A 400Mpa-grade low-temperature marine steel plate with good wear resistance and a preparation method thereof

Abstract

The invention discloses a 400Mpa-grade low-temperature marine engineering steel plate with good wear resistance and a manufacturing method thereof, belonging to the field of steel material preparation. The chemical composition mass percentage of the steel plate is as follows: C: 0.06% to 0.1%, Si: 0.7% to 1.0%, Mn: 0.5% to 0.9%, P: ≤0.012%, S: ≤0.003%, Cu: 0.3% to 0.5%, Ni: 0.4% to 0.7%, Cr: 0.55% to 0.75%, Nb: 0.015% to 0.04%, Al: 0.02% to 0.05%, and the balance is Fe and unavoidable impurities. The method includes smelting, continuous casting process, rolling process, and cooling process. The invention can better solve the problems of easy wear and low-temperature brittle fracture of traditional 400Mpa-grade marine engineering structure steel in polar regions.

Description

Technical Field

[0001] The invention belongs to the field of steel material preparation, in particular to a composition design of a 400Mpa-level low-temperature marine engineering steel plate with good wear resistance and a manufacturing method thereof. Background Art

[0002] The Arctic Ocean's oil and gas reserves account for about a quarter of the world's reserves, making it the region with the greatest potential for oil and gas resource development in the world today. In recent years, due to the continuous intensification of global warming, the melting of sea ice has led to an increasing human resource development in the region. As an important stakeholder in Arctic affairs, the development of oil and gas resources in the Arctic region is also of great strategic significance to China's sustainable development and energy security. Since polar marine engineering equipment often has to withstand dynamic and continuous impact loads on the ice during operation, severe ice load wear will occur over a long period of time. Therefore, marine engineering steel plates are required to have not only high strength and low-temperature toughness, but also good wear resistance.

[0003] The patent with publication number CN112322976A, "A rare earth wear-resistant steel NM400 coil with excellent low-temperature toughness and its production method", proposes a production method for rare earth wear-resistant steel NM400 coil with excellent low-temperature toughness. Through the design of rare earth microalloying composition, combined with TMCP online quenching technology and two-stage cooling process, the coil produced has excellent low-temperature toughness and wear resistance. However, its maximum thickness is only 9mm, which cannot meet the demand for thick steel plates in the field of marine engineering equipment.

[0004] The patent with publication number CN107475635B, "A low temperature resistant high impact toughness steel for wind power and its production method", proposes a production method for low temperature resistant high impact toughness steel for wind power, which improves the low temperature impact toughness of ordinary CrMo steel through the composite effect of Al and Ni. However, it can only guarantee good impact toughness above -40℃, which cannot meet the working requirements of marine engineering equipment in extremely cold areas with a temperature of -60℃.

[0005] The patent "A high-strength FH40 marine engineering steel plate and its production method" with publication number CN103695769B proposes FH40 grade marine engineering steel plate with a thickness of 60 to 100 mm, a -60°C V-type impact energy of 180 to 240 J, and good low-temperature toughness, but it cannot guarantee high wear resistance when used in low-temperature environments. Summary of the invention

[0006] The purpose of the present invention is to address the deficiencies of the prior art and propose a FH40 marine engineering steel plate with good wear resistance and low-temperature toughness and a manufacturing method thereof. The steel plate of the present invention has high strength (yield strength ≥ 390Mpa, tensile strength 510-660Mpa), high elongation (elongation after fracture ≥ 20%), high hardness (Vickers hardness value ≥ 230), excellent low-temperature toughness (-60℃ impact absorption energy ≥ 100J), excellent Z-direction section shrinkage (≥ 65%) and other characteristics, which can better solve the problems of easy wear and low-temperature brittle fracture of traditional 400Mpa-grade marine engineering structure steel in polar regions.

[0007] The purpose of the present invention is achieved through the following technical solutions:

[0008] The invention discloses a 400Mpa-grade low-temperature marine engineering steel plate with good wear resistance. The chemical composition mass percentage of the steel plate is as follows: C: 0.06%-0.1%, Si: 0.7%-1.0%, Mn: 0.5%-0.9%, P: ≤0.012%, S: ≤0.003%, Cu: 0.3%-0.5%, Ni: 0.4%-0.7%, Cr: 0.55%-0.75%, Nb: 0.015%-0.04%, Al: 0.02%-0.05%, and the balance is Fe and unavoidable impurities.

[0009] Carbon equivalent Ceq = C + Mn / 6 + (Cu + Ni) / 15 + (Cr + Mo + V) / 5 ≤ 0.4%;

[0010] Crack sensitivity coefficient Pcm=C+Si / 30+Mn / 20+Cu / 20+Ni / 60+Cr / 20+Mo / 15+V / 10+5B≤0.22%.

[0011] The thickness of the steel plate is 50 to 60 mm.

[0012] The mechanism of action of each alloy component in the steel of the present invention is as follows:

[0013] C: C plays a role of solid solution strengthening in steel. It is an important element that affects the strength, hardness, toughness and hardenability of steel, and expands the γ phase region within a certain range. If the carbon content is higher than 0.1%, the low-temperature toughness of the steel will drop sharply; if the carbon content is too low, the hardenability of the steel will be insufficient, the hardness will be too low, and the wear resistance will be insufficient. Considering the product performance, the C content is selected to be 0.06% to 0.1%.

[0014] Si: Si mainly exists in austenite in the form of solid solution, which shifts the pearlite isothermal transformation "C" curve to the right, prolongs the incubation period of pearlite transformation, promotes the formation of bainite, increases the hardenability of steel, and can improve the strength of the solid solution in steel, especially the yield strength, thereby improving the wear resistance of steel. In addition, a certain amount of Si inhibits the precipitation of carbides and can improve the mechanical and thermal stability of supercooled austenite. However, when the silicon content is too high, the plasticity, toughness and ductility of the steel will be significantly reduced. When it exceeds 1%, blocky ferrite structure will appear in the steel, reducing the low-temperature toughness of the steel. Therefore, the preferred Si element content is 0.7% to 1.0%.

[0015] Mn: Mn is an important element in controlled rolling. It mainly plays a role in refining grains, improving strength and toughness. Manganese can reduce the phase transformation temperature Ar3, expand the processing temperature range, increase the reduction pass and deformation amount in the austenite deformation zone, and fully refine the austenite grains. However, when the Mn element is added to the steel in an amount exceeding 0.9%, the bainite phase transformation zone will be reduced, resulting in a reduction in the amount of bainite transformation during controlled cooling to form martensite, resulting in an increase in hardness and a decrease in toughness and plasticity. Therefore, the Mn element content is selected to be 0.5% to 0.9%.

[0016] Cu: The Cu element can effectively improve the strength, toughness and corrosion resistance of steel materials, especially the atmospheric corrosion resistance. It can also optimize welding and processing performance. The disadvantage is that it is easy to produce hot brittleness during hot processing. When the Cu content exceeds 0.5%, the plasticity will be significantly reduced. The preferred Cu element content is 0.3% to 0.5%.

[0017] Ni: Ni can inhibit the precipitation of carbides in steel, improve the stability of austenite, and expand the austenite transformation zone. At the same time, the lattice constant of Ni is similar to that of γ-Fe, so it can form a continuous solid solution, produce grain refinement and solid solution strengthening, which is beneficial to improve the hardenability of steel, thereby improving the strength and hardness of steel. In addition, Ni will also significantly improve the low-temperature fracture toughness of steel. Therefore, the Ni content is selected to be 0.4% to 0.7%.

[0018] Cr: Cr is a ferrite and carbide forming element and one of the most important elements for wear-resistant materials. Cr can improve the hardenability of steel and produce various forms of carbide precipitation with C. It can also replace Fe to form carbides with complex chemical structures such as (FeCr)3C, (FeCr)7C and (FeCr)23C. The different forms of carbide precipitation caused by Cr can significantly increase the wear resistance of low-temperature marine steel. However, too high Cr content will damage the low-temperature toughness of marine steel. The preferred Cr content here is 0.55% to 0.75%.

[0019] Nb: Nb is a common microalloying element in low-temperature marine steel. It can form fine carbides with C, inhibit the coarsening of grains during the austenitization process of steel, pin austenite grain boundaries and dislocations, thereby significantly refining the deformed austenite structure and producing dispersed precipitation strengthening, improving the strength and toughness of steel; it can also prevent the growth of recrystallized grains and inhibit the recrystallization process, increase the temperature of the non-recrystallization zone, and expand the window of the two-stage interval. In the present invention, the Nb content is 0.015% to 0.04%.

[0020] Al: Al is used as a deoxidizer during steelmaking to refine grains and inhibit the aging of low-carbon steel. In particular, it can reduce the brittle transition temperature of steel and improve the toughness of steel at low temperatures. In addition, the addition of Al can significantly increase the stacking fault energy and limit the movement of dislocations in metal materials, thereby increasing the dislocation density and achieving the effect of strengthening the foundation of metal materials. However, excessive Al content will produce large oxide inclusions, which will have an adverse effect on the toughness of steel, so the preferred Al content is 0.02% to 0.05%.

[0021] P, S: P and S elements are generally considered to be unavoidable harmful elements in the metal material composition system. When the S content is high, it will increase the hot brittleness tendency of the metal material, easily cause brittle cracking during pressure processing at high temperature, and easily segregate to form banded structure, deteriorating the continuity of the microstructure. When the P content is high, it will significantly reduce the toughness and plasticity of the steel, making it difficult to process the steel in the later stage, and at the same time cause the cold brittleness of the steel at a lower temperature. Therefore, when designing the composition, it is necessary to control the P content as little as possible to ≤0.012%, and the S content to ≤0.003%.

[0022] The method for preparing the above-mentioned 400Mpa-grade low-temperature marine steel plate with good wear resistance comprises the following steps:

[0023] 1) Steel cleanliness control and continuous casting billet quality control technology through smelting and continuous casting processes

[0024] The production process adopts desulfurization slag stripping, double slag converter dephosphorization, LF furnace deep desulfurization, white slag refining deoxidation, and slab continuous casting. The whole process adopts full protection casting to reduce secondary oxidation during continuous casting, prevent molten steel from absorbing nitrogen, and optimize the flow field of the tundish. In order to reduce the internal center segregation of the slab, the end strong cooling and light pressure reduction technology are used in the continuous casting process. The cooling water volume at the last stage of the continuous casting end is increased to 170-200L / min, and the pressure reduction is 6-9mm. The tundish superheat and other aspects are optimized and controlled. The tundish superheat is controlled at 20℃~30℃, and the continuous casting billet thickness is 250-300mm. At the same time, the stacking slow cooling technology is used after the slab is off the line. The stacking slow cooling rate is 15.2-16.8℃ / h. The slab temperature (charging temperature) when loaded into the heating furnace after slow cooling is controlled at 490-550℃.

[0025] 2) Rolling process

[0026] Push the continuous casting billet into the heating furnace for four-stage heating, with a preheating temperature of 760-810°C and a preheating time of 20-25 minutes. The temperature of the first heating section is 1000-1100°C and the heating time is 40-45 minutes. The temperature of the second heating section is 1130-1190°C and the heating time is 30-35 minutes. The temperature of the equalizing section is 1040-1120°C and the heating time is 55-65 minutes. Appropriately reduce the heating section time to avoid abnormal growth of the cast structure during the heating stage.

[0027] After the continuous casting billet is discharged from the furnace, high-pressure water dephosphorization is carried out for 3-4 times. The rolling process adopts a two-stage controlled rolling technology. The first-stage rolling temperature is 1030-1100℃. The temperature of the continuous casting billet in the first stage is relatively high, and a large reduction is adopted. The minimum single-pass reduction rate in the first stage is ≥15%. The thickness of the intermediate billet is controlled to be 2-3 times the thickness of the finished steel plate. The intermediate billet is water-cooled at a cooling rate of 3.5-5.2℃ / s to prevent the intermediate billet grains from continuing to grow and play a role in maintaining grain refinement. The second-stage rolling temperature is 770-830℃, and the minimum single-pass reduction rate in this stage is ≥13%. The final rolling temperature is 750-810℃. The first-stage rolling is carried out in the austenite recrystallization zone, and a large reduction rolling is carried out. When the austenite deformation reaches the critical dynamic recrystallization deformation, the deformed austenite grains are recrystallized between rolling passes, and the rolling is repeated and gradually refined. The second stage rolling is carried out in the two-phase region below Ar3. The untransformed austenite grains are further elongated, forming deformation bands with larger deformation in the grains. Ferrite nucleates on the deformation bands and grain boundaries and transforms into small equiaxed grains. The microstructure after the phase transformation is composed of low-carbon bainite grains and ferrite grains, and the grains are more refined.

[0028] 3) Cooling process

[0029] The water temperature for controlled cooling after rolling is 730-780℃, the terminal temperature is 440-520℃, and the cooling rate is 8.5-11℃ / s. The controlled cooling process is used to control the growth of deformed austenite grains and ferrite grains after phase transformation to achieve the requirement of improving the strength of steel without compromising toughness.

[0030] 4) Slow cooling process: After controlled cooling, immediately put it into the slow cooling tank, the temperature entering the tank shall not be lower than 400℃, and the slow cooling time shall not be less than 24h.

[0031] Beneficial effects of the present invention:

[0032] 1. Aiming at the dynamic and continuous impact loads of ice layers in ice areas faced by marine structures working in polar regions, the present invention proposes a new component system design and corresponding production process, and the developed 400Mpa-grade steel plate has excellent low-temperature toughness, yield strength ratio, ductility and wear resistance. The steel plate produced has a yield strength of ≥390MPa, a tensile strength of 510-660MPa, a cross-sectional elongation of ≥20%, a Z-direction cross-sectional shrinkage of ≥65%, a -60℃ Charpy impact energy of the steel plate core of ≥100J, and a Vickers hardness of ≥230.

[0033] 2. Compared with traditional low-temperature marine engineering steel, the present invention reduces the use of precious metals such as V and Ti, and adopts controlled rolling and controlled cooling technology, eliminating the need for heat treatment steps such as heating, quenching, and tempering, thereby improving production efficiency while reducing costs, saving energy, reducing consumption, and protecting the environment.

[0034] 3. The microstructure of the steel plate is a mixed structure of bainite + ferrite. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Figure 1 This is a metallographic photograph of the steel plate prepared in Example 2 (500 times). DETAILED DESCRIPTION

[0036] The following non-limiting embodiments may enable a person skilled in the art to more fully understand the present invention, but are not intended to limit the present invention in any way.

[0037] The following examples are used to specifically illustrate the present invention. These examples are only general descriptions of the present invention and do not limit the present invention.

[0038] The chemical composition of the steel of the embodiment of the present invention is shown in Table 1, the smelting and billet pretreatment parameters of the steel of the embodiment of the present invention are shown in Table 2, the slab heating process of the steel plate of the embodiment of the present invention is shown in Table 3, the rolling and cooling process of the steel plate of the embodiment of the present invention is shown in Table 4, and the mechanical properties of the steel plate of the embodiment of the present invention are shown in Table 5.

[0039] Table 1 wt% of ingredients in each example

[0040] Example C Si Mn P S Cu Ni Cr Nb Alt 1 0.069 0.71 0.62 0.009 0.002 0.35 0.67 0.55 0.038 0.039 2 0.064 0.74 0.66 0.01 0.002 0.42 0.63 0.58 0.023 0.041 3 0.076 0.81 0.53 0.007 0.001 0.34 0.44 0.55 0.019 0.043 4 0.061 0.76 0.68 0.008 0.001 0.41 0.59 0.66 0.021 0.047 5 0.089 0.96 0.55 0.011 0.002 0.5 0.69 0.61 0.036 0.028 6 0.066 0.85 0.77 0.008 0.003 0.41 0.52 0.65 0.027 0.036 7 0.098 0.92 0.51 0.008 0.003 0.42 0.58 0.66 0.031 0.041 8 0.091 0.79 0.68 0.009 0.001 0.38 0.65 0.56 0.029 0.021 9 0.072 0.99 0.82 0.006 0.002 0.39 0.46 0.62 0.027 0.031 10 0.075 0.83 0.58 0.007 0.002 0.49 0.61 0.61 0.016 0.044 11 0.081 0.82 0.57 0.008 0.003 0.41 0.62 0.75 0.019 0.033 12 0.091 0.87 0.71 0.012 0.001 0.33 0.52 0.59 0.024 0.035

[0041] Table 2 Steel smelting and billet pretreatment parameters

[0042]

[0043] Table 3 Heating process

[0044]

[0045]

[0046] Table 4 Rolling and cooling process

[0047]

[0048] Table 5 Mechanical properties of steel plates

[0049]

Claims

1. A 400Mpa low-temperature marine steel plate with good wear resistance, characterized in that: The chemical composition mass percentage of the steel plate is as follows: C: 0.06% to 0.1%, Si: 0.7% to 1.0%, Mn: 0.5% to 0.9%, P: ≤ 0.012%, S: ≤ 0.003%, Cu: 0.3% to 0.5%, Ni: 0.4% to 0.7%, Cr: 0.55% to 0.75%, Nb: 0.015% to 0.04%, Al: 0.02% to 0.05%, and the balance is Fe and unavoidable impurities; The method for preparing the 400Mpa-grade low-temperature marine steel plate with good wear resistance comprises the following steps: 1) Smelting and continuous casting process 2) Rolling process The continuous casting billet is pushed into the heating furnace for four-stage heating, wherein the preheating temperature is 760-810°C and the preheating time is 20-25 minutes; the temperature of the heating section 1 is 1000-1100°C and the heating time is 40-45 minutes; the temperature of the heating section 2 is 1130-1190°C and the heating time is 30-35 minutes; the temperature of the soaking section is 1040-1120°C and the heating time is 55-65 minutes; After the continuous casting billet is discharged from the furnace, high-pressure water is used for dephosphorization. The rolling process adopts a two-stage controlled rolling technology. The first-stage rolling temperature is 1030-1100°C, and the minimum single-pass reduction rate in the first stage is ≥15%; the intermediate billet thickness is controlled to be 2-3 times the thickness of the finished steel plate; the intermediate billet is water-cooled, and the cooling rate is 3.5-5.2°C / s; the second-stage rolling temperature is 770-830°C, and the minimum single-pass reduction rate in the second stage is ≥13%; the final rolling temperature is 750-810°C; 3) Cooling process The water inlet temperature for controlled cooling after rolling is 730-780°C, the end temperature is 440-520°C, and the cooling rate is 8.5-11°C / s; 4) Slow cooling process After controlled cooling, place it in a slow cooling tank. The temperature entering the tank should not be lower than 400℃ and the slow cooling time should not be less than 24h.

2. The steel plate according to claim 1, characterized in that: Carbon equivalent Ceq≤0.4%.

3. The steel plate according to claim 1, characterized in that: The crack sensitivity coefficient Pcm of the steel plate is ≤0.22%.

4. The steel plate according to claim 1, characterized in that: The steel plate has a yield strength of ≥390 MPa, a tensile strength of 510-660 MPa, a cross-sectional elongation of ≥20%, a Z-direction cross-sectional shrinkage of ≥65%, a -60°C Charpy impact energy of ≥100 J, and a Vickers hardness of ≥230.

5. The steel plate according to claim 1, characterized in that: The thickness of the steel plate is 50-60 mm.

6. The steel plate according to claim 1, characterized in that In step 1), desulphurization slag stripping, double slag converter dephosphorization, LF furnace deep desulfurization, white slag refining deoxidation, and continuous casting processes are adopted for production, and full-protection casting is adopted throughout the process; terminal strong cooling and reduction technology are adopted during the continuous casting process, the cooling water volume in the last section of the continuous casting end is increased to 170-200L / min, the reduction amount is 6-9mm, the superheat of the tundish is controlled at 20℃~30℃, the thickness of the continuous casting billet is 250-300mm, and at the same time, the stacking slow cooling technology is adopted after the slab comes off the line, and the stacking slow cooling rate is 15.2-16.8℃ / h. The temperature of the slab when it is loaded into the heating furnace after slow cooling is controlled at 490-550℃.

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