A heat treatment process for improving HIC resistance of Q345R (HIC) steel plate

Abstract

The application relates to the technical field of metallurgy, in particular to a heat treatment process for improving the HIC resistance of Q345R (HIC) steel plates. C3 The steel plate is heated to A r1 The steel plate is cooled to B s Temperature in two stages: the first stage is rapid cooling, the cooling speed is 25-30 DEG C / s, and the second stage is slow cooling, the cooling speed is 2-5 DEG C / s; and then the steel plate is air-cooled to room temperature. The application controls the phase change degree of the steel plate by adopting the two-stage cooling mode, changes the final structure of the steel plate, reduces the banded structure, improves the acid resistance of the Q345R (HIC) steel plate, and has good social and economic benefits.

Description

Technical Field

[0001] This invention relates to the field of metallurgical technology, specifically to a heat treatment process for improving the HIC resistance of Q345R (HIC) steel plates. Background Technology

[0002] Q345R (HIC) steel is a container steel with resistance to hydrogen-induced cracking. Its common microstructure consists of ferrite and pearlite. During solidification, due to the selective crystallization of alloying elements, the mass fraction of manganese in the interdendritic region is significantly higher than that within the dendrites. Since manganese is a stabilizing element in the austenite region, the manganese-rich region contains austenite (A). r3 The transformation temperature must be lower than that of the manganese-depleted region. As the temperature decreases, ferrite preferentially nucleates and grows in the manganese-depleted region, and multiple ferrite grains will connect, forming a growth plane at the growth front. Because carbon has very low solubility in ferrite, as austenite transforms into ferrite, carbon in the ferrite is gradually expelled into the austenite in the manganese-rich region, thus forming pearlite bands in subsequent phase transformations. The presence of banded structures makes the microstructure of Q345R (HIC) steel plate uneven, severely affecting its performance. Since Q345R (HIC) steel is mainly used in harsh environments containing wet hydrogen sulfide, such as the petrochemical industry, it is highly susceptible to hydrogen sulfide corrosion leading to hydrogen-induced cracking. Banded structures exacerbate the sensitivity of Q345R (HIC) steel to hydrogen-induced cracking, promoting crack initiation and propagation along the banded structure. The more severe the banded structure, the greater the sensitivity to hydrogen-induced cracking, thereby reducing the steel plate's resistance to HIC and affecting its service life.

[0003] Q345R (HIC) steel plates are typically heat-treated using normalizing heat treatment. For example, CN 115896417 A discloses a method for preparing extra-thick Q345R steel plates with HIC resistance, employing normalizing heat treatment with the following process parameters: heating temperature 900-910℃, holding time 2.0-2.5 min / mm, followed by air cooling to 400±50℃ and then air cooling to room temperature. However, because the air cooling rate is still relatively slow, a significant amount of ferrite still precipitates from the austenite, resulting in poor elimination of banded structures. CN 115652046 A discloses a heat treatment process for eliminating banded structures in steel, employing solution quenching followed by multiple cyclic quenching and tempering, which significantly extends the production process and increases production costs. Summary of the Invention

[0004] To address the high production cost of existing heat treatment processes for Q345R (HIC) steel plates, this invention provides a heat treatment process to improve the HIC resistance of Q345R (HIC) steel plates. By employing a two-stage cooling method, the degree of phase transformation of the steel plate is controlled, thereby altering the final microstructure of the steel plate. This reduces the banded structure and improves the acid resistance of Q345R (HIC) steel plates, resulting in significant social and economic benefits.

[0005] The technical solution of this invention is as follows:

[0006] A heat treatment process for improving the HIC resistance of Q345R (HIC) steel plates includes the following steps:

[0007] Heat the steel plate to A C3 The furnace temperature should be 30-50℃ for at least 30 minutes, followed by two stages of cooling.

[0008] The first stage involves rapid cooling, at a rate of 25-30℃ / s, to A. r1 temperature;

[0009] In the second stage, the cooling rate is reduced to 2-5℃ / s, cooling to B. s temperature;

[0010] It was then air-cooled to room temperature.

[0011] The key phase transition temperature involved in this process is A. C3、 A r1 and B s ,in,

[0012] A C3 : Austenitizing temperature during heating;

[0013] A r1 The initial phase transformation temperature at which austenite transforms into pearlite during the cooling process;

[0014] B s The initial phase transformation temperature at which ferrite transforms into bainite during the cooling process.

[0015] Furthermore, the chemical composition and weight percentage of Q345R (HIC) steel plate are as follows: 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, with the balance being Fe and unavoidable impurities.

[0016] Furthermore, the weight percentage of Ca in Q345R (HIC) steel plate is ≤0.025%.

[0017] Furthermore, the thickness of Q345R (HIC) steel plates is 8-80mm, such as 8mm, 10mm, 12mm, 15mm, 20mm, 25mm, 30mm, 35mm, 40mm, 45mm, 50mm, 60mm, 70mm, 80mm, etc.

[0018] Furthermore, for steel plates with a thickness greater than 20mm, the furnace time should be controlled at 14-16 min / cm.

[0019] Furthermore, the billet is stacked and slowly cooled, then cold-charged into a furnace for heating and rolling to obtain steel plates for heat treatment.

[0020] Furthermore, the slow cooling time for stacking is more than 48 hours.

[0021] Furthermore, the heating time for cold-charged billets is controlled at 8-14 min / cm billet thickness, and the exit temperature is 1155-1225℃.

[0022] 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℃; preferably, steel plates with a thickness >20mm are cooled by water after rolling, and the final cooling temperature is 640-700℃.

[0023] Furthermore, the banded microstructure of Q345R (HIC) steel plate is grade 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%.

[0024] Furthermore, the final metallographic structure of Q345R (HIC) steel plate is 40%-45% pearlite + 20%-30% bainite + 25%-35% ferrite.

[0025] The beneficial effects of this invention are as follows:

[0026] (1) The heat treatment process of this invention can significantly reduce the banded structure level, alleviate the promoting effect of banded structure on hydrogen-induced cracking of Q345R (HIC) steel plate, thereby improving the HIC resistance of the steel plate, increasing the HIC resistance qualification rate of the steel plate, and achieving the effect of enhancing the service life of the steel plate in acid environment. The produced steel plate has stable quality, banded structure level 0-1.0, excellent hydrogen-induced cracking resistance, and can reach and far exceed the standard level of CLR≤5.0%, CTR≤1.5%, and CSR≤0.5%.

[0027] (2) The heat treatment process of the present invention involves heating the steel plate to its A C3 The steel plate is heated to 30-50℃ to fully austenitize it, and then rapidly cooled to A. r1Temperature can reduce the residence time of the steel plate in the austenite and ferrite dual-phase region, reduce the amount of ferrite precipitation, thereby inhibiting the carbon removal of ferrite to the surrounding area and inhibiting the formation of banded structures. At the same time, due to the faster cooling rate, it prevents rapid grain growth, which is beneficial to ensuring the strength properties of the steel plate; the second stage slows down the cooling rate, cooling to B. s The temperature is then further cooled to room temperature to allow sufficient time for pearlite and granular bainite to form, ensuring the strength and plasticity of the steel plate.

[0028] (3) The process of the present invention is simple, stable and has a short production cycle. It can significantly reduce production costs and shorten the process. At the same time, Q345R (HIC) steel has good acid resistance, the uniformity of the steel plate structure is improved, and the comprehensive performance of the steel plate is guaranteed. It has great market promotion and application prospects. Attached Figure Description

[0029] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, for those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0030] Figure 1 This is a metallographic photograph of the Q345R (HIC) steel plate prepared in Example 1.

[0031] Figure 2 This is a metallographic photograph of the Q345R (HIC) steel plate prepared in Comparative Example 1.

[0032] Figure 3 This is a metallographic photograph of the Q345R (HIC) steel plate prepared in Example 2.

[0033] Figure 4 The image shows a metallographic photograph of the Q345R (HIC) steel plate prepared in Comparative Example 2. Detailed Implementation

[0034] To enable those skilled in the art to better understand the technical solutions of this invention, the technical solutions of the embodiments of this invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this invention, and not all embodiments. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without creative effort should fall within the scope of protection of this invention.

[0035] Example 1

[0036] A 230mm thick billet with the following chemical composition and weight percentages (C: 0.18%, Si: 0.30%, Mn: 1.25%, P: 0.007%, S: 0.001%, Nb: 0.030%, V: 0.040%, Ca: 0.0025%, with the balance being Fe and unavoidable impurities) was slowly cooled for 48 hours after being removed from the production line. It was then cold-loaded into a heating furnace and heated for 207 minutes at a furnace exit temperature of 1200℃. The billet was then rolled, with the descaling high-pressure water pressure controlled at 23MPa during rolling to ensure effective descaling. The finishing rolling temperature was controlled at 850℃ to obtain a 12mm thick steel plate.

[0037] Calculate the austenitizing temperature A of the steel plate. C3 The initial phase transformation temperature A, which is 846℃, is the temperature at which austenite begins its transformation into pearlite. r1 The initial phase transformation temperature B, which is 695℃, is the temperature at which ferrite begins to transform into bainite. s It is 626℃.

[0038] The steel plate is subjected to the following heat treatment:

[0039] The steel plate was heated to 880℃, held for 30 minutes, cooled to 695℃ at a rate of 30℃ / s, and then cooled to 626℃ at a rate of 3℃ / s, followed by air cooling to room temperature.

[0040] The metallographic structure of the final steel plate was observed, and the results are as follows: Figure 1 As shown, the steel plate has a banded microstructure of grade 0.5 and a metallographic structure of 40% pearlite + 25% bainite + 35% ferrite. Following the method specified in NACE TM0284 standard, three samples were taken from the steel plate for HIC resistance testing, with CLR of 0%, CTR of 0%, and CSR of 0%. The mechanical properties of the steel plate were tested: yield strength of 480 MPa, tensile strength of 596 MPa, elongation of 26%, and impact energy at -30℃ of 228 J.

[0041] Comparative Example 1

[0042] 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, in which the steel plate is heated to 880°C, held at that temperature for 30 minutes, and then air-cooled to room temperature.

[0043] The metallographic structure of the final steel plate was observed, and the results are as follows: Figure 2As shown, the steel plate has a banded microstructure of grade 5 and a metallographic structure of 75% ferrite + 25% pearlite. Following the method specified in NACE TM0284 standard, three samples were taken from the steel plate for HIC resistance testing, with a CLR of 5%, CTR of 1.0%, and CSR of 0.5%. The mechanical properties of the steel plate were tested: yield strength of 368 MPa, tensile strength of 531 MPa, elongation of 27%, and impact energy at -30℃ of 186 J.

[0044] Example 2

[0045] A 230mm thick billet with the following chemical composition and weight percentages (C: 0.18%, Si: 0.35%, Mn: 1.28%, P: 0.007%, S: 0.001%, Nb: 0.035%, V: 0.040%, Ca: 0.0025%, with the balance being Fe and unavoidable impurities) was slowly cooled for 54 hours after being removed from the production line. It was then cold-loaded into a heating furnace and heated for 230 minutes, exiting at a furnace temperature of 1210℃. The billet was then rolled, with the descaling high-pressure water pressure controlled at 24MPa during rolling to ensure effective descaling. The finishing rolling temperature was controlled at 840℃ to obtain a 60mm thick steel plate.

[0046] Calculate the austenitizing temperature A of the steel plate. C3 The initial phase transformation temperature A of austenite to pearlite is 857℃. r1 The initial phase transformation temperature B, which is 704℃, is the temperature at which ferrite begins to transform into bainite. s It is 636℃.

[0047] The steel plate is subjected to the following heat treatment:

[0048] The steel plate was heated to 900℃, held for 90 minutes, cooled to 704℃ at a rate of 30℃ / s, and then cooled to 636℃ at a rate of 4℃ / s, followed by air cooling to room temperature.

[0049] The metallographic structure of the final steel plate was observed, and the results are as follows: Figure 3 As shown, the steel plate has a banded microstructure of grade 0 and a metallographic structure of 45% pearlite + 25% bainite + 30% ferrite. Following the method specified in NACE TM0284 standard, three samples were taken from the steel plate for HIC resistance testing, with CLR of 0%, CTR of 0%, and CSR of 0%. The mechanical properties of the steel plate were tested: yield strength of 435 MPa, tensile strength of 570 MPa, elongation of 28%, and impact energy at -30℃ of 235 J.

[0050] Comparative Example 2

[0051] 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, in which the steel plate is heated to 900°C, held at that temperature for 90 minutes, and then air-cooled to room temperature.

[0052] The metallographic structure of the final steel plate was observed, and the results are as follows: Figure 4 As shown, the steel plate has a banded microstructure of grade 5 and a metallographic structure of 75% ferrite + 25% pearlite. Following the method specified in NACE TM0284 standard, three samples were taken from the steel plate for HIC resistance testing, with a CLR of 4.5%, CTR of 1.5%, and CSR of 0%. The mechanical properties of the steel plate were tested: yield strength of 334 MPa, tensile strength of 512 MPa, elongation of 31%, and impact energy at -30℃ of 150 J.

[0053] Although the present invention has been described in detail with reference to the accompanying drawings and preferred embodiments, the present invention is not limited thereto. Various equivalent modifications or substitutions can be made to the embodiments of the present invention by those skilled in the art without departing from the spirit and essence of the invention, and such modifications or substitutions should all be within the scope of the present invention. Any variations or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention should also be covered within the protection scope of the present invention.

Claims

1. A heat treatment process for improving the HIC resistance of Q345R (HIC) steel plates, characterized in that, The chemical composition and weight percentage of Q345R (HIC) steel plate are as follows: 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, with the balance being Fe and unavoidable impurities; After the billet is stacked and cooled slowly, cold-charged into the furnace for heating, and rolled, a steel plate for heat treatment is obtained. The heating time in the furnace is controlled at 8-14 min / cm of billet thickness, and the furnace exit temperature is 1155-1225℃. During the rolling process, the descaling high-pressure water pressure is controlled at ≥21MPa, and the finishing rolling temperature is 820-870℃. The heat treatment process includes the following steps: Heat the steel plate to A C3 The furnace temperature should be 30-50℃ for at least 30 minutes, followed by two stages of cooling. The first stage involves rapid cooling, at a rate of 25-30℃ / s, to A. r1 temperature; In the second stage, the cooling rate is reduced to 2-5℃ / s, cooling to B. s temperature; Then air-cooled to room temperature; The banded structure of Q345R (HIC) steel plate is grade 0-1.0, with CLR of 0%, CTR of 0%, and CSR of 0%.

2. The heat treatment process as described in claim 1, characterized in that, The weight percentage of Ca in Q345R (HIC) steel plate is ≤0.025%.

3. The heat treatment process as described in claim 1, characterized in that, The thickness of Q345R (HIC) steel plate is 8-80mm.

4. The heat treatment process as described in claim 1, characterized in that, Steel plates with a thickness greater than 20mm are cooled by water after rolling, with a final cooling temperature of 640-700℃.

5. The heat treatment process as described in 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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