Cold-rolled steel coil spheroidizing annealing temperature control process

Abstract

The invention discloses a cold-rolled steel coil spheroidizing annealing temperature control process which comprises the step of performing spheroidizing annealing on a cold-rolled steel coil, and the spheroidizing annealing treatment process sequentially comprises the steps of free heating, first-time heating and maintaining, second-time heating and maintaining, first-time free cooling and maintaining, first-time cooling and maintaining, second-time cooling and maintaining and second-time free cooling and maintaining. According to the spheroidizing annealing temperature control process, the organization structure and the mechanical property of the cold-rolled steel coil can be improved, the quality of a cold-rolled steel coil product is improved, and the market requirement is met.

Description

Technical Field

[0001] The present invention relates to a temperature control process for spheroidizing annealing of cold-rolled steel coils, belonging to the technical field of metal materials. Background Art

[0002] Cold-rolled steel coils play an important role in the iron and steel industry and are widely used in multiple fields such as automobiles, construction, household appliances, and machinery. With the increasingly high-quality and personalized requirements of customers, the cold-rolling process of cold-rolled steel coils needs to be updated and optimized according to market demands to meet the requirements of high-quality cold-rolled steel coil products that meet market requirements.

[0003] Currently, the processing technology of cold-rolled steel coils has relatively high technical requirements. Generally, spheroidizing annealing treatment is carried out after cold rolling to eliminate work hardening generated by cold rolling, restore the plasticity and toughness of the steel, and improve the organizational structure. However, the spheroidizing annealing process does not match the rolling, which will reduce the organizational structure and mechanical properties of the steel coil, such as generating deformation or cracks, affecting the subsequent use of cold-rolled steel coils; in addition, the existing spheroidizing annealing process has a long process cycle and high energy consumption. Summary of the Invention

[0004] At least aiming at one of the above problems existing in the prior art, the present invention provides a temperature control process for spheroidizing annealing of cold-rolled steel coils, which can improve the organizational structure and mechanical properties of cold-rolled steel coils, enhance the quality of cold-rolled steel coil products, and meet market demands.

[0005] To achieve the above object, the present invention adopts the following technical solution: A temperature control process for spheroidizing annealing of cold-rolled steel coils, comprising: subjecting the cold-rolled steel coil to spheroidizing annealing, and the spheroidizing annealing treatment process: successively free heating, first heating and maintaining, second heating and maintaining, first free cooling and maintaining, first cooling and maintaining, second cooling and maintaining, second free cooling and maintaining;

[0006] Free heating to 340 - 360 °C; first heating to 670 - 690 °C and maintaining for 60 - 90 min; second heating to 770 - 785 °C and maintaining for 90 - 120 min; first free cooling to 725 - 730 °C and maintaining for 60 - 90 min; first cooling to 705 - 710 °C and maintaining for 30 - 60 min; second cooling to and maintaining at 620 - 635 °C and maintaining for 20 - 40 min; second free cooling to 220 - 260 °C and maintaining for 20 - 40 min.

[0007] Preferably, the heating rate of the first heating is greater than that of the second heating. The first heating is rapid heating, and the second heating is slow heating.

[0008] Preferably, the rapid heating rate is 75 - 85 °C / h; the slow heating rate is 12 - 20 °C / h.

[0009] Preferably, the rate of the first cooling is less than that of the second cooling; the first cooling is slow cooling and the second cooling is rapid cooling.

[0010] Preferably, the slow cooling rate is 5 - 10 °C / h; the rapid cooling rate is 25 - 35 °C / h.

[0011] Preferably, the first natural cooling is water-cooled natural cooling and the second natural cooling is air-cooled natural cooling.

[0012] Preferably, the rate of water-cooled natural cooling is 0.1 - 0.2 °C / s; the rate of air-cooled natural cooling is 0.5 - 2 °C / s.

[0013] Preferably, it further includes that the pickled hot-rolled steel coil is cold-rolled to obtain a cold-rolled steel coil.

[0014] Preferably, the cold-rolling process is multi-pass cold-rolling and at least one intermediate annealing is performed.

[0015] Preferably, the reduction rate of single-pass cold-rolling in the cold-rolling process is 14 - 20%, and when the total reduction rate of continuous multi-pass cold-rolling is 37 - 50%, one intermediate annealing is performed and the intermediate annealing is at least once.

[0016] Preferably, the intermediate annealing process is: heating to 600 - 720 °C at a rate of 30 - 50 °C / h, holding for 30 - 120 min, and then furnace cooling to 130 - 150 °C.

[0017] Preferably, it further includes that the cold-rolled steel coil after spheroidizing annealing is subjected to temper rolling to obtain the target cold-rolled steel coil.

[0018] Advantages of the present invention: For the spheroidizing annealing temperature control process of the cold-rolled steel coil of the present invention, by adjusting the temperature of the spheroidizing annealing of the cold-rolled steel coil, combining heating in a multi-stage heating manner and cooling in a multi-stage cooling manner, a reasonable annealing process is formulated, which can significantly improve the mechanical properties of the cold-rolled steel coil material, and there is no deformation or cracking phenomenon in the subsequent processes; in the cooling process of the present invention, water-cooled natural cooling is first adopted and then air-cooled natural cooling, which can refine grains, effectively improve the microstructure of the cold-rolled steel coil, and further enhance the mechanical properties of the cold-rolled steel coil material; the spheroidizing annealing temperature control process of the cold-rolled steel coil of the present invention improves the performance of the finished parts of the cold-rolled steel coil balls, achieving a better use effect. Specific embodiments

[0019] The following is a clear and complete description of the technical solutions in the implementation of the present invention. The described embodiments are only a part of the embodiments of the present invention, rather than all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts fall within the scope of protection of the present invention. For those conditions not specified in the embodiments, they are carried out according to the conventional conditions or the conditions recommended by the manufacturer.

[0020] The T8 hot-rolled steel coil of the present invention is used as the raw material, with a specification of 4 mm × 1500 mm. The mass percentages of the chemical components of the steel coil are as follows: C 0.62% - 0.70%, Si 0.17% - 0.37%, Mn 0.90% - 1.20%, P ≤ 0.035%, S ≤ 0.035%, and the rest are Fe and inevitable impurities; the microstructure is uniformly fine and is a fully pearlitic structure.

[0021] The spheroidizing annealing temperature control process of the cold-rolled steel coil of the present invention includes: performing spheroidizing annealing on the cold-rolled steel coil. The spheroidizing annealing treatment process is carried out in sequence: freely heating up to 340 - 360 °C, then heating up to 670 - 690 °C for the first time and holding for 60 - 90 min, then heating up to 770 - 785 °C for the second time and maintaining for 90 - 120 min, then freely cooling down to 725 - 730 °C for the first time and holding for 60 - 90 min, then cooling down to 705 - 710 °C for the first time and holding for 30 - 60 min, cooling down to and maintaining at 620 - 635 °C for the second time and holding for 20 - 40 min, and then freely cooling down to 220 - 260 °C for the second time and holding for 20 - 40 min.

[0022] Among them, the first heating-up is rapidly heated at a rate of 75 - 85 °C / h, and the second heating-up is slowly heated at a rate of 12 - 20 °C / h; the first cooling-down is slowly cooled at a rate of 5 - 10 °C / h, and the second cooling-down is rapidly cooled at a rate of 25 - 35 °C / h.

[0023] Among them, the first free cooling is water-cooled and freely cooled at a rate of 0.1 - 0.2 °C / s, and the second free cooling is air-cooled and freely cooled at a rate of 0.5 - 2 °C / s.

[0024] Among them, it also includes that the hot-rolled steel coil after pickling is cold-rolled through multiple passes and at least one intermediate annealing to obtain a cold-rolled steel coil; the specific cold-rolling treatment process is that the reduction rate of a single-pass cold-rolling is 14 - 20%, and when the total reduction rate of multiple-pass cold-rolling is 37 - 50%, an intermediate annealing is carried out once, and the intermediate annealing is at least once. The intermediate annealing process is: heating up to 600 - 720 °C at a rate of 30 - 50 °C / h and holding for 30 - 120 min, and then cooling in the furnace to 130 - 150 °C.

[0025] Among them, after spheroidizing annealing, leveling treatment is carried out. During the leveling treatment process, a constant rolling force mode of 400 - 600 tons is adopted, the tension of the uncoiling section is 0.03 - 0.06 KN / mm 2 , and the tension of the coiling section is 0.04 - 0.08 KN / mm 2 .

[0026] Example 1

[0027] A temperature control process for spheroidizing annealing of cold-rolled steel coils is as follows:

[0028] Using a T8 hot-rolled steel coil as the raw material, with a specification of 4 mm × 1500 mm, after pickling, cold rolling treatment is carried out;

[0029] Cold rolling treatment process: The pickled hot-rolled steel coil is cold-rolled through a cold rolling mill in multiple passes. The reduction rates of the first two passes of cold rolling are 15.3%, 13%, and 16% in sequence, and the total reduction rate is 44.3%. Then, intermediate annealing is carried out once: heating to 650 °C at a rate of 30 - 50 °C / h and holding for 50 min, then cooling in the furnace to 150 °C, and then continuing with two passes of cold rolling, with reduction rates of 15% and 13% in sequence, to obtain the cold-rolled steel coil;

[0030] Spheroidizing annealing process: The cold-rolled steel coil is carried out in an annealing furnace. First, it is freely heated to 350 °C, then rapidly heated to 680 °C at a rate of 85 °C / h and held for 80 min, then slowly heated to 778 °C at a rate of 20 °C / h and held for 110 min, then water-cooled freely at a rate of 0.2 °C / s to 727 °C and held for 75 min, then slowly cooled at a rate of 10 °C / h to 708 °C and held for 45 min, rapidly cooled at a rate of 35 °C / h to 628 °C and held for 30 min, and then air-cooled freely at a rate of 2 °C / s to 240 °C and held for 30 min;

[0031] Leveling treatment: During the leveling treatment process, a constant rolling force mode of 600 tons is adopted, and the tension of the uncoiling section is 0.06 KN / mm 2 , and the tension of the coiling section is 0.08 KN / mm 2 ; Leveling to a cold-rolled finished steel strip of 1.1 mm × 1500 mm, there is no deformation or cracking phenomenon in the subsequent processes.

[0032] Example 2

[0033] A temperature control process for spheroidizing annealing of cold-rolled steel coils is as follows:

[0034] Using a T8 hot-rolled steel coil as the raw material, with a specification of 4 mm × 1500 mm, after pickling, cold rolling treatment is carried out;

[0035] Cold rolling process: The pickled hot-rolled steel coil is cold-rolled through a cold rolling mill in multiple passes. The reduction ratios of the first two passes of cold rolling are 20% and 17.6% in sequence, and the total reduction ratio is 37.6%. Then, it is annealed in the middle once: heated to 670°C at a rate of 40°C / h and held for 30 min, and then cooled in the furnace to 140°C. Then, it is cold-rolled in two more passes, and the reduction ratios are 17.8% and 19.4% in sequence, and the total reduction ratio is 37.2%. Then, it is annealed in the middle again: heated to 650°C at a rate of 40°C / h and held for 30 min, and then cooled in the furnace to 130°C to obtain the cold-rolled steel coil;

[0036] Spheroidizing annealing process: The cold-rolled steel coil is processed in an annealing furnace. It is heated freely to 340°C, then rapidly heated to 670°C at a rate of 80°C / h and held for 60 min. Then, it is slowly heated to 770°C at a rate of 15°C / h and held for 90 min. Then, it is cooled freely by water cooling at a rate of 0.15°C / s to 725°C and held for 60 min. Then, it is slowly cooled to 705°C at a rate of 8°C / h and held for 30 min. It is rapidly cooled to 620°C at a rate of 30°C / h and held for 20 min. Then, it is cooled freely by air cooling at a rate of 1°C / s to 220°C and held for 20 min;

[0037] Skin pass rolling treatment. The skin pass rolling process adopts a 500-ton constant rolling force mode. The tension in the uncoiling section is 0.04 KN / mm 2 , and the tension in the coiling section is 0.06 KN / mm 2 ; It is skin passed to a cold-rolled finished steel strip of 1 mm × 1500 mm, and there is no deformation or cracking phenomenon in the subsequent processes.

[0038] Example 3

[0039] A temperature control process for spheroidizing annealing of cold-rolled steel coils is as follows:

[0040] Using a T8 hot-rolled steel coil as the raw material, with a specification of 4 mm × 1500 mm, it is pickled and then cold-rolled;

[0041] Cold rolling process: The pickled hot-rolled steel coil is cold-rolled through a cold rolling mill in multiple passes. The reduction ratios of the first two passes of cold rolling are 16%, 14%, and 20% in sequence, and the total reduction ratio is 50%. Then, it is annealed in the middle once: heated to 690°C at a rate of 30°C / h and held for 90 min, and then cooled in the furnace to 130°C. Then, it is cold-rolled in a single pass, and the reduction ratio is 17.3% to obtain the cold-rolled steel coil;

[0042] Spheroidizing annealing process: The cold-rolled steel coil is processed in an annealing furnace. It is heated freely to 360°C first, then rapidly heated to 690°C at a rate of 75°C / h and held for 90 min. Then it is slowly heated to 785°C at a rate of 12°C / h and held for 120 min. Then it is cooled freely by water cooling at a rate of 0.1°C / s to 730°C and held for 90 min. Then it is slowly cooled to 710°C at a rate of 5°C / h and held for 60 min. It is rapidly cooled to 635°C at a rate of 25°C / h and held for 40 min. Then it is cooled freely by air cooling at a rate of 0.5°C / s to 260°C and held for 40 min;

[0043] Skin pass treatment: In the skin pass treatment process, a 400-ton constant rolling force mode is adopted. The tension in the uncoiling section is 0.03 KN / mm 2 , and the tension in the coiling section is 0.04 KN / mm 2 ; The cold-rolled finished steel strip is skinned to 1.3 mm × 1500 mm, and there is no deformation or cracking in the subsequent processes.

[0044] Comparative Example 1

[0045] A spheroidizing annealing temperature control process for cold-rolled steel coils, which is different from Example 3 in that: the spheroidizing annealing treatment process is: heated freely to 350°C, then heated to 690°C at a rate of 75°C / h and held for 90 min; then heated to 785°C at a rate of 12°C / h and held for 120 min. After the holding is completed, it is slowly cooled to 635°C at a rate of 5°C / h, and then cooled freely by air cooling to 260°C; there are deformation and cracking phenomena in the subsequent processes.

[0046] Comparative Example 2

[0047] A spheroidizing annealing temperature control process for cold-rolled steel coils, which is different from Example 3 in that: the spheroidizing annealing treatment process is: heated to 350°C at a rate of 50°C / h, then heated to 650°C at a rate of 75°C / h and held for 120 min, then heated to 785°C at a rate of 50°C / h and held for 480 min. After the holding is completed, it is slowly cooled to 635°C at a rate of 5°C / h, and then cooled freely by air cooling to 260°C; there are deformation and cracking phenomena in the subsequent processes.

[0048] Comparative Example 3

[0049] A temperature control process for spheroidizing annealing of cold-rolled steel coils, which is different from Example 3 in that: the spheroidizing annealing process is as follows: freely raise the temperature to 360°C, then rapidly raise the temperature to 785°C at a rate of 50°C / h and hold for 270 min, then freely cool by water cooling at a rate of 0.1°C / s to 730°C and hold for 90 min, then slowly cool at a rate of 5°C / h to 710°C and hold for 60 min, rapidly cool at a rate of 25°C / h to 635°C and hold for 40 min, then freely cool by air cooling at a rate of 0.5°C / s to 260°C and hold for 40 min; there are cracking and deformation phenomena in the subsequent processes.

[0050] Comparative Example 4

[0051] A temperature control process for spheroidizing annealing of cold-rolled steel coils, which is different from Example 3 in that: the spheroidizing annealing process is as follows: freely raise the temperature to 360°C, then rapidly raise the temperature to 785°C at a rate of 50°C / h and hold for 270 min, then freely cool by water cooling at a rate of 0.1°C / s to 730°C and hold for 90 min, then slowly cool at a rate of 5°C / h to 635°C and hold for 120 min, then freely cool by air cooling at a rate of 0.5°C / s to 260°C and hold for 40 min; there are deformation and cracking phenomena in the subsequent processes.

[0052] Comparative Example 5

[0053] A temperature control process for spheroidizing annealing of cold-rolled steel coils, which is different from Example 3 in that: the spheroidizing annealing process is as follows: freely raise the temperature to 360°C, then rapidly raise the temperature to 785°C at a rate of 50°C / h and hold for 600 min, after the holding ends, slowly cool to 635°C at a rate of 5°C / h, and then freely cool by air cooling to 260°C; there are deformation and cracking phenomena in the subsequent processes.

[0054] Comparative Example 6

[0055] A temperature control process for spheroidizing annealing of cold-rolled steel coils, which is different from Example 3 in that: the spheroidizing annealing process is as follows: freely raise the temperature to 280°C, then rapidly raise the temperature to 550°C at a rate of 50°C / h and hold for 120 min, then slowly raise the temperature to 735°C at a rate of 50°C / h and hold for 180 min, then slowly cool at a rate of 5°C / h to 710°C and hold for 90 min, rapidly cool at a rate of 25°C / h to 600°C and hold for 40 min, then freely cool by air cooling at a rate of 0.5°C / s to 260°C and hold for 40 min; there are cracking and deformation phenomena in the subsequent processes.

[0056] Comparative Example 7

[0057] A temperature control process for spheroidizing annealing of cold-rolled steel coils, which is different from Example 3 in that: the process of spheroidizing annealing is as follows: freely heat up to 360°C, then rapidly heat up to 690°C at a rate of 75°C / h and hold for 90 min, then slowly heat up to 785°C at a rate of 12°C / h and hold for 120 min, then freely cool down to 730°C by air cooling at a rate of 0.1°C / s and hold for 90 min, then slowly cool down to 710°C at a rate of 5°C / h and hold for 60 min, rapidly cool down to 635°C at a rate of 25°C / h and hold for 40 min, then freely cool down to 260°C by air cooling at a rate of 0.5°C / s and hold for 40 min; slight deformation and cracking phenomena occur in the subsequent processes.

[0058] Comparative Example 8

[0059] A temperature control process for spheroidizing annealing of cold-rolled steel coils, which is different from Example 3 in that: the process of spheroidizing annealing is as follows: freely heat up to 360°C, then rapidly heat up to 690°C at a rate of 75°C / h and hold for 90 min, then slowly heat up to 785°C at a rate of 12°C / h and hold for 120 min, then slowly cool down to 710°C at a rate of 5°C / h and hold for 150 min, rapidly cool down to 635°C at a rate of 25°C / h and hold for 40 min, then freely cool down to 260°C by air cooling at a rate of 0.5°C / s and hold for 40 min; deformation and cracking phenomena occur in the subsequent processes.

[0060] In the above Examples 1 to 3 and Comparative Examples 1 to 8, the mass percentages of the chemical components of the T8 hot-rolled steel coil raw materials used are: C 0.67%, Si 0.25%, Mn 0.96%, P 0.024%, S 0.028%, and the rest are Fe and inevitable impurities. The microstructure is uniform and fine, and it is a fully pearlite structure.

[0061] The cold-rolled steel coils after spheroidizing annealing in Examples 1 to 6 and Comparative Examples 1 to 8 of the present invention were subjected to performance tests. Among them, the hardness test was carried out in accordance with GB / T 231.2-2022 "Brinell hardness test for metallic materials"; the spheroidization rate was carried out in accordance with JB / T 5074-2007 "Rating of spheroidized structures in low and medium carbon steels"; the tensile test was carried out in accordance with GB / T 228.1-2021 "Tensile testing of metallic materials - Part 1: Method of test at room temperature", and the results are shown in Table 1.

[0062] Hardness HB Spheroidization rate % Yield strength MPa Tensile strength MPa Elongation rate % Example 1 163 95 882 1145 17 Example 2 158 90 857 1085 14 Example 3 168 92 865 1118 15 Comparative example 1 193 70 731 847 7 Comparative example 2 197 68 726 824 6 Comparative example 3 201 69 739 845 7 Comparative example 4 191 79 795 905 9 Comparative example 5 199 65 757 876 8 Comparative example 6 204 72 723 853 7 Comparative example 7 189 86 831 985 12 Comparative example 8 194 81 808 933 10

[0063] As can be seen from Table 1 above, in the spheroidizing annealing process of Embodiments 1 to 3 of the present invention, free heating, first heating and maintaining, second heating and maintaining, first free cooling and maintaining, first cooling and maintaining, second cooling and maintaining, and second free cooling and maintaining are carried out in sequence, so that the organizational structure and mechanical properties of the T8 cold-rolled steel coil are improved, and there is no deformation or cracking phenomenon in the subsequent processes, which can improve the quality of the cold-rolled steel coil product, meet the market demand, and enhance the market competitiveness; further, in the spheroidizing annealing process, first water cooling and then air cooling for free cooling is beneficial to the improvement of the organizational structure of the cold-rolled steel coil and the enhancement of the mechanical properties of the cold-rolled steel coil material.

[0064] In summary, the temperature control process for spheroidizing annealing of the cold-rolled steel coil of the present invention can improve the organizational structure and mechanical properties of the cold-rolled steel coil, and improve the performance of the finished parts of the cold-rolled steel coil ball, achieving a better use effect.

[0065] For those skilled in the art, it is obvious that the present invention is not limited to the details of the above exemplary embodiments, and without departing from the spirit and basic characteristics of the present invention, the present invention can be implemented in other specific forms. Therefore, from any point of view, the embodiments should be regarded as exemplary and non-limiting. The scope of the present invention is defined by the appended claims rather than the above description. Therefore, all changes falling within the meaning and scope of the equivalent elements of the claims are intended to be included in the present invention.

[0066] In addition, it should be understood that although this specification is described according to embodiments, not every embodiment only contains an independent technical solution. This narrative way of the specification is only for clarity. Those skilled in the art should regard the specification as a whole, and the technical solutions in each embodiment can also be appropriately combined to form other embodiments that can be understood by those skilled in the art.

Claims

1. A temperature control process for spheroidizing annealing of cold-rolled steel coils, characterized in that, Including: Subject the cold-rolled steel coil to spheroidizing annealing. The spheroidizing annealing process is as follows: free heating, first heating and maintaining, second heating and maintaining, first free cooling and maintaining, first cooling and maintaining, second cooling and maintaining, second free cooling and maintaining in sequence; Free heat up to 340 - 360 °C; first heat up to 670 - 690 °C and maintain for 60 - 90 min; second heat up to 770 - 785 °C and maintain for 90 - 120 min; first free cool to 725 - 730 °C and maintain for 60 - 90 min; first cool to 705 - 710 °C and maintain for 30 - 60 min; second cool to and maintain at 620 - 635 °C and maintain for 20 - 40 min; second free cool to 220 - 260 °C and maintain for 20 - 40 min.

2. The temperature control process for spheroidizing annealing of cold-rolled steel coils according to claim 1, characterized in that The first free cooling is water-cooled free cooling, and the second free cooling is air-cooled free cooling.

3. The temperature control process for spheroidizing annealing of cold-rolled steel coils according to claim 2, characterized in that, The rate of water-cooled free cooling is 0.1 - 0.2 °C / s; the rate of air-cooled free cooling is 0.5 - 2 °C / s.

4. A spheroidizing annealing temperature control process for cold-rolled steel coils according to claim 1, characterized in that, The rate of the first heating is greater than the rate of the second heating.

5. A temperature control process for spheroidizing annealing of cold-rolled steel coils according to claim 4, characterized in that, The rate of the first heating is 75 - 85 °C / h, and the rate of the second heating is 12 - 20 °C / h.

6. A spheroidizing annealing temperature control process for cold-rolled steel coils according to claim 1, characterized in that, The rate of the first cooling is less than the rate of the second cooling.

7. A temperature control process for spheroidizing annealing of cold-rolled steel coils according to claim 6, characterized in that The rate of the first cooling is 5 - 10 °C / h, and the rate of the second cooling is 25 - 35 °C / h.

8. A spheroidizing annealing temperature control process for cold-rolled steel coils according to claim 1, characterized in that, It also includes obtaining the cold-rolled steel coil by cold rolling the pickled hot-rolled steel coil; the cold rolling process is multi-pass cold rolling with at least one intermediate annealing.

9. A temperature control process for spheroidizing annealing of cold-rolled steel coils according to claim 8, characterized in that, The reduction rate of single-pass cold rolling in the cold rolling process is 14 - 20%, and when the total reduction rate of continuous multi-pass cold rolling is 37 - 50%, one intermediate annealing is carried out, and the intermediate annealing is at least once.

10. A temperature control process for spheroidizing annealing of cold-rolled steel coils according to claim 8 or 9, characterized in that, The intermediate annealing process is: heat up to 600 - 720 °C at a rate of 30 - 50 °C / h, hold for 30 - 120 min, and then cool in the furnace to 130 - 150 °C.