Production method of thin-gauge Q420qD bridge steel with thickness of less than 12mm

Abstract

The invention discloses a production method of thin-gauge Q420qE bridge steel with the thickness smaller than 12 mm. The thin-gauge Q420qE bridge steel comprises, by weight, 0.09%-0.11% of C, 0.22%-0.32% of Si, 1.55%-1.68% of Mn, smaller than or equal to 0.015% of P, smaller than or equal to 0.005% of S, 0.036%-0.046% of Nb, 0.031%-0.041% of V, 0.010%-0.020% of Ti, 0.05%-0.15% of Cr, 0.020%-0.035% of Als and the balance Fe and inevitable impurities. The invention further discloses main steps and process parameters. According to the method, the mechanical property and the impact property are guaranteed, meanwhile, a natural cooling mode is adopted, the plate shape problem caused by non-uniform cooling of the thin steel plate due to accelerated cooling is avoided, and the high strength-thin specification-plate shape matching problem is effectively solved.

Description

Technical Field

[0001] The invention belongs to the technical field of Q420qE bridge steel production, and in particular relates to a production method of thin-gauge Q420qD bridge steel with a thickness of less than 12 mm. Background Art

[0002] Bridge steel is widely used in large-span, heavy-load bridges for highways, railways, and dual-use highways and railways. It is a special steel for bridge construction. With the large-scale construction of bridges and their advancement to high-altitude and cold regions, the requirements for high strengthening and low-temperature toughness of bridge steel are getting higher and higher. With the continuous popularization of the concept of green development and the widespread application of thermomechanical rolling technology, more and more bridge steel plates are delivered in the thermomechanical rolling state. Due to the advantages of Q420q-level bridge steel such as higher strength, reduced use, and small installation workload, the demand for Q420q-level bridge steel is increasing.

[0003] GB / T714-2015 "Structural Steel for Bridges" stipulates that Q420q can only be delivered by thermomechanical rolling, and strict regulations are made on the chemical composition, especially the carbon content (carbon content ≤ 0.11%), which increases the difficulty of production. The impact energy requirement for grade D and grade E bridge steel is greater than 120J. For specifications with a thickness of ≤12mm, the main production difficulty lies in the difficulty of rolling, the difficulty in controlling the yield strength ratio, and the difficulty in ensuring the impact energy. In addition, the specifications are thin and the control of the plate shape is extremely difficult. Therefore, the production of such thin-sized bridge steel is very difficult at present, and the overall qualified rate is low.

[0004] Chinese patent CN 110735085 A discloses "a method for manufacturing thin-gauge Q345qE and Q370qE steel plates". This patent does not control rolling. The steel plate cooling stage adopts natural air cooling method, and does not spray water to accelerate cooling. The steel plate has good performance, good plate shape control, reduces the difficulty of manufacturing thin-gauge Q345qE and Q370qE, and is suitable for thin thickness specifications. The disadvantages are that the first is that the blank needs to be rolled, which increases energy consumption and reduces production efficiency. The second is that it is only used to produce Q345q and Q370q bridge steels, and is not suitable for Q420q bridge steels.

[0005] Chinese patent CN 114525453 A discloses "a thin-gauge bridge steel and its production method". This patent provides a method for producing thin-gauge bridge steel by composite rolling. The disadvantage is that it is suitable for producing composite bridge steel plates, but not for conventional bridge steel.

[0006] Chinese patent CN 102766806 B discloses "an ultra-wide thin-gauge structural steel plate for bridges and its production method". Through normalizing, Q345q bridge steel with a thickness of 6-12mm is produced. The disadvantage is that the carbon equivalent is high after normalizing, and it is only applicable to Q345q bridge steel. Summary of the invention

[0007] The purpose of the present invention is to provide a production method of thin-gauge Q420qD bridge steel with a thickness of less than 12 mm, wherein the mechanical properties of the steel plate meet the following requirements: yield strength ≥420MPa, tensile strength ≥540MPa, elongation after fracture ≥19%, impact energy at -40°C ≥120J, and in particular, the steel plate has a good plate shape.

[0008] In order to solve the above technical problems, the present invention adopts the following technical solutions:

[0009] The present invention provides a production method for thin-gauge Q420qD bridge steel with a thickness of less than 12 mm, and the main steps and process parameters are as follows:

[0010] S1 Hot metal pretreatment: Hot metal is desulfurized and dephosphorized by KR stirring method, S≤0.005%, P≤0.015% in hot metal;

[0011] S2 converter smelting: the temperature of molten iron entering the furnace is greater than 1260℃, ensuring that the P content of the molten steel discharged from the furnace is ≤0.010% and the S content is ≤0.004%;

[0012] S3 LF refining: LF stage accurately controls the composition of molten steel, deoxidation and alloying, S≤0.004%;

[0013] S4 RH vacuum treatment: In this treatment mode, the vacuum time is maintained for more than 20 minutes, the pure degassing time is greater than 15 minutes, and the soft blowing time is greater than 15 minutes, ensuring that the hydrogen content of the molten steel is ≤1.6ppm, the oxygen content is ≤30ppm, and the nitrogen content is ≤50ppm;

[0014] S5 slab continuous casting: dynamic soft pressure, electromagnetic stirring, protective pouring, constant casting speed process is adopted to finally produce 250mm thick continuous casting slab, the low center segregation of the slab is controlled below Class C 3.0, and the slab is pile-cooled or pit-cooled for more than 36 hours after it comes off the line;

[0015] S6 Heating: The slab is heated in a walking beam heating furnace. The atmosphere in the furnace is strictly controlled to ensure the heating temperature and heating time of the slab. The heating temperature is 1210℃~1240℃. The total time in the furnace is greater than 270min, of which the heating section time is greater than 120min and the soaking section time is greater than 30min, to ensure that the alloy elements are fully dissolved and the slab temperature is uniform.

[0016] S7 rolling: rolling adopts two-stage controlled rolling, usually called rough rolling stage and finishing rolling stage; rough rolling is carried out on a 3800mm rough rolling mill, the start rolling temperature is above 1170℃, and the relative reduction rate of a single pass is controlled at more than 15% for at least two passes; the start rolling thickness of finishing rolling is 4 times the thickness of the finished product, the start rolling temperature of finishing rolling is less than 1000℃, and the final rolling temperature is less than 800℃;

[0017] S8 cooling: the steel plate is cooled naturally after rolling without accelerated cooling;

[0018] The chemical composition of the bridge steel is as follows by weight: C: 0.09-0.11%, Si: 0.22-0.32%, Mn: 1.55-1.68%, P: ≤0.015%, S: ≤0.005%, Nb: 0.036-0.046%, V: 0.031-0.041%, Ti: 0.010-0.020%, Cr: 0.05-0.15%, Als: 0.020-0.035%, and the remainder is Fe and unavoidable impurities.

[0019] Furthermore, the chemical composition of the bridge steel is C: 0.010%, Si: 0.24%, Mn: 1.58%, P: 0.013%, S: 0.002%, Nb: 0.042%, V: 0.036%, Ti: 0.015%, Cr: 0.07%, Als: 0.022% by weight, and the remainder is Fe and unavoidable impurities.

[0020] Furthermore, the chemical composition of the bridge steel is C: 0.010%, Si: 0.25%, Mn: 1.60%, P: 0.012%, S: 0.002%, Nb: 0.041%, V: 0.035%, Ti: 0.014%, Cr: 0.08%, Als: 0.025%, and the remainder is Fe and unavoidable impurities.

[0021] Furthermore, the chemical composition of the bridge steel is as follows by weight: C: 0.010%, Si: 0.26%, Mn: 1.62%, P: 0.014%, S: 0.002%, Nb: 0.042%, V: 0.036%, Ti: 0.013%, Cr: 0.08%, Als: 0.023%, and the remainder is Fe and unavoidable impurities.

[0022] Furthermore, the produced thin-gauge Q420qD bridge steel plates with a thickness of less than 12 mm meet the following requirements: yield strength ≥ 420 MPa, tensile strength ≥ 540 MPa, elongation after fracture ≥ 19%, and impact energy at -40°C ≥ 120 J, and in particular have good plate shape.

[0023] Compared with the prior art, the beneficial technical effects of the present invention are:

[0024] Applicable thickness is ≤12mm.

[0025] The present invention adopts a low-carbon-Mn-Nb-V-Ti-Cr microalloying design in terms of composition, which can effectively prevent the abnormal growth of austenite grains during the heating and rolling stages, ensure the grain size of austenite, and ensure the grain size of subsequent natural cooling phase transformation. The Cr element plays a strengthening role in the subsequent natural cooling process to ensure the strength. In addition, the present invention adopts a natural cooling method after rolling, which avoids the plate shape problem caused by uneven cooling of thin-gauge steel plates caused by accelerated cooling, thereby improving the plate shape qualification rate of thin-gauge steel plates.

[0026] The invention obtains a microstructure of ferrite+pearlite+a small amount of bainite and a grain size of 11-12 by controlling the rolling temperature and natural cooling. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] The present invention will be further described below in conjunction with the accompanying drawings.

[0028] Figure 1 This is the optical metallographic structure diagram of the near surface of the steel plate in Example 2 of the present invention.

[0029] Figure 2 This is a high-magnification image of the impact fracture of the steel plate of Example 2 of the present invention.

[0030] Figure 3 This is the shape of the steel plate of Examples 1-3 of the present invention. DETAILED DESCRIPTION

[0031] The present invention is described in more detail below with reference to the accompanying drawings using examples. These examples are merely descriptions of the best mode of carrying out the present invention and do not impose any limitation on the scope of the present invention.

[0032] The chemical composition of the steel plate of the present invention is C: 0.09-0.11%, Si: 0.22-0.32%, Mn: 1.55-1.68%, P: ≤0.015%, S: ≤0.005%, Nb: 0.036-0.046%, V: 0.031-0.041%, Ti: 0.010-0.020%, Cr: 0.05-0.15%, Als: 0.020-0.035%, and the balance is Fe and unavoidable impurities. In addition, the present invention also provides a method for manufacturing the steel plate.

[0033] The main steps and process parameters are as follows:

[0034] S1 Hot metal pretreatment: Hot metal is desulfurized and dephosphorized by KR stirring method, S≤0.005%, P≤0.015% in hot metal;

[0035] S2 converter smelting: the temperature of molten iron entering the furnace is greater than 1260℃, ensuring that the P of the molten steel out of the furnace is ≤0.010%, and S is ≤0.004%.

[0036] S3 LF refining: LF stage accurately controls the composition of molten steel, deoxidation and alloying, S≤0.004%.

[0037] S4 RH vacuum treatment: using this treatment mode, the vacuum time is maintained for more than 20 minutes, the pure degassing time is greater than 15 minutes, and the soft blowing time is greater than 15 minutes, ensuring that the hydrogen content of the de-situ molten steel is ≤1.6ppm, the oxygen content is ≤30ppm, and the nitrogen content is ≤50ppm.

[0038] S5 slab continuous casting: dynamic light pressure, electromagnetic stirring, protective pouring, constant casting speed process is adopted to finally produce 250mm thick continuous casting slabs. The low-multiple center segregation of the slabs is controlled below Class C 3.0. The slabs are pile-cooled or pit-cooled for more than 36 hours after leaving the production line.

[0039] S6 Heating: The slab is heated in a walking beam heating furnace, and the atmosphere in the furnace is strictly controlled to ensure the heating temperature and heating time of the slab, which is 1210℃~1240℃. The total time in the furnace is greater than 270min, of which the heating section time is greater than 120min and the soaking section time is greater than 30min, to ensure that the alloy elements are fully dissolved and the slab temperature is uniform.

[0040] S7 rolling: rolling adopts two-stage controlled rolling, usually called rough rolling stage and finishing rolling stage. Rough rolling is carried out in a 3800mm rough rolling mill, the start rolling temperature is above 1170℃, and the relative reduction rate of a single pass is controlled at more than 15% for at least two passes. The start rolling thickness of finishing rolling is 4 times the thickness of the finished product, the start rolling temperature of finishing rolling is less than 1000℃, and the final rolling temperature is less than 800℃.

[0041] S8 cooling: The steel plate is cooled naturally after rolling without accelerated cooling.

[0042] The present invention is described in detail below in conjunction with practical embodiments.

[0043] Table 1 lists the chemical composition of the examples, and Table 2 lists the rolling process parameters of the examples.

[0044] Table 1 Chemical composition of the present invention (wt%)

[0045] Example C Si Mn P S Nb V Ti Cr Als 1 0.10 0.24 1.58 0.013 0.002 0.042 0.036 0.015 0.07 0.022 2 0.10 0.25 1.60 0.012 0.002 0.041 0.035 0.014 0.08 0.025 3 0.10 0.26 1.62 0.014 0.002 0.042 0.036 0.013 0.08 0.023

[0046] Table 2 Rolling parameters of the embodiment of the present invention

[0047]

[0048] The mechanical properties, low temperature impact properties and cold bending properties of the steel plate of the embodiment of the present invention were tested, and the results are shown in Table 3.

[0049] Table 3 Mechanical properties of steel plates according to the present invention

[0050]

[0051]

[0052] The present invention adopts low carbon-Mn-Nb-V-Ti-Cr microalloying design combined with a specific process flow to produce a thin specification Q420qE bridge steel plate with a thickness of less than 12 mm. The mechanical properties of the steel plate meet the following requirements: yield strength ≥420MPa, tensile strength ≥540MPa, elongation after fracture ≥19%, -40℃ impact energy ≥120J, and in particular, has a good plate shape. It can be used to manufacture large-scale highway and railway large-span and heavy-load bridges.

[0053] 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 producing thin-gauge Q420qD bridge steel with a thickness of less than 12 mm, characterized in that: The main steps and process parameters are as follows: S1 Hot metal pretreatment: Hot metal is desulfurized and dephosphorized by KR stirring method, S≤0.005%, P≤0.015% in hot metal; S2 converter smelting: the temperature of molten iron entering the furnace is greater than 1260℃, ensuring that the P content of the molten steel discharged from the furnace is ≤0.010% and the S content is ≤0.004%; S3 LF refining: LF stage accurately controls the composition of molten steel, deoxidation and alloying, S≤0.004%; S4 RH vacuum treatment: In this treatment mode, the vacuum time is maintained for more than 20 minutes, the pure degassing time is greater than 15 minutes, and the soft blowing time is greater than 15 minutes, ensuring that the hydrogen content of the molten steel is ≤1.6ppm, the oxygen content is ≤30ppm, and the nitrogen content is ≤50ppm; S5 slab continuous casting: dynamic soft pressure, electromagnetic stirring, protective pouring, constant casting speed process is adopted to finally produce 250mm thick continuous casting slab, the low center segregation of the slab is controlled below Class C 3.0, and the slab is pile-cooled or pit-cooled for more than 36 hours after it comes off the line; S6 Heating: The slab is heated in a walking beam heating furnace. The atmosphere in the furnace is strictly controlled to ensure the heating temperature and heating time of the slab. The heating temperature is 1210℃~1240℃. The total time in the furnace is greater than 270min, of which the heating section time is greater than 120min and the soaking section time is greater than 30min, to ensure that the alloy elements are fully dissolved and the slab temperature is uniform. S7 rolling: rolling adopts two-stage controlled rolling, usually called rough rolling stage and finishing rolling stage; rough rolling is carried out on a 3800mm rough rolling mill, the start rolling temperature is above 1170℃, and the relative reduction rate of a single pass is controlled at more than 15% for at least two passes; the start rolling thickness of finishing rolling is 4 times the thickness of the finished product, the start rolling temperature of finishing rolling is less than 1000℃, and the final rolling temperature is less than 800℃; S8 cooling: the steel plate is cooled naturally after rolling without accelerated cooling; The chemical composition of the bridge steel is as follows by weight: C: 0.09-0.11%, Si: 0.22-0.32%, Mn: 1.55-1.68%, P: ≤0.015%, S: ≤0.005%, Nb: 0.036-0.046%, V: 0.031-0.041%, Ti: 0.010-0.020%, Cr: 0.05-0.15%, Als: 0.020-0.035%, and the remainder is Fe and unavoidable impurities.

2. The method for producing thin-gauge Q420qD bridge steel with a thickness of less than 12 mm according to claim 1, characterized in that: The chemical composition of the bridge steel is as follows by weight: C: 0.010%, Si: 0.24%, Mn: 1.58%, P: 0.013%, S: 0.002%, Nb: 0.042%, V: 0.036%, Ti: 0.015%, Cr: 0.07%, Als: 0.022%, and the remainder is Fe and unavoidable impurities.

3. The method for producing thin-gauge Q420qD bridge steel with a thickness of less than 12 mm according to claim 1, characterized in that: The chemical composition of the bridge steel is as follows by weight: C: 0.010%, Si: 0.25%, Mn: 1.60%, P: 0.012%, S: 0.002%, Nb: 0.041%, V: 0.035%, Ti: 0.014%, Cr: 0.08%, Als: 0.025%, and the remainder is Fe and unavoidable impurities.

4. The method for producing thin-gauge Q420qD bridge steel with a thickness of less than 12 mm according to claim 1, characterized in that: The chemical composition of the bridge steel is as follows by weight: C: 0.010%, Si: 0.26%, Mn: 1.62%, P: 0.014%, S: 0.002%, Nb: 0.042%, V: 0.036%, Ti: 0.013%, Cr: 0.08%, Als: 0.023%, and the remainder is Fe and unavoidable impurities.

5. The method for producing thin-gauge Q420qD bridge steel with a thickness of less than 12 mm according to claim 1, characterized in that: The produced thin-gauge Q420qD bridge steel plates with a thickness of less than 12 mm meet the following requirements: yield strength ≥420MPa, tensile strength ≥540MPa, elongation after fracture ≥19%, and impact energy at -40℃ ≥120J, especially having a good plate shape.

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