Quenched and tempered high-strength container steel Q690DR and production method thereof

Abstract

The invention discloses quenched and tempered high-strength container steel Q690DR. The thickness of a steel plate ranges from 120 mm to 200 mm, and the quenched and tempered high-strength container steel Q690DR comprises, by mass, 0.08 wt%-0.16 wt% of C, 0.17 wt%-0.30 wt% of Si, 1.40 wt%-1.7 wt% of Mn, smaller than or equal to 0.018 wt% of P, smaller than or equal to 0.005 wt% of S, smaller than or equal to 3.5 wt% of Cr + Mo + Ni, smaller than or equal to 0.1 wt% of V + Nb + Ti, 0.001 wt%-0.002 wt% of B, 0.015 wt%-0.050 wt% of Als and the balance Fe and residual elements. According to the steel plate obtained through water-cooling die casting, heating, rolling and quenched-tempered heat treatment, tempered sorbite serves as the main structure of the upper surface and the lower surface, a batten bainite tempered structure, a granular bainite tempered structure and tempered sorbite serve as the part with the 1 / 4 thickness, and the granular bainite tempered structure mainly serves as the part with the 1 / 2 thickness. The yield strength is controlled to be 720-760 MPa, the tensile strength is controlled to be 830-850 MPa, the elongation is controlled to be 22-24%, the transverse V-shaped impact stability at-40 DEG C is 100 J or above, and the use requirement of the high-strength container steel is completely met.

Description

Technical Field

[0001] The invention belongs to the technical field of ultra-thick steel plate production, and specifically relates to a quenched and tempered high-strength container steel Q690DR and a production method thereof. Background Art

[0002] The high strength of Q690DR steel plate ensures the pressure bearing capacity of the container, and its good low temperature toughness ensures safe use in low temperature environment. Since Q690DR steel plate can still maintain high impact absorption energy at -40℃, it is very suitable for manufacturing equipment working in low temperature environment, such as liquefied natural gas (LNG) storage tanks, refrigeration equipment, polar exploration equipment and other low temperature environment equipment.

[0003] For example, Chinese patent CN115747616B discloses a smelting method of P690QL2 marine tank steel, whose chemical composition by mass percentage is C: 0.07% ~ 0.09%, Si: 0.15% ~ 0.25%, Mn: 1.0% ~ 1.3%, P ≤ 0.013%, S ≤ 0.002%, Nb: 0.010% ~ 0.020%, V: 0.040% ~ 0.050%, Ti: 0.0 10%~0.020%, Ni: 1.0%~1.30%, Cr: 0.30%~0.50%, Mo: 0.30%~0.60%, Cu≤0.20%, Al: 0.040%~0.070%, Ca: 0.00050%~0.0050%, N≤0.0050%, H≤0.0003%, B: 0.0020%~0.0030%, the balance is Fe and inevitable impurities. This patent is limited to a smelting method, emphasizing that the thickness of 6-180mm P690QL2 can be smelted by this method, and there is no subsequent material forming process for smelting. The maximum thickness of the steel plate that can be produced is not clearly stated in the application examples. In addition, this solution uses continuous casting billets to form materials, and the rolling compression ratio is limited, especially for the internal quality of extra-thick plates and the effect of grain refinement is limited.

[0004] For example, publication number CN111850401A discloses a low-cost, high-strength and toughness pressure vessel steel plate and its production method, wherein the chemical composition of the steel plate is as follows by mass percentage: C: 0.13% to 0.18%, Si: 0.25% to 0.35%, Mn: 1.10% to 1.30%, P≤0.010%, S≤0.002%, Mo: 0.25% to 0.35%, Al: 0.030% to 0.040%, Ti: 0.070% to 0.130%. The technical solution adds 0.25% to 0.35% Mo and 0.070% to 0.130% Ti. Although the addition of a large amount of Ti element can improve the strength of the steel, it is easy to form compounds with elements such as C, N, O, and S, especially with a strong affinity for O, which affects the purity of the molten steel and has an adverse effect on the low-temperature impact performance and welding; and the production thickness of the steel plate is only 6-36mm.

[0005] In view of this, the present invention is proposed. Summary of the invention

[0006] In order to solve the above-mentioned technical defects, the purpose of the present invention is to provide a quenched and tempered high-strength container steel Q690DR, the yield strength of the steel plate is controlled at 720-760MPa, the tensile strength is controlled at 830-850MPa, the elongation is controlled at 22-24%, and the -40°C lateral V-shaped impact is stable at more than 100J, which fully meets the use requirements of high-strength container steel.

[0007] Another object of the present invention is to provide a method for producing quenched and tempered high-strength container steel Q690DR.

[0008] To achieve the above-mentioned purpose, the technical solution adopted by the present invention is: a quenched and tempered high-strength container steel Q690DR, the steel plate thickness is 120mm~200mm, and contains the following chemical components in mass percentage: C: 0.08~0.16, Si: 0.17~0.30, Mn: 1.40~1.7, P≤0.018, S≤0.005, Cr+Mo+Ni≤3.5, V+Nb+Ti≤0.1, B: 0.001~0.002, Als: 0.015~0.050, and the rest are Fe and residual elements.

[0009] It should be noted that Ni can reduce the resistance to dislocation movement in steel, relax stress, and then change the substructure of the matrix organization. The lattice constant of Ni is similar to that of γ-iron, so it can form a continuous solid solution, while lowering the critical point and increasing the stability of austenite; Mo can reduce the overheating tendency of steel, and combined with Ni can greatly improve the hardenability, improve strength, hardness, thermal stability, inhibit and reduce temper brittleness, and improve impact toughness.

[0010] The production method of the quenched and tempered high-strength container steel Q690DR steel plate comprises the following steps:

[0011] ① Water-cooled mold casting: select an ingot mold with a thickness of ≥900mm, control the superheat at 30-35℃, the main body casting time is 12-20min, the cap casting time is 2-6min, the cap is removed after 7h, and the ingot is removed after 9h; use thick steel ingots to ensure a large compression ratio of the steel plate and the quality of the core.

[0012] ② Heating: After the steel ingot is put into the furnace, it is simmered for 2 hours. The heating rate below 1000℃ is less than 100℃ / h, and the heating rate above 1000℃ is less than 130℃ / h. The maximum heating temperature of the steel ingot is controlled at ≤1320℃, and the heating time is 0.8~1.0min / mm;

[0013] ③ Rolling: Two-stage rolling is adopted, the first-stage rolling temperature is 1050℃~1150℃, the pass reduction is 70~80mm, the steel is dried 1.5~2 times the thickness of the finished product, the second-stage rolling temperature is ≤950℃, the final rolling temperature is 820~920℃, and the steel is cooled by ACC after rolling, and the red-return temperature is 580~600℃;

[0014] ④Quenching and tempering heat treatment: quenching heating temperature 880~920℃, holding time 2~2.4min / mm, control quenching cooling rate 0.4~1.1℃ / S; tempering holding temperature 550~620℃, holding time 3~4min / mm.

[0015] The steel plate obtained by this scheme has a main structure of tempered bainite on the upper and lower surfaces, a tempered lath bainite structure + a tempered granular bainite structure + a tempered bainite structure at 1 / 4 thickness, and a tempered granular bainite structure at 1 / 2 thickness. Its yield strength is controlled at 720-760MPa, the tensile strength is controlled at 830-850MPa, the elongation is controlled at 22-24%, and the -40℃ transverse V-shaped impact is stable at more than 100J, which fully meets the use requirements of high-strength container steel. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] The present invention will be further described in detail below in conjunction with the accompanying drawings.

[0017] Figure 1 This is the metallographic structure diagram of the surface of the 140 mm thick steel plate of the present invention.

[0018] Figure 2 This is the metallographic structure diagram of the 140 mm thick steel plate at 1 / 4 of the thickness of the present invention.

[0019] Figure 3 It is the metallographic structure diagram of the 140 mm thick steel plate at 1 / 2 thickness of the present invention. DETAILED DESCRIPTION

[0020] In order to make the purpose, technical scheme and advantages of the embodiments of the present invention clearer, the technical scheme in the embodiments of the present invention will be described clearly and completely below. If the specific conditions are not specified in the embodiments, they are carried out according to conventional conditions or conditions recommended by the manufacturer. If the manufacturer of the reagents or instruments used is not specified, they are all conventional products that can be purchased commercially.

[0021] The features and performance of the present invention are further described in detail below in conjunction with the embodiments.

[0022] A quenched and tempered high-strength container steel Q690DR is produced with the chemical composition percentages shown in the following table:

[0023]

[0024] The production of the steel includes the following process points:

[0025] a. KR hot metal pretreatment: the hot metal arriving at the station must be slag-free before and after slag-free to ensure that the slag layer thickness on the liquid surface is ≤20mm. After the hot metal is desulfurized by KR stirring, the hot metal S is ≤0.005%, the desulfurization cycle is ≤21min, and the desulfurization temperature drop is ≤20℃;

[0026] b. Converter smelting: 100 / 120 ton top and bottom double-blown converter is used, S≤0.005%, P≤0.080% in the molten iron, the molten iron temperature is ≥1270℃, the error of molten iron loading is controlled at ±1t, high-quality scraps are strictly used for scrap steel, the process gun position is controlled at 1.0-1.3m in the early stage, 1.2-1.6m in the middle stage, and 1.0-1.1m in the late stage, the slag basicity R is controlled at 2.5-4.0, the steel tapping target P≤0.015%, C≥0.05%, S≤0.012%, silicon aluminum barium calcium, manganese iron alloy, silicon iron alloy and lime, fluorite are added to the ladle during the steel tapping process, slag stopper is used to block the slag before steel tapping, slag stopper cone is used to block the slag before steel tapping is completed, and the slag layer thickness is guaranteed to be ≤30mm. Argon blowing is required throughout the converter steel tapping process;

[0027] c. Argon blowing treatment: Aluminum wire is added to the argon station at one time. Argon is blown strongly for 3 minutes at the argon station with a flow rate of 200-500NL / min. The diameter of the naked eye of the steel liquid surface is controlled at 300-500mm. The temperature away from the argon station shall not be lower than 1570℃.

[0028] d.LF refining: During the refining process, argon is blown throughout the whole process, and refined slag is added. The basicity is controlled at 4.0-6.0. Deoxidizers mainly composed of calcium carbide, aluminum particles, and ferrosilicon powder are added. The amount added is appropriate according to the oxygen content in the molten steel and the white slag production. During the heating process, appropriate current is selected for heating according to the rhythm surplus and temperature conditions. The heating time is controlled twice, the first heating is 7-12 minutes, and the second heating is 6-10 minutes. During the second heating process, deoxidizer is added, and the number of slag sticking is required to be greater than 6 times. Silicon calcium wire is added before leaving the station, and argon must be turned off before adding silicon calcium wire;

[0029] e. VD refining: VD vacuum must be below 67Pa, pressure holding time must be ≥15min, soft blowing for 2-5min or no blowing after breaking vacuum, molten steel must not be exposed during soft blowing, vacuum pumping time for online package is 1.7min, covering agent must cover the steel liquid surface, argon must be turned off before adding covering agent, and steel temperature is 1565±15℃;

[0030] f. Mold casting: select an ingot mold with a thickness of ≥900mm, control the superheat at 30-35℃, the main body pouring time is 12-20min, the cap casting time is 2-6min, the protective slag is added at 70kg / piece, the cap is removed after 7h, and the ingot is removed after 9h;

[0031] g. Heating: After the steel ingot is put into the furnace, it is simmered for 2 hours. The heating rate below 1000℃ is less than 100℃ / h, and the heating rate above 1000℃ is less than 130℃ / h. The heating temperature of the steel ingot is controlled at ≤1320℃, and the heating time is 0.8~1.0min / mm;

[0032] h. Rolling: Two-stage rolling is adopted. The first-stage rolling temperature is 1050℃~1150℃, the pass reduction is 70~80mm, and the steel is dried 1.5~2 times the thickness of the finished product. The second-stage rolling temperature is ≤950℃, the final rolling temperature is 820~920℃, and the steel is cooled by ACC after rolling. The temperature of returning to red is 580~600℃.

[0033] i. Stack cooling: The stack slow cooling temperature shall not be lower than 450℃, and the stack cooling time shall be ≥48 hours;

[0034] j. Quenching and tempering: quenching heating temperature 880 ~ 920 ℃, holding time 2 ~ 2.4min / mm, quenching cooling rate controlled at 0.4 ~ 1.1℃ / S, of which the cooling rate of 200mm thick steel plate is controlled at 0.42℃ / S and the cooling rate of 140mm thick steel plate is controlled at 0.62℃ / S; tempering holding temperature 550 ~ 620℃, holding time 3 ~ 4min / mm.

[0035] The mechanical properties of the steel plate obtained in the example were tested, and the test results are shown in the following table:

[0036]

[0037] This time, two batches of 140mm and 200mm thick Q690DR were trial-produced. Through reasonable chemical composition design and production process control, the quenched and tempered high-strength container steel Q690DR steel plate was successfully developed. Its yield strength is controlled at 720-760MPa, tensile strength is controlled at 830-850MPa, elongation is controlled at 22-24%, and the -40℃ transverse V-shaped impact is stable at more than 100J, which fully meets the use requirements of high-strength container steel.

[0038] The metallographic structure of the steel plate obtained in the example was tested. The upper and lower surface structures were mainly tempered bainite, the 1 / 4 thickness was tempered lath bainite + granular bainite + tempered bainite, and the 1 / 2 thickness was mainly tempered granular bainite. The test results are shown in the attached figure.

Claims

1. A quenched and tempered high-strength container steel Q690DR, characterized in that: The thickness of the steel plate is 120mm~200mm, and it contains the following chemical compositions in mass percentage: C: 0.08~0.16, Si: 0.17~0.30, Mn: 1.40~1.7, P≤0.018, S≤0.005, Cr+Mo+Ni≤3.5, V+Nb+Ti≤0.1, B: 0.001~0.002, Als: 0.015~0.050, and the others are Fe and residual elements; the yield strength is controlled at 720~760MPa, the tensile strength is controlled at 830~850MPa, the elongation is controlled at 22~24%, and the -40℃ lateral V-shaped impact is stable at more than 100J.

2. The method for producing the quenched and tempered high-strength container steel Q690DR according to claim 1, characterized in that Including water-cooled mold pouring, heating, rolling and quenching and tempering heat treatment, as follows: ① Water-cooled mold casting: select an ingot mold with a thickness of ≥900mm, control the superheat at 30-35℃, the main body casting time is 12-20min, the cap casting time is 2-6min, the cap is removed after 7h, and the ingot is removed after 9h; ② Heating: After the steel ingot is put into the furnace, it is simmered for 2 hours. The heating rate below 1000℃ is less than 100℃ / h, and the heating rate above 1000℃ is less than 130℃ / h. The maximum heating temperature of the steel ingot is controlled at ≤1320℃, and the heating time is 0.8~1.0min / mm; ③ Rolling: Two-stage rolling is adopted, the first-stage rolling temperature is 1050℃~1150℃, the pass reduction is 70~80mm, the steel is dried 1.5~2 times the thickness of the finished product, the second-stage rolling temperature is ≤950℃, the final rolling temperature is 820~920℃, and the steel is cooled by ACC after rolling, and the red-return temperature is 580~600℃; ④Quenching and tempering heat treatment: quenching heating temperature 880~920℃, holding time 2~2.4min / mm, control quenching cooling rate 0.4~1.1℃ / S; tempering holding temperature 550~620℃, holding time 3~4min / mm.

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