Method for controlling low yield ratio of railway bridge steel Q420qE through relaxation after rolling

Abstract

The invention discloses a method for controlling the low yield ratio of railway bridge steel Q420qE through relaxation after rolling. The method comprises the following steps: (1) a smelting-continuous casting production process flow: KR desulfurization, converter treatment, LF refining, RH vacuum treatment, slab continuous casting and slow cooling; (2) heating; (3) rolling comprises rough rolling and finish rolling; (4) a relaxation process: after finish rolling, the steel plate is relaxed on a roller way, namely short-distance reciprocating swing is carried out for 70-100 seconds, and the release temperature of the steel plate is 700-720 DEG C; and (5) laminar cooling equipment is adopted for cooling. According to the method for controlling the low yield ratio of the railway bridge steel Q420qE through relaxation after rolling, the manufactured railway bridge steel Q420qE steel plate is pure in steel quality, uniform in mechanical property and excellent in low-temperature (-40 DEG C) impact toughness, the yield ratio is smaller than or equal to 0.85, and the method is suitable for manufacturing of a main body structure of a large-span bridge.

Description

Technical Field

[0001] The invention belongs to the technical field of metallurgical plate production, and in particular relates to a method for controlling the low yield strength ratio of railway bridge steel Q420qE through post-rolling relaxation. Background Art

[0002] The rapid development of railway construction in my country has not only led to a substantial increase in the demand for bridge steel, but also increasingly higher quality requirements, requiring steel plates to have high toughness, excellent weldability and good seismic resistance. The yield ratio indicates the closeness between yield strength and tensile strength, and is an important performance indicator of the material. Steel plates with a high yield ratio will break soon after yielding, while steel plates with a low yield ratio will experience greater strain hardening after yielding and will not break until they reach a higher tensile strength. Therefore, bridge steels with a low yield ratio are suitable for the main structure of large-span bridges to improve the safety of bridges.

[0003] With the development of high-speed and heavy-load railways, high-strength bridge steels such as 420MPa grade have gradually been promoted and applied. However, as the strength level increases, its yield strength ratio is more difficult to control. The carbon content of 345-370MPa grade steel generally adopts a medium carbon design (≤0.14%), and the structure is mainly ferrite + pearlite. The yield strength ratio is generally low and easy to control; the carbon content of 420-500MPa grade steel generally adopts a low carbon design (≤0.11%), and a low-carbon bainite structure is obtained by controlling the rolling and cooling process. Although the yield strength, tensile strength, elongation after fracture, low-temperature toughness, etc. can meet the technical requirements, the yield strength ratio is relatively high. The present invention provides a method for controlling the low yield strength ratio of railway bridge steel Q420qE by post-rolling relaxation. Summary of the invention

[0004] The purpose of the present invention is to provide a method for controlling the low yield strength ratio of railway bridge steel Q420qE by post-rolling relaxation. The railway bridge steel Q420qE steel plate manufactured by the present invention not only has pure steel quality, uniform mechanical properties, and excellent low-temperature (-40°C) impact toughness, but also has a yield strength ratio of ≤0.85, and is suitable for the manufacture of main structures of large-span bridges.

[0005] In order to solve the above technical problems, the present invention adopts the following technical solutions:

[0006] The present invention provides a method for controlling the low yield strength ratio of railway bridge steel Q420qE by post-rolling relaxation, comprising:

[0007] (1) Smelting-continuous casting production process: KR desulfurization-converter-LF refining-RH vacuum treatment-slab continuous casting-slow cooling; the composition of molten steel for the casting machine is C: 0.09-0.11%, Si: 0.20-0.30%, Mn: 1.55-1.65%, P: ≤0.010%, S≤0.005%, Als: 0.020-0.040%, Nb: 0.035-0.045%, V: 0.035-0.045%, Ti: 0.005-0.015%, and the rest is Fe and unavoidable impurities;

[0008] (2) During the heating process, the slab is put into the furnace; the slab time in the furnace and the temperature out of the furnace are strictly controlled, the total time in the furnace is ≥220min, and the temperature out of the furnace is 1220±20℃;

[0009] (3) Rolling includes rough rolling and finishing rolling. For rough rolling, the starting rolling temperature is ≥1070°C. According to the width of the product, the widening task is completed with as few passes as possible. According to the thickness of the product, the relative reduction rate of a single pass is controlled at ≥14% for at least two passes. For finishing rolling, the starting rolling temperature is 860-950°C, and the finishing rolling has at least two passes with a reduction rate of ≥12%. The final rolling temperature of finishing rolling is 770-840°C.

[0010] (4) Relaxation process: After finishing rolling, the steel plate is relaxed on the roller, i.e., it swings back and forth for 70-100 seconds in a short distance, and the release temperature of the steel plate is 700-720°C;

[0011] (5) The cooling adopts laminar cooling equipment, and the cooling mode adopts the self-learning calculation result of the secondary system. The final cooling temperature is 600-660℃, and the cooling speed is 10-20℃ / s.

[0012] Furthermore, the yield strength ratio satisfies ≤0.85.

[0013] Further, the molten iron is subjected to a desulfurization pretreatment, and the molten iron is decarbonized and dephosphorized to obtain molten steel by top and bottom combined blowing converter smelting. Argon is blown throughout the converter smelting process, scrap steel is added to the converter, and the converter tapping temperature is 1640°C; then the molten steel after converter smelting is subjected to LF refining outside the furnace, and the refining temperature is ≥1560°C. After LF refining outside the furnace, the molten steel is subjected to RH vacuum treatment, and the vacuum time is maintained for ≥8min; slab continuous casting is carried out according to the smelting chemical composition, and the superheat is 23°C, and then the slab is cleaned, slowly cooled, and continuously Ingot quality inspection; the slab heating temperature is 1225℃, the heating time is 235min, and the heated slab is subjected to high-pressure water dephosphorization; the starting temperature of rough rolling is 1090℃, the average starting temperature of finishing rolling is 930℃, and the average final temperature of finishing rolling is 820℃; after finishing rolling, the steel plate is relaxed on the roller, that is, short-range reciprocating swing for 70 seconds, the steel plate release temperature is 720℃, and the average temperature of the steel plate is reduced to 650℃ after laminar cooling; the thickness of the finished steel plate is 10.0mm, and finally the product quality inspection and warehousing are carried out.

[0014] Further, the molten iron is subjected to a desulfurization pretreatment, and the molten iron is decarbonized and dephosphorized to obtain molten steel by top and bottom combined blowing converter smelting. Argon is blown throughout the converter smelting process, scrap steel is added to the converter, and the converter tapping temperature is 1652°C; then the molten steel after converter smelting is subjected to LF refining outside the furnace, and the refining temperature is ≥1564°C. After LF refining outside the furnace, the molten steel is subjected to RH vacuum treatment, and the vacuum time is maintained for ≥10min; slab continuous casting is carried out according to the smelting chemical composition, and the superheat is 23°C, and then the slab is cleaned, slowly cooled, and continuously Ingot quality inspection; the slab heating temperature is 1230℃, the heating time is 245min, and the heated slab is subjected to high-pressure water dephosphorization; the starting temperature of rough rolling is 1075℃, the average starting temperature of finishing rolling is 920℃, and the average final temperature of finishing rolling is 820℃; after finishing rolling, the steel plate is relaxed on the roller, that is, short-range reciprocating swing for 82 seconds, the steel plate release temperature is 715℃, and the average temperature of the steel plate is reduced to 640℃ after laminar cooling; the thickness of the finished steel plate is 24.0mm, and finally the product quality inspection and warehousing are carried out.

[0015] Further, the molten iron is pre-treated for desulfurization, and the molten iron is decarbonized and dephosphorized to obtain molten steel by top and bottom combined blowing converter smelting. Argon is blown throughout the converter smelting process, scrap steel is added to the converter, and the converter tapping temperature is 1661°C; then the molten steel after converter smelting is subjected to LF refining outside the furnace, and the refining temperature is ≥1564°C. After LF refining outside the furnace, the molten steel is subjected to RH vacuum treatment, and the vacuum time is maintained for ≥10min; slab continuous casting is carried out according to the smelting chemical composition, and the superheat is 28°C, and then the slab is cleaned, slowly cooled, and continuously Ingot quality inspection; the slab heating temperature is 1210℃, the heating time is 250min, and the heated slab is subjected to high-pressure water dephosphorization; the starting temperature of rough rolling is 1080℃, the average starting temperature of finishing rolling is 910℃, and the average final temperature of finishing rolling is 810℃; after finishing rolling, the steel plate is relaxed on the roller, that is, short-range reciprocating swing for 85 seconds, the steel plate release temperature is 710℃, and the average temperature of the steel plate is reduced to 635℃ after laminar cooling; the thickness of the finished steel plate is 36.0mm, and finally the product quality inspection and warehousing are carried out.

[0016] Further, the molten iron is subjected to a desulfurization pretreatment, and the molten iron is decarbonized and dephosphorized to obtain molten steel by top and bottom combined blowing converter smelting. Argon is blown throughout the converter smelting process, scrap steel is added to the converter, and the converter tapping temperature is 1660°C; then the molten steel after converter smelting is subjected to LF refining outside the furnace, and the refining temperature is ≥1562°C. After LF refining outside the furnace, the molten steel is subjected to RH vacuum treatment, and the vacuum time is maintained for ≥10min; slab continuous casting is carried out according to the smelting chemical composition, and the superheat is 22°C, and then the slab is cleaned, slowly cooled, and continuously Ingot quality inspection; the slab heating temperature is 1215℃, the heating time is 252min, and the heated slab is subjected to high-pressure water dephosphorization; the starting temperature of rough rolling is 1075℃, the average starting temperature of finishing rolling is 890℃, and the average final temperature of finishing rolling is 800℃; after finishing rolling, the steel plate is relaxed on the roller, that is, the short-range reciprocating swing is 93 seconds, the steel plate release temperature is 700℃, and the average temperature of the steel plate is reduced to 625℃ after laminar cooling; the thickness of the finished steel plate is 44.0mm, and finally the product quality inspection and warehousing are carried out.

[0017] Further, the molten iron is subjected to a desulfurization pretreatment, and the molten iron is decarbonized and dephosphorized to obtain molten steel by top and bottom combined blowing converter smelting. Argon is blown throughout the converter smelting process, scrap steel is added to the converter, and the converter tapping temperature is 1667°C; then the molten steel after converter smelting is subjected to LF refining outside the furnace, and the refining temperature is ≥1563°C. After LF refining outside the furnace, the molten steel is subjected to RH vacuum treatment, and the vacuum time is maintained for ≥10min; slab continuous casting is carried out according to the smelting chemical composition, and the superheat is 26°C, and then the slab is cleaned, slowly cooled, and continuously Ingot quality inspection; the slab heating temperature is 1220℃, the heating time is 235min, and the heated slab is dephosphorized by high-pressure water; the starting temperature of rough rolling is 1090℃, the average starting temperature of finishing rolling is 890℃, and the average final temperature of finishing rolling is 790℃; after finishing rolling, the steel plate is relaxed on the roller, that is, it swings back and forth for 100 seconds in a short distance, the steel plate release temperature is 700℃, and the average temperature of the steel plate is reduced to 610℃ after laminar cooling; the thickness of the finished steel plate is 50.0mm, and finally the product quality inspection and warehousing are carried out.

[0018] Compared with the prior art, the beneficial technical effects of the present invention are:

[0019] The railway bridge steel Q420qE steel plate manufactured by the present invention not only has pure steel quality, uniform mechanical properties, excellent low temperature (-40°C) impact toughness, but also satisfies the yield strength ratio of ≤0.85, and is suitable for the main structure manufacturing of large-span bridges. At the same time, the preparation method of the present invention is simple and suitable for industrial production. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] The present invention will be further described below in conjunction with the accompanying drawings.

[0021] Figure 1 This is a microstructure diagram of the comparative example of the present invention, the structure is bainite, which is not easy to control the low yield ratio;

[0022] Figure 2 This is the microstructure diagram of Example 2 of the present invention. The structure is ferrite + pearlite, which is easy to achieve low yield ratio control. DETAILED DESCRIPTION

[0023] The present invention is described in more detail below through specific examples and comparative examples. The examples include the first step, the second step, the third step, the fourth step, and the fifth step of the technical solution, and the comparative examples include the first step, the second step, the third step, and the fifth step of the technical solution. The examples are merely descriptions of the best mode of implementation of the present invention and do not limit the scope of the present invention in any way.

[0024] Comparative Example

[0025] The molten iron is pre-treated for desulfurization, and the top and bottom combined blown converter is used to decarbonize and dephosphorize the molten iron to obtain molten steel. Argon is blown throughout the converter smelting process, and scrap steel is added to the converter. The converter tapping temperature is 1642℃. Then the molten steel after converter smelting is subjected to LF furnace refining, and the refining temperature is ≥1563℃. After LF furnace refining, the molten steel is subjected to RH vacuum treatment, and the vacuum time is maintained for ≥8min. The slab is continuously cast according to the smelting chemical composition shown in Table 1, and the superheat is 25℃. After that, the slab is cleaned, slowly cooled, and the quality of the continuous casting slab is checked. The slab heating temperature is 1230℃, the heating time is 245min, and the heated slab is dephosphorized by high-pressure water. The starting temperature of rough rolling is 1085℃, the average starting temperature of finishing rolling is 920℃, and the average final rolling temperature of finishing rolling is 810℃. After finishing rolling, the average temperature of the steel plate is reduced to 630℃ by laminar cooling. The thickness of the finished steel plate is 36.0mm, and the product quality is finally inspected and put into storage.

[0026] Example 1

[0027] The molten iron is pre-treated for desulfurization, and the top and bottom combined blown converter is used to decarbonize and dephosphorize the molten iron to obtain molten steel. Argon is blown throughout the converter smelting process, scrap steel is added to the converter, and the converter tapping temperature is 1640℃. Then the molten steel after converter smelting is subjected to LF furnace refining, and the refining temperature is ≥1560℃. After LF furnace refining, the molten steel is subjected to RH vacuum treatment, and the vacuum time is maintained for ≥8min. The slab is continuously cast according to the smelting chemical composition shown in Table 1, and the superheat is 23℃. After that, the slab is cleaned, slowly cooled, and the quality of the continuous casting slab is checked. The slab heating temperature is 1225℃, the heating time is 235min, and the heated slab is dephosphorized by high-pressure water. The rough rolling start temperature is 1090℃, the finishing rolling start average temperature is 930℃, and the finishing rolling end average temperature is 820℃. After finishing rolling, the steel plate is relaxed (short-distance reciprocating swing) on ​​the roller for 70 seconds, and the steel plate release temperature is 720°C. After laminar cooling, the average temperature of the steel plate is reduced to 650°C. The thickness of the finished steel plate is 10.0mm, and the product quality is finally tested and put into storage.

[0028] Example 2

[0029] The molten iron is pre-treated for desulfurization, and the top and bottom combined blown converter is used to decarbonize and dephosphorize the molten iron to obtain molten steel. Argon is blown throughout the converter smelting process, and scrap steel is added to the converter. The converter tapping temperature is 1652℃. Then the molten steel after converter smelting is subjected to LF furnace refining, and the refining temperature is ≥1564℃. After LF furnace refining, the molten steel is subjected to RH vacuum treatment, and the vacuum time is maintained for ≥10min. The slab is continuously cast according to the smelting chemical composition shown in Table 1, and the superheat is 23℃. After that, the slab is cleaned, slowly cooled, and the quality of the continuous casting slab is checked. The slab heating temperature is 1230℃, the heating time is 245min, and the heated slab is dephosphorized by high-pressure water. The rough rolling start temperature is 1075℃, the finishing rolling start average temperature is 920℃, and the finishing rolling end average temperature is 820℃. After finishing rolling, the steel plate is relaxed (short-distance reciprocating swing) on ​​the roller for 82 seconds, and the release temperature of the steel plate is 715°C. After laminar cooling, the average temperature of the steel plate is reduced to 640°C. The thickness of the finished steel plate is 24.0mm, and the product quality is finally tested and put into storage.

[0030] Example 3

[0031] The molten iron is pre-treated for desulfurization, and the top and bottom combined blown converter is used to decarbonize and dephosphorize the molten iron to obtain molten steel. Argon is blown throughout the converter smelting process, and scrap steel is added to the converter. The converter tapping temperature is 1661℃. Then the molten steel after converter smelting is subjected to LF furnace refining, and the refining temperature is ≥1564℃. After LF furnace refining, the molten steel is subjected to RH vacuum treatment, and the vacuum time is maintained for ≥10min. The slab is continuously cast according to the smelting chemical composition shown in Table 1, and the superheat is 28℃. After that, the slab is cleaned, slowly cooled, and the quality of the continuous casting is checked. The slab heating temperature is 1210℃, the heating time is 250min, and the heated slab is dephosphorized by high-pressure water. The rough rolling start temperature is 1080℃, the finishing rolling start average temperature is 910℃, and the finishing rolling end average temperature is 810℃. After finishing rolling, the steel plate is relaxed (short-distance reciprocating swing) on ​​the roller for 85 seconds, and the release temperature of the steel plate is 710°C. After laminar cooling, the average temperature of the steel plate is reduced to 635°C. The thickness of the finished steel plate is 36.0mm, and the product quality is finally tested and put into storage.

[0032] Example 4

[0033] The molten iron is pre-treated for desulfurization, and the top and bottom combined blown converter is used to decarbonize and dephosphorize the molten iron to obtain molten steel. Argon is blown throughout the converter smelting process, scrap steel is added to the converter, and the converter tapping temperature is 1660℃. Then the molten steel after converter smelting is subjected to LF furnace refining, and the refining temperature is ≥1562℃. After LF furnace refining, the molten steel is subjected to RH vacuum treatment, and the vacuum time is maintained for ≥10min. The slab is continuously cast according to the smelting chemical composition shown in Table 1, and the superheat is 22℃. After that, the slab is cleaned, slowly cooled, and the quality of the continuous casting slab is checked. The slab heating temperature is 1215℃, the heating time is 252min, and the heated slab is dephosphorized by high-pressure water. The rough rolling start temperature is 1075℃, the finishing rolling start average temperature is 890℃, and the finishing rolling end average temperature is 800℃. After finishing rolling, the steel plate is relaxed (short-distance reciprocating swing) on ​​the roller for 93 seconds, and the steel plate release temperature is 700°C. After laminar cooling, the average temperature of the steel plate is reduced to 625°C. The thickness of the finished steel plate is 44.0mm, and the product quality is finally tested and put into storage.

[0034] Example 5

[0035] The molten iron is pre-treated for desulfurization, and the top and bottom combined blown converter is used to decarbonize and dephosphorize the molten iron to obtain molten steel. Argon is blown throughout the converter smelting process, scrap steel is added to the converter, and the converter tapping temperature is 1667℃. Then the molten steel after converter smelting is subjected to LF furnace refining, and the refining temperature is ≥1563℃. After LF furnace refining, the molten steel is subjected to RH vacuum treatment, and the vacuum time is maintained for ≥10min. The slab is continuously cast according to the smelting chemical composition shown in Table 1, and the superheat is 26℃. After that, the slab is cleaned, slowly cooled, and the quality of the continuous casting slab is checked. The slab heating temperature is 1220℃, the heating time is 235min, and the heated slab is dephosphorized by high-pressure water. The rough rolling start temperature is 1090℃, the finishing rolling start average temperature is 890℃, and the finishing rolling end average temperature is 790℃. After finishing rolling, the steel plate is relaxed (short-range reciprocating swing) on ​​the roller for 100 seconds, and the release temperature of the steel plate is 700°C. After laminar cooling, the average temperature of the steel plate is reduced to 610°C. The thickness of the finished steel plate is 50.0mm, and the product quality is finally tested and put into storage.

[0036] Table 1 Chemical composition (wt%) of comparative examples and embodiments 1 to 5 of the present invention

[0037] Chemical Elements C Si Mn P S V Ti Nb Als Comparative Example 0.10 0.25 1.60 0.008 0.002 0.040 0.012 0.036 0.025 Example 1 0.09 0.23 1.58 0.009 0.001 0.038 0.010 0.040 0.030 Example 2 0.10 0.22 1.53 0.010 0.003 0.036 0.011 0.038 0.029 Example 3 0.11 0.26 1.59 0.008 0.004 0.041 0.010 0.039 0.031 Example 4 0.10 0.28 1.60 0.007 0.001 0.039 0.012 0.036 0.036 Example 5 0.11 0.25 1.61 0.010 0.002 0.040 0.014 0.040 0.027

[0038] Table 2 Mechanical properties of the comparative examples and embodiments 1 to 5 of the present invention

[0039]

[0040] The embodiments described above are only descriptions of the preferred modes of the present invention, and are not intended to limit the scope of the present invention. Without departing from the design spirit of the present invention, various modifications and improvements made to the technical solutions of the present invention by ordinary technicians in this field should all fall within the protection scope determined by the claims of the present invention.

Claims

1. A method for controlling the low yield strength ratio of railway bridge steel Q420qE by post-rolling relaxation, characterized in that: include: (1) Smelting-continuous casting production process: KR desulfurization-converter-LF refining-RH vacuum treatment-slab continuous casting-slow cooling; the composition of molten steel for the casting machine is C: 0.09-0.11%, Si: 0.20-0.30%, Mn: 1.55-1.65%, P: ≤0.010%, S≤0.005%, Als: 0.020-0.040%, Nb: 0.035-0.045%, V: 0.035-0.045%, Ti: 0.005-0.015%, and the rest is Fe and unavoidable impurities; (2) During the heating process, the slab is put into the furnace; the slab time in the furnace and the temperature out of the furnace are strictly controlled, the total time in the furnace is ≥220min, and the temperature out of the furnace is 1220±20℃; (3) Rolling includes rough rolling and finishing rolling. For rough rolling, the starting rolling temperature is ≥1070°C. According to the width of the product, the widening task is completed with as few passes as possible. According to the thickness of the product, the relative reduction rate of a single pass is controlled at ≥14% for at least two passes. For finishing rolling, the starting rolling temperature is 860-950°C, and the finishing rolling has at least two passes with a reduction rate of ≥12%. The final rolling temperature of finishing rolling is 770-840°C. (4) Relaxation process: After finishing rolling, the steel plate is relaxed on the roller, i.e., it swings back and forth for 70-100 seconds in a short distance, and the release temperature of the steel plate is 700-720°C; (5) The cooling adopts laminar cooling equipment, and the cooling mode adopts the self-learning calculation result of the secondary system. The final cooling temperature is 600-660℃, and the cooling speed is 10-20℃ / s.

2. The method for controlling the low yield strength ratio of railway bridge steel Q420qE by post-rolling relaxation according to claim 1, characterized in that: The yield strength ratio meets the requirement of ≤0.

85.

3. The method for controlling the low yield strength ratio of railway bridge steel Q420qE by post-rolling relaxation according to claim 1, characterized in that: The molten iron is pre-treated for desulfurization, and the top and bottom combined blowing converter is used to decarbonize and dephosphorize the molten iron to obtain molten steel. Argon is blown throughout the converter smelting process, scrap steel is added to the converter, and the converter tapping temperature is 1640℃; then the molten steel after converter smelting is subjected to LF refining outside the furnace, and the refining temperature is ≥1560℃. After LF refining outside the furnace, the molten steel is subjected to RH vacuum treatment, and the vacuum time is maintained for ≥8min; slab continuous casting is carried out according to the smelting chemical composition, and the superheat is 23℃, and then the slab is cleaned, slowly cooled, and the continuous casting billet is The slab is heated to 1225℃ for 235min and dephosphorized by high-pressure water. The starting temperature of rough rolling is 1090℃, the average starting temperature of finishing rolling is 930℃, and the average final temperature of finishing rolling is 820℃. After finishing rolling, the steel plate is relaxed on the roller, i.e., it swings back and forth for 70 seconds. The release temperature of the steel plate is 720℃. After laminar cooling, the average temperature of the steel plate is reduced to 650℃. The thickness of the finished steel plate is 10.0mm. Finally, the product quality is tested and put into storage.

4. The method for controlling the low yield strength ratio of railway bridge steel Q420qE by post-rolling relaxation according to claim 1, characterized in that: The molten iron is pre-treated for desulfurization, and the top and bottom combined blowing converter is used to decarbonize and dephosphorize the molten iron to obtain molten steel. Argon is blown throughout the converter smelting process, scrap steel is added to the converter, and the converter tapping temperature is 1652℃; then the molten steel after converter smelting is subjected to LF refining, and the refining temperature is ≥1564℃. After LF refining, the molten steel is subjected to RH vacuum treatment, and the vacuum time is maintained for ≥10min; slab continuous casting is carried out according to the smelting chemical composition, and the superheat is 23℃, and then The slabs are cleaned, slowly cooled, and the quality of the continuous casting slabs is checked; the slab heating temperature is 1230℃, the heating time is 245min, and the heated slabs are dephosphorized by high-pressure water; the rough rolling start temperature is 1075℃, the finishing rolling start average temperature is 920℃, and the finishing rolling end average temperature is 820℃; after finishing rolling, the steel plate is relaxed on the roller, that is, the short-range reciprocating swing is 82 seconds, the steel plate release temperature is 715℃, and the average temperature of the steel plate is reduced to 640℃ after laminar cooling; The finished steel plate is 24.0mm thick and is finally inspected for product quality and put into storage.

5. The method for controlling the low yield strength ratio of railway bridge steel Q420qE by post-rolling relaxation according to claim 1, characterized in that: The molten iron is pre-treated for desulfurization, and the top and bottom combined blowing converter is used to decarbonize and dephosphorize the molten iron to obtain molten steel. Argon is blown throughout the converter smelting process, scrap steel is added to the converter, and the converter tapping temperature is 1661°C; then the molten steel after converter smelting is subjected to LF refining, and the refining temperature is ≥1564°C. After LF refining, the molten steel is subjected to RH vacuum treatment, and the vacuum time is maintained for ≥10min; slab continuous casting is carried out according to the smelting chemical composition, and the superheat is 28°C, and then The slabs are cleaned, slowly cooled, and the quality of the continuous casting slabs is checked; the slab heating temperature is 1210℃, the heating time is 250min, and the heated slabs are dephosphorized with high-pressure water; the rough rolling start temperature is 1080℃, the finishing rolling start average temperature is 910℃, and the finishing rolling end average temperature is 810℃; after finishing rolling, the steel plate is relaxed on the roller, that is, the short-range reciprocating swing is 85 seconds, the steel plate release temperature is 710℃, and the average temperature of the steel plate is reduced to 635℃ after laminar cooling; The thickness of the finished steel plate is 36.0mm, and the product quality is finally inspected and put into storage.

6. The method for controlling the low yield strength ratio of railway bridge steel Q420qE by post-rolling relaxation according to claim 1, characterized in that: The molten iron is pre-treated for desulfurization, and the top and bottom combined blowing converter is used to decarbonize and dephosphorize the molten iron to obtain molten steel. Argon is blown throughout the converter smelting process, scrap steel is added to the converter, and the converter tapping temperature is 1660℃; then the molten steel after converter smelting is subjected to LF refining, and the refining temperature is ≥1562℃. After LF refining, the molten steel is subjected to RH vacuum treatment, and the vacuum time is maintained for ≥10min; slab continuous casting is carried out according to the smelting chemical composition, and the superheat is 22℃, and then The slabs are cleaned, slowly cooled, and the quality of the continuous casting slabs is checked; the slab heating temperature is 1215℃, the heating time is 252min, and the heated slabs are dephosphorized by high-pressure water; the rough rolling start temperature is 1075℃, the finishing rolling start average temperature is 890℃, and the finishing rolling end average temperature is 800℃; after finishing rolling, the steel plate is relaxed on the roller, that is, the short-range reciprocating swing is 93 seconds, the steel plate release temperature is 700℃, and the average temperature of the steel plate is reduced to 625℃ after laminar cooling; The finished steel plate is 44.0mm thick and is finally inspected for product quality and put into storage.

7. The method for controlling the low yield strength ratio of railway bridge steel Q420qE by post-rolling relaxation according to claim 1, characterized in that: The molten iron is pre-treated for desulfurization, and the top and bottom combined blowing converter is used to decarbonize and dephosphorize the molten iron to obtain molten steel. Argon is blown throughout the converter smelting process, scrap steel is added to the converter, and the converter tapping temperature is 1667℃; then the molten steel after converter smelting is subjected to LF refining outside the furnace, and the refining temperature is ≥1563℃. After LF refining outside the furnace, the molten steel is subjected to RH vacuum treatment, and the vacuum time is maintained for ≥10min; slab continuous casting is carried out according to the smelting chemical composition, and the superheat is 26℃, and then the slab is cleaned, slowly cooled, and the continuous casting billet is The slab is heated to 1220℃ for 235min and dephosphorized by high-pressure water. The starting temperature of rough rolling is 1090℃, the average starting temperature of finishing rolling is 890℃, and the average final temperature of finishing rolling is 790℃. After finishing rolling, the steel plate is relaxed on the roller, i.e., it swings back and forth for 100 seconds. The release temperature of the steel plate is 700℃, and the average temperature of the steel plate is reduced to 610℃ after laminar cooling. The thickness of the finished steel plate is 50.0mm, and finally the product quality is tested and put into storage.

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