A heat treatment method for low expansion alloy plate

Abstract

The invention discloses a heat treatment method for a low expansion alloy plate, comprising: hot rolling a low expansion alloy blank, the final rolling temperature is 800-900°C, and a hot rolled plate is obtained; the hot rolled plate is loaded into a heating furnace, and the first heat treatment is performed at a heating temperature of 800-850°C, and after cooling to 200-250°C, the second heat treatment is performed at a heating temperature of 300-350°C. The method of the invention can achieve a lower linear thermal expansion coefficient, and can improve the surface quality and mechanical properties of the low expansion alloy plate.

Description

Technical Field

[0001] The invention relates to the technical field of stainless steel alloy production and manufacturing, and in particular to a heat treatment method for a low expansion alloy plate. Background Art

[0002] Low expansion alloys have the characteristic of extremely low linear thermal expansion coefficient in the operating temperature range. They are widely used in precision instrument parts, composite material molds, thermal bimetallic low expansion layers, electronic component sealing materials, and LNG carriers, etc. They are key materials for scientific and technological development and national defense and military industry. At present, low expansion alloys used in high-end fields mainly rely on imports, and their high prices restrict the development of related industries. Low expansion alloys have high requirements on the linear thermal expansion coefficient during use, and they also need to take into account mechanical properties and ensure product surface quality. Therefore, the formulation of heat treatment process has become an important problem in the localization of this alloy.

[0003] The Chinese invention patent application with application number 201410162936.4 discloses a low expansion magnetic shielding alloy and its preparation method, and the process steps include: high purity raw material preparation → component proportioning → vacuum induction furnace smelting and vacuum consumable remelting → forging processing → sampling → heat treatment → performance testing. Its heat treatment system is: high purity hydrogen protection, heating to 970±10℃ with the furnace, keeping warm for 2.5~3h, cooling to 550±10℃ at 200~250℃ / h, and fast cooling to below 300℃ before being taken out of the furnace.

[0004] However, the alloy composition of low expansion magnetic shielding alloy is different from that of low expansion alloy. The performance indicators of low expansion magnetic shielding alloy are expansion coefficient and magnetic property, while the performance indicators of low expansion alloy are expansion coefficient and mechanical property. Therefore, low expansion magnetic shielding alloy and low expansion alloy have different requirements for heat treatment temperature, time and cooling rate. In addition, the above patent application adopts a single heating treatment, which cannot achieve the purpose of eliminating internal stress, reducing expansion coefficient and stabilizing performance. Summary of the invention

[0005] The object of the present invention is to provide a method for heat treatment of a low expansion alloy plate.

[0006] To achieve the above purpose, the specific technical solution of a low expansion alloy plate heat treatment method of the present invention is:

[0007] A method for heat treating a low expansion alloy plate, comprising:

[0008] The low expansion alloy billet is hot rolled at a final rolling temperature of 800-900°C to obtain a hot rolled plate;

[0009] The hot-rolled sheet is placed in a heating furnace, subjected to a first heat treatment at a heating temperature of 800 to 850°C, cooled to 200 to 250°C, and then subjected to a second heat treatment at a heating temperature of 300 to 350°C.

[0010] Optionally, the thickness of the low expansion alloy blank is 120-400 mm.

[0011] Optionally, the charging temperature of the hot-rolled plate into the heating furnace is 500-600°C.

[0012] Optionally, the thickness H of the hot-rolled plate is 5 mm ≤ H < 20 mm.

[0013] Optionally, the holding time of the first heat treatment is 40 minutes.

[0014] Optionally, the thickness H of the hot-rolled plate is 20 mm ≤ H ≤ 120 mm.

[0015] Optionally, the holding time of the first heat treatment is 2×H minutes.

[0016] Optionally, the insulation time of the second heat treatment is 10 to 12 hours.

[0017] Optionally, the low expansion alloy billet comprises, by weight percentage: C≤0.015%, Si≤0.25%, Mn0.25%-0.45%, P≤0.005%, S≤0.003%, Ni 35.5%-36.5%, Mg≤0.005%, Zr≤0.01%, and the balance is Fe and unavoidable impurities.

[0018] A low expansion alloy sheet obtained by adopting the above-mentioned low expansion alloy sheet heat treatment method.

[0019] It can be seen from the above technical solution that the heat treatment method of a low expansion alloy plate of the present invention has at least the following beneficial effects:

[0020] The heat treatment method for the low expansion alloy plate of the present invention can achieve a lower linear thermal expansion coefficient and can improve the surface quality and mechanical properties of the low expansion alloy plate, with no defects such as cracks, deep pits, heavy skin, etc. on the surface and excellent mechanical properties.

[0021] The heat treatment method for low expansion alloy sheet of the present invention provides a suitable heat treatment process for sheets of different thickness specifications, and can obtain a low and stable linear thermal expansion coefficient. In addition, the heat treatment method for low expansion alloy sheet of the present invention further eliminates residual stress by adding a second heat treatment, thereby preventing the low expansion alloy sheet from deforming during use. DETAILED DESCRIPTION

[0022] In order to fully understand the purpose, features and effects of the present invention, the present invention is described in detail through the following specific embodiments. Except for the following contents, the process method of the present invention adopts conventional methods or devices in the art. Unless otherwise specified, the following terms have the meanings commonly understood by those skilled in the art.

[0023] In order to make the low expansion alloy plate have an extremely low and stable thermal expansion coefficient in the range of 20-100°C and obtain excellent mechanical properties, the inventors of the present invention conducted in-depth research from the following two aspects: 1) controlling the chemical composition of the alloy and reducing the proportion of impurity elements; 2) adopting a two-stage heat treatment process to obtain ideal organizational properties by controlling the heating rate, holding time and cooling method.

[0024] Based on the above inventive concept, the present invention provides a method for heat treatment of a low expansion alloy sheet, comprising:

[0025] The low expansion alloy billet is hot rolled at a final rolling temperature of 800-900°C to obtain a hot rolled plate;

[0026] The hot-rolled sheet is placed in a heating furnace and subjected to a first heat treatment at a heating temperature of 800 to 850°C. After cooling to 200 to 250°C, a second heat treatment is conducted at a heating temperature of 300 to 350°C.

[0027] As a preferred embodiment, the heat treatment method of the low expansion alloy plate of the present invention comprises the following steps:

[0028] (1) Prepare the blank

[0029] The heat treatment method of the present invention is directed to a low expansion alloy slab, the element composition (wt%) of which is:

[0030] C≤0.015%, Si≤0.25%, Mn 0.25%~0.45%, P≤0.005%, S≤0.003%, Ni35.5%~36.5%, Mg≤0.005%, Zr≤0.01%, and the balance is Fe and unavoidable impurities.

[0031] The chemical composition of the alloy has a great influence on the expansion coefficient. In the low expansion alloy of the present invention, except for the main elements Si, Mn, Ni, and Fe, the addition of other elements will increase the expansion coefficient and cause problems such as rolling cracking. Therefore, the present invention strictly limits the content of impurity elements (such as C, P, S, etc.).

[0032] The smelting process route of the ingot can adopt "induction furnace + electroslag remelting" or "electric furnace or converter + refining outside the furnace". The obtained ingot is rolled or forged to obtain a rough rolled billet or forged billet as a billet. The specific smelting process and rolling or forging process can refer to the relevant schemes in the prior art, which will not be described here.

[0033] Preferably, the thickness of the blank is 120-400 mm.

[0034] Preferably, the surface of the blank is ground to ensure that the surface of the blank has no defects such as cracks.

[0035] (2) Hot rolling treatment

[0036] The billet is sent to the rolling mill for rolling, and the final rolling temperature is controlled within the range of 800-900°C. After rolling, the billet is taken out of the furnace and air-cooled to room temperature. The thickness (H) of the obtained hot-rolled plate is 5-120mm.

[0037] The rolling passes are 10 to 16 times, and the specific number of passes can be reasonably selected by the technicians in this field according to the thickness of the billet and the thickness of the finished product. When the billet thickness is less than 200mm, the pass reduction does not exceed 15mm. The total reduction rate is not less than 70%.

[0038] The inventors found that the reasonable arrangement of the rolling passes and the reduction in each pass in the rolling process can prevent the billet from being too warped and unable to bite into the billet during the rolling process. The final rolling temperature of 800-900°C is to avoid the problem of plate shape caused by too low temperature. The total reduction rate is not less than 70% to control the recrystallized grain size and ensure the mechanical properties of the product.

[0039] (3) First heat treatment

[0040] The hot rolled sheet is fed into a heating furnace for the first heat treatment. The charging temperature of the hot rolled sheet is 500-600°C (for example, 500°C, 510°C, 520°C, 530°C, 540°C, 550°C, 560°C, 570°C, 580°C, 590°C or 600°C, etc.), and the temperature is increased to 800-850°C (for example, 800°C, 810°C, 820°C, 830°C, 840°C or 850°C, etc.) at a heating rate not exceeding 300°C / h for the first heat treatment.

[0041] The holding time of the first heat treatment is determined according to the thickness H of the hot-rolled plate. If the thickness H of the hot-rolled plate is 5mm≤H<20mm, the holding time is 40 minutes; if the thickness H of the hot-rolled plate is 20mm≤H≤120mm, the holding time is (2×H) minutes. After the first heat treatment, the hot-rolled plate is taken out of the furnace and cooled to 200-250°C (200°C, 210°C, 220°C, 230°C, 240°C or 250°C, etc.) at a cooling rate of 240-300°C / h (for example, 240°C / h, 260°C / h, 280°C / h, 300°C / h, etc.).

[0042] The inventors have found that when performing the first heat treatment, the furnace charging temperature and heating rate of the present invention can ensure that the plate is fully and evenly heated and avoid thermal cracks. The heating time is determined according to the thickness of the plate, which ensures that the plate is fully recrystallized on the one hand, and prevents excessive grain growth and performance degradation on the other hand. The cooling rate of 240 to 300 ° C / h can prevent large internal stress from being generated by direct quenching.

[0043] (4) Second heat treatment

[0044] The plate which has undergone the first heat treatment is hot charged into a furnace, heated to 300-350°C (for example, 300°C / h, 310°C / h, 320°C / h, 330°C / h, 340°C / h or 350°C / h) at a heating rate of 300-350°C / h (for example, 300°C / h, 310°C / h, 320°C / h, 330°C / h, 340°C / h or 350°C / h, etc.), subjected to a second heat treatment, kept at this temperature for 10-12 hours, and cooled to room temperature at a cooling rate of 300-350°C / h (for example, 300°C / h, 310°C / h, 320°C / h, 330°C / h, 340°C / h or 350°C / h, etc.) after being taken out of the furnace.

[0045] The inventors found that when performing the second heat treatment, directly charging the furnace at 200-250°C can save energy and time, without lowering the temperature to room temperature and then heating it up. The heating speed is controlled at 300-350°C / h, which can ensure the temperature uniformity of the plate. Long-term heat preservation can significantly eliminate the internal stress of the plate, avoid the influence of internal stress on the expansion coefficient, and ensure the stability of mechanical properties and expansion coefficient.

[0046] In order to make the low expansion alloy sheet have a lower linear thermal expansion coefficient, excellent mechanical properties and surface quality, the inventor of the present invention comprehensively considered the heat treatment process of the low expansion alloy sheet, and thus proposed the above heat treatment method. In the heat treatment method, the hot rolling treatment, the first heat treatment and the second heat treatment are coordinated and acted together as a whole, thereby ensuring that the obtained low expansion alloy sheet has a lower linear thermal expansion coefficient, excellent mechanical properties and surface quality. The details are as follows:

[0047] First, the rolling process controls the rolling parameters. The deformation energy storage is controlled by the total reduction rate and the number of rolling passes, thereby affecting the recrystallization structure. The plate shape is controlled by the final rolling temperature and the deformation amount of the pass to ensure the continuity of the production process.

[0048] Secondly, a relatively stable rolled structure can be obtained by controlling the composition and rolling process in the early stage. The main purpose of the first heat treatment is recrystallization annealing. By keeping the temperature above the recrystallization temperature for a certain period of time, the structure is completely transformed into a recrystallized structure. At this step, attention should be paid to the furnace charging temperature and heating temperature to avoid thermal cracks on the surface of the material due to excessive furnace temperature or too fast heating. At the same time, in order to obtain excellent mechanical properties, the heating time needs to be adjusted according to the plate thickness to obtain the desired grain size. After the insulation is completed, the cooling rate needs to be controlled to avoid internal stress caused by rapid cooling.

[0049] Again, the second heat treatment can be directly heated up after the plate temperature drops to 200-250℃ to speed up the production rhythm. The main purpose of this heat treatment is to eliminate the internal stress of the material in the previous production process, so it needs to be kept at low temperature for a long time. The thermal expansion coefficient of the material after this step of heat treatment is further reduced. The uniformity of the expansion coefficient and mechanical properties of the same plate is further improved.

[0050] Example

[0051] The present invention is further described below by way of examples, but the present invention is not limited to the scope of the examples. The experimental methods in the following examples without specifying specific conditions are carried out according to conventional methods and conditions, or selected according to the product specifications.

[0052] The initial rolled billet or forging billet used in the following embodiments is prepared by the applicant's upstream production line according to conventional methods.

[0053] The detection methods used in the following embodiments are as follows:

[0054] Yield strength: GB / T 228.1 Tensile test of metallic materials Part 1: Room temperature test method

[0055] Tensile strength: GB / T 228.1 Tensile test for metallic materials Part 1: Room temperature test method

[0056] Elongation: GB / T 228.1 Tensile test for metallic materials Part 1: Room temperature test method

[0057] Hardness (HRB): GB / T 230.1 Rockwell hardness test for metallic materials Part 1: Test method (A, B, C, D, E, F, G, H, K, N, T scale)

[0058] Linear thermal expansion coefficient: GB / T 4339 Determination of thermal expansion characteristic parameters of metal materials

[0059] Example 1

[0060] (1) The thickness of the initial rolling slab is 150 mm, and the chemical composition is as follows:

[0061] element C Si Mn P S Ni Mg Zr wt% 0.009 0.13 0.35 ≤0.005 ≤0.003 35.7 ≤0.005 ≤0.01

[0062] (2) The bloom is fed into a rolling mill for rolling. The hot rolling final rolling temperature is 840°C, the hot rolled plate thickness is 6 mm, and after hot rolling, the bloom is air-cooled to room temperature.

[0063] (3) The hot-rolled plate is sent to the heating furnace for the first heat treatment. The charging temperature of the hot-rolled plate is 550°C, the heating temperature of the first heat treatment is 810°C, the first heat treatment is carried out, the holding time is 40 minutes, and after leaving the furnace, it is cooled to 220°C at a cooling rate of 280°C / h.

[0064] (4) The plate hot charging furnace is heated to 330°C at a heating rate of 300°C / h, kept at this temperature for 12h, and cooled to room temperature at a cooling rate of 320°C / h after being taken out of the furnace.

[0065] After testing, the yield strength of the plate is 245MPa, the tensile strength is 435MPa, the elongation is 43%, and the hardness (HRB) is 66, 67, 69. The linear thermal expansion coefficient is 0.89×10 -6 / ℃(20-100℃), good surface quality.

[0066] Example 2

[0067] (1) The thickness of the initial rolling slab is 200 mm, and the chemical composition is as follows:

[0068] element C Si Mn P S Ni Mg Zr wt% 0.011 0.15 0.33 ≤0.005 ≤0.003 35.9 ≤0.005 ≤0.01

[0069] (2) The bloom is fed into a rolling mill for rolling. The hot rolling final rolling temperature is 840°C, the hot rolled plate thickness is 20 mm, and after hot rolling, the bloom is air-cooled to room temperature.

[0070] (3) The hot-rolled plate is sent to the heating furnace for the first heat treatment. The charging temperature of the hot-rolled plate is 560°C, and the temperature is raised to 840°C at a heating rate not exceeding 300°C / h for the first heat treatment. The holding time is 40 minutes. After leaving the furnace, it is cooled to 250°C at a cooling rate of 240°C / h.

[0071] (4) The plate is hot-charged in the furnace, and the temperature is raised to 310°C at a heating rate of 350°C / h, kept at this temperature for 12 hours, and cooled to room temperature at a cooling rate of 300°C / h after being taken out of the furnace.

[0072] After testing, the yield strength of the plate is 254MPa, the tensile strength is 462MPa, the elongation is 41%, and the hardness (HRB) is 68, 67, 68. The linear thermal expansion coefficient is 0.92×10 -6 / ℃(20-100℃), good surface quality.

[0073] Example 3

[0074] (1) The thickness of the forging billet is 350 mm and the chemical composition is as follows:

[0075] element C Si Mn P S Ni Mg Zr wt% 0.011 0.16 0.35 ≤0.005 ≤0.003 35.9 ≤0.005 ≤0.01

[0076] (2) The bloom is fed into a rolling mill for rolling. The hot rolling final rolling temperature is 860° C. The hot rolled plate thickness is 80 mm. After hot rolling, the bloom is air-cooled to room temperature.

[0077] (3) The hot-rolled plate is sent to the heating furnace for the first heat treatment. The charging temperature of the hot-rolled plate is 570°C, and the temperature is raised to 850°C at a heating rate not exceeding 300°C / h for the first heat treatment. The holding time is 160 minutes. After leaving the furnace, it is cooled to 250°C at a cooling rate of 300°C / h.

[0078] (4) The plate is hot-charged in the furnace, and the temperature is raised to 350°C at a heating rate of 330°C / h, kept at this temperature for 12 hours, and cooled to room temperature at a cooling rate of 350°C / h after being taken out of the furnace.

[0079] After testing, the yield strength of the plate is 235MPa, the tensile strength is 431MPa, the elongation is 45%, and the hardness (HRB) is 62, 61, 63. The linear thermal expansion coefficient is 0.97×10 -6 / ℃(20-100℃), good surface quality.

[0080] The above embodiments are preferred implementation modes of the present invention, but the implementation modes of the present invention are not limited to the above embodiments. Any other substitutions, modifications, combinations, changes, simplifications, etc. made without departing from the spirit and principles of the present invention shall be equivalent replacement modes and shall be included in the protection scope of the present invention.

Claims

1. A method for heat treatment of a low expansion alloy sheet, characterized in that: include: The low expansion alloy billet is hot rolled, and the final rolling temperature is 800-900°C to obtain a hot rolled plate; The hot rolled plate is loaded into a heating furnace and subjected to a first heat treatment at a heating temperature of 800-850°C. After that, the hot rolled plate is taken out of the furnace and cooled to 200-250°C at a cooling rate of 240-300°C / h. Then, the hot rolled plate is subjected to a second heat treatment at a heating temperature of 300-350°C. The holding time of the second heat treatment is 10-12 hours. The low expansion alloy blank comprises, by weight percentage, C≤0.015%, Si≤0.25%, Mn 0.25%~0.45%, P≤0.005%, S≤0.003%, Ni 35.5%~36.5%, Mg≤0.005%, Zr≤0.01%, and the balance is Fe and unavoidable impurities.

2. The heat treatment method of low expansion alloy sheet according to claim 1, characterized in that: The thickness of the low expansion alloy blank is 120-400 mm.

3. The heat treatment method of low expansion alloy sheet according to claim 1, characterized in that: The charging temperature of the hot rolled plate into the heating furnace is 500-600°C.

4. The heat treatment method of low expansion alloy sheet according to claim 1, characterized in that: The thickness H of the hot-rolled plate is 5 mm ≤ H < 20 mm.

5. The heat treatment method of low expansion alloy sheet according to claim 4, characterized in that: The holding time of the first heat treatment is 40 minutes.

6. The heat treatment method of low expansion alloy sheet according to claim 1, characterized in that: The thickness H of the hot-rolled plate is 20 mm ≤ H ≤ 120 mm.

7. The heat treatment method of low expansion alloy sheet according to claim 6, characterized in that: The holding time of the first heat treatment is 2×H minutes.

8. A low expansion alloy sheet obtained by the low expansion alloy sheet heat treatment method according to any one of claims 1 to 7.