Low-temperature-resistant 235MPa-grade carbon structural steel and preparation method thereof

Abstract

The invention discloses low-temperature-resistant 235MPa-grade carbon structural steel and a preparation method thereof. The low-temperature-resistant 235MPa-grade carbon structural steel comprises the following chemical components in percentage by weight: 0.06 to 0.12 percent of C, 0.10 to 0.35 percent of Si, 0.70 to 1.20 percent of Mn, less than or equal to 0.025 percent of P, less than or equal to 0.015 percent of S, 0.010 to 0.020 percent of Nb, more than or equal to 0.015 percent of Als and the balance of Fe and inevitable impurities. Through the composition design of low C, Nb and Al composite microalloying, the controlled rolling and air cooling process is adopted, the steel plate with the thickness of 6-80 mm is obtained, a microstructure with ferrite and pearlite as main components is obtained, and the 235 MPa grade carbon structural steel has the good characteristics of low cost, low yield ratio and high toughness, is suitable for manufacturing equipment in high latitude areas and has high social value.

Description

Technical Field

[0001] This invention relates to the field of metallurgical technology, and more specifically, to a low-temperature resistant 235MPa grade carbon structural steel and its preparation method. Background Technology

[0002] 235MPa grade carbon structural steel is a type of steel with low carbon content, good plasticity and weldability. It possesses moderate strength and toughness, and is relatively inexpensive, making it widely used in industry and daily life. As human activity extends to higher latitudes, the requirements for the impact toughness of 235MPa grade carbon structural steel are becoming increasingly stringent. Many domestic and international standards, such as ASTM A36, EN10025-2, GB / T700, GB / T19879, GB / T712, and GB / T4171, include steel grades of this strength level. However, except for Q235GJE, Ship E, and Q235NHE, which have explicit requirements for low-temperature toughness at -40℃, the impact temperature of other steel grades of the same strength level is not lower than -20℃. Currently, there are no reports on 235MPa grade carbon structural steel plates resistant to -40℃ low-temperature impact and their manufacturing methods. Summary of the Invention

[0003] The purpose of this invention is to overcome the above-mentioned shortcomings of the prior art and provide a low-temperature resistant 235MPa grade carbon structural steel and its preparation method.

[0004] The present invention solves its technical problems by adopting the following technical solutions.

[0005] This invention provides a low-temperature resistant 235MPa grade carbon structural steel. The chemical composition and weight percentage of the low-temperature resistant 235MPa grade carbon structural steel are as follows: C: 0.06~0.12%, Si: 0.10~0.35%, Mn: 0.70~1.20%, P≤0.025%, S≤0.015%, Nb: 0.010~0.020%, Als: ≥0.015%, with the remainder being Fe and unavoidable impurities.

[0006] The present invention also provides a method for preparing the above-mentioned low-temperature resistant 235MPa grade carbon structural steel, which adopts the following smelting process: converter smelting - LF refining - continuous casting - heating - controlled rolling - post-rolling cooling - straightening - inspection - marking - warehousing.

[0007] The present invention has the following beneficial effects: This invention provides a low-temperature resistant 235MPa grade carbon structural steel. Through composition design of low C, Nb+Al composite microalloying, and the use of controlled rolling + air cooling process, a steel plate with a thickness of 6~80mm is obtained, resulting in a microstructure mainly composed of ferrite + pearlite, and realizing the good characteristics of low cost, low yield strength ratio and high toughness of the steel plate. This invention possesses excellent comprehensive performance, with a yield strength ≥235MPa, tensile strength 370~500MPa, yield-to-tensile ratio ≤0.85, elongation ≥25%, longitudinal impact energy ≥60J at -40℃, and good weldability. It is suitable for manufacturing equipment in high-latitude regions and has high social value. Attached Figure Description

[0008] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of the present invention and should not be regarded as a limitation on the scope. For those skilled in the art, other related drawings can be obtained based on these drawings without creative effort.

[0009] Figure 1 Metallographic image of the low-temperature resistant 235MPa grade carbon structural steel prepared in Example 1 at 1 / 4 thickness; Figure 2 Metallographic image of the low-temperature resistant 235MPa grade carbon structural steel prepared in Example 5 at 1 / 4 thickness; Figure 3 The metallographic structure of the low-temperature resistant 235MPa grade carbon structural steel prepared in Example 7 is shown at 1 / 4 of its thickness. Detailed Implementation

[0010] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below. Where specific conditions are not specified in the embodiments, conventional conditions or conditions recommended by the manufacturer shall apply. Reagents or instruments whose manufacturers are not specified are all conventional products that can be purchased commercially.

[0011] The following is a detailed description of a low-temperature resistant 235MPa grade carbon structural steel and its preparation method provided by an embodiment of the present invention.

[0012] In a first aspect, the present invention provides a low-temperature resistant 235MPa grade carbon structural steel, the chemical composition and weight percentage of which are: C: 0.06~0.12%, Si: 0.10~0.35%, Mn: 0.70~1.20%, P≤0.025%, S≤0.015%, Nb: 0.010~0.020%, Als: ≥0.015%, with the remainder being Fe and unavoidable impurities.

[0013] To prepare 235MPa grade carbon structural steel resistant to low-temperature impact at -40℃, this invention proposes to increase the Mn content in the chemical composition of traditional 235MPa grade carbon structural steel to above 0.70%, preferably 0.70~1.20%, reduce the sulfur and phosphorus content, and add Nb and Al microalloying elements to achieve grain refinement and precipitation strengthening, thereby improving the low-temperature impact toughness of the 235MPa grade carbon structure.

[0014] In some alternative embodiments, the microstructure of the low-temperature resistant 235MPa grade carbon structural steel is ferrite + pearlite.

[0015] In some alternative embodiments, the mechanical properties of the low-temperature resistant 235MPa grade carbon structural steel are as follows: yield strength ≥235MPa, tensile strength 370~500MPa, yield ratio ≤0.85, elongation ≥25%, and longitudinal impact energy at -40℃ ≥60J.

[0016] In some alternative implementations, the thickness of the low-temperature resistant 235MPa grade carbon structural steel is 6~80mm.

[0017] Secondly, the present invention also provides a method for preparing the above-mentioned low-temperature resistant 235MPa grade carbon structural steel, which adopts the following smelting process: converter smelting - LF refining - continuous casting - heating - controlled rolling - post-rolling cooling - straightening - inspection - marking - warehousing.

[0018] In some alternative embodiments, the continuous casting process and the heating process include: smelting according to the chemical composition and continuously casting billets of the required thickness, requiring a compression ratio of not less than 3; sending the continuously cast billets to a heating furnace, heating at a temperature of 1200~1260℃, and controlling the total heating time at 1.0~1.1min / mm.

[0019] In some alternative embodiments, the rolling process includes: controlling the roughing rolling start temperature to be ≥1000℃, the finishing rolling start temperature to be 850~920℃, the intermediate billet thickness to be heated to be not less than twice the finished product thickness; and the final rolling temperature to be 780~860℃.

[0020] In some alternative implementations, the post-rolling cooling process includes air cooling the rolled steel sheet.

[0021] In some alternative implementations, the straightening process includes 1-3 passes of post-rolling hot straightening.

[0022] Traditional 235MPa grade carbon structural steel is not resistant to low temperatures, mainly due to the following reasons: 1) The Mn content in 235MPa grade carbon structural steel is relatively low (not greater than 0.70%). Mn can dissolve in ferrite, and the addition of Mn lowers the austenite phase transformation temperature, increases the ferrite crystal nucleation rate, and reduces the ferrite grain growth rate, thereby refining the grains; 2) The sulfur and phosphorus content in 235MPa grade carbon structural steel is relatively high (sulfur not greater than 0.050%, phosphorus not greater than 0.045%). Sulfur easily forms FeS or MnS, weakening grain boundaries and reducing the steel's ductility and toughness, while phosphorus significantly increases the steel's brittle transition temperature and reduces its low-temperature impact toughness; 3) 235MPa grade carbon structural steel lacks nitrogen-fixing elements such as Nb, V, Ti, and Al, and cannot form corresponding carbonitridium compounds to achieve grain refinement and precipitation strengthening, etc. Therefore, the corresponding preparation technology cannot meet the low-temperature resistance performance of -40℃.

[0023] To overcome the aforementioned problems in existing technologies, this invention provides a 235MPa grade carbon structural steel plate and its preparation method. The method increases the Mn content in the chemical composition of traditional 235MPa grade carbon structural steel to 0.70~1.20%, reduces the sulfur and phosphorus content, and adds Nb and Al microalloying elements to achieve grain refinement and precipitation strengthening. Furthermore, the preparation process specifies a compression ratio ≥3 to better achieve grain and microstructure refinement during rolling; it specifies heating temperature and time to ensure a more uniform and fine austenite microstructure during heating; it specifies the roughing rolling start temperature, intermediate billet thickness, finish rolling start temperature, and final rolling temperature to better achieve controlled rolling and refine austenite grains; and it specifies post-rolling air cooling to obtain a purer ferrite + pearlite microstructure in the 235MPa grade carbon structural steel. Through these reasonable limitations in composition and process, the low-temperature impact toughness of the 235MPa grade carbon structural steel is significantly improved.

[0024] The present invention will be further described below with reference to embodiments.

[0025] The following embodiment provides a method for producing low-temperature resistant 235MPa grade carbon structural steel, employing the following smelting route: smelting, continuous casting, heating, rolling, and cooling, specifically: 1) Smelt and continuously cast billets of the required thickness according to chemical composition, with a compression ratio of not less than 3; 2) Heating: The continuously cast billet is sent to the heating furnace, and the heating temperature is 1200~1260℃. The total heating time is controlled at 1.0~1.1min / mm. 3) Rolling: Two-stage rolling is adopted. The roughing rolling start temperature is ≥1000℃, the finishing rolling start temperature is 850~920℃, and the thickness of the intermediate billet before heating is not less than twice the thickness of the finished product; the final rolling temperature is 780~860℃. 4) Air cooling after rolling; 5) Hot straightening: 1-3 passes of hot straightening after rolling; The chemical composition, process parameters, and performance of each embodiment are shown in Tables 1, 2, and 3: Table 1. Chemical composition (wt%) of 235MPa grade carbon structural steel prepared in Examples 1-7

[0026] Table 2. Process parameters for the 235MPa grade carbon structural steel prepared in Examples 1-7

[0027] Table 3. Test results of 235MPa grade carbon structural steel prepared in Examples 1-7

[0028] Combined with Table 3 above and Figures 1-3 It can be seen that the microstructure of the 235MPa grade carbon structural steel plate prepared using the schemes of Examples 1-7 of this invention is a "ferrite + pearlite" structure. Therefore, the 235MPa grade carbon structural steel produced by this invention has excellent low-temperature impact performance, with a longitudinal impact energy KV2 > 60J at -40℃.

[0029] The chemical composition, process parameters and performance of Comparative Examples 1-6 are shown in Tables 4, 5 and 6, respectively.

[0030] Table 4. Chemical composition (wt%) of 235MPa grade carbon structural steel prepared in Comparative Examples 1-6

[0031] Table 5 shows the chemical composition (wt%) of the 235MPa grade carbon structural steel prepared in Comparative Examples 1-6.

[0032] Table 6 shows the chemical composition (wt%) of the 235MPa grade carbon structural steel prepared in Comparative Examples 1-6.

[0033] As can be seen from Table 6 above, for 235MPa grade carbon structural steel with the same plate thickness, the test results are significantly lower than those of the example.

[0034] As can be seen from the above, the present invention provides a low-temperature resistant 235MPa grade carbon structural steel and its preparation method. By improving the composition and preparation process of traditional 235MPa grade carbon structural steel, the low-temperature impact toughness of the 235MPa grade carbon structural steel is significantly improved.

[0035] The above are merely preferred embodiments of the present invention and are not intended to limit the present invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A low-temperature-resistant 235 MPa-grade carbon structural steel, characterized in that, The chemical composition and weight percentage of the low-temperature-resistant 235MPa-grade carbon structural steel are as follows: C: 0.06-0.12%, Si: 0.10-0.35%, Mn: 0.70-1.20%, P≤0.025%, S≤0.015%, Nb: 0.010-0.020%, Als: ≥0.015%, and the rest is Fe and inevitable impurities.

2. The low-temperature-resistant 235 MPa-grade carbon structural steel according to claim 1, characterized in that, The metallographic structure of the low-temperature-resistant 235MPa-grade carbon structural steel is ferrite+pearlite structure.

3. The low-temperature-resistant 235 MPa-grade carbon structural steel according to claim 1, characterized in that, The mechanical properties of the low-temperature-resistant 235MPa-grade carbon structural steel are as follows: yield strength≥235MPa, tensile strength 370-500MPa, yield strength ratio≤0.85, elongation≥25%, and longitudinal impact energy at-40℃≥60J.

4. The cryogenic resistant 235 MPa grade carbon structural steel as claimed in claim 1, wherein, The thickness specification of the low-temperature-resistant 235MPa-grade carbon structural steel is 6-80mm.

5. A method of producing the cryogenic-resistant 235 MPa-grade carbon structural steel according to any one of claims 1 to 4, characterized in that, The following smelting procedure is adopted: converter smelting-LF refining-continuous casting-heating-control rolling-rolling cooling-straightening-inspection-marking-warehousing.

6. The production method according to claim 5, wherein The continuous casting procedure comprises: smelting according to the chemical composition and continuously casting the required thickness of the blank, and the compression ratio is required to be not less than 3.

7. The preparation method according to claim 5, characterized in that, The heating procedure comprises: sending the continuously cast blank to the heating furnace, the heating temperature is 1200-1260℃, and the total heating time is controlled according to 1.0-1.1min / mm.

8. The preparation method according to claim 5, characterized in that, The rolling procedure comprises: controlling the rough rolling open rolling temperature to be≥1000℃, the finish rolling open rolling temperature to be 850-920℃, and the intermediate blank waiting temperature thickness to be not less than 2 times of the finished product thickness; and the final rolling temperature is 780-860℃.

9. The preparation method according to claim 5, characterized in that, The rolling cooling procedure comprises: air cooling the rolled steel plate.

10. The method of claim 5, wherein, The straightening procedure comprises: 1-3 times of hot straightening after rolling.

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