A low yield ratio high strength and toughness offshore steel and its preparation method

Abstract

A low yield ratio high-strength and tough offshore steel and its preparation method belong to the field of metallurgical technology. The basic design idea adopted in the present invention is that on the basis of C+Ni+Cr+Mo+V+Cu series steel, the carbon content is controlled below 0.1%. Through appropriate ratios of Cr, Ni, Mo and Cu, it generates dispersed second-phase strengthening and fine-grain strengthening to improve strength and toughness, so that the steel can ensure the required strength and good low-temperature toughness without increasing costs. During the preparation process, through two-stage rolling and quenching+tempering heat treatment, the ratio of soft and hard phases is regulated to improve the plasticity and toughness of the material and achieve a low yield ratio. A low yield ratio 690MPa grade high-toughness offshore steel with a yield strength ≥690MPa, a tensile strength ≥830MPa, a yield ratio ≤0.87, an elongation after fracture A≥18%, a room-temperature transverse impact toughness ≥100J, and a -40°C transverse impact toughness ≥80J is prepared.

Description

Technical Field

[0001] The present invention belongs to the field of metallurgical technology, and particularly relates to a low yield ratio high-strength and tough offshore steel and a preparation method thereof. Background Art

[0002] Offshore steel is applied in fields such as shipbuilding and ocean engineering, and is an important material for manufacturing ocean equipment. The service environment of ocean engineering platforms is harsh. It is necessary to consider the influence of various factors such as strong winds, surges, tides, ice impacts, and earthquakes, and also to consider adapting to various sea conditions. The changeable and complex ocean service environment determines that the steel for ocean engineering must have properties such as high strength, high toughness, fatigue resistance, resistance to lamellar tearing, good weldability, seawater corrosion resistance, and good cold workability. At the same time, during the application of high-strength steel for ocean engineering, as the strength increases, the yield ratio of the material will continuously increase, the plasticity will decrease, and the safety will decrease. Therefore, while maintaining a relatively high strength, the offshore steel should ensure a low yield ratio and good low-temperature toughness.

[0003] Patent CN114480960A discloses "A low yield ratio low-temperature toughness 800MPa grade high-strength steel and its production process", which adopts an ultra-low C, high Mn, Cr content, alloying with Mo, B, Ti, and a composition design without adding Ni and V. The TMCP + tempering process is used to achieve a yield strength of 690 grade, the steel plate thickness reaches 25 - 30mm, the yield strength ≥ 690MPa, the tensile strength ≥ 770 - 910MPa, the longitudinal low-temperature impact energy at -40°C ≥ 180J, and the yield ratio is in the range of 0.88 - 0.89.

[0004] Patent CN116219318A discloses "A low yield ratio ultra-low temperature toughness extra-thick offshore steel plate and its manufacturing method", which adopts a composition design with low C, low Mn, high Ni combined with Cr and Nb elements to obtain a steel plate with a yield strength above 550MPa, a tensile strength of 640 - 820MPa, a Charpy impact energy at -80°C ≥ 80J, and a yield ratio ≤ 0.84. Not only is the strength level limited, but also two high-temperature rollings are required, the heating time is too long, and the energy consumption is large.

[0005] CN112647019A discloses "a manufacturing method for low yield ratio steels with different strength levels". The alloy element contents are as follows: C: 0.06 - 0.12%, Mn: 1.40 - 1.60%, Si: 0.10 - 0.20%, S ≤ 0.005%, P ≤ 0.015%, Nb: 0.020 - 0.050%, Ti: 0.080 - 0.120%, Alt: 0.020 - 0.040%, N ≤ 0.006%, and the rest are Fe and inevitable impurities. The hot continuous rolling controlled rolling and controlled cooling + quenching process is adopted, with a thickness specification of 2 - 16 mm, a tensile strength above 700 MPa, a yield strength between 580 - 689 MPa, a longitudinal low-temperature impact energy at -40°C ≤ 138 J, and a yield ratio between 0.77 - 0.83.

[0006] Patent CN114032459A discloses "a preparation method for high-strength and high-toughness medium and heavy plates with a yield strength of 690 MPa grade and a low yield ratio". A composite alloying design of low carbon, medium and low nickel, and high Cu, Cr, and Mo is adopted, along with Nb + V + Ti composite microalloying. Through multi-step heat treatment, medium and heavy plates with a thickness specification of 10 - 50 mm, a yield strength ≥ 690 MPa, and a yield ratio ≤ 0.85 are obtained. Although this method also reaches 690 MPa, it adopts a multi-step heat treatment process, resulting in a long production cycle and low production efficiency.

[0007] Therefore, the present invention designs a new composition design. Through quenching + tempering heat treatment, a composition design of low C, medium and low Ni, and Cr + Mo + V + Cu microalloying is adopted. By controlling the contents of strong carbide-forming elements such as Cr, Mo, Cu, and V, the diffusion of carbon is hindered, thereby slowing down the formation rate of austenite, refining the grain size, coordinating the quenching + tempering process to regulate the ratio of soft and hard phases, forming fine and dispersed precipitated phases, and hindering grain growth, so as to obtain high-strength and high-toughness low yield ratio steel plates with a yield strength above 690 MPa and a low-temperature toughness value of above 80 J at -40°C. Summary of the Invention

[0008] The present invention aims to provide a low yield ratio high strength and toughness offshore steel and its preparation method. The basic design idea adopted is that on the basis of C+Ni+Cr+Mo+V+Cu series steel, the carbon content is controlled below 0.1%. Too low carbon content will inevitably reduce the strength of the steel. The content of Ni element is increased to improve the low temperature toughness of the steel as much as possible. Ni element is an element that stabilizes austenite and can greatly improve the low temperature toughness of the steel plate. At the same time, through the appropriate ratio of Cr, Ni, Mo and Cu, it produces dispersed second phase strengthening and fine grain strengthening to improve strength and toughness, so that the steel can ensure the required strength and good low temperature toughness without increasing the cost. The preparation process realizes a low yield ratio through two-stage rolling and quenching+tempering heat treatment. Rapid cooling after quenching can inhibit the transformation of austenite into soft phases such as pearlite and promote the formation of martensite or bainite. These hard phases are beneficial to improving the strength of the material. High temperature tempering can promote the release of internal stress generated during the quenching process, and at the same time decompose hard phases such as martensite to form stable structures such as bainite and tempered sorbite. Among them, with the precipitation of carbides and the decomposition of martensite, the content of soft phase ferrite also gradually increases. By regulating the ratio of hard and soft phases, the plasticity and toughness of the material are improved and the yield ratio is reduced. Finally, a low yield ratio high strength and toughness offshore steel with good comprehensive mechanical properties is obtained.

[0009] To achieve the above object, the technical solution of the present invention is as follows:

[0010] A low yield ratio high strength and toughness offshore steel, the elemental composition of which is by mass percentage: C 0.08% - 0.1%, Si 0.30% - 0.35%, Mn 1.05% - 1.25%, Ni 1.80% - 2.10%, Mo 0.30% - 0.35%, Cr 0.13% - 0.16%, Ti 0.008% - 0.010%, V 0.015% - 0.030%, Nb 0.045% - 0.050%, B 0.0010% - 0.0013%, Cu 0.08% - 0.12%, Al 0.055% - 0.060%, P≤0.010%, S≤0.0040%, carbon equivalent CET≤0.31, and the balance is Fe and unavoidable impurities;

[0011] The yield strength of the offshore steel is ≥690 MPa, the tensile strength is ≥830 MPa, the yield ratio is ≤0.87, the elongation after fracture A≥18%, the transverse impact energy at room temperature is ≥100 J, and the transverse impact energy at -40°C is ≥80 J.

[0012] The preparation method of the low yield ratio high strength and toughness offshore steel includes smelting and continuous casting → heating → controlled rolling → controlled cooling → tempering, and the steps are as follows:

[0013] Step 1. smelting and continuous casting: smelting the raw materials to obtain molten iron; smelting the ingot according to the composition of the offshore steel billet, and the thickness of the ingot is not less than 150mm; the molten iron is subjected to desulfurization treatment, converter smelting, LF refining and RH vacuum treatment, and then continuously cast into billets, the sulfur content of the molten iron after desulfurization treatment is controlled to be ≤0.0040%, the converter adopts the double slag method to control P≤0.010%, the LF refining adopts white slag operation desulfurization and deoxidation, the vacuum degree of RH vacuum treatment is ≤3.0mbar, the vacuum holding time is ≥15min, calcium treatment is carried out after the vacuum ends, and then static stirring is carried out, the static stirring time is ≥12min, and the continuous casting controls the superheat of the tundish to be 10℃~15℃; the continuous casting implements full-process protection casting, adopts large reduction (single-pass reduction is more than 20mm) rolling technology to improve the internal quality of the continuous casting billet, and uses the end-stage electric stirring to control segregation;

[0014] Step 2. Heating: After continuous casting into ingots, the ingots with a thickness of more than 150 mm are heated in a soaking furnace, the heating temperature is controlled to be 1200°C-1250°C, the furnace time is 110min-120min, and the tapping temperature is 1100°C-1180°C;

[0015] Step 3. controlled rolling: rolling the heated ingot; the rolling is divided into two stages, rough rolling and finishing rolling, and the width is controlled to be 160mm-180mm during rolling; 0-3 vertical rolling processes are performed between the finishing and rough rolling stages; the starting rolling temperature of rough rolling is 1050℃-1120℃, and after 3-5 rolling passes, the reduction rate of the last 2-3 passes is ≥9%; large reduction rolling is adopted to ensure that the reduction rate of each pass is set as much as possible according to the maximum rolling capacity; the cumulative compression ratio of rough rolling is not less than 1.5, and the thickness of the intermediate billet is ≥64mm; the starting rolling temperature of the finishing rolling stage is 827℃-835℃, and after 3-5 rolling passes, the final rolling temperature is ≥800℃; the rolled steel plate is obtained, and the thickness of the rolled steel plate is 20mm-100mm;

[0016] Step 4. Controlled cooling: After rolling, laminar cooling is used, with a cooling rate of 5℃ / s-10℃ / s. The water inlet temperature is 775℃-798℃, and the red-return temperature is 600℃~660℃;

[0017] Step 5. Quenching and tempering: the steel plate after controlled cooling treatment is subjected to quenching and tempering heat treatment; the quenching temperature is the phase change temperature AC3±(10℃~30℃), the holding time is 15min-60min, and after the holding is completed, it is cooled by water cooling to room temperature; then tempering treatment is carried out, the tempering temperature is 660℃-680℃, the holding time is 30min-60min, and air cooling or wind cooling is carried out to room temperature.

[0018] The mechanism of component design in the present invention is as follows:

[0019] C: It is the main element to ensure strength and hardness. If the C content is too high, a large amount of hardened structure will be produced, affecting the low-temperature impact toughness and welding performance. If the C content is too low, the strength of the steel plate will decrease. Therefore, in the present invention, the content range of C is controlled to be 0.08% - 0.10%.

[0020] Si: It plays a role in solid solution strengthening and can improve the strength of the steel plate. At the same time, as a deoxidizer, Si can reduce the O content. However, in the present invention, Al is used for deoxidation, which can reduce the Si element content. But when the mass fraction of Si is relatively high, the toughness and plasticity of the steel will be reduced. Adding an appropriate amount of Si to the steel can inhibit the growth of carbides and improve the toughness of the steel. Therefore, in the present invention, the content range of Si is controlled to be 0.30% - 0.35%.

[0021] Mn: It is also one of the deoxidizers and can effectively improve the hot working performance and anti-tearing ability of the steel. Mn has the effect of delaying the transformation of austenite to ferrite in the steel, which is beneficial to refining ferrite, improving strength and toughness. When the content of Mn is low, the above effects are not significant, and the strength and toughness of the steel plate are relatively low. If it is too high, it will cause segregation of continuous casting billets and poor toughness. Therefore, considering the comprehensive addition of alloys in the present invention, the content range of Mn is controlled to be 1.05% - 1.25%.

[0022] Ni: In the steel, it can expand the austenite region, enabling the steel to obtain more austenite structure during the quenching process. Since austenite is the precursor of hard phase structures such as martensite formed after quenching, it can indirectly increase the hardness and depth after quenching and improve the hardenability; it can also refine the grains of the soft phase ferrite to form a finer structure, improving the strength and toughness of the steel. In addition, the combined action with residual Cr and P in the steel will help improve the corrosion resistance of the steel. Therefore, in the present invention, the content range of Ni is controlled to be 1.80% - 2.10%.

[0023] Mo: In the steel, it can increase the stability of austenite and delay the decomposition rate of austenite during the cooling process, so that the steel can form a deeper hardened layer during quenching, increasing the hardenability. Molybdenum can inhibit the growth of grains, obtain finer grains and more grain boundaries, and can hinder the propagation of cracks, thereby improving strength and toughness. But if it is too high, it is easy for carbides to precipitate along the grain boundaries, and cracks will be generated and propagated along the grain boundaries, which will instead inhibit the toughness. Therefore, in the present invention, the content range of Mo is controlled to be 0.30% - 0.35%.

[0024] Cr: Chromium can form a continuous solid solution with iron, narrow the austenite phase region, and slow down the decomposition rate of austenite, so that the steel is more likely to obtain a deeper hardened layer during the quenching process. In addition, chromium can also form various carbides with carbon, and these carbides help refine the structure and improve the hardenability of the steel during the quenching process. But if the content is too high, it will affect the toughness of the steel and cause temper brittleness. Therefore, in the present invention, the content range of Cr is controlled to be 0.13% - 0.16%.

[0025] B: Solution boron promotes the formation of fine bainite structure after controlled rolling and controlled cooling. This kind of fine bainite contains more stable dislocations and inherits the deformation dislocations generated in austenite during its formation. This fine structure significantly improves the strength and toughness of the steel. It can also have a synergistic effect with other alloying elements (such as niobium, titanium, etc.) in the steel, further improving the hardenability and mechanical properties of the steel. The content range of B controlled in the present invention is 0.0010% - 0.0013%.

[0026] Cu: Dissolved in the steel, it can increase the resistance to dislocation movement, thus achieving the effect of solid solution strengthening and improving the strength and hardness of the steel. The addition of Cu element can also make the grains in the steel smaller and increase the grain boundary area, thereby improving the hardness and strength of the steel. The refined grains can more effectively hinder the movement of dislocations, but the content of Cu should not be too high, as a higher content is more likely to cause brittle fracture. Therefore, the content range of Cu controlled in the present invention is 0.08% - 0.12%.

[0027] Al: It is an important deoxidizing element in the steelmaking process. Adding a small amount of Al to the steel can also effectively reduce the inclusion content in the steel and refine the grains. However, too much Al will promote the generation of surface cracks in continuous casting billets, produce internal aluminum-based inclusions, and reduce the quality of slab. Therefore, the content range of total Al controlled in the present invention is 0.055% - 0.060%.

[0028] Ti: It has the functions of refining grains and precipitation strengthening. Ti combines with carbon element to form stable carbides, which play a role in pinning austenite grain boundaries and maintaining the refinement of austenite grains. Ti combines with S to form Ti4C2S2 or (MnTi)S, reducing the content of inclusion MnS in the steel and improving the non-uniformity of the structure, thereby improving the impact toughness of the steel. Therefore, the content range of Ti controlled in the present invention is 0.008% - 0.010%.

[0029] V: It is an element for refining grains and has a high solid solubility in the steel. It mainly affects the structure and properties of the steel by forming V(C,N), precipitating and separating in the ferrite at the austenite grain boundary, and can inhibit the recrystallization of austenite and prevent the growth of grains during the rolling process, thus playing a role in refining ferrite grains, improving the strength and toughness of the steel, but the addition amount should not be too much. Therefore, the content range of V controlled in the present invention is 0.015% - 0.030%.

[0030] P, S: They are unfavorable elements in the steel, seriously affecting the strength, toughness and corrosion resistance of the steel, etc., and their content needs to be controlled as low as possible. Therefore, the content range of P controlled in the present invention is below 0.010%, and the content range of S controlled is below 0.0040%.

[0031] The beneficial effects of the present invention:

[0032] The present invention provides a low yield ratio high-strength and tough offshore steel and a preparation method thereof, adopting the basic design idea of low C, high Ni and microalloying of Cr+Mo+V+Cu. Based on the C+Ni+Cr+Mo+V+Cu series steel, the carbon content is controlled below 0.1%. Too low carbon content will inevitably reduce the strength of the steel. The content of Ni element is increased to improve the low-temperature toughness of the steel as much as possible. Ni element is an element that stabilizes austenite and can greatly improve the low-temperature toughness of the steel plate. At the same time, through the appropriate ratio of Cr, Ni, Mo and Cu, it produces dispersed second-phase strengthening and fine-grain strengthening to improve strength and toughness, so that the steel can ensure the required strength and good low-temperature toughness without increasing costs. The preparation process realizes a low yield ratio through two-stage rolling and quenching+tempering heat treatment. Rapid cooling after quenching can inhibit the transformation of austenite into soft phases such as pearlite and promote the formation of martensite or bainite. These hard phases are beneficial to improving the strength of the material. High-temperature tempering can promote the release of internal stress generated during the quenching process, and at the same time decompose hard phases such as martensite to form stable structures such as bainite and tempered sorbite. Among them, with the precipitation of carbides and the decomposition of martensite, the content of soft-phase ferrite also gradually increases. By regulating the ratio of hard and soft phases, the plasticity and toughness of the material are improved, and the yield ratio is reduced. The finally obtained offshore steel has a yield strength ≥690 MPa, a tensile strength ≥830 MPa, a yield ratio ≤0.87, an elongation after fracture A≥18%, a room-temperature transverse impact toughness ≥100 J, and a -40°C transverse impact toughness ≥80 J, and has good mechanical properties. Description of the Drawings

[0033] Figure 1 It is the microstructure morphology at the 1 / 2 position of the steel plate in the quenched and tempered state in Example 1 of the present invention;

[0034] Figure 2 It is the microstructure morphology at the 1 / 2 position of the steel plate in the quenched and tempered state in Example 2 of the present invention;

[0035] Figure 3 It is the microstructure morphology at the 1 / 2 position of the steel plate in the quenched and tempered state in Example 3 of the present invention;

[0036] Figure 4 It is the microstructure morphology at the 1 / 2 position of the steel plate in the quenched and tempered state in Example 4 of the present invention;

[0037] Figure 5 It is the microstructure morphology at the 1 / 2 position of the steel plate in the quenched and tempered state in Example 5 of the present invention. Detailed Embodiments

[0038] The present invention will be further described in detail below with reference to the drawings and specific embodiments.

[0039] Example 1

[0040] A low yield ratio and high strength marine steel, the steel plate thickness is 20mm, its chemical composition and mass percentage are shown in Table 1, the balance is Fe and unavoidable impurities. It includes smelting and continuous casting, heating, controlled rolling, controlled cooling and quenching and tempering processes, and the specific process steps are as follows:

[0041] (1) Smelting and continuous casting: Smelt 150mm thick ingots according to chemical composition requirements. The raw materials are melted in proportion to obtain molten iron. After desulfurization treatment, the molten iron is smelted in a converter, LF refined and RH vacuum treated, and then continuously cast into ingots. The sulfur content of the molten iron after desulfurization is controlled at ≤0.0040%. The converter adopts the double slag method to control P≤0.010%. LF refining uses white slag operation for desulfurization and deoxidation. The vacuum degree of RH vacuum treatment is ≤3.0mbar, and the vacuum holding time is ≥15min. After the vacuum is over, calcium treatment is carried out, and then static stirring is carried out. The static stirring time is ≥12min. The continuous casting controls the superheat of the tundish to be 10℃~15℃. Continuous casting implements full protection casting, adopts rolling technology with a single pass reduction of 20mm to improve the internal quality of the continuous casting ingot, and uses end-stage electric stirring to control segregation.

[0042] (2) Heating: After continuous casting into billets, the 150 mm thick billets were heated in a soaking furnace, the heating temperature was controlled at 1200°C, the furnace time was 110 min, and the tapping temperature was 1150°C;

[0043] (3) Controlled rolling: Slab rolling is divided into two stages: rough rolling and finishing rolling. The width is controlled at 160mm during rolling. Two vertical rolling processes are performed in the middle (180-68-160 is the vertical rolling reduction process for controlling the width). The starting temperature of rough rolling is 1050-1120℃. After four rolling passes, large reduction rolling is adopted to ensure that the reduction rate of each pass is set as much as possible according to the maximum rolling capacity. The reduction rate of the last two passes is ≥15%. The reduction rates of the passes are 13.3%, 23.1%, 20%, and 20% respectively. The cumulative reduction ratio of rough rolling is 2.3, and the thickness of the intermediate slab is 64mm. The starting temperature of the finishing rolling stage is 827℃, the final rolling temperature is 805℃, and the reduction rates of the passes are 18.7%, 19.2%, 19.0%, 17.6%, 17.8%, and 11.5% respectively.

[0044] (4) Controlled cooling: The billet is rolled to a finished product thickness of 20 mm. After rolling, it is cooled by laminar cooling at a cooling rate of 8°C / s. The water inlet temperature is 778°C and the red-return temperature is 610°C.

[0045] (5) Quenching and tempering: After rolling, the steel plate is subjected to quenching and tempering heat treatment. The quenching temperature is 860°C, the holding time is 15 minutes, and after the holding is completed, it is cooled to room temperature by water cooling in a water tank; the tempering temperature is 660°C, the holding time is 30 minutes, and air cooling is performed to room temperature.

[0046] The performance indicators of the steel plate in this embodiment are shown in Table 1. The core structure of the steel plate is as follows: Figure 1 As shown, the metallographic structure of the steel is a mixed structure of 26.5% ferrite, lath bainite and granular bainite, and the grain size is 25μm-35μm.

[0047] Example 2

[0048] A low yield ratio and high strength marine steel, the steel plate thickness is 40mm, its chemical composition and mass percentage are shown in Table 1, the balance is Fe and unavoidable impurities. It includes smelting and continuous casting, heating, controlled rolling, controlled cooling and quenching and tempering processes, and the specific process steps are as follows:

[0049] (1) Smelting and continuous casting: Smelt 150mm thick ingots according to chemical composition requirements. The raw materials are melted in proportion to obtain molten iron. After desulfurization treatment, the molten iron is smelted in a converter, LF refined and RH vacuum treated, and then continuously cast into ingots. The sulfur content of the molten iron after desulfurization is controlled at ≤0.0040%. The converter adopts the double slag method to control P≤0.010%. LF refining uses white slag operation for desulfurization and deoxidation. The vacuum degree of RH vacuum treatment is ≤3.0mbar, and the vacuum holding time is ≥15min. After the vacuum is over, calcium treatment is carried out, and then static stirring is carried out. The static stirring time is ≥12min. The continuous casting controls the superheat of the tundish to be 10℃~15℃. Continuous casting implements full protection casting, adopts rolling technology with a single pass reduction of 20mm to improve the internal quality of the continuous casting ingot, and uses end-stage electric stirring to control segregation.

[0050] (2) Heating: After continuous casting into billets, the 150 mm thick billets were heated in a soaking furnace, the heating temperature was controlled at 1200 °C, the furnace time was 113 min, and the tapping temperature was 1150 °C;

[0051] (3) Controlled rolling: Slab rolling is divided into two stages: rough rolling and finishing rolling. The width is controlled at 160mm during rolling. Two vertical rolling processes are performed in the middle (180-68-160 is the vertical rolling reduction process for controlling the width). The starting temperature of rough rolling is 1050-1120℃. After four rolling passes, large reduction rolling is adopted to ensure that the reduction rate of each pass is set as much as possible according to the maximum rolling capacity. The reduction rate of the last two passes is ≥15%. The reduction rates of the passes are 13.3%, 23.1%, 20%, and 15% respectively. The cumulative reduction ratio of rough rolling is 2.2, and the thickness of the intermediate slab is 68mm. The starting temperature of the finishing rolling stage is 827℃, the final rolling temperature is 805℃, and the reduction rates of the passes are 17.6%, 18%, and 13% respectively.

[0052] (4) Controlled cooling: The billet is rolled to a finished product thickness of 40 mm. After rolling, it is cooled by laminar cooling at a cooling rate of 9 °C / s. The water inlet temperature is 785 °C and the red-return temperature is 611 °C.

[0053] (5) Quenching and tempering: After rolling, the steel plate is subjected to quenching and tempering heat treatment. The quenching temperature is 900°C, the holding time is 20 minutes, and after the holding is completed, it is cooled by water tank water cooling to room temperature; the tempering temperature is 680°C, the holding time is 60 minutes, and air cooling is performed to room temperature.

[0054] The performance indicators of the steel plate in this embodiment are shown in Table 1. The core structure of the steel plate is as follows: Figure 2 As shown, the metallographic structure of the steel is a mixed structure of 34.5% ferrite and granular bainite, and the grain size is 20μm-30μm.

[0055] Example 3

[0056] A low yield ratio and high strength marine steel, the steel plate thickness is 60mm, its chemical composition and mass percentage are shown in Table 1, the balance is Fe and unavoidable impurities. It includes smelting and continuous casting, heating, controlled rolling, controlled cooling and quenching and tempering processes, and the specific process steps are as follows:

[0057] (1) Smelting and continuous casting: Smelt 180mm thick ingots according to chemical composition requirements. The raw materials are melted in proportion to obtain molten iron. After desulfurization treatment, the molten iron is smelted in a converter, LF refined and RH vacuum treated, and then continuously cast into ingots. The sulfur content of the molten iron after desulfurization is controlled at ≤0.0040%. The converter adopts the double slag method to control P≤0.010%. LF refining uses white slag operation for desulfurization and deoxidation. The vacuum degree of RH vacuum treatment is ≤3.0mbar, and the vacuum holding time is ≥15min. After the vacuum is over, calcium treatment is carried out, and then the static stirring time is ≥12min. The continuous casting controls the superheat of the tundish to be 10℃~15℃. The continuous casting implements full protection casting, adopts the rolling technology with a single pass reduction of 20mm to improve the internal quality of the continuous casting ingot, and uses the end-stage electric stirring to control segregation.

[0058] (2) Heating: After continuous casting into billets, the 180 mm thick continuous casting billets are heated in a soaking furnace, the heating temperature is controlled at 1200 °C, the furnace time is 120 min, and the tapping temperature is 1180 °C;

[0059] (3) Controlled rolling: Slab rolling is divided into two stages: rough rolling and finishing rolling. The starting rolling temperature of rough rolling is 1050℃-1120℃. After 4 rolling passes, large reduction rolling is adopted to ensure that the reduction rate of each pass is set as much as possible according to the maximum rolling capacity. The reduction rate of the last two passes is ≥15%. The reduction rates of the passes are 16.7%, 16.7%, 16%, and 14.3% respectively. The cumulative reduction ratio of rough rolling is 2, and the thickness of the intermediate slab is 90mm. The starting rolling temperature of the finishing rolling stage is 834℃, the final rolling temperature is 805℃, and the reduction rates of the passes are 10%, 9.8%, 8.2%, 7.5%, and 3.2% respectively.

[0060] (4) Controlled cooling: The billet is rolled to a finished product thickness of 60 mm. After rolling, it is cooled by laminar cooling at a cooling rate of 10°C / s. The water inlet temperature is 792°C and the red-return temperature is 600°C.

[0061] (5) Quenching and tempering: After rolling, the steel plate is subjected to quenching and tempering heat treatment. The quenching temperature is 920℃, the holding time is 20min, and after the holding is completed, it is cooled by water tank water cooling to room temperature; the tempering temperature is 660℃, the holding time is 60min, and air cooling is performed to room temperature.

[0062] The performance indicators of the steel plate in this embodiment are shown in Table 1. The core structure of the steel plate is as follows: Figure 3 As shown, the metallographic structure of the steel is a mixed structure of 59.5% ferrite and granular bainite, and the grain size is 10μm-20μm.

[0063] Example 4

[0064] A low yield ratio and high strength marine steel, the steel plate thickness is 80mm, its chemical composition and mass percentage are shown in Table 1, the balance is Fe and unavoidable impurities. It includes smelting and continuous casting, heating, controlled rolling, controlled cooling and quenching and tempering processes, and the specific process steps are as follows:

[0065] (1) Smelting and continuous casting: Smelt 250mm thick ingots according to chemical composition requirements. The raw materials are melted in proportion to obtain molten iron. After desulfurization treatment, the molten iron is smelted in a converter, LF refined and RH vacuum treated, and then continuously cast into ingots. The sulfur content of the molten iron after desulfurization is controlled at ≤0.0040%. The converter adopts the double slag method to control P≤0.010%. LF refining uses white slag operation for desulfurization and deoxidation. The vacuum degree of RH vacuum treatment is ≤3.0mbar, and the vacuum holding time is ≥15min. After the vacuum is over, calcium treatment is carried out, and then the static stirring time is ≥12min. The continuous casting controls the superheat of the tundish to be 10℃~15℃. The continuous casting implements full protection casting, adopts the rolling technology with a single pass reduction of 30mm to improve the internal quality of the continuous casting ingot, and uses the end-stage electric stirring to control segregation.

[0066] (2) Heating: After continuous casting into billets, the 250 mm thick continuous casting billets are heated in a soaking furnace, the heating temperature is controlled at 1200 °C, the furnace time is 120 min, and the steel tapping temperature is 1180 °C;

[0067] (3) Controlled rolling: Slab rolling is divided into two stages: rough rolling and finishing rolling. The starting rolling temperature of rough rolling is 1050℃-1120℃. After 4 rolling passes, large reduction rolling is adopted to ensure that the reduction rate of each pass is set as much as possible according to the maximum rolling capacity. The reduction rate of the last two passes is ≥15%. The reduction rates of the passes are 12%, 13.6%, 15.8%, 15.6%, and 17.2% respectively. The cumulative reduction ratio of rough rolling is 2.1, and the thickness of the intermediate slab is 120mm. The starting rolling temperature of the finishing rolling stage is 834℃, the final rolling temperature is 805℃, and the reduction rates of the passes are 12.5%, 9.5%, 8.4%, and 8% respectively.

[0068] (4) Controlled cooling: The billet is rolled to a finished product thickness of 80 mm. After rolling, it is cooled by laminar cooling at a cooling rate of 5°C / s. The water inlet temperature is 798°C and the red-return temperature is 633°C.

[0069] (5) Quenching and tempering: After rolling, the steel plate is subjected to conditioning heat treatment. The quenching temperature is 890°C, the holding time is 30 minutes, and after the holding is completed, it is cooled to room temperature by water cooling in a water tank; the tempering temperature is 680°C, the holding time is 60 minutes, and air cooling is performed to room temperature.

[0070] The performance indicators of the steel plate in this embodiment are shown in Table 1. The core structure of the steel plate is as follows: Figure 4 As shown, the metallographic structure of the steel is a mixed structure of 64.3% ferrite, tempered troostite and granular bainite, and the grains are fine, with a size of 5μm-15μm.

[0071] Example 5

[0072] A low yield ratio and high strength and toughness marine steel, the steel plate thickness is 100mm, its chemical composition and mass percentage are shown in Table 1, the balance is Fe and unavoidable impurities. It includes smelting and continuous casting, heating, controlled rolling, controlled cooling and quenching and tempering processes, and the specific process steps are as follows:

[0073] (1) Smelting and continuous casting: Smelt 300mm thick ingots according to chemical composition requirements. The raw materials are melted in proportion to obtain molten iron. After desulfurization treatment, the molten iron is smelted in a converter, LF refined and RH vacuum treated, and then continuously cast into ingots. The sulfur content of the molten iron after desulfurization is controlled at ≤0.0040%. The converter adopts the double slag method to control P≤0.010%. LF refining uses white slag operation for desulfurization and deoxidation. The vacuum degree of RH vacuum treatment is ≤3.0mbar, and the vacuum holding time is ≥15min. After the vacuum is over, calcium treatment is carried out, and then the static stirring time is ≥12min. The superheat of the continuous casting ladle is controlled to be 10℃~15℃. The continuous casting implements full protection casting, adopts the rolling technology with a single pass reduction of 20mm to improve the internal quality of the continuous casting ingot, and uses the end-stage electric stirring to control segregation.

[0074] (2) Heating: After continuous casting into billets, the 300-mm-thick continuous casting billets are heated in a soaking furnace. The heating temperature is controlled at 1200 °C, the residence time in the furnace is 120 min, and the tapping temperature is 1170 °C;

[0075] (3) Controlled rolling: The slab rolling is divided into two stages: rough rolling and finish rolling. The rough rolling starting temperature is 1050 °C - 1120 °C. After 4 passes of rolling, large reduction ratios are adopted to ensure that the reduction ratios of each pass are set as large as possible according to the maximum rolling capacity. The reduction ratios of the last two passes are ≥9%, and the reduction ratios of each pass are 6.7%, 10.7%, 12%, 9.1%, and 10% in sequence. The cumulative reduction ratio of rough rolling is 1.6, and the thickness of the intermediate slab is 180 mm. The starting temperature of the finish rolling stage is 834 °C, the finishing temperature is 805 °C, and the reduction ratios of each pass are 16.7%, 16.7%, 12%, and 9.1% in sequence;

[0076] (4) Controlled cooling: The steel billet is rolled to the finished thickness of 100 mm. After rolling, laminar cooling is adopted for cooling, and the cooling rate is 10 °C / s. The water inlet temperature is 798 °C, and the recrystallization temperature is 660 °C.

[0077] (5) Quenching and tempering: The rolled steel plate is subjected to quenching and tempering heat treatment. The quenching temperature is 900 °C, the holding time is 60 min. After holding, water cooling in a water tank is adopted for cooling to room temperature; the tempering temperature is 660 °C, the holding time is 30 min, and air cooling is carried out to room temperature.

[0078] In this embodiment, the performance indexes of the steel plate are shown in Table 1, and the microstructure morphology of the core of the steel plate is as Figure 5 shown: The metallographic structure of the steel is a mixed structure of 64.4% ferrite, tempered sorbite and granular bainite, with fine grains, and the size is 4 μm - 13 μm.

[0079] Table 1 Chemical composition and performance comparison table of the steel plate in the embodiment of the present invention

[0080]

[0081]

[0082] In addition to the above embodiments, the present invention also includes other implementation manners. All technical solutions formed by equivalent transformation or equivalent substitution shall fall within the protection scope of the claims of the present invention.

Claims

1. A method for preparing low yield ratio and high toughness marine engineering steel, characterized in that: Including smelting and continuous casting → heating → controlled rolling → controlled cooling → quenching and tempering, the steps are as follows: Step 1. Smelting and continuous casting: smelting the raw materials to obtain molten iron; the molten iron is subjected to desulfurization treatment, converter smelting, LF refining and RH vacuum treatment, and then continuously cast into billets; wherein, the molten iron is smelted into billets according to the composition of marine steel, and the thickness of the billets is not less than 150mm; the single pass reduction in the continuous casting process is more than 20mm; Step 2. Heating: heating the ingot in a soaking furnace and then taking it out of the furnace; wherein the heating temperature is 1200°C-1250°C; Step 3. Controlled rolling: rolling the heated ingot to obtain a rolled steel plate; the rolling process is divided into finishing rolling and rough rolling, and 0-3 vertical rolling processes are performed between the finishing rolling and rough rolling stages; wherein the rough rolling is performed for 3-5 passes, and the reduction rate of the last 2-3 passes is ≥9%; the finishing rolling is performed for 3-5 passes; Step 4. Controlled cooling: The rolled steel plate is cooled by laminar cooling at a cooling rate of 5°C / s-10°C / s; Step 5. Quenching and tempering: the steel plate after controlled cooling treatment is subjected to quenching and tempering heat treatment and then water-cooled to room temperature, and then tempered and cooled to room temperature; In step 2, during the heating process, the time in the furnace is 110min-120min; after the heating is completed, the tapping temperature is 1100°C-1180°C; In step 4, during cooling, the water inlet temperature is 775°C-798°C, and the red-returning temperature is 600°C-660°C; In step 5, during the quenching and tempering heat treatment, the quenching temperature is the phase transition temperature AC3±(10°C~30°C), and the holding time is 15min-60min; The tempering temperature is 660℃-680℃, and the holding time is 30min-60min. The cooling after tempering is carried out by controlled cooling or air cooling. The element composition of the marine engineering steel is calculated by mass percentage as follows: C 0.08%-0.1%, Si 0.30%-0.35%, Mn 1.05%-1.25%, Ni 1.80%-2.10%, Mo 0.30%-0.35%, Cr 0.13%-0.16%, Ti 0.008%-0.010%, V 0.015%-0.030%, Nb 0.045%-0.050%, B 0.0010%-0.0013%, Cu 0.08%-0.12%, Al 0.055%-0.060%, P≤0.010%, S≤0.0040%, carbon equivalent CET≤0.31, and the balance is Fe and unavoidable impurities; The marine engineering steel has a yield strength of ≥690MPa, a tensile strength of ≥830MPa, a yield strength ratio of ≤0.87, an elongation after fracture A of ≥18%, a transverse impact energy at room temperature of ≥100J, and a transverse impact energy at -40°C of ≥80J.

2. The method for preparing a low yield ratio and high toughness marine engineering steel according to claim 1, characterized in that: In step 1, the sulfur content of the molten iron after desulfurization treatment is controlled at ≤0.0040%; the converter adopts the double slag method to control P≤0.010%; LF refining adopts white slag operation for desulfurization and deoxidation; the vacuum degree of RH vacuum treatment is ≤3.0mbar, and the vacuum holding time is ≥15min; calcium treatment is carried out after the vacuum is ended, and then static stirring is carried out, and the static stirring time is ≥12min; continuous casting implements full casting protection, and the superheat of the tundish is controlled at 10℃~15℃ during the continuous casting process.

3. The method for preparing a low yield ratio and high toughness marine engineering steel according to claim 1, characterized in that: In step 3, the width is controlled to be 160 mm-180 mm during the rolling process; In the rough rolling stage, the starting rolling temperature is 1050℃-1120℃, the cumulative compression ratio of rough rolling is not less than 1.5, and the thickness of the intermediate billet is ≥64mm; In the finishing rolling stage, the starting rolling temperature is 827℃-835℃, and the final rolling temperature is ≥800℃; The thickness of the steel plate after rolling is 20mm-100mm.

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