WO2023029282A1

WO2023029282A1 - Production method for high-strength steel plate for engineering machinery

Info

Publication number: WO2023029282A1
Application number: PCT/CN2021/136991
Authority: WO
Inventors: 彭宁琦; 杨建华; 周文浩; 李红英; 倪川皓; 罗登; 刘丹; 张勇伟; 刘海浪; 姚建华; 赵军; 钱亚军; 张青学
Original assignee: 湖南华菱湘潭钢铁有限公司; 中南大学; 中联重科股份有限公司
Priority date: 2021-08-30
Filing date: 2021-12-10
Publication date: 2023-03-09
Also published as: CN113846260B; CN113846260A

Abstract

A production method for a high-strength steel plate for engineering machinery. The components of the steel and the weight percentages thereof are as follows: C＝0.10％-0.25％, Si ≤ 0.80％, Mn＝0.80％-1.20％, P ≤ 0.010％, S ≤ 0.0015％, Al＝0.005％-0.015％, Cr ≤ 1.0％, Mo ≤ 1.0％, Ni＝0.30％-2.0％, Cu ≤ 0.30％, Ti ≤ 0.015％, Nb ≤ 0.020％, V ≤ 0.080％, B ≤ 0.0040％, N ≤ 0.0040％, O ≤ 0.0010％, and H ≤ 0.00015％, with the balance being Fe at ≥95％ and inevitable impurities. The process steps for the steel plate comprise: smelting, refining, continuous casting, rolling, and heat treating. The structure of the steel plate comprises a lath martensite matrix and a small amount of residual austenite, wherein 5％-10％ of the residual austenite is on the upper surface of the steel plate, and 2％-5％ of the residual austenite is on the lower surface of the steel plate. The steel plate of the present invention has good comprehensive mechanical properties and pure steel quality, has homogeneous properties in the transverse and longitudinal structures and a rolling direction, has a good plate shape and good surface quality, and has low residual stress and exhibits excellent whole plate bending capacity in the rolling direction. The steel plate may be used in the manufacturing of large engineering machinery such as cranes and pump trucks.

Description

A production method of high-strength steel plate for construction machinery

technical field

The invention belongs to the technical field of metallurgy and relates to a production method of high-strength low-alloy steel wide and thick steel plates for engineering machinery.

Background technique

Construction machinery refers to the general term for the mechanical equipment required for the construction of national defense, transportation, construction, water conservancy, mining, energy industry and other projects. Since the main components of construction machinery often bear complex and variable heavy loads, the quality requirements for their materials are extremely strict. Low-alloy high-strength steel has good strength and toughness, weldability, crack resistance, fatigue resistance and formability, and is widely used in the field of construction machinery steel.

With the advancement of modern equipment manufacturing technology and the increasing requirements of large-scale engineering projects to improve construction efficiency, reduce energy consumption, and save costs, construction machinery is developing in the direction of large-scale and lightweight construction, which requires the application of higher-strength steel. At present, low-alloy high-strength steel for construction machinery has gradually developed from Q550, Q690 to Q960, Q1100, and even Q1300. As the yield strength of steel increases to above 960MPa, its plasticity and toughness decrease obviously, and the welding performance deteriorates. According to the classical plasticity theory, when the material is bent, the outer surface is stretched to produce plastic deformation, and when the elongation limit of the material is exceeded, rupture will occur. Therefore, for high-strength steel plates with yield strength ≥ 960MPa, due to low plasticity and high cold bending resistance, cracks and inaccurate dimensional accuracy are prone to occur during bending, and this has become a prominent problem in the actual use of this type of steel. Therefore, while meeting the high-strength requirements of steel plates for construction machinery, how to better improve the plasticity and bending capacity of steel plates through new processes or new technologies is a difficult problem that international wide and thick plate iron and steel enterprises hope to solve.

For low-alloy high-strength steels dominated by martensite matrix, increasing the amount of retained austenite can effectively improve the plasticity and toughness of steel, which has long been accepted by scholars. But how to improve the stability of retained austenite in quenched steel? It is generally believed that in addition to adding more carbon and alloying elements and controlling the supercooled austenite grain size and cooling rate to reduce the martensitic transformation start temperature M _s and end temperature M _f to increase the amount of retained austenite In addition, there are mainly two ways: (1) Quenching the supercooled austenite to a certain temperature between M _s and M _f , holding it for a period of time and then continuing to cool, can make the untransformed austenite more stable, Subsequent transformation will be carried out at a lower temperature, and the amount of transformation will be less than that of continuous cooling, that is, the thermal stabilization of austenite. Speer et al. used this phenomenon to propose a Q+P heat treatment process, that is, Quenching + Partitioning. In order to enhance the carbon distribution effect and hinder the precipitation of Fe ₃ C, the Q+P steel they designed contains 1% to 2% Si. This is applied in TRIP (Transformation-Induced Plastic) steel. But there is no doubt that the reduction in the amount of martensite is not conducive to the strength of steel. Xu Zuyao introduced the precipitation hardening mechanism on the basis of the Q+P process, and proposed the Q+P+T process, that is, adding carbide-forming elements Nb, Mo, etc. to the steel, and then after the carbon is distributed from the martensite to the retained austenite , continue to temper at a certain temperature (Tempering) to precipitate complex carbides on the martensite matrix to obtain higher strength and toughness; (2) plastically deform the austenite before quenching or subject it to compressive stress , can make the subsequent martensite transformation difficult, and the amount of transformation is reduced, that is, the mechanical stabilization of austenite. Retained austenite in low-carbon steel is closely related to the mechanical stabilization of austenite. When austenite transforms into martensite, volume expansion occurs, so that the untransformed austenite is lost due to the additional pressure of surrounding martensite. Continue to transform the conditions, so even if it is cooled to below the M _f point, 100% martensite cannot be obtained, and a part of retained austenite remains.

The DQ process (Direct quenching), developed half a century ago, makes full use of the flattened grains and high dislocations of deformed metastable austenite, so that small and uniform crystal clusters (Packet) and plates can be obtained during martensitic transformation. Block and a larger number of retained austenite and austenite film (Austenite film), so as to obtain high strength and improve the ductility and toughness of steel. The technology of producing high-strength steel and ultra-high-strength steel by DQ process is relatively mature. The developed ultra-fast cooling equipment mainly includes UFC of CRM, Super-OLAC of JFE, MULPIC of SIEMENS VAI, ADCOS-PM of RAL of Northeastern University, etc. . Based on the concept of Q&P, Thomas et al. developed a new DQ+P process and introduced it into the practice of hot rolling production line. The research of Li Xiaolei et al. showed that, using the DQ+P process, even for low-carbon steel with a carbon content of only 0.078%, a retained austenite as high as 7.2% can be obtained. Another typical application of the DQ+P process is the online heat treatment equipment HOP developed by JFE, which has been used to develop some high-tech products such as high-strength steel plates for earthquake-resistant buildings with low yield ratio, and the effect is remarkable.

Searching the existing technical patents shows that high-strength steel plates with a yield strength of 960 MPa and above are mainly produced by Q+T or DQ+T processes, and the structure design is usually a relatively simple tempered martensite or tempered sorbite structure, and the residual austenite The body is decomposed in a large amount during the tempering process, and its performance is characterized by high yield ratio and elongation usually between 8% and 15%, such as Chinese patents CN103882332A, CN104513936B, CN102747303B, CN102505096B, etc. However, in order to improve the bending performance, users desire to increase the elongation of steel to 15% or more. Therefore, especially for Q1100 steel and Q1300 steel with higher strength levels, it is difficult to achieve such a high elongation requirement by the traditional tempering process. Chinese patent CN102226248B adopts Q+P process, which can obtain more than 10% retained austenite in the martensite matrix, but the strength of the steel is low, and when the elongation is 15%, the yield strength only reaches 870MPa. Chinese patent CN102925802B also adopts the Q+P process, which adds higher Mn and more microalloying elements Nb, V, and Ti to compensate for the strength loss caused by the reduction of martensite, but Mn and Nb are prone to central segregation , is especially harmful to high-strength steel, and adding excessive Ti (0.035% to 0.045%) will form undeformable large-sized TiN and TiC particles with sharp edges and corners, which is not only unfavorable to uniform elongation, but also seriously affects the fatigue properties of high-strength steel . Chinese patent CN107557548B adopts Q+P+T process in the laboratory to obtain Cr-Ni-Mo low-alloy ultra-high-strength steel with yield strength ≥ 1200 MPa and elongation ≥ 15%, but it adds 0.20% to 0.40% carbon and relatively Too much Cr, Ni, Mo and other alloying elements will obviously deteriorate the weldability of steel. Chinese patent CN104046908B adopts DQST process (that is, DQ+P process), which can obtain ultra-high strength, good low-temperature plastic toughness, delayed crack resistance and weldability with less alloying elements, but compared with offline quenching, online quenching steel plate It is more difficult to control the shape, and there is inevitably a certain difference in the quenching temperature of the head, middle and tail of the steel plate, resulting in poor performance uniformity in the direction of rolling, especially for wide and thick plates below 16mm in thickness, and there are certain limitations in practical application. Chinese patent CN106191673B, in order to preserve the retained austenite, adopts Q+ strong straightening process to produce a quenched ultra-high strength steel plate, and uses strong cold straightening instead of low temperature tempering to remove quenching residual stress, but strong cold Straightening equipment, and the thickness of the plate is limited to 4-20mm due to the straightening ability.

Therefore, the main problem of the prior art is: for quenched martensitic steel, if the traditional tempering process is adopted, the plasticity is low, and if the thermal stabilization or mechanical stabilization of austenite is used to improve the plasticity, the strength is reduced; or At the cost of sacrificing welding performance, fatigue performance, plate shape accuracy, surface quality, performance uniformity, etc.; or it will cause problems such as high residual stress, especially microscopic stress. In addition, most of the existing technologies focus on improving the plasticity of the steel plate to improve the bending performance of the steel plate. The bending ability also has an impact that cannot be ignored; and when the steel plate is bent, how to match the impact of strong plasticity on improving the springback of the steel plate has never been involved.

Contents of the invention

The purpose of the present invention is to provide a production method of high-strength steel plate for engineering machinery, by using a lower carbon equivalent and ingenious heat treatment process design to control the distribution of residual austenite on the thickness of the steel plate, and at the same time reduce the adverse effects on the bending performance of the steel The inclusions, central segregation and banded structure improve the flatness, surface quality and uniformity of structure and performance of the steel plate, so that the produced steel plate has good strong-plastic matching and excellent bending ability of the whole plate along the rolling direction.

Technical scheme of the present invention:

A production method of a high-strength steel plate for engineering machinery, the chemical composition weight percentage of the steel is C=0.10%-0.25%, Si≤0.80%, Mn=0.80%-1.20%, P≤0.010%, S≤0.0015%, Al=0.005%～0.015%, Cr≤1.0%, Mo≤1.0%, Ni=0.30%～2.0%, Cu≤0.30%, Ti≤0.015%, Nb≤0.020%, V≤0.080%, B≤0.0040% , N≤0.0040%, O≤0.0010%, H≤0.00015%, the balance is ≥95% Fe and unavoidable impurities; the structure of the steel plate is lath martensite matrix and a small amount of retained austenite, among which There are 5% to 10% retained austenite on the surface, and 2% to 5% retained austenite on the lower surface of the steel plate; the production process steps include:

(1) Smelting: use converter smelting and slag blocking to tap; control endpoint O≤0.03%;

(2) Refining: RH vacuum light treatment deoxidation, line feeding precipitation deoxidation, LF heating desulfurization and alloying, vacuum deep treatment degassing, and then soft blowing time ≥ 12min;

(3) Continuous casting: full protection pouring is carried out; the superheat of the tundish is controlled at 5-15°C, and dynamic light reduction is adopted to control the low power of the billet to meet the Mannesmann standard center segregation level 1-2;

(4) Rolling: Heat the slab to 1100-1250°C, and then roll it after descaling with high-pressure water; after cooling to 300°C, heat straightening is completed, and then the cooling rate is lower than 15°C/h Slowly cool to room temperature, carry out hydrogen expansion treatment;

(5) Heat treatment: Carry out flaw detection and surface shot blasting on the steel plate before heat treatment; then heat to 820-930°C, the heating time is (1.0-2.0)min/mm×H+(10-20)min, then start quenching and cooling, control The cooling rate between the quenching start temperature T ₁ and the cooling stop temperature T ₂ is 10-30°C/s, and the T ₂ on the upper surface falls within the temperature range of M _f + (20-50)°C through water ratio control, The T ₂ of the lower surface falls within the temperature range of M _f- (20～50)℃; after quenching, the steel plate is quickly sent to the tempering furnace for heat preservation, the heat preservation temperature is 200～300 ℃, and the heat preservation time is (2.0～4.0)min /mm×H+(10～20)min; where H is the thickness of the steel plate, in mm; M _f is the end temperature of the martensitic transformation, which varies with the quenching cooling rate between 250 and 350°C.

Further, for high-strength steel of steel grade Q960, preferably, C=0.10%-0.15%, CEV≤0.52%.

Further, for high-strength steel with steel grade Q1100, preferably, C=0.15%-0.20%, CEV≤0.62%.

Further, for high-strength steel of steel grade Q1300, preferably, C=0.20%-0.25%, CEV≤0.72%.

The CEV is carbon equivalent, CEV=C+Mn/6+(Cr+Mo+V)/5+(Ni+Cu)/15.

go a step further,

The composition of the high-strength steel is preferably Si=0.30%-0.50%, Nb=0.010%-0.020%. More preferably, Ti≤0.008%, most preferably Ti≤0.003% and N≤0.0030%. Further preferably, the content of solid solution boron is controlled. When the thickness of the steel plate is ≤20mm, B ^* =0.0008%-0.0015%; when the thickness of the steel plate is between 20-50mm, B ^* =0.0012%-0.0020%; At 50mm, B ^* =0.0015%～0.0025%; where B ^* =B-0.77×N+Ti/5. Still more preferably, the unavoidable impurities include As≦0.008%, Sn≦0.005%, Sb≦0.005%. The above are all calculated by weight percentage.

In the step (1), preferably, a top-bottom combined blowing converter is used, and argon is blown at the bottom during the whole process of smelting and tapping; the selected molten iron is desulfurized by KR pretreatment, and the control S≤0.005%, and P≤0.15% ;The selected slagging materials include lime, lightly burned dolomite, fluorite and iron ore, and the slag volume is controlled to be ≥82kg/t; the selected coolant includes alloy materials of heavy scrap, molybdenum, nickel, and copper, and the control Molten iron ratio ≥ 90%.

In the step (2), preferably, the vacuum degree of light vacuum treatment deoxidation is ≤10kPa, and the treatment time is ≥12min; the treatment time of heating up desulfurization is ≤60min; If the hydrogen is less than 0.00012%, it is best to use a VD furnace for strong stirring, and the bottom blowing argon flow rate is ≥600L/min. More preferably, O≤0.004% after vacuum deoxidation, and then adopt the method of feeding calcium wire first and then aluminum wire for precipitation deoxidation, and control Ca=0.0015%-0.0040%.

In the step (3), preferably, the moisture content of the mold flux and the covering agent is lower than 0.25%; the baking temperature of the tundish is not lower than 1100°C, and the baking time is 150-300 minutes; the dynamic light reduction rate is controlled at 1.0- Between 1.2mm/m; the billet is cooled by stacking, it is best to add a heat preservation cover, when the temperature is below 300 ℃, remove the stack, and clean the upper and lower surfaces of the billet with flame, the cleaning depth is ≥ 5mm, and the cleaning The temperature is preferably ≥150°C;

In the step (4), preferably, the furnace temperature in the preheating section of the heating furnace is ≤600°C, the time in the preheating section is ≥120min, and the soaking temperature in the soaking section is controlled at 1150-1200°C; The gas ratio is ≤20%, and the pressure in the furnace is controlled within the range of 5-20Pa; the pressure of the descaling water is guaranteed to be ≥22MPa, and the surface temperature of the slab is between 880-980°C during rolling; the rolling in the recrystallization zone is controlled Compression ratio ≥ 2.5, stretch ratio ≥ 1.5, control rolling compression ratio in non-recrystallized area ≤ 3.0, pass reduction rate ≤ 15%; control final rolling temperature 820 ~ 860 ℃; after rolling, when the steel plate is cooled to 300 ~ 350 ℃ After straightening, the steel plate is quickly hoisted into the slow cooling pit or stacked to cool to room temperature, and the cooling time is controlled to be ≥ 24h.

In the step (5), preferably, a normalizing pretreatment process is added before the heat treatment, the normalizing temperature is 850-930°C, and the final heat treatment of this step is performed after normalizing; the heating temperature of the final heat treatment is controlled at 820-850°C , and then air-cooled to 600-650°C before starting quenching; after quenching and heat preservation, air-cool or stack-cool to room temperature, and decide whether to add a cold straightening process according to the flatness of the steel plate.

Technical principle of the present invention:

In the composition design of steel, the present invention adopts low Mn, low S, P, O, N, H, low As, Sn, Sb, and controls B, Ti, Nb, V, Al nitride, carbide, oxidation The types and forms of compounds and sulfides, and adding hardenable elements such as Cr, Mo, Ni, Cu, B, etc., the steel has high purity and low carbon equivalent, so that the steel has good comprehensive mechanical properties. Specifically, the setting basis of each component is as follows:

C: Carbon is a solid solution strengthening element. The strength of quenched martensitic steel mainly depends on the carbon content in martensite; carbon is also a hardenability element, which is helpful for low-temperature martensite transformation; It can effectively improve the strength of steel; at the same time, as an austenite stabilizing element, the carbon content has a great influence on the amount of retained austenite. Therefore, in order to obtain high-strength steel with a certain strength and improve its plasticity at the same time, it is necessary to ensure at least a lower limit of carbon content, but carbon is not good for the weldability of steel, and if the carbon content is too high, it will lead to serious central segregation and an increase in the ductile-brittle transition temperature. High, quenching is prone to cracks and other problems, so the carbon content should be reduced as much as possible under the condition that the strength requirements of the steel can be met. The present invention controls the carbon content between 0.10% and 0.25%, and for Q960 steel, the preferred carbon content is 0.10% to 0.15%; for Q1100 steel, the preferred carbon content is 0.15% to 0.20%; for Q1300 steel, the preferred carbon content is 0.20% to 0.25%.

Si: Silicon also has strong solid solution strengthening effect, but too high silicon content is not good for the surface quality of steel, and it is easy to generate unevenly distributed iron oxide scales that are difficult to remove, and because it promotes the coarseness of crystal groups and lath groups It is detrimental to the low temperature toughness of the martensite matrix. In the present invention, it is further considered that silicon can effectively hinder the formation of carbides. On the one hand, it is beneficial to the stability of retained austenite, increasing the amount of retained austenite, which is beneficial to the plasticity of steel; on the other hand, it can delay the Partitioning process of martensite Decomposition, improve the tempering stability of 200 ~ 300 ℃ heat preservation, inhibit low temperature temper brittleness. Therefore, the present invention designs that the silicon content is not more than 0.80%, and preferably the silicon content is between 0.30-0.50%.

Mn: Manganese is a hardenability element, which reduces the critical quenching speed and M _s point of steel, improves the stability and quantity of retained austenite, and at the same time refines the packet structure scale and increases the orientation difference between block structures, thereby It is beneficial to improve the strength and ductility of the steel plate. However, manganese is prone to segregation during the solidification process of molten steel, especially when the carbon content in the steel is high, the conjugate segregation phenomenon will cause severe central segregation of the billet, resulting in the formation of abnormal structures in the center of the steel plate and affecting weldability, etc. More importantly, the long strip-shaped MnS inclusions with more segregation areas are easy to gather hydrogen atoms, causing cracks to nucleate at MnS and propagate along the segregation area, forming serious delayed cracks; in addition, Mn also promotes the formation of harmful elements at grain boundaries The segregation on the steel increases the temper brittleness tendency of the steel. Therefore, it is extremely important to select an appropriate range of Mn content for high-strength martensitic steel. The present invention controls the manganese content between 0.80% and 1.20%.

P: Phosphorus is a harmful element in steel, which is not good for plasticity and toughness, and also affects welding performance, especially after segregation. Theoretically, the lower the requirement, the better, but considering the operability and economy of steelmaking, the present invention controls the phosphorus content to not more than 0.010%.

S: Sulfur is also a harmful element in steel, especially combined with Mn to form MnS inclusions. During hot rolling, the plasticity of MnS makes MnS extend along the rolling direction to form long strips of MnS inclusions along the rolling direction, which is serious It will damage the transverse mechanical properties of high-strength steel plates, as well as Z-direction properties, bending properties along the rolling direction, and hydrogen-induced delayed crack resistance. The present invention does not require the content of Ca and other sulfide modifying elements, because calcium is very easy to evaporate in vacuum, and feeding calcium wire after vacuum degassing often causes secondary pollution of molten steel and brings more inclusions. The present invention reduces the amount of MnS inclusions by reducing Mn content and ultra-low S control, controls the degree of segregation, and controls the temperature range for a large amount of MnS precipitation to control its size and distribution. Further, the present invention uses Ca precipitation deoxidation to retain a small amount of CaO and CaS in molten steel, and utilizes BN particles precipitated at higher temperatures to become the nucleation core of MnS, thereby significantly reducing strip-shaped MnS inclusions. In the present invention, hot metal pretreatment and LF+VD refining desulfurization are preferred, and the ultra-low sulfur requirement of ≤0.0015% is effectively controlled.

Al: Aluminum is a strong deoxidizing element. In order to control the oxygen content in steel to ≤0.0010%, sufficient aluminum content (≥0.005%) is required, but too much aluminum will lead to more Al ₂ O ₃ etc. in the steelmaking process Inclusions remain, and in order to prevent AlN from competing with BN and (Nb, V) (C, N) in the present invention, the upper limit of aluminum should be controlled. Therefore, the present invention controls the aluminum content between 0.005% and 0.015%.

Cr: Chromium is a hardenability element, which has a particularly strong effect on delaying bainite transformation. Chromium can improve the segregation of manganese; shift the decomposition temperature of retained austenite to high temperature; increase the orientation difference between martensite lath clusters. Therefore, adding a certain amount of chromium to martensitic steel will not affect the plasticity and toughness of the steel while increasing the strength. However, if the chromium content is too high, it will lead to an increase in carbon equivalent, which will deteriorate the weldability of the steel, especially easy to form upper bainite structure in the welding heat-affected zone and make it embrittled. In addition, chromium also increases the tendency to temper embrittlement. Therefore, the present invention controls the chromium content to be no more than 1.0%.

Mo: Molybdenum is a hardenability element, which inhibits the formation of massive ferrite, promotes martensite transformation, and can refine the size of martensite laths, but the orientation difference between lath clusters is small, so an appropriate amount of molybdenum The content can improve the toughness of martensite, but if it exceeds a certain amount, the toughness will be reduced instead. In addition, molybdenum also has the effect of reducing the tendency of temper brittleness; compared with chromium, the carbide particles of molybdenum are finer. In quenched steel, chromium and molybdenum are usually added at the same time to make the steel have sufficient hardenability and obtain better comprehensive mechanical properties. The present invention controls the content of molybdenum to not more than 1.0%.

Ni: Nickel is an austenite stabilizing element, which can significantly improve the low-temperature toughness of steel and effectively prevent copper embrittlement of steel. It is a very important element in the present invention and needs to be added at least 0.30%. Nickel is a precious metal with a high price, and if the content is too high, it is easy to cause the iron oxide scale to be pressed in and affect the surface quality of the steel plate. The present invention determines that the nickel content ranges from 0.30% to 2.0%.

Cu: Copper is an austenite stabilizing element that increases the strength and low temperature toughness of steel. Copper has excellent resistance to atmospheric corrosion and has a significant improvement on stress corrosion. However, if the amount of copper added is too much, it is easy to cause copper brittleness, the surface quality of the slab and internal cracking problems. The present invention controls the copper content to not more than 0.30%.

Ti: Titanium and nitrogen have a strong affinity to form TiN. In order to avoid liquid precipitation of TiN in molten steel, the content of titanium and nitrogen should be controlled. At the same time, the welding should be controlled according to the control level and requirements of nitrogen (N≤0.0040%) The obvious embrittlement phenomenon caused by the coherent precipitation of excessive titanium in martensite with TiC, so the present invention stipulates that Ti≤0.015%; further considering that when the Ti/N ratio is around 2.0 during welding, it has better toughness, the present invention is preferred Ti≤0.008%; because TiN precipitates are hard, have sharp corners, and are not easily deformed, which is unfavorable to bending performance. Therefore, it is more preferable in the present invention not to add titanium, only retain the remaining titanium, and control Ti≤0.003% and N≤0.0030%.

Nb: The solute dragging effect of niobium and the nanoscale precipitates have a pinning effect on the austenite grain boundary, hindering the recrystallization and grain growth of austenite, and refining the austenite during rolling and quenching heating The role of grains, experiments show that 0.01% niobium can start to play a role. However, for high-strength steels with high carbon content, excessive niobium cannot cause sufficient solid solution and lose its effective effect, and even micron-sized NbC is often left in the central segregation part, which is quite unfavorable for cold bending performance and increases the risk of hydrogen-induced cracks . Therefore, the present invention controls the upper limit of niobium to not more than 0.020%, and adds vanadium to control the austenite grain size by relying on the composite precipitation of niobium and vanadium. In the present invention, the preferred control range of niobium content is 0.010%-0.020%.

V: Vanadium not only has the effect of precipitation strengthening, but also has the effect of refining grains together with niobium, but when the addition amount is greater than 0.080%, the amount of precipitates is large, which affects the ductile-brittle transition temperature of steel, so the present invention limits V≤0.080 %.

B: Boron is a hardenable element. A small amount of boron in solid solution is easy to be adsorbed on the grain boundary, which can significantly prevent the phase transformation of ferrite and pearlite, improve the stability of austenite, and most importantly, reduce The hardness gradient of the small quenched steel improves the over-quenching of the surface layer, and the more solid solution, the more obvious the effect. However, the solid solubility of boron in α-Fe is limited, and the formed borides such as Fe ₂₃ C ₃ B ₃ are unfavorable to the toughness of steel. Therefore, the present invention controls B≤0.0040%, and considering the influence of BN and TiN, the solid solution boron B ^* is about B ^* =B-0.77×N+Ti/5, so preferably, when the steel plate thickness is ≤20mm, B ^* ＝0.0008%～0.0015%; when the steel plate thickness is between 20～50mm, B ^* ＝0.0012%～0.0020%; when the steel plate thickness is ≥50mm, B ^* ＝0.0015%～0.0025%.

N: Nitrogen is an inevitable element in steel. Since solid solution or free nitrogen is very detrimental to the impact toughness of steel, especially the aging impact toughness, it is necessary to form nitrides to avoid the existence of free nitrogen. In the present invention, TiN and BN are mainly formed, which have the effect of hindering the growth of austenite grains at high temperatures of 1100-1400°C, but this effect can be realized only with a very small amount of nitrogen, especially the relative mass of BN is small, which is harmful to For the same weight of N, its precipitation volume fraction is more. Considering the control ability and economy of steelmaking, the present invention controls N≤0.0040%, and when titanium is not intentionally added, N≤0.0030% is preferred.

O: Oxygen is a harmful element with high content and many inclusions, which is unfavorable to the bending performance of the steel plate. The present invention adopts RH vacuum deoxidation, wire feeding precipitation deoxidation, diffusion deoxidation during LF furnace white slag desulfurization, and controls the oxygen content of converter tapping from the source, and adopts slag blocking, soft blowing, protective pouring, and avoids secondary Measures such as oxidation can control O≤0.0010%.

H: Hydrogen is a harmful gas element. Hydrogen-induced delayed cracking is one of the main reasons for the failure of martensitic high-strength steel during cutting and bending. The invention strictly controls H≤0.00015%, forms hydrogen traps through nano-precipitated phases, reduces the amount and accumulation of diffusible hydrogen, and effectively prevents the occurrence of hydrogen-induced delayed cracks.

Impurity elements As, Sn, Sb: Arsenic, tin, and antimony are difficult to remove in the steelmaking process, and need to be controlled from the quality of raw and auxiliary materials such as molten iron, scrap steel, slag, and alloy materials. For the high-strength steel of the present invention, it is more harmful, and it is best to control As≤0.008%, Sn≤0.005%, and Sb≤0.005%.

In terms of process formulation, in order to meet the composition design requirements of steel, the present invention adopts purified steelmaking, including: (a) using high-quality raw and auxiliary materials, such as using heavy waste, limiting the content of P in molten iron, using low-moisture mold slag and covering (b) use molten iron pretreatment, LF refining and VD strong stirring vacuum treatment for ultra-low S smelting; (c) use top-bottom combined blowing converter and smelting with large amount of slag to remove S, and strictly prevent slag from tapping Back to P; (d) Large amount of slag in the converter, blowing argon at the bottom of the whole process, increasing the ratio of molten iron, prohibiting supplementary blowing, tapping without deoxidation to maintain the activity of oxygen in molten steel, controlling LF refining time, ensuring the vacuum degree and maintenance of vacuum degassing Measures such as vacuum time, adding alloy materials and wire feeding after avoiding vacuum, and carrying out full-process protective pouring are all beneficial to control the N content; (e) Adopt three deoxidation methods of vacuum, precipitation and diffusion, and limit the O content at the end of the converter to control molten steel Overoxidation, control of aluminum content and use of calcium deoxidation to control the type and form of oxides, ensure soft blowing time to fully float oxidized inclusions, avoid secondary oxidation of molten steel after vacuum, etc., effectively realize the control of low O and low inclusions; (f) In order to control H ≤ 0.00015%, first ensure that the hydrogen concentration of molten steel after vacuum degassing is ≤ 0.00012%, and then use the whole process of protective pouring, control the moisture content of mold slag and covering agent, bake the tundish, and stack the slabs Cool and cover the heat preservation cover and steel plate at a temperature below 300°C and slowly cool to room temperature for hydrogen expansion.

On the basis of reducing the content of easily segregated elements, the invention adopts low degree of superheat and dynamic soft reduction to control the center segregation of the slab, and improves the degree of segregation through the phase transformation process and homogeneous process of multiple cooling and heating. In order to improve the surface quality of the steel plate, the present invention cleans the entire slab of the cast slab, which can not only remove the near-surface defects that will be worsened by subsequent rolling, but also reduce the thickness of the primary oxide scale, which is beneficial to descaling and removal. At the same time, control the heating temperature, ensure the slight positive pressure of the heating furnace, reduce or eliminate the use of coke oven gas that contains more S and water vapor but has a higher calorific value, ensure the descaling water pressure, and operate in the temperature range where the secondary oxide scale is easy to peel off Descaling treatment before rolling, surface shot blasting before heat treatment and many other measures to prevent scale from being pressed in. The present invention makes it easy to control the rolling plate shape by controlling the rolling reduction and the final rolling temperature, and ensures the flatness of the finished steel plate through hot straightening, heat treatment to release stress or even cold straightening, especially when the martensitic transformation is basically completed It is extremely beneficial to control the flatness of the sheet after hot straightening. The present invention effectively eliminates the banded structure by controlling the center segregation degree, the rolling temperature and flattening degree in the transverse and longitudinal directions, and adopts the normalizing pretreatment process, so that the grains are refined and the transverse and longitudinal structures are uniform. High-temperature rolling and off-line heat treatment process make the performance of the steel plate uniform in the direction of rolling. The formulation of these processes is conducive to improving the bending performance of the steel plate, especially the bending ability along the rolling direction.

In the final heat treatment process of the present invention, by controlling the quenching cooling rate and the cooling stop temperature of the upper and lower surfaces of the steel plate, the upper surface of the steel plate is subjected to the Q+P process, and the lower surface is subjected to the Q+on-line T process, thereby controlling the residual austenite on the thickness of the steel plate distribution, so that the upper surface of the steel plate has 5% to 10% retained austenite, and the lower surface of the steel plate has 2% to 5% retained austenite, so that the upper surface of the steel plate has better plasticity and the lower surface has higher strength. When the whole plate is bent, the upper surface becomes the outer surface of the bend, and the lower surface becomes the inner surface of the bend. In this way, the deformation of the inner and outer surfaces during the bending process is fully utilized, and there is not only enough plasticity to prevent cracks on the outer surface, but also It has enough strength to restrain springback and improve bending dimensional accuracy. And immediately after a long time of low temperature insulation, the slight buckling deformation caused by different cooling processes on the upper and lower surfaces of the steel plate can be recovered, and the stress generated can basically be released, and this leads to a small tensile residual on the upper surface of the steel plate The stress does not increase the difficulty of bending the steel plate inwardly. In order to obtain more, more stable and more dispersed retained austenite, the process of heating in the critical zone and air cooling to a certain temperature before quenching is used to make the initial austenite grains finer, more grain boundaries, and alloy elements A concentration gradient is generated and carbon is fully enriched. The results show that the steel plate produced in this way not only has higher plasticity, but also has higher strength and toughness.

Beneficial effects of the present invention:

(1) The Q960, Q1100, Q1300 high-strength steel plates produced according to the method of the present invention have good comprehensive mechanical properties: the strength meets the requirements of GB/T 16270 and GB/T 28909 standards; elongation after fracture and impact toughness are far greater than the standard value , elongation after breaking ≥ 15%, -40 ℃ Charpy (V-type) impact energy ≥ 47J; 5 times thick and wide cold bending 180 ° without cracks (the upper surface of the steel plate is bent towards the lower surface. The thickness of the steel plate is ≤ 16mm, bending The core diameter is 3 times the plate thickness; the steel plate thickness > 16mm and ≤ 25mm, the bending center diameter is 4 times the plate thickness; the steel plate thickness is > 25mm, thinned to 25mm, the upper surface of the steel plate is retained, and the bending center diameter is 100mm).

(2) The low-alloy high-strength steel produced by the present invention has pure steel quality, uniform horizontal and vertical structure and rolling direction, good plate shape and surface quality, low residual stress, and excellent bending ability of the whole plate along the rolling direction ; And the composition design idea is reasonable, the carbon equivalent is low, and the addition of alloy elements is less, so that the steel has good welding performance and good economy. It can be applied to the manufacture and lightweight of key load-bearing components of large-scale construction machinery such as cranes and pump trucks, and has broad application prospects.

(3) The main equipment required for the production method of the present invention are top-bottom combined blowing converter, KR, LF, RH, VD, straight arc continuous casting machine with dynamic light reduction, walking beam type heating furnace, high-pressure water purifier Scale box, wide and thick plate rolling mill, hot straightening machine, shot blasting machine, quenching and tempering line, normalizing furnace, cold leveling machine, etc., these equipments are equipped or have similar equipment in most wide and thick plate steel mills today, so the present invention The method can be popularized and applied, and has strong practicability; and the method of the present invention provides clear and specific production steps and processes for realizing the composition, structure, performance and quality requirements of steel, has strong operability, and at the same time, the production rhythm is reasonable and continuous production is possible. , Bulk supply.

(4) The low-alloy high-strength steel plate produced by the present invention can effectively promote the development of the construction machinery equipment manufacturing industry to the green manufacturing road of large-scale, lightweight and high-efficiency. In addition, the technology of purified steelmaking, steel plate surface quality control, structure and performance regulation and other technologies involved in the present invention can be used for reference in the construction of pressure vessels, wind power, hydropower, nuclear power, tools and molds, heat-resistant, wear-resistant and other high-strength steels or ultra-high-strength steels. development and production.

Description of drawings

Fig. 1 is a schematic diagram showing the quantity and morphology of retained austenite in the upper and lower surface structures of the steel plate according to the embodiment of the present invention and its bending.

Fig. 2 is a schematic diagram of bending forming of the whole steel plate along the rolling direction according to the embodiment of the present invention.

Detailed ways

The present invention will be further described below with embodiment.

The chemical composition, steel grade and plate thickness of the steel plate in the embodiment of the present invention are shown in Table 1, and the balance is Fe and unavoidable impurities.

The process steps of the steel plate in the embodiment of the present invention include: smelting (hot metal pretreatment), refining, continuous casting, rolling, heat treatment (normalizing pretreatment), and its key process steps include:

(1) Smelting: A 150t top-bottom combined blowing converter is used for smelting. The molten iron in all examples is desulfurized by KR pretreatment. The selected coolant includes trimming and plate waste, as well as ferromolybdenum, nickel plate and copper plate. The slagging material includes lime , lightly burned dolomite, fluorite and iron ore, bottom-blown argon throughout the smelting process and tapping process, when the blowing process reaches about 4/5, use the secondary gun to measure the temperature and take samples, and then reach the end goal in one blowing process, and the output The steel end adopts the sliding plate to stop the slag. The specific process parameters are shown in Table 2.

(2) Refining: After tapping, transport the ladle furnace to the RH workstation for light treatment and deoxidation, then break the vacuum, feed calcium wire + aluminum wire or aluminum wire for precipitation deoxidation, let it stand for a few minutes, and then transport it to LF for heating to make white slag After desulfurization and alloying, VD is used for vacuum deep treatment and degassing. After the degassing reaches the target, it will leave the station, and ensure a period of soft blowing to fully float the inclusions. During this period, no alloy material and wire feeding will be added. The specific process parameters are shown in Table 3.

(3) Continuous casting: adopt straight-arc continuous casting machine and full protection of submerged nozzle, long nozzle and tundish baking to cast high-quality slab, and control the low magnification of the slab to meet the Mannesmann standard center segregation 1~2 Grade, otherwise it is judged as unqualified product; after the continuous casting slab is cut and deburred, it is stacked off-line and cooled, and covered with a heat preservation cover, and then unstacked when it is cooled below 300°C, and the upper and lower surfaces of the entire board are flame-cleaned , cleaning depth of 5 ~ 8mm. The specific process parameters are shown in Table 5.

(4) Rolling: A walking beam heating furnace is adopted to control the heating rate in the preheating section and the soaking temperature in the soaking section, as well as control the fuel and furnace pressure of the heating furnace; The rated pressure of scale and descaling water is 25MPa, the actual pressure is not lower than 22MPa, and then two-stage controlled rolling is carried out to control the degree of reduction and widening, and the high-pressure water before rolling is controlled when the surface temperature of the slab is between 880 and 980°C Descaling; heat straightening after rolling, and after straightening, the steel plate is quickly lifted into the slow cooling pit or stacked to cool to room temperature. The specific process parameters are shown in Table 4.

(5) Heat treatment: Carry out flaw detection and surface shot blasting to the steel plate before heat treatment, and except embodiment 1 and embodiment 6, other embodiments all carry out normalizing pretreatment; Heating temperature, heating time, quenching start temperature (through air cooling), cooling speed and stop cooling temperature of the upper and lower surfaces of the steel plate; immediately keep the steel plate for a period of time after quenching, and then air cool or stack cool to room temperature, and according to the level of the steel plate The straightness determines whether to add a cold straightening process. The specific process parameters are shown in Table 6.

The microstructure of the steel plate in the embodiment of the present invention is observed, and it is mainly composed of lath martensite matrix and a small amount of retained austenite, wherein the upper surface of the steel plate has 5% to 10% of retained austenite, and the lower surface of the steel plate has 2% to 10%. 5% retained austenite, as shown in Figure 1, the white color in the structure photo is retained austenite, and the steel plate is bent 90° from the upper surface to the lower surface.

Table 7 shows the results of tensile, Charpy impact and cold bending tests on the steel plate of the embodiment of the present invention (note: the upper surface of the steel plate is bent toward the lower surface), wherein the direction, shape, position and code of the sample are shown in Table 8. It can be seen that the steel plate produced by the method of the present invention has high strength and toughness, high plasticity (elongation after fracture) and good cold bending performance, and exhibits excellent bending ability of the whole plate along the rolling direction, as shown in FIG. 2 .

The chemical composition (percentage by weight, %) of each embodiment steel plate of table 1

Table 2 The smelting process parameters of the steel plates of each embodiment

Refining process parameters of each embodiment steel plate of table 3

The rolling process parameter of each embodiment steel plate of table 4

The continuous casting process parameter of each embodiment steel plate of table 5

The heat treatment process parameter of each embodiment steel plate of table 6

The mechanical property of each embodiment steel plate of table 7

The mechanical testing sample situation of each embodiment steel plate of table 8

Claims

A production method of high-strength steel plates for construction machinery, characterized in that: the composition weight percentage of steel is C=0.10%～0.25%, Si≤0.80%, Mn=0.80%～1.20%, P≤0.010%, S≤ 0.0015%, Al=0.005%～0.015%, Cr≤1.0%, Mo≤1.0%, Ni=0.30%～2.0%, Cu≤0.30%, Ti≤0.015%, Nb≤0.020%, V≤0.080%, B ≤0.0040%, N≤0.0040%, O≤0.0010%, H≤0.00015%, the balance is ≥95% Fe and unavoidable impurities; the structure of the steel plate is lath martensite matrix and a small amount of retained austenite, The upper surface of the steel plate has 5% to 10% retained austenite, and the lower surface of the steel plate has 2% to 5% retained austenite; the process steps include:

(1) Smelting: use converter smelting and slag blocking to tap; control endpoint O≤0.03%;

(2) Refining: RH vacuum light treatment deoxidation, line feeding precipitation deoxidation, LF heating desulfurization and alloying, vacuum deep treatment degassing, and then soft blowing time ≥ 12min;

(3) Continuous casting: full protection pouring is carried out; the superheat of the tundish is controlled at 5-15°C, and dynamic light reduction is adopted to control the low power of the billet to meet the Mannesmann standard center segregation level 1-2;

(4) Rolling: Heat the slab to 1100-1250°C, and then roll it after descaling with high-pressure water; after cooling to 300°C, heat straightening is completed, and then the cooling rate is lower than 15°C/h Slowly cool to room temperature, carry out hydrogen expansion treatment;

(5) Heat treatment: Carry out flaw detection and surface shot blasting on the steel plate before heat treatment; then heat to 820-930°C, the heating time is (1.0-2.0)min/mm×H+(10-20)min, then start quenching and cooling, control The cooling rate between the quenching start temperature T 1 and the cooling stop temperature T 2 is 10-30°C/s, and the T 2 on the upper surface falls within the temperature range of M f + (20-50)°C through water ratio control, The T 2 of the lower surface falls within the temperature range of M f -(20～50)℃; after quenching, the steel plate is quickly sent to the tempering furnace for heat preservation, the heat preservation temperature is 200～300 ℃, and the heat preservation time is (2.0～4.0)min /mm×H+(10～20)min; where H is the thickness of the steel plate, in mm; M f is the end temperature of the martensitic transformation, which varies with the quenching cooling rate between 250 and 350°C.
The production method of a high-strength steel plate for construction machinery according to claim 1, characterized in that: for high-strength steel with a steel grade of Q960, C=0.10% to 0.15% in the composition weight percentage of the steel, and CEV≤0.52 %.
The production method of a high-strength steel plate for construction machinery according to claim 1, characterized in that: for high-strength steel with a steel grade of Q1100, C=0.15% to 0.20% in the composition weight percentage of the steel, and CEV≤0.62 %.
The production method of a high-strength steel plate for construction machinery according to claim 1, characterized in that: for high-strength steel with a steel grade of Q1300, C=0.20% to 0.25% in the composition weight percentage of the steel, and CEV≤0.72 %.
The production method of a high-strength steel plate for construction machinery according to claim 1, characterized in that: Si=0.30%-0.50% in the composition weight percentage of the high-strength steel.
The production method of a high-strength steel plate for construction machinery according to claim 1, characterized in that: Nb=0.010%～0.020% in the composition weight percentage of the high-strength steel.
The production method of a high-strength steel plate for construction machinery according to claim 1, characterized in that: Ti≤0.008% in the composition weight percentage of the high-strength steel.
The production method of a high-strength steel plate for construction machinery according to claim 1, characterized in that: Ti≤0.003% and N≤0.0030% in the composition weight percentage of the high-strength steel.
The production method of a high-strength steel plate for construction machinery according to claim 1, characterized in that: when the thickness of the steel plate is ≤ 20 mm, control B * = 0.0008% to 0.0015%; when the thickness of the steel plate is between 20 and 50 mm , control B * = 0.0012% ~ 0.0020%; when the thickness of the steel plate is ≥ 50mm, control B * = 0.0015% ~ 0.0025%; where B * is solid solution boron, B * = B-0.77×N+Ti/5.
The production method of high-strength steel plate for construction machinery according to claim 1, characterized in that: the inevitable impurities include As≤0.008%, Sn≤0.005%, Sb≤0.005%.
The production method of a high-strength steel plate for engineering machinery according to claim 1, characterized in that: in the step (1) smelting process, a top-bottom combined blowing converter is used, and bottom blowing is controlled throughout the smelting and tapping process Argon; the selected molten iron is pretreated and desulfurized by KR, and the S≤0.005% and P≤0.15% are controlled; the selected slagging materials include lime, light-burned dolomite, fluorite and iron ore, and the slag content is controlled to be ≥82kg/t; The selected coolant includes heavy scrap (Heavy scrap), molybdenum, nickel, and copper alloy materials, and the molten iron ratio is controlled to be ≥90%.
The production method of high-strength steel for construction machinery according to claim 1, characterized in that: said step (2) refining process, control KR vacuum light treatment deoxidation vacuum degree ≤ 10kPa, processing time ≥ 12min; control LF temperature rise The treatment time of desulfurization is ≤60min; the vacuum degree of controlled vacuum deep treatment and degassing is ≤67Pa, and the vacuum keeping time is controlled so that the hydrogen is less than 0.00012%.
The production method of high-strength steel for construction machinery according to claim 12, characterized in that: said step (2) refining process, vacuum light treatment deoxidation, O≤0.004% after vacuum deoxidation, and then use the calcium wire first and then Precipitate deoxidation by feeding aluminum wire, control Ca=0.0015%～0.0040%.
The production method of high-strength steel for construction machinery according to claim 12, characterized in that: in the refining process of the step (2), VD furnace is selected for vacuum deep treatment and degassing to carry out strong stirring, and the flow rate of bottom-blown argon gas is controlled to be ≥ 600L/min.
The production method of high-strength steel for construction machinery according to claim 1, characterized in that: in the step (3) continuous casting process, the moisture content of mold flux and covering agent is controlled to be lower than 0.25%; the tundish baking temperature is controlled Not lower than 1100°C, baking time 150-300min; control the dynamic light reduction rate at 1.0-1.2mm/m; the billet is cooled by stacking, when it is cooled to below 300°C, the stacking is removed, and then the billet is placed on, The lower surface shall be flame-cleaned for the entire board, and the cleaning depth shall be ≥5mm.
The production method of high-strength steel for construction machinery according to claim 15, characterized in that: in the step (3) continuous casting process, after the billets are stacked and cooled, a heat preservation cover is added.
The production method of high-strength steel for construction machinery according to claim 15, characterized in that: in the step (3) continuous casting process, the flame cleaning temperature is ≥ 150°C.
The production method of high-strength steel for construction machinery according to claim 1, characterized in that: in the step (4) rolling process, the furnace temperature in the preheating section of the heating furnace is controlled to be ≤600°C, and the time in the preheating section is ≥120min, The soaking temperature in the soaking section is controlled at 1150-1200°C; the proportion of coke oven gas in the fuel used in the heating furnace is controlled to be ≤20%, and the pressure in the furnace is controlled to be within the range of 5-20Pa; the descaling water pressure is guaranteed to be ≥22MPa, and the surface of the slab Descaling is required when rolling at a temperature between 880 and 980°C; the rolling reduction ratio in the recrystallization zone is controlled to be ≥ 2.5, the width ratio is ≥ 1.5, the rolling compression ratio in the non-recrystallized zone is controlled to be ≤ 3.0, and the pass reduction rate is ≤ 15 %; Control the final rolling temperature at 820-860°C; heat straightening is carried out when the steel plate is cooled to 300-350°C after rolling. .
The production method of high-strength steel for construction machinery according to claim 1, characterized in that: in the step (5) of the heat treatment process, the heating temperature is controlled at 820-850°C, and then quenching is started after air cooling to 600-650°C; quenching After heat preservation, air cool or stack cool to room temperature, and decide whether to add cold straightening process according to the flatness of the steel plate.
The production method of high-strength steel for construction machinery according to claim 1, characterized in that: in the step (5) of the heat treatment process, a normalizing pretreatment process is added before the heat treatment, and the normalizing temperature is 850-930°C.