CN118653109B - A method for avoiding delayed cracking of low alloy high strength steel plate after cutting - Google Patents

Abstract

The invention relates to a method for avoiding delayed cracking after cutting a low-alloy high-strength steel plate, which comprises the following chemical components: 0.14 to 0.19 percent of C, 0.15 to 0.35 percent of Si, 1.05 to 1.45 percent of Mn, less than or equal to 0.02 percent of P, less than or equal to 0.005 percent of S, 0.010 to 0.040 percent of Nb, 0.010 to 0.025 percent of Ti, 0.5 to 0.8 percent of Mo, 0.02 to 0.05 percent of Als, 0.005 to 0.010 percent of Ca, 0.0008 to 0.0018 percent of B, 0.3 to 0.5 percent of Cr, 0.3 to 0.5 percent of Ni, and the balance of Fe and impurities. The production process of the steel plate comprises the following steps: molten steel smelting, external refining +RH vacuum degassing, micro Ti treatment, B alloying, continuous casting, casting blank pit heating and slow cooling, casting blank reheating, control cooling, hot straightening, steel plate stacking and slow cooling, quenching, tempering and cutting. The invention effectively solves the problem of delayed cracking after steel cutting.

Description

Method for avoiding delayed cracking after cutting of low-alloy high-strength steel plate

Technical Field

The invention relates to the technical field of steel plate surface quality control, in particular to a method for preventing delayed cracking after cutting of a low-alloy high-strength steel plate for 1000 MPa-level engineering machinery with a thickness specification of 40-80 mm.

Background

In recent years, with the rapid development of industries such as hydropower, engineering machinery, wind power construction and the like, the demand for large-thickness steel plates is increasing. The low alloy high strength engineering machinery steel is widely used due to low alloy element content (low cost), high strength and high toughness (good comprehensive performance), the later processing process of the steel plate can adopt laser cutting, plasma cutting, flame cutting and other methods to cut, wherein the thickness of the steel plate corresponding to the laser cutting and the plasma cutting is generally below 40mm, so that certain use limitation exists, and the flame cutting is widely used due to low cost, easy operation, high cutting efficiency and wide range of the thickness of the steel plate which can be cut. The steel plate for the large-thickness high-strength engineering machinery is often cut by flame, but a flame section is easy to form delayed cracks after flame cutting, no obvious sign exists before the delayed cracks are generated, sudden brittle fracture can be caused after the delayed cracks are expanded, loss is caused for a user, and meanwhile, disastrous results can be caused. The fire section delayed cracking is caused by the fact that the cutting surface of the steel plate is affected by cutting thermal circulation to generate tissue transformation (such as segregation bands and large-size sharp-angle inclusions exist in tissues), hydrogen atoms are easy to capture, and therefore hydrogen embrittlement is initiated and delayed cracking is formed. The development of engineering machinery and other fields is severely restricted by the existence of the problems, and how to solve the problems becomes a matter which needs to be considered urgently by steel manufacturing enterprises.

The invention patent of China with the application number of CN201510453518.5 discloses a control method for avoiding delayed cracking of high-carbon steel in large thickness in winter, which is characterized in that a high-carbon steel plate designed by high C, mn components is subjected to normalizing treatment and flame cutting and then is subjected to slow cooling, and then is subjected to slow cooling pit stress relief treatment after flame cutting, so that a martensitic structure is reduced, the stress concentration of the structure is reduced, and the delayed cracking of the stress after flame cutting is avoided, but the high-carbon steel plate subjected to flame cutting is placed in a slow cooling pit or a single-body furnace with the temperature of 300-350 ℃, and is subjected to heat preservation for 1.5-2.5 hours and then is cooled to below 100 ℃ along with the furnace, so that the production cost of a user is increased.

The Chinese patent application No. CNC 202210040898.X discloses a method for preventing cutting delay cracks of large-thickness low-alloy martensitic steel, which adopts reasonable content proportion of Ti, nb, mo, al and B elements to control hardenability and inclusion size of the large-thickness steel plate, adopts a secondary quenching and low-temperature tempering heat treatment process to avoid formation and expansion of delay cracks, adopts flame cutting to preheat the steel plate before flame cutting, covers refractory heat-insulating cotton after wind-shielding cutting or moves into a heat-insulating pit to be slowly cooled, effectively avoids stress to cause delay cracks after cutting, but limits the thickness of the steel plate to be 70mm at most, has strict requirement on Ti/N ratio, adopts a secondary quenching process, and increases heat treatment production cost.

Disclosure of Invention

The invention provides a method for avoiding delayed cracking after cutting of a low-alloy high-strength steel plate, which aims at the high-strength steel plate for engineering machinery with the thickness specification of more than 1000MPa grade of 40-80 mm, and adopts the method for optimizing continuous casting, heating, rolling, heat treatment and cutting processes, so that the problem of delayed cracking after cutting of the steel is effectively solved, and the cut steel plate does not have delayed cracking after being placed for more than 60 days.

In order to achieve the above purpose, the invention is realized by adopting the following technical scheme:

A method for avoiding delayed cracking after cutting low-alloy high-strength steel plate comprises the steps of controlling chemical components of the steel plate and controlling the production process of the steel plate, wherein the chemical components of the steel plate comprise ：C 0.14%～0.19%,Si 0.15%～0.35%,Mn 1.05%～1.45%,P≤0.02%,S≤0.005%,Nb 0.010%～0.040%,Ti 0.010%～0.025%,Mo 0.5%～0.8%,Als 0.02%～0.05%,Ca 0.005%～0.010%,B 0.0008%～0.0018%,Cr 0.3%～0.5%,Ni 0.3%～0.5%, percent by weight of Fe and unavoidable impurities, the total content of impurity elements is lower than 0.05 percent, and the production process of the steel plate comprises molten steel smelting, external refining+RH vacuum degassing, micro Ti treatment, B alloying, continuous casting, casting blank pit heating and slow cooling, casting blank reheating, controlled cooling, hot straightening, steel plate stacking slow cooling, quenching, tempering and cutting.

Further, the production process of the steel plate comprises the following steps:

1) Smelting molten steel until casting blank enters a pit, heating and slowly cooling:

Controlling the S content to be lower than 0.015% during molten iron pretreatment, controlling the P content to be less than or equal to 0.02% during converter smelting, controlling the converter smelting end point C content to be 0.14% -0.19%, blowing argon gas to be 25% -35 min during converter tapping, feeding Si-Ca wires to perform external refining treatment during LF refining, maintaining RH vacuum degassing for more than 25min, adding Al according to the Als content requirement after the degassing treatment, adding Ti-Fe to perform micro Ti treatment, adding B-Fe alloy to perform B alloying, controlling the superheat degree to be 15-25 ℃ during continuous casting, controlling the continuous casting pulling rate to be 0.8-1.3 m/min, controlling the electromagnetic stirring current intensity of a secondary cooling zone to be 100-150A during continuous casting, controlling the secondary cooling specific water amount to be 1.3-2.0L/kg, feeding the casting blank under heavy pressure at a horizontal fan-shaped section, feeding the casting blank into a pit after the casting blank is taken down, controlling the temperature to be not lower than 600 ℃ to be 600-650 ℃ and cooling time to be 48-72 h, and cooling the casting blank after the casting blank is taken down to the pit to be cooled down below 150 ℃;

2) The casting blank is reheated, wherein the temperature of a preheating section is 900-1050 ℃, the temperatures of a soaking section and a heating section are 1225-1250 ℃, the total heating time of the soaking section and the heating section is 4-5.5 hours, and the total furnace time is 6-8 hours;

3) The rolling and hot straightening are carried out in two stages, wherein the initial rolling temperature of rough rolling is more than or equal to 1120 ℃, the final rolling temperature of rough rolling is 980-1025 ℃, the rough rolling is not more than 6 times, the rolling reduction rate of at least the first 2 times is more than 15%, at least 2 times of descaling water is sprayed in the process of waiting for the intermediate billet to be heated, the descaling time of each time is 1-2 min, the descaling water pressure is 15-20 MPa, the initial rolling temperature of finish rolling is 860-920 ℃, the final rolling temperature of finish rolling is 820-850 ℃, the rolling reduction rate of at least the first 2 times is not more than 20%, the rolling speed of slow rolling is 1-2 m/s, the straightening temperature of hot straightening is 750-800 ℃, the straightening force is 3000-4500 kN, the bending roll amount is 1.2-1.8 mm, and the tilting value is 3.5-5.5 mm;

4) Stacking and slowly cooling the steel plates, namely immediately stacking and slowly cooling after hot straightening is finished, wherein the stacking temperature is not lower than 400 ℃, and the slowly cooling time is not less than 48 hours;

5) Quenching, wherein the quenching heating comprises a low temperature section, a high temperature section and a cooling section, the temperature of the low temperature section is 620-680 ℃, the temperature of the low temperature section is 2.0-3.5 min/mm in a furnace, the temperature of the high temperature section is 880-920 ℃, the temperature of the high temperature section is 1.5-1.8 min/mm in the furnace, the temperature of the cooling section is 760-810 ℃, the temperature of the cooling section is 0.5-0.9 min/mm in the furnace, the quenching average cooling speed is 20-45 ℃ per second, and the quenching finishing temperature is 180-250 ℃;

6) Tempering, wherein the tempering heating temperature is 150-180 ℃, and the tempering heat preservation time is 2.0-3.5 min/mm;

7) Cutting, wherein the preheating temperature of the steel plate is 100-200 ℃, the cutting is performed at normal temperature, the energy of a welding line is controlled below 20kJ/cm during cutting, the cutting edge and a heat affected zone are covered by an asbestos cloth heat insulation blanket after cutting, and the steel plate is slowly cooled to room temperature.

Further, the finished steel plate is more than 1000MPa grade steel plate.

Further, the thickness of the finished steel plate is 40-80 mm, the thickness of the casting blank is 250-300 mm, and the thickness of the intermediate blank is 2.5-3.5 times of the thickness of the finished steel plate.

Further, in the step 7), the steel plate is preheated by a flame gun, an electronic heating pad or a heating furnace.

Compared with the prior art, the invention has the beneficial effects that:

1) In the process of external refining, si-Ca wire is fed to conduct external refining treatment, so that the content of oxygen and sulfur in the molten steel can be effectively reduced, sulfide and oxide inclusions are changed into spherical inclusions containing calcium, and meanwhile, the quantity of inclusions is reduced, thereby achieving the purposes of purifying the molten steel, improving the quality of steel and reducing the local internal stress of the steel plate during rolling. The quality defects of the central part and the corner part of the casting blank can be effectively reduced by controlling the continuous casting superheat degree and the blank drawing rate, the average carbon segregation index can be reduced by optimizing the current intensity of electromagnetic stirring in a secondary cooling area and the secondary cooling specific water quantity, thereby inhibiting segregation, and the trend of the central crack deterioration of the casting blank is inhibited by limiting the secondary cooling intensity. The casting blank pit slow cooling is beneficial to reducing the center segregation of the casting blank and effectively reducing the H content in the casting blank.

2) When the casting blank is heated, the temperature of the preheating section is controlled to ensure that carbide of Ti and Nb is quickly and fully dissolved in the matrix and fully diffused, and the high heating temperature is adopted to ensure the in-furnace time and the total in-furnace time of the soaking section and the heating section, so that the full diffusion of elements such as C, mn, mo, H and the like is effectively promoted and the structural uniformity of the rolled steel plate is ensured.

3) The rolling process is controlled in two stages, the final rolling temperature of the rough rolling stage is controlled, the intermediate billet is ensured to be in an austenite recrystallization temperature range in the process of waiting for temperature, the tissue uniformity is ensured, the rolling reduction rate of the two stages and the thickness of the intermediate billet are limited, the tissue uniformity in the thickness direction of the rolled steel plate is ensured, scale water is sprayed in the process of waiting for temperature of the intermediate billet, austenite grain growth can be restrained, meanwhile, temperature gradients are generated on the inner surface and the outer surface of the steel billet, the penetration of rolling deformation to the thickness center is promoted, grains at the 1/2 thickness position are thinned, the improvement of the core tissue of the thick steel plate is facilitated, the tissue stress generated by tissue non-uniformity is reduced, the deformation penetration amount of the core of the steel plate is improved by adopting slow rolling, the tissue uniformity is further improved, the straightening process parameters are optimized, and the full release of the internal stress is promoted.

4) The hot straightened steel plate adopts stacking slow cooling, and simultaneously limits the slow cooling time and temperature so as to accelerate the diffusion of C, mn and H elements in the core part of the steel plate, inhibit the segregation of the C, mn and H elements and slow down the poor structural stress of the core part of the steel plate and the occurrence of hydrogen induced cracks of the steel plate.

5) The quenching and heating of the steel plate comprises a low-temperature section, a high-temperature section and a cooling section. The temperature of the low-temperature section and the furnace time are controlled, and the purpose of the method is to continuously promote outward diffusion of H element in the steel plate and reduce the H content of the steel plate on one hand, and promote massive precipitation of Nb and Ti composite carbides through long-time heat preservation on the other hand, and the carbides provide massive hydrogen traps for the interior of the steel plate. The high temperature section temperature and the furnace time are limited, and the purpose of the method is to rapidly heat the steel plate to the austenitizing temperature, provide tissue guarantee for obtaining martensite for subsequent quenching of the steel plate, and simultaneously prevent coarsening of crystal grains and coarsening of Nb and Ti composite carbides caused by overhigh temperature, so that the number of hydrogen traps in the steel plate is greatly reduced, and the risk of cracks is increased. The temperature of the cooling section and the furnace time are limited, and the purpose of the cooling section is to enable austenite to be reduced to be above Ar3 temperature in the cooling section, namely, the full austenitizing state of the steel plate is ensured, the quenched steel plate is fully hardened, meanwhile, the lower the temperature of the steel plate for quenching is, the lower the stress level of the steel plate is in the quenching process, and the cracking condition of the steel plate caused by the quenching process can be avoided. Meanwhile, in order to avoid quenching cracks caused by overlarge steel plate stress due to overlarge cooling speed in the quenching process, the average quenching cooling speed is designed to be 20-45 ℃ per second, and the quenching finishing temperature is limited, so that the stress in the steel plate quenching process is reduced, the quenching cracks are avoided, and meanwhile, the austenite in the steel plate is ensured to fully finish martensitic transformation.

6) The method comprises the steps of preheating and cutting a steel plate, limiting welding line energy, preventing coarsening of a heat affected zone structure caused by overlarge input energy, and cracking caused by overlarge internal stress after the steel plate is cooled, covering a cutting edge and the heat affected zone by using an asbestos cloth heat insulation blanket after cutting, slowly cooling to room temperature, avoiding a wet environment in a slow cooling process, and strictly preventing liquids such as water and the like from being contacted, so as to prevent cracking caused by structural stress caused by rapid cooling of the steel plate after cutting.

7) After the process is adopted, the cut steel plate is placed for more than 60 days, and no delayed crack defect occurs.

Detailed Description

The invention relates to a method for avoiding delayed cracking after cutting a low-alloy high-strength steel plate, which comprises the following chemical components in percentage by weight, wherein the balance of ：C 0.14%～0.19%,Si 0.15%～0.35%,Mn 1.05%～1.45%,P≤0.02%,S≤0.005%,Nb 0.010%～0.040%,Ti 0.010%～0.025%,Mo 0.5%～0.8%,Als 0.02%～0.05%,Ca 0.005%～0.010%,B 0.0008%～0.0018%,Cr 0.3%～0.5%,Ni 0.3%～0.5%, is Fe and unavoidable impurities, and the total content of impurity elements is lower than 0.05%.

The main elements in the steel plate have the following functions:

Carbon is an important element for ensuring the hardenability of the steel plate, is also an important factor for determining the carbon equivalent, and has great influence on the strength, toughness, plasticity and weldability of the steel. The welding performance of the steel plate is affected by the excessive carbon content. The carbon content is too low, which affects the overall strength of the steel sheet. The invention controls the carbon content to be 0.14% -0.19% on the premise of ensuring the strength required by the steel plate.

Si is a necessary element for deoxidation, has a certain solid solution strengthening effect, and the content of Si is too high, so that the toughness, the uniformity of the ultra-thick plate performance and the weldability of the steel are affected, and the content of the Si is controlled to be 0.15% -0.35%.

Mn is an important solid solution strengthening element in steel, can reduce the transformation temperature, refine the microstructure, improve the toughness while strengthening the steel plate, and can improve the hardenability. Too high a manganese content can cause segregation and is detrimental to welding and toughness. The manganese content is controlled to be 1.05% -1.45%.

Ni can improve the low-temperature toughness and the plasticity of steel. For extra thick plates, the low-temperature impact performance of 1/4 and 1/2 of the thickness direction can be improved, the alloy element is an indispensable alloy element, ni can delay pearlite transformation, promote the formation of medium-temperature transformation structures and reduce the cold speed limit of thick steel plates. However, the price of nickel is high, and the production cost is comprehensively considered, and the Ni content is controlled to be 0.3% -0.5%.

Cr can increase the hardenability of steel, refine the structure and reduce the ductile-brittle transition temperature, and when being used together with Mn, the Cr can improve the hardenability of steel and improve the mechanical property of steel, and the segregation trend of Cr is smaller than that of Mn, and the Cr replaces part of Mn, so that the segregation of the core part of the steel plate can be reduced, the internal quality of the steel plate can be improved and the uniformity of the mechanical property can be improved, and the chromium content is controlled to be 0.3-0.5%.

Mo is capable of remarkably increasing the hardenability and hardenability of steel, refining the microstructure of the quenched steel and improving the toughness. However, molybdenum is a noble alloy, and in order to reduce the production cost and ensure the performance, less molybdenum or no molybdenum can be selected according to the thickness of the steel plate, so that the content of the molybdenum is controlled to be 0.50% -0.80%.

Nb is one of common elements in modern microalloyed steel, has good fine grain strengthening and precipitation strengthening effects, and can delay austenite recrystallization, but excessive Nb can increase production cost and control difficulty of continuous casting process. According to the invention, the Nb content range is 0.01% -0.04%, and meanwhile, a reasonable TMCP process is matched, so that a uniform composite phase mainly comprising acicular ferrite structure can be obtained, and the finished steel plate has good toughness.

Ti is a strong solid N element and exists in the form of TiN in the continuous casting billet. The fine TiN particles can effectively inhibit the growth of austenite grains when the continuous casting billet is reheated, and are beneficial to improving the solid solubility of Nb in austenite and improving the impact toughness of a welding heat affected zone. However, when the Ti addition exceeds a certain value, tiN particles coarsen, thereby improving the stress concentration level of the particle interface and the matrix. Therefore, the Ti content is controlled to be 0.01% -0.025%.

B is an effective element for obviously improving the hardenability of steel, is extremely easy to segregate at grain boundaries, can prevent carbon from being precipitated, and can play an obvious role by a trace amount of boron. When the boron content is too high, boron carbonitride is easy to form, toughness is reduced, thermal embrittlement is caused, and the content of the boron carbonitride is controlled to be 0.0008% -0.0018%.

Ca is used as deoxidizing and desulfurizing purifying agent for steel to improve the form of non-metal inclusion and is widely used in calcium treating clean steel. By adding trace amounts of calcium elements to the steel, dispersed, thermally stable calcium-containing oxide particles of the second phase can be formed in the steel. The research results show that the dispersed calcium-containing oxide particles pin the austenite grain boundary migration of the CGHAZ in the welding thermal cycle process, limit the growth of austenite grains, obtain finer welding CGHAZ grain size and further improve the toughness of the welding CGHAZ of the micro-calcium steel. However, too high Ca can be biased at grain boundaries, and adversely affect the hardenability and impact toughness of steel, and the content of Ca is controlled to be 0.005% -0.0010%.

Als-is usually used as a deoxidizer in steel and also has the effect of refining the structure if AlN is formed. When the Al content exceeds 0.05%, excessive aluminum oxide inclusions may reduce the cleanliness of the steel. If the Al content is too low, the deoxidization will be insufficient, and oxides will be formed from easily oxidized elements such as Ti, and the lower limit of the Al content is set to 0.02%.

P, S is an unavoidable impurity element in steel, and too high P, S affects toughness and weldability of the steel sheet, so that the lower the steel sheet should be, the better. But cannot be unlimited low for smelting process considerations. Therefore, the upper limit of P, S content is set to 0.02% and 0.005% respectively.

The method is suitable for the low-alloy high-strength steel plate for the engineering machinery with the thickness specification of 40-80 mm and the strength level of more than 1000MPa, and the whole process flow from molten steel smelting to cutting of the steel plate comprises molten steel smelting, external refining +RH vacuum degassing, micro Ti treatment, B alloying, continuous casting, casting blank pit heating and slow cooling, casting blank reheating, controlled rolling and controlled cooling, hot straightening, steel plate stacking slow cooling, quenching, tempering and steel plate cutting.

The invention mainly controls the following technical processes:

1) Smelting molten steel until casting blank enters a pit, heating and slowly cooling:

Raw materials are pretreated by KR molten iron, the content of S is controlled to be lower than 0.015%, slag is removed, the raw materials enter a converter after slag skimming, the P content is controlled to be less than or equal to 0.02% by adopting a double slag method in converter smelting, the C content is controlled to be 0.14% -0.19% by controlling the end point of converter smelting, argon is blown for 25-35 min during converter tapping, and the removal of impurities in molten steel can be promoted by blowing argon and ballasting before continuous casting, so that the uniformity of molten steel components is improved.

In the process of external refining (LF refining), the Si-Ca wire is fed to conduct external refining treatment on molten steel, so that the content of oxygen and sulfur in the molten steel can be effectively reduced, sulfide and oxide inclusions are changed into calcium-containing spherical inclusions, the number of inclusions is reduced, the purposes of purifying the molten steel and improving the quality of steel are achieved, and the local internal stress of a steel plate during rolling is reduced. Maintaining RH vacuum degassing for more than 25min, adding Al according to Als content after degassing, adding Ti-Fe for micro-titanium treatment, and adding B-Fe alloy, wherein the alloy is added into molten steel to avoid floating on slag layer.

During continuous casting of the slab, the continuous casting superheat degree is controlled to be 15-25 ℃, the continuous casting blank drawing speed is controlled to be 0.8-1.3 m/min, and quality defects of the core part and the corner part of the casting blank can be effectively reduced through the control of the continuous casting superheat degree and the continuous casting blank drawing speed. The electromagnetic stirring current intensity of the secondary cooling area in the continuous casting stage is controlled to be 100-150A, the secondary cooling specific water quantity is controlled to be 1.3-2.0L/kg, so that the average carbon segregation index is reduced, segregation is inhibited, and meanwhile, the trend of crack deterioration in the center of a casting blank is inhibited by controlling the secondary cooling intensity. And the casting billet is put under heavy pressing at the horizontal sector section, namely the solidification end, the rolling reduction of the continuous casting billet is 30-45 mm, and the heavy pressing can help to reduce billet segregation, refine austenite grains and reduce internal structure defects.

And after the casting blank is taken off line, slowly cooling the casting blank in a pit, wherein the pit inlet temperature is not lower than 600 ℃, the slow cooling temperature is 600-650 ℃, the slow cooling time is 48-72H, and the casting blank is taken out of the pit after the pit is cooled to 150 ℃, and the casting blank pit inlet slow cooling is beneficial to reducing the center segregation of the casting blank and effectively reducing the H content in the casting blank.

2) The casting blank is reheated, namely the casting blank (the thickness is 250-300 mm) is sent into a step heating furnace to be heated, the temperature interval of a preheating section is 900-1050 ℃, the purpose is to promote carbide of Ti and Nb to be quickly and fully dissolved in a matrix and fully diffused, the temperatures of a soaking section and a heating section are 1225-1250 ℃, the total heating time of the soaking section and the heating section is 4-5.5 hours, the total furnace time is 6-8 hours, the high heating temperature is adopted, the furnace time of the soaking section and the heating section is ensured, the total furnace time is improved, and the purpose is to effectively promote full diffusion of elements such as C, mn, mo, H and the like, and the uniformity of rolled steel plate tissues is ensured.

3) Rolling and cooling control and hot straightening:

The first stage is recrystallization rolling (rough rolling), the initial rolling temperature of rough rolling is more than or equal to 1120 ℃, the final rolling temperature interval of rough rolling is 980-1025 ℃, the rough rolling is not more than 6 times, and the reduction rate of the first 2 times is preferably more than 15%. The thickness of the intermediate blank is 2.5-3.5 times of the thickness of the finished steel plate. In the process of heating the intermediate blank, 2-pass descaling water is sprayed for 1-2 min, and the pressure of a descaler is 15-20 MPa, so that the aim of inhibiting austenite grains from growing is to generate a temperature gradient on the inner and outer surfaces of the blank, promote the penetration of rolling deformation to the thickness center, refine grains at the 1/2 thickness position, improve the core structure of a thick steel plate and reduce the structure stress generated by uneven structure.

The second stage is non-recrystallization rolling (finish rolling), the finish rolling initial temperature range is 860-920 ℃, the finish rolling final temperature range is 820-850 ℃, the finish rolling is not more than 6 times, and the reduction rate of the first 2 times of the pass reduction system is preferably more than 20%. In the finish rolling stage, slow rolling is adopted, and the rolling speed is 1-2 m/s.

The final rolling temperature in the rough rolling stage is controlled to ensure that the intermediate billet is always in an austenite recrystallization temperature range in the process of waiting for temperature, thereby ensuring the uniformity of the structure, and the rolling reduction in the two stages is limited to ensure the uniformity of the structure in the thickness direction of the rolled steel plate.

The straightening temperature in hot straightening is 750-800 ℃, the straightening force is 3000-4500 kN, the bending roll amount is set to be 1.2-1.8 mm, the tilting value is set to be 3.5-5.5 mm, and the internal stress is promoted to be fully released by optimizing the straightening process parameters.

4) And (3) stacking and slow cooling the steel plates, namely immediately stacking and slow cooling the steel plates after hot straightening, wherein the stacking temperature is not lower than 400 ℃, the slow cooling time is not less than 48 hours, and the stacking and slow cooling accelerates the diffusion of C, mn and H elements in the core part of the steel plates, inhibits the segregation of the C, mn and H elements and slows down the structural stress difference in the core part of the steel plates and the occurrence of hydrogen-induced cracks of the steel plates.

5) Quenching;

In the invention, the quenching and heating of the steel plate comprises a low-temperature section, a high-temperature section and a cooling section.

The temperature of the low-temperature section of the quenching heat treatment furnace is 620-680 ℃, and the time of the low-temperature section is 2.0-3.5 min/mm, so that on one hand, H in the steel plate is continuously promoted to be outwards diffused, the H content of the steel plate is reduced, and on the other hand, nb and Ti composite carbides are promoted to be greatly precipitated through long-time heat preservation, and a great amount of hydrogen traps are provided for the inside of the steel plate by the carbides. Considering that the Nb and Ti composite carbide coarsens due to the over-high temperature, and cracks appear due to stress aggregation in the subsequent forming process of the steel plate, the temperature of the low temperature Duan Baowen is not more than 680 ℃.

The temperature of the high-temperature section is 880-920 ℃, the time of the high-temperature section in a furnace is 1.5-1.8 min/mm, the purpose of the high-temperature section is to rapidly heat the steel plate to an austenitizing temperature and provide tissue guarantee for obtaining martensite for subsequent quenching of the steel plate, but the excessive temperature can lead to coarsening of crystal grains and further coarsening of Nb and Ti composite carbide, so that the number of hydrogen traps in the steel plate is greatly reduced, the risk of cracks is increased, and therefore, the heat preservation time of the high-temperature section is not more than 1.8min/mm.

The temperature of the cooling section is 760-810 ℃, the time of the cooling section is 0.5-0.9 min/mm, the purpose is to enable austenite to be reduced to be above Ar3 temperature in the cooling section, namely, the full austenitizing state of the steel plate is ensured, the quenched steel plate is fully hardened, meanwhile, the lower the quenching temperature of the steel plate is, the lower the stress level of the steel plate in the quenching process is, and the cracking condition of the steel plate caused by the quenching process can be avoided.

The steel plate is quenched in a rolling quenching machine after being discharged from a quenching heat treatment furnace, so as to obtain sufficient martensitic transformation, meanwhile, the excessive stress of the steel plate caused by excessive cooling speed is avoided, quenching cracks appear, the average cooling speed of quenching is 20-45 ℃ per second, the quenching finishing temperature is 180-250 ℃, the aim is to reduce the stress of the steel plate in the quenching process, avoid the occurrence of quenching cracks, and simultaneously ensure that the austenitic transformation of the steel plate can be fully completed.

6) Tempering is a main technical means for reducing the residual stress of quenching, and fully releases the residual stress of the steel plate on the premise of ensuring the hardness of the steel plate, and the tempering heating temperature is controlled to be 150-180 ℃, and the tempering heat preservation time is controlled to be 2.0-3.5 min/mm.

7) Cutting, wherein the preheating temperature of the steel plate is 100-200 ℃, a flame burning gun and an electronic heating pad can be adopted, heating can be carried out by using a heating furnace, welding is carried out at normal temperature, the overheat of a welding area is controlled, the energy of a welding line is controlled below 20kJ/cm (the coarsening of a heat affected zone tissue is caused by overlarge control input energy, the overlarge internal stress is caused after the steel plate is cooled, cracks are generated), the cutting edge and the heat affected zone are covered by an asbestos cloth heat insulation blanket after cutting, the steel plate is slowly cooled to the room temperature, the moisture environment is avoided in the slow cooling process, and liquids such as water are strictly forbidden (the rapid cooling of the steel plate after cutting is avoided, and the cracks are caused by the tissue stress).

According to the invention, through optimizing smelting, heating, rolling, heat treatment and cutting processes, the problem that low alloy high strength steel for engineering machinery with the thickness specification of 40-80 mm and the strength level of more than 1000MPa generates delayed cracks after cutting is finally solved, and the cut steel plate does not generate delayed cracks after being placed for more than 60 days.

Embodiments of the present invention will be further described with reference to examples for more intuitively embodying the present invention. The following examples are only preferred embodiments of the present invention, but the scope of the present invention is not limited thereto, and any technical solutions that can be obviously obtained by those skilled in the art within the technical scope of the present invention, including simple changes or equivalent substitutions, are within the scope of the present invention.

Examples

Table 1 shows chemical components in each example steel, table 2 shows smelting, continuous casting and casting blank pit entry slow cooling process parameters of each example steel, table 3 shows heating process parameters of each example casting blank, table 4 shows rolling process parameters of each example steel plate, table 5 shows hot straightening and stacking slow cooling process parameters of each example steel plate, table 6 shows heat treatment process parameters of each example steel plate, and table 7 shows cutting process parameters and surface states of each example steel plate.

Table 1 chemical composition (wt.%)

Examples	C	Si	Mn	Mo	Als	B	Ti	Nb	Ni	Cr	Ca
												1	0.17	0.16	1.21	0.56	0.033	0.0013	0.018	0.025	0.42	0.39	0.007
2	0.14	0.32	1.19	0.65	0.045	0.0008	0.021	0.034	0.39	0.44	0.008
												3	0.16	0.27	1.33	0.79	0.025	0.0017	0.023	0.015	0.36	0.43	0.006
4	0.18	0.33	1.09	0.74	0.037	0.0010	0.014	0.036	0.49	0.38	0.008
												5	0.17	0.23	1.43	0.62	0.047	0.0013	0.019	0.032	0.38	0.47	0.010
6	0.19	0.29	1.26	0.55	0.036	0.0009	0.017	0.026	0.32	0.37	0.005

Note that the impurity element P in the steel is less than or equal to 0.02 percent, and S is less than or equal to 0.005 percent.

Table 2 parameters of process for smelting, continuous casting and slow cooling of cast blank in pit

Examples	Argon blowing time/min	RH vacuum degassing time/min	Degree of superheat/°c	Pulling speed/(m.min -1)	Electromagnetic stirring current intensity/A	Continuous casting billet reduction/mm	Secondary cooling specific water quantity/(L/kg)	Pit entry temperature/°c	Slow cooling temperature/°c	Slow cooling time/h	Pit temperature/°c
												1	28	28	17	0.9	120	36	1.6	620	630	50	130
2	25	25	23	1.3	140	33	2.0	615	640	70	150
												3	26	27	25	1.2	135	39	1.4	620	635	48	145
4	32	30	21	1.0	150	45	1.8	605	645	65	110
												5	30	32	24	1.1	140	35	1.9	640	650	72	120
6	33	28	20	0.8	125	38	1.7	630	640	50	130

TABLE 3 heating process parameters of casting blanks

Table 4 rolling process parameters of steel sheet

Examples	Rough rolling start temperature/°c	Rough rolling finishing temperature/°c	Rough rolling pass	First pass reduction/%	Second pass reduction/%	Finish rolling start temperature/°c	Finish rolling finishing temperature/°c	Finish rolling pass	First pass reduction rate/%	Second pass reduction rate/%	2-Pass descaling time/min	Descaling water pressure/MPa	Thickness/mm of intermediate blank	Rolling speed/m/s
															1	1130	1012	4	18	20	910	840	5	22	24	1.0、1.6	18、20	200	1.5
2	1125	1015	5	16	18	900	820	6	22	25	1.2、1.8	15、19	140	1.3
															3	1120	1025	6	16	20	885	825	5	22	23	1.3、2.0	16、18	180	1.6
4	1120	1005	5	16	18	880	830	5	22	23	1.2、1.8	15、18	200	1.7
															5	1135	996	4	17	19	900	840	6	22	25	1.7、1.9	16、20	150	1.2
6	1135	985	5	18	16	890	845	5	22	25	1.2、1.6	18、20	150	1.8

TABLE 5 thermal straightening and Stacking Cold Process parameters for Steel plates

Examples	Temperature of thermal correction/°c	Straightening force/kN	Roll bending amount/mm	Tilting value/mm	Stacking temperature/°c	Stacking time/h
							1	750	3200	1.2	3.6	500	50
2	785	4200	1.6	5.5	520	52
							3	770	4500	1.7	4.8	560	50
4	780	3500	1.5	4.5	530	49
							5	775	3600	1.8	3.8	525	60
6	760	4300	1.3	5.3	570	48

TABLE 6 Steel plate heat treatment Process parameters

Examples	Low temperature section temperature/°c	The low temperature section is in furnace time/min/mm	High temperature section temperature/°c	The high temperature section is in furnace time/min/mm	Temperature of cooling section/°c	The temperature reduction section is at furnace time/min/mm	Average cooling rate/°c/s	Quenching end temperature/°c	Tempering temperature/°c	Tempering time/min/mm
											1	660	2.8	880	1.7	780	0.6	25	200	160	2.5
2	680	3.2	905	1.5	800	0.8	30	230	180	3.0
											3	630	3.3	910	1.5	770	0.9	35	185	170	3.3
4	650	2.1	920	1.8	790	0.7	28	190	160	3.1
											5	670	2.6	885	1.6	810	0.8	40	220	150	2.2
6	665	2.8	890	1.7	805	0.7	36	230	165	2.6

TABLE 7 Steel plate cutting Process parameters and surface State after cutting

Examples	Plate thickness/mm	Preheating temperature/°c	Weld line energy/kJ/cm	Days of placement	With or without cracks
						1	60	150	18	65	Without any means for
2	40	180	15	80	Without any means for
						3	70	200	19	60	Without any means for
4	80	180	16	70	Without any means for
						5	45	178	18	100	Without any means for
6	50	190	17	120	Without any means for

The foregoing is only a preferred embodiment of the present invention, but the scope of the present invention is not limited thereto, and any person skilled in the art, who is within the scope of the present invention, should make equivalent substitutions or modifications according to the technical scheme of the present invention and the inventive concept thereof, and should be covered by the scope of the present invention.

Claims (4)

1. A method for avoiding delayed cracking after cutting a low-alloy high-strength steel plate is characterized by comprising the steps of controlling chemical components of the steel plate and controlling the production process of the steel plate, wherein the chemical components of the steel plate comprise ：C 0.14%～0.19%,Si 0.15%～0.35%,Mn 1.05%～1.45%,P≤0.02%,S≤0.005%,Nb 0.010%～0.040%,Ti 0.010%～0.025%,Mo 0.5%～0.8%,Als 0.02%～0.05%,Ca 0.005%～0.010%,B 0.0008%～0.0018%,Cr 0.3%～0.5%,Ni 0.3%～0.5%, percent by weight of Fe and unavoidable impurities, and the total content of impurity elements is lower than 0.05 percent;

the production process of the steel plate comprises the following steps:

1) Smelting molten steel until casting blank enters a pit, heating and slowly cooling:

Controlling the S content to be lower than 0.015% during molten iron pretreatment, controlling the P content to be less than or equal to 0.02% during converter smelting, controlling the converter smelting end point C content to be 0.14% -0.19%, blowing argon gas to be 25% -35 min during converter tapping, feeding Si-Ca wires to perform external refining treatment during LF refining, maintaining RH vacuum degassing for more than 25min, adding Al according to the Als content requirement after the degassing treatment, adding Ti-Fe to perform micro Ti treatment, adding B-Fe alloy to perform B alloying, controlling the superheat degree to be 15-25 ℃ during continuous casting, controlling the continuous casting pulling rate to be 0.8-1.3 m/min, controlling the electromagnetic stirring current intensity of a secondary cooling zone to be 100-150A during continuous casting, controlling the secondary cooling specific water amount to be 1.3-2.0L/kg, feeding the casting blank under heavy pressure at a horizontal fan-shaped section, feeding the casting blank into a pit after the casting blank is taken down, controlling the temperature to be not lower than 600 ℃ to be 600-650 ℃ and cooling time to be 48-72 h, and cooling the casting blank after the casting blank is taken down to the pit to be cooled down below 150 ℃;

2) The casting blank is reheated, wherein the temperature of a preheating section is 900-1050 ℃, the temperatures of a soaking section and a heating section are 1225-1250 ℃, the total heating time of the soaking section and the heating section is 4-5.5 hours, and the total furnace time is 6-8 hours;

3) The rolling and cooling are controlled, wherein the rolling is carried out in two stages, the initial rolling temperature of rough rolling is more than or equal to 1120 ℃, the final rolling temperature of rough rolling is 980-1025 ℃, the rolling speed of rough rolling is not more than 6 times, the rolling reduction rate of at least the first 2 times is more than 15%, at least 2 times of descaling water is sprayed in the process of waiting for the intermediate billet, the descaling time of each time is 1-2 min, the descaling water pressure is 15-20 MPa, the initial rolling temperature of finish rolling is 860-920 ℃, the final rolling temperature of finish rolling is 820-850 ℃, the rolling reduction rate of at least the first 2 times is more than 20%, the rolling speed of slow rolling is 1-2 m/s, the straightening temperature of hot straightening is 750-800 ℃, the straightening force is 3000-4500 kN, the bending roll amount is 1.2-1.8 mm, and the tilting value is 3.5-5.5 mm;

4) Stacking and slowly cooling the steel plates, namely immediately stacking and slowly cooling after hot straightening is finished, wherein the stacking temperature is not lower than 400 ℃, and the slowly cooling time is not less than 48 hours;

5) Quenching, wherein the quenching heating comprises a low temperature section, a high temperature section and a cooling section, the temperature of the low temperature section is 620-680 ℃, the temperature of the low temperature section is 2.0-3.5 min/mm in a furnace, the temperature of the high temperature section is 880-920 ℃, the temperature of the high temperature section is 1.5-1.8 min/mm in the furnace, the temperature of the cooling section is 760-810 ℃, the temperature of the cooling section is 0.5-0.9 min/mm in the furnace, the quenching average cooling speed is 20-45 ℃ per second, and the quenching finishing temperature is 180-250 ℃;

6) Tempering, wherein the tempering heating temperature is 150-180 ℃, and the tempering heat preservation time is 2.0-3.5 min/mm;

7) Cutting, wherein the preheating temperature of the steel plate is 100-200 ℃, the cutting is performed at normal temperature, the energy of a welding line is controlled below 20kJ/cm during cutting, the cutting edge and a heat affected zone are covered by an asbestos cloth heat insulation blanket after cutting, and the steel plate is slowly cooled to room temperature.

2. The method for preventing delayed cracking after cutting of low-alloy high-strength steel sheet according to claim 1, wherein the finished steel sheet is 1000 MPa-grade or more.

3. The method for preventing delayed cracking after cutting of the low-alloy high-strength steel plate according to claim 1, wherein the thickness of the finished steel plate is 40-80 mm, the thickness of a casting blank is 250-300 mm, and the thickness of an intermediate blank is 2.5-3.5 times that of the finished steel plate.

4. The method for preventing delayed cracking after cutting of low alloy high strength steel sheet according to claim 1, wherein in step 7), the steel sheet is preheated by flame gun, electric heating pad or furnace.