of the present application adopts low-temperature normalizing + tempering process, the normalizing temperature is controlled at 780-820℃, which is different from the current normalizing process route of full austenitization (the normalizing temperature is generally controlled at 30-50℃ above Ar3 temperature, i.e. 860-900℃). By reducing 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 adopted after normalizing, so that a part of unsolved ferrite soft phase in the form of fine needle or granular dispersion is retained in the steel, which is different from the full austenite structure of conventional normalizing. Through such a heat treatment mode, the steel plate finally obtains a two-phase structure of fine tempered bainite + needle or granular dispersed ferrite, which can make the steel have good strength and toughness matching, effectively reduce the yield ratio of the material, and improve the low-temperature toughness of the material.

[0012] (3) The present application produces a steel plate with a thickness of 8-20mm by controlling rolling and heat treatment temperature, especially the normalizing temperature, the mechanical properties are yield strength≥440MPa, tensile strength≥600MPa, yield ratio≤0.75, impact energy at-60℃≥200J, and the ductile-brittle transition temperature is not higher than-80℃, the product has good corrosion resistance and good performance. BRIEF DESCRIPTION OF DRAWINGS

[0013] Figure 1 The heart microstructure diagram of the 440MPa normalizing steel plate for ship structure produced in Example 1 is shown in the figure.

[0014] Figure 2 The heart microstructure diagram of the 440MPa normalizing steel plate for ship structure produced in Example 2 is shown in the figure. DETAILED DESCRIPTION

[0015] The composition control range, the best implementation mode and other main contents of the present application will be further described in combination with examples as follows:

[0016] According to the chemical composition design range of the present application, three examples of 440MPa normalizing structural steel for ship body are disclosed. The chemical composition of the examples is shown in Table 1; the test results of the mechanical properties of the steel are shown in Table 3, wherein the 8mm thick steel plate adopts a 5x10x55mm non-standard impact specimen. The heart microstructure of the steel plate is shown in the figures. Figure 1 、 Figure 2 .

[0017] Examples 1-3:

[0018] The chemical composition and process of Examples 1-3 are all in accordance with the method scheme of the present application, and the process steps are as follows:

[0019] (1) Billet heating: the heating temperature is controlled at 1160-1180℃, the soaking time is 53-60min,

[0020] (2) Rolling process: controlled rolling process is adopted, rough rolling temperature is 980-1050℃, continuous two passes reduction is ≥18%; finish rolling final rolling temperature is 770-810℃, low temperature large reduction is adopted, and air cooling is conducted to room temperature after rolling.

[0021] (3) Heat treatment: normalizing control temperature is 790-810℃, time control is plate thickness mm×min / mm+(20-30)min; tempering control temperature is 620-630℃, time control is plate thickness mm×min / mm+(30-40)min;

[0022] Comparative Example 1:

[0023] The chemical composition range of Comparative Example 1 is not within the range of the present application, the heating temperature and normalizing temperature are all beyond the requirements of the present application, and the comparison difference of the process parameters with the process parameters of the present application is shown in Table 2.

[0024] Table 1 Chemical composition (wt%) of steel plates produced by the examples and the comparative example

[0025] .

[0026] Table 2 Process parameters of steel plates produced by the examples and the comparative example

[0027] .

[0028] Table 3 Mechanical properties of steel produced by the examples and the comparative example

[0029] .

[0030] It can be known by comparison that the element composition mass percentage and process parameters of the steel produced by Example 1-Example 3 all meet the requirements of the present application technical solution, and the mechanical properties of the steel also meet the requirements; the strength, -60℃ impact energy and other indexes of the steel of Comparative Example 1 all have obvious decrease compared with the steel produced by the examples, the mechanical properties are less abundant, and it is difficult to stably meet the demand of 440MPa ship body steel.

[0031] It can be known by comparison that based on the special steel element composition and rolling heat treatment process, the present application realizes excellent mechanical properties of the steel plate, the yield strength is ≥440MPa, the tensile strength is ≥600MPa, the elongation is ≥26%, the -60℃ impact energy is ≥200J, and the ductile-brittle transition temperature is not higher than -80℃.

Claims

1. A method for producing a normalized high-strength steel for 440 MPa grade hull structures, the steel being produced according to a technological route comprising the following stages: smelting - moulding - rolling - rapid cooling - finishing - normalization - tempering, characterized in that: The chemical composition of the steel is, in mass percentage, 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 inevitable impurities; The produced steel plate has a thickness of 8~20mm, the microstructure of the steel is bainite tempering structure + ferrite, the average grain size of the ferrite is ≤10μm; the yield strength of the steel is ≥440MPa grade, 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 method comprises the following key process steps 1) blank heating: the heating temperature is not more than 1180℃, and the furnace time is controlled to be 200~280min; 2) rolling: the two-stage controlled rolling process is adopted in the rolling process, the rough rolling temperature is not lower than 980℃, and the average reduction rate of each pass is ≥18%; the final rolling temperature of the finishing rolling is 770~840℃, and the rolling is followed by air cooling; 3) heat treatment: the normalizing and tempering process is adopted, the normalizing temperature is controlled to be 780~820℃, and the time is controlled to be plate thickness mm×min / mm + (20~30)min / mm; the tempering is controlled to be 600~640℃, and the time is controlled to be plate thickness mm×min / mm + (30~40)min.

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