A low-alloy high-strength steel and its production method

Abstract

This invention discloses a low-alloy high-strength steel and its production method. The chemical composition and weight percentage of the low-alloy high-strength steel are as follows: C 0.14-0.18%, Si 0.20-0.35%, Mn 0.65-0.75%, P≤0.025%, S≤0.018%, Als 0.010-0.035%, Ti 0.04-0.06%, Ca 0.01-0.015%, N 0.0020-0.0050%, with the remainder being Fe and unavoidable trace elements. The low-alloy high-strength steel was developed by increasing the microalloy Ti content and reducing the alloy Mn content. Furthermore, the rolling process was optimized to address the behavior characteristics of the Ti second phase during hot rolling, achieving the production of Q420 steel with a Mn content reduced to 0.65-0.75%, while ensuring its excellent impact performance.

Description

Technical Field

[0001] This invention belongs to the field of medium plate production technology, specifically relating to a low-alloy high-strength steel and its production method. Background Technology

[0002] Q420 steel is widely used in large power plant equipment, locomotives and rolling stock, heavy machinery, and mining machinery construction projects. Reducing the cost of this type of steel is not only of great significance to improving the economic benefits of enterprises, but will also generate positive social benefits.

[0003] According to the national standard GB / T 1591-2018, the chemical composition requirements for Q420 steel are (mass percentage): C ≤0.20%, Si ≤0.55%, Mn ≤1.70%, P ≤0.035%, S ≤0.035%, Cr ≤0.30%, Ni ≤0.80%, Cu ≤0.40%, N ≤0.015%, Mo ≤0.2%, Ti ≤0.05%. However, the Mn content in existing Q420 steel is relatively high, exceeding 1.0%, leading to higher production costs.

[0004] For example, Chinese patent CN 114807768 A discloses a Q420 low-alloy high-strength structural steel and its production method. The composition design of this invention adopts (by mass percentage): C 0.14~0.17%, Si 0.39~0.45%, Mn 1.25~1.40%, P≤0.030%, S≤0.030%, N 0.009~0.015%, V 0.040~0.060%, with the remainder being Fe and unavoidable impurities. Chinese patent CN 103966410 A discloses a method for producing Q420 grade thick plates by high-temperature rolling. The composition of this invention (by mass percentage) is: C 0.14–0.18%, Si 0.25–0.45%, Mn 1.3–1.8%, P ≤0.02%, S ≤0.004%, N 0.003–0.008%, V 0.05–0.08%, with the remainder being Fe and unavoidable impurities. All of the above patents utilize V for alloying and have a relatively high Mn content, resulting in higher production costs.

[0005] Chinese patent CN 118703911 A discloses a Q420 grade improved toughness high corrosion-resistant steel plate, its production method, and its application. The invention uses the following composition (by mass percentage): C 0.06–0.08%, Si 0.15–0.40%, Mn 1.05–1.25%, P ≤0.020%, S ≤0.003%, Nb 0.01–0.02%, Ti 0.01–0.02%, Als 0.02–0.05%, Cu 0.30–0.70%, Ni 0.7–1.5%, Sb 0.08–0.30%, with the remainder being Fe and unavoidable impurities. The Mn content in its composition is also relatively high.

[0006] How to reduce the production cost of Q420 steel while ensuring its performance is a pressing issue that is currently receiving much attention. Summary of the Invention

[0007] To address the aforementioned technical problems, this invention provides a low-alloy high-strength steel and its production method. The invention develops a low-alloy high-strength steel by increasing the microalloyed Ti content and reducing the alloyed Mn content. Furthermore, the rolling process is optimized to address the behavior characteristics of the Ti second phase during hot rolling, achieving the production of Q420 steel with a Mn content reduced to 0.65–0.75%, while ensuring its excellent impact performance.

[0008] The technical solution adopted in this invention is as follows:

[0009] A low-alloy high-strength steel, wherein the chemical composition and weight percentage of the low-alloy high-strength steel are as follows: C 0.14-0.18%, Si 0.20-0.35%, Mn 0.65-0.75%, P ≤0.025%, S ≤0.018%, Als 0.010-0.035%, Ti 0.04-0.06%, Ca 0.01-0.015%, N 0.0020-0.0050%, with the remainder being Fe and unavoidable trace elements; the composition meets the requirements of GB / T 1591-2018 standard.

[0010] The microstructure of the low-alloy high-strength steel is ferrite and pearlite, with a grain size greater than or equal to grade 11.

[0011] The low-alloy high-strength steel has a yield strength ≥420MPa, longitudinal impact energy at 20℃ ≥150J, impact energy at 0℃ ≥120J, and impact energy at -20℃ ≥60J.

[0012] The thickness of the low-alloy high-strength steel is 15-20 mm.

[0013] The present invention also provides a method for producing the aforementioned low-alloy high-strength steel, the method comprising the following steps: continuous casting into slabs after smelting → natural cooling → hot rolling → water cooling → hot straightening of steel plates → stacking cooling;

[0014] During the hot rolling process, the cumulative deformation rate below 900℃ shall not be less than 50%.

[0015] The smelting process includes: blast furnace smelting → hot metal pretreatment → converter smelting → deoxidation and alloying → LF refining.

[0016] In the LF refining step, white slag is produced and maintained for ≥10 min. Then, ferrotitanium is added, and after the composition meets the standard, calcium wire is fed in and soft argon is blown for ≥8 min.

[0017] In the LF refining step, 200-300 kg of refining slag, 400-600 kg of lime, and 50-100 kg of fluorite are added. After the LF furnace is energized and the slag is dissolved for 6-8 minutes, deoxidizers such as Al particles are added to create white slag.

[0018] In the hot rolling step, the heating temperature is controlled at 1180-1200℃ and the heating time is greater than 2 hours; after heating, rough rolling and finish rolling are carried out.

[0019] In the hot rolling process, the roughing rolling start temperature is ≥1000℃ and the finishing rolling temperature is ≥950℃; the finishing rolling start temperature is ≤930℃ and the finishing rolling temperature is 830±20℃.

[0020] In the water cooling step, the outlet water temperature is less than 800℃, and the steel plate reddening temperature is 720±20℃.

[0021] The low-alloy high-strength steel provided by this invention is developed by increasing the microalloyed Ti content and decreasing the alloy Mn content. Increasing the Ti content to 0.04-0.06% results in a TiC particle precipitation temperature approaching approximately 800°C. Since the production process ensures a relatively large deformation rate and a slow cooling rate around 800°C, it promotes TiC particle precipitation, thus producing precipitation strengthening. While Mn primarily plays a solid solution strengthening role, reducing the Mn content leads to a decrease in strength, when the Mn content is reduced to 0.65-0.75%, under the current process, the strength increase caused by TiC particle precipitation just compensates for the strength decrease caused by the reduced Mn content. Therefore, the low-alloy high-strength steel of this invention meets the strength grade of Q420 steel. Simultaneously, due to the precipitation of fine TiC particles, the ductile-brittle transition temperature of Q420 steel will decrease, which is beneficial for low-temperature impact toughness.

[0022] Compared with the prior art, the present invention has the following beneficial effects:

[0023] This invention uses relatively inexpensive titanium, increases the Ti content while reducing the manganese content, and promotes the precipitation of TiC particles through a designed rolling process, thereby ensuring the strength and good impact toughness of Q420 steel and achieving the production of Q420 steel with a Mn content reduced to 0.65-0.75%. Attached Figure Description

[0024] Figure 1 This is a microstructure photograph of a quarter-thickness section of the Q420 steel in Example 1. Detailed Implementation

[0025] The present invention will be further described below with reference to embodiments.

[0026] The present invention provides a low-alloy high-strength steel with the following chemical composition and weight percentage: C 0.14-0.18%, Si 0.20-0.35%, Mn 0.65-0.75%, P ≤0.025%, S ≤0.018%, Als 0.010-0.035%, Ti 0.04-0.06%, Ca 0.01-0.015%, N 0.0020-0.0050%, with the remainder being Fe and unavoidable trace elements.

[0027] The production process of the low-alloy high-strength steel is as follows: blast furnace smelting → hot metal pretreatment → converter smelting → deoxidation and alloying → LF refining → continuous casting into slabs → natural cooling → heating → hot rolling → water cooling → hot straightening of steel plates → stacking cooling → finished product.

[0028] In the LF refining step, white slag is produced and maintained for ≥10 min. Then, ferrotitanium is added, and after the composition meets the standard, calcium wire is fed in and soft argon is blown for ≥8 min.

[0029] In the LF refining step, 200-300 kg of refining slag, 400-600 kg of lime, and 50-100 kg of fluorite are added. After the LF furnace is energized and the slag is dissolved for 6-8 minutes, deoxidizers such as Al particles are added to create white slag.

[0030] In the hot rolling step, the heating temperature is controlled at 1180–1200℃, and the heating time is greater than 2 hours; after heating, rough rolling and finish rolling are carried out. The rough rolling start temperature is ≥1000℃ and the finish rolling temperature is ≥950℃; the finish rolling start temperature is ≤930℃ and the finish rolling temperature is 830±20℃, and the cumulative deformation rate below 900℃ is not less than 50%.

[0031] The present invention will now be described in detail with reference to the embodiments.

[0032] Example 1

[0033] A low-alloy high-strength steel has a hot rolling process divided into two stages: rough rolling with an initial rolling temperature of 1000℃ and a final rolling temperature of 950℃, and finish rolling with an initial rolling temperature of 880℃ and a final rolling temperature of 840℃, with a cumulative deformation rate of 50% below 900℃. The water cooling process has an outlet water temperature of approximately 760℃ and a reheating temperature of 690℃.

[0034] The chemical composition of the 16mm thick Q420 steel produced is as follows: C 0.16%, Si 0.24%, Mn 0.71%, P 0.012%, S ≤0.007%, Als 0.023%, Ti 0.052%, Ca 0.014%, N 0.0050%; the microstructure is ferrite and pearlite, with a grain size of grade 12; the steel plate has a yield strength of 464 MPa, a tensile strength of 566 MPa, an elongation of 24%, and an average impact energy of 189 J at 20℃, 184 J at 0℃, and 114 J at -20℃.

[0035] Example 2

[0036] A low-alloy high-strength steel has a hot rolling process divided into two stages: rough rolling with an initial rolling temperature of 1000℃ and a final rolling temperature of 950℃, and finish rolling with an initial rolling temperature of 920℃ and a final rolling temperature of 830℃, with a cumulative deformation rate of 50% below 900℃. The water cooling process has an outlet water temperature of approximately 770℃ and a reheating temperature of 730℃.

[0037] The chemical composition of the 16mm thick Q420 steel produced is as follows: C 0.16%, Si 0.24%, Mn 0.71%, P 0.012%, S ≤0.007%, Als 0.023%, Ti 0.052%, Ca 0.014%, N 0.0050%; the microstructure is ferrite and pearlite, with a grain size of grade 12; the steel plate has a yield strength of 458 MPa, a tensile strength of 557 MPa, an elongation of 26%, and an average impact energy of 164 J at 20℃, 127 J at 0℃, and 71 J at -20℃.

[0038] Comparative Example 1

[0039] A medium plate with low manganese content has a hot rolling process divided into two stages: rough rolling with an initial rolling temperature of 1020℃ and a final rolling temperature of 960℃, and finish rolling with an initial rolling temperature of 950℃ and a final rolling temperature of 850℃, with a cumulative deformation rate of 50% below 930℃. The water cooling process has an outlet water temperature of approximately 770℃ and a reddening temperature of 700℃.

[0040] The chemical composition of the produced 20mm thick steel plate is as follows: C 0.16%, Si 0.30%, Mn 0.75%, P 0.012%, S≤0.007%, Als 0.023%, Ti 0.050%, Ca 0.014%, N 0.0050%; the microstructure is ferrite and pearlite, with a grain size of grade 12; the steel plate has a yield strength of 386 MPa, a tensile strength of 549 MPa, an elongation of 33%, and an average longitudinal impact energy of 108 J at 0℃.

[0041] The above detailed description of a low-alloy high-strength steel and its production method with reference to the embodiments is illustrative rather than limiting. Several embodiments can be listed according to the defined scope. Therefore, changes and modifications without departing from the overall concept of the present invention should be within the protection scope of the present invention.

Claims

1. A low-alloy high-strength steel, characterized in that, The chemical composition and weight percentage of the low-alloy high-strength steel are as follows: C 0.14-0.18%, Si 0.20-0.35%, Mn 0.65-0.75%, P ≤0.025%, S ≤0.018%, Als 0.010-0.035%, Ti 0.04-0.06%, Ca 0.01-0.015%, N 0.0020-0.0050%, with the remainder being Fe and unavoidable trace elements; The longitudinal impact energy of the low-alloy high-strength steel at 20℃ is ≥150J, the impact energy at 0℃ is ≥120J, and the impact energy at -20℃ is ≥60J. The production method of the low-alloy high-strength steel includes the following steps: smelting and continuous casting into slabs → natural cooling → hot rolling → water cooling → hot straightening of steel plates → stacking cooling. In the hot rolling process, the cumulative deformation rate below 900℃ shall not be less than 50%; In the hot rolling step, the heating temperature is controlled at 1180-1200℃ and the heating time is greater than 2 hours; after heating, rough rolling and finish rolling are performed. Roughing rolling start temperature ≥1000℃, finishing rolling temperature ≥950℃; finishing rolling start temperature ≤930℃, finishing rolling temperature 830±20℃; In the water cooling step, the steel plate's red-hot temperature is 720±20℃.

2. The low-alloy high-strength steel according to claim 1, characterized in that, The yield strength of the low-alloy high-strength steel is ≥420MPa.

3. The low-alloy high-strength steel according to claim 1 or 2, characterized in that, The thickness of the low-alloy high-strength steel is 15-20 mm.

4. The method for producing low-alloy high-strength steel according to any one of claims 1-3, characterized in that, The production method includes the following steps: smelting and continuous casting into slabs → natural cooling → hot rolling → water cooling → hot straightening of steel plates → stacking cooling. During the hot rolling process, the cumulative deformation rate below 900℃ shall not be less than 50%.

5. The production method according to claim 4, characterized in that, The smelting process includes: blast furnace smelting → hot metal pretreatment → converter smelting → deoxidation and alloying → LF refining.

6. The production method according to claim 5, characterized in that, In the LF refining step, white slag is produced and maintained for ≥10 min. Then, ferrotitanium is added, and after the composition meets the standard, calcium wire is fed in and soft argon is blown for ≥8 min.

7. The production method according to claim 4, characterized in that, In the water cooling step, the outlet water temperature is less than 800℃.

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