Heat treatment process for improving anti-HIC characteristic of Q345R (HIC) steel plate

Abstract

The invention relates to the technical field of metallurgy, in particular to a heat treatment process for improving the HIC resistance of a Q345R (HIC) steel plate, which comprises the following steps of: heating the steel plate to 30-50 DEG C above AC3, keeping in-furnace time for more than or equal to 30 minutes, and then cooling in two stages: in the first stage, quickly cooling, namely cooling to Ar1 temperature at the speed of 25-30 DEG C / s; in the second stage, the cooling speed is reduced, and cooling is conducted to the Bs temperature at the speed of 2-5 DEG C / s; and then air-cooling to room temperature. The phase change degree of the steel plate is controlled by adopting a two-stage cooling mode, so that the final structure of the steel plate is changed, the purposes of reducing the banded structure and improving the acid resistance of the Q345R (HIC) steel plate are achieved, and good social and economic benefits are achieved.

Description

Technical Field

[0001] The invention relates to the technical field of metallurgy, and in particular to a heat treatment process for improving the HIC resistance of a Q345R (HIC) steel plate. Background Art

[0002] Q345R (HIC) steel is a container steel with hydrogen-induced cracking resistance. Its common structures are ferrite and pearlite. During the solidification process, due to the selective crystallization of alloy elements, the mass fraction of manganese between dendrites is significantly higher than that within dendrites. As a stabilizing element in the austenite region, manganese in the manganese-rich region r3 The transformation temperature should be lower than that of the manganese-depleted zone. When the temperature is lowered, ferrite will preferentially nucleate and grow in the manganese-depleted zone, and multiple ferrite grains will be connected and form a growth plane at the growth front. Since the solubility of carbon in ferrite is very small, as the austenite transforms to the ferrite phase, the carbon in the ferrite is gradually discharged into the austenite in the manganese-rich zone, and then forms a pearlite band in the subsequent phase transformation. The existence of the banded structure makes the Q345R (HIC) steel plate structure uneven, which seriously affects the performance of the steel plate. Since Q345R (HIC) steel is mainly used in harsh environments containing wet hydrogen sulfide such as the petrochemical industry, it is very easy to be corroded by hydrogen sulfide and cause hydrogen-induced cracking. The banded structure will increase the sensitivity of Q345R (HIC) steel to hydrogen-induced cracking, promote the initiation and expansion of cracks along the banded structure, and the more serious the banded structure, the greater the sensitivity to hydrogen-induced cracking, thereby reducing the anti-HIC performance of the steel plate and affecting the service life of the steel plate.

[0003] Q345R (HIC) steel plate heat treatment usually adopts normalizing heat treatment, for example, CN 115896417 A discloses a preparation method of Q345R extra thick steel plate with anti-HIC characteristics, adopts normalizing heat treatment, and the process parameters are as follows: heating temperature 900-910℃, holding time 2.0-2.5min / mm, air cooling to 400±50℃ after holding, and then air cooling to room temperature. Since the cooling rate of air cooling is still slow, more ferrite is still precipitated in austenite first, so the elimination effect of banded structure is not good. CN 115652046 A discloses a heat treatment process for eliminating banded structure in steel, which adopts solid solution quenching plus multiple cycles of quenching and tempering, which greatly prolongs the production process and increases production costs. Summary of the invention

[0004] In view of the technical problem of high production cost in the existing Q345R (HIC) steel plate heat treatment process, the present invention provides a heat treatment process for improving the HIC resistance of the Q345R (HIC) steel plate. By adopting a two-stage cooling method, the phase change degree of the steel plate is controlled, thereby changing the final structure of the steel plate, achieving the purpose of reducing the banded structure and improving the acid resistance of the Q345R (HIC) steel plate, and having good social and economic benefits.

[0005] The technical solution of the present invention is as follows: A heat treatment process for improving the HIC resistance of Q345R (HIC) steel plate comprises the following steps: Heat the steel plate to A C3 Above 30-50℃, in the furnace for ≥30min, then cool in two stages: In the first stage, rapid cooling is performed at a rate of 25-30℃ / s to cool to A r1 temperature; In the second stage, the cooling rate is reduced to 2-5℃ / s to cool to B s temperature; Then air cool to room temperature.

[0006] The important phase transition temperature involved in this process is A C3、 A r1 and B s ,in, A C3 : Austenitizing temperature during heating; A r1 : The starting phase transformation temperature of austenite to pearlite during cooling; B s : The starting phase transformation temperature of ferrite to bainite during cooling.

[0007] Further, the chemical composition and weight percentage of the Q345R (HIC) steel plate are C: 0.17%-0.19%, Si: 0.25%-0.45%, Mn: 1.20%-1.35%, P≤0.008%, S≤0.001%, Nb≤0.050%, V≤0.050%, Ca / S≥2.0, and the balance is Fe and unavoidable impurities.

[0008] Furthermore, the weight percentage of Ca in the Q345R (HIC) steel plate is ≤0.025%.

[0009] Furthermore, the thickness of Q345R (HIC) steel plate is 8-80 mm, for example, 8 mm, 10 mm, 12 mm, 15 mm, 20 mm, 25 mm, 30 mm, 35 mm, 40 mm, 45 mm, 50 mm, 60 mm, 70 mm, 80 mm, etc.

[0010] Furthermore, for steel plates with a thickness of >20 mm, the furnace time is controlled at 14-16 min / cm.

[0011] Furthermore, the ingots are stacked and slowly cooled, cold loaded into a furnace for heating, and rolled to obtain steel plates for heat treatment.

[0012] Furthermore, the stacking slow cooling time is more than 48 hours.

[0013] Furthermore, the heating time of the cold-charged furnace is controlled according to 8-14 min / cm of the ingot thickness, and the furnace discharge temperature is 1155-1225°C.

[0014] Furthermore, during the rolling process, the descaling high-pressure water pressure is controlled to be ≥21MPa to ensure the descaling effect; the finishing rolling temperature is 820-870°C; preferably, the steel plate with a thickness of >20mm is water-cooled after rolling, and the final cooling temperature is 640-700°C.

[0015] Furthermore, the banded structure of the Q345R (HIC) steel plate is level 0-1.0, the crack length percentage (CLR) is 0%, the crack thickness percentage (CTR) is 0%, and the crack sensitivity percentage (CSR) is 0%.

[0016] Furthermore, the final metallographic structure of the Q345R (HIC) steel plate is 40%-45% pearlite + 20%-30% bainite + 25%-35% ferrite.

[0017] The beneficial effects of the present invention are: (1) The heat treatment process of the present invention can significantly reduce the level of banded structure and reduce the promoting effect of banded structure on hydrogen-induced cracking of Q345R (HIC) steel plate, thereby improving the anti-HIC performance of the steel plate and the qualified rate of the anti-HIC performance of the steel plate, thereby achieving the effect of enhancing the service life of the steel plate in an acid environment. The quality of the produced steel plate is stable, the banded structure is level 0-1.0, and the anti-hydrogen-induced cracking performance is excellent, which can reach and far exceed the standard level of CLR≤5.0%, CTR≤1.5%, and CSR≤0.5%.

[0018] (2) The heat treatment process of the present invention is to heat the steel plate to its A C3 Above 30-50℃, the steel plate is fully austenitized, and then quickly cooled to A r1 Temperature can reduce the residence time of the steel plate in the dual phase region of austenite and ferrite, reduce the precipitation of ferrite, thereby inhibiting the ferrite from expelling carbon to the surrounding area and inhibiting the formation of banded structure. At the same time, due to the fast cooling rate, the rapid growth of grains is prevented, which is beneficial to ensure the strength performance of the steel plate; the second stage slows down the cooling rate and cools to B stemperature, and then continue to air cool to room temperature, so that pearlite and granular bainite have sufficient formation time to ensure the strength and plasticity of the steel plate.

[0019] (3) The process of the present invention is simple, has high stability, and has a short production cycle. It can significantly reduce production costs and shorten the process. While enabling Q345R (HIC) steel to obtain better acid resistance, the uniformity of the steel plate's structure is improved, thereby ensuring the comprehensive performance of the steel plate and having great market promotion and application prospects. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, for ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0021] Figure 1 This is a metallographic photograph of the Q345R (HIC) steel plate prepared in Example 1.

[0022] Figure 2 This is a metallographic photograph of the Q345R (HIC) steel plate prepared in Comparative Example 1.

[0023] Figure 3 This is a metallographic photograph of the Q345R (HIC) steel plate prepared in Example 2.

[0024] Figure 4 This is a metallographic photograph of the Q345R (HIC) steel plate prepared in Comparative Example 2. DETAILED DESCRIPTION

[0025] In order to enable those skilled in the art to better understand the technical solutions in the present invention, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work should fall within the scope of protection of the present invention.

[0026] Example 1 The ingot with a thickness of 230 mm, a chemical composition and its weight percentage of C: 0.18%, Si: 0.30%, Mn: 1.25%, P: 0.007%, S: 0.001%, Nb: 0.030%, V: 0.040%, Ca: 0.0025%, and the remainder being Fe and unavoidable impurities, was stacked and slowly cooled for 48 hours off the line, and was cold loaded into a heating furnace with a heating time of 207 min and a furnace outlet temperature of 1200°C. Then, it was rolled. During the rolling process, the descaling high-pressure water pressure was controlled to 23 MPa to ensure the descaling effect, and the finishing rolling temperature was controlled to 850°C to obtain a steel plate with a thickness of 12 mm.

[0027] Calculate the austenitizing temperature A of the steel plate C3 The starting phase transformation temperature of austenite to pearlite is 846℃. r1 The starting phase transformation temperature of ferrite to bainite is 695℃. s It is 626℃.

[0028] The steel plate is heat treated as follows: The steel plate was heated to 880 °C, kept at this temperature for 30 min, cooled to 695 °C at a rate of 30 °C / s, then cooled to 626 °C at a rate of 3 °C / s, and then air-cooled to room temperature.

[0029] Observe the metallographic structure of the final steel plate, the results are as follows Figure 1 As shown, the banded structure of the steel plate is grade 0.5, and the metallographic structure is 40% pearlite + 25% bainite + 35% ferrite. According to the method specified in the NACE TM0284 standard, three samples were taken from the steel plate for HIC resistance test, with CLR of 0%, CTR of 0%, and CSR of 0%. The mechanical properties of the steel plate were tested, and the yield strength was 480MPa, the tensile strength was 596MPa, the elongation was 26%, and the impact energy at -30℃ was 228J.

[0030] Comparative Example 1 Comparative Example 1 is basically the same as Example 1, except that a different heat treatment process is used. The heat treatment process of Comparative Example 1 is normalizing treatment, where the steel plate is heated to 880° C., kept at this temperature for 30 minutes, and then air-cooled to room temperature.

[0031] Observe the metallographic structure of the final steel plate, the results are as follows Figure 2 As shown, the banded structure of the steel plate is level 5, and the metallographic structure is 75% ferrite + 25% pearlite. According to the method specified in the NACE TM0284 standard, three samples were taken from the steel plate for HIC resistance test, with CLR of 5%, CTR of 1.0%, and CSR of 0.5%. The mechanical properties of the steel plate were tested, and the yield strength was 368MPa, the tensile strength was 531MPa, the elongation was 27%, and the impact energy at -30℃ was 186J.

[0032] Example 2 The ingot with a thickness of 230 mm, a chemical composition and its weight percentage of C: 0.18%, Si: 0.35%, Mn: 1.28%, P: 0.007%, S: 0.001%, Nb: 0.035%, V: 0.040%, Ca: 0.0025%, and the remainder being Fe and unavoidable impurities, was stacked and slowly cooled for 54 hours off the line, and was cold loaded into a heating furnace with a heating time of 230 min and a furnace outlet temperature of 1210°C. Then, it was rolled. During the rolling process, the descaling high-pressure water pressure was controlled to 24 MPa to ensure the descaling effect, and the finishing rolling temperature was controlled to 840°C to obtain a steel plate with a thickness of 60 mm.

[0033] Calculate the austenitizing temperature A of the steel plate C3 The starting phase transformation temperature of austenite to pearlite is 857℃. r1 The starting phase transformation temperature of ferrite to bainite is 704℃. s It is 636℃.

[0034] The steel plate is heat treated as follows: The steel plate was heated to 900 °C, kept at this temperature for 90 min, cooled to 704 °C at a rate of 30 °C / s, then cooled to 636 °C at a rate of 4 °C / s, and then air-cooled to room temperature.

[0035] Observe the metallographic structure of the final steel plate, the results are as follows Figure 3 As shown, the banded structure of the steel plate is level 0, and the metallographic structure is 45% pearlite + 25% bainite + 30% ferrite. According to the method specified in the NACE TM0284 standard, three samples were taken from the steel plate for HIC resistance test, with CLR of 0%, CTR of 0%, and CSR of 0%. The mechanical properties of the steel plate were tested, with a yield strength of 435MPa, a tensile strength of 570MPa, an elongation of 28%, and an impact energy of 235J at -30℃.

[0036] Comparative Example 2 Comparative Example 2 is basically the same as Example 2, except that a different heat treatment process is used. The heat treatment process of Comparative Example 2 is normalizing treatment, where the steel plate is heated to 900° C., kept at this temperature for 90 minutes, and then air-cooled to room temperature.

[0037] Observe the metallographic structure of the final steel plate, the results are as follows Figure 4As shown, the banded structure of the steel plate is level 5, and the metallographic structure is 75% ferrite + 25% pearlite. According to the method specified in the NACE TM0284 standard, three samples were taken from the steel plate for HIC resistance test, and the CLR was 4.5%; the CTR was 1.5%, and the CSR was 0%. The mechanical properties of the steel plate were tested, and the yield strength was 334MPa, the tensile strength was 512MPa, the elongation was 31%, and the impact energy at -30℃ was 150J.

[0038] Although the present invention has been described in detail with reference to the accompanying drawings and in combination with the preferred embodiments, the present invention is not limited thereto. Without departing from the spirit and essence of the present invention, a person of ordinary skill in the art may make various equivalent modifications or substitutions to the embodiments of the present invention, and these modifications or substitutions shall be within the scope of the present invention. Any person of ordinary skill in the art may easily think of changes or substitutions within the technical scope disclosed by the present invention, and these shall be within the scope of protection of the present invention.

Claims

1. A heat treatment process for improving the HIC resistance of Q345R (HIC) steel plate, characterized in that: The steps include: Heat the steel plate to A C3 Above 30-50℃, in the furnace for ≥30min, then cool in two stages: The first stage is rapid cooling, cooling to A at a rate of 25-30℃ / s. r1 temperature; In the second stage, the cooling rate is reduced to 2-5℃ / s to cool to B s temperature; Then air cool to room temperature.

2. The heat treatment process according to claim 1, characterized in that: The chemical composition and weight percentage of Q345R (HIC) steel plate are C: 0.17%-0.19%, Si: 0.25%-0.45%, Mn: 1.20%-1.35%, P≤0.008%, S≤0.001%, Nb≤0.050%, V≤0.050%, Ca / S≥2.0, and the balance is Fe and unavoidable impurities.

3. The heat treatment process according to claim 2, characterized in that: The weight percentage of Ca in Q345R (HIC) steel plate is ≤0.025%.

4. The heat treatment process according to claim 1, characterized in that: The thickness of Q345R (HIC) steel plate is 8-80mm.

5. The heat treatment process according to claim 1, characterized in that: The ingots are slowly cooled by stacking, cold loaded into a furnace for heating, and rolled to obtain steel plates for heat treatment.

6. The heat treatment process according to claim 5, characterized in that: The heating time of the cold charging furnace is controlled according to 8-14min / cm of the billet thickness, and the furnace discharge temperature is 1155-1225℃.

7. The heat treatment process according to claim 5, characterized in that: During the rolling process, the descaling high-pressure water pressure is controlled to be ≥21MPa, and the finishing rolling temperature is 820-870℃.

8. The heat treatment process according to claim 7, characterized in that: Steel plates with a thickness of >20mm are cooled by water after rolling, and the final cooling temperature is 640-700℃.

9. The heat treatment process according to claim 1, characterized in that: The banded structure of Q345R (HIC) steel plate is grade 0-1.0, CLR is 0%, CTR is 0%, and CSR is 0%.

10. The heat treatment process according to claim 1, characterized in that: The final metallographic structure of Q345R (HIC) steel plate is 40%-45% pearlite + 20%-30% bainite + 25%-35% ferrite.

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