Production method of low-temperature container steel 06Ni7DR with thickness of 10-100 mm

Abstract

The invention discloses a production method of low-temperature container steel 06Ni7DR with the thickness of 10-100 mm. The steel plate comprises the following chemical components in percentage by mass (wt%): 0.04-0.07% of C, 0.4-0.7% of Mn, 0.05-0.30% of Si, less than or equal to 0.008% of P, less than or equal to 0.003% of S, 6.5-7.5% of Ni, less than or equal to 0.03% of Nb, less than or equal to 0.01% of V, less than or equal to 0.50% of Cr, less than or equal to 0.10% of Mo and the balance of Fe and inevitable impurities. A low C + Ni and Mo alloying component system is adopted, rolling, controlled cooling, stack cooling, subcritical quenching and tempering heat treatment are carried out, the obtained low-temperature container steel 06Ni7DR with the thickness being 10-100 mm is obtained, the yield strength is larger than or equal to 560 MPa, the tensile strength is 680-820 MPa, and the low-temperature impact energy at the temperature of-196 DEG C is larger than or equal to 100 J. On the premise that the alloy cost is reduced, the process design is reasonable, and the requirement of low-temperature container steel which is stable and reliable in performance and completely meets the requirement can be obtained by adopting a short production process.

Description

Technical Field

[0001] The invention belongs to the field of medium and thick plate production, and specifically relates to a production method of 06Ni7DR steel for low-temperature containers with a thickness of 10 to 100 mm. Background Art

[0002] With the development of industrial technology, the production, storage, and transportation of various liquefied petroleum gases, liquid ammonia, liquid oxygen, liquid hydrogen, liquid nitrogen, etc., as well as the development of marine engineering and cold regions, have put forward higher and higher requirements for low-temperature steel. As a special equipment steel, the main indicator of pressure vessel steel is low-temperature toughness, including low-temperature impact toughness and tough-brittle transition temperature. At present, the most widely used low-temperature container steel is 06Ni9DR, but due to the continuous increase in the market Ni gold price, 06Ni9DR is expensive, and every 1% reduction in Ni content in LNG steel can reduce production costs by 5%. Under the premise of ensuring performance, the development of low-Ni steel plates has become an important direction.

[0003] However, after the Ni content is reduced, how to ensure the reasonable matching of -196℃ low-temperature impact toughness and strength is a difficult problem facing the metallurgical industry, and there is no public report yet. Therefore, under the premise of advocating green environmental protection, energy saving and cost reduction, the production method of low-temperature container storage tanks is particularly important. In view of this, the present invention is specially proposed. Summary of the invention

[0004] In order to meet the above technical requirements, the purpose of the present invention is to provide a production method of 06Ni7DR steel for low-temperature containers with a thickness of 10 to 100 mm. The solution has low alloy cost, reasonable process design, short entire production process, and the obtained low-temperature container steel has excellent performance.

[0005] To achieve the above purpose, the technical solution adopted by the present invention is as follows: low temperature steel is required to have high strength and high toughness, and must also have good welding performance, easy processing performance, etc. In view of its special requirements, in addition to adding a certain amount of solid solution strengthening element Ni to the steel, it is necessary to adopt microalloying technology to add carbide forming elements to the steel, and utilize the precipitation strengthening and grain refinement effects produced by them to greatly improve the comprehensive performance of the steel. In addition to the key alloying element Ni that improves low temperature toughness, and the basic permanent elements C, Si, Mn, P, and S, a trace amount of Mo alloying element needs to be added.

[0006] Therefore, the present invention provides a method for producing 06Ni7DR steel for low-temperature containers with a thickness of 10 to 100 mm, using the following chemical components in mass percentage: C: 0.04 to 0.07%, Mn: 0.4 to 0.7%, Si: 0.05 to 0.30%, P≤0.008%, S≤0.003%, Ni: 6.5 to 7.5%, Nb≤0.03%, V≤0.01%, Cr≤0.50%, Mo≤0.10%, and the balance is Fe and unavoidable impurities;

[0007] The production method of the above-mentioned low-temperature container steel 06Ni7DR includes rolling, controlled cooling, stack cooling and heat treatment, which are specifically as follows:

[0008] 1) Rolling: Two-stage rolling is adopted. According to the relationship curve between the austenite recrystallization termination temperature and the element solid solution content of the refined grain, the rolling temperature in the recrystallization zone is determined to be above 980℃. The rolling temperature in the first stage is 1020~1060℃. High reduction rolling is adopted to ensure that the rolling force fully penetrates and bridges the internal segregation and loose defects. The total reduction rate is ≥60%. When the temperature of the steel plate drops to 1000℃~1020℃, the rolling is stopped. The steel plate swings on the roller table to dry the steel. According to the thickness of the finished steel plate, the intermediate billet drying thickness value, the finishing rolling temperature value entering the second stage, the final rolling temperature value, the temperature value when entering ACC cooling and the temperature value of the steel plate returning to red are determined by the following parameter table:

[0009]

[0010]

[0011] 2) Controlled cooling: Slow cooling after rolling can easily cause grain growth. The water entry temperature and red-return temperature of the steel plate refer to the requirements in Table 1. Control the cooling rate to 5-10℃ / s, water temperature to 20-25℃, water ratio to 2-2.05, and roller speed to 0.8-1.3m / s;

[0012] 3) Stack cooling: After the steel plate is off the production line, it is placed in a slow cooling pit for stacking and slow cooling. The stack cooling temperature is ≥350°C and the stack cooling time is ≥48 hours.

[0013] 4) Heat treatment: including sub-temperature quenching and tempering, wherein the sub-temperature quenching process is carried out in a non-oxidizing quenching furnace, the first heating and holding temperature is 790-800°C, the holding time is 1.0-1.2min / mm, the second heating and holding temperature is 800-810°C, the holding time is 0.8-1.0min / mm, after the holding is completed, a quenching machine is used to accelerate cooling to room temperature, and the cooling rate is controlled at 0.1-5°C / S; the tempering process is carried out in a normalizing furnace with nitrogen protection, the tempering temperature is 600-640°C, the holding time is 3.0-5.0min / mm, and the holding is completed and air-cooled to room temperature.

[0014] It should be noted that, compared with the existing heat treatment process, the present invention adopts a lower quenching temperature to ensure that the grain size of the steel plate is relatively small. On the one hand, the reduction of grain diameter increases the density of large-angle grain boundaries in the plastic zone at the crack front, which is conducive to hindering the rapid expansion of cracks, and the grain boundary itself also has an inhibitory effect on crack expansion. The increase in grain boundary area can improve the toughness of the material; on the other hand, since the reversed austenite is mainly distributed at interfaces such as grain boundaries, the smaller the grain size, the more uniformly dispersed the distribution of the reversed austenite, and the low-temperature toughness of the steel is also improved.

[0015] The 10-100 mm thick low-temperature container steel 06Ni7DR obtained by the present invention has a yield strength of ≥560MPa, a tensile strength of 680-820MPa, and a -196°C low-temperature impact energy of ≥100J. It has stable and reliable performance and fully meets the requirements of low-temperature container steel. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] The technical features of the present invention are further described below in conjunction with the accompanying drawings and embodiments:

[0017] Figure 1 Schematic diagram of the metallographic structure of the 12 mm thick quenched and tempered surface of the embodiment of the present invention (×

[0018] 500).

[0019] Figure 2 Schematic diagram of the metallographic structure of 1 / 4 of the 12 mm thick quenched and tempered state of the embodiment of the present invention (×

[0020] 500).

[0021] Figure 3 Schematic diagram of the metallographic structure of 1 / 2 of the 12 mm thick quenched and tempered state of the embodiment of the present invention (×

[0022] 500).

[0023] Figure 4 Schematic diagram of the metallographic structure of the 30 mm thick quenched and tempered surface of the embodiment of the present invention (×

[0024] 500).

[0025] Figure 5 Schematic diagram of the metallographic structure of 1 / 4 of the 30 mm thick quenched and tempered state of the embodiment of the present invention (×

[0026] 500).

[0027] Figure 6 Schematic diagram of the metallographic structure of 1 / 2 of the 30 mm thick tempered state in an embodiment of the present invention (×500).

[0028] Figure 7 This is a schematic diagram of the metallographic structure of the 100 mm thick tempered surface of an embodiment of the present invention (×500).

[0029] Figure 8 Schematic diagram of the metallographic structure of 1 / 4 of the 100mm tempered state in an embodiment of the present invention (×500).

[0030] Fig. 9 This is a schematic diagram of the metallographic structure of 1 / 2 of the 100mm quenched and tempered state of an embodiment of the present invention (×500).

[0031] 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.

[0032] The features and performance of the present invention are further described in detail below in conjunction with the embodiments.

[0033] Several cryogenic container steels 06Ni7DR with different thicknesses are produced using the mass percentage chemical composition shown in the following table:

[0034]

[0035] The production of the steel includes rolling, controlled cooling, stack cooling and heat treatment of 350mm thick continuous casting billets, as follows:

[0036] a. Rolling: Type II controlled rolling is adopted, and the rolling temperature is controlled between 1000 and 1020°C. High reduction rolling is adopted, and the rolling is fully penetrated to bridge the internal segregation and loose defects. The total reduction rate is 62%; when the temperature of the rolled piece meets the finishing temperature in Table 1, the second stage of rolling begins, and the first pass uses water cooling in front of the rolling mill. During the rolling process, attention should be paid to ensuring a good plate shape, so as to ensure deformation penetration so that the internal grains of austenite are flattened and elongated, increase the effective area of ​​the grain boundary and effectively form a large number of deformation bands, provide more nucleation points for austenite phase transformation, and achieve the purpose of refining austenite grains; the process parameters during rolling are shown in Table 1.

[0037] Table 1

[0038]

[0039] b. Controlled cooling: Since slow cooling after rolling can easily cause grain growth, the water inlet temperature and the red-return temperature refer to the requirements in Table 1, cooling rate>8℃ / s, water temperature 23℃, water ratio 2.0, flow rate 180m3 / h; roller speed: 12mm thick steel plate roller speed 1.3m / s, 30mm thick steel plate roller speed 1.1m / s, 100mm thick steel plate roller speed 0.8m / s.

[0040] c. Stack cooling: After the steel plate comes off the line, it is placed in a slow cooling pit for stacking and slow cooling. The stack cooling temperature is ≥350℃ and the stack cooling time is ≥48 hours to eliminate the mechanical and structural stress generated during the rolling of the steel plate.

[0041] d. Heat treatment: including sub-temperature quenching and tempering. The sub-temperature quenching process is carried out in a non-oxidizing quenching furnace. The heat treatment process is LT, which is different from the previous QLT (high temperature quenching + sub-temperature quenching + tempering) process. A lower quenching temperature is used to obtain a steel plate with a finer grain size. On the one hand, the reduction in grain diameter increases the density of large-angle grain boundaries in the plastic zone at the crack front, which is beneficial to hinder the rapid expansion of cracks. The grain boundary itself also has an inhibitory effect on crack expansion. The increase in grain boundary area can improve the toughness of the material; on the other hand, since the reversed austenite is mainly distributed at interfaces such as grain boundaries, the smaller the grain size, the more uniform and dispersed the distribution of the reversed austenite, and the low-temperature toughness of the steel is also improved. The insulation temperature of the first heating and insulation zone is 790-800℃, and the insulation time is 1.0min / mm. The insulation temperature of the second heating and insulation zone is 800-810℃, and the insulation time is 0.8min / mm. After the insulation is completed, a quenching machine is used to accelerate cooling to room temperature, the cooling water temperature is 15-22℃, the quenching water pressure is 1.0MPa, and the cooling rate is 0.1-5℃ / S; the tempering process is carried out in a normalizing furnace protected by nitrogen, the tempering temperature is 600-620℃, the insulation time is 3.0min / mm, and the insulation is completed and air-cooled to room temperature. The mechanical properties of the steel plate obtained in the example were analyzed according to GB / T713, and the performance indicators are shown in Table 2 below.

[0042] Table 2 Mechanical properties of 06Ni7DR steel plate

[0043]

[0044]

[0045] It can be seen from the above table that the low-temperature container steel 06Ni7DR obtained by this scheme fully meets the requirements of yield strength ≥560MPa, tensile strength 680~820MPa, -196℃ low-temperature impact energy ≥100J, and fully meets the needs of low-temperature container steel; the metallographic structure test results are shown in the attached figure.

[0046] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A method for producing 06Ni7DR steel for low-temperature containers with a thickness of 10 to 100 mm, characterized in that: The following chemical composition by mass percentage is adopted: C: 0.04-0.07%, Mn: 0.4-0.7%, Si: 0.05-0.30%, P≤0.008%, S≤0.003%, Ni: 6.5-7.5%, Nb≤0.03%, V≤0.01%, Cr≤0.50%, Mo≤0.10%, and the balance is Fe and unavoidable impurities; The production method of the above-mentioned low-temperature container steel 06Ni7DR includes rolling, controlled cooling, stack cooling and heat treatment, which are specifically as follows: 1) Rolling: Two-stage rolling is adopted. According to the relationship curve between the austenite recrystallization termination temperature and the element solid solution content of the refined grain, the rolling temperature in the recrystallization zone is determined to be above 980°C. The rolling temperature in the first stage is 1020-1060°C. High reduction rolling is adopted to ensure that the rolling force fully penetrates and bridges the internal segregation and loose defects. The total reduction rate is ≥60%. When the temperature of the steel plate drops to 1000°C-1020°C, the rolling is stopped. The steel plate is swung on the roller table to dry the steel. According to the thickness of the finished steel plate, the intermediate billet drying thickness value, the finishing rolling temperature value entering the second stage, the final rolling temperature value, the temperature value when entering ACC cooling, and the temperature value for returning the steel plate to red are determined through the following parameter table; 2) Controlled cooling: Slow cooling after rolling can easily cause grain growth. The water entry temperature and red-return temperature of the steel plate refer to the requirements in Table 1. Control the cooling rate to 5-10℃ / s, water temperature to 20-25℃, water ratio to 2-2.05, and roller speed to 0.8-1.3m / s; 3) Stack cooling: After the steel plate is off the production line, it is placed in a slow cooling pit for stacking and slow cooling. The stack cooling temperature is ≥350°C and the stack cooling time is ≥48 hours. 4) Heat treatment: including sub-temperature quenching and tempering, wherein the sub-temperature quenching process is carried out in a non-oxidizing quenching furnace, the first heating and holding temperature is 790-800°C, the holding time is 1.0-1.2min / mm, the second heating and holding temperature is 800-810°C, the holding time is 0.8-1.0min / mm, after the holding is completed, a quenching machine is used to accelerate cooling to room temperature, and the cooling rate is controlled at 0.1-5°C / S; The tempering process is carried out in a normalizing furnace protected by nitrogen, the tempering temperature is 600-640°C, the holding time is 3.0-5.0 min / mm, and the holding is completed and then air-cooled to room temperature.

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