Dual-phase low-temperature steel and heat treatment process thereof

Abstract

The invention discloses double-phase low-temperature steel and a heat treatment process thereof, and relates to the technical field of metallurgy, and the double-phase low-temperature steel comprises the following chemical components in percentage by mass: 0.010-0.025% of C, 3.6-4.4% of Mn, 0.6-1.0% of Ni, 0.01-0.05% of Nb, less than or equal to 0.1% of Si, less than or equal to 0.002% of S, less than or equal to 0.005% of P and the balance of Fe and inevitable impurities. According to the method, the steel plate is heated to 770-810 DEG C, austenitizing can be completed through the heat preservation time of 30-60 min, meanwhile, alloy elements are evenly distributed sufficiently, cooling is accelerated in a water cooling mode after austenitizing, carbide precipitation is inhibited, and a quenched martensite structure is obtained through quenching. According to the tempering process, the steel plate is heated to 620-635 DEG C, heat preservation is conducted for 60-90 min, reversed austenite with the volume fraction being about 10% can be formed, and alloy elements are enriched so that enough heat stability can be obtained.

Description

Technical Field

[0001] The invention relates to the technical field of metallurgy, and in particular to a dual-phase low-temperature steel and a heat treatment process thereof. Background Art

[0002] Currently, the commonly used nickel-based low-temperature steel uses Ni as the main alloying element, and the high content of Ni can greatly reduce the tough-brittle transition temperature. Depending on the use temperature, the Ni content of this type of low-temperature steel can be 1.5%, 3.5%, 5% and 9%, and the Charpy impact test temperatures are -80℃, -110℃, -120℃ and -196℃, respectively, and the yield strength is higher than 275MPa, 345MPa, 390MPa and 575MPa, respectively. Among them, 5Ni and 9Ni steels are the most widely used.

[0003] Ni is the most commonly used alloying element for low-temperature steel. This element can form an α or γ solid solution with Fe. While improving the strength-toughness match of the ferrite phase, it can also improve toughness by stabilizing the austenite phase, significantly reducing the toughness-brittle transition temperature. However, the large addition of Ni increases the material cost. As another beneficial element for low-temperature steel, Mn can reduce the toughness-brittle transition temperature and improve low-temperature toughness, and has a significant price advantage over Ni. If a portion of Ni can be replaced with an appropriate amount of Mn, the alloy cost of low-temperature steel can be reduced and the economy of low-temperature steel can be improved.

[0004] In recent years, high manganese low temperature steel has been developed at home and abroad. The manganese content is as high as 25%, and it also contains relatively high C and Cr elements. It has excellent performance at -196℃ or even lower temperatures, and is expected to become a new generation of economical low temperature materials to replace 9Ni steel. However, when high manganese low temperature steel is compared with 5Ni steel, it is difficult to reflect the economic advantage. In this case, it is necessary to develop corresponding economical alternative materials. Summary of the invention

[0005] The technical problem to be solved by the present invention is to overcome the disadvantages of the prior art and provide a dual-phase low-temperature steel and a heat treatment process thereof.

[0006] In order to solve the above technical problems, the technical solution of the present invention is as follows: A dual-phase low-temperature steel, whose chemical composition and mass percentage are: C: 0.010%-0.025%, Mn: 3.6%-4.4%, Ni: 0.6%-1.0%, Nb: 0.01%-0.05%, Si: ≤0.1%, S: ≤0.002%, P: ≤0.005%, and the balance is Fe and unavoidable impurities.

[0007] As a preferred embodiment of the dual-phase low-temperature steel of the present invention, the dual-phase low-temperature steel has a dual-phase structure of tempered martensite + austenite, and the volume fraction of austenite is 9%-12%.

[0008] As a preferred embodiment of the dual-phase low-temperature steel of the present invention, the dual-phase low-temperature steel has a steel plate thickness of 12-40 mm, a yield strength of 463-531 MPa, and a -120°C Charpy impact absorption energy of 102-134 J.

[0009] The present invention also provides a heat treatment process for dual-phase low-temperature steel, comprising: Quenching: heating the steel plate to a first preset temperature and keeping it warm, then water cooling it to room temperature; Tempering: Heat the steel plate to the second preset temperature and keep it warm, then air cool it to room temperature.

[0010] As a preferred solution of the heat treatment process of the dual-phase low-temperature steel of the present invention, wherein: the first preset temperature is 770-810°C.

[0011] As a preferred solution of the heat treatment process of the dual-phase low-temperature steel of the present invention, the holding time of the quenching process is 30 to 60 minutes.

[0012] As a preferred solution of the heat treatment process of the dual-phase low-temperature steel of the present invention, wherein: the second preset temperature is 620-635°C.

[0013] As a preferred solution of the heat treatment process of the dual-phase low-temperature steel of the present invention, the holding time of the tempering process is 60 to 90 minutes.

[0014] The beneficial effects of the present invention are: During the heat treatment of the present invention, the steel plate is first heated to 770-810°C, which is in the austenite phase region. The holding time of 30-60 minutes can make the austenitization complete and make the alloy elements distributed uniformly enough. Too high temperature or too long holding time will cause coarse grains and reduce toughness. After austenitization, the cooling is accelerated by water cooling to inhibit the precipitation of carbides, and quenching is performed to obtain a quenched martensite structure. In the tempering heat treatment process, the steel plate is heated to 620-635°C, which is in the two-phase region of the composition of the present invention. During the holding process of 60-90 minutes, a reverse transformation austenite with a volume fraction of about 10% can be formed, and the alloy elements are enriched to obtain sufficient thermal stability. It can still maintain a face-centered cubic structure at a temperature of -120°C without phase change, which is a beneficial second phase for improving low-temperature toughness. Another function of tempering is to restore the martensitic structure and improve the toughness of the matrix itself. After heat treatment, a dual-phase structure of tempered martensite + austenite is obtained. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings required for use in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other accompanying drawings can be obtained based on these accompanying drawings without paying creative labor.

[0016] Figure 1 This is a scanning electron microscope image of the metallographic structure of the dual-phase low-temperature steel in Example 1. DETAILED DESCRIPTION

[0017] In order to make the content of the present invention more clearly understood, the present invention is further described in detail below according to specific implementation modes and in combination with the accompanying drawings.

[0018] The present invention provides a dual-phase low-temperature steel, the chemical composition and mass percentage of which are: C: 0.010%-0.025%, Mn: 3.6%-4.4%, Ni: 0.6%-1.0%, Nb: 0.01%-0.05%, Si: ≤0.1%, S: ≤0.002%, P: ≤0.005%, and the balance is Fe and inevitable impurities. The dual-phase low-temperature steel has a dual-phase structure of tempered martensite + austenite, wherein the volume fraction of austenite is 9%-12%.

[0019] Among them, the C element can improve the strength through solid solution strengthening or precipitation strengthening, and can stabilize the austenite phase, but in order to reduce the tough-brittle transition temperature of the material, the C content should be reduced as much as possible. In addition, C is not conducive to the weldability of the material. Therefore, the present invention controls the C content to 0.010%-0.025%.

[0020] Mn is the main alloying element of the low-temperature steel of the present application. Mn is a ferrite strengthening element and also an austenite stabilizing element. Therefore, Mn can replace the more expensive Ni to a certain extent. In order to make the material have excellent low-temperature toughness without adding the Ni element, the Mn addition amount needs to be higher than the content range of low-alloy steel, but too high Mn content will increase the degree of segregation, increase the difficulty of smelting and increase the material cost. The present invention controls the Mn content to 3.6%-4.4%.

[0021] After adding a suitable amount of Mn, a small amount of Ni is also required. Ni is mainly used to stabilize austenite. In the present invention, the Ni content is controlled at 0.6%-1.0%. In the present application, a trace amount of Nb is added, which can hinder the grain boundary migration during high-temperature heating in the form of second phase precipitation, thereby refining the grains and improving the mechanical properties. The addition amount is controlled in the range of 0.01%-0.05%.

[0022] Si is a deoxidizing element in the steelmaking process, but Si also causes the ductile-brittle transition temperature to rise, so the upper limit of Si content needs to be controlled for low-temperature steel. The present invention controls Si to ≤ 0.1%.

[0023] S is easy to form MnS with Mn, and P is easy to segregate at the grain boundary and reduce the grain boundary bonding force. In order to improve the low-temperature toughness of the material, S and P need to be controlled to the minimum. The present invention controls S≤0.002% and P≤0.005%.

[0024] The thickness of the dual-phase low-temperature steel plate is 12-40mm, the yield strength is 463-531MPa, and the -120℃ Charpy impact absorption energy is 102-134J.

[0025] The present invention also provides a heat treatment process for dual-phase low-temperature steel, comprising: Quenching: Heat the steel plate to 770-810℃ and keep it warm for 30-60min, then cool it to room temperature with water; Tempering: Heat the steel plate to 620-635℃ and keep it warm for 60-90min, then air cool it to room temperature.

[0026] The technical solution of the present invention is further described below by way of embodiments.

[0027] Example 1: This example is a dual-phase low-temperature steel with a thickness of 20 mm, and the chemical composition and its mass fraction are 0.021% C, 3.6% Mn, 1.0% Ni, 0.02% Nb, 0.06% Si, 0.002% S, 0.004% P, and the balance Fe and impurity elements. The steel plate is heated to 770°C and kept at this temperature for 60 minutes, then water-cooled to room temperature, and then the steel plate is heated to 630°C and kept at this temperature for 70 minutes, then air-cooled to room temperature. Figure 1 It can be seen that the steel plate has a dual-phase structure of tempered martensite (light color) + austenite (dark color), in which the volume fraction of austenite is 11%, the yield strength is 516MPa, and the Charpy impact absorption energy at -120℃ is 111J.

[0028] Example 2: This example is a dual-phase low-temperature steel with a thickness of 12 mm, and the chemical composition and its mass fraction are 0.01% C, 4.4% Mn, 0.6% Ni, 0.01% Nb, 0.07% Si, 0.001% S, 0.005% P, and the balance Fe and impurity elements. The steel plate is heated to 810°C and kept warm for 30 minutes, then water-cooled to room temperature, and then heated to 635°C and kept warm for 60 minutes, and then air-cooled to room temperature. The volume fraction of austenite in the steel plate structure is 12%, the yield strength is 463 MPa, and the -120°C Charpy impact absorption energy is 134 J.

[0029] Example 3: This example is a dual-phase low-temperature steel with a thickness of 40 mm, and the chemical composition and its mass fraction are 0.025% C, 4.1% Mn, 0.7% Ni, 0.05% Nb, 0.1% Si, 0.001% S, 0.004% P, and the balance Fe and impurity elements. The steel plate is heated to 800°C and kept warm for 40 minutes, then water-cooled to room temperature, and then heated to 620°C and kept warm for 90 minutes, and then air-cooled to room temperature. The volume fraction of austenite in the steel plate structure is 9%, the yield strength is 531 MPa, and the -120°C Charpy impact absorption energy is 102 J.

[0030] Therefore, the technical solution of the present application first heats the steel plate to 770-810°C during heat treatment. This temperature range is in the austenite phase region. The holding time of 30-60 minutes can make the austenitization complete and make the alloy elements distributed uniformly enough. Too high temperature or too long holding time will cause coarse grains and reduce toughness. After austenitization, water cooling is used to accelerate cooling, inhibit the precipitation of carbides, and quench to obtain quenched martensite structure. In the tempering heat treatment process, the steel plate is heated to 620-635°C. This temperature range is in the two-phase region of the composition of the present invention. During the 60-90min holding process, a reverse transformation austenite with a volume fraction of about 10% can be formed, and the alloy elements are enriched to obtain sufficient thermal stability. It can still maintain a face-centered cubic structure at a temperature of -120°C without phase change, which is a beneficial second phase for improving low-temperature toughness. Another function of tempering is to restore the martensitic structure and improve the toughness of the matrix itself. After heat treatment, a dual-phase structure of tempered martensite + austenite is obtained.

[0031] In addition to the above embodiments, the present invention may also have other implementation modes; any technical solutions formed by equivalent replacement or equivalent transformation shall fall within the protection scope required by the present invention.

Claims

1. A dual-phase low-temperature steel, characterized in that: Its chemical composition and mass percentage are: C: 0.010%-0.025%, Mn: 3.6%-4.4%, Ni: 0.6%-1.0%, Nb: 0.01%-0.05%, Si: ≤0.1%, S: ≤0.002%, P: ≤0.005%, and the remainder is Fe and unavoidable impurities.

2. The heat treatment process for dual-phase low-temperature steel according to claim 1, characterized in that: The dual-phase low-temperature steel has a dual-phase structure of tempered martensite+austenite, and the volume fraction of austenite is 9%-12%.

3. The heat treatment process for dual-phase low-temperature steel according to claim 1, characterized in that: The dual-phase low-temperature steel has a plate thickness of 12-40 mm, a yield strength of 463-531 MPa, and a -120°C Charpy impact absorption energy of 102-134 J.

4. A heat treatment process for the dual-phase low-temperature steel according to claims 1 to 3, characterized in that: include: Quenching: heating the steel plate to a first preset temperature and keeping it warm, then water cooling it to room temperature; Tempering: Heat the steel plate to the second preset temperature and keep it warm, then air cool it to room temperature.

5. The heat treatment process for dual-phase low-temperature steel according to claim 4, characterized in that: The first preset temperature is 770-810°C.

6. The heat treatment process for dual-phase low-temperature steel according to claim 4, characterized in that: The holding time of the quenching process is 30 to 60 minutes.

7. The heat treatment process for dual-phase low-temperature steel according to claim 4, characterized in that: The second preset temperature is 620-635°C.

8. The heat treatment process for dual-phase low-temperature steel according to claim 4, characterized in that: The holding time of the tempering process is 60 to 90 minutes.

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