CN114807761B - EH36 grade marine engineering steel having high ductility and method for manufacturing same - Google Patents

Abstract

The invention relates to EH36 grade marine engineering steel with high ductility, which is characterized by comprising the following chemical components in percentage by weight: c:0.05 to 0.08 percent, si:0.10 to 0.30 percent of Mn:1.00% -1.50%, nb:0.01 to 0.04 percent, V:0.02% -0.05%, ti:0.005% -0.02%, P: less than or equal to 0.01 percent, S: less than or equal to 0.01 percent, als:0.01% -0.05%, and the balance of Fe and unavoidable impurities. The chemical composition of the invention does not add noble alloy elements such as Ni and the like; the microstructure and grain size of the steel plate are regulated and controlled by controlling the sulfur and phosphorus content and adopting continuous casting billet induction heating, two-stage controlled rolling, relaxation and controlled cooling processes, so that the steel plate with high yield, stable strength and toughness and particularly high ductility is obtained.

Description

EH36 grade marine engineering steel with high ductility and manufacturing method thereof

technical field

The invention relates to the field of iron and steel material preparation.

Background technique

In recent years, although marine accidents involving marine engineering equipment have been decreasing, serious marine disasters have occurred from time to time, often causing huge property losses and loss of life. Therefore, it is important not only to provide measures to prevent collision and stranding of marine engineering equipment, but also to discuss how to minimize losses in the event of a shipwreck. Development has become an inevitable trend, and the near-sea industry is gradually transforming into the marine industry, completing the upgrade from offshore to deep-sea. Once a marine accident involving marine engineering equipment occurs, it will cause huge property losses and loss of life. Therefore, it is important not only to provide measures to prevent collision and grounding of marine engineering equipment, but also to minimize losses in the event of a shipwreck.

For this reason, the development of mobile deep-sea platforms, FPSOs and ocean-going engineering ships not only requires that ocean engineering steels have good strength-toughness matching, stability, and low-temperature toughness, but also specifically requires steel plates to have high ductility to cope with ships and ocean engineering equipment. Possible accidents of collision and grounding. High ductility is an important index in the concept of "high safety serviceability" of steel for marine engineering, mainly to improve the safety of collisions. The better the ductility of structural materials, the more energy absorbed during collisions. American ABS Classification Society At present, the high ductility performance index has been included in the specification. The purpose is to allow the steel plate to absorb the energy of the collision when ships and marine engineering equipment collide and run aground, making it less prone to structural instability and thus improving the anti-collision capability of the equipment. A safeguard and reduces the risk of severe marine pollution due to cargo oil and bunker spills. Japan's Nippon Steel and Sumitomo Metals have developed such steel plates for marine engineering ("NSafe-Hull" series) with high ductility and anti-collision functions, which are currently in the trial application stage. However, my country's ocean engineering has not yet formed a large-scale application of such steel plates, and steel companies have not conducted systematic research. Steel plates for ocean engineering with high ductility and anti-collision functions will belong to the new demand for steel for ocean engineering. This invention will contribute to safer and more reliable offshore operations and transportation.

Public reports of relevant patents:

The invention patent with the publication number CN105803330B discloses a structural steel plate for normalizing hulls and its preparation method. The patent uses Nb, V, Ti to refine grain elements, TMCP + normalizing heat treatment to prepare EH36 grade high-strength ship plates, and manufactures The cycle is longer, the energy consumption is high, and the production cost is increased. The elongation rate of the steel plate after breaking is 24-32%.

The invention patent with publication number CN108517462A proposes a high ductility EH40 ship plate steel and its preparation method. The patent is designed with low-carbon and micro-alloy strengthening components, and adopts two-stage rolling plus water-cooling-air-cooling-water-cooling three-stage cooling. The production process of this technology is complex, requiring high equipment control capabilities, long production cycle, and narrow process window. .

The invention patent with the publication number CN110714171A discloses a high-ductility EH420 grade ship plate steel and its production method. The ultra-fast cooling technology with a speed of 60°C/s can obtain a composite structure of soft phase ferrite plus hard bainite, but this technology is difficult, and it is difficult to achieve the required cooling rate conditions in actual industrial production, and the steel plate thickness Conditions are limited.

The invention patent with the publication number CN201910182915 proposes a medium and thick plate production process, which includes the following steps: steelmaking; continuous casting; liquid core rolling; billet cutting; online induction heating; full longitudinal rolling; cooling; finishing Shearing; in the online induction heating step, the surface temperature of the continuous casting slab is 1050-1150°C by induction heating, the average temperature of the continuous casting slab is 1150-1200°C, and the heating rate of the compensation heating is not less than the maximum of the continuous casting machine However, the purpose of the induction heating step in this technology is mainly to compensate for the heating and soaking of the corners of the continuous casting slab.

Contents of the invention

The technical problem to be solved by the present invention is to provide a high ductility EH36 marine engineering steel and its manufacturing method, which have high yield, stable strength and toughness, and especially high ductility.

To achieve the above object, the present invention adopts the following technical solutions:

EH36 grade marine engineering steel with high ductility, chemical composition by weight percentage: C: 0.05% ~ 0.08%, Si: 0.10% ~ 0.30%, Mn: 1.00% ~ 1.50%, Nb: 0.01% ~ 0.04% , V: 0.02% to 0.05%, Ti: 0.005% to 0.02%, P: ≤0.01%, S: ≤0.01%, Als: 0.01% to 0.05%, and the rest are Fe and unavoidable impurities.

The performance of EH36 grade steel for marine engineering with high ductility: elongation of the steel plate after breaking ≥ 32%, maximum force elongation ≥ 28%, yield strength ≥ 360MPa, tensile strength 490-630MPa, Charpy at -40°C Impact energy ≥200J, zero plastic transition temperature NDTT<-40℃; microstructure is ferrite and pearlite, in which the average ferrite grain size is 5.0-10.0μm, and the ferrite content ranges from 85-95%.

The action mechanism of each alloy component in steel, where the percentage symbol % represents the weight percentage:

C: The final structure of the high-ductility EH36 steel plate in the present invention is a F/B dual-phase structure. For the F/B dual-phase steel, the C content is an important factor affecting the ratio of the two phases. Under the same TCMP conditions, the lower the C content of the steel plate, the higher the ferrite ratio. The elongation performance of dual-phase steel mainly depends on the proportion of soft-phase ferrite. During the overall coordinated deformation process, the stress generated by plastic deformation of bainite can be released by inducing the deformation of surrounding soft-phase ferrite to avoid excessive bainite. Early occurrence of stress concentration and necking. Considering the high ductility characteristics as the product's invention purpose, a lower C content is required to ensure a high ferrite ratio in the final structure. However, the C content should not be lower than 0.05%. Too low C content will not only lead to high austenite grain boundary mobility, but also bring big problems to the uniform and refined structure of the TMCP process. It is easy to form a mixed crystal structure and cause an increase in the yield ratio. , Too low C content also causes the reduction of grain boundary bonding force, resulting in low low-temperature impact toughness of steel plate and deterioration of low-temperature impact toughness in welding heat-affected zone. Considering the above factors, it is preferable to control the C content at 0.05-0.08%.

Si: It is the main deoxidizing component in the steelmaking process. In order to obtain a sufficient deoxidizing effect, it must contain more than 0.10%, but if it exceeds the upper limit, it will reduce the toughness of the base metal and welded parts. At the same time, the ductile-brittle transition temperature can be increased, so the preferred Si content is 0.10-0.30%.

Mn: It is an essential element to ensure the strength and toughness of steel. In order to improve the strength and toughness of the material of the present invention, the Mn content ranges from 1.00 to 1.50%.

Nb: Nb has a very strong affinity with C, N, and O, and forms corresponding extremely stable compounds with it. Nb can refine the grains of steel, reduce the overheating sensitivity of steel, and under certain conditions, it can improve the strength and toughness of steel, especially under the condition of high heating rate induction heating, fine Nb carbonitrides are precipitated Refine austenite grains. The Nb content in the present invention is controlled at 0.01-0.04%.

V: V is a ferrite refining element, which improves the strength, yield ratio and low temperature toughness of steel, improves the welding performance of steel, and can also increase the thermal strength and creep resistance of steel. However, the V content should not be too high. If it is too high, the toughness of the steel will be reduced, which is not conducive to the creep performance of the steel. The V content of the present invention is controlled at 0.02-0.05%.

Ti: Ti can not only improve the strength of steel, refine grains, reduce aging sensitivity and cold brittleness, but also a small amount of titanium can improve welding performance. Ti plays a role in the form of TiN. When it is less than 0.005%, the effect is small, and when it exceeds 0.04%, it is easy to form large particles of TiN and lose its effect. Therefore, the Ti content is preferably 0.005 to 0.02%.

P: It is an element that adversely affects low-temperature toughness and ductility. It can segregate in the center of the slab and aggregate at grain boundaries to damage low-temperature toughness. The material of the present invention is controlled at no more than 0.01%.

S: is an element that adversely affects low-temperature toughness and ductility, and can form sulfide inclusions and become a source of cracks. The material of the present invention is controlled at no more than 0.01%.

Als: As a deoxidizing and grain-refining element that must be added in the present invention, the added content is more than 0.01%, but when it exceeds 0.08%, hot cracks in the slab are likely to occur, and the toughness of the steel is reduced. Als content is controlled at 0.01% to 0.05%.

Manufacturing method of EH36 steel for marine engineering with high ductility, process flow: smelting, continuous casting, slab heating, rolling, straightening, relaxation, cooling; where:

1) Slab heating: Induction heating, with a heating rate of 5.5-7℃·s ^-1 , the temperature of the continuous casting slab is rapidly raised to 1150-1180℃, and the temperature uniformity of the casting slab is controlled within the range of ±30℃;

2) Rolling process: Two-stage rolling is adopted for the continuous casting slab. The rolling temperature of the first stage is in the austenite recrystallization temperature zone of 950-1000°C, and the average single-pass reduction rate is above 10%, so that the austenite The tenite is fully recrystallized and the austenite grain size is refined; the rolling temperature of the second stage is 750-800°C in the austenite non-recrystallized temperature zone, and the average single-pass reduction rate reaches 15-20%. Fully deform the austenite grains, provide energy storage and location for the phase deformation nuclei, increase the phase deformation nuclei rate, and further reduce the ferrite grains by deformation in the two-phase region;

3) Relaxation phase transformation control after rolling: the steel plate is air-cooled to 680-730°C before entering the cooling system, increasing the relaxation time in the α/γ two-phase region after final rolling, and fully nucleating pro-eutectoid ferrite to ensure The proportion of ferrite structure in the final structure is greater than 85%;

4) Cooling process: adopt a rapid laminar cooling system with an average cooling rate of 3-10°C/s, control the temperature of reddening at 550-600°C, control the composition and size of the phase transition structure, and cool the steel plate slowly after cooling. Time ≥ 24h.

The method of longitudinal magnetic flux induction heating is adopted in the heating process of the slab.

The thickness range of the finished steel plate is 20-50 mm.

Compared with prior art, the beneficial effect of the present invention is:

The chemical composition of the steel plate in the present invention does not add precious alloy elements such as Ni; by controlling the content of sulfur and phosphorus, the microstructure and grain size of the steel plate are regulated by adopting continuous casting slab induction heating, two-stage controlled rolling, relaxation and controlled cooling process, A steel plate with high yield, stable strength and toughness, and especially high ductility is obtained. The tensile elongation of the steel plate after fracture is ≥32%, the maximum elongation rate is ≥28%, the yield strength is ≥360MPa, the tensile strength is 490-630MPa, the Charpy impact energy at -40°C is ≥200J, and the zero plastic transition temperature NDTT<-40°C ; The microstructure is ferrite and pearlite, in which the average ferrite grain size is 5.0-10.0 μm, and the ferrite content ranges from 85 to 95%. The mechanical properties and high service safety performance of the steel plate can meet the service conditions of marine engineering equipment.

Description of drawings

Figure 1 is a microstructure diagram of the present invention.

Detailed ways

The specific embodiment of the present invention is further described below:

EH36 grade marine engineering steel with high ductility, chemical composition by weight percentage: C: 0.05% ~ 0.11%, Si: 0.10% ~ 0.30%, Mn: 1.00% ~ 1.50%, Nb: 0.01% ~ 0.04% , V: 0.02% to 0.05%, Ti: 0.005% to 0.02%, P: ≤0.01%, S: ≤0.01%, Als: 0.01% to 0.05%, and the rest are Fe and unavoidable impurities.

Manufacturing method of EH36 steel for marine engineering with high ductility, process flow: smelting, continuous casting, slab heating, rolling, straightening, relaxation, cooling; where:

(1) Smelting process: smelting according to the composition range, smelting and continuous casting to obtain continuous casting slabs, LF and RH refining furnaces each need 10 to 30 minutes, the superheat of molten steel in the tundish is ≤25°C, and the whole process is protected for casting; A, B, C, D type inclusions meet the requirements of: A≤0.5, B≤0.5, C≤1.0, D≤1.0;

(2) Billet heating: Induction heating process: The microstructure change in the austenitization process has an important influence on the microstructure and properties of the subsequent steel. The method of longitudinal magnetic flux induction heating is adopted, and the heating rate is 5.5-7℃·s-1, Rapidly raise the temperature of the continuous casting slab to 1150-1180°C, and control the temperature uniformity of the casting slab within the range of ±30°C. Under the condition of induction heating, due to the skin effect, the surface layer of the continuous casting slab is quenched and fine-grained and the temperature is rapidly raised to Austrian in a short time. The heat in the densitic phase zone then conducts rapidly inward. The faster the heating rate, the greater the degree of superheat, and austenite nucleates rapidly and massively at the grain boundaries of cementite and ferrite. At the same time, when the heating rate is fast, the austenite grains do not have sufficient time to grow, which weakens the growth trend of austenite.

(3) Rolling process: Two-stage rolling is adopted for the continuous casting slab. The rolling temperature of the first stage is in the austenite recrystallization temperature zone of 950-1000°C, and the average single-pass reduction rate is above 10%. The austenite is fully recrystallized and the austenite grain size is refined; the rolling temperature of the second stage is 750-800°C in the austenite non-recrystallized temperature zone, and the average single-pass reduction rate reaches 15-20% , so that the austenite grains are fully deformed, providing energy storage and location for the phase deformation nuclei, increasing the phase deformation nuclei rate, and the deformation in the two-phase region further reduces the ferrite grains, and finally achieves the purpose of refining the grains.

(4) Straightening process: After the final rolling, the steel plate is straightened in a straightening machine, the purpose is to improve the flatness of the steel plate, and at the same time prevent problems such as warping of the steel plate during the cooling process.

(5) Relaxation phase transition control after rolling: Control the relaxation time of the α/γ two-phase region by adjusting the start-cooling temperature, and then adjust the ratio and morphology of the F/B two-phase structure of the steel plate. The steel plate is air-cooled to 680-730°C before entering the cooling system. The purpose is to increase the relaxation time in the α/γ two-phase region after final rolling, which is conducive to the sufficient nucleation of pro-eutectoid ferrite and ensures the ferrite in the final structure. The tissue ratio is greater than 85%. During relaxation, the carbon element in supercooled austenite develops from uniform distribution to aggregation, and the area where carbon migrates out forms proeutectoid ferrite, that is, the long-range diffusion of carbon emission from proeutectoid ferrite to austenite. Therefore, the longer the relaxation time of the steel plate before cooling, the more fully this phase transformation and carbon diffusion process will be carried out, and the higher the degree of C element accumulation in the austenite region around the proeutectoid ferrite will be, and in the subsequent Bainite is formed during the laminar rapid cooling process. Therefore, in the present invention, the C content is required to be less than 0.08%, which can effectively reduce the carbon enrichment phenomenon of the surrounding austenite during the nucleation and growth of proeutectoid ferrite, which is beneficial to the dispersion distribution of bainite and avoids the deformation process produce local stress concentrations.

(6) Cooling process: A rapid laminar cooling system with an average cooling rate of 3-10°C/s is adopted, and the reddening temperature is controlled at 550-600°C. The purpose is to control the composition and size of the phase-transition structure. Cold, slow cooling time ≥ 24h.

Example

Table 1 is the chemical composition of steel embodiment steel for ocean engineering;

Table 2 is the smelting process of embodiment steel;

Table 3 is the rolling process of embodiment steel;

Table 4 is the mechanical property of embodiment steel;

Table 5 shows the low-temperature performance and NDTT temperature of steels in the examples of the present invention.

Table 1: Steel Chemical Composition

Example C Si mn Nb V Ti P S als 1 0.05 0.21 1.41 0.021 0.026 0.012 0.005 0.001 0.03 2 0.06 0.14 1.48 0.025 0.031 0.007 0.004 0.001 0.01 3 0.07 0.10 1.05 0.012 0.036 0.012 0.005 0.001 0.02 4 0.08 0.12 1.21 0.015 0.021 0.005 0.005 0.001 0.02 5 0.05 0.18 1.13 0.013 0.042 0.012 0.005 0.001 0.03 6 0.08 0.26 1.31 0.022 0.033 0.018 0.004 0.001 0.05 7 0.07 0.16 1.16 0.023 0.029 0.015 0.005 0.001 0.04 8 0.06 0.29 1.36 0.023 0.032 0.012 0.005 0.001 0.03

Table 2: Smelting process

Table 3: Rolling process

Table 4: Conventional tensile mechanical properties and ferrite content and average grain size

Table 5: Low temperature performance and NDTT temperature

Example Finished product thickness/mm NDTT/℃ Impact energy(-40℃)Akv/J 1 twenty two -50 236 2 25 -50 242 3 29 -45 221 4 32 -45 251 5 35 -45 232 6 45 -50 230 7 48 -50 242 8 50 -50 228

The above is only the basic principle of the present invention, and does not limit the present invention in any way. All equivalent changes and modifications according to the present invention are within the scope of the technical protection scheme of this patent.

Claims (1)

1. The EH 36-grade ocean engineering steel with high ductility is characterized in that the elongation after the stretch breaking of a steel plate is more than or equal to 32%, the maximum force elongation is more than or equal to 28%, the yield strength is more than or equal to 360MPa, the tensile strength is 490-630 MPa, the Charpy impact energy at minus 40 ℃ is more than or equal to 200J, and the zero plastic transition temperature NDTT is less than minus 40 ℃; the microstructure is ferrite and pearlite, wherein the average grain size of the ferrite is 5.0-10.0 mu m, the ferrite content range is 85-95%, and the chemical components are as follows by weight percent: c:0.05 to 0.07 percent, si:0.10 to 0.14 percent, mn:1.00% -1.50%, nb:0.01 to 0.04 percent, V:0.031% -0.05%, ti:0.005% -0.02%, P: less than or equal to 0.01 percent, S: less than or equal to 0.01 percent, als:0.01% -0.05%, and the balance of Fe and unavoidable impurities; the manufacturing method comprises the following steps:

the process flow comprises the following steps: smelting, continuous casting, heating a casting blank, rolling, straightening, relaxing and cooling;

1) Heating a casting blank: induction heating at a heating rate of 5.5-7deg.C.s ^-1 The continuous casting blank is quickly heated to 1150-1180 ℃ and the casting blank temperature is highThe uniformity of the degree is controlled within the range of +/-30 ℃; a longitudinal magnetic flux induction heating mode is adopted in the casting blank heating process;

2) The rolling process comprises the following steps: adopting two-stage rolling to the continuous casting billet, wherein the first-stage rolling temperature is 950-1000 ℃ austenite recrystallization temperature zone, and the average single-pass reduction rate is more than 10%; in the austenite unrecrystallized temperature region with the rolling temperature of 750-800 ℃ in the second stage, the average single-pass reduction rate reaches 15-20%;

3) Relaxation phase transition control after rolling: cooling the steel plate to 680-730 ℃ and then entering a cooling system, increasing relaxation time in an alpha/gamma two-phase region after finish rolling, and fully nucleation the proeutectoid ferrite to ensure that the ferrite structure proportion of a final structure is more than 85%; carbon elements in supercooled austenite are gradually accumulated and developed from uniform distribution during relaxation, and a region from which carbon migrates forms proeutectoid ferrite which is spread to the carbon discharge long-range of the austenite; the carbon enrichment phenomenon of surrounding austenite in the process is effectively reduced when the eutectoid ferrite is nucleated and grown, so that the diffusion distribution of bainite is facilitated, and the local stress concentration in the deformation process is avoided;

4) The cooling process comprises the following steps: and (3) adopting a rapid laminar cooling system with an average cooling speed of 3-10 ℃/s, controlling the reddening temperature to 550-600 ℃, controlling the phase change structure composition and size, and slowly cooling the cooled steel plate for more than or equal to 24 hours.