Low-cost 500MPa-grade steel for engineering machinery and production method of low-cost 500MPa-grade steel

Abstract

According to the low-cost 500MPa-grade steel for the engineering machinery and the production method of the low-cost 500MPa-grade steel, rolling is carried out by adopting a large compression ratio, a specific water cooling process is combined, and a good plate shape is achieved while the performance is guaranteed. The chemical components comprise the following components in percentage by mass: 0.06-0.08% of C; 0.20 to 0.35 percent of Si; mn: 1.55 to 1.70%; less than or equal to 0.015% of P; s < = 0.003%; 0.035% to 0.050% of Nb; 0.15% to 0.25% of Cr; ti: 0.008% to 0.015%; 0.025% to 0.040% of Al; the carbon equivalent CEV of the components ranges from 0.348 to 0.418, the welding crack sensitivity Pcm is smaller than or equal to 0.189, the structure of the steel plate is composed of less pearlite, ferrite and granular bainite, and the impact toughness at the low temperature of minus 50 DEG C can be kept.

Description

Technical Field

[0001] The present invention belongs to the technical field of engineering machinery steel, and specifically relates to a low-cost 500MPa grade engineering machinery steel and a production method thereof. Background Art

[0002] With the development of economy, the demand for construction machinery and equipment is increasing day by day, and the use of engineering construction is gradually developing from economically developed areas to remote and cold areas. High strength and high toughness will become the main development direction of steel for construction machinery in the future. At the same time, the use of construction machinery and equipment will inevitably be subject to strong impact, melting of permafrost, collision during use at high altitudes and cold weather, etc., which will bring new challenges to the future use of equipment. Equipment users require that construction machinery and equipment not only meet the requirements of high strength, but also need to have higher low-temperature toughness to meet the requirements of equipment use in cold and other areas.

[0003] In the patent application with publication number CN108624744A, a Q500qE bridge steel plate and its production method are disclosed, wherein the composition C: 0.045-0.07%, Si: 0.20-0.40%, Mn: 1.6-1.8%, P≤0.018%, S≤0.006%, Cr: 0.25-0.35%, Nb: 0.02-0.04%, Cu: 0.15-0.25%, Ni: 0.15-0.25%, Mo: 0.15-0.25%, Ti: 0.01-0.25%, Als: 0.015-0.045%. The preparation method of the composition has the following problems: firstly, the carbon equivalent is high, and welding cracking and unstable impact performance are prone to occur; secondly, the alloy cost is high and is not suitable for industrial mass production, which affects the production rhythm and efficiency.

[0004] The patent application with publication number CN114182165A discloses a low yield ratio and high toughness Q500qE bridge steel and its production method. The key production process requires the use of TMCP+tempering process, which increases the heat treatment process and increases the cost. It is not suitable for large-scale industrial production requirements.

[0005] According to current data, most of the processes in patents for steel plates of this grade and performance are complicated and can only guarantee -20℃ impact performance. The present invention mainly optimizes and improves the impact performance and plate shape in terms of process technology, ensuring that steel plates of this specification have high impact toughness and good plate shape without reducing strength. Summary of the invention

[0006] The purpose of the present invention is to provide a low-cost 500MPa grade steel for engineering machinery and a method for manufacturing the same, by means of a simple composition design and a conventional TMCP production process, to achieve efficient industrialized production.

[0007] The present invention is achieved through the following technical scheme: a low-cost 500MPa grade steel for engineering machinery and a production method thereof, which comprises the following components in mass percentage: C: 0.06-0.08%; Si: 0.20-0.35%; Mn: 1.55-1.70%; P≤0.015%; S≤0.003%; Nb: 0.035-0.050%; Cr: 0.15-0.25%; Ti: 0.008-0.015%; Al: 0.025-0.040%; the rest is bulk Fe and unavoidable impurities.

[0008] A low-cost 500MPa steel for engineering machinery and a production method thereof include a smelting step, a billet heating step, a rolling step, a water cooling step, and a hot straightening step, characterized in that: a) In the smelting step, the converter smelting steel P≤0.010%, S≤0.003%, spot blowing is not allowed, RH vacuum pressure≤100Pa, vacuum treatment time≥10min, the bottom needs to be blown with argon gas for ≥10min after wire feeding to ensure the floating time of gas and inclusions, the end temperature of static blowing is ≥1580℃, the continuous casting superheat is 15-25℃, the original austenite grain size of the ingot is guaranteed to be small, the pulling speed range is 0.45m / min-0.75m / min, accompanied by electric stirring ≥10mm light pressure to improve the density of the internal structure; b) Billet heating part: Billets need to be hot-delivered, and the hot-delivery temperature is ≥200℃. The temperature of the three heating and soaking sections is divided into 4 sections. The first heating section is 1020-1060℃, the second section is 1200-1240℃, and the third section is 1260-1300℃. The soaking temperature of the billet is 1250-1270℃ to ensure the uniformity of the temperature inside and outside the billet. The total heating time of the billet is 1.1-1.5t (unit: min / mm), where t is the billet thickness; c) Rolling part: It is characterized by adopting two-stage controlled rolling. In the first stage rolling step, the thickness of the initial rolling ingot is ≥250mm, the thickness of the intermediate ingot is 80~120mm, the first stage compression ratio is guaranteed to be ≥2.0, the initial rolling temperature is ≥960℃, the rolling is carried out at a temperature above the recrystallization zone, the reduction rate of each of the last three passes is ≥15%, and the reduction rate of the last pass is ≥20%. The second stage adopts even-pass rolling, the final rolling end temperature is 780~820℃, the reduction rate of each pass is ≥10%, and the compression ratio of this stage is ≥3.0, which promotes the precipitation of second phase particles and obtains more nucleation points for ferrite nucleation, where t is the thickness of the finished product; d) Water cooling part: water inlet temperature ≥760℃ to prevent ferrite from precipitating first. ACC adopts 2kg water cooling mode, head shielding, horizontal spraying, side spraying and tail blowing to prevent uneven water cooling of the whole plate and the appearance of blackheads. The water group adopts nozzle groups 1 / 4 / 7 / 8 / 9 / 10, 13-30 groups are connected, the upper spray flow rate is 100~120m3 / h, the roller speed is 0.4~0.7m / s, the water ratio is 1.8~2.1, and the cooling rate is controlled at 8~14℃ / s to ensure the continuity and uniformity of water cooling while preventing the bending after water cooling from being unable to be straightened. The final cooling temperature is 390±20℃; e) Hot straightening part: After water cooling, relax for 10~15S to reduce the temperature difference between the inside and outside of the steel plate and eliminate stress, and then use 2 rounds of hot straightening with a straightening force between 10000KN~15000KN.

[0009] Among them, the yield strength of the steel plate is 530-580MPa, the tensile strength is 640-690MPa, the deformation elongation is ≥20%, the longitudinal impact energy at -50℃ is ≥250J, and the yield strength ratio is ≤0.85.

[0010] Among them, the thickness of the steel plate is between 25-40mm.

[0011] Advantages of the present invention: (1) Based on the existing equipment capacity, a simple and reasonable composition and process design are used to develop 500-level engineering machinery steel plates, breaking the traditional complex process and improving production efficiency. A low-carbon composition design is adopted, and micro-alloying such as Nb and Ti is used. At the same time, a certain amount of Cr is added to improve strength and hardenability, and the amount of precious metals such as Mo and Ni added is reduced, thereby reducing alloy costs.

[0012] (2) The present invention adopts a large compression ratio for rolling from the ingot to the finished product. This method can ensure a high degree of overall grain refinement in the thickness direction and ensure industrial production efficiency.

[0013] (3) The present invention adopts specific water cooling parameters, which can ensure performance while having a good plate shape after cold straightening.

[0014] (4) The 500MPa grade steel plate produced by the present invention has good comprehensive performance, carbon equivalent CEV≤0.418, welding crack sensitivity Pcm≤0.189, better welding performance, and elongation ≥20%, -20℃ longitudinal impact energy ≥250J. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Figure 1 Typical organization of the steel plate of the present invention (500×); Figure 2 The present invention produces the steel plate offline. DETAILED DESCRIPTION

[0016] The invention discloses a low-cost 500MPa grade steel for engineering machinery and a production method thereof. The production method comprises the steps of smelting, heating of ingot casting, rolling, water cooling, hot straightening and the like. The chemical composition of the steel plate and its mass percentage are as follows: C: 0.06-0.08%; Si: 0.20-0.35%; Mn: 1.55-1.70%; P≤0.015%; S≤0.003%; Nb: 0.035-0.050%; Cr: 0.15-0.25%; Ti: 0.008-0.015%; Al: 0.025-0.040%; the rest are bulk Fe and unavoidable impurities. Among them, carbon equivalent CEV=C+Mn / 6+(Cr+Mo+V) / 5+(Ni+Cu) / 15≤0.418, welding crack sensitivity Pcm=C+Si / 30+Mn / 20+Cu / 20+Ni / 60+Cr / 20+Mo / 15+V / 10+5B≤0.189. Example 1

[0017] A low-cost 500MPa grade steel for engineering machinery and a production method thereof. The chemical composition and mass percentage of the steel plate with a thickness of 25mm are C: 0.064%; Si: 0.315%; Mn: 1.59%; P: 0.011%; S: 0.0021%; Nb: 0.039%; Cr: 0.188%; Ti: 0.012%; Al: 0.031%; the rest are bulk Fe and unavoidable impurities.

[0018] The production steps and process parameters of the steel plate of the present invention are: 1. The smelting steps of the present invention are as follows: in converter smelting, steel tapping P: 0.008%, S: 0.0027%, no spot blowing, RH vacuum pressure ≤ 100Pa, vacuum treatment time 16min, bottom static blowing time of argon gas after wire feeding 18min, static blowing end temperature 1596°C, continuous casting superheat 18-24°C, pulling speed range 0.58m / min-0.72m / min accompanied by electric stirring and 14mm soft pressure reduction; 2. The slab heating step of the present invention: the slab hot delivery temperature is 248°C, and a three-stage heating plus a soaking stage heating method is adopted. The heating temperatures are 1046°C, 1226°C, 1287°C, and the soaking stage temperature is 1266°C. The heating time of each section is 104min, 94min, 78min, and 85min, respectively. The thickness of the casting is 261mm, and the total heating time is 361min (1.38t). 3. The rolling part of the present invention is characterized in that the rolling adopts two-stage controlled rolling. In the first stage rolling step, the thickness of the initial rolling cast billet is 261 mm, the thickness of the intermediate billet is 85 mm, the first stage rolling temperature is 1041°C, and the single pass reduction rates of the last three passes are 17%, 19.5%, and 22.3% respectively. The second stage adopts even-pass rolling, the final rolling end temperature is 807°C, and the single pass reduction rate is ≥11.5%, wherein t is the finished product thickness; 4. The water cooling part of the present invention: the water inlet temperature is 778℃, ACC adopts 2kg water cooling mode, adopts head shielding, horizontal spraying, side spraying, and tail blowing, the water group adopts nozzle groups 1 / 4 / 7 / 8 / 9 / 10, 13 groups-30 groups are connected, and the upper spray flow rate is 105m 3 / h, roller speed 0.65m / s, water ratio 1.9, controlled cooling rate 17℃ / s, final cooling temperature 387℃; 5. The hot straightening part of the present invention: after water cooling and relaxation for 11S, two hot straightenings are adopted, and the straightening forces are 118000KN and 123000KN respectively.

[0019] The steel plate obtained in this embodiment has a yield strength of 550 MPa, a tensile strength of 667 MPa, an elongation of 20.5%, and a longitudinal impact energy of 305 / 299 / 295 J at -40°C, respectively. Example 2

[0020] A low-cost 500MPa grade steel for engineering machinery and a production method thereof. The chemical composition and mass percentage of the steel plate with a thickness of 40mm are C: 0.075%; Si: 0.303%; Mn: 1.61%; P: 0.012%; S: 0.0019%; Nb: 0.040%; Cr: 0.23%; Ti: 0.011%; Al: 0.028%; the rest are bulk Fe and unavoidable impurities.

[0021] The production steps and process parameters of the steel plate of the present invention are: 1. The smelting steps of the present invention are as follows: in converter smelting, the steel tapping P is 0.007%, S is 0.0024%, no spot blowing, RH vacuum pressure ≤ 100Pa, vacuum treatment time is 19min, the bottom needs static blowing of argon for 14min after wire feeding, static blowing end temperature is 1603℃, continuous casting superheat is 16-22℃, pulling speed range is 0.51m / min-0.67m / min, accompanied by electric stirring and 12mm light pressure reduction; 2. The slab heating step in the present invention: the slab hot delivery temperature is 237°C, and a three-stage heating plus a soaking stage heating method is adopted. The heating temperatures are 1025°C, 1229°C, 1288°C, and the soaking stage temperature is 1267°C. The heating times are 98min, 109min, 112min, and 121min, respectively. The thickness of the casting is 305mm, and the total heating time is 440min (1.44t). 3. The rolling part of the present invention is characterized in that the rolling adopts two-stage controlled rolling. In the first stage rolling step, the thickness of the initial rolling billet is 305 mm, the thickness of the intermediate billet is 120 mm, the first stage rolling temperature is 1021°C, and the single pass reduction rates of the last three passes are 16%, 17.3%, and 21.7% respectively. The second stage adopts even-pass rolling, the final rolling end temperature is 785°C, and the single pass reduction rate is ≥10.8%, wherein t is the finished product thickness; 4. The water cooling part of the present invention: the water inlet temperature is 766℃, ACC adopts 2kg water cooling mode, adopts head shielding, horizontal spraying, side spraying, and tail blowing, the water group adopts nozzle groups 1 / 4 / 7 / 8 / 9 / 10, 13 groups to 30 groups are connected, and the upper spray flow rate is 117m 3 / h, roller speed 0.48m / s, water ratio 2.0, controlled cooling rate 11℃ / s, final cooling temperature 395℃; 5. The hot straightening part of the present invention: after water cooling, relax for 13S, adopt 2 passes of hot straightening, and the straightening forces are 12500KN and 14400KN respectively.

[0022] The steel plate obtained in this embodiment has a yield strength of 533 MPa, a tensile strength of 672 MPa, an elongation of 22.4%, and a longitudinal impact energy of 285 / 279 / 278 J at -40°C, respectively.

[0023] As attached Figure 1 As shown in the figure, the steel plate produced by this method is a relatively fine pearlite + ferrite + granular bainite structure after metallographic inspection, without obvious segregation band, ensuring good low temperature impact toughness and good strength. Figure 2 As shown, the steel plate produced by this method has good plate shape and flatness.

Claims

1. A low-cost 500MPa grade steel for engineering machinery, characterized by: The components include the following mass percentages: C: 0.06-0.08%; Si: 0.20-0.35%; Mn: 1.55-1.70%; P≤0.015%; S≤0.003%; Nb: 0.035-0.050%; Cr: 0.15-0.25%; Ti: 0.008-0.015%; Al: 0.025-0.040%; the rest is Fe and unavoidable impurities; among them, carbon equivalent CEV=C+Mn / 6+(Cr+Mo+V) / 5+(Ni+Cu) / 15≤0.418, welding crack sensitivity Pcm=C+Si / 30+Mn / 20+Cu / 20+Ni / 60+Cr / 20+Mo / 15+V / 10+5B≤0.

189.

2. The method for producing a low-cost 500MPa grade engineering machinery steel according to claim 1, characterized in that: The steps include: 1) Smelting part: In converter smelting, the steel P≤0.010%, S≤0.003%, spot blowing is not allowed, RH vacuum pressure≤100Pa, vacuum treatment time≥10min, the bottom needs to be blown with argon gas for ≥10min after wire feeding, the end temperature of static blowing is ≥1580℃, continuous casting superheat is 15-25℃, the pulling speed range is 0.45m / min-0.75m / min, accompanied by electric stirring and ≥10mm light pressure; 2) Billet heating part: Billets need to be hot-delivered, and the hot-delivery temperature is ≥200℃. The temperature of the three heating and soaking sections is divided into 4 sections. The first heating section is 1020-1060℃, the second section is 1200-1240℃, and the third section is 1260-1300℃. The soaking temperature of the billet is 1250-1270℃. The total heating time of the billet is 1.1-1.5t, where t is the billet thickness. 3) Rolling part: It is characterized by adopting two-stage controlled rolling. In the first stage rolling step, the thickness of the initial rolling ingot is ≥250mm, the thickness of the intermediate ingot is 80~120mm, the initial rolling temperature of the first stage is ≥960℃, the reduction rate of each of the last three passes is ≥15%, and the reduction rate of the last pass is ≥20%. The second stage adopts even-pass rolling, the final rolling end temperature is 780~820℃, and the reduction rate of each pass is ≥10%, where t is the thickness of the finished product; 4) Water cooling part: water inlet temperature ≥ 760℃, ACC adopts 2kg water cooling mode, adopts head shielding, horizontal spraying, side spraying, and tail blowing, the water group adopts nozzle group 1 / 4 / 7 / 8 / 9 / 10, 13-30 groups are connected, and the upper spray flow rate is 100~120m 3 / h, roller speed 0.4~0.7m / s, water ratio 1.8~2.1, controlled cooling speed 8~20℃ / s, final cooling temperature 390±20℃; 5) Hot straightening part: After water cooling, relax for 10~15S and then use 2 times of hot straightening. The straightening force is between 10000KN~15000KN.

3. The method for producing low-cost 500MPa grade engineering machinery steel according to claim 2, characterized in that: The prepared steel plate has a yield strength of 530-580MPa, a tensile strength of 640-690MPa, a deformation elongation ≥20%, a -50°C longitudinal impact energy ≥250J, and a yield strength ratio ≤0.

85.

4. The method for producing a low-cost 500MPa grade engineering machinery steel according to claim 2, characterized in that: The thickness of the steel plate is between 25-40mm.

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