Production method of 420-grade preheating-free welding marine steel

Abstract

The invention discloses a production method of 420-grade preheating-free welding maritime work steel, 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, 1.45%-1.55% 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 0.010% of Al, 0.15%-0.25% of Ni, 0.10%-0.18% of Cu, 0.10%-0.18% of Cr, less than or equal to 0.38% of Ceq = [C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15], and the balance of Fe and inevitable impurities. A low-carbon and high-manganese component design is adopted, the S420MLO super-thick steel plate with the maximum thickness capable of reaching 100 mm is produced through a continuous casting billet, the carbon equivalent Ceq is smaller than or equal to 0.38, 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, energy is saved, consumption is reduced, and the method is environmentally friendly.

Description

Technical Field

[0001] The invention belongs to the technical field of metallurgy and relates to a production method of 420-grade preheat-free welding marine engineering steel. Background Art

[0002] As offshore oil production moves to deep sea and offshore, offshore platforms and deep-sea jackets are becoming larger and larger. To ensure structural safety, higher requirements are placed on the performance and metallurgical quality of marine engineering steel, which requires not only high strength but also excellent toughness and weldability. Taking the Haiji No. 2 jacket of China's Liuhua project as an example, its design height reaches 340m. In order to achieve weight reduction and lightweight, 420-grade marine steel has been used in batches to replace the original 355-grade marine steel. However, 460-grade marine steel is still in the research and development stage and has not yet been able to be supplied in batches.

[0003] The existing technology of traditional marine engineering steel production usually requires preheating to 80~120℃ before welding to ensure toughness after 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 production method of 420-grade preheat-free welded marine engineering steel, the representative steel grade is S420MLO, the thickness is 50-100mm, and in particular, no preheating before welding can be achieved.

[0005] The technical solution of the present invention: A production method for 420-grade preheat-free welding marine engineering steel, wherein the chemical composition mass percentage of the steel is C=0.04%~0.06%, Si=0.15%~0.50%, Mn=1.45%~1.55%, P≤0.008%, S≤0.003%, Ti=0.012%~0.02%, Al≤0.010%, Ni=0.15%~0.25%, Cu=0.10%~0.18%, Cr=0.10%~0.18%, and the rest is Fe and unavoidable impurities, Ceq=[C+Mn / 6+(Cr+Mo+V) / 5+(Ni+Cu) / 15]≤ 0.38; the yield strength of steel is 418~463Mpa, the tensile strength is 552~576MPa, the elongation is 22.67~26.31%, the impact energy of the parent material at -50℃ is 322~362J, and the impact energy of welding at -40℃ is 239~325J; 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 1.5 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 to adjust the composition. 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 continuous casting billets 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 rough rolling and final rolling temperature are ≥980℃, and the second stage finishing rolling and final rolling temperature are controlled at 781℃~800℃; (4) Cooling: Use water cooling for rapid cooling, and the red-return temperature after cooling is 425~465℃.

[0006] The technical principle of the present invention is as follows: by adjusting the oxygen content of the converter and the order of adding alloys during the steelmaking process, the inclusions in the molten steel are modified to form fine inclusions mainly composed of titanium, magnesium, calcium oxides and composite inclusions, which provide ferrite and bainite phase deformation nuclei in the subsequent rolling cooling and welding cooling phase transformations, promote phase transformation, and effectively cut 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 is required; 2) The present invention improves the base material and welding toughness of thick steel plates produced by the TMCP process through composition and inclusion control technology, has simple process, low production cost, energy saving and consumption reduction, improved welding efficiency, improved welding environment, and provides metallurgical ideas for the TMCP process to produce high-strength, tough and easy-to-weld extra-thick steel plates; 3) The method of the present invention produces a new type of economical, low-carbon and environmentally friendly steel with a yield strength of 418-463 MPa, a tensile strength of 552-576 MPa, an elongation of 22.67-26.31%, a -50°C parent material impact energy of 322-362 J, and a -40°C welding impact energy of 239-325 J. The steel can be widely used in the manufacture of thick plates in shipbuilding, wind power, bridges, construction and other fields. BRIEF DESCRIPTION OF THE DRAWINGS

[0008] Figure 1 Metallographic structure photograph of the steel plate of Example 1 of the present invention.

[0009] Figure 2 Metallographic structure photograph of the steel plate of Comparative Example 1 of the present invention. DETAILED DESCRIPTION

[0010] According to the production method of a 420-grade preheat-free welding marine engineering steel of the present invention, 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 a group of embodiments and comparative examples. The chemical composition of the steel plates in the embodiments and comparative examples is shown in Table 1.

[0011] The key process steps and parameters of the example and comparative example steel plates are as follows. Example 1

[0012] Steel plate thickness 100mm, production process steps: (1) Smelting: The smelting raw materials are pre-treated by KR molten iron and then smelted in a converter. No aluminum iron is added when the converter is tapped, but 230 kg of ferrosilicon and 1.5 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 86 ppm after the slag is sintered for the first time. First, 316 kg of titanium iron is added, then 35 m of aluminum wire is fed, and finally 100 m of calcium wire is fed. Then white slag is made to adjust the composition. After refining, VD vacuum treatment is carried out to break the air and set the hydrogen content at 1.2 ppm. Finally, it is cast into a continuous casting billet with a thickness of 300 mm. (2) Heating: reheat the continuous casting billet to 1135°C, soaking time 26 min; (3) Rolling: After leaving the furnace, two-stage rolling is carried out: the first stage of rough rolling with a final rolling temperature of 1023°C, and the second stage of fine rolling with a final rolling temperature of 784°C; (4) Cooling: Rapid cooling by water cooling is adopted, and the red-return temperature after cooling is 426℃.

[0013] Comparative Example 1 Steel plate thickness 100mm, production process steps: (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 after arriving at the station. After refining, VD vacuum treatment is carried out to break the air and determine the hydrogen content to be 1.1 ppm. Finally, it is cast into a continuous casting billet with a thickness of 300 mm. (2) Heating: reheat the continuous casting billet to 1132°C, soaking time 25 min; (3) Rolling: After leaving the furnace, two-stage rolling is carried out: the first stage of rough rolling with a final rolling temperature of 1015°C, and the second stage of fine rolling with a final rolling temperature of 788°C; (4) Cooling: Rapid cooling by water cooling is adopted, and the red-return temperature after cooling is 432℃. Example 2

[0014] Steel plate thickness 80mm, production process steps: (1) Smelting: The smelting raw materials are pre-treated by KR molten iron and then smelted in a converter. No aluminum iron is added when the converter is tapped. 236 kg of ferrosilicon and 1.5 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 slag is smelted for the first time. 352 kg of titanium iron is added first, followed by 36 m of aluminum wire and finally 100 m of calcium wire. Then white slag is made to adjust the composition. After refining, VD vacuum treatment is carried out to break the air and set the hydrogen content at 1.1 ppm. Finally, it is cast into a continuous casting billet with a thickness of 300 mm. (2) Heating: reheat the continuous casting billet to 1132°C, soaking time 25 min; (3) Rolling: After leaving the furnace, two-stage rolling is carried out: the first stage of rough rolling with a final rolling temperature of 1006°C, and the second stage of fine rolling with a final rolling temperature of 791°C; (4) Cooling: Rapid cooling by water cooling is adopted, and the red-return temperature after cooling is 443℃.

[0015] Comparative Example 2 Steel plate thickness 80mm, production process steps: (1) Smelting: The smelting raw materials are pre-treated by KR molten iron and then smelted in a converter. No aluminum iron is added when the converter is tapped. 236 kg of ferrosilicon and 1.5 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 slag is smelted for the first time. 352 kg of titanium iron is added first, followed by 36 m of aluminum wire and finally 100 m of calcium wire. Then white slag is made to adjust the composition. After refining, VD vacuum treatment is carried out to break the air and set the hydrogen content at 1.1 ppm. Finally, it is cast into a continuous casting billet with a thickness of 300 mm. (2) Heating: reheat the continuous casting billet to 1186°C, soaking time 26 min; (3) Rolling: After leaving the furnace, two-stage rolling is carried out: the first stage of rough rolling with a final rolling temperature of 1015°C, and the second stage of fine rolling with a final rolling temperature of 789°C; (4) Cooling: Rapid cooling by water cooling is adopted, and the red-return temperature after cooling is 446℃. Example 3

[0016] Steel plate thickness 50mm, production process steps: (1) Smelting: The smelting raw materials are pre-treated by KR molten iron and then smelted in a converter. No aluminum iron is added when the converter is tapped. 218 kg of ferrosilicon and 1.5 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 96 ppm after the slag is sintered for the first time. 364 kg of titanium iron is added first, followed by 34 m of aluminum wire and finally 100 m of calcium wire. Then white slag is made to adjust the composition. After refining, VD vacuum treatment is carried out to break the air and set the hydrogen content at 1.5 ppm. Finally, it is cast into a continuous casting billet with a thickness of 300 mm. (2) Heating: reheat the continuous casting billet to 1135°C, soaking time 24 min; (3) Rolling: After leaving the furnace, two-stage rolling is carried out: the first stage of rough rolling with a final rolling temperature of 1012°C, and the second stage of fine rolling with a final rolling temperature of 798°C; (4) Cooling: Rapid cooling by water cooling is adopted, and the red-return temperature after cooling is 464℃.

[0017] Comparative Example 3 Steel plate thickness 50mm, production process steps: (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 after arriving 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: reheat the continuous casting billet to 1208°C, soaking time 25 min; (3) Rolling: After leaving the furnace, two-stage rolling is carried out: the first stage of rough rolling with a final rolling temperature of 1023°C, and the second stage of fine rolling with a final rolling temperature of 815°C; (4) Cooling: Rapid cooling by water cooling is adopted, and the red-return temperature after cooling is 605℃.

[0018] The comprehensive mechanical properties test results of the steel plates of the embodiment and the comparative example can be seen in Table 2.

[0019] 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.

[0020] Table 1 Chemical composition and Ceq (wt%) of steel plates in the examples and comparative examples .

[0021] Table 2 Comprehensive mechanical properties of the steel plates of the examples and comparative examples .

[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 S420MLO 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 is large granular and plate-like bainite, as shown in FIG. Figure 2 As shown; Comparative Example 2 has the same composition and smelting process as Example 2, but adopts high-temperature heating, which causes partial dissolution of effective inclusions formed in the smelting process, and fails to provide sufficient nucleation particles during the cooling phase transformation, resulting in a decrease in the impact properties of the parent material and the toughness after welding does not meet the requirements; and the steel in Comparative Example 3 is Q420qE bridge plate, which adopts traditional smelting process and rolling method, and its toughness of preheat-free welding also does not meet the requirements.

Claims

1. A method for producing 420 grade preheat-free welded marine steel, characterized in that: The chemical composition of steel is C=0.04%~0.06%, Si=0.15%~0.50%, Mn=1.45%~1.55%, P≤0.008%, S≤0.003%, Ti=0.012%~0.02%, Al≤0.010%, Ni=0.15%~0.25%, Cu=0.10%~0.18%, Cr=0.10%~0.18%, Ceq=[C+Mn / 6+(Cr+Mo+V) / 5+(Ni+Cu) / 15] ≤ 0.38, the rest is Fe and unavoidable impurities; the yield strength of the steel is 418~463Mpa, the tensile strength is 552~576MPa, the elongation is 22.67~26.31%, the impact energy of the parent material at -50℃ is 322~362J, and the impact energy of the welding at -40℃ is 239~325J; 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 1.5 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 to adjust the composition. 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 continuous casting billets 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 rough rolling and final rolling temperature are ≥980℃, and the second stage finishing rolling and final rolling temperature are controlled at 781℃~800℃; (4) Cooling: Use water cooling for rapid cooling, and the red-return temperature after cooling is 425~465℃.

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