590MPa-grade ultralow-carbon bainite steel and preparation method thereof

Abstract

The invention relates to 590MPa-grade ultralow-carbon bainite steel and a preparation method thereof, and belongs to the technical field of metal materials. The problems that existing 590MPa-grade steel is poor in matching of obdurability and weldability, and cannot have good weldability and high yield ratio while high obdurability is met are solved. The invention relates to 590 MPa grade ultra-low carbon bainite steel, which comprises the following chemical components in percentage by weight: 0.02 to 0.03 percent of C, 1.0 to 1.5 percent of Mn, 1.0 to 1.3 percent of Mo, 0.03 to 0.07 percent of Nb, 2.0 to 4.0 percent of Ni, 0.3 to 0.7 percent of Cr, 0.01 to 0.03 percent of Ti, 0.2 to 0.4 percent of Al and the balance of Fe and inevitable impurity elements. The prepared steel has good obdurability, excellent welding performance and low yield ratio at the same time.

Description

Technical Field

[0001] The present invention relates to the technical field of metal materials, and in particular to a 590MPa grade ultra-low carbon bainite steel and a preparation method thereof. Background Art

[0002] In the shipbuilding industry, the high strength and toughness of hull steel is the key to ensure the safety and durability of ship structures. In the prior art, in order to meet the strength requirements of 590MPa, a high content of carbon (C) and alloy elements such as Ni, Cr, Mo, and V are usually added to the hull steel. Specifically, the existing 590MPa-level hull steel has a carbon content ranging from 0.06-0.11%, and the total amount of alloy exceeds 4.3%. However, this high-carbon and high-alloy content design scheme, although it improves the strength of the steel, also significantly increases the difficulty and complexity of welding. In practical applications, in order to avoid defects such as cracks generated during welding, preheating and post-heating processes are usually required, which not only greatly increases the difficulty of welding construction, but also easily causes cracks due to residual stress after welding, causing safety hazards, and greatly increases the construction cost of the ship; according to statistics, during the construction of the entire ship, the cost of the welding process accounts for about 30-50% of the construction cost. Therefore, the increase in welding difficulty directly leads to a significant increase in the cost of material application.

[0003] In addition, due to the high strength level, high-strength hull structure steel usually adopts tempering heat treatment, and the structure is tempered martensite or tempered martensite + bainite. The steel plate after tempering heat treatment has a high yield strength ratio (≥0.93). The high yield strength ratio will reduce the service safety margin of the steel and increase the service risk.

[0004] In recent years, ultra-low carbon bainitic steel has gradually attracted widespread attention as a new type of high-strength, tough and easy-to-weld steel. This type of steel effectively eliminates the adverse effects of carbon on welding performance by significantly reducing the carbon content, thereby significantly improving welding efficiency. At present, ultra-low carbon bainitic steel has been widely used in many fields such as oil pipelines and marine engineering, and has demonstrated excellent performance. Therefore, the development of an ultra-low carbon bainitic steel with high strength, low yield ratio and easy welding is the direction of exploration in the field of hull steel. Summary of the invention

[0005] In view of the above analysis, the present invention aims to provide a 590MPa grade ultra-low carbon bainite steel and a preparation method thereof, which at least solves one of the following problems existing in the existing 590MPa grade steel: 1. The existing 590MPa grade steel has poor matching between strength and toughness and weldability, and cannot have good welding performance while meeting high strength and toughness. 2. The existing 590MPa grade steel has a relatively high yield strength ratio.

[0006] The purpose of the present invention is mainly achieved through the following technical solutions:

[0007] The invention provides a 590MPa grade ultra-low carbon bainite steel, the chemical composition of which comprises by weight: C: 0.02-0.03%, Mn: 1.0-1.5%, Mo: 1.0-1.3%, Nb: 0.03-0.07%, Ni: 2.0-4.0%, Cr: 0.3-0.7%, Ti: 0.01-0.03%, Al: 0.2-0.4%, and the rest are Fe and inevitable impurity elements.

[0008] Further, the chemical composition includes by weight: C: 0.02-0.03%, Mn: 1.0-1.45%, Mo: 1.17-1.3%, Nb: 0.03-0.07%, Ni: 2.80-4.0%, Cr: 0.46-0.70%, Ti: 0.01-0.03%, Al: 0.2-0.4%, and the rest are Fe and unavoidable impurity elements.

[0009] Furthermore, the mass ratio of the Ni element to the Al element satisfies: 6≤Ni / Al≤14.

[0010] Furthermore, the microstructure of the steel is granular bainite, and the content of granular bainite is 100%.

[0011] The present invention also provides a method for preparing 590MPa grade ultra-low carbon bainite steel, which is used to prepare the above 590MPa grade ultra-low carbon bainite steel, including molten iron pretreatment - converter smelting - LF refining - continuous casting - continuous casting billet heating - controlled rolling and controlled cooling - tempering heat treatment;

[0012] After KR desulfurization, the S content of the molten iron is ≤0.002%.

[0013] Furthermore, the continuous casting process adopts full-process protective casting, the target superheat of the molten steel in the middle package is ≤25°C, and after continuous casting, it enters the slow cooling pit for treatment.

[0014] Furthermore, the heating temperature of the continuous casting billet is 1200-1250° C., and the heating time is 1.8-2.2 hours.

[0015] Furthermore, the controlled rolling and controlled cooling adopts two-stage rolling, the first stage rolling is rough rolling, the starting rolling temperature is 1150-1180°C, the final rolling temperature is 1000-1100°C, and the cumulative deformation is ≥60%.

[0016] Furthermore, in the two-stage rolling, the second stage rolling is finish rolling, the starting rolling temperature is 900-980° C., the final rolling temperature is 750-850° C., and the cumulative deformation is ≥35%.

[0017] Furthermore, in the controlled rolling and controlled cooling, the cooling rate is 1 to 10°C / s.

[0018] Compared with the prior art, the present invention can achieve at least one of the following beneficial effects:

[0019] 1. The present invention provides a 590MPa grade ultra-low carbon bainitic steel and a preparation method thereof. The steel adopts a low-carbon design, and the strength of the steel no longer depends on the carbon content and the total amount of alloy elements, but on the dislocation strengthening, fine grain strengthening and precipitation strengthening of the Ni3Al phase in the bainite structure. After the carbon content is greatly reduced, the cementite in the bainite ferrite matrix is ​​basically eliminated, and the toughness, weldability and yield strength ratio of the steel are further improved. The prepared steel has good toughness and excellent welding performance.

[0020] 2. The present invention substantially eliminates cementite in the bainitic ferrite matrix by reducing the carbon content, and through the synergistic effect of reasonable steel composition design and a preparation method with strict control of process parameters, the prepared 590MPa grade ultra-low carbon bainitic steel achieves a good match between strength and toughness and weldability, with a yield strength of ≥590Mpa, a tensile strength of ≥850MPa, and an elongation of ≥22%. At the same time, the steel plate has good weldability, the impact energy of the welded joint at -20°C is ≥80J, and the CTOD of the welded joint at -20°C is ≥0.35mm.

[0021] 3. In the preparation method of a 590MPa grade ultra-low carbon bainitic steel provided by the present invention, the preparation process is simple, no post-rolling heat treatment is required, and the structure obtained after controlled rolling and controlled cooling is uniform and the performance is stable.

[0022] 4. The present invention prepares an ultra-low carbon bainite structure through controlled rolling and controlled cooling technology, and relies on the coordination of soft and hard phases in the bainite structure to prepare a 590MPa grade ultra-low carbon bainite steel with a low yield ratio of 0.73-0.77.

[0023] In the present invention, the above-mentioned technical solutions can also be combined with each other to achieve more preferred combination solutions. Other features and advantages of the present invention will be described in the subsequent description, and some advantages can become obvious from the description, or can be understood by practicing the present invention. The purpose and other advantages of the present invention can be realized and obtained through the contents particularly pointed out in the description and the drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] The drawings are only for the purpose of illustrating particular embodiments and are not to be considered limiting of the present invention. Like reference symbols denote like components throughout the drawings.

[0025] Figure 1 This is the microstructure diagram of 1# steel in the embodiment of the present invention. DETAILED DESCRIPTION

[0026] The preferred embodiments of the present invention are described in detail below in conjunction with the accompanying drawings, wherein the accompanying drawings constitute a part of this application and are used together with the embodiments of the present invention to illustrate the principles of the present invention, but are not used to limit the scope of the present invention.

[0027] The invention provides a 590MPa grade ultra-low carbon bainite steel. The chemical composition of the 590MPa grade ultra-low carbon bainite steel is calculated by mass percentage as follows: C: 0.02-0.03%, Mn: 1.0-1.5%, Mo: 1.0-1.3%, Nb: 0.03-0.07%, Ni: 2.0-4.0%, Cr: 0.3-0.7%, Ti: 0.01-0.03%, Al: 0.2-0.4%, and the rest is Fe and inevitable impurity elements.

[0028] Wherein, the mass ratio of Ni element to Al element satisfies: 6≤Ni / Al≤14.

[0029] The reasons for limiting the composition of the ingot of the 590 MPa grade polar hull steel and the method for producing the same in the present invention are explained below, and the mass percentage in the composition is expressed only in %.

[0030] C: Carbon is an element that ensures the strength of the steel plate and significantly affects the weldability of the material; the carbon element ensures that the steel forms a full bainite structure. The reduction of carbon content can significantly improve the welding performance of the steel. When the C content is 0.02% to 0.03%, the weldability and toughness of the steel are well matched.

[0031] Mn: Manganese is a basic element for the formation of bainite, which causes an obvious bend in the supercooled austenite transformation curve and significantly delays the high-temperature transformation, separating the upper and lower C curves of the steel; Mn easily forms MnS impurities with S in the steel, which damages the performance of the steel; therefore, the present invention selects the addition amount of Mn to be 1.0% to 1.5%.

[0032] Mo: Mo element can reduce the bainite transformation temperature, promote bainite phase transformation, and shorten the bainite transformation time; the present invention selects the addition amount of Mo to be 1.0% to 1.3%.

[0033] Nb: Nb element can inhibit the deformation and recrystallization behavior of high-temperature austenite, increase the recrystallization temperature, expand the non-recrystallization zone, increase the deformation accumulation during rolling in the non-recrystallization zone, introduce high-density dislocations, and promote microstructure refinement; the present invention selects the addition amount of Nb to be 0.03% to 0.07%.

[0034] Ni: Ni element mainly reduces the ductile-brittle transition temperature and improves the strength and toughness of steel; therefore, the present invention selects the addition amount of Ni to be 2.0% to 4.0%.

[0035] Cr: Cr element can significantly reduce the transformation temperature of bainite, affect the C curve of bainite transformation, and improve the strength of bainite steel; therefore, the present invention selects the addition amount of Cr to be 0.3% to 0.7%.

[0036] Ti: Titanium precipitates can inhibit the austenite grain growth process during the heating process of the ingot and the welding thermal cycle, thereby improving the toughness of the steel plate and the welding heat-affected zone. When the Ti content is too low, the effect is weakened; when the content is higher than 0.04%, large particles of TiN are easily formed and the effect is lost; therefore, the present invention selects the addition amount of Ti to be controlled at 0.01-0.03%.

[0037] Al: Al plays a deoxidizing role in steel to ensure the purity of molten steel. At the same time, it ensures a certain solid solution aluminum content and forms a dispersed nano-scale Ni3Al phase during the steel tempering process, providing a strong strengthening effect. Therefore, the present invention selects the addition amount of Al to be 0.2-0.4%.

[0038] Preferably, a 590MPa grade ultra-low carbon bainitic steel, the chemical composition of which is measured by weight: C: 0.02-0.03%, Mn: 1.0-1.45%, Mo: 1.17-1.3%, Nb: 0.03-0.07%, Ni: 2.80-4.0%, Cr: 0.46-0.70%, Ti: 0.01-0.03%, Al: 0.2-0.4%, and the rest is Fe and unavoidable impurity elements.

[0039] The present invention also provides a method for preparing the above-mentioned 590MPa grade ultra-low carbon bainitic steel, comprising the following steps:

[0040] S1: Hot metal pretreatment

[0041] The impurity elements in the steel are reduced by pretreatment of molten iron to ensure that the S content is ≤ 0.002% after KR desulfurization; a high S content will produce inclusions and affect the performance of the steel.

[0042] S2: converter smelting + LF refining

[0043] High-quality scrap steel is added for converter smelting, white slag is made through LF furnace to refine molten steel, the composition of molten steel is adjusted to reach the target value, and the content of inclusions in the steel is strictly controlled through degassing, desulfurization during refining and argon blowing in the later stage of refining; finally, Ti wire and Ca wire are fed to improve the recovery rate of Ti and Ca, prevent Ti from precipitating at too high temperature, and avoid the formation of large titanium oxide inclusions.

[0044] S3: Continuous Casting

[0045] The continuous casting process adopts full-process protective casting, and the target superheat of the molten steel in the middle package is ≤25℃. After continuous casting, it enters the slow cooling pit for treatment; excessive superheat will lead to an increase in inclusions in the molten steel, affecting the purity and performance of the casting.

[0046] S4: Continuous casting heating

[0047] The continuous casting billet is heated at a temperature of 1200 to 1250° C. and a heating time of 1.8 to 2.2 h.

[0048] S5: Controlled rolling and controlled cooling

[0049] The heated continuous casting billet is subjected to controlled rolling and controlled cooling: the controlled rolling adopts a two-stage rolling method: the first stage is rough rolling, the starting rolling temperature is 1150-1180°C, the final rolling temperature is 1000-1100°C, and the cumulative deformation is ≥60%; the second stage is finishing rolling, the starting rolling temperature is 900-980°C, the final rolling temperature is 750-850°C, and the cumulative deformation is ≥35%; the billet after finish rolling is cooled to room temperature, and the cooling rate is 1-10°C / s; it should be noted that heating causes plastic deformation of the steel, and the starting rolling temperatures of rough rolling and finishing rolling are determined based on 80% of the solidus temperature in the alloy phase.

[0050] S6: Tempering heat treatment

[0051] The rolled steel plate is placed in a heating furnace for tempering heat treatment at a temperature of 600°C to 650°C for 2-3 hours. After the tempering, the steel plate is air-cooled to obtain a dispersed nano-scale Ni3Al precipitation phase, which plays a strong precipitation strengthening role and ensures the high strength source of the material.

[0052] It should be noted that the present invention adopts a low-carbon design, and the strength of the steel no longer depends on the carbon content and the total amount of alloying elements, but relies on dislocation strengthening, fine grain strengthening and precipitation strengthening of Ni3Al phase in the bainite structure. After the carbon content is greatly reduced, the cementite in the bainite ferrite matrix is ​​basically eliminated, and the toughness, weldability and yield strength ratio of the steel are further improved. The prepared steel has good toughness and excellent welding performance.

[0053] The preparation method of the present invention is highly practical, and can obtain ultra-low carbon bainite steel with consistent organizational type, stable performance, and good strength and toughness matching within a wide process range. The bainite morphology in the obtained ultra-low carbon bainite steel is granular bainite, and the content is 100%. The granular bainite structure consists of a matrix ferrite structure (soft phase) and MA islands (hard phase) distributed in the matrix. The yield strength is mainly determined by the strength of the soft phase, and the tensile strength is mainly determined by the hard phase. The soft and hard phases in the granular bainite are well matched, which ensures the strength of the prepared steel plate while reducing the yield strength ratio.

[0054] The hull structural steel prepared by the present invention has a yield strength of ≥590Mpa (such as 624-658MPa), a tensile strength of ≥850MPa (such as 852-874MPa), an elongation of ≥22% (such as 22.0-26%), and a yield strength ratio of ≤0.77 (such as 0.73-0.77). At the same time, the steel plate has good weldability, the impact energy of the welded joint at -20°C is ≥80J, and the CTOD of the welded joint at -20°C is ≥0.35mm.

[0055] The advantages of the precise control of the chemical composition, content and preparation process parameters of the present invention will be demonstrated below with specific examples and comparative examples.

[0056] Example

[0057] This embodiment discloses six types (1#-4# steels) of 590MPa grade ultra-low carbon bainitic steels, and selects two types (5# and 6#) of steels as comparative steels.

[0058] 1#-4# all use the same process:

[0059] S1: Hot metal pretreatment

[0060] The impurity elements in the steel are reduced by pretreatment of molten iron to ensure that the S content is ≤ 0.002% after KR desulfurization; a high S content will produce inclusions and affect the performance of the steel.

[0061] S2: converter smelting + LF refining

[0062] High-quality scrap steel is added for converter smelting, white slag is made through LF furnace to refine molten steel, the composition of molten steel is adjusted to reach the target value, and the content of inclusions in the steel is strictly controlled through degassing, desulfurization during refining and argon blowing in the later stage of refining; finally, Ti wire and Ca wire are fed to improve the recovery rate of Ti and Ca, prevent Ti from precipitating at too high temperature, and avoid the formation of large titanium oxide inclusions.

[0063] S3: Continuous Casting

[0064] The continuous casting process adopts full-process protective casting, the target superheat of the molten steel in the middle package is ≤25℃, and it enters the slow cooling pit for treatment after continuous casting.

[0065] S4: Continuous casting heating

[0066] The continuous casting billet is heated at a temperature of 1200 to 1250° C. and a heating time of 1.8 to 2.2 h.

[0067] S5: Controlled rolling and controlled cooling

[0068] The heated continuous casting billet is subjected to controlled rolling and controlled cooling: the controlled rolling adopts a two-stage rolling method: the first stage is rough rolling, the starting rolling temperature is 1150-1180°C, the final rolling temperature is 1000-1100°C, and the cumulative deformation is ≥60%; the second stage is finish rolling, the starting rolling temperature is 900-980°C, the final rolling temperature is 750-850°C, and the cumulative deformation is ≥35%; the billet after finish rolling is cooled to room temperature, and the cooling rate is 1-10°C / s.

[0069] S6: Tempering heat treatment

[0070] The rolled steel plate is placed in a heating furnace for tempering heat treatment at a temperature of 600°C to 650°C for 2-3 hours, and then air-cooled to room temperature.

[0071] The mass percentages of the elemental components of 1# to 4# steels and 6# steel all meet the requirements of the present invention, while the mass percentages of the elemental components of 5# steel do not meet the requirements of the present invention. The differences in their elemental compositions are shown in Table 1. The process parameters of 6# steel do not meet the requirements of the present invention, and the differences in their process parameters are shown in Table 2. The microstructure of 1# steel is shown in FIG. Figure 1 shown.

[0072] Table 1 Chemical composition of 1#-6# steel (wt, %)

[0073] Steel serial number C Mn Mo Nb Ni Cr Ti Al Ni / Al 1# 0.02 1.00 1.17 0.04 4.00 0.46 0.01 0.37 10.81 2# 0.02 1.45 1.30 0.07 2.80 0.61 0.03 0.20 14 3# 0.03 1.30 1.25 0.03 3.10 0.70 0.02 0.40 7.75 4# 0.03 1.50 1.00 0.06 2.00 0.30 0.02 0.31 6.45 5# 0.07 0.35 0.20 0.01 2.76 0.85 0.01 0.06 46 6# 0.02 1.20 1.10 0.05 2.70 0.37 0.01 0.28 9.64

[0074] Table 2 1#-6# steel preparation process

[0075]

[0076] The mechanical properties of 1#~6# steels are shown in Table 3.

[0077] Table 3 shows the mechanical properties of 1#-6# steel

[0078]

[0079]

[0080] The weldability test of 1#-6# steel was carried out at a line energy of 120±5KJ / cm. No preheating was performed before welding and no heat treatment was performed after welding. The welding groove was a single-sided V-shape. The performance of the welded joint is shown in Table 4.

[0081] Table 4 Weldability of 1#-6# steel

[0082]

[0083] By comparison, it can be seen that the smelting methods and process parameters of 1#~5# steels are the same or similar, but the mass percentage of the elemental composition of 1#~4# steels all meet the requirements of the present invention, and the mass percentage of the elemental composition of 5# steel does not meet the requirements of the present invention. The strength and other mechanical properties of 5# steel are consistent with those of 1#~4# steels, but the yield strength is relatively high, and the weldability is significantly lower than that of 1#~4# steels, making it difficult to meet the requirements of easy welding. The mass percentage of the elemental composition of 1#~4# and 6# steels is the same or similar, but the smelting methods and process parameters of 1#~4# steels all meet the requirements of the present invention, and the smelting methods and process parameters of 6# steel do not meet the requirements of the present invention. The strength and other mechanical properties of 6# steel are significantly lower than those of 1#~4#, making it difficult to meet the 590MPa grade requirements.

[0084] By comparison, it can be seen that the present invention achieves excellent mechanical properties of the steel plate through the synergistic effect of reasonable steel composition design and strict control of process parameters in the preparation method, with yield strength ≥590MPa, tensile strength ≥850MPa, elongation ≥22%, and yield strength ratio ≤0.77. At the same time, the steel plate has good weldability, the impact energy of the welded joint at -20℃ is ≥80J, and the CTOD of the welded joint at -20℃ is ≥0.35mm.

[0085] The above description is only a preferred specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by any technician familiar with the technical field within the technical scope disclosed by the present invention should be covered within the protection scope of the present invention.

Claims

1. A 590MPa grade ultra-low carbon bainitic steel, characterized in that: The chemical composition includes by weight: C: 0.02-0.03%, Mn: 1.0-1.5%, Mo: 1.0-1.3%, Nb: 0.03-0.07%, Ni: 2.0-4.0%, Cr: 0.3-0.7%, Ti: 0.01-0.03%, Al: 0.2-0.4%, and the rest are Fe and unavoidable impurity elements.

2. The 590MPa grade ultra-low carbon bainitic steel according to claim 1, characterized in that: The chemical composition includes by weight: C: 0.02-0.03%, Mn: 1.0-1.45%, Mo: 1.17-1.3%, Nb: 0.03-0.07%, Ni: 2.80-4.0%, Cr: 0.46-0.70%, Ti: 0.01-0.03%, Al: 0.2-0.4%, and the rest are Fe and unavoidable impurity elements.

3. The 590MPa grade ultra-low carbon bainitic steel according to claim 1, characterized in that: The mass ratio of the Ni element to the Al element satisfies: 6≤Ni / Al≤14.

4. The 590MPa grade ultra-low carbon bainitic steel according to claim 1, characterized in that: The microstructure of the steel is granular bainite, and the content of granular bainite is 100%.

5. A method for preparing 590MPa ultra-low carbon bainitic steel, used for preparing the 590MPa ultra-low carbon bainitic steel according to any one of claims 1 to 4, characterized in that: Including molten iron pretreatment - converter smelting - LF refining - continuous casting - continuous casting billet heating - controlled rolling and controlled cooling - tempering heat treatment; After KR desulfurization, the S content of the molten iron is ≤0.002%.

6. The preparation method according to claim 5, characterized in that: The continuous casting process adopts full-process protective casting, the target superheat of the molten steel in the middle package is ≤25°C, and after continuous casting, it enters the slow cooling pit for treatment.

7. The preparation method according to claim 6, characterized in that: The continuous casting billet is heated at a temperature of 1200 to 1250° C. and for a heating time of 1.8 to 2.2 hours.

8. The preparation method according to claim 7, characterized in that: The controlled rolling and controlled cooling adopts two-stage rolling, the first stage rolling is rough rolling, the starting rolling temperature is 1150-1180°C, the final rolling temperature is 1000-1100°C, and the cumulative deformation is ≥60%.

9. The preparation method according to claim 8, characterized in that: In the two-stage rolling, the second stage rolling is finish rolling, the starting rolling temperature is 900-980° C., the final rolling temperature is 750-850° C., and the cumulative deformation is ≥35%.

10. The preparation method according to claim 9, characterized in that: In the controlled rolling and controlled cooling, the cooling rate is 1 to 10° C. / s.

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