Duplex stainless steel tube blank and preparation method thereof

Abstract

The invention relates to a duplex stainless steel tube blank and a preparation method thereof. The preparation method comprises the following steps: pouring molten steel obtained by smelting into a steel ingot; the steel ingot is forged by controlling the pass deformation according to the temperature of the steel ingot; and performing post-treatment on the steel ingot according to the surface temperature of the forged steel ingot to obtain the pipe blank. The pipe blank comprises the following components in percentage by mass: less than or equal to 0.028% of C, 0.40-0.80% of Si, less than or equal to 1.00% of Mn, less than or equal to 0.020% of P, less than or equal to 0.010% of S, 22.4-22.8% of Cr, 5.2-5.8% of Ni, 3.1-3.5% of Mo, 0.16-0.20% of N, 0.01-0.03% of Al and the balance of Fe; mn / N > = 4, and [O] content < = 0.0035%. The duplex stainless steel tube blank has excellent hot working plasticity and can be used for producing high-quality duplex stainless steel forgings.

Description

Technical Field

[0001] The present invention belongs to the technical field of forging, and particularly relates to a duplex stainless steel tube blank and a preparation method thereof. Background Art

[0002] Duplex stainless steel is a structure in which austenite (γ) and ferrite (α) coexist. During hot working, the thermal expansion coefficients and deformation resistances of the two phases are significantly different, which easily leads to internal stress concentration; the optimal hot working temperature range of duplex stainless steel is relatively narrow, and outside this range, the plasticity is likely to decrease, thereby increasing the risk of cracking.

[0003] Cracking during processing easily leads to an increase in the rejection rate, and rework will affect production efficiency. Material replacement will result in high costs; hot working cracking will cause cracks to form inside or on the surface of the material, which will significantly reduce the mechanical properties of the component, such as strength and toughness. Components with cracks may suddenly break due to stress concentration during subsequent use, especially in high-pressure, corrosive, or alternating load environments; that is, components with cracks pose a great safety hazard during use.

[0004] Therefore, to solve the problem of hot working cracking of duplex stainless steel, comprehensive research needs to be carried out from aspects such as material properties and processing technology. Summary of the Invention

[0005] In order to solve all or part of the above problems, the purpose of the present invention is to provide a duplex stainless steel tube blank and a preparation method thereof.

[0006] According to one aspect of the present invention, a duplex stainless steel tube blank is provided. Calculated by mass percentage, it includes the following components: C ≤ 0.028%, Si: 0.40 - 0.80%, Mn ≤ 1.00%, P ≤ 0.020%, S ≤ 0.010%, Cr: 22.4 - 22.8%, Ni: 5.2 - 5.8%, Mo: 3.1 - 3.5%, N: 0.16 - 0.20%, Al: 0.01 - 0.03%, and the balance is Fe; and Mn / N ≥ 4, and the content of [O] ≤ 0.0035%.

[0007] Furthermore, it has two phases of austenite and ferrite.

[0008] The present invention also provides a preparation method of a duplex stainless steel tube blank. The duplex stainless steel tube blank is as described in any one of the above, and the preparation method includes:

[0009] Pouring the molten steel obtained by smelting into an ingot;

[0010] Performing forging treatment on the ingot according to the pass deformation amount controlled by the temperature of the ingot;

[0011] The steel ingot is post-treated according to the surface temperature of the forged steel ingot to obtain the duplex stainless steel tube blank.

[0012] Further, before pouring the molten steel obtained by smelting into a steel ingot, the preparation method further includes: smelting the molten steel by using an intermediate frequency furnace, an argon oxygen refining furnace and an external refining furnace.

[0013] Further, in the argon oxygen refining furnace smelting, deoxidation is carried out by means of Si / Al composite deoxidation to reduce the oxygen content in the molten steel to less than 0.0035%.

[0014] Further, before forging the steel ingot according to the pass deformation amount controlled by the temperature of the steel ingot, the preparation method further includes:

[0015] Putting the steel ingot into a heating furnace and heating it to 1250 °C.

[0016] Further, the forging treatment of the steel ingot according to the pass deformation amount controlled by the temperature of the steel ingot is specifically:

[0017] Controlling the pass deformation amount ≤ 30%, and carrying out the first-stage forging on the steel ingot with a temperature in the range of 1200 °C - 1250 °C until the temperature of the steel ingot is less than 1200 °C; controlling the pass deformation amount ≥ 50% again, and carrying out the second-stage forging on the steel ingot with a temperature between 1100 °C - 1200 °C until the temperature of the steel ingot is less than or equal to 1100 °C; finally controlling the pass deformation amount ≤ 20%, and carrying out the third-stage forging on the steel ingot with a temperature less than or equal to 1100 °C.

[0018] Further, the post-treatment of the steel ingot according to the surface temperature of the forged steel ingot to obtain the duplex stainless steel tube blank is specifically:

[0019] If the surface temperature of the forged steel ingot is greater than 950 °C, an online solution treatment process is adopted to carry out solution treatment on the steel ingot, and the water temperature is controlled below 80 °C to carry out water cooling on the steel ingot after solution treatment to obtain the duplex stainless steel tube blank;

[0020] If the surface temperature of the forged steel ingot is less than or equal to 950 °C, an offline solution treatment process is adopted to carry out solution treatment on the steel ingot, and the steel ingot after solution treatment is water cooled to below 200 °C and then air cooled to obtain the duplex stainless steel tube blank.

[0021] Further, the specific process of if the surface temperature of the forged steel ingot is less than or equal to 950 °C, an offline solution treatment process is adopted to carry out solution treatment on the steel ingot, and the steel ingot after solution treatment is water cooled to below 200 °C and then air cooled to obtain the duplex stainless steel tube blank is:

[0022] If the surface temperature of the steel ingot after forging is less than or equal to 950 °C, then put the steel ingot into a heating furnace and heat it to 1050 °C, and set the holding time for heat preservation, where the holding time is determined by the radius of the steel ingot; and take out the steel ingot, water-cool the steel ingot to below 200 °C and then air-cool it to obtain the duplex stainless steel tube blank.

[0023] Further, the value of the holding time is equal to the value of the radius of the steel ingot in millimeters multiplied by any value within the range of one to two, and the unit of the holding time is minutes.

[0024] As can be seen from the above technical solutions, a duplex stainless steel tube blank and a preparation method thereof provided by the present invention have the following beneficial effects:

[0025] The duplex stainless steel tube blank of the present invention has two phases of austenite and ferrite; the duplex stainless steel tube blank of the present invention has excellent hot working plasticity and can be used to produce high-quality duplex stainless steel forgings. Description of the Drawings

[0026] Figure 1 It is a flowchart of a method for preparing a duplex stainless steel tube blank according to an embodiment of the present invention. Detailed Description of the Invention

[0027] In order to fully understand the purpose, features and effects of the present invention, the present invention will be described in detail through the following specific embodiments. Except for the following content, the process methods of the present invention all adopt conventional methods or devices in the art. Unless otherwise specified, the following terms have the meanings commonly understood by those skilled in the art.

[0028] When a numerical range is disclosed herein, the above range is considered continuous and includes the minimum value and the maximum value of the range, as well as each value therebetween. Further, when the range refers to an integer, it includes each integer between the minimum value and the maximum value of the range. In addition, when multiple ranges are provided to describe features or characteristics, these ranges can be combined. In other words, unless otherwise specified, all ranges disclosed herein should be understood to include any and all sub-ranges subsumed therein.

[0029] A duplex stainless steel tube blank according to an embodiment of the present invention, calculated by mass percentage, includes the following components: C ≤ 0.028%, Si: 0.40 - 0.80%, Mn ≤ 1.00%, P ≤ 0.020%, S ≤ 0.010%, Cr: 22.4 - 22.8%, Ni: 5.2 - 5.8%, Mo: 3.1 - 3.5%, N: 0.16 - 0.20%, Al: 0.01 - 0.03%, and the balance is Fe; and Mn / N ≥ 4, and the [O] content ≤ 0.0035%.

[0030] For the above-mentioned ratio of the duplex stainless steel tube blank in this embodiment, a small amount of C can improve the strength through solid solution strengthening. However, the carbides formed by carbon and chromium will cause local chromium depletion and reduce the corrosion resistance. Therefore, in this embodiment, the C content is limited to be less than or equal to 0.028%, and this content can reduce the precipitation of carbides and avoid intergranular corrosion during welding or at high temperatures.

[0031] Si, as a deoxidizer, can reduce oxide inclusions during melting and improve high-temperature oxidation resistance; Al, as an auxiliary deoxidizer, can refine the grain size. However, excessive Si and Al may promote brittle phases. Therefore, the contents of Si and Al need to be controlled appropriately to balance deoxidation and avoid brittleness. In this embodiment, the Si content is 0.40 - 0.80%, and the Al content is 0.01 - 0.03%.

[0032] Mn can stabilize austenite (partially replace nickel) and promote the dissolution of nitrogen. Excessive Mn may reduce the pitting corrosion resistance. Therefore, in this embodiment, the Mn content is controlled below 1.00%, and the synergy optimization of Mn and N is: Mn / N ≥ 4.

[0033] N is a strong austenite stabilizer, which plays a role in maintaining the duplex structure, can significantly improve the pitting corrosion resistance, and enhance the solid solution strengthening strength. However, too high N may lead to the precipitation of nitrides and reduce the hot working performance of the steel. Therefore, in this embodiment, the N content is controlled at 0.16 - 0.20%.

[0034] Cr will form a Cr2O3 passivation film to provide basic corrosion resistance and promote the formation of ferrite phase. Cr needs to be balanced with austenite-forming elements (Ni, N) to maintain the duplex ratio. Therefore, in this embodiment, the Cr content must be controlled at 22.4 - 22.8%.

[0035] Ni is an austenite phase stabilizing element, which can improve toughness and processing performance. Mo is a key alloying element in duplex stainless steel, which can significantly enhance the pitting corrosion and crevice corrosion resistance. However, both Ni and Mo belong to precious metal elements, and too high content will increase the manufacturing cost. Therefore, in this embodiment, the Ni content is controlled at 5.2 - 5.8%, and the Mo content is controlled at 3.1 - 3.5%.

[0036] Oxygen is an impurity element, which is easy to form oxide inclusions and reduce toughness and corrosion resistance. In this embodiment, the oxygen content is controlled below 0.0035%, which can reduce the volume fraction and size of hard oxide inclusions, reduce the risk of stress concentration, and reduce the initiation of microcracks during high-temperature deformation.

[0037] The duplex stainless steel tube blank of this embodiment has two phases, austenite and ferrite.

[0038] The duplex stainless steel tube blank of this embodiment has excellent hot working plasticity and can be used to produce high-quality duplex stainless steel forgings.

[0039] An embodiment of the present invention also provides a method for preparing a duplex stainless steel tube blank, where the duplex stainless steel tube blank is as described in any one of the above embodiments. As Figure 1 shown, the preparation method of this embodiment includes:

[0040] Step S001: Pour the molten steel obtained by smelting into an ingot;

[0041] Step S002: Forge the ingot according to the pass deformation amount controlled by the temperature of the ingot;

[0042] Step S003: Post-treat the ingot according to the surface temperature of the forged ingot to obtain a duplex stainless steel tube blank.

[0043] Among them, before pouring the molten steel obtained by smelting into an ingot in step S001, the preparation method of this embodiment further includes: smelting with an intermediate frequency furnace, an argon oxygen refining furnace, and secondary refining outside the furnace to obtain molten steel. Among them, in the argon oxygen refining furnace smelting stage, deoxidation is carried out by the Si / Al composite deoxidation method to reduce the oxygen content in the molten steel to less than 0.0035%; after the argon oxygen refining furnace smelting, the molten steel is poured into an ingot after adjusting the alloy composition through the secondary refining process outside the furnace.

[0044] Before forging the ingot according to the pass deformation amount controlled by the temperature of the ingot in step S002, the preparation method of this embodiment further includes putting the ingot into a heating furnace and heating it to 1250°C. After being heated by the heating furnace, the highest temperature of the ingot is 1250°C.

[0045] Among them, step S002 forging the ingot according to the pass deformation amount controlled by the temperature of the ingot is specifically: controlling the pass deformation amount ≤ 30%, and forging the ingot with a temperature in the range of 1200°C - 1250°C in the first stage until the temperature of the ingot is less than 1200°C; controlling the pass deformation amount ≥ 50% again, and forging the ingot with a temperature between 1100°C - 1200°C in the second stage until the temperature of the ingot is less than or equal to 1100°C; finally, controlling the pass deformation amount ≤ 20%, and forging the ingot with a temperature less than or equal to 1100°C in the third stage.

[0046] In this embodiment, the forging process of the ingot includes a first stage (initial stage), a second stage (stable stage), and a third stage (final forging stage). Each stage is divided according to temperature. The first stage is for an ingot with a temperature range of 1200°C - 1250°C after being heat-treated in a heating furnace. The second stage is for an ingot with a temperature range of 1100°C - 1200°C after being forged in the first stage. The third stage is for an ingot with a temperature less than or equal to 1100°C after being forged in the second stage. Among them, small deformation is used in the first stage of forging, that is, the pass deformation amount ≤ 30%; large deformation is used in the second stage of forging, that is, the pass deformation amount ≥ 50%; small deformation is used in the third stage of forging, that is, the pass deformation amount ≤ 20%.

[0047] Among them, step S003 post-processes the ingot according to the surface temperature of the forged ingot to obtain a duplex stainless steel tube blank. Specifically: if the surface temperature of the forged ingot is greater than 950°C, an online solution treatment process is adopted to solution-treat the ingot, and the water temperature is controlled below 80°C to perform water cooling on the solution-treated ingot to obtain a duplex stainless steel tube blank; if the surface temperature of the forged ingot is less than or equal to 950°C, an offline solution treatment process is adopted to solution-treat the ingot, and the solution-treated ingot is water-cooled to below 200°C and then air-cooled to obtain a duplex stainless steel tube blank.

[0048] Among them, if the surface temperature of the forged ingot is less than or equal to 950°C, an offline solution treatment process is adopted to solution-treat the ingot, and the solution-treated ingot is water-cooled to below 200°C and then air-cooled to obtain a duplex stainless steel tube blank. Specifically: if the surface temperature of the forged ingot is less than or equal to 950°C, the ingot is placed in a heating furnace and heated to 1050°C, and the holding time is set for holding. The holding time t is determined by the radius R of the ingot; and the ingot is taken out, water-cooled to below 200°C and then air-cooled to obtain a duplex stainless steel tube blank.

[0049] Regarding the aforementioned holding time t, the value of the holding time is equal to the value of the radius of the ingot in millimeters multiplied by any value within the range of one to two. The unit of the holding time is minutes, that is, t = R * (1 - 2) min, and the unit of R is millimeters (mm).

[0050] The duplex stainless steel tube blank prepared in the embodiment of the present invention has two phases of austenite and ferrite, and the tube blank prepared in the embodiment of the present invention has excellent hot working plasticity and can be used to produce high-quality duplex stainless steel forgings.

[0051] The preparation method of the embodiment of the present invention will be described below through Example 1, Example 2, and Example 3. Among them, the compositions of the tube blanks in Example 1, Example 2, and Example 3 are shown in Table 1 below:

[0052] Table 1: Composition Table of Tube Blanks in Example 1, Example 2 and Example 3

[0053]

[0054]

[0055] Example 1:

[0056] The actual composition of the duplex stainless steel tube blank is shown in Table 1. The preparation method is as follows: Molten steel is obtained through the process of "medium frequency furnace + AOD + secondary refining outside the furnace", and the oxygen content is 0.0033%; the radius of the tube blank of the ingot cast is 140 mm; the "multi-stage temperature-controlled forging" process is adopted: the pass deformation amount in the initial stage of forging is 28%; the pass deformation amount in the stable stage of forging is 50%; the pass deformation amount in the final forging stage is 17%; the surface temperature of the tube blank at the end of forging is 990 °C, and online solution heat treatment is adopted.

[0057] Example 2:

[0058] The actual composition of the duplex stainless steel tube blank is shown in Table 1. The preparation method is as follows: Molten steel is obtained through the process of "medium frequency furnace + AOD + secondary refining outside the furnace", and the oxygen content is 0.0032%; the radius of the tube blank of the ingot cast is 110 mm, and the "multi-stage temperature-controlled forging" process is adopted: the pass deformation amount in the initial stage of forging is 26%; the pass deformation amount in the stable stage of forging is 52%; the pass deformation amount in the final forging stage is 19%; the surface temperature of the tube blank at the end of forging is 1010 °C, and online solution heat treatment is adopted.

[0059] Example 3:

[0060] The actual composition of the duplex stainless steel tube blank is shown in Table 1. The preparation method is as follows: Molten steel is obtained through the process of "medium frequency furnace + AOD + secondary refining outside the furnace", and the oxygen content is 0.0033%; the radius of the tube blank of the ingot cast is 90 mm, and the "multi-stage temperature-controlled forging" process is adopted: the pass deformation amount in the initial stage of forging is 27%; the pass deformation amount in the stable stage of forging is 55%; the pass deformation amount in the final forging stage is 18%; the surface temperature of the tube blank at the end of forging is 940 °C, and off-line solution heat treatment is adopted. After the tube blank is loaded into the furnace, it is heated to 1050 °C, the set holding time t = 90 * 1.5 = 135 min, water-cooled after being taken out of the furnace, and taken out for air-cooling to room temperature after being cooled to 190 °C on the surface of the tube blank.

[0061] The present invention has been disclosed above in preferred embodiments. However, those skilled in the art should understand that these embodiments are only used to illustrate the present invention and should not be construed as limiting the scope of the present invention. It should be noted that any equivalent changes and substitutions to these embodiments should be regarded as covered within the scope of the claims of the present invention. Therefore, the protection scope of the present invention should be subject to the scope defined in the claims.

Claims

1. A duplex stainless steel tube, characterized in that: Calculated by mass percentage, it includes the following components: C≤0.028%, Si: 0.40-0.80%, Mn≤1.00%, P≤0.020%, S≤0.010%, Cr: 22.4-22.8%, Ni: 5.2-5.8%, Mo: 3.1-3.5%, N: 0.16-0.20%, Al: 0.01-0.03%, and the balance is Fe; and Mn / N≥4, [O] content≤0.0035%.

2. The duplex stainless steel tube according to claim 1, characterized in that: It has two phases: austenite and ferrite.

3. A method for preparing a duplex stainless steel tube blank, characterized in that: The duplex stainless steel tube blank is as described in any one of claims 1-2, and the preparation method comprises: pouring the smelted molten steel into steel ingots; Controlling the deformation amount of each pass according to the temperature of the steel ingot to forge the steel ingot; The steel ingot is post-treated according to the surface temperature of the steel ingot after forging to obtain the duplex stainless steel tube blank.

4. The preparation method according to claim 3, characterized in that: Before the smelted molten steel is cast into steel ingots, the preparation method further comprises: smelting the molten steel by medium frequency furnace smelting, argon oxygen refining furnace smelting and refining outside the furnace.

5. The preparation method according to claim 4, characterized in that: During smelting in an argon-oxygen refining furnace, deoxidation is carried out by Si / Al composite deoxidation to reduce the oxygen content in the molten steel to less than 0.0035%.

6. The preparation method according to claim 3, characterized in that: Before forging the steel ingot by controlling the deformation amount of each pass according to the temperature of the steel ingot, the preparation method further comprises: The steel ingot is placed in a heating furnace and heated to 1250°C.

7. The preparation method according to claim 6, characterized in that: The forging process of the steel ingot is specifically performed by controlling the deformation amount of each pass according to the temperature of the steel ingot: The deformation amount per pass is controlled to be ≤30%, and the first stage forging is performed on the steel ingot with a temperature in the range of 1200℃-1250℃, until the temperature of the steel ingot is less than 1200℃; the deformation amount per pass is controlled to be ≥50%, and the second stage forging is performed on the steel ingot with a temperature between 1100℃-1200℃, until the temperature of the steel ingot is less than or equal to 1100℃; finally, the deformation amount per pass is controlled to be ≤20%, and the third stage forging is performed on the steel ingot with a temperature less than or equal to 1100℃.

8. The preparation method according to claim 3, characterized in that: The post-processing of the steel ingot according to the surface temperature of the steel ingot after forging to obtain the duplex stainless steel tube blank is specifically as follows: If the surface temperature of the steel ingot after forging is greater than 950° C., an online solution treatment process is adopted to treat the steel ingot, and the water temperature is controlled below 80° C. to water cool the steel ingot after the solution treatment to obtain the duplex stainless steel tube blank; If the surface temperature of the steel ingot after forging is less than or equal to 950° C., an offline solution treatment process is adopted to treat the steel ingot, and the steel ingot after the solution treatment is water-cooled to below 200° C. and then air-cooled to obtain the duplex stainless steel tube blank.

9. The preparation method according to claim 8, characterized in that: If the surface temperature of the forged steel ingot is less than or equal to 950° C., an offline solution treatment process is adopted to treat the steel ingot, and the steel ingot after the solution treatment is water-cooled to below 200° C. and then air-cooled to obtain the duplex stainless steel tube blank, specifically: If the surface temperature of the steel ingot after forging is less than or equal to 950° C., the steel ingot is placed in a heating furnace and heated to 1050° C., and a holding time is set for holding, wherein the holding time is determined by the radius of the steel ingot; and the steel ingot is taken out, water-cooled to below 200° C., and then air-cooled to obtain the duplex stainless steel tube blank.

10. The preparation method according to claim 9, characterized in that: The value of the holding time is equal to the value of the radius of the steel ingot in millimeters multiplied by any value in the range of one to two, and the unit of the holding time is minutes.

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