Delayed cracking resistant steel plate and preparation method thereof

Abstract

The invention provides a delayed cracking resistant high-strength steel plate which comprises the following components in percentage by mass: 0.014 to 0.017 percent of C, 0.10 to 0.20 percent of Si, 0.80 to 1.05 percent of Mn, 0.010 to 0.018 percent of Nb, 0.20 to 0.30 percent of Cr, 0.0008 to 0.0016 percent of B and the balance of Fe. The invention further provides a preparation method of the delayed-cracking-resistant high-strength steel plate, and the preparation method comprises the following steps: mixing the raw materials according to the component proportion, smelting and casting to obtain an initial plate blank; the initial plate blank is heated and then subjected to rough rolling and finish rolling in sequence; and the plate blank obtained through finish rolling is rapidly cooled, and finally heat treatment is conducted. According to the delayed cracking resistant high-strength steel plate and the preparation method thereof, the risk of delayed cracking after the steel plate is cooled is greatly reduced, so that the steel plate has the characteristics of high strength, high toughness, low cost, high weldability and the like.

Description

Technical Field

[0001] The invention relates to the technical field of microalloys, and in particular to a delayed cracking resistant steel plate and a preparation method thereof. Background Art

[0002] Delayed cracking is a delayed destruction phenomenon of a material under specific conditions, mainly occurring in metal materials, especially high-strength steel. This phenomenon is usually related to factors such as the material's microstructure, stress state, and hydrogen content. Delayed cracking can be divided into multiple stages, including the incubation period, slow expansion period, and sudden fracture period. The incubation period and slow expansion period are closely related to the accumulation and diffusion of hydrogen, so they are also called hydrogen-induced cracking.

[0003] With the advancement of manufacturing technology and the increase in demand in industries such as engineering machinery, coal mining machinery, marine engineering, super high-rise buildings, hydropower and nuclear power, and pressure vessels, the application of high-strength steel plates has become more and more extensive in order to reduce costs, increase "strength" and reduce weight to achieve the goal of energy saving and consumption reduction. The commonly used steel grades for engineering and coal mining machinery have rapidly increased from Q345 and Q460 to Q550, Q690, and even higher. However, as the strength increases, the delayed fracture sensitivity of steel also increases. The high sensitivity to hydrogen-induced delayed fracture has become an important factor restricting the promotion and application of high-strength steel grades. Summary of the invention

[0004] The technical problem solved by the present invention is to provide a high-strength steel plate resistant to delayed cracking. The steel plate provided in the present application has the characteristics of high strength, high toughness and good stability, and can significantly reduce the risk of delayed cracking of the steel plate.

[0005] In view of this, the present application provides a delayed cracking resistant high-strength steel plate, which comprises, by mass percentage:

[0006] C: 0.014~0.017%, Si: 0.10~0.20%, Mn: 0.80~1.05%, Nb: 0.010~0.018%, Cr: 0.20~0.30%, B: 0.0008~0.0016%, and the balance is Fe.

[0007] Preferably, it includes: C: 0.015~0.016%, Si: 0.12~0.18%, Mn: 0.90~1.0%, Nb: 0.012~0.016%, Cr: 0.21~0.28%, B: 0.0010~0.0015%, and the balance Fe.

[0008] Preferably, it includes: C: 0.015%, Si: 0.15%, Mn: 1.0%, Nb: 0.013%, Cr: 0.22%, B: 0.0014%, P: 0.015%, S: 0.005%, and the balance Fe.

[0009] The present application also provides a method for preparing the delayed cracking resistant high-strength steel plate, comprising the following steps:

[0010] The raw materials are mixed according to the composition ratio, then smelted and cast to obtain an initial slab;

[0011] The initial slab is heated, and then subjected to rough rolling and finish rolling in sequence;

[0012] The slab obtained by finish rolling is rapidly cooled and finally heat treated.

[0013] Preferably, the heating temperature is 1120-1160° C., and the heating rate is ≥8 min / cm.

[0014] Preferably, the starting rolling temperature of the rough rolling is 1050-1100°C, the starting rolling temperature of the finish rolling is 950-1000°C, and the final rolling temperature of the finish rolling is 850-900°C.

[0015] Preferably, the starting temperature of the rapid cooling is 810-850°C, the average cooling rate is 5-40°C / s, and the termination temperature of the rapid cooling is ≤150°C or 600-650°C.

[0016] Preferably, the termination temperature of the rapid cooling is ≤150°C, and the heat treatment is the first tempering; the termination temperature of the rapid cooling is 600-650°C, and the heat treatment includes quenching and second tempering performed sequentially.

[0017] Preferably, the quenching temperature is 900-920° C. and the quenching time is 5-15 min.

[0018] Preferably, the temperature of the first tempering is 590-650° C., and the time is 15-25 min; the temperature of the second tempering is 530-610° C., and the time is 15-25 min.

[0019] The present application provides a method for preparing a delayed cracking resistant high-strength steel plate, which comprises: C: 0.014-0.017%, Si: 0.10-0.20%, Mn: 0.80-1.05%, Nb: 0.010-0.018%, Cr: 0.20-0.30%, B: 0.0008-0.0016%, and the balance Fe; the present application adjusts the ratio of C, Mn, Cr, and Nb, and reduces the content of Mn to below 1.05wt%, thereby significantly reducing the banded structure caused by Mn segregation and improving the uniformity of the structure, and by adding a small amount of Nb element, the recrystallization termination temperature is increased, the grains are refined, the crack arrest ability of the steel plate is significantly improved, and the possibility of delayed cracking is reduced.

[0020] The present application also provides a method for preparing a delayed cracking resistant high-strength steel plate, which first heats the initial slab, then performs rough rolling and finish rolling in sequence, and finally cools the obtained slab quickly to obtain the delayed cracking resistant steel plate after heat treatment; the preparation method of the delayed cracking resistant high-strength steel plate provided in the present application realizes the refinement of the original austenite grain size through the combination of chemical composition and process, reduces the amount of elements easily segregated, improves the uniformity of the structure, is conducive to obtaining a high-strength steel plate with high strength, high toughness, low cost, high weldability and high stability, and reduces the risk of delayed cracking of the high-strength steel plate. DETAILED DESCRIPTION

[0021] In order to further understand the present invention, preferred embodiments of the present invention are described below in conjunction with examples. However, it should be understood that these descriptions are only for further illustrating the features and advantages of the present invention, rather than limiting the claims of the present invention.

[0022] In view of the problem that the high strength of high-strength steel plates in the prior art cannot be taken into account at the same time as reducing the delayed fracture sensitivity, the present application provides a delayed cracking resistant high-strength steel plate and a preparation method thereof, which adjusts the composition and preparation process of the high-strength steel plate to make the obtained high-strength steel plate have the characteristics of high strength, high toughness, low cost, high weldability, high stability and delayed cracking resistance. Specifically, the embodiment of the present invention discloses a delayed cracking resistant high-strength steel plate, which comprises, in mass percentage:

[0023] C: 0.014~0.017%, Si: 0.10~0.20%, Mn: 0.80~1.05%, Nb: 0.010~0.018%, Cr: 0.20~0.30%, B: 0.0008~0.0016%, and the balance is Fe.

[0024] Specifically, the high-strength steel plate includes: C: 0.015~0.016%, Si: 0.12~0.18%, Mn: 0.90~1.0%, Nb: 0.012~0.016%, Cr: 0.21~0.28%, B: 0.0010~0.0015%, and the balance Fe.

[0025] For example, the C content described in this application is 0.014%, 0.015%, 0.016%, and 0.017%.

[0026] For example, the Si content described in this application is 0.11%, 0.12%, 0.13%, 0.14%, 0.15%, 0.16%, 0.17%, 0.18%, and 0.19%.

[0027] For example, the Mn content in this application is 0.81%, 0.82%, 0.83%, 0.84%, 0.85%, 0.86%, 0.87%, 0.88%, 0.89%, 0.90%, 0.91%, 0.92%, 0.93%, 0.94%, 0.95%, 0.96%, 0.97%, 0.98%, 0.99%, 1.00%, 1.01%, 1.02%, 1.03%, and 1.04%.

[0028] For example, the Nb content in the present application is 0.011%, 0.012%, 0.013%, 0.014%, 0.015%, 0.016%, and 0.017%.

[0029] For example, the Cr content in this application is 0.21%, 0.22%, 0.23%, 0.24%, 0.25%, 0.26%, 0.27%, 0.28%, and 0.29%.

[0030] For example, the content of B in this application is 0.0009%, 0.0010%, 0.0011%, 0.0012%, 0.0013%, 0.0014%, and 0.0015%.

[0031] Furthermore, the present application also provides a method for preparing a delayed cracking resistant high-strength steel plate, comprising the following steps:

[0032] The raw materials are mixed according to the composition ratio, then smelted and cast to obtain an initial slab;

[0033] The slab is heated, and then subjected to rough rolling and finish rolling in sequence;

[0034] The slab obtained by finish rolling is rapidly cooled and finally heat treated.

[0035] In the preparation process of high-strength steel plates, the present application first mixes the raw materials according to the component ratio, then melts and casts them to obtain an initial slab; the raw materials are selected from raw materials well known to those skilled in the art, and the melting and casting are carried out in a manner well known to those skilled in the art, and the present application has no special restrictions on this.

[0036] After obtaining the initial slab, the present application heats the slab, the heating temperature is 1120-1160°C, and the heating rate is ≥8 min / cm; specifically, the heating rate is 1130-1155°C, more specifically, the heating temperature is 1140-1150°C, and the heating rate is specifically 10-15 min / cm.

[0037] The present application then carries out rough rolling on the initial slab, and the starting temperature of the rough rolling is 1050-1100°C, specifically, the starting temperature of the rough rolling is 1080-1100°C, more specifically, the starting temperature of the rough rolling in the present application is 1060°C, 1070°C, 1080°C, 1090°C or 1100°C; the starting temperature of the finishing rolling is 950-1000°C, and the final rolling temperature of the finishing rolling is 850-900°C; specifically, the starting temperature of the finishing rolling is 970-990°C, and the final rolling temperature of the finishing rolling is 850-880°C; for example, the starting temperature of the finishing rolling in the present application is 960°C, 970°C, 980°C, 990°C, and the final rolling temperature of the finishing rolling is 860°C, 870°C, 880°C, 890°C.

[0038] According to the present invention, rapid cooling is carried out after finish rolling, the starting temperature of the rapid cooling is 810-850°C, the average cooling rate is 5-40°C / s, and the termination temperature of the rapid cooling is ≤150°C or 600-650°C; specifically, the starting temperature of the rapid cooling is 810-840°C, the average cooling rate is 8-38°C / s, and the termination temperature of the rapid cooling is 100-140°C or 630-650°C; more specifically, the starting temperature of the rapid cooling is 820°C, 830°C, and the average cooling rate is 8-12°C / s, 10-15°C / s, 28-36°C / s, 30-38°C / s, 32-40°C / s.

[0039] In the present application, the rapidly cooled slab is finally subjected to heat treatment, and the termination temperature of the rapid cooling is ≤150°C, then the heat treatment is the first tempering, and the termination temperature of the rapid cooling is 600~650°C, then the heat treatment includes quenching and second tempering performed in sequence; specifically, the temperature of the first tempering is 590~650°C, and the time is 15~25min, more specifically, the temperature of the first tempering is 597~633°C, and the time is 16~24min, more specifically, the temperature of the first tempering is 597~603°C, 607~613°C, 627~633°C. The quenching temperature is 900-920°C, and the time is 5-15min; the second tempering temperature is 530-610°C, and the time is 15-25min; specifically, the quenching temperature is 907-913°C, and the time is 7-13min, and the second tempering temperature is 537-603°C, and the time is 16-24min; more specifically, the quenching temperature is 909-912°C, and the time is 9-11min, and the second tempering temperature is 543-597°C. When the termination temperature of the above-mentioned rapid cooling is ≤150°C, the structure is lath bainite, and the structure remains unchanged after tempering, and the internal stress is reduced; or, when the termination temperature of the rapid cooling is 600-650°C, the structure is ferrite + pearlite, and after quenching, it is lath bainite or lath bainite + martensite, and after tempering, it is lath bainite + troostite, and the internal stress is reduced.

[0040] The invention realizes the refinement of the original austenite grain size by adopting reasonable chemical composition and process design, reduces the addition amount of elements prone to segregation, improves the uniformity of the organization, and has the characteristics of high strength, high toughness, low cost, high weldability, high stability and the like.

[0041] In order to further understand the present invention, the delayed cracking resistant steel plate and the preparation method thereof provided by the present invention are described in detail below in conjunction with embodiments. The protection scope of the present invention is not limited by the following embodiments.

[0042] Example 1 Preparation of 20 mm thick high-strength steel plate with delayed cracking resistance

[0043] A delayed cracking resistant high-strength steel plate, the chemical composition and weight percentage content (wt%) of the steel are:

[0044] C: 0.016wt%, Si: 0.17wt%, Mn: 0.90wt%, P: 0.014wt%, S: 0.005wt%, Nb: 0.014wt%, Cr: 0.24wt%, B: 0.0013wt%, the rest are Fe and unavoidable impurities;

[0045] The manufacturing method of delayed cracking resistant high-strength steel plate comprises the following steps: slab heating - high-pressure water descaling cooling - double-stand reversible wide and thick plate rolling mill - rapid cooling - heat treatment; the specific steps are as follows:

[0046] 1) Slab heating

[0047] The slab with a thickness of 250 mm was discharged from the furnace at a temperature of 1155°C and a heating rate of 10 min / cm;

[0048] 2) Rolling

[0049] The heated slab is subjected to rough rolling and finish rolling in sequence to obtain a 20 mm slab; the starting rolling temperature of the rough rolling is 1100°C, the starting rolling temperature of the finish rolling is 990°C, and the final rolling temperature of the finish rolling is 870°C;

[0050] 3) Cooling control system

[0051] The rolled slab is cooled, with the starting temperature at 840°C, the average cooling rate in the cooling zone at 34°C / s, and the cooling end temperature at ≤150°C;

[0052] 4) Heat treatment

[0053] The cooled slab is tempered at a tempering temperature of 630° C. for a holding time of 20 minutes to obtain a delayed cracking resistant steel plate.

[0054] The properties of the steel plate prepared in this example were tested, and the test results are listed in Table 1.

[0055] Example 2 Preparation of 20 mm thick high-strength steel plate with delayed cracking resistance

[0056] A delayed cracking resistant high-strength steel plate, the chemical composition and weight percentage content (wt%) of the steel are:

[0057] C: 0.014wt%, Si: 0.15wt%, Mn: 0.99wt%, P: 0.013wt%, S: 0.005wt%, Nb: 0.016wt%, Cr: 0.23wt%, B: 0.0014wt%, and the rest are Fe and unavoidable impurities.

[0058] The manufacturing method of delayed cracking resistant high-strength steel plate comprises the following steps: slab heating - high-pressure water descaling cooling - double-stand reversible wide and thick plate rolling mill - rapid cooling - heat treatment; the specific steps are as follows:

[0059] 1) Slab heating

[0060] The slab with a thickness of 250 mm was heated to a furnace temperature of 1155°C and a heating rate of 8 min / cm;

[0061] 2) Rolling

[0062] The heated slab is subjected to rough rolling and finish rolling in sequence to obtain a slab with a thickness of 20 mm; the starting rolling temperature of the rough rolling is 1100°C, the starting rolling temperature of the finish rolling is 990°C, and the final rolling temperature of the finish rolling is 880°C;

[0063] 3) Cooling control system

[0064] The rolled slab is cooled, the cooling start temperature is 830°C, the average cooling rate is 12°C / s, and the cooling end temperature is 640°C;

[0065] 4) Heat treatment

[0066] The cooled slab was quenched and tempered at a quenching temperature of 910° C. for a holding time of 10 minutes and a tempering temperature of 600° C. for a holding time of 20 minutes to obtain a delayed cracking resistant steel plate.

[0067] The properties of the steel plate prepared in this example were tested, and the test results are listed in Table 1.

[0068] Example 3 Preparation of 20 mm thick high-strength steel plate with delayed cracking resistance

[0069] A delayed cracking resistant high-strength steel plate, the chemical composition and weight percentage content (wt%) of the steel are:

[0070] C: 0.015wt%, Si: 0.16wt%, Mn: 1.01wt%, P: 0.013wt%, S: 0.003wt%, Nb: 0.015wt%, Cr: 0.24wt%, B: 0.0013wt%, and the rest are Fe and unavoidable impurities.

[0071] The manufacturing method of delayed cracking resistant high-strength steel plate comprises the following steps: slab heating - high-pressure water descaling cooling - double-stand reversible wide and thick plate rolling mill - rapid cooling - heat treatment; the specific steps are as follows:

[0072] 1) Slab heating

[0073] The slab with a thickness of 250 mm was heated to a furnace temperature of 1155°C and a heating rate of 8 min / cm;

[0074] 2) Rolling

[0075] The heated slab is subjected to rough rolling and finish rolling in sequence to obtain a slab with a thickness of 20 mm, wherein the starting rolling temperature of the rough rolling is 1100° C., the starting rolling temperature of the finish rolling is 1000° C., and the final rolling temperature of the finish rolling is 890° C.;

[0076] 3) Cooling control system

[0077] The rolled slab is cooled, the cooling start temperature is 840°C, the average cooling rate is 36°C / s, and the cooling end temperature is ≤150°C;

[0078] 4) Heat treatment

[0079] The cooled slab is tempered at a tempering temperature of 600° C. for a holding time of 20 minutes to obtain a delayed cracking resistant steel plate.

[0080] The properties of the steel plate prepared in this example were tested, and the test results are listed in Table 1.

[0081] Example 4 Preparation of 25 mm thick high-strength steel plate with delayed cracking resistance

[0082] A delayed cracking resistant high-strength steel plate, the chemical composition and weight percentage content (wt%) of the steel are:

[0083] C: 0.017wt%, Si: 0.14wt%, Mn: 1.02wt%, P: 0.015wt%, S: 0.003wt%, Nb: 0.015wt%, Cr: 0.21wt%, B: 0.0016wt%, and the rest are Fe and unavoidable impurities.

[0084] The manufacturing method of delayed cracking resistant high-strength steel plate comprises the following steps: slab heating - high-pressure water descaling cooling - double-stand reversible wide and thick plate rolling mill - rapid cooling - heat treatment; the specific steps are as follows:

[0085] 1) Slab heating

[0086] The slab with a thickness of 250 mm was heated to a furnace temperature of 1155°C and a heating time of 8 min / cm;

[0087] 2) Rolling

[0088] The heated slab is subjected to rough rolling and finish rolling in sequence to obtain a steel plate with a thickness of 25 mm, wherein the starting rolling temperature of the rough rolling is 1080-1100° C., the starting rolling temperature of the finish rolling is 960° C., and the final rolling temperature of the finish rolling is 860° C.;

[0089] 3) Cooling control system

[0090] The rolled slab is cooled, the cooling start temperature is 820°C, the average cooling rate is 10°C / s, and the cooling end temperature is 640°C;

[0091] 4) Heat treatment

[0092] The cooled slab was quenched and tempered at a quenching temperature of 910° C. for a holding time of 10 minutes and a tempering temperature of 540° C. for a holding time of 20 minutes to obtain a delayed cracking resistant steel plate.

[0093] The properties of the steel plate prepared in this example were tested, and the test results are listed in Table 1.

[0094] Example 5 Preparation of 25 mm thick high-strength steel plate with delayed cracking resistance

[0095] A delayed cracking resistant high-strength steel plate, the chemical composition and weight percentage content (wt%) of the steel are:

[0096] C: 0.014wt%, Si: 0.17wt%, Mn: 0.97wt%, P: 0.012wt%, S: 0.002wt%, Nb: 0.016wt%, Cr: 0.25wt%, B: 0.0013wt%, and the rest are Fe and unavoidable impurities.

[0097] The manufacturing method of delayed cracking resistant high-strength steel plate comprises the following steps: slab heating - high-pressure water descaling cooling - double-stand reversible wide and thick plate rolling mill - rapid cooling - heat treatment; the specific steps are as follows:

[0098] 1) Slab heating

[0099] The slab with a thickness of 250 mm was heated to a furnace temperature of 1155°C and a heating rate of 8 min / cm;

[0100] 2) Rolling

[0101] The heated slab is subjected to rough rolling and finish rolling in sequence, the starting rolling temperature of the rough rolling is 1100°C, the starting rolling temperature of the finish rolling is 980°C, and the final rolling temperature of the finish rolling is 850°C;

[0102] 3) Cooling control system

[0103] The rolled steel plate is cooled, the cooling start temperature is 830°C, the average cooling rate is 32°C / s, and the cooling end temperature is ≤150°C;

[0104] 4) Heat treatment

[0105] The cooled steel plate was tempered at a tempering temperature of 610° C. for a holding time of 20 minutes to obtain a delayed cracking resistant steel plate.

[0106] The properties of the steel plate prepared in this example were tested, and the test results are listed in Table 1.

[0107] Comparative Example 1

[0108] The preparation method is the same as that of Example 1, except that the composition of the steel plate is adjusted, including: C: 0.16wt%, Si: 0.30wt%, Mn: 1.35wt%, P: ≤0.020wt%, S: ≤0.010wt%, Cr: 0.23wt%, Ti: 0.021wt%, B: 0.0018wt%, and the rest is Fe and unavoidable impurities.

[0109] The properties of the steel plate prepared in this example were tested, and the test results are listed in Table 1.

[0110] Comparative Example 2

[0111] The preparation method is the same as that of Example 1, except that the composition of the steel plate is adjusted, including: C: 0.09wt%, Si: 0.21wt%, Mn: 1.59wt%, P: ≤0.020wt%, S: ≤0.010wt%, Nb: 0.38wt%, Ti: 0.022wt%, B: 0.0019wt%, and the rest is Fe and unavoidable impurities.

[0112] The properties of the steel plate prepared in this example were tested, and the test results are listed in Table 1.

[0113] Table 1 Performance data of steel plates prepared in Examples

[0114]

[0115] The above embodiments are only used to help understand the method and core idea of ​​the present invention. It should be noted that, for those skilled in the art, several improvements and modifications can be made to the present invention without departing from the principles of the present invention, and these improvements and modifications also fall within the scope of protection of the claims of the present invention.

[0116] The above description of the disclosed embodiments enables one skilled in the art to implement or use the present invention. Various modifications to these embodiments will be apparent to one skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the present invention. Therefore, the present invention will not be limited to the embodiments shown herein, but rather to the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A delayed cracking resistant high-strength steel plate, comprising, by mass percentage: C: 0.014~0.017%, Si: 0.10~0.20%, Mn: 0.80~1.05%, Nb: 0.010~0.018%, Cr: 0.20~0.30%, B: 0.0008~0.0016%, and the balance is Fe.

2. The delayed cracking resistant high-strength steel plate according to claim 1, characterized in that: Including: C: 0.015~0.016%, Si: 0.12~0.18%, Mn: 0.90~1.0%, Nb: 0.012~0.016%, Cr: 0.21~0.28%, B: 0.0010~0.0015%, and the balance Fe.

3. The delayed cracking resistant high-strength steel plate according to claim 1, characterized in that: Including: C: 0.015%, Si: 0.15%, Mn: 1.0%, Nb: 0.013%, Cr: 0.22%, B: 0.0014%, P: 0.015%, S: 0.005%, and the balance Fe.

4. The method for preparing the delayed cracking resistant high-strength steel plate according to claim 1, comprising the following steps: The raw materials are mixed according to the composition ratio, then smelted and cast to obtain an initial slab; The initial slab is heated, and then subjected to rough rolling and finish rolling in sequence; The slab obtained by finish rolling is rapidly cooled and finally heat treated.

5. The preparation method according to claim 1, characterized in that: The heating temperature is 1120-1160° C., and the heating rate is ≥8 min / cm.

6. The preparation method according to claim 1, characterized in that: The starting rolling temperature of the rough rolling is 1050-1100°C, the starting rolling temperature of the finish rolling is 950-1000°C, and the final rolling temperature of the finish rolling is 850-900°C.

7. The preparation method according to claim 1, characterized in that: The starting temperature of the rapid cooling is 810-850°C, the average cooling rate is 5-40°C / s, and the termination temperature of the rapid cooling is ≤150°C or 600-650°C.

8. The preparation method according to claim 1 or 7, characterized in that: The termination temperature of the rapid cooling is ≤150°C, and the heat treatment is the first tempering; the termination temperature of the rapid cooling is 600-650°C, and the heat treatment includes quenching and second tempering performed in sequence.

9. The preparation method according to claim 8, characterized in that: The quenching temperature is 900-920° C. and the quenching time is 5-15 minutes.

10. The preparation method according to claim 8, characterized in that: The temperature of the first tempering is 590-650° C., and the time is 15-25 min; the temperature of the second tempering is 530-610° C., and the time is 15-25 min.

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