420MPa grade low alloy high strength steel with high work hardening rate and manufacturing method

Abstract

The present invention relates to a 420MPa-grade low-alloy high-strength steel with a high work hardening rate and a manufacturing method thereof. The chemical composition of the steel is calculated by mass percentage as follows: C: 0.08% to 0.1%, Si: 0.3% to 0.5%, Mn: 1.5% to 1.7%, P≤0.018%, S≤0.008%, Al: 0.02% to 0.05%, Cr: 0.1% to 0.2%, Nb: 0.045% to 0.065%, Ti: 0.01% to 0.03%, N≤0.006%, and the remainder is Fe and unavoidable impurities, wherein Nb+Ti: 0.05% to 0.08%. The present invention is based on a C-Si-Mn system, Nb and Ti are added and a small amount of Cr is combined, and the solid solution strengthening and grain refinement effects thereof are utilized to adjust the microstructure to reduce the yield strength ratio of the steel plate, so that the yield strength ratio of the steel plate is controlled within 0.6 to 0.8.

Description

Technical Field

[0001] The invention relates to a cold-rolled hot-dip galvanized 420MPa low-alloy high-strength steel for automobiles with a high work hardening rate and a manufacturing method thereof. Background Art

[0002] In recent years, in order to continuously pursue lightweight and improve safety of automobiles, the competition for automobile steel has become increasingly fierce. The use of aluminum alloys, magnesium alloys and composite materials in automobiles has continued to increase. This competition has strongly promoted the development of high-strength and application technologies for various types of automobile steel. In the early days, high-strength micro-alloyed steel, as a typical representative of traditional high-strength steel, significantly solved the strength problem of automobile steel with its good strength and toughness, good forming performance, excellent welding performance and low production cost. It was widely used in automobile body and collision-related structural parts. Typical parts include front and rear longitudinal beams, door inner reinforcement beams, floor beams, roof beams and chassis suspension parts. Unfortunately, with the continuous increase in strength, the forming performance of low-alloy high-strength steel is poor. Therefore, the design of complex phase organization has led to the birth of dual-phase steel. Dual-phase steel has gradually developed into a new type of high-strength stamping high-strength steel with good formability due to its low yield ratio, high initial work hardening rate, excellent strength and ductility. It has become an important part of modern automobile steel and is widely used in various safety parts such as automobile bumpers, doors, wheels and longitudinal beams of the body. Then, the manufacturing process of some automotive parts needs to go through flanging and hole expansion processes, which require the steel plate to have good flanging and hole expansion performance. The flanging and hole expansion performance of ferrite-martensite dual-phase steel is poor. From this point of view, the diversification and customization of the performance of high-strength steel for automobiles are becoming more and more important, that is, choosing the right material for the right part to achieve the rationalization and maximization of material application. Therefore, it is urgent to develop a high-strength steel product with performance between low-alloy high-strength steel and dual-phase steel, which can take into account both high yield ratio and high strength plasticity, and high work hardening rate, so as to meet the stamping needs of complex formed parts.

[0003] Traditional 420MPa grade hot-dip galvanized low-alloy high-strength steel is developed from ordinary carbon steel and low-carbon steel. Its composition is based on ordinary carbon steel by adding micro-alloying elements such as Ti and Nb, and mainly uses traditional strengthening methods such as solid solution strengthening, precipitation strengthening and fine grain strengthening to achieve the purpose of improving the strength of steel. Its organizational structure mainly includes ferrite, pearlite and a small amount of carbide. The yield strength ratio is generally between 0.7 and 0.9, and the work hardening index n is generally about 0.13. Due to the high yield strength ratio, its plasticity and work hardening properties are poor, resulting in poor formability, especially in the production process of relatively complex parts in drawing or deep drawing, this type of steel still cannot meet the processing requirements.

[0004] In the prior art, patent CN112725704A discloses a hot-dip galvanized 420MPa low-alloy high-strength steel for automobiles and its production method. The produced hot-dip galvanized 420MPa low-alloy high-strength steel has a tensile strength of ≤590MPa and a yield strength ratio of ≥0.9, which has typical traditional high-strength low-alloy steel performance. Its work hardening rate is relatively low, so its tensile forming performance is poor. Patent CN111826578A discloses a 420MPa cold-rolled low-alloy high-strength steel and its manufacturing method. The produced 420MPa cold-rolled low-alloy high-strength steel has a tensile strength of ≤590MPa and a high yield strength ratio, which still belongs to the performance category of traditional high-strength low-alloy steel; secondly, the low-alloy high-strength steel is a cold-rolled product, and its corrosion resistance is poor compared with galvanized products. Summary of the invention

[0005] The purpose of the present invention is to provide a 420MPa grade low alloy high strength steel with a high work hardening rate and a manufacturing method, which can take into account both high yield ratio and high strength plasticity, and high work hardening rate, so as to meet the stamping requirements of complex formed parts.

[0006] To achieve the above object, the present invention is implemented through the following technical solutions:

[0007] A 420MPa grade low alloy high strength steel with a high work hardening rate, the chemical composition of the steel is calculated by mass percentage:

[0008] C: 0.08%~0.1%, Si: 0.3%~0.5%, Mn: 1.5%~1.7%, P≤0.018%, S≤0.008%, Al: 0.02%~0.05%, Cr: 0.1%~0.2%, Nb: 0.045%~0.065%, Ti: 0.01%~0.03%, N≤0.006%, the balance is Fe and unavoidable impurities, among which Nb+Ti: 0.05%~0.08%.

[0009] The microstructure of the steel is calculated by volume percentage: ferrite is 79% to 90%, the average grain size is 2 to 5 μm, pearlite is 9% to 20%, cementite and nano-carbide precipitation phase is 1% to 3%, among which the nano-carbide precipitation phase has a particle size of 5 to 10 nm, and the nano-carbide precipitation phase is dispersed in the ferrite matrix.

[0010] The mechanical properties of the hot-dip galvanized finished steel plate produced from the low-alloy high-strength steel are as follows: yield strength 420-570MPa, tensile strength 590-700MPa, yield strength ratio 0.6-0.8, A80 longitudinal elongation after fracture ≥20%, and work hardening index n≥0.15.

[0011] 420MPa grade low alloy high strength steel:

[0012] The main role of C in microalloyed steel is to provide strength and form strengthening phase particles. The C content has a great influence on the mechanical properties of microalloyed steel. Generally speaking, increasing the C content can increase the amount of pearlite and improve the strength and hardness of the steel. The strength of the steel plate will increase with the increase of the C content, but the plasticity, toughness and weldability will decrease accordingly. In order to achieve good formability and welding properties of low-alloy high-strength steel, low-carbon components are generally used. The upper limit of the C content is set at 0.1% to ensure that the steel plate has the necessary strength level, and the lower limit of the C content is set at 0.8% to form fine dispersed nano-precipitated particles with Nb and Ti, and play a role in grain refinement and precipitation strengthening.

[0013] Mn and Cr are typical austenite stabilizing elements, which can expand the austenite phase area and facilitate hot rolling process control. Mn and Cr also play a role in solid solution strengthening and further improve the strength of steel. However, too high Mn content can easily lead to the formation of banded structure, affecting the comprehensive performance of the steel plate. Therefore, the Mn content is 1.5-1.7% and the Cr content is 0.1-0.2%.

[0014] As a non-carbide forming element, Si can avoid the formation of coarse carbides in the matrix during the cooling process and improve the plasticity and toughness of the steel plate. However, excessive Si will form an oxide film on the surface of the steel during hot rolling and annealing, which will cause problems in the hot rolling surface quality control and hot dip galvanizing surface control. Therefore, the Si content is controlled at 0.3% to 0.5%.

[0015] As microalloying elements, Nb and Ti are strong carbide-forming elements. They mainly produce grain refinement and precipitation strengthening effects and further improve strength through partial Nb solid solution and precipitation of Nb and Ti carbides. Therefore, the Nb content is designed to be 0.045-0.065%, the Ti content is 0.01-0.03%, and the Nb+Ti content is between 0.05-0.08% to ensure effective precipitation strengthening.

[0016] The solid solution temperatures of titanium and niobium carbides are very close. Generally, when titanium and niobium steel is heated to 1200℃ and heated for 2 hours, 90% of titanium and niobium carbides in the steel can be dissolved in austenite. However, the solid solution temperature of titanium nitride is very high, and it begins to precipitate in the liquid phase. Solid solution hardly occurs at the general slab heating temperature. Therefore, during the high-temperature heating process, the role of titanium is mainly that its undissolved titanium nitride particles are distributed on the austenite grain boundaries, hindering the growth of austenite grains; and the role of niobium is mainly that its solid solution state of niobium can inhibit the growth of austenite grains through the solute drag effect at the grain boundaries. In addition, during the rolling process, titanium carbonitride and niobium carbonitride will precipitate in the form of nano-precipitated particles on dislocations, grain boundaries and subgrain boundaries to hinder the dynamic recrystallization process of austenite, which is conducive to grain refinement. Generally, there are four main precipitation modes of titanium carbide and niobium carbide in steel: precipitation in high-temperature austenite zone, precipitation in low-temperature austenite zone, interphase precipitation during the transformation of austenite to ferrite, and supersaturated precipitation in the ferrite zone. The size of the first type of precipitated carbides is about tens of nanometers, which mainly prevents the growth of austenite grains; while the latter three types of precipitation can obtain carbides with a size of about a few nanometers through process control, producing a strong precipitation strengthening effect and significantly improving the yield strength of the material. In addition, the addition of titanium to niobium-containing steel can further increase the solid solubility of niobium in the austenite state due to the formation of titanium nitride, and exert the grain refinement and precipitation strengthening effects of niobium.

[0017] A method for manufacturing 420MPa-grade low-alloy high-strength steel with a high work hardening rate, which is produced according to the following process flow: steelmaking, continuous casting, slab heating, hot rolling, laminar cooling, coiling, pickling, cold rolling, surface cleaning, continuous annealing, hot-dip galvanizing and skin pass, and tension straightening;

[0018] The hot rolling and cold rolling production process includes:

[0019] 1) The ingot is heated and kept warm in a walking beam heating furnace at a temperature of 1220-1260°C, and dephosphorized with high-pressure water after being taken out of the furnace;

[0020] 2) The inlet temperature of the roughing mill is 1150-1200°C, the final rolling temperature of the finishing mill is 860-920°C, and the coiling is performed after laminar cooling, and the coiling temperature is 560-620°C;

[0021] The hot-dip galvanizing process comprises:

[0022] 1) The cold hardened coil is first surface cleaned and then enters the continuous annealing furnace;

[0023] 2) In the continuous annealing furnace, the heating section temperature is 800-840°C; the holding section temperature is 800-840°C, the holding section dew point temperature is -20--40°C; the rapid cooling temperature is 450-500°C, and the furnace nose dew point temperature is -20--30°C;

[0024] 3) The temperature of the strip entering the zinc pot is 450-460°C; the Al content of the zinc pot is 0.2%-0.24% when producing GI coating, and 0.13%-0.15% when producing GA coating.

[0025] In the hot rolling process, the hot rolled strip is pickled in a pickling tank to remove the surface oxide scale, and then cold rolled by a cold rolling unit, and the total cold rolling reduction rate is 60% to 70%.

[0026] In the polishing process, the polishing elongation is 0.6% to 0.9%; in the straightening process, the straightening elongation is ≤0.2%.

[0027] Compared with the prior art, the present invention has the following beneficial effects:

[0028] The present invention is based on the conventional C-Si-Mn system, adds microalloying elements Nb and Ti and a small amount of Cr, and utilizes its solid solution strengthening and grain refinement effects to regulate the microstructure and reduce the yield strength ratio of the steel plate, so that the yield strength ratio of the steel plate is controlled at 0.6 to 0.8, which is between the same level of low-alloy high-strength steel and dual-phase steel. The present invention adopts the addition of microalloying elements Nb and Ti, and utilizes its precipitation strengthening effect to improve the work hardening rate of the steel plate, so that the work hardening rate is increased to more than 0.15, so that the steel plate exhibits better forming performance in the process of processing parts that require higher tensile forming. The low-carbon design is adopted to further reduce its cold crack sensitivity and welding performance.

[0029] The 420MPa grade low alloy high strength steel produced by the present invention has a high work hardening rate and a relatively low yield strength ratio, which means that the low alloy high strength steel exhibits good strain hardening performance and is particularly suitable for parts that need to bear high tensile forming loads. A good work hardening rate n value can avoid premature failure of the steel plate due to local shrinkage, and the deformation can be dispersed to a larger area. Compared with traditional micro-alloy steels of the same grade, it exhibits better machinability. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 This is the microstructure diagram of the high work hardening rate cold-rolled hot-dip galvanized 420MPa low-alloy high-strength steel for automobiles produced in Example 1.

[0031] Figure 2 This is a transmission electron microscopy dark field phase image of nano-carbide precipitation particles of the cold-rolled hot-dip galvanized 420MPa low-alloy high-strength steel for automobiles with a high work hardening rate produced in Example 1. DETAILED DESCRIPTION

[0032] The present invention is described in detail below in conjunction with the accompanying drawings, but it should be noted that the implementation of the present invention is not limited to the following embodiments.

[0033] A method for manufacturing 420MPa-grade low-alloy high-strength steel with a high work hardening rate, which is produced according to the following process flow: steelmaking, continuous casting, slab heating, hot rolling, laminar cooling, coiling, pickling, cold rolling, surface cleaning, continuous annealing, hot-dip galvanizing and skin pass, and tension straightening;

[0034] The control of hot rolling structure will have a decisive influence on the final performance of the steel plate after annealing. Therefore, the hot rolling process adopts controlled rolling and controlled cooling in order to obtain fine grains and uniformly dispersed second phase particles. The specific hot rolling and cold rolling production process includes:

[0035] 1) The ingot is heated and kept warm in a walking beam heating furnace at a temperature of 1220-1260°C, so that the micro-alloy elements in the steel are fully dissolved in the matrix during high-temperature austenitization, and dephosphorized with high-pressure water after being taken out of the furnace;

[0036] 2) Two-stage control of the recrystallization zone and the non-recrystallization zone is adopted. Multiple rough rolling is performed in the austenite recrystallization zone (1050-1150°C). The dynamic deformation and dynamic recrystallization of austenite in this temperature range are utilized to refine the austenite grains. The inlet temperature of the rough rolling mill is 1150-1200°C. Multiple finishing rolling is performed in the austenite non-recrystallization zone (860-1000°C) to obtain deformed austenite. The accumulated large amount of plastic deformation leads to a significant increase in dislocation density and induces ferrite phase transformation, thereby refining the grains and achieving the purpose of fine grain strengthening. The final rolling temperature of the finishing mill is 860-920°C, and the coiling is performed after laminar cooling. The coiling temperature is 560-620°C. The purpose is to obtain a hot-rolled plate structure with fine structure and dispersed distribution of the second phase. During this cooling process, Nb and Ti dissolved in the matrix will be fully precipitated in the form of carbonitride particles through interphase precipitation and supersaturated precipitation, and the control of the final rolling temperature and coiling temperature is very important for the control of the amount and size of precipitation. The hot-rolled strip is pickled in the pickling tank to remove the surface iron oxide scale, and then cold-rolled by a 5-stand cold rolling mill. If the cold rolling compression ratio is too low, the deformation of the matrix structure is small, and the strength of the product after annealing is low; if the cold rolling compression ratio is too large, it is easy to cause the hardness after annealing to be high and the yield strength ratio is too high. Therefore, it is more reasonable to use a cold rolling compression ratio of 60%-70%.

[0037] 3) The hot-rolled strip is pickled in a pickling tank to remove the surface iron oxide scale, and then cold-rolled by a cold rolling unit, with a total cold rolling reduction rate of 60% to 70%.

[0038] The hot-dip galvanizing annealing process is a key step in determining the final performance of the product. Therefore, in addition to meeting the requirements of hot-dip galvanizing, the reasonable control of the annealing process also requires regulating the final proportion of each phase of the matrix and the full precipitation of the second phase particles. The hot-dip galvanizing process includes:

[0039] 1) The cold hardened coil is first surface cleaned and then enters the continuous annealing furnace;

[0040] 2) In the continuous annealing furnace, the heating section temperature is 800-840°C; the holding section temperature is 800-840°C, the holding section dew point temperature is -20--40°C; the rapid cooling temperature is 450-500°C, and the furnace nose dew point temperature is -20--30°C;

[0041] Due to the addition of microalloying elements titanium and niobium, the recrystallization temperature of the strip steel is slightly increased. Therefore, it is necessary to appropriately increase the continuous annealing temperature to ensure sufficient recrystallization of the steel plate. Therefore, the temperature of the heating section is controlled at 800-840°C; the temperature of the holding section is 800-840°C.

[0042] The dew point temperature of the holding section is -20~-40℃, in order to prevent the external oxidation of Si, Mn and Cr alloy elements, which will deteriorate the quality of the plate surface and affect the subsequent plating performance. After annealing, the strip is cooled to above the galvanizing temperature by direct rapid cooling. On the one hand, in order to avoid the formation of proeutectoid ferrite and the segregation of carbon in ferrite to austenite, thereby affecting the precipitation of titanium niobium carbide particles in the ferrite matrix; on the other hand, in order to cool the Nb and Ti carbonitrides that have not been fully precipitated in the matrix to a relatively low temperature and precipitate them into fine second phase particles in the subsequent equalization section and galvanizing process, the rapid cooling outlet temperature is designed to be 450℃~500℃. The dew point temperature of the furnace nose is -20~-30℃, also in order to prevent the external oxidation of alloy elements and cause leakage defects.

[0043] 3) The temperature of the strip entering the zinc pot is 450-460°C; the Al content of the zinc pot is 0.2%-0.24% when producing GI coating, and 0.13%-0.15% when producing GA coating.

[0044] In the finishing process, the finishing elongation is 0.6% to 0.9%; in the straightening process, the straightening elongation is ≤0.2%.

[0045] The present invention uses a high-toughness matrix with a low carbon content combined with finely dispersed carbonitrides to achieve an ideal match between strength and plasticity. Therefore, in order to obtain nano-scale particles with as much precipitation as possible and finer particle size, the key to ferrite-based Nb-Ti microalloyed steel is to ensure that the dissolved microalloying elements are fully precipitated in a reasonable temperature range through a reasonable controlled rolling and controlled cooling process and hot-dip galvanizing annealing process, thereby obtaining a certain volume fraction and second-phase particles with most particle sizes distributed below ten nanometers, combining the good plasticity and toughness of ferrite with the nano-precipitation phase, and achieving good comprehensive mechanical properties of microalloyed steel.

[0046] In this embodiment, the chemical composition of the hot-dip galvanized 420MPa low-alloy high-strength steel with high work hardening rate is shown in Table 1, the process parameter control is shown in Table 2, the mechanical properties of the finished steel plate are shown in Table 3, and the morphology of the finished steel plate of Example 1 is shown in Table 3. Figure 1 , Figure 2 .

[0047] Table 1 Chemical composition of steel (wt%)

[0048]

[0049]

[0050] Table 2 Process parameter control

[0051]

[0052] Table 3 Phase ratio and phase size in steel plate

[0053]

[0054]

[0055] Table 4 Mechanical properties of finished steel plates

[0056]

[0057] The finished steel plates of Examples 1-10 have good processing properties and have proven their value in many applications in the automotive industry, such as typical parts applications on automobile bodies: side beams, cross beams, and pillars.

Claims

1. A 420MPa grade low alloy high strength steel with high work hardening rate, characterized in that: The chemical composition of steel is calculated by mass percentage: C: 0.08% ~ 0.1%, Si: 0.3% ~ 0.5%, Mn: 1.5% ~ 1.7%, P ≤ 0.018%, S ≤ 0.008%, Al: 0.02% ~ 0.05%, Cr: 0.1% ~ 0.2%, Nb: 0.045% ~ 0.065%, Ti: 0.01% ~ 0.03%, N ≤ 0.006%, the balance is Fe and unavoidable impurities, of which Nb + Ti: 0.05% ~ 0.08%; The microstructure of the steel is calculated by volume percentage, with ferrite 79% to 90%, average grain size 2 to 5 μm, pearlite 9% to 20%, cementite and nano-carbide precipitation phase 1% to 3%, wherein the nano-carbide precipitation phase has a particle size of 5 to 10 nm, and the nano-carbide precipitation phase is dispersed on the ferrite matrix; The mechanical properties of the hot-dip galvanized finished steel sheet produced from the low-alloy high-strength steel are as follows: yield strength 420-570MPa, tensile strength 590-700MPa, yield strength ratio 0.6-0.8, A80 longitudinal elongation ≥20%, work hardening index n≥0.15; The manufacturing method of the 420MPa grade low alloy high strength steel with a high work hardening rate is produced according to the following process flow: steelmaking, continuous casting, slab heating, hot rolling, laminar cooling, coiling, pickling, cold rolling, surface cleaning, continuous annealing, hot dip galvanizing and skin pass, and straightening; The hot rolling and cold rolling production process includes: 1) The ingot is heated and kept warm in a walking beam heating furnace at a temperature of 1220-1260°C, and dephosphorized with high-pressure water after being taken out of the furnace; 2) The inlet temperature of the roughing mill is 1150-1200°C, the final rolling temperature of the finishing mill is 860-920°C, and the coiling is performed after laminar cooling, and the coiling temperature is 572-620°C; The hot-dip galvanizing process comprises: 1) The cold hardened coil is first surface cleaned and then enters the continuous annealing furnace; 2) In the continuous annealing furnace, the heating section temperature is 800-840°C; the holding section temperature is 800-840°C, the holding section dew point temperature is -20--40°C; the rapid cooling temperature is 450-500°C, and the furnace nose dew point temperature is -20--30°C; 3) The temperature of the strip entering the zinc pot is 450-460°C; the Al content of the zinc pot is 0.2%-0.24% when producing GI coating, and 0.13%-0.15% when producing GA coating; In the finishing process, the finishing elongation is 0.6% to 0.9%; in the straightening process, the straightening elongation is ≤0.2%.

2. The 420MPa grade low alloy high strength steel with high work hardening rate according to claim 1, characterized in that: In the hot rolling process, the hot rolled strip is pickled in a pickling tank to remove the surface oxide scale, and then cold rolled by a cold rolling unit, and the total cold rolling reduction rate is 60% to 70%.

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