Production method of high-toughness high-resistivity low-alloy steel medium plate

Abstract

The invention discloses a production method of a high-toughness high-resistivity low-alloy steel medium plate. Steel comprises the following chemical components in percentage by weight: 0.06%-0.10% of C, 0.90%-1.30% of Si, 1.45%-1.7% of Mn, less than or equal to 0.015% of P, less than or equal to 0.004% of S, 0.020%-0.060% of Alt, 0.008%-0.020% of Ti, 1.3%-1.5% of Mn / Si, more than or equal to 3.42% of Ti / N and the balance of Fe and inevitable impurities. Through high-temperature finish rolling, the finish rolling temperature is 50 DEG C or above higher than the Ar3 temperature point, and complete recrystallization is ensured; after rolling, double-stage controlled cooling is adopted, the pearlite transformation temperature is skipped to 650-700 DEG C through fast cooling, and ferrite is promoted to grow sufficiently through slow cooling; the microstructure of the finally obtained steel plate is 85%-92% of ferrite and 8%-15% of pearlite, the grain size of the ferrite is 10-12 grades, high toughness and high resistivity are well matched, the alloy production cost is low, and the process is simple.

Description

Technical Field

[0001] In the field of metal material processing technology, this invention specifically relates to a medium-thick plate of high resistivity low alloy steel with ferrite as the main component and its production method, which is particularly suitable for fields requiring low iron loss and high formability, such as structural components of power equipment and electromagnetic shielding cabins. Background Technology

[0002] Conventional low-alloy steels (such as Q345) have a resistivity of only 0.25-0.30 μΩ·m, resulting in high iron losses in electromagnetic equipment. High-silicon electrical steels (Si>3.0%) are difficult to adapt to the low-temperature and extremely cold environments due to their low-temperature brittleness.

[0003] Patent application CN02136192.4 discloses "Soft Magnetic Structural Steel and its Manufacturing Method." This method employs an ultra-low C and high silicon composition design, and adds alloys such as Ni and Cu. Through two-stage rolling and normalizing treatment, the grain size is controlled to be 15-30 μm, and the product has Akv ≥130 J at -20℃. Patent application CN118547223A discloses "A High-Strength, Low Yield-to-Toughness, High-Toughness Soft Magnetic Steel and its Production Method." This method employs a high Ni, Cu, V, and La composition design, and the produced steel plate has a yield strength ≥390 MPa, tensile strength 570-660 MPa, yield-to-toughness ratio ≤0.78%, elongation ≥25.0%, and resistivity ≥0.40 μΩ·m.

[0004] The technology involved in the aforementioned patents mainly involves adding higher levels of Si and precious alloys Ni and Cu. On the one hand, high Si content makes the steel plate more prone to cracking and reduces its toughness; on the other hand, the higher content of precious alloys increases production costs. The production process employs normalizing or normalizing + tempering heat treatment, which is complex. Summary of the Invention

[0005] This invention aims to provide a low-cost, high-toughness, and high-resistivity method for producing medium-thick low-alloy steel plates. The produced steel plates have a thickness of 20~50mm and the main performance indicators are: yield strength ≥350MPa, tensile strength ≥470MPa, elongation ≥25%, low-temperature impact strength ≥80J at -40℃, and the microstructure of the steel plate is 85%~92% ferrite + 8%~15% pearlite, with a ferrite grain size of 10~12.

[0006] The technical solution of this invention: A method for producing medium-thick plates of high-toughness, high-resistivity low-alloy steel, comprising the following production process: hot metal pretreatment – ​​converter smelting – LF ladle refining – RH or VD vacuum treatment – ​​continuous casting – heating – rolling – cooling – post-treatment – ​​performance testing – flaw detection; the chemical composition of the steel by weight percentage is C=0.06%~0.10%, Si=0.90%~1.30%, Mn=1.45%~1.7%, P≤0.015%, S≤0.004%, Alt=0.020%~0.060%, Ti=0.008%~0.020%, Mn / Si=1.3~1.5, Ti / N≥3.42, with the balance being Fe and unavoidable impurities; key process steps include: (1) Heating: Heating temperature 1200~1250℃, heating time 180~240min; (2) Rolling: Rough rolling start temperature ≥1050℃, cumulative reduction rate ≥65%; Finish rolling start temperature 880~910℃, finishing rolling temperature 840~870℃; (3) Cooling: The first stage is water cooling to 700~730℃ at a cooling rate of 10~15℃ / s, and the second stage is slow cooling to 600℃ at a rate of ≤1℃ / s; (4) Post-treatment: When the thickness is ≤40mm, it is stacked at 450~550℃ and when the thickness is >40mm, it is placed in a slow cooling pit at 500~550℃ and cooled to 300℃ at a cooling rate of ≤20℃ / h.

[0007] Furthermore, the Ti content satisfies Ti / N≥3.42, forming a TiN precipitate with a size of 20~50nm.

[0008] The final microstructure of the steel plate is as follows: 85%~92% ferrite + 8%~15% pearlite, with ferrite grain size of grade 10~12 and average size of 12~18μm; 8%~15% fine lamellar pearlite with lamellar spacing of 100~150nm; and 20~50nm TiN particles uniformly distributed in the ferrite matrix. The steel plate has a yield strength ≥350MPa, tensile strength ≥470MPa, elongation ≥25%, low-temperature impact resistance ≥80J at -40℃, and resistivity ≥0.45μΩ·m.

[0009] The principle of this invention: (1) Composition-structure synergistic control principle: Mn / Si=1.3~1.5: Increasing the Si content can significantly improve resistivity by utilizing its solid solution strengthening, but the ductility and toughness will decrease accordingly; while increasing the Mn content can offset the brittleness tendency of Si, and at the same time expand the γ phase region to promote complete recrystallization during high-temperature rolling. Controlling Ti / N≥3.42 preferentially forms 20~50nm TiN particles, pinning austenite grain boundaries to inhibit grain coarsening, refining ferrite grains to level 10~12, and fixing N atoms to prevent age embrittlement. Using low C (0.06%~0.10%) + medium Si (0.9%~1.3%) content, under the premise of ensuring resistivity ≥0.45μΩ·m, the low-temperature brittleness caused by high Si is avoided.

[0010] (2) Rolling-cooling phase transformation kinetics principle: High-temperature final rolling at a temperature 50°C higher than Ar3 temperature point is used to ensure complete recrystallization of austenite and provide a uniform nucleation substrate for subsequent phase transformation. The first stage of cooling after rolling is carried out at a cooling rate of 10~15°C / s to skip the pearlite transformation "nose temperature" (650~700°C) and inhibit pearlite formation; the second stage of slow cooling is carried out at a cooling rate of ≤1°C / s to provide sufficient diffusion time in the ferrite phase transformation region, promote the full growth of ferrite to 85%~92%, and completely avoid bainite.

[0011] The innovations and beneficial effects of this invention are as follows: High-temperature final rolling at 840-870℃ ensures complete recrystallization, as the final rolling temperature is more than 50℃ higher than the Ar3 temperature. A two-stage controlled cooling process skips the pearlite transformation "nose temperature," followed by slow cooling to promote full ferrite growth, achieving a ferrite proportion of ≥85% and effectively controlling the microstructure to completely avoid bainite. For steel plates ≤40mm thick, residual heat at 550-600℃ is used for stacking cooling to stabilize the microstructure; for steel plates >40mm thick, slow cooling pits are used to prevent thermal stress cracking in the core of the thick plate.

[0012] Table 1. Performance comparison between steel plates produced by the method of this invention and traditional Q345 steel plates. . Detailed Implementation

[0013] The present invention will be further described below with reference to the embodiments.

[0014] Example 1: Production method of 25mm thick steel plate The steel's chemical composition by weight percentage is C = 0.08, Si = 1.15, Mn = 1.60, Ti = 0.015, Al = 0.040, P = 0.012, S = 0.003, with the balance being Fe and other trace elements. Key process steps include: (1) Heating: Heating temperature 1230℃, heating time 180min.

[0015] (2) Rolling: Rough rolling starts at 1080℃ with a reduction rate of 67% and an intermediate billet of 100mm; finish rolling starts at 900℃ and finishes at 855℃.

[0016] (3) Cooling: After rolling, water cooling is applied at 12℃ / s to 720℃, and then air cooling is applied at 0.8℃ / s to 600℃.

[0017] (4) Post-treatment: Stack at 560℃ and cover with insulation cotton.

[0018] The steel plate performance test results are as follows: resistivity 0.48μΩ·m, yield strength 375MPa, tensile strength 490MPa, elongation 28%, impact energy at -40℃ 98J; microstructure is 90% ferrite (grain size grade 11) + 10% pearlite.

[0019] Example 2: Production method of 50mm thick steel plate The steel's chemical composition by weight percentage is C=0.07, Si=1.05, Mn=1.68, Ti=0.012, Al=0.035, P=0.010, S=0.002, with the balance being Fe and other trace elements. Key process steps include: (1) Heating: Heating temperature 1240℃, heating time 240min.

[0020] (2) Rolling: Rough rolling starts at 1060℃ with a reduction rate of 65% and an intermediate billet of 120mm; finish rolling starts at 890℃ and finishes at 850℃.

[0021] (3) Cooling: After rolling, water cool to 730℃ at 10℃ / s, and then air cool to 600℃ at 0.5℃ / s.

[0022] (4) Post-treatment: 580℃ into the slow cooling pit, and cooled to 300℃ at 15℃ / h.

[0023] The obtained steel plate performance test results are as follows: resistivity 0.46μΩ·m, yield strength 360MPa, tensile strength 480MPa, elongation 27%, impact energy at -40℃ 85J; microstructure is 87% ferrite (grain size grade 10) + 13% pearlite.

Claims

1. A method for producing medium-thick plates of high-toughness, high-resistivity low-alloy steel, wherein the steel production process route is: hot metal pretreatment - converter smelting - LF ladle refining - RH or VD vacuum treatment - continuous casting - heating - rolling - cooling - post-treatment - performance testing - flaw detection, characterized in that: The steel's chemical composition by weight percentage is C=0.06%~0.10%, Si=0.90%~1.30%, Mn=1.45%~1.7%, P≤0.015%, S≤0.004%, Alt=0.020%~0.060%, Ti=0.008%~0.020%, Mn / Si=1.3~1.5, Ti / N≥3.42, with the balance being Fe and unavoidable impurities; Key process steps include: (1) Heating: Heating temperature 1200~1250℃, heating time 180~240min; (2) Rolling: Rough rolling start temperature ≥1050℃, cumulative reduction rate ≥65%; Finish rolling start temperature 880~910℃, finishing rolling temperature 840~870℃; (3) Cooling: The first stage is water cooling to 700~730℃ at a cooling rate of 10~15℃ / s, and the second stage is slow cooling to 600℃ at a rate of ≤1℃ / s; (4) Post-treatment: When the thickness is ≤40mm, it is stacked at 450~550℃ and when the thickness is >40mm, it is placed in a slow cooling pit at 500~550℃ and cooled to 300℃ at a cooling rate of ≤20℃ / h.

2. The method for producing medium-thick plates of high-toughness, high-resistivity low-alloy steel according to claim 1, characterized in that: The Ti content satisfies Ti / N≥3.42, forming TiN precipitates with a size of 20~50nm.

3. The method for producing medium-thick plates of high-toughness, high-resistivity, low-alloy steel according to claim 1, characterized in that: The final microstructure of the steel plate was 85%~92% ferrite + 8%~15% pearlite, with a ferrite grain size of 10~12.