Preparation method of preheating-welding-free extra-thick steel S355MLO for ocean engineering

Abstract

The invention discloses a preparation method of preheating-welding-free extra-thick steel S355MLO for ocean engineering, and belongs to the technical field of metal materials. The steel comprises the following chemical components in percentage by mass: 0.04%-0.06% of C, 0.15%-0.50% of Si, 1.35%-1.45% of Mn, less than or equal to 0.008% of P, less than or equal to 0.003% of S, 0.012%-0.02% of Ti, less than or equal to 0.015% of Al, 0.15%-0.25% of Ni, less than or equal to 0.18% of Cu, less than or equal to 0.18% of Cr, less than or equal to 0.35% of Ceq = [C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15] and the balance of Fe and inevitable impurities. The low-carbon and high-manganese component design is adopted, the S355MLO super-thick steel plate with the maximum thickness of 100 mm is produced through a continuous casting billet, the carbon equivalent Ceq is smaller than or equal to 0.35, and high strength, high toughness and high welding performance are integrated. The base metal and welding toughness of the thick steel plate produced by the TMCP process are improved through the component and inclusion regulation and control technology, the process is simple, the production cost is low, the production efficiency is high, preheating before welding can be avoided, and energy conservation and consumption reduction are achieved.

Description

Technical Field

[0001] The invention belongs to the technical field of metallurgy and relates to a method for preparing preheat-free welding extra-thick marine engineering steel. Background Art

[0002] As offshore oil development technology matures, offshore oil extraction has become the main way of oil supply in the future. Although my country has a long coastline and rich oil resources on the seabed, its oil extraction capacity is still far behind that of developed countries, and large-scale offshore oil extraction is imminent; at the same time, due to the characteristics of large thickness, high quality requirements, and high production difficulty of structural steel plates for marine engineering, a large amount of ultra-thick and high-quality marine engineering steel in China still needs to be imported from abroad. Ultra-thick marine engineering steel plates with a thickness of more than 80mm are currently produced by many foreign steel mills using steel ingots to improve toughness by meeting the compression ratio requirements of more than 4.0, but the process has a long production process, high energy consumption, and high production costs, so the price is very expensive.

[0003] In order to ensure toughness after welding, traditional marine engineering steel usually needs to be preheated to 80~120℃ before welding. However, due to the large-scale marine engineering equipment and the characteristics of outdoor welding, preheating before welding has become increasingly difficult to achieve. Summary of the invention

[0004] The object of the present invention is to provide a preparation method of preheat-free welding extra-thick marine engineering steel S355MLO, the representative steel type is S355MLO, the thickness is 50-100mm, and in particular, no preheating before welding can be achieved.

[0005] The technical solution of the present invention: A preparation method of preheat-free welding extra-thick marine engineering steel S355MLO, the chemical composition mass percentage of the steel is C=0.04%~0.06%, Si=0.15%~0.50%, Mn=1.35%~1.45%, P≤0.008%, S≤0.003%, Ti=0.012%~0.02%, Al≤0.015%, Ni=0.15%~0.25%, Cu≤0.18%, Cr≤0.18%, Ceq=[C+Mn / 6+(Cr+Mo+V) / 5+(Ni+Cu) / 15]≤ 0.35, the rest is Fe and unavoidable impurities; the yield strength of the steel is 388~416Mpa, the tensile strength is 525~554MPa, the elongation is 23.82%~25.38%, the impact energy of the parent material at -50℃ is 328~356J, and the impact energy of the welding at -40℃ is 252~324J; the key process steps are as follows: (1) Smelting: The smelting raw materials are pre-treated by KR molten iron and then smelted in a converter. It is forbidden to add aluminum iron when tapping the converter. 200-300 kg of ferrosilicon and 1.35 t of metallic manganese are added. No aluminum wire is fed after tapping the converter. It is not allowed to add aluminum-containing deoxidizer when the LF furnace arrives at the station for the first time. The oxygen content is controlled at 80-100 ppm after the first electrification of slag. First, 300-400 kg of titanium iron is added, and then 30-50 m of aluminum wire is fed, and finally 100 m of calcium wire is fed. Then white slag is made and the composition is adjusted. After refining, VD vacuum treatment is carried out to break the air and determine the hydrogen content ≤2.0 ppm. Finally, it is cast into a continuous casting billet with a thickness of ≥300 mm. (2) Heating: reheat the continuous casting billet to 1100~1140℃, soaking time 20~30min; (3) Rolling: After leaving the furnace, two-stage rolling is carried out: the first stage of rough rolling has a final rolling temperature of ≥980°C, and the second stage of fine rolling has a final rolling temperature controlled at 801~820°C; (4) Cooling: Use water cooling for rapid cooling, and the red-return temperature after cooling is 470~550℃.

[0006] The steel prepared by the above method has a yield strength of 388~416MPa, a tensile strength of 525~554MPa, an elongation of 23.82%~25.38%, a -50℃ base metal impact energy of 328~356J, and a -40℃ welding impact energy of 252~324J; The technical principle of the present invention is as follows: the present invention adjusts the oxygen content of the converter and the order of adding alloys during the steelmaking process, modifies the inclusions in the molten steel, forms fine inclusions mainly composed of titanium, magnesium, calcium oxides and composite inclusions, provides ferrite and bainite phase deformation nuclei in the subsequent rolling cooling and welding cooling phase transformation, promotes phase transformation, and effectively cuts the original austenite grains, thereby achieving the purpose of refining the grains and improving the low-temperature toughness and welding toughness of the steel plate.

[0007] Beneficial effects of the present invention: 1) The present invention uses continuous casting billets to produce high-toughness marine engineering steel plates with a low-temperature impact requirement of -40°C or below, with a maximum thickness of 100 mm. While ensuring product quality, no preheating before welding can be achieved.

[0008] 2) The present invention improves the base material and welding toughness of thick steel plates produced by TMCP process through composition and inclusion control technology.

[0009] 3) The method of the present invention produces a new type of economical, low-carbon and environmentally friendly steel, which can also be widely used in the manufacture of thick plates in many fields such as shipbuilding, wind power, bridges, and construction. The process is simple, the production cost is low, energy saving and consumption reduction are reduced, welding efficiency is improved, and the welding environment is improved. BRIEF DESCRIPTION OF THE DRAWINGS

[0010] Figure 1 Metallographic structure photograph of the steel plate of Example 1 of the present invention.

[0011] Figure 2 Metallographic structure photograph of the steel plate of Comparative Example 1 of the present invention. DETAILED DESCRIPTION

[0012] According to the method for preparing the preheat-free welded extra-thick marine engineering steel S355MLO, the steel is smelted in a 120t converter, refined in an LF furnace, vacuum treated in a VD furnace, and cast into a 300mm thick continuous casting billet, and then rolled into a finished product on a 5000mm double-frame medium and thick plate production line. The present invention is further described below by examples and comparative examples. The chemical composition of the steel plates in the examples and comparative examples is shown in Table 1.

[0013] The key process parameters of the steel plates of the embodiments and comparative examples are as follows. Example 1

[0014] The thickness of the steel plate is 100 mm. The preparation process is as follows: (1) Smelting: The smelting raw materials are pre-treated with KR molten iron and then smelted in a converter. No aluminum iron is added when the converter is tapped. 263 kg of ferrosilicon and 1.35 t of metallic manganese are added for deoxidation. No aluminum wire is fed after the converter is tapped. No aluminum-containing deoxidizer is added when the LF furnace is powered on for the first time at the station. The oxygen content is controlled at 92 ppm after the first power-on slag is regulated. First, 324 kg of ferrotitanium is added, then 36 m of aluminum wire is fed, and finally 100 m of calcium wire is fed. Then, white slag is produced and the composition is adjusted; after refining, VD vacuum treatment is carried out to break the air and determine the hydrogen content to be 1.6ppm; finally, it is cast into a continuous casting billet with a thickness of 300mm; (2) Heating: The continuous casting billet is reheated to 1132℃, and the soaking time is 27min; (3) Rolling: After leaving the furnace, two stages of rolling are carried out: the first stage rough rolling and the final rolling temperature are 1035℃, and the second stage fine rolling and the final rolling temperature are 802℃; (4) Cooling: Rapid cooling by water cooling, the red-returning temperature after cooling is 476℃.

[0015] Comparative Example 1 The thickness of the steel plate is 100 mm. The preparation process is as follows: (1) Smelting: the smelting raw materials are pre-treated with KR molten iron and then smelted in a converter. When the converter is tapped, 601 kg of aluminum iron is added for deoxidation. Normal deoxidation and alloying are carried out when the LF furnace is powered on for the first time at the station. After refining, VD vacuum treatment is carried out to break the air and determine the hydrogen content to be 1.2 ppm. Finally, it is cast into a continuous casting billet with a thickness of 300 mm. (2) Heating: the continuous casting billet is reheated to 1136 °C, and the soaking time is 28 min. (3) Rolling: two-stage rolling is carried out after the billet is taken out of the furnace: the first stage rough rolling final rolling temperature is 1025 °C, and the second stage fine rolling final rolling temperature is 803 °C. (4) Cooling: water cooling is used for rapid cooling, and the red-return temperature after cooling is 474 °C. Example 2

[0016] The thickness of the steel plate is 80 mm. The preparation process is as follows: (1) Smelting: The smelting raw materials are pre-treated with KR molten iron and then smelted in a converter. No aluminum iron is added when the converter taps the steel. 274 kg of ferrosilicon and 1.35 t of metallic manganese are added for deoxidation. No aluminum wire is fed after the converter taps the steel. No aluminum-containing deoxidizer is added when the LF furnace is powered on for the first time at the station. After the first power-on slag is oxidized, the oxygen content is controlled at 89 ppm. First, 342 kg of ferrotitanium is added, then 33 m of aluminum wire is fed, and finally 100 m of calcium wire is fed. Then Make white slag and adjust the composition; after refining, perform VD vacuum treatment to break the air and determine the hydrogen content to 1.2ppm; finally cast it into a continuous casting billet with a thickness of 300mm; (2) Heating: reheat the continuous casting billet to 1135℃, and soak it for 25min; (3) Rolling: after leaving the furnace, carry out two-stage rolling: the first stage rough rolling final rolling temperature is 1023℃, and the second stage fine rolling final rolling temperature is 808℃; (4) Cooling: rapid cooling by water cooling, the red return temperature after cooling is 486℃.

[0017] Comparative Example 2 The thickness of the steel plate is 80 mm. The preparation process is as follows: (1) Smelting: The smelting raw materials are pre-treated with KR molten iron and then smelted in a converter. No aluminum iron is added when the converter taps the steel. 274 kg of ferrosilicon and 1.35 t of metallic manganese are added for deoxidation. No aluminum wire is fed after the converter taps the steel. No aluminum-containing deoxidizer is added when the LF furnace is powered on for the first time at the station. After the first power-on slag is oxidized, the oxygen content is controlled at 89 ppm. First, 342 kg of ferrotitanium is added, then 33 m of aluminum wire is fed, and finally 100 m of calcium wire is fed. Then Make white slag and adjust the composition; after refining, perform VD vacuum treatment to break the air and determine the hydrogen content to 1.2ppm; finally cast it into a continuous casting billet with a thickness of 300mm; (2) Heating: reheat the continuous casting billet to 1193℃, and soak it for 26min; (3) Rolling: after leaving the furnace, carry out two-stage rolling: the first stage rough rolling final rolling temperature is 1020℃, and the second stage fine rolling final rolling temperature is 810℃; (4) Cooling: rapid cooling by water cooling, the red return temperature after cooling is 483℃. Example 3

[0018] The thickness of the steel plate is 50 mm. The preparation process is as follows: (1) Smelting: The smelting raw materials are pre-treated with KR molten iron and then smelted in a converter. No aluminum iron is added when the converter is tapped. 275 kg of ferrosilicon and 1.35 t of metallic manganese are added for deoxidation. No aluminum wire is fed after the converter is tapped. No aluminum-containing deoxidizer is added when the LF furnace is powered on for the first time at the station. The oxygen content is controlled at 88 ppm after the first power-on slag is quenched. First, 324 kg of ferrotitanium is added, then 35 m of aluminum wire is fed, and finally 100 m of calcium wire is fed. Then Make white slag and adjust the composition; after refining, perform VD vacuum treatment to break the air and set the hydrogen content to 1.5ppm; finally cast it into a continuous casting billet with a thickness of 300mm; (2) Heating: reheat the continuous casting billet to 1138℃, and soak it for 22min; (3) Rolling: after leaving the furnace, carry out two-stage rolling: the first stage rough rolling final rolling temperature is 1015℃, and the second stage fine rolling final rolling temperature is 812℃; (4) Cooling: rapid cooling by water cooling, the red return temperature after cooling is 528℃.

[0019] Comparative Example 3 The thickness of the steel plate is 50 mm. The preparation process is as follows: (1) Smelting: the smelting raw materials are pre-treated with KR molten iron and then smelted in a converter. When the converter is tapped, 600 kg of aluminum iron is added for deoxidation. Normal deoxidation and alloying are carried out when the LF furnace is powered on for the first time at the station. After refining, VD vacuum treatment is carried out to break the air and determine the hydrogen content to be 1.6 ppm. Finally, it is cast into a continuous casting billet with a thickness of 300 mm. (2) Heating: the continuous casting billet is reheated to 1208°C, and the soaking time is 25 minutes. (3) Rolling: After being discharged from the furnace, two-stage rolling is carried out: the first stage rough rolling has a final rolling temperature of 1023°C, and the second stage fine rolling has a final rolling temperature of 815°C. (4) Cooling: rapid cooling is carried out by water cooling, and the red-returning temperature after cooling is 605°C.

[0020] The comprehensive mechanical properties of the steel plates of the embodiments and comparative examples are shown in Table 2.

[0021] Among them, both the embodiment and the comparative example adopt the flat position welding method, the welding method is SAW, no preheating is performed before welding, the heat input is 35KJ / cm, the impact specimen size is 10mm×10mm×55mm, the parent material impact is the core position, and the welding impact position is the heat affected zone FL+2mm.

[0022] The base materials and welding performance of the steel plates of Examples 1, 2 and 3 of the present invention are good. The metallographic structure of the steel plates of the examples is observed, and the microstructure is mainly composed of acicular ferrite + granular bainite, with fine and uniform grains. Figure 1 As shown. The low temperature impact toughness and welding toughness of the comparative steel plate are poorer than those of the embodiment. Comparative Example 1 is ordinary S355MLO marine steel, and the 100mm extra-thick steel plate produced by traditional deoxidation and rolling process, the parent material and the impact performance after welding can not meet the requirements, and the metallographic structure has large pieces of granular and plate-like bainite, such as Figure 2As shown; Comparative Example 2 has the same composition and smelting process as Example 2, but high temperature heating is used, which causes the effective inclusions formed in the smelting process to partially dissolve, and does not provide sufficient nucleation particles during the cooling phase change, resulting in a decrease in the impact performance of the parent material, and the toughness after welding does not meet the requirements; and the steel in Comparative Example 3 is Q420qE, a bridge plate, which uses traditional smelting technology and rolling methods, and its toughness for preheat-free welding does not meet the requirements. Therefore, it is difficult to stably produce marine steel plates that meet the requirements of low-temperature toughness and preheat-free welding toughness in industrial production using traditional smelting and rolling processes, which fully demonstrates the ingenuity, consideration and uniqueness of the present invention in composition and process design.

[0023] Table 1 Chemical composition and Ceq (wt, %) of steel plates prepared in Examples and Comparative Examples .

[0024] Table 2 Comprehensive mechanical properties of steel plates prepared in Examples and Comparative Examples .

Claims

1. A method for preparing S355MLO ultra-thick marine engineering steel without preheating welding, characterized in that: The chemical composition mass percentage of the steel is C=0.04%~0.06%, Si=0.15%~0.50%, Mn=1.35%~1.45%, P≤0.008%, S≤0.003%, Ti=0.012%~0.02%, Al≤0.015%, Ni=0.15%~0.25%, Cu≤0.18%, Cr≤0.18%, Ceq=[C+Mn / 6+(Cr+Mo+V) / 5+(Ni+Cu) / 15] ≤ 0.35, and the rest is Fe and unavoidable impurities; the yield strength of the steel is 388~416Mpa, the tensile strength is 525~554MPa, the elongation is 23.82%~25.38%, the impact energy of the base material at -50℃ is 328~356J, and the welding impact energy at -40℃ is 252~324J; the key process steps are as follows: (1) Smelting: The smelting raw materials are pre-treated by KR molten iron and then smelted in a converter. It is forbidden to add aluminum iron when tapping the converter. 200-300 kg of ferrosilicon and 1.35 t of metallic manganese are added. No aluminum wire is fed after tapping the converter. It is not allowed to add aluminum-containing deoxidizer when the LF furnace arrives at the station for the first time. The oxygen content is controlled at 80-100 ppm after the first electrification of slag. First, 300-400 kg of titanium iron is added, and then 30-50 m of aluminum wire is fed, and finally 100 m of calcium wire is fed. Then white slag is made and the composition is adjusted. After refining, VD vacuum treatment is carried out to break the air and determine the hydrogen content ≤2.0 ppm. Finally, it is cast into a continuous casting billet with a thickness of ≥300 mm. (2) Heating: reheat the continuous casting billet to 1100~1140℃, soaking time 20~30min; (3) Rolling: After leaving the furnace, two-stage rolling is carried out: the first stage of rough rolling has a final rolling temperature of ≥980°C, and the second stage of fine rolling has a final rolling temperature controlled at 801~820°C; (4) Cooling: Rapid cooling is carried out by water cooling, and the red-return temperature after cooling is 470~550℃; Yield strength / Mpa, tensile strength / MPa, elongation / %, parent material impact energy -50℃ / J, welding impact energy -40℃ / J.

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