EH690 high-strength steel for maritime work and manufacturing process thereof

Abstract

The invention discloses EH690 high-strength steel for maritime work and a manufacturing process of the EH690 high-strength steel, the EH690 high-strength steel mainly comprises the chemical components of C, Si, Mn, P, S, Cr, Mo, V, Ti, Nb, Ni, B and Al, the residual elements comprise Cu and part of gas elements, and the balance is Fe and inevitable impurities. The process of the high-strength steel comprises the steps of adopting an EBT electric arc furnace, LF furnace refining, VD vacuum refining, die casting process route, forging, pre-heat treatment of normalizing and final heat treatment process of quenching and tempering. The longitudinal impact toughness value of the prepared low-temperature steel at-40 DEG C is larger than or equal to 69 J, the transverse impact toughness value is larger than or equal to 46 J, and the low-temperature steel has high strength, good low-temperature impact performance, good welding performance and corrosion resistance.

Description

Technical Field

[0001] The invention relates to the technical field of special equipment metallurgy, and in particular to EH690 high-strength steel for marine engineering and a forging heat treatment process thereof. Background Art

[0002] With the development of marine resources, the mechanical properties of ultra-high strength steel plates for marine engineering are becoming more and more demanding. The standard versions of classification societies in various countries have been continuously updated, not only putting forward higher requirements in terms of mechanical properties such as tensile and impact, but also adding requirements for low yield ratio. EH690 steel is used for offshore platforms. Offshore platforms are special places for operations on the ocean, and their service life is 50% longer than that of ships. Due to the special use environment of EH690 steel, the material requires higher strength, good low-temperature impact resistance, good welding performance and corrosion resistance. The "China Classification Society Material and Welding Specification" standard stipulates the composition and performance of EH690, but to obtain EH690 forgings with better performance, it is necessary to design and optimize the composition ratio of the material, and select reasonable smelting, forging and heat treatment processes in combination with the product. Summary of the invention

[0003] The purpose of the present invention is to provide EH690 high-strength steel for marine engineering and its forging heat treatment process, so as to ensure that the prepared high-strength steel can meet the material use requirements in terms of strength and low-temperature impact performance.

[0004] To achieve the above purpose, the technical solution adopted by the present invention is: A kind of EH690 high-strength steel for marine engineering, whose chemical composition is measured by weight percentage and consists of the following components: C: 0.14-0.16; Si: 0.30-0.55%; Mn: 1.4-1.6%; P: ≤0.015%; S: ≤0.005%; Cr: 0.45-0.6%; Ni: 0.25-0.4%; Mo: 0.30-0.35%; Ti≤0.02%; V: 0.06-0.10%; B≤0.005%; Als: 0.02-0.04%; Nb: 0.2-0.4%; Cu: ≤0.1%; gas element [H] ≤2PPm; [O] ≤30ppm; [N] ≤80ppm; the balance is Fe and unavoidable impurities.

[0005] A high-strength EH690 steel for marine engineering, the composition of which meets the above requirements, adopts the EBT arc furnace-LF furnace refining-VD furnace vacuum refining-die casting process route to smelt and cast the steel ingot to form a cylindrical billet. The carbon equivalent Cet and welding sensitivity Pcm in the chemical composition design follow the following principles: Cet=C+(Mn+Mo) / 10+(Cu+Cr) / 20+Ni / 40≤0.4%; Pcm=C+Si / 30+Mn / 20+Cu / 20+Ni / 60+Cr / 20+Mo / 15+V / 10+5B≤0.33%.

[0006] The forging of the billet is divided into billet opening, forging and forming, which is specifically divided into the following steps: Step 1: Chamfering, heat the billet to 850℃ and keep it for 3h, then heat it to 1220℃ at a rate of ≤120℃ / h, keep it for 10h, ensure that the billet is evenly burned through, take it out of the furnace, press the jaws, chamfer, chop the riser end and the nozzle end, chamfer it to the outer diameter size D1 in the upper and lower flat anvils, and the final forging temperature is ≥800℃; Step 2: Upsetting: the blank treated in step 1 is upset in the Z direction as a whole with an upsetting cover plate, and the blank is upset to an outer diameter of D2 and a height of H1. The upsetting is performed in 3 times, and the pressing amount is controlled at 250-350mm each time. After each upsetting, the pressure is maintained, and the final forging temperature is ≥800℃; Step 3: Heat the billet processed in step 2 to 1200°C, keep it warm for 4 hours, take it out of the furnace, stretch and upset it in the X direction, stretch and upset it in the Y direction, stretch it in the Z direction, forge it to the product blanking size, and the final forging temperature is ≥800°C; if one fire cannot complete the deformation in the X direction, Y direction, and Z direction, during the forging process, return to the furnace for heating when the temperature is lower than 800°C, and continue forging after heating through in the temperature range of 1150-1200°C until the deformation in the three directions is completed; Step 4: sawing the blank processed in step 3 into blanks; Step 5: Heat the billet obtained in step 4 to 1200°C, keep it at this temperature for 6 hours, take it out of the furnace, and forge it; Step 6: The blank in step 5 is stretched to ¨a, with a height of H2, and then upset to ¨b, with a height of H3, punched with a punch with a diameter of D3, the core material is removed, the hole is expanded to the size of the center hole of D4, and reshaped to obtain a workpiece that meets the design size requirements; Step 7: The workpiece of step 6 is further heat treated to obtain EH690 high-strength steel for marine engineering that meets the design requirements.

[0007] The heat treatment in step 7 comprises the following steps: Load the workpiece into the furnace, the furnace temperature is ≤300℃, and the furnace spacing is ≥30mm; Preparatory heat treatment, normalizing, heating at a rate of ≤100℃ / h to 910±10℃, directly keeping the temperature for 6h after reaching the temperature, and air cooling after taking out of the furnace; Final heat treatment: quenching + tempering. The workpiece after preliminary heat treatment is loaded into the furnace with a loading temperature of ≤300℃, heated to 900±10℃ at a speed of ≤100℃ / h, kept warm for 7h after reaching temperature, and water-cooled after taking out of the furnace. When the workpiece temperature drops below 300℃, the workpiece is loaded into the furnace for the third time with a loading temperature of ≤300℃, heated to 560±10℃ at a speed of ≤100℃ / h, kept warm for 9h after reaching temperature, and air-cooled after taking out of the furnace.

[0008] In order to obtain good structure and comprehensive mechanical properties of the workpiece in the above quenching, the water temperature before the workpiece is quenched into water is required to be ≤40℃; during the quenching operation, the workpiece is immediately shaken up and down by a steel loader after entering the water, and the workpiece cannot float to the surface of the water to achieve the effect of breaking the air film, so that the workpiece can be cooled evenly and quickly, and the shaking time is not less than two minutes.

[0009] Beneficial effects of the invention: The product EH690 high-strength steel of the invention is optimized on the basis of meeting the composition and performance requirements of EH690 in the "China Classification Society Material and Welding Specifications". The addition of V element is to refine the grains and improve the strength; the selection of Mo element is to increase the toughness of the material and improve the corrosion resistance of the steel; The patent of this invention optimizes the composition ratio of EH690 high-strength steel and selects reasonable smelting, forging and heat treatment process methods to obtain steel ingots that meet the design requirements. BRIEF DESCRIPTION OF THE DRAWINGS

[0010] Figure 1 : Schematic diagram of the blank structure of the chamfered edge in manufacturing process step 1; Figure 2 : Schematic diagram of the roughened blank structure in step 2 of the manufacturing process; Figure 3 : Schematic diagram of the billet structure after three-way forging in manufacturing process step 3; Figure 4 : Microstructure picture of the finished workpiece; Figure 5 : Low magnification metallographic image of the finished workpiece. DETAILED DESCRIPTION

[0011] A high-strength EH690 steel for marine engineering, whose chemical composition, measured by weight percentage, consists of the following components: C: 0.15; Si: 0.35; Mn: 1.39; P: 0.009; S: 0.002; Cr: 0.46; Ni: 0.26; Mo: 0.3; Ti: 0.002; V: 0.07; B: 0.0001; Als: 0.019; Nb: 0.031; Cu: 0.07; gas elements [H] ≤ 2PPm; [O] ≤ 30ppm; [N] ≤ 80ppm; the balance is Fe and unavoidable impurities.

[0012] The production process of EH690 high-strength steel for marine engineering, the composition of the high-strength steel is as described above, the raw materials are smelted and poured into steel ingots by EBT electric arc furnace-LF furnace refining-VD vacuum refining-die casting process to form cylindrical billets, the forging of billets is divided into blanking and forging and forming, which is specifically divided into the following steps; Step 1: Chamfering, such as Figure 1 As shown, a 15t steel ingot with the above chemical composition is placed in a heating furnace for heating, heated to 850℃ and kept for 3h, then heated to 1220℃ at a rate of ≤120℃ / h, kept for 10h, ensuring that the billet is evenly burned through, taken out of the furnace, pressed, chamfered, chopped off the riser end and the nozzle end, and chamfered in the upper and lower flat anvils to an outer diameter size D1≈1100mm. The final forging temperature is ≥800℃.

[0013] Step 2: Upsetting: The blank treated in step 1 is upset in the Z direction with an upsetting cover plate, and the blank is upset to an outer diameter of D2 = 1500mm and a height of H1 = 950mm. The upsetting is performed in 3 times, and the pressing amount is controlled at 250-350mm each time. After each upsetting, the pressure must be maintained. The final forging temperature is ≥800℃.

[0014] Step 3: Heat the billet processed in step 2 to 1200℃, keep it warm for 4h, take it out of the furnace, stretch and upset it in the X direction, stretch and upset it in the Y direction, stretch it in the Z direction, and forge it to the product blanking size of ¨600*4780, such as Figure 3 As shown. Final forging temperature ≥ 800℃.

[0015] Step 4: If one fire cannot complete the deformation in the X, Y and Z directions, when the temperature is lower than 800℃, it should be returned to the furnace for heating, and continue forging after being heated through in the temperature range of 1150-1200℃ until the deformation in the three directions is completed. The final forging temperature is ≥800℃.

[0016] Step 5: Saw the blank processed in step 4 into blanks of ¨600*325.

[0017] Step 6: Heat the ¨600*325 billet processed in step 5 to 1200℃, keep it warm for 6h, take it out of the furnace, and forge it.

[0018] Step 7: The ¨600*325 billet in step 6 is drawn to ¨a=540, with a height of H2=400, and then upset to ¨b=614, with a height of H3=310. It is punched with a punch with a diameter of D3=φ280, the core material is removed, and the hole is expanded with a rack to a center hole size of D4=φ350. After shaping, a forging that meets the design size requirements is obtained.

[0019] The forgings forged in the above steps are subjected to heat treatment, including the following steps: Step 1: Normalizing, load the forged parts into the furnace, the loading distance is ≥30mm, the loading temperature is ≤300℃, heat to 910±10℃ at a speed of ≤100℃ / h, keep warm for 6h after reaching the temperature, and air cool after taking out of the furnace.

[0020] Step 2: Quenching. Cool the forgings normalized in step 1 to ≤300℃ and load them into the furnace. Heat them to 900±10℃ at a speed of ≤100℃ / h. Keep them warm for 7h after reaching the temperature. Cool them in water after they are taken out of the furnace. In order to obtain good organization and comprehensive mechanical properties, the water temperature of the forgings before quenching in water is required to be ≤40℃. After the forgings enter the water, they are immediately shaken up and down by a steel loader. The forgings cannot float to the surface of the water to achieve the effect of breaking the air film, so that the forgings can be cooled evenly and quickly. The shaking time is not less than two minutes.

[0021] Step 3: Tempering. When the temperature of the forging drops below 300°C, the quenched forging is loaded into the furnace for the third time. The charging temperature of the furnace is ≤300℃, and it is heated to 560±10℃ at a speed of ≤100℃ / h. After reaching the temperature, it is directly kept warm for 9 hours and then air-cooled when it is taken out of the furnace.

[0022] The mechanical properties of the forgings prepared according to the present invention were tested, including room temperature yield strength (Rel) and tensile strength (Rm), section shrinkage (Z%), elongation after forging (A%), -30℃ low temperature impact toughness (Kv2), -40℃ low temperature impact toughness (Kv2). The test results are as follows: The first group of specimens: room temperature yield strength (Rel) is 738 MPa, tensile strength (Rm) is 802 MPa, section shrinkage (Z%) is 77, elongation after forging (A%) is 25, -30℃ low temperature impact toughness (Kv2) is 208J, 210J, 201J in the longitudinal direction, and 263J, 277J, 294J in the transverse direction, -40℃ low temperature impact toughness (Kv2) is 203J, 187J, 187J in the longitudinal direction, and 240J, 280J, 291J in the transverse direction.

[0023] The second group of specimens: room temperature yield strength (Rel) is 741MPa, tensile strength (Rm) is 82MPa, section shrinkage (Z%) is 76, elongation after forging (A%) is 23, -30℃ low temperature impact toughness (Kv2) is 209J, 196J, 196J in the longitudinal direction, 266J, 276J, 282J in the transverse direction, -40℃ low temperature impact toughness (Kv2) is 177J, 192J, 204J in the longitudinal direction, 264J, 208J, 265J in the transverse direction.

[0024] The patent of this invention optimizes the composition ratio of EH690 high-strength steel for marine engineering and reasonably selects the smelting, forging and heat treatment processes, so that the EH690 high-strength steel has good comprehensive mechanical properties and strong low-temperature impact toughness, and has the performance of serving in extremely cold environments for marine platform steel.

Claims

1. A high-strength EH690 steel for marine engineering, characterized in that The chemical composition of the EH690 high-strength steel is composed of the following components in weight percentage: C: 0.14-0.16; Si: 0.30-0.55%; Mn: 1.4-1.6%; P: ≤0.015%; S: ≤0.005%; Cr: 0.45-0.6%; Ni: 0.25-0.4%; Mo: 0.30-0.35%; Ti≤0.02%; V: 0.06-0.10%; B≤0.005%; Als: 0.02-0.04%; Nb: 0.2-0.4%; Cu: ≤0.1%; gas elements [H] ≤2PPm; [O] ≤30ppm; [N] ≤80ppm; the balance is Fe and unavoidable impurities.

2. A process for producing EH690 high-strength steel for marine engineering, the components of the high-strength steel being as described in claim 1, characterized in that: The steel ingot is smelted and poured using the EBT arc furnace - LF furnace refining - VD furnace vacuum refining - die casting process to form a cylindrical billet. The billet forging is divided into billet opening, forging and forming, which is specifically divided into the following steps: Step 1: Chamfering, heat the billet to 850℃ and keep it for 3h, then heat it to 1220℃ at a rate of ≤120℃ / h, keep it for 10h, ensure that the billet is evenly burned through, take it out of the furnace, press the jaws, chamfer, chop the riser end and the nozzle end, chamfer it to the outer diameter size D1 in the upper and lower flat anvils, and the final forging temperature is ≥800℃; Step 2: Upsetting: the blank treated in step 1 is upset in the Z direction with an upsetting cover plate, and the blank is upset to an outer diameter of D2 and a height of H1. The upsetting is performed in 3 times, and the pressing amount is controlled at 250-350mm each time. After each upsetting, the pressure is maintained, and the final forging temperature is ≥800℃; Step 3: Heat the billet processed in step 2 to 1200°C, keep it warm for 4 hours, take it out of the furnace, stretch and upset it in the X direction, stretch and upset it in the Y direction, stretch it in the Z direction, forge it to the product blanking size, and the final forging temperature is ≥800°C; if one fire cannot complete the deformation in the X direction, Y direction, and Z direction, during the forging process, return to the furnace for heating when the temperature is lower than 800°C, and continue forging after heating through in the temperature range of 1150-1200°C until the deformation in the three directions is completed; Step 4: sawing the blank processed in step 3 into blanks; Step 5: Heat the billet obtained in step 4 to 1200°C, keep it at this temperature for 6 hours, take it out of the furnace, and forge it; Step 6: The blank in step 5 is stretched to ¨a, with a height of H2, and then upset, upset to ¨b, with a height of H3, punched with a punch with a diameter of D3, the core material is removed, the hole is expanded to the size of the center hole of D4, and reshaped to obtain a workpiece that meets the design size requirements; Step 7: The workpiece of step 6 is further heat treated to obtain EH690 high-strength steel for marine engineering that meets the design requirements.

3. The manufacturing process of EH690 high-strength steel for marine engineering according to claim 2 is characterized in that: The heat treatment in step 7 comprises the following steps: Load the workpiece into the furnace, the furnace temperature is ≤300℃, and the furnace spacing is ≥30mm; Preparatory heat treatment, normalizing, heating at a rate of ≤100℃ / h to 910±10℃, directly keeping the temperature for 6h after reaching the temperature, and air cooling after taking out of the furnace; Final heat treatment: quenching + tempering. The workpiece after preliminary heat treatment is loaded into the furnace with a loading temperature of ≤300℃, heated to 900±10℃ at a speed of ≤100℃ / h, kept warm for 7h after reaching temperature, and water-cooled after taking out of the furnace. When the workpiece temperature drops below 300℃, the workpiece is loaded into the furnace for the third time with a loading temperature of ≤300℃, heated to 560±10℃ at a speed of ≤100℃ / h, kept warm for 9h after reaching temperature, and air-cooled after taking out of the furnace.

4. The manufacturing process of EH690 high-strength steel for marine engineering according to claim 3 is characterized in that: The temperature of the water used for water cooling after the final heat treatment is required to be ≤40℃; during the quenching operation, the workpiece should be shaken up and down by a steel loader immediately after entering the water. The workpiece cannot float to the surface of the water and the shaking time should be no less than two minutes.

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