A weather-resistant Q235NHB medium plate produced by a double-stand rolling mill

Abstract

The invention provides a weather-resistant Q235NHB medium plate produced by a double-stand rolling mill, with a metallographic structure of F+P, a banded structure of 0.5, a grain size of 9 to 9.5, and a yield strength of R p0.2 The product has a tensile strength of 295 MPa or 300 MPa, a tensile strength Rm of 400 to 415 MPa, an elongation A of 34-47%, and an impact energy Ak of ​​298 J, 299 J, and 307 J at 0°C. The product performance exceeds the national standard by more than 15-20%. The product of the present invention has good performance and low cost.

Description

[0001] This application is a divisional application, parent application number: 202210280309.5, application date: 2022.03.21, invention name: Method for producing weather-resistant Q235NHB medium plate by double-stand rolling mill and Q235NHB medium plate. Technical Field

[0002] The invention relates to the field of metallurgy, and in particular to a weather-resistant Q235NHB medium plate produced by a double-stand rolling mill. Background Art

[0003] Q235NHB middle plate is atmospheric corrosion resistant steel, commonly used in bridges, vehicles, towers, railways, oil derricks, harbor buildings, oil production platforms and chemical and petroleum equipment and other steel structures exposed to the atmosphere for a long time. However, at present, the production cost of weather-resistant Q235NHB middle plate is high, the performance is relatively low, and the cost performance is low, which affects the use of the product.

[0004] In summary, the prior art has the following problems: the production cost of weather-resistant Q235NHB mid-plate is high, the performance is relatively low, and the cost performance is low. Summary of the invention

[0005] The invention provides a method for producing weather-resistant Q235NHB medium plate by a double-stand rolling mill and the Q235NHB medium plate, so as to solve the problems of high production cost, relatively low performance and low cost performance of the weather-resistant Q235NHB medium plate.

[0006] To this end, the present invention proposes a method for producing weather-resistant Q235NHB medium plate by a double-stand rolling mill, and the method for producing weather-resistant Q235NHB medium plate by a double-stand rolling mill comprises the following process steps:

[0007] Converter smelting, refining, slab continuous casting, inspection, slab heating, descaling, rough rolling, finishing rolling, straightening and storage and stacking;

[0008] The chemical composition weight percentage of weather-resistant Q235NHB middle plate is:

[0009] C: 0.05-0.09Wt%, Si: 0.10-0.20Wt%, Mn: 0.45-0.55Wt%, P≤0.018Wt%, S≤0.010Wt%, Alt: 0.020-0.040Wt%, Cu: 0.25-0.35Wt%, Cr: 0.40-0.60Wt%, Ni: 0.07-0.12Wt%; the remainder is inevitable trace elements;

[0010] Among them, in converter smelting, the S of molten iron entering the furnace is required to be ≤0.030Wt%, and argon is blown from the bottom throughout the smelting process; the molten steel is refined and deoxidized in the LF furnace without calcium treatment;

[0011] The heating section is 1030-1230°C, the soaking section is 1140-1230°C, and the heating time is set to 2.5-3.5 hours.

[0012] Furthermore, the rough rolling temperature is 1030-1200°C, and the blank thickness is controlled to be 3 times the finished product thickness.

[0013] Furthermore, the start rolling temperature of the finishing rolling is 850-990°C, and the final rolling temperature is 810-890°C.

[0014] Furthermore, no post-machine water cooling is used in the production process.

[0015] Furthermore, the rolled steel plates are pile-cooled in the finished product warehouse for more than 24 hours to naturally eliminate the residual stress inside and outside the steel plates and optimize the internal and external appearance quality of the steel plates.

[0016] Furthermore, the stack cooling temperature is 220-280°C.

[0017] The present invention also proposes a weather-resistant Q235NHB medium plate produced by a double-stand rolling mill, wherein the chemical composition weight percentage of the weather-resistant Q235NHB medium plate is:

[0018] C: 0.05-0.09Wt%, Si: 0.10-0.20Wt%, Mn: 0.45-0.55Wt%, P≤0.018Wt%, S≤0.010Wt%, Alt: 0.020-0.040Wt%, Cu: 0.25-0.35Wt%, Cr: 0.40-0.60Wt%, Ni: 0.07-0.12Wt%; the remainder is inevitable trace elements;

[0019] After rolling, the steel plates are put into the finished product warehouse for stack cooling for more than 24 hours.

[0020] The double-stand rolling mill produces weather-resistant Q235NHB medium plates using the method described above.

[0021] Furthermore, the thickness of the weather-resistant Q235NHB medium plate produced by the double-stand rolling mill is 16-30 mm.

[0022] Furthermore, the thickness of the weather-resistant Q235NHB medium plate produced by the double-stand rolling mill is 16 mm, 20 mm or 30 mm.

[0023] Furthermore, the chemical composition weight percentage of the weather-resistant Q235NHB middle plate is:

[0024] C: 0.06Wt%, Si: 0.15Wt%, Mn: 0.486Wt%, P: 0.014Wt%, S: 0.003Wt%, Alt: 0.024Wt%, Cu: 0.309Wt%, Cr: 0.443Wt%, Ni: 0.011Wt%; the remainder are inevitable trace elements.

[0025] Beneficial effects:

[0026] The present invention reduces the amount of alloys, especially the more expensive metals, without reducing the performance of the product through component design, thereby reducing costs. In addition, through reasonable component design, specific smelting process, reasonable heating system, and rolling system, cracks in the billet are avoided and the quality of the product is guaranteed. By stacking, reasonable stacking time and temperature are set, and the heat treatment process is omitted, which not only ensures the quality and performance of the product, but also reduces the cost. In terms of rolling technology, it is required that the starting and final temperature ranges and rolling passes in the rough rolling and finishing rolling stages be reasonably designed, and the rolling capacity advantage of the 2800 double-frame medium plate rolling mill is fully utilized to optimize the pass arrangement and increase the reduction (rate) to achieve grain refinement, optimize the structure, and ensure product performance and appearance quality.

[0027] The product performance of the present invention reaches the yield strength R p0.2 ≥235MPa, tensile strength Rm is 360~510MPa, elongation A≥25%, 0℃ impact energy Ak≥47J. Furthermore, the performance yield strength R p0.2 ≥290MPa, tensile strength Rm is ≥395MPa, elongation A ≥33%, 0℃ impact energy Ak ≥229J. Furthermore, the performance yield strength R p0.2 The product performance reaches 295MPa or 300MPa, the tensile strength Rm is 400 to 415MPa, the elongation A is 34-47%, and the impact energy Ak at 0℃ is 298J, 299J, and 307J. The performance of the above products exceeds the national standard by more than 15-20%, and the use of steel can be reduced while ensuring strength or performance. DETAILED DESCRIPTION

[0028] In order to have a clearer understanding of the technical features, purposes and effects of the present invention, the present invention is now described.

[0029] The weather-resistant Q235NHB medium plate production method of the present invention is: (molten iron pretreatment) → converter smelting → argon blowing after the furnace → LF refining → slab continuous casting (+ electric stirring + soft pressing) → stack cooling and inspection → slab heating → descaling → rough rolling → finishing rolling → straightening → stack cooling → sampling → steel plate inspection (ultrasonic flaw detection) → marking → warehousing.

[0030] The applicant uses a 2800mm (2800mm is the maximum width of the finished product that can be produced) double-stand medium plate rolling mill production line to produce weather-resistant Q235NHB medium plates. The double-stand medium plate rolling mill production line is a non-continuous rolling, including one rolling mill for rough rolling and one rolling mill for finishing rolling. Rough rolling is repeated in a "vertical, horizontal and vertical" manner. Before rough rolling, the incoming material has a thickness (217-220mm), a width (1810-1830mm) and a length (1700-2500mm). After 8-10 passes of repeated rolling, the intermediate billet has a thickness (55-80mm) and a width (1900-2100mm), and then enters finishing rolling.

[0031] After 6-8 passes of repeated rolling, the finished product is formed with a thickness of 16-40mm (according to the contract thickness, the maximum thickness that can be produced is 60mm according to actual capacity), a width of 1800-2000mm (according to the contract width, the maximum width that can be produced is 2800mm according to actual capacity), and a length of 8000-10000mm (according to the contract length, the maximum length that can be produced is 150000mm according to actual capacity).

[0032] Main points of converter smelting production:

[0033] A. The requirement for molten iron entering the furnace is S≤0.030Wt%, and argon is blown from the bottom throughout the smelting process.

[0034] B. The molten steel is refined and deoxidized in the LF furnace without calcium treatment.

[0035] The stacking and slow cooling process before and after rolling and the billet stacking before rolling are beneficial to reduce the heating cost.

[0036] The heating process requires that the heating time and the temperature range of each heating section should be reasonably designed to meet the characteristics of the pusher type heating furnace in use, and the heating temperature of each heating section should be strictly controlled on site, especially when producing small batches of multiple steel types together. The billet heating quality should be able to better meet the rolling requirements and provide temperature conditions for the "large reduction" pass in the rough rolling stage. Heating temperature control requirements: the heating section is 1030~1230℃, the soaking section is 1140~1230℃, and the heating time is set to 2.5-3.5 hours (about 210 minutes in actual production).

[0037] In terms of rolling technology, the design of the starting and ending temperatures in the rough rolling stage must be reasonable, in line with the rolling characteristics of the 2800 double-stand medium plate mill, and the rolling process temperature must be strictly controlled on site. According to the actual production steel temperature, the starting temperature of rough rolling is 1030-1200℃, and the thickness of the blank is controlled at 3 times the thickness of the finished product. After rough rolling and steel conversion, more than two large reductions (20-30mm) should be used, the number of passes should be reduced, and the single-pass reduction rate should be increased (the pass reduction rate is generally 10-17%, and the pass reduction rate after large reduction is 20-30%, such as 20%, 22%, 25%, 28%, 30%). During rolling, the residence time between passes should be reduced as much as possible to achieve crushing and refining of grains and optimize the performance of finished products.

[0038] In terms of rolling technology, the design of the start and final temperature ranges in the finishing rolling stage must be reasonable, and the 2800 double-stand medium plate mill should be fully utilized to match the rolling characteristics. In addition, the temperature drop during the rolling process must be strictly controlled on site to ensure the desired metallographic structure of the finished product, so that product performance can be ensured from the rolling technology. According to the actual production steel temperature, the start rolling temperature of the finishing rolling is 850-990°C, the final rolling temperature is 810-890°C, the number of passes is reduced, and the single-pass reduction rate is increased (the reduction rate is generally 11-20%, such as 11%, 15%, 18%, 20%). After continuous rolling, no water cooling or post-machine controlled cooling water curtain is required during the production process.

[0039] During normal production, there is no need to use the post-rolling ultra-fast cooling device, which reduces production consumption.

[0040] Online automatic surface inspection equipment is used on the finishing line to ensure product surface quality.

[0041] The rolled steel plates are placed in the finished product warehouse for cooling for more than 24 hours to naturally eliminate the residual stress inside and outside the steel plates, optimize the internal and external quality of the steel plates, and eliminate the need for heat treatment.

[0042] After cooling, each product is ultrasonically inspected to ensure good internal quality.

[0043] Specific examples of the present invention are shown in Table 1, Table 2, Table 3 and Table 4.

[0044] Table 1: Actual chemical composition of the product (wt%, Examples 3 to 5 are the same smelting number with different finished product thicknesses)

[0045] Examples C Si Mn P S Cu Cr Ni Alt Example 1 0.069 0.15 0.486 0.014 0.003 0.309 0.443 0.011 0.024 Example 2 0.070 0.14 0.471 0.014 0.003 0.295 0.442 0.009 0.023 Example 3 0.072 0.16 0.482 0.016 0.002 0.310 0.441 0.010 0.025 Example 4 0.072 0.16 0.482 0.016 0.002 0.310 0.441 0.010 0.025 Example 5 0.072 0.16 0.482 0.016 0.002 0.310 0.441 0.010 0.025

[0046] Table 2: Specific process parameters of each embodiment

[0047]

[0048]

[0049] Table 3 Product performance (qualified):

[0050]

[0051] The metallographic structure, inclusions, banded structure and grain size of the above five examples are shown in Table 4

[0052] Table 4 Metallographic structure, inclusions, banded structure and grain size of Q235NHB medium plate

[0053]

[0054] The above description is only an illustrative embodiment of the present invention and is not intended to limit the scope of the present invention. The components of the present invention can be combined with each other without conflict, and any equivalent changes and modifications made by any technician in the field without departing from the concept and principle of the present invention should fall within the scope of protection of the present invention.

Claims

1. A weather-resistant Q235NHB medium plate produced by a double-stand rolling mill, characterized in that: The chemical composition weight percentage of weather-resistant Q235NHB middle plate is: C: 0.05~0.09Wt%, Si: 0.10~0.20Wt%, Mn: 0.45~0.55Wt%, P≤0.018Wt%, S≤0.010Wt%, Alt: 0.020~0.040Wt%, Cu: 0.25~0.35Wt%, Cr: 0.40~0.60Wt%, Ni: 0.07~0.12Wt; the balance is iron and inevitable trace elements; The method used by the double-stand rolling mill to produce weather-resistant Q235NHB medium plates includes the following process steps: Converter smelting, refining, slab continuous casting, inspection, slab heating, descaling, rough rolling, finishing rolling, straightening and storage and stacking; After rough rolling and steel conversion, the maximum reduction of 20~30mm should be adopted for two or more times to increase the single-pass reduction rate. The pass reduction rate after adopting the large reduction rate is 20~30%; The starting temperature of finishing rolling is 850-990℃, and the final rolling temperature is 810-890℃; the finishing rolling reduction rate is 11-20%; no water cooling or post-machine controlled cooling water curtain is used in the production process; Weather-resistant Q235NHB medium plate produced by double-stand rolling mill, metallographic structure: F + P, banded structure: 0.5; Grain size: 9~9.5; Weather-resistant Q235NHB medium plate produced by double-stand rolling mill, yield strength R p0.2 The steel plate has reached 300MPa, the tensile strength Rm is 400 to 415MPa, the elongation A is 34-47%, and the impact energy Ak at 0℃ is 298 J, 299 J, and 307J. The performance of the weather-resistant Q235NHB medium plate produced by the double-stand rolling mill exceeds the national standard by more than 15-20%; The thickness of the weather-resistant Q235NHB medium plate produced by the double-stand rolling mill is 16-30 mm.

2. The weather-resistant Q235NHB medium plate produced by the double-stand rolling mill according to claim 1 is characterized in that: The thickness of the weather-resistant Q235NHB medium plate produced by the double-stand rolling mill is 16 mm, 20 mm or 30 mm.

3. The weather-resistant Q235NHB medium plate produced by the double-stand rolling mill according to claim 1, characterized in that: The chemical composition weight percentage of weather-resistant Q235NHB middle plate is: C: 0.06Wt%, Si: 0.15Wt%, Mn: 0.486Wt%, P: 0.014Wt%, S: 0.003Wt%, Alt: 0.024Wt%, Cu: 0.309Wt%, Cr: 0.443Wt%, Ni: 0.011Wt%; the remainder are inevitable trace elements.

4. The weather-resistant Q235NHB medium plate produced by the double-stand rolling mill according to claim 1, characterized in that: After rolling, the steel plates are put into the finished product warehouse for stack cooling for more than 24 hours to naturally eliminate the residual stress inside and outside the steel plates and optimize the internal and external appearance quality of the steel plates.

5. The weather-resistant Q235NHB medium plate produced by the double-stand rolling mill according to claim 1, characterized in that: The stack cooling temperature is 220-280℃.

6. The weather-resistant Q235NHB medium plate produced by the double-stand rolling mill according to claim 1, characterized in that: The chemical composition weight percentage of the weather-resistant Q235NHB middle plate is: C: 0.072Wt%, Si: 0.16Wt%, Mn: 0.482Wt%, P: 0.016Wt%, S: 0.002Wt%, Alt: 0.025Wt%, Cu: 0.310Wt%, Cr: 0.441Wt%, Ni: 0.010Wt; the balance is iron and inevitable trace elements.

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