CN113549846A

CN113549846A - 550 MPa-grade marine steel with excellent low-temperature performance and manufacturing method thereof

Info

Publication number: CN113549846A
Application number: CN202110788226.2A
Authority: CN
Inventors: 金耀辉; 赵坦; 朱隆浩; 王�华; 陈华; 王东旭; 韩鹏; 于浩男; 李家安; ***
Original assignee: Angang Steel Co Ltd
Current assignee: Angang Steel Co Ltd
Priority date: 2021-07-13
Filing date: 2021-07-13
Publication date: 2021-10-26

Abstract

550 MPa-grade marine steel with excellent low-temperature performance and a manufacturing method thereof are disclosed, wherein the steel comprises the following chemical components in percentage by weight: 0.05 to 0.09 percent of C, 0.1 to 0.3 percent of Si, 1.25 to 1.60 percent of Mn1, less than or equal to 0.012 percent of P, less than or equal to 0.002 percent of S, 0.25 to 0.40 percent of Cu0, 0.65 to 0.85 percent of Ni0, 0.15 to 0.30 percent of Cr0.030 to 0.045 percent of Nb0.030 to 0.045 percent of Mo0.30 to 0.40 percent of Ti0.008 to 0.020 percent of Ti, 0.02 to 0.04 percent of Alt0, and the balance of Fe and inevitable impurities. The yield strength of the steel plate is more than or equal to 550MPa, the tensile strength is 640-820 MPa, and the elongation after fracture is more than or equal to 16%; the low-temperature toughness is excellent, and the impact energy at minus 60 ℃ is more than or equal to 100J; the tissue performance is uniform.

Description

550 MPa-grade marine steel with excellent low-temperature performance and manufacturing method thereof

Technical Field

The invention relates to the field of steel material preparation, in particular to 550 MPa-level ultrahigh-strength marine steel with large thickness and excellent low-temperature performance and a manufacturing method thereof.

Background

The steel is used as a key structural material of ocean engineering equipment and widely applied to offshore wind power, production platforms, submarine pipelines and the like. The service period of the marine engineering equipment is generally 30 years, which is 50% longer than that of the traditional ship, and the service environment of the marine engineering equipment is very severe, so that the marine engineering equipment is not only subjected to the action of self gravity load, but also influenced by sea conditions such as sea storms, ocean currents, seabed earthquakes and the like. In recent years, with the rapid development of offshore oil and natural gas, the requirement on high-strength, high-toughness and thick-specification steel for the ocean platform is increasing, and due to the special service environment of the steel, the steel for the ocean platform has strict requirements on the low-temperature impact toughness, the lamellar tearing resistance and other properties of the steel. In order to meet the requirements of ocean engineering on high-performance and high-service-safety steel plates, the ultrahigh-strength marine steel with excellent low-temperature performance is urgently needed to be developed.

Patent document CN109112429B discloses a FH550 grade thick plate with excellent low-temperature toughness and a manufacturing method thereof, wherein the sulfur and phosphorus content is controlled by adding proper Cu, Cr, Mo, Ni and microalloy elements, and three-stage controlled rolling and controlled cooling processes are adopted to produce FH550 ship and maritime work steel plate with the maximum thickness of 80mm, but the Cu content (0.5 wt.% to 0.7 wt.%) is high, which easily causes the phenomenon of copper brittleness, and the Cu content of the steel plate in a TMCP state specified in the relevant ship regulations is usually not higher than 0.55 wt.%, so that the application range of the product is limited.

The patent document CN103276301A discloses that "a steel with yield strength not less than 550MPa for low-temperature engineering and a production method thereof" is a steel plate with thickness of 80-100 mm manufactured by adopting continuous casting and TMCP process, the yield strength is not less than 550MPa, and the steel plate has excellent low-temperature impact toughness and impact energy at-60 ℃ is more than 100J. However, the Mn element content in the component elements is high (1.63-2.00%), MnS is easily formed to influence impact toughness, and meanwhile, the excessive Mn element content easily causes central segregation of a casting blank to reduce the core performance, and the steel plate core toughness level is not provided, so that the low-temperature performance of the steel plate cannot be comprehensively represented.

The patent document with publication number CN111534740A, "a 550MPa anti-fatigue high-strength high-toughness steel plate and a manufacturing method thereof," discloses a 550MPa anti-fatigue high-strength high-toughness steel plate and a manufacturing method thereof, but the invention patent adopts a two-stage rolling, quenching and tempering process, the manufacturing process and the working procedure are complex, the alloy and time cost is high, and the delivery period of the steel plate is long.

The patent document CN104404384B entitled "Steel plate for ocean engineering platform with 550 MPa-level Low compression ratio and high toughness and production method" discloses a method for producing a high-strength and high-toughness ocean engineering steel plate by adopting TMCP + T process under the conditions of low carbon content and low compression ratio. The high-temperature heat treatment process is adopted after rolling, so that the production period is prolonged; meanwhile, Al element is not added into the steel plate, but the lower limit of the content of the Al element is required by the international standard and relevant classification society specifications which are followed by the steel for ocean engineering at present, so that the application range of the product is limited.

Aiming at the defects, the 550 MPa-grade marine steel with the maximum thickness of 80mm and excellent low-temperature performance is obtained by adopting the coupled design of alloy component design, smelting, rolling control and cooling control.

Disclosure of Invention

The invention aims to provide 550 MPa-grade marine steel with excellent low-temperature performance and a manufacturing method thereof, and the steel plate has ultrahigh strength, the yield strength is more than or equal to 550MPa, the tensile strength is 640-820 MPa, and the elongation after fracture is more than or equal to 16%; the low-temperature toughness is excellent, and the impact energy at the 1/4 thickness and 1/2 thickness positions at 60 ℃ below zero is more than or equal to 100J; the tissue performance is uniform.

In order to achieve the purpose, the invention adopts the following technical scheme:

550 MPa-grade marine steel with excellent low-temperature performance comprises the following chemical components in percentage by weight: 0.05 to 0.09 percent of C, 0.1 to 0.3 percent of Si, 1.25 to 1.60 percent of Mn, less than or equal to 0.012 percent of P, less than or equal to 0.002 percent of S, 0.25 to 0.40 percent of Cu, 0.65 to 0.85 percent of Ni, 0.15 to 0.30 percent of Cr, 0.030 to 0.045 percent of Nb, 0.30 to 0.40 percent of Mo, 0.008 to 0.020 percent of Ti, 0.02 to 0.04 percent of Alt, and the balance of Fe and inevitable impurities.

The action mechanism of each alloy component in the steel is as follows:

c: the carbon-containing steel is an essential element for ensuring the strength, has an obvious effect on improving the strength of steel through solid solution strengthening and precipitation strengthening, but the increase of the carbon content seriously affects the welding performance and the low-temperature toughness of the steel, and the C content is preferably controlled to be 0.05-0.09% from the viewpoint of product performance.

Si: si: although Si is a solid solution strengthening element, although it is advantageous to improve the strength of the steel sheet and the oxidation resistance at high temperatures, Si promotes packet size coarsening, seriously deteriorates low-temperature toughness, elongation and weldability of the ultrahigh-strength steel sheet, and the Si content is preferably 0.1% to 0.3% in consideration of the economical efficiency and workability of steel making.

Mn: the most important alloying elements in the steel not only increase the strength of the steel sheet, but also have the effects of enlarging the austenite phase region, lowering the Ar3 point temperature, and refining ferrite grains to improve the low-temperature toughness of the steel sheet, but when the Mn element is too high in mass, the Mn element segregation deteriorates the low-temperature toughness of the core of the thick plate, and the performance of the welding heat affected zone is lowered, so the Mn content is preferably in the range of 1.25% to 1.60%.

P: the material P is an element which brings adverse effect on an impact value, can be segregated in the central part of a slab, is aggregated in a grain boundary and the like, and can damage low-temperature toughness, and the material P is controlled to be not higher than 0.012%.

S: the material is an element which brings adverse effect to an impact value, can form sulfide inclusions to become a crack source, and is controlled to be not higher than 0.002%.

Cu: cu mainly plays a role in solid solution and precipitation strengthening in steel, and a proper amount of Cu improves the strength without reducing the toughness and improves the corrosion resistance of the steel. Meanwhile, the proper amount of Cu added into the thick steel plate can also precipitate epsilon-Cu in the process of tempering after rolling, thereby improving the strength of the steel plate. However, when the Cu content is too high, hot embrittlement of the steel occurs during heating, the surface quality of the steel sheet deteriorates, toughness of the base material and the heat affected zone decreases, and a large amount of ε -Cu precipitates, resulting in a sharp increase in the yield ratio. Use with Ni can avoid hot shortness. The Cu content ranges from 0.25% to 0.40%.

Ni: the nickel is dissolved in austenite, thereby inhibiting austenite recrystallization, refining austenite grains and improving the low-temperature toughness of the steel plate. However, the production cost is obviously increased along with the increase of the content of nickel, so the content of Ni is controlled to be 0.55-0.85 percent by comprehensively considering the performance and the production cost of the steel plate.

Cr: the element capable of improving the hardenability and strength of the steel sheet is an element which narrows the austenite phase region, is a weak carbide-forming element, can form carbide in the steel or can be dissolved in ferrite, and Cr is an effective element for improving the hardenability of the steel, and can form a continuous solid solution with Fe. Cr is an element for effectively improving the strength of the steel plate, so that ferrite phase transformation obviously shifts to the right, the cooling rate interval of bainite phase transformation is widened, and the formation of a medium-temperature transformation structure is promoted. However, too high Cr content increases the tendency to temper brittleness and increases the difficulty of welding, while too low Cr content does not effectively exert its strengthening effect. The content of Cr in the invention is controlled to be 0.15-0.30%.

Nb: the niobium is added to promote the grain refinement of the steel rolling microstructure, can simultaneously improve the strength and the toughness, can effectively refine the microstructure by inhibiting austenite recrystallization in the controlled rolling process, and strengthens a matrix by precipitation. In the welding process, the segregation and precipitation of niobium atoms can prevent austenite grains from coarsening during heating, ensure that a finer heat affected zone structure is obtained after welding, and improve the welding performance. The Nb content is preferably controlled to 0.030% to 0.045%.

Mo: mo is an element for narrowing the austenite phase region, and also suppresses the decomposition of austenite, delays the transformation of grain boundary ferrite, and contributes to the formation of a bainite structure. Mo can improve the hardenability of steel, and Mo is a strong solid solution strengthening element, and can obviously improve the strength of steel through solid solution strengthening. The effects of phase change strengthening and dislocation strengthening are generated, and the strength and the structural uniformity of the steel are obviously improved. When the Mo content is lower than 0.10%, the improvement on the strength and the structure uniformity of the steel is not obvious; however, since the content of Mo is too high, on one hand, the cost is increased, and on the other hand, the toughness and the welding performance of the steel are reduced, the content of Mo is controlled to be 0.30-0.40% in the invention.

Ti: the nitride, carbide, or carbonitride is formed by the trace amount of the component, and has an effect of refining crystal grains and improving toughness of the base material. However, when the content exceeds 0.025%, the toughness of the base material and the welding heat-affected zone is lowered, and therefore, the content is preferably controlled to 0.008% to 0.020%.

And (3) Alt: the content of the deoxidizing and grain refining element to be added in the present invention is 0.01% or more, but if it exceeds 0.08%, hot cracking of the cast slab is likely to occur, and the toughness of the steel is lowered. The preferred content of Alt is controlled between 0.02 percent and 0.04 percent.

The yield strength of the marine steel plate is more than or equal to 550MPa, the tensile strength is 640-820 MPa, the elongation after fracture is more than or equal to 16%, and the impact energy at minus 60 ℃ is more than or equal to 100J.

The maximum thickness of the finished marine steel plate is 80 mm.

A manufacturing method of 550 MPa-grade marine steel with excellent low-temperature performance comprises the following steps:

1) smelting, continuous casting and slow cooling of casting blank: the method is characterized by adopting the processes of molten iron deep desulfurization, converter smelting, external refining, vacuum treatment and continuous casting to produce the continuous casting, wherein the superheat degree target value of the tundish molten steel is 20-30 ℃, the whole process is protected for casting, the soft reduction is 5-9 mm in cooperation with the soft reduction process, a continuous casting blank is obtained after continuous casting, the thickness of the continuous casting blank is 250-360 mm, the continuous casting blank is subjected to stacking and slow cooling after being taken off a line, the stacking temperature is more than or equal to 650 ℃, and the stacking time is more than or equal to 60 hours;

2) the rolling process comprises the following steps: the four-stage controlled rolling is adopted, the temperature of a casting blank soaking section ranges from 1130 ℃ to 1180 ℃, the first stage is high-temperature controlled rolling, the rolling temperature is 1000 ℃ to 1060 ℃ after widening is finished, the single-pass reduction rate of longitudinal rolling is more than or equal to 15%, the penetration of the reduction to a steel plate core part is ensured, and the as-cast structure of a plate blank is improved. The initial rolling temperature of the second stage is 900-950 ℃, the single-pass reduction rate is more than or equal to 12%, the cumulative reduction rate is more than or equal to 25%, the initial rolling temperature of the third stage is 840-880 ℃, the single-pass reduction rate is more than or equal to 12%, the cumulative reduction rate is more than or equal to 36%, the initial rolling temperature of the fourth stage is 740-780 ℃, the single-pass reduction rate is more than or equal to 10%, the cumulative reduction rate is more than or equal to 40%, and the final rolling temperature is 700-750 ℃;

3) and (3) a cooling process: after the steel plate is straightened, controlled cooling is carried out, a DQ + ACC rapid cooling system with the average cooling speed of more than or equal to 4 ℃/s is adopted for cooling, the starting cooling temperature is 680-730 ℃, and the temperature of red returning is 250-350 ℃;

4) the slow cooling process comprises the following steps: and (3) immediately placing the steel plate into a slow cooling pit after the controlled cooling is finished, and carrying out heat preservation and cooling on the slow cooling pit by adopting an alternative stacking mode with hot steel plates of more than 300 ℃, wherein the stacking time is more than or equal to 24 hours.

After the continuous casting blank in the step 1) is off-line, hot acid etching (the etching solution is 1:1 hydrochloric acid aqueous solution, the temperature is 75 +/-5 ℃, the etching time is 40min) is adopted for evaluation, and the evaluation is carried out according to a Mannesmann standard map, wherein the evaluation result is not more than 2 grade.

Compared with the prior art, the invention has the beneficial effects that:

1) the product steel plate produced by the process of the invention is subjected to clean smelting, and the casting blank segregation and the grain size of the rolled steel plate are controlled by combining a specific continuous casting process and a controlled rolling and controlled cooling process, so that the 550 MPa-level marine steel with the impact toughness of more than or equal to 100J at 60 ℃ is realized.

2) The invention gives full play to the technical equipment advantages of a wide and thick plate rolling mill, combines with a continuous casting slab with the thickness of 250-360 mm, and develops a 550 MPa-level ultrahigh-strength marine steel thick plate product with excellent low-temperature performance, wherein the maximum thickness of the finished product is 80 mm.

3) Reasonable straightening process and cooling control process are adopted to match, the plate flatness is guaranteed, and the unevenness of the steel plate within 2 meters is less than or equal to 6 mm.

(4) The microstructure of the steel plate is a bainite structure.

Drawings

FIG. 1 is a photograph (500 times) of a metallographic structure of example 1.

Detailed Description

The following examples are intended to illustrate the invention in detail, and are intended to be a general description of the invention, and not to limit the invention.

The chemical components of the steel of the embodiment of the invention are shown in Table 1, the smelting and continuous casting process parameters and the casting blank evaluation results of the steel of the embodiment of the invention are shown in Table 2, the rolling and cooling process of the steel plate of the embodiment of the invention is shown in Table 3, the mechanical properties of the steel plate of the embodiment of the invention are shown in Table 4, and the unevenness of the steel plate of the embodiment of the invention is shown in Table 5.

TABLE 1 chemical composition wt% of steel of examples of the invention

Examples	C	Si	Mn	P	S	Cu	Ni	Cr	Nb	Mo	Ti	Alt
													1	0.071	0.24	1.32	0.008	0.002	0.38	0.85	0.24	0.039	0.359	0.016	0.034
2	0.082	0.29	1.25	0.01	0.002	0.33	0.73	0.22	0.037	0.398	0.012	0.029
													3	0.065	0.16	1.45	0.009	0.002	0.35	0.76	0.28	0.031	0.318	0.011	0.024
4	0.088	0.13	1.31	0.008	0.002	0.28	0.69	0.25	0.044	0.332	0.01	0.037
													5	0.074	0.20	1.41	0.012	0.002	0.34	0.75	0.2	0.034	0.301	0.018	0.027
6	0.079	0.27	1.36	0.011	0.001	0.32	0.65	0.23	0.038	0.309	0.009	0.022
													7	0.061	0.20	1.51	0.009	0.001	0.31	0.79	0.19	0.035	0.339	0.013	0.035
8	0.056	0.19	1.59	0.01	0.002	0.27	0.58	0.16	0.042	0.341	0.014	0.031

TABLE 2 evaluation results of smelting and stacking process parameters and casting blank of steel according to the present invention

Table 3 rolling and cooling process of steel plate according to the embodiment of the present invention

TABLE 4 mechanical Properties of Steel sheets according to examples of the present invention

TABLE 5 unevenness of steel sheets according to examples of the present invention

Claims

1. The 550 MPa-grade marine steel with excellent low-temperature performance is characterized by comprising the following chemical components in percentage by weight: 0.05 to 0.09 percent of C, 0.1 to 0.3 percent of Si, 1.25 to 1.60 percent of Mn, less than or equal to 0.012 percent of P, less than or equal to 0.002 percent of S, 0.25 to 0.40 percent of Cu, 0.65 to 0.85 percent of Ni, 0.15 to 0.30 percent of Cr, 0.030 to 0.045 percent of Nb, 0.30 to 0.40 percent of Mo, 0.008 to 0.020 percent of Ti, 0.02 to 0.04 percent of Alt, and the balance of Fe and inevitable impurities.

2. The 550 MPa-grade marine steel with excellent low-temperature performance as claimed in claim 1, wherein the yield strength of the steel plate of the marine steel is not less than 550MPa, the tensile strength is 640-820 MPa, the elongation after fracture is not less than 16%, and the impact energy at-60 ℃ is not less than 100J.

3. The 550MPa grade marine steel with excellent low temperature performance according to claim 1, wherein the finished product of the marine steel plate has a maximum thickness of 80 mm.

4. A method for manufacturing 550MPa grade marine steel with excellent low temperature performance according to any one of claims 1 to 3, comprising the steps of:

1) the rolling process comprises the following steps: rolling is controlled by adopting four stages, wherein the temperature of a soaking section of a casting blank is 1130-1180 ℃, high-temperature rolling is controlled in the first stage, the rolling starting temperature is 1000-1060 ℃, the single-pass reduction rate of longitudinal rolling is more than or equal to 15%, the rolling starting temperature of the second stage is 900-950 ℃, the single-pass reduction rate is more than or equal to 12%, the cumulative reduction rate is more than or equal to 25%, the rolling starting temperature of the three stages is 840-880 ℃, the single-pass reduction rate is more than or equal to 12%, the cumulative reduction rate is more than or equal to 36%, the rolling starting temperature of the four stages is 740-780 ℃, the single-pass reduction rate is more than or equal to 10%, the cumulative reduction rate is more than or equal to 40%, and the final rolling temperature is 700-750 ℃;

2) and (3) a cooling process: after the steel plate is straightened, controlled cooling is carried out, a DQ + ACC rapid cooling system with the average cooling speed of more than or equal to 4 ℃/s is adopted for cooling, the starting cooling temperature is 680-730 ℃, and the temperature of red returning is 250-350 ℃;

3) the slow cooling process comprises the following steps: and (3) immediately placing the steel plate into a slow cooling pit after the controlled cooling is finished, and carrying out heat preservation and cooling on the slow cooling pit by adopting an alternative stacking mode with hot steel plates of more than 300 ℃, wherein the stacking time is more than or equal to 24 hours.

5. The method for manufacturing the 550 MPa-grade marine steel with excellent low-temperature performance according to claim 4, further comprising the following steps of smelting, continuous casting and slow cooling of a casting blank: the superheat degree target value of the tundish molten steel is 20-30 ℃, the whole process is protected and cast, a soft reduction process is matched, the soft reduction is 5-9 mm, the thickness of a continuous casting blank is 250-360 mm, the continuous casting blank is stacked and slowly cooled after being off-line, the stacking temperature is more than or equal to 650 ℃, and the stacking time is more than or equal to 60 hours; after the continuous casting billet is off-line, the hot acid etching is adopted for evaluation, the evaluation is carried out by referring to a Mannesmann standard map, and the rating result is less than or equal to level 2.