Normalizing heat treatment process of high-temperature fast-fired 345-grade container steel

Abstract

This invention discloses a high-temperature rapid-heat normalizing heat treatment process for 345 grade container steel. The alloy composition of the steel is as follows (mass percentage): C = 0.16%–0.18%, Si = 0.3%–0.4%, Mn = 1.2%–1.7%, P ≤ 0.02%, S ≤ 0.01%, Nb ≤ 0.05%, V ≤ 0.05%, Ti ≤ 0.03%, Cr ≤ 0.3%, Mo ≤ 0.08%, Ni ≤ 0.3%, Cu ≤ 0.3%, with the remainder being Fe and unavoidable impurities. This invention increases the heating rate of the steel plate by ≥ 30°C above the conventional normalizing process temperature (i.e., the high-temperature rapid-heat normalizing process temperature = austenitizing temperature + 60~100°C) by increasing the temperature difference, thereby shortening the heat treatment time, enabling rapid austenitization, reducing holding time, and refining the grain size. Combined with optimized rolling processes, a finer and more uniform microstructure is obtained without reducing the steel plate strength, improving impact toughness.

Description

Technical Field

[0001] This invention relates to the field of heat treatment technology, and specifically to a high-temperature rapid-fire normalizing heat treatment process for 345 grade container steel. Background Technology

[0002] The target temperature for conventional normalizing of 345 grade container steel is generally within the range of 860~880℃. The furnace time varies depending on the thickness of the normalized plate in a conventional heat treatment furnace; medium-thick plates can achieve complete austenitization at 1.8 times their thickness. Currently, for steel plates ≥50mm thick, complete austenitization requires a furnace time of 2 times or more of the plate thickness, resulting in longer normalizing heating times and slower heating rates. To improve the impact toughness of the steel plate, alloys can be added to refine the grain size, but this increases costs. Therefore, a high-temperature, rapid-heating normalizing heat treatment process for 345 grade container steel is needed, optimizing the conventional normalizing process without changing the composition. This would improve the impact toughness of the steel plate without reducing its strength. Summary of the Invention

[0003] This invention aims to provide a high-temperature rapid-fire normalizing heat treatment process for 345 grade container steel. This method is used to produce 345 grade container steel and improves the impact toughness of the steel plate by using a high-temperature rapid-fire normalizing process.

[0004] The technical solution of this invention: A high-temperature rapid-fired normalizing heat treatment process for 345 grade container steel, characterized in that: the alloy composition of the steel is C=0.16%~0.18% by mass, Si=0.3%~0.4%, Mn=1.2%~1.7%, P≤0.02%, S≤0.01%, Nb≤0.05%, V≤0.05%, Ti≤0.03%, Cr≤0.3%, Mo≤0.08%, Ni≤0.3%, Cu≤0.3%, with the remainder being Fe and unavoidable impurities; including the following process steps: 1) Rolling process: The steel tapping temperature of the heating furnace is 1180~1250℃; the rough rolling reduction is at least three times ≥35mm, the intermediate billet thickness is > finished product thickness +50mm; the finishing rolling start temperature is ≥800℃, the finishing rolling temperature is 760-810℃, and the final cooling temperature is 670~710℃. 2) High-temperature rapid normalizing process: The high-temperature rapid normalizing process temperature = austenitizing temperature + 60~100℃, the heating coefficient is 1.4~1.8 times the plate thickness in min / mm, and the holding time is 15~25min. The high-temperature rapid normalizing process temperature is ≥30℃ higher than the conventional normalizing process temperature; after exiting the furnace, it is cooled in still or slightly flowing air.

[0005] The innovative principle and beneficial effects of this invention are as follows: Conventional normalizing process temperature = austenitizing temperature + 30~50℃, conventional normalizing process heating rate 1.8~2.2 times plate thickness min / mm, holding time 25~45min; High-temperature rapid normalizing process temperature = austenitizing temperature + 60~100℃, high-temperature rapid normalizing process heating rate 1.4~1.8 times plate thickness min / mm, holding time 15~25min; after exiting the furnace, cooling is performed in still or slightly flowing air. Compared with the prior art, this invention, under conditions ≥30℃ higher than the conventional normalizing process temperature, increases the heating rate of the steel plate by increasing the temperature difference, enabling rapid austenitization. The high-temperature rapid normalizing process shortens the heating time by 10~50min and the holding time by 5~15min compared to the conventional normalizing process. Rapid heating shortens the high-temperature holding time and refines the grain size. Simultaneously, the rolling process is optimized using solid solution strengthening and grain refinement techniques: increasing the furnace exit temperature improves the alloy solid solution strengthening effect; large reduction in rough rolling increases the thickness of intermediate slabs, improving the rolled strength margin of the steel plate and refining the original austenite grain size. By increasing the normalizing heat treatment temperature and shortening the furnace time and holding time, and supplementing the optimized rolling process, a finer and more uniform microstructure is obtained without reducing strength. This improves the impact toughness of the steel plate while simultaneously increasing heat treatment efficiency and achieving energy conservation and consumption reduction. Attached Figure Description

[0006] Figure 1 The image shows the metallographic microstructure of the steel plate in Example 1.

[0007] Figure 2 The image shows the metallographic microstructure of steel plate #5, which is a control example. Detailed Implementation

[0008] The present invention will be further described in detail below with reference to a set of embodiments and comparative examples.

[0009] A high-temperature rapid-fired normalizing heat treatment process for 345 grade container steel, wherein the alloy composition of the steel is C=0.17%, Si=0.35%, Mn=1.56%, P≤0.0157%, S≤0.0011%, Nb≤0.0468%, V≤0.0267%, Ti≤0.0179%, Cr≤0.2006%, Mo≤0.0074%, Ni≤0.0146%, with the remainder being Fe and unavoidable impurities; the process includes the following steps: 1) Rolling process: The steel tapping temperature of the heating furnace is 1190~1220℃; the rough rolling reduction is at least three passes ≥35mm, the intermediate billet is 140mm; the finishing rolling opening temperature is ≥820℃, the finishing rolling temperature is 780~810℃, and the final cooling temperature is 670~700℃. 2) High-temperature rapid normalizing process: Normalizing temperature 910±10℃, total furnace time 120~130min. After being taken out of the furnace, cool in still or slightly flowing air.

[0010] The calculated austenitizing temperature was 845℃. Sample steel plates #1 to #4 are examples produced according to the process method of this invention; sample steel plate #5 is a control example produced using conventional normalizing process. Rolling process parameters are shown in Table 1, high-temperature rapid normalizing process parameters are shown in Table 2, and the steel performance test results are shown in Table 3.

[0011] Performance data: Steel plate samples were taken from the tail 1 / 4 position; strength and impact samples were machined from the inner 1 / 4 thickness position. Delivery condition performance standards: Yield strength ≥305MPa, tensile strength 490-620MPa, elongation ≥20%; minimum transverse impact at -20℃ 29J, average impact 41J. Maximum mold-welded condition performance standards: Yield strength ≥305MPa, tensile strength 490-620MPa, elongation ≥20%; minimum transverse impact at -20℃ 29J, average impact 41J. Mold welding process: 606~620℃, heating rate 80℃ / s, holding time 600min, air cooling after removal from the furnace.

[0012] Table 1 Rolling process parameters .

[0013] Table 2. High-Temperature Rapid Normalizing Process Parameters .

[0014] Table 3. Performance test results of steel .

[0015] Table 3 shows that the average impact of steel plates #1, #2, #3, and #4 produced using the high-temperature rapid-heat normalizing heat treatment process for 345 grade container steel according to this patent in both the delivered and mold-welded states is ≥130J. The impact of steel plate #5 produced using the conventional normalizing process in the delivered state is ≤108J, with an average impact of 95J. In the mold-welded state, the average impact is ≤34J, with an average impact of 25J. Therefore, the impact performance of steel plates produced using the high-temperature rapid-heat normalizing heat treatment process for 345 grade container steel according to this patent is significantly better than that of steel plates produced using the conventional normalizing process in both the delivered and mold-welded states.

[0016] The strength of steel plates 1#, 2#, 3#, and 4# produced using the high-temperature rapid-fire normalizing heat treatment process of 345 grade container steel according to this patent in the delivery state is better than that of steel plate 5# produced by the conventional normalizing process, while the strength in the mold-welded state is not much different between the two.

[0017] The metallographic microstructure of the steel plates produced by both heat treatment processes is ferrite + pearlite. The No. 1 steel plate produced using a high-temperature rapid-fire normalizing heat treatment process for 345 grade container steel, as per this patent, has a grain size of grade 9 (e.g., ...). Figure 1 The grain size of 5# steel plates produced by conventional normalizing process is grade 8~8.5 (e.g., Figure 2 The proportion of large-size grains is higher than that of steel plates produced by the high-temperature rapid normalizing process. The steel plates produced by the method of this invention have finer and more uniform grain sizes.

[0018] The metallographic microstructures of steel plates #2 to #4 in Examples are similar to those of steel plate #1, and are omitted here.

Claims

1. A high-temperature rapid-fire normalizing heat treatment process for 345 grade container steel, characterized in that: The alloy composition of the steel is as follows (mass percentage): C = 0.16%–0.18%, Si = 0.3%–0.4%, Mn = 1.2%–1.7%, P ≤ 0.02%, S ≤ 0.01%, Nb ≤ 0.05%, V ≤ 0.05%, Ti ≤ 0.03%, Cr ≤ 0.3%, Mo ≤ 0.08%, Ni ≤ 0.3%, Cu ≤ 0.3%, with the remainder being Fe and unavoidable impurities. The process includes the following steps: 1) Rolling process: The steel tapping temperature of the heating furnace is 1180~1250℃; the rough rolling reduction is at least three times ≥35mm, the intermediate billet thickness is > finished product thickness +50mm; the finishing rolling start temperature is ≥800℃, the finishing rolling temperature is 760~810℃, and the final cooling temperature is 670~710℃. 2) High-temperature rapid normalizing process: The temperature of the high-temperature rapid normalizing process is equal to the austenitizing temperature + 60~100℃, the heating coefficient is 1.4~1.8 times the plate thickness min / mm, and the holding time is 15~25min; the temperature of the high-temperature rapid normalizing process is ≥30℃ higher than that of the conventional normalizing process; after exiting the furnace, it is cooled in still or slightly flowing air.

2. The high-temperature rapid-fire normalizing heat treatment process for 345 grade container steel according to claim 1, characterized in that: The alloy composition of the steel is as follows (by mass percentage): C=0.17%, Si=0.35%, Mn=1.56%, P≤0.0157%, S≤0.0011%, Nb≤0.0468%, V≤0.0267%, Ti≤0.0179%, Cr≤0.2006%, Mo≤0.0074%, Ni≤0.0146%, with the remainder being Fe and unavoidable impurities. The process includes the following steps: 1) Rolling process: The steel tapping temperature of the heating furnace is 1190~1220℃; the rough rolling reduction is at least three passes ≥35mm, the intermediate billet is 140mm; the finishing rolling opening temperature is ≥820℃, the finishing rolling temperature is 780~810℃, and the final cooling temperature is 670~700℃. 2) High-temperature rapid normalizing process: Normalizing temperature 910±10℃, total furnace time 120~130min. After being taken out of the furnace, cool in still or slightly flowing air.

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