A 550mpa grade high toughness quenched and tempered steel plate and a method of manufacturing the same

Abstract

The application relates to a 550MPa-grade high-toughness quenched and tempered steel plate and a manufacturing method thereof, wherein the chemical components in the steel are calculated according to percentage by weight as follows: C 0.06%-0.10%, Si 0.15%-0.4%, Mn 1.35%-1.65%, P<=0.012%, S<=0.002%, Ni 0.15%-0.35%, Cr 0.15%-0.40%, Nb 0.015%-0.03%, V 0.02%-0.04%, Mo 0.15%-0.30%, Ti 0.007%-0.020%, B 0.0007%-0.002%, Alt 0.018%-0.045%, and the balance is Fe and inevitable impurities. The steel plate has the characteristics of ultrahigh strength (yield strength >=550MPa, tensile strength 640-820MPa, and post-break elongation >=16%), excellent low-temperature toughness (impact energy >=100J at 1 / 2-40 DEG C, and >=100J at -40 DEG C after welding), uniform structure and performance, and good plate shape (unevenness <=4mm within 2m of the steel plate).

Description

Technical Field

[0001] This invention belongs to the field of steel material preparation, and particularly relates to a 550MPa high-toughness quenched and tempered steel plate and its manufacturing method. Background Technology

[0002] Steel, as a key structural material for marine engineering equipment, is widely used in offshore wind power, production platforms, and subsea pipelines. Marine engineering equipment typically has a service life of 30 years, 50% longer than traditional ships. The service environment of marine engineering equipment is extremely harsh, subject not only to its own weight but also to the effects of sea conditions such as wind, waves, ocean currents, and submarine earthquakes. In recent years, with the rapid development of offshore oil and gas exploration, the demand for high-strength, high-toughness, and thick-gauge steel for marine platforms has been increasing. Due to their special service environment, marine platform steel has stringent requirements for properties such as low-temperature impact toughness and resistance to lamellar tearing. Low-temperature impact toughness is one of the important properties of marine platform steel and a key factor that must be considered in marine platform design. To meet the needs of marine engineering for high-performance, high-service-safety steel plates, there is an urgent need to develop ultra-high-strength marine engineering steel with excellent low-temperature performance.

[0003] The invention patent CN109112429B, entitled "FH550 grade thick plate with excellent low-temperature toughness and its manufacturing method", produces FH550 steel plates for ships and marine engineering with a maximum thickness of 80mm by adding appropriate Cu, Cr, Mo, Ni and microalloying elements, controlling the sulfur and phosphorus content, and using a three-stage controlled rolling combined with controlled cooling process. However, its Cu content (0.5wt.% to 0.7wt.%) is relatively high, which easily causes the phenomenon of "copper embrittlement". At the same time, relevant ship regulations stipulate that the Cu content of TMCP state steel plates is usually not higher than 0.55wt.%, which limits the scope of use of this patent.

[0004] The invention patent CN103276301A, entitled "A Low-Temperature Engineering Steel with a Yield Strength ≥ 550 MPa and its Production Method," describes steel plates with a thickness of 80–100 mm manufactured using continuous casting + TMCP technology. These plates exhibit a yield strength greater than or equal to 550 MPa and excellent low-temperature impact toughness, with an impact energy greater than 100 J at -60℃. However, the high Mn content (1.63–2.00%) in the composition elements easily leads to MnS formation, affecting impact toughness. Furthermore, excessive Mn content can cause segregation in the center of the cast billet, reducing core performance. Additionally, the patent does not provide information on the core toughness level of the steel plate, thus failing to comprehensively characterize its low-temperature performance.

[0005] The invention patent CN104404384B, entitled "A 550MPa Grade Low Compression Ratio High Toughness Steel Plate for Marine Engineering Platforms and its Production Method," discloses a method for producing high-strength and high-toughness marine engineering steel plates using the TMCP+T process under low carbon content and low compression ratio conditions. The post-rolling high-temperature heat treatment process increases the production cycle. Furthermore, the steel plates produced by this invention patent do not contain added Al, while current international standards and relevant classification society regulations for marine engineering steels all impose minimum Al content requirements, limiting the application scope of this invention patent. Summary of the Invention

[0006] The purpose of this invention is to propose a 550MPa grade high-toughness quenched and tempered steel plate and its manufacturing method. The steel plate of this invention has ultra-high strength (yield strength ≥ 550MPa, tensile strength 640~820MPa, elongation after fracture ≥ 16%), excellent low-temperature toughness (impact energy at -40℃ ≥ 100J, tensile strength after welding 640~820MPa, impact absorption energy at -40℃ after welding ≥ 100J), excellent low-temperature fracture toughness (CTOD characteristic value of base metal at -10℃ ≥ 1mm, CTOD characteristic value of heat-affected zone at -10℃ ≥ 0.4mm), uniform microstructure, and good plate shape (unevenness within 2 meters of the steel plate ≤ 4mm).

[0007] To achieve the above objectives, the present invention employs the following technical solution:

[0008] This invention has conducted extensive and systematic experimental research on several aspects, including alloy element screening and proportioning, steel cleanliness control, continuous casting process, heating and controlled rolling and cooling processes, optimization of tempering heat treatment processes, and matching of microstructure strength and toughness. Ultimately, a compositional system and its manufacturing process that meet the objectives of this invention were determined. The specific technical solution is as follows:

[0009] A 550MPa grade high-toughness quenched and tempered steel plate, the chemical composition of which, by weight percentage, is: C 0.06%–0.10%, Si 0.15%–0.4%, Mn 1.35%–1.65%, P≤0.012%, S≤0.002%, Ni 0.15%–0.35%, Cr 0.15%–0.40%, Nb 0.015%–0.03%, V 0.02%–0.04%, Mo 0.15%–0.30%, Ti 0.007%–0.020%, B 0.0007%–0.002%, Alt 0.018%–0.045%; the balance being Fe and unavoidable impurities.

[0010] The steel plate has a yield strength ≥550MPa, tensile strength 640~820MPa, elongation at section ≥16%, Z-direction reduction of area ≥65%, transverse Charpy impact energy of the steel plate core at -40℃ ≥100J, post-weld tensile strength 640~820MPa, post-weld transverse Charpy impact energy at -40℃ ≥100J, CTOD characteristic value of the base material at -10℃ ≥1mm, and CTOD characteristic value of the heat-affected zone at -10℃ ≥0.4mm.

[0011] The mechanisms of action of each alloy component in the steel of this invention are as follows:

[0012] C is an essential element for ensuring strength. It has a significant effect on improving the strength of steel through solid solution strengthening and precipitation strengthening. However, an increase in carbon content seriously affects the weldability and low-temperature toughness of steel. From the perspective of product performance, it is preferable to control the C content at 0.06% to 0.10%.

[0013] Si: As a solid solution strengthening element, although Si is beneficial to improving the strength of steel plates and their oxidation resistance at high temperatures, Si promotes packet size coarsening, which seriously impairs the low-temperature toughness, elongation and weldability of ultra-high strength steel plates. Considering the economics and operability of steelmaking, the preferred Si content is 0.15% to 0.4%.

[0014] Mn: As the most important alloying element in steel, in addition to improving the strength of steel plates, it also has the functions of expanding the austenite phase region, lowering the Ar3 point temperature, refining ferrite grains, and improving the low-temperature toughness of steel plates. However, when the quality of Mn is too high, the segregation of Mn will result in poor low-temperature toughness in the core of the thick plate and a decrease in the performance of the weld heat-affected zone. Therefore, the preferred Mn content range is 1.35% to 1.65%.

[0015] P: is an element that has an adverse effect on impact value. It can impair low-temperature toughness by segregating in the center of the slab and agglomerating at grain boundaries. The P content of the material in this invention is controlled to be no higher than 0.012%.

[0016] S: is an element that has an adverse effect on the impact value and can form sulfide inclusions, which can become crack initiation sites. The material in this invention is controlled to be no higher than 0.002%.

[0017] Ni: Nickel can significantly reduce the transformation temperature range of intermediate-temperature transformation products, expand the cooling range for producing stable lath bainite structures, and limit the long-range diffusion of carbon elements by reducing the transformation temperature and accelerating the transformation rate, resulting in a more uniform transformation structure. Nickel can also improve the low-temperature toughness of steel plates. However, as the nickel content increases, the cost will increase significantly. Therefore, considering both the performance of the steel plate and the production cost, the Ni content in this invention is controlled at 0.15% to 0.35%.

[0018] Cr (Cr): Improves the hardenability and strength of steel plates. It is an element that shrinks the austenite phase region and a weak carbide-forming element. In steel, it can form carbides and also dissolve in ferrite. Cr is also an effective element in improving the hardenability of steel, as it can form a continuous solid solution with Fe. Cr effectively increases the strength of steel plates by significantly shifting the ferrite phase transformation to the right, widening the cooling rate range of the bainite phase transformation, and promoting the formation of intermediate-temperature transformation structures. However, excessively high Cr content increases the tendency for temper brittleness and increases welding difficulty, while excessively low content cannot effectively exert its strengthening effect. In this invention, the Cr content is controlled at 0.15%–0.40%.

[0019] Niobium (Nb) is added to refine the grain size after quenching, thereby effectively refining the microstructure and strengthening the matrix through precipitation. The combined addition of Nb and boron effectively inhibits austenite recrystallization during rolling, refining the grains while also suppressing carbon diffusion. This effectively inhibits the precipitation of M23(C,B)6 at austenite grain boundaries, ensuring effective segregation of boron at ferrite nucleation sites, improving the hardenability of the steel, and giving it higher strength and better low-temperature toughness. The addition of Nb and the formation of Nb(C,N) in the steel suppress the appearance of coarse M23(C,B)6 structures. This reduces carbon diffusion, improves boron utilization efficiency, and promotes microstructure homogenization. However, when the Nb content exceeds a certain range, MA islands will form in the welded HAZ, which is detrimental to toughness. The Nb content is preferably controlled between 0.015% and 0.03%.

[0020] V: Adding V to steel can refine the grain structure and improve strength and toughness. The effect is not significant when the addition amount is less than 0.02%; when it is greater than 0.10%, the toughness and weldability of the steel decrease. With an appropriate nitrogen content, V can fully precipitate, significantly reducing the particle size and spacing in the steel, resulting in precipitation strengthening and thus improving strength. Therefore, this invention controls the V content to be between 0.02% and 0.04%.

[0021] Mo: Mo is an element that shrinks the austenite phase region, inhibits the decomposition of austenite, and delays the transformation of grain boundary ferrite, thus promoting the formation of bainite. Mo can improve the hardenability of steel and is also a strong solid solution strengthening element, which can significantly improve the strength of steel through solid solution strengthening. It produces phase transformation strengthening and dislocation strengthening effects, significantly improving the strength and microstructure uniformity of steel. When the Mo content is below 0.10%, the improvement on the strength and microstructure uniformity of steel is not significant; however, if the Mo content is too high, it will increase the cost on the one hand, and reduce the toughness and weldability of steel on the other hand. Therefore, in this invention, the Mo content is controlled at 0.15% to 0.30%.

[0022] Ti: When present in trace amounts, it forms nitrides, carbides, or carbonitrides, which can refine the grains and improve the toughness of the base material. However, when the content exceeds 0.025%, it will reduce the toughness of the base material and the heat-affected zone of the weld. Therefore, it is preferable to control the content between 0.007% and 0.020%.

[0023] B: It can improve the hardenability and strength of steel plates, but if the B content is too high, it will form coarse BN particles that are detrimental to hardenability and toughness. It will also affect the weldability and surface quality of steel plates. Therefore, the B content in this invention is controlled at 0.0007% to 0.002%.

[0024] Alt: As a necessary deoxidizing and grain-refining element added in this invention, the content should be above 0.01%, but exceeding 0.08% can easily cause hot cracking in the cast billet, and at the same time reduce the toughness of the steel. The preferred Alt content is controlled between 0.018% and 0.045%.

[0025] The maximum thickness of the finished steel plate is 80mm.

[0026] The microstructure of the steel plate is tempered bainite with a grain size ≥ 7.

[0027] The unevenness of the steel plate within 2 meters should be ≤4mm.

[0028] A method for manufacturing a 550MPa grade high-toughness quenched and tempered steel plate specifically includes the following steps:

[0029] 1) Smelting and slow cooling of billets: Select smelting raw materials, mainly molten iron or high-quality recycled steel, with P, S and other elements as low as possible. Control the smelting composition according to the target value, strictly control the content of residual elements, and avoid exceeding the carbon equivalent limit. The LF and RH refining furnaces need to be treated for 20-40 minutes each. The superheat of the molten steel in the tundish should be 22-28℃. Strictly control the content of residual elements. The gas content should be controlled as [N]≤40ppm; [O]≤10ppm, [H]≤2.0ppm. Protect the casting process throughout. The inclusions of A, B, C and D in the steel should meet the following requirements: A≤1.0, B≤1.0, C≤1.0, D≤1.0. The continuous casting billet forming process adopts the light reduction technology with a light reduction of 6-8mm. After the continuous casting billet is removed from the line, it is stacked and slowly cooled with a stacking temperature ≥600℃ and a time ≥48h.

[0030] 2) Heating: The continuously cast billet is heated in four sections: heating section 1, heating section 2, heating section 3, and soaking section. The upper and lower parts of each section are controlled separately. The lower heating temperature of heating section 1 is 1000-1120℃, and the upper heating temperature of heating section 1 is 1050-1180℃; the lower heating temperature of heating section 2 is 1130-1180℃, and the upper heating temperature of heating section 2 is 1180-1220℃; the lower heating temperature of heating section 3 is 1150-1200℃, and the upper heating temperature of heating section 3 is 1200-1250℃; the lower heating temperature of the soaking section is 1050-1150℃, and the upper heating temperature of the soaking section is 1150-1200℃; the heating time is 10-12 min / mm.

[0031] 3) Rolling: After the billet exits the furnace, high-pressure water descaling is performed 1-2 times to remove surface iron oxide scale and lower the continuous casting billet temperature. The initial rolling temperature is 1000-1050℃, with a single-pass reduction rate ≥15% except for the widening pass, improving the as-cast microstructure of the slab, reducing the billet thickness before heating, and shortening the heating time of the steel plate. The intermediate billet thickness is 2-2.5 times the finished product thickness. The intermediate billet is cooled by inter-stand water cooling at a rate ≥5℃ / s. The second-stage initial rolling temperature is 850-900℃, with a single-pass reduction rate ≥11%, and the final rolling temperature is 800-850℃. Through two-stage rolling, the phase transformation microstructure is controlled to be uniform and refined, preparing the initial microstructure for tempering heat treatment.

[0032] 4) Cooling: After final rolling, the steel plate is directly cooled using a DQ+ACC rapid cooling system with an average cooling rate of ≥10℃ / s and a reddening temperature of 200~300℃. This is combined with a large deformation rolling process and post-rolling cooling to refine the original microstructure.

[0033] 5) Quenching and tempering: The steel plate is subjected to quenching heat treatment, wherein the quenching heating temperature is controlled between 880 and 920℃, the heating rate is 1.4 to 1.8 min / mm, the holding time is 20 to 40 min, and the plate is quenched to room temperature after being taken out of the furnace. The quenching process is used to obtain the uniformity of the original austenite grain size on the full thickness section of the thick steel plate.

[0034] The steel plate is tempered at a temperature of 550–610℃ for 3–4.5 min / mm in the furnace, and then air-cooled after being taken out of the furnace to obtain the finished steel plate. By controlling the tempering process and adjusting the composition, uniformity and refinement of the phase transformation structure and the precipitation behavior of multiphase particles, the low-temperature impact toughness and resistance to low-temperature fracture (CTOD) of the steel grade are improved.

[0035] Compared with the prior art, the beneficial effects of the present invention are:

[0036] 1) The steel plate produced by the process of this invention has good comprehensive mechanical properties through the coupled design of alloy composition, deformation and heat treatment processes. It solves the problem of the difficulty in matching and harmonizing the strength, plasticity and low temperature toughness (-40℃) of 550MPa grade steel plate. The produced steel plate has a yield strength ≥550MPa, tensile strength 640~820MPa, elongation at section ≥16%, Z-direction reduction of area ≥65%, transverse Charpy impact energy of the steel plate base metal core at -40℃ ≥100J, post-weld tensile strength 640~820MPa, post-weld transverse Charpy impact energy at -40℃ ≥100J, low temperature fracture CTOD characteristic value of the base metal at -10℃ ≥1mm, and low temperature fracture CTOD characteristic value of the heat-affected zone at -10℃ ≥0.4mm.

[0037] 2) This invention fully leverages the technical and equipment advantages of the heavy plate rolling mill and heat treatment unit to develop ultra-high strength and high toughness marine engineering steel thick plates with a maximum thickness of 80mm.

[0038] 3) By combining a reasonable rolling process with an offline quenching and tempering process, compared with the TMCP or TMCP+T process for steel plates of the same strength, offline quenching can precisely control the starting temperature of quenching and ensure uniform temperature across the entire plate; at the same time, it is easier to control the flatness of the plate, and the flatness of the steel plate within 2 meters can be ≤4mm.

[0039] 4) The microstructure of the steel plate is tempered bainite with a grain size ≥ 7. Attached Figure Description

[0040] Figure 1 Metallographic image of Example 1 (500x magnification).

[0041] Figure 2 This is a grain size photograph (100x) of Example 1. Detailed Implementation

[0042] To make the objectives, technical solutions, and advantages of this invention clearer, the specific implementation methods of this invention will be further described below in conjunction with the embodiments. The following embodiments are used to specifically illustrate the content of this invention. These embodiments are only general descriptions of the content of this invention and do not limit the content of this invention.

[0043] The chemical composition of the steel in this embodiment is shown in Table 1; the carbon equivalent and welding crack sensitivity coefficient of the steel in this embodiment are shown in Table 2; the smelting process parameters of the steel in this embodiment are shown in Table 3; the slab heating process of the steel plate in this embodiment is shown in Table 4; the rolling and cooling process of the steel plate in this embodiment is shown in Table 5; the quenching and tempering heat treatment process of the steel plate in this embodiment is shown in Table 6; the mechanical properties of the steel plate in this embodiment are shown in Table 7; the welding performance of the steel in this embodiment is shown in Table 8; the low-temperature fracture performance of the steel in this embodiment is shown in Table 9; and the unevenness and grain size of the steel plate in this embodiment are shown in Table 10.

[0044] Table 1. Chemical composition (wt%) of steel in the embodiments of the present invention.

[0045] Example C Si Mn P S Ni Cr Nb V Mo Ti B Als 1 0.071 0.23 1.51 0.011 0.002 0.16 0.21 0.017 0.033 0.189 0.013 0.0014 0.037 2 0.061 0.31 1.56 0.012 0.003 0.18 0.26 0.027 0.029 0.201 0.009 0.0009 0.041 3 0.079 0.19 1.42 0.011 0.002 0.21 0.33 0.023 0.038 0.171 0.007 0.0011 0.025 4 0.089 0.16 1.48 0.012 0.001 0.29 0.24 0.029 0.026 0.151 0.008 0.001 0.031 5 0.072 0.21 1.49 0.008 0.002 0.24 0.29 0.019 0.024 0.231 0.007 0.0008 0.021 6 0.062 0.28 1.59 0.007 0.002 0.28 0.17 0.026 0.031 0.251 0.012 0.0012 0.029 7 0.069 0.26 1.45 0.009 0.003 0.19 0.19 0.021 0.036 0.201 0.011 0.0013 0.027 8 0.081 0.17 1.53 0.009 0.001 0.26 0.16 0.016 0.028 0.199 0.015 0.0015 0.024 9 0.071 0.29 1.54 0.008 0.002 0.22 0.23 0.028 0.032 0.169 0.013 0.0009 0.039 10 0.082 0.24 1.47 0.01 0.001 0.23 0.29 0.019 0.025 0.191 0.01 0.0007 0.021

[0046] Table 2 Carbon equivalent and welding crack sensitivity coefficient of steel in the embodiments of the present invention

[0047]

[0048]

[0049] Table 3. Smelting process parameters of steel in the embodiments of the present invention.

[0050]

[0051] Table 4. Slab heating process of steel plate in embodiments of the present invention.

[0052]

[0053] Table 5 Steel Plate Rolling and Cooling Processes in Embodiments of the Invention

[0054]

[0055] Table 6. Tempering and heat treatment process of steel plate in embodiments of the present invention.

[0056]

[0057] Table 7 Mechanical properties of steel plates in embodiments of the present invention

[0058]

[0059]

[0060] Table 8 Welding performance of steel in embodiments of the present invention

[0061]

[0062] Table 9 Low-temperature fracture properties of steel in the embodiments of the present invention

[0063]

[0064] Table 10. Unevenness and grain size of steel plates in embodiments of the present invention.

[0065]

[0066]

[0067] To illustrate the present invention, the present invention has been appropriately and sufficiently described above through embodiments. The above embodiments are only for illustrating the present invention and are not intended to limit the present invention. Those skilled in the art can make various changes and modifications without departing from the spirit and scope of the present invention. Any modifications, equivalent substitutions, improvements, etc., should be included within the protection scope of the present invention. The patent protection scope of the present invention should be defined by the claims.

Claims

1. A method of manufacturing a 550 MPa grade high toughness quenched and tempered steel plate, characterized by, The chemical composition of the steel is calculated by weight percentage: C 0.06%-0.10%, Si 0.15%-0.4%, Mn 1.35%-1.65%, P≤0.012%, S≤0.002%, Ni 0.15%-0.35%, Cr 0.21%-0.40%, Nb 0.015%-0.03%, V 0.02%-0.04%, Mo 0.15%-0.231%, Ti 0.007%-0.020%, B 0.0007%-0.002%, Alt 0.018%-0.045%; the balance is Fe and inevitable impurities; The manufacturing method comprises the following steps: 1) smelting and slab slow cooling: LF and RH refining furnaces each need to be treated for 20-40 min, the tundish molten steel superheat is 22-28 ℃, the light press-down technology is used in the continuous casting slab forming process, the light press-down amount is 6-8 mm, the continuous casting slab is stacked and slowly cooled after being discharged, the stacking temperature is≥600 ℃, and the time is≥48 h; 2) heating: the continuous casting slab is heated in 4 sections, the 4 heating sections are heating section 1, heating section 2, heating section 3 and soaking section respectively, the upper and lower sections of the furnace are controlled respectively, the lower heating temperature of heating section 1 is 1000-1120 ℃, the upper heating temperature of heating section 1 is 1050-1180 ℃; the lower heating temperature of heating section 2 is 1130-1180 ℃, the upper heating temperature of heating section 2 is 1180-1220 ℃; the lower heating temperature of heating section 3 is 1150-1200 ℃, the upper heating temperature of heating section 3 is 1200-1250 ℃; the lower heating temperature of soaking section is 1050-1150 ℃, the upper heating temperature of soaking section is 1150-1200 ℃; the heating time is 10-12 min / mm; 3) rolling: the open rolling temperature is 1000-1050 ℃, the single pass reduction rate is≥15% except the spreading pass, the intermediate slab is cooled by the inter-stand cooling water at a rate of≥5 ℃ / s, the second stage open rolling temperature is 850-900 ℃, the single pass reduction rate is≥11%, and the finish rolling temperature is 821-850 ℃; 4) cooling: the steel plate is directly cooled after finish rolling, the DQ+ACC rapid cooling system with an average cooling rate of≥10 ℃ / s is used, and the re-red temperature is 200-300 ℃; 5) quenching and tempering: the quenching heating temperature is controlled between 880-920 ℃, the heating rate is 1.4-1.8 min / mm, the holding time is 20-40 min, and the steel plate is quenched to room temperature after being discharged from the furnace; the tempering temperature is 550-560 ℃, the furnace time is 3-4.5 min / mm, the steel plate is air-cooled after being discharged, and the steel plate finished product is obtained.

2. The method of producing a 550 MPa grade high toughness quenched and tempered steel plate according to claim 1, characterized by, The maximum thickness of the steel plate finished product is 80 mm.

3. The method of producing a 550 MPa grade high toughness quenched and tempered steel plate according to claim 1, characterized in that, The microstructure of the steel plate is tempered bainite, and the grain size is≥7 levels.

4. The method of producing a 550 MPa grade high toughness quenched and tempered steel plate according to claim 1, characterized in that, The unevenness of the steel plate within 2 m is≤4 mm.

5. The method of producing a 550 MPa grade high toughness quenched and tempered steel plate according to claim 1, wherein In step 1), the gas content of the molten steel is controlled as follows: [N]≤40 ppm, [O]≤10 ppm, [H]≤2.0 ppm; the A, B, C and D type inclusions in the steel meet the requirements of: A type≤1.0, B type≤1.0, C type≤1.0, and D type≤1.0.

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