A low crack sensitive coefficient 550mpa grade low alloy high strength steel plate and a manufacturing method thereof

Abstract

The present application relates to a kind of low crack sensitivity coefficient 550MPa grade low alloy high-strength steel plate and its manufacturing method, belong to metallurgical technical field.The chemical composition of the steel plate is C:0.05~0.09%, Si:0.10~0.30%, Mn:1.40~1.60%, Nb:0.040%~0.060%, Ti:0.008%~0.018%, Al:0.020~0.040%, B:≤0.0010%, Ca:0.0005~0.0015%, P:≤0.012%, S:≤0.002%, O:≤0.002%, N:≤0.0035%, H:≤0.0001%, the balance is Fe and inevitable impurity element;Carbon equivalent CEV:≤0.37%, Pcm≤0.19%.Production thickness is 50~70mm, microstructure is mainly uniform acicular ferrite and bainite mixed structure.From the angle of element composition design, base material organization, rolling deformation, small water cooling etc., it is guaranteed that the steel plate has high strength, high toughness, excellent cold bending performance and welding cracking resistance, can be widely used in engineering machinery, high-rise building, bridge, ocean platform etc., and the manufacture of large-scale engineering machinery equipment or steel structure that has strict requirements to formability, weldability.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of metallurgy, and particularly relates to a yield strength 550 MPa grade alloy steel plate. BACKGROUND

[0002] The yield strength 550 MPa grade high-strength steel plate is mainly applied to key stress parts of engineering machinery such as high-rise buildings, engineering crane buckets, concrete pump trucks, coal mine conveying devices and hydraulic supports, and is particularly high in requirements for welding crack sensitivity, fracture toughness and surface quality. Too high carbon equivalent easily increases the fracture risk of key parts of engineering machinery during welding or application, which to some extent puts forward a more stringent requirement for low carbon equivalent and low crack sensitivity coefficient on composition design.

[0003] Chinese patent publication No. CN 113604737 A discloses a Q550D high-strength steel plate and a preparation method thereof. Nitrogen element with relatively low price is used to reduce alloy cost. Too high nitrogen element (≥0.01%) in molten steel can cause the generation of surface star cracks of continuous casting billets, increase the cost of manual grinding, reduce the yield of casting billets and the rolling plate material rate, and shorten the production cycle without vacuum degassing refining in the steelmaking process. The hydrogen content in the molten steel is high, hydrogen-induced delayed cracks are generated during the cutting and welding of the steel plate, which causes quality disputes and affects the customer's use cycle. The thickness specification of the rolled steel plate is not specified. In comparison, the present application can effectively control N≤0.0035% and H≤0.00010% in the molten steel, inhibit the generation of surface star cracks of continuous casting billets and hydrogen-induced delayed cracks during the cutting and welding of the steel plate, increase the yield of casting billets and the rolling plate material rate, and ensure the customer's use cycle.

[0004] Chinese patent publication No. CN 114570898 A discloses a production method of a low yield strength ratio low alloy high-strength steel plate. The composition contains C: 0.09-0.18%, Nb: 0.3-0.5%, V: 0.3-0.8%, and Cu: 0.1-0.25%. The high content of carbon and other elements increases the carbon equivalent and crack sensitivity coefficient, which is not conducive to the improvement of the weldability of the steel plate, and easy to produce welding cracks, low production efficiency and addition of precious alloy element Cu, further increasing the raw material cost of the steel plate.

[0005] Chinese Patent Publication No. CN 114672725 A discloses a TMCP delivery Q550D engineering machinery steel and its preparation method. This is a VN microalloyed thin-gauge engineering machinery steel with a C content of 0.12–0.15%, N content of 80–100 ppm, carbon equivalent (CEV) of 0.39–0.44%, and Pcm content of 0.22–0.25%. While increasing the steel plate strength, the high carbon equivalent content is detrimental to improving the weldability of the steel plate. Furthermore, the carbon content is in the peritectic steel range, which easily exacerbates the segregation of the center of mass within the continuously cast billet; high nitrogen content easily causes star cracks on the surface of the continuously cast billet, increasing the cost of manual grinding, reducing the billet yield, and decreasing the rolled plate yield; the production thickness of the steel plate is only 12–20 mm, and the thin plate limits its application range.

[0006] Chinese Patent Publication No. CN 112725703A discloses a low yield strength ratio Q550D high-strength steel plate and its manufacturing method. This invention employs a controlled rolling and cooling + low-temperature quenching + tempering process, but this increases production costs and lengthens the production cycle, severely impacting delivery time and resulting in insignificant economic benefits. Furthermore, the steel plate thickness involved is only 20–30 mm.

[0007] Chinese Patent Publication No. CN 113943890A discloses a low-energy-consumption Q550D thick steel plate and its production method. It also employs a quenching and tempering process, which increases energy consumption costs and lengthens the production cycle. Furthermore, it uses a low-carbon, high-manganese design (C: 0.075–0.095%, Mn: 1.70–1.60%). The increased Mn content can compensate for the decrease in strength caused by the reduced C content; however, Mn is a segregating element, and excessively high Mn content can exacerbate segregation in the center of the continuously cast billet, making subsequent steel plate cutting and welding prone to cracking, thus affecting the safety of the steel plate in use. Summary of the Invention

[0008] The purpose of this invention is to provide a low-alloy steel plate with a yield strength of 550 MPa and low crack sensitivity, as well as its manufacturing method. Low carbon and low alloy content control ensures weldability and reduces crack sensitivity, while the manufacturing method achieves high strength in the steel plate.

[0009] The technical scheme of the present application is as follows: the chemical composition of the steel plate is as follows in percentage by mass: C: 0.05-0.09%, Si: 0.10-0.30%, Mn: 1.40-1.60%, Nb: 0.040%-0.060%, Ti: 0.008%-0.018%, Al: 0.020-0.040%, B: ≤0.0010%, Ca: 0.0005-0.0015%, P: ≤0.012%, S: ≤0.002%, O: ≤0.002%, N: ≤0.0035%, H: ≤0.0001%, and the balance is Fe and inevitable impurities; the carbon equivalent CEV is ≤0.37%, and Pcm is ≤0.19%.

[0010] The production thickness of the yield strength 550MPa low alloy high strength steel plate is 50-70mm.

[0011] The element composition of the steel is limited as follows:

[0012] C: Carbon can significantly improve the strength of the steel plate as a solid solution element, but it has adverse effects on the weldability, plasticity and toughness, and cold bending performance of the steel plate. Based on the need for matching the weldability and cold bending performance of the steel plate, the carbon content is controlled to be 0.05-0.09% in the present application.

[0013] Si: Si plays a solid solution strengthening role in the steel. However, too high Si content will deteriorate the toughness of the martensitic high strength steel and increase the cold crack sensitivity of the steel plate, and therefore, the Si content is controlled to be 0.10-0.30% in the present application.

[0014] Mn: Mn improves the hardenability, promotes the martensitic transformation, and improves the strength in the steel. However, Mn is a main segregation element, and too high Mn content will cause center segregation of the continuous casting billet to form MnS, which has adverse effects on the toughness, lamellar tearing resistance and welding performance of the steel plate. The present application specifies that the Mn addition amount is in the range of 1.40-1.60%.

[0015] Nb: Nb can produce significant grain refinement, precipitation strengthening and precipitation strengthening effects. Nb dissolved in austenite can improve the hardenability, and the Nb carbide precipitation phase has the effect of refining the grains and improving the strength of the matrix. The Nb content is controlled to be 0.040-0.060% in the present application.

[0016] Ti: Ti(C / N) has a high dissolution temperature, which is usually above 1400℃, and has a strong pinning effect on the grain boundary at high temperature, which prevents the coarsening of austenite grains during the heating process. High-temperature rolling is required in the present application, and the Ti content is controlled to be 0.008-0.018%.

[0017] Al: Al is a deoxidizing and grain refining element. The present application specifies that the Al content is 0.020-0.040%.

[0018] Ca: Inclusion modification element, can react with long strip MnS to form spherical CaS, change the anisotropy of the steel plate, and also make the Al2O3 inclusions generated by Al deoxidation change into spherical low melting point inclusions, promote the floating removal of inclusions, and improve the plasticity and toughness of the steel plate. The content of Ca in the application is controlled to be 0.0005-0.0015%.

[0019] P: Harmful element of steel, easy to segregate, and has adverse effect on the plasticity and toughness of the material. The higher the content of P, the higher the brittleness of the steel plate. Limit P: ≤0.012%.

[0020] S: Harmful element of steel, easy to segregate. It can easily form MnS segregation inclusions with Mn element, leading to delamination cracking of the steel plate. Limit S: ≤0.002%.

[0021] O, N, H: Harmful gas elements, high content, many inclusions, reduce the plasticity and toughness of the steel plate, and the risk of cracking of the steel plate is high. The application strictly controls the content of O ≤0.002%, the content of N ≤0.0035%, and the content of H ≤0.0001%.

[0022] CEV: The application adopts the carbon equivalent formula CEV=C+Mn / 6+(Cr+Mo+V) / 5+(Ni+Cu) / 15. The carbon equivalent has a great influence on the strength and weldability of the steel. High CEV has high strength, but the weldability is reduced. In order to ensure the weldability of the steel plate, the application controls CEV ≤0.37%.

[0023] Pcm: The application adopts the cold crack sensitivity coefficient formula Pcm=C+Si / 30+(Mn+Cr+Cu) / 20+Ni / 60+Mo / 15+V / 10+B / 5. The cold crack sensitivity coefficient has a great influence on the weldability of the steel. High Pcm has low weldability, and vice versa. The application controls Pcm ≤0.19%.

[0024] The application further provides a manufacturing method of the above-mentioned yield strength 550MPa grade low crack sensitivity low alloy steel plate, which relates to the following specific process:

[0025] (1) Smelting: The following chemical components are initially smelted by using an electric furnace or converter, and further fine-tuned by using secondary refining and VD or vacuum degassing treatment, and after degassing of the molten steel, trace Ca treatment and a certain time of static argon blowing treatment are performed. The chemical components are as follows: C: 0.05-0.09%, Si: 0.10-0.30%, Mn: 1.40-1.60%, Nb: 0.040%-0.060%, Ti: 0.008%-0.018%, Al: 0.020-0.040%, B: ≤0.0010%, Ca: 0.0005-0.0015%, P: ≤0.012%, S: ≤0.002%, O: ≤0.002%, N: ≤0.0035%, H: ≤0.0001%, carbon equivalent CEV: ≤0.37%, Pcm: ≤0.19%.

[0026] (2) Continuous casting: The molten steel is preferably cast by using a 300-450 mm section continuous casting machine with low superheat and argon protection, and the superheat of the molten steel is controlled at 10-25°C; during casting, the casting speed fluctuation is controlled at ±0.05 m / min, and the non-sine oscillation of the liquid surface of the crystallizer is controlled at ±5 mm; the thickness of the protective slag layer is 70-90 mm, and the thickness of the liquid slag is 7-11 mm; the low-magnification center segregation of the continuous casting billet is ≤C1.0 grade, and the center porosity is ≤1.0 grade.

[0027] (3) Heating: The continuous casting billet is preferably heated to the soaking temperature of 1200-1250°C by using a pusher-type heating furnace, wherein the temperature of the first heating section is 650-750°C, the temperature of the second heating section is 750-1150°C, the temperature of the third heating section is 1200-1260°C, and the temperature of the fourth soaking section is 1200-1250°C; the heating time of the continuous casting billet in the furnace is 8-14 min / cm, so that the alloy elements in the steel are fully solid-solved to ensure the uniformity of the components and properties of the final product.

[0028] (4) Rolling: After the continuous casting billet is discharged, high-pressure water descaling treatment is performed, and then the continuous casting billet is subjected to rough rolling + finishing rolling in stages. In order to increase the rolling deformation of the core of the steel plate, rough rolling is performed in the high-temperature zone, the rough rolling starting temperature is 1150-1210°C, and the finishing temperature is 1000-1100°C; the thickness ratio of the steel plate after rough rolling to the finished product thickness is controlled at 2-3 times, and the number of passes with a reduction rate of 15-20% is not less than 3, and the total reduction amount of the rough rolling passes is 50-70%; the rough rolling starting temperature is 880-920°C, and the finishing temperature is 820-850°C, and the total reduction rate of the passes is 55%-65%.

[0029] (5) Cooling: in order to avoid grain growth and not produce large stress, after rolling, two-stage small water quantity mode is used to control cooling, ACC and DQ laminar flow water group number is 17-20, roller speed is 0.5-1.0 m / min, cooling speed is 12-20 ℃ / s; cooling start temperature is 780-840 ℃, final cooling temperature is 500-600 ℃, laminar flow up and down single water quantity is 150-250 / 300-400 m / h. 3 / h.

[0030] Compared with the prior art, the present application has the following advantages:

[0031] The present application provides a low crack sensitivity low alloy steel plate with yield strength of 550 MPa and a manufacturing method thereof, and the steel plate has good welding crack resistance.

[0032] The present application adopts low-carbon low-carbon equivalent low-crack sensitivity coefficient alloy design, cancels the addition of valuable alloy elements such as Ni, Mo, Cu and V, and the carbon equivalent of the steel plate is lower (≤0.37%) and the crack sensitivity coefficient is lower (≤0.19%), which greatly reduces the cracking problem in the welding process of the steel plate; through the controlled rolling and controlled cooling process, the problems of long traditional quenching and tempering process flow, high energy consumption and large alloy addition amount are solved; through the optimization of the controlled cooling process, the DQ+ACC two-stage small water quantity mode is used to control cooling, and the problem of uneven performance of the steel plate head and tail and transverse and longitudinal direction in the traditional controlled rolling and controlled cooling process is solved.

[0033] In order to increase the rolling deformation of the core of the steel plate, rough rolling is carried out at a high temperature zone, and the rough rolling final rolling temperature is 1000-1100 ℃. The present application fully utilizes Nb / Ti micro-alloying to ensure grain refinement of the steel plate during high-temperature large deformation rolling and improve the strength and toughness.

[0034] Unlike the traditional two-stage rolling which ensures the reduction amount in the finish rolling stage, the rolling deformation amount of the present application is mainly completed in the rough rolling stage. The ratio of the thickness of the steel plate after rough rolling to the finished product thickness is controlled to be below 2-3 times, and the pass number of the reduction rate of 15-20% is not less than 3.

[0035] The yield strength of the 50-70 mm steel plate of the present application is ≥550 MPa, the tensile strength is ≥700 MPa, the elongation is ≥20%, the low temperature Charpy impact energy at -40 ℃ is ≥100 J, the cold bending performance of the steel plate is excellent, D=3a, and there is no crack in 180° cold bending. The microstructure of the steel plate is mainly fine and uniform acicular ferrite and bainite mixed structure, which meets the application needs of engineering machinery Q550 steel plate.

[0036] The method of the present application can be applied to other high-strength steel plates, such as high-strength marine ship plate steel, high-rise building steel, bridge steel, engineering machinery steel, etc. BRIEF DESCRIPTION OF DRAWINGS

[0037] Figure 1is the microstructure of the steel plate thickness 1 / 4 of Example 3, which is fine and uniform acicular ferrite and bainite mixture.

[0038] Figure 2 is the microstructure of the steel plate thickness 1 / 2 of Example 3, which is also fine and uniform acicular ferrite and bainite mixture. DETAILED DESCRIPTION

[0039] The application is further described in detail below with examples, which are exemplary and intended to explain the application, and cannot be understood as limiting the application.

[0040] The production process of the high-strength low-alloy steel plate of the embodiment is: converter smelting -> LF furnace external refining -> VD or RH vacuum degassing -> Ca treatment -> continuous casting -> heating -> controlled rolling and controlled cooling.

[0041] The manufacturing method of the yield strength 550 MPa low crack sensitivity low-alloy steel plate of the embodiments 1-4 of the application comprises the following steps:

[0042] (1) Smelting: 150 tons of converter smelting is adopted, then the molten steel is sent to the LF furnace for refining and treated by RH vacuum degassing, and the Ca treatment is performed after breaking the vacuum. The final composition of the molten steel is controlled as shown in Table 1.

[0043] (2) Continuous casting: the molten steel is cast into a 370 mm thick continuous casting billet. The casting temperature is controlled at 10-25 ℃ above the liquidus. During casting, the speed fluctuation is controlled within ±0.05 m / min, and the non-sine vibration of the liquid surface in the crystallizer is controlled within ±5 mm; the thickness of the protective slag layer is 70-90 mm, and the thickness of the liquid slag is 7-11 mm; the center segregation of the continuous casting billet is ≤C1.0 grade, and the center porosity is ≤1.0 grade. The related process parameters are shown in Table 2.

[0044] (3) Controlled rolling: the continuous casting billet obtained in step (2) is loaded into a pusher-type heating furnace, heated to 1200-1250 ℃, and the heating time is 8-14 min / cm, so that the alloy elements in the steel are fully solid-solved to ensure the uniformity of the composition and performance of the final product. After the billet is discharged, it is treated by high-pressure water descaling, and then subjected to two-stage controlled rolling of rough rolling and finish rolling. In order to increase the rolling deformation of the core of the steel plate, the rough rolling is carried out at a high temperature zone, and the rough rolling final rolling temperature is 1000-1100 ℃. The thickness ratio of the steel plate after rough rolling to the finished product thickness is controlled at 2-3 times, and the number of passes with a reduction rate of 15-20% is not less than 3, and the total reduction amount of the rough rolling passes is 50-70%; the steel plate finish rolling opening rolling temperature is 880-920 ℃, and the final rolling temperature is 820-850 ℃, and the total reduction rate of the passes is 55%-65%. The related process parameters are shown in Table 3.

[0045] (4) Controlled cooling: In order to avoid grain growth and not produce large stress, ACC+DQ two-section small water amount cooling is adopted, ACC and DQ laminar flow water group number is 17-20, roller speed is 0.5-1.0 m / min, cooling speed is 12-20 ℃ / s; cooling starting temperature is 780-840 ℃, final cooling temperature is 500-600 ℃, laminar flow up and down single water amount is 150-250 / 300-400 m / h. The relevant process parameters are shown in Table 4. 3

[0046] The specific components and manufacturing process parameters are shown in Tables 1-4. The mechanical properties of the steel plates of each example are shown in Table 5.

[0047] Figure 1 Microstructure metallographic photos of the steel plate of Example 3 are given. The base body structure of the steel plate is fine and uniform acicular ferrite and bainite mixture, which ensures that the steel plate has excellent strength and toughness and welding cracking resistance.

[0048] The present application adopts high-cleanliness steelmaking continuous casting process, controlled rolling, ACC+DQ two-section small water amount controlled cooling process, controls from the aspects of chemical composition design, base material structure, rolling deformation, small water amount cooling, etc., ensures that the steel plate has high strength, high toughness, excellent cold bending performance and welding cracking resistance, and can be widely applied to the manufacturing of large-scale engineering machinery equipment or steel structure which has strict requirements on formability and weldability, such as engineering machinery, high-rise buildings, bridges, offshore platforms, etc.

[0049] Table 1 Chemical composition (wt%) of high-strength steel plates of each example

[0050] Example C Si Mn P S Al Nb V Ti B Ca O N H CEV Pcm 1 0.06 0.25 1.51 0.008 0.001 0.030 0.050 0.003 0.012 0.0002 0.0016 0.0010 0.0029 0.0001 0.32 0.15 2 0.08 0.20 1.48 0.010 0.001 0.025 0.045 0.003 0.015 0.0003 0.0012 0.0008 0.0026 0.0001 0.34 0.16 3 0.06 0.23 1.53 0.007 0.001 0.032 0.048 0.004 0.012 0.0002 0.0012 0.0012 0.0030 0.0001 0.33 0.15 4 0.07 0.22 1.50 0.008 0.001 0.033 0.047 0.002 0.013 0.0002 0.0014 0.0011 0.0029 0.0001 0.33 0.16

[0051] Table 2 Continuous casting process parameters of high-strength steel plates of each example

[0052] Example Superheat, °C Drawing speed, m / min Mold flux layer thickness, mm Liquid slag thickness, mm Center segregation Center porosity 1 18 0.57 75 8.5 C 0.5 0.5 2 15 0.55 80 9 C 0.5 0.5 3 16 0.55 78 8 C 0.5 0.5 4 15 0.56 75 8.5 C 0.5 0.5

[0053] Table 3 Controlled rolling process parameters of high-strength steel plates of each example

[0054] Example Product thickness specification, mm Coarse rolling finish rolling temperature, °C Coarse rolling ≥ 30 mm pass Finish rolling opening rolling temperature, °C Finish rolling temperature, °C 1 50 1090 4 900 840 2 57 1050 4 890 850 3 65 1030 3 890 830 4 70 1030 4 880 830

[0055] Table 4 Controlled cooling process parameters of high-strength steel plates of each example

[0056] Example Product thickness specification, mm Cooling start temperature, °C DQ + ACC laminar flow water group number, groups Roller speed, m / min Cooling rate, °C / s Final cooling temperature, °C 1 50 820 118 0.64 17 560 2 57 827 18 0.65 18 530 3 65 810 20 0.58 19 500 4 70 815 20 0.55 19 520

[0057] Table 5 Mechanical properties of high-strength steel plates of each example

[0058] ​

Claims

1. A method for manufacturing a 550 MPa grade low alloy high strength steel plate having a low crack sensitivity, characterized by: The steel plate has the following chemical components in percentage by mass: C: 0.05-0.09%, Si: 0.10-0.30%, Mn: 1.40-1.60%, Nb: 0.040%-0.060%, Ti: 0.008%-0.018%, Al: 0.020-0.040%, B: ≤0.0010%, Ca: 0.0005-0.0015%, P: ≤0.012%, S: ≤0.002%, O: ≤0.002%, N: ≤0.0035%, H: ≤0.0001%, and the balance of Fe and inevitable impurities; the carbon equivalent CEV is ≤0.37%, and Pcm is ≤0.19%; the production thickness of the steel plate is 50-70 mm, and the microstructure is mainly a uniform mixed structure of acicular ferrite and bainite. The manufacturing method comprises the following steps: I. smelting The steel is smelted by an electric furnace or a converter, and the composition is further adjusted by using external refining and vacuum degassing treatment; after the molten steel is degassed, Ca treatment and static soft argon blowing are performed; II. continuous casting The molten steel is cast into a billet, and the low-magnification center segregation of the continuous casting billet is ≤C1.0 level, and the center porosity is ≤1.0 level; III. heating The continuous casting billet is heated in a furnace, and the structure is completely austenitized to fully solid-solute the alloy elements in the steel; IV. rolling After the continuous casting billet is discharged from the furnace, the billet is descaled by high-pressure water, and is subjected to rough rolling and finishing rolling in stages; in order to increase the rolling deformation of the center of the steel plate, the rough rolling is performed in a high-temperature zone, the rough rolling start rolling temperature is 1150-1210°C, and the finish rolling temperature is 1000-1100°C; the thickness ratio of the steel plate after rough rolling to the finished product thickness is controlled to be 2-3 times, and the pass times of the rolling reduction rate of 15-20% is not less than 3, and the total rolling reduction amount of the rough rolling is 50-70%; the finish rolling start rolling temperature is 880-920°C, the finish rolling temperature is 820-850°C, and the total rolling reduction rate is 55%-65%; V. cooling After rolling, the cooling is controlled by two-stage small water flow, ACC and DQ laminar water group number 17-20, roller speed 0.5-1.0 m / min, cooling speed 12-20 ℃ / s; cooling start temperature 780-840 ℃, final cooling temperature 500-600 ℃, laminar up and down single water flow 150-250 / 300-400 m 3 / h.

2. The method of claim 1, wherein: The yield strength of the steel plate is ≥550 MPa, the tensile strength is ≥700 MPa, and the elongation is ≥20%; The -40°C low-temperature Charpy impact energy is ≥100 J; the steel plate has excellent cold bending performance, D=3a, and no cracks occur in 180° cold bending.

3. The method of claim 1, wherein: In step II, the continuous casting is performed by using a 300-450 mm section thickness continuous casting machine to perform low superheat argon protection pouring, and the superheat of the molten steel is controlled to be 10-25°C; during pouring, the drawing speed fluctuation is controlled to be within ±0.05 m / min, and the non-sine vibration of the crystallizer liquid surface is controlled to be within ±5 mm; the thickness of the protective slag layer is 70-90 mm, and the thickness of the liquid slag is 7-11 mm.

4. The method of claim 1, wherein: In step III, the continuous casting billet is heated to the soaking temperature of 1200-1250°C by using a pusher-type heating furnace, wherein the first heating section temperature is 650-750°C, the second heating section temperature is 750-1150°C, the third heating section temperature is 1200-1260°C, and the fourth soaking section temperature is 1200-1250°C; the continuous casting billet is heated in the furnace for 8-14 min / cm.

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