Patent Publication Number: US-2019169708-A1

Title: 1900 MPa GRADE PRESS HARDENING STEEL BY MEDIUM THIN SLAB CASTING AND DIRECT ROLLING AND METHOD FOR PRODUCING THE SAME

Description:
BACKGROUND OF THE INVENTION 
     1. Technical Field 
     The present invention relates to a steel for automobile parts and a producing method thereof, and in particular, to a press hardening steel by medium and thin slab casting and direct rolling and having a tensile strength of 1900 MPa or more and a production method thereof. The producing method is adapted for a product having a thickness range of 2 to 10 mm. 
     2. Background 
     With the development of automobile industry and the gradual development of automobile design and manufacturing in a direction of energy conservation, environmental protection and safety in the automobile industry, lightweight automobile designs have become the tendency of automobile design for a long time now and in future. 
     The researches show that there was a linear relationship between an overall weight and energy consumption of an automobile. According to statistics, fuel efficiency can be increased by 6% to 8% for every 10% reduction in automobile weight. One of the most important ways to reduce the weight of an automobile is to use a high-strength and ultra-high-strength steel, so that a curb weight of the automobile can be greatly reduced without compromising a collision safety and the comfort. However, as the strength continues to increase, formability of a steel sheet will become worse, especially for an ultra-high-strength steel of above 1900 MPa. During the forming process, there will be problems such as cracking, spingback and low dimensional accuracy of parts. Furthermore, higher requirements are imposed on stamping equipment, that is, a large-tonnage punching machine and a high-wearing die are required, and a life cycle of the die is greatly affected. At present, there is no cold forming stamping equipment and die capable of forming 1900 MPa or above in the country. 
     At present, a tensile strength of existing press hardening steel in the country and abroad cannot reach 1900 MPa or more, and all of them are cold-rolled annealed or pre-coated after being cold-rolled annealed. The production processes include: hot metal desulphurization→converter steelmaking→external refining→continuous casting→slab heating→hot rolling→pickling+cold rolling→continuous annealing→(pre-coating)→finishing packaging→blanking→heating→die stamping and quenching. There is a shortage of long production process and high cost. For some anti-collision or load-bearing parts, multiple parts combined with members are used to improve the anti-collision and load-carrying capacity, which leads to greatly increased raw material cost and processing cost. 
     With the development of iron and steel industry, a medium and thin slab casting and direct rolling process has been greatly developed. The medium and thin slab casting and direct rolling process can directly produce steel sheet and strip with a nominal thickness of more than 2.0-10 mm. Some thin-specification parts only adopting cold-rolled high-strength steels or members composed of multiple parts for strengthening have been gradually replaced by direct rolling ultra-high-strength steel sheet using a slab casting and direct rolling process. For example, Chinese Patent Publication No. CN 102965573A has developed a high-strength steel for engineering structures with a yield strength (R eL ) of 700 MPa or more and a tensile strength (R m ) of 750 MPa or more. The steel sheet has the chemical composition of: C: 0.15-0.25%, Si≤0.10%, Mn: 1.00-1.80%, P≤0.020%, S≤0.010%, Ti: 0.09-0.20%, Als: 0.02-0.08%, N≤0.008%, and the balance of Fe and inevitable impurities, in terms of % by mass. The invention steel sheet can be produced by a production method including: smelting and continuous casting into a slab, soaking, and controlling the soaking temperature to be 1200-1300° C. and a soaking time to be 20-60 min; hot rolling, and controlling a rolling temperature to be not lower than 1200° C. and a finishing rolling temperature to be 870-930° C.; performing laminar cooling, cooling to a batching temperature at a cooling speed of not lower than 20° C./s; and performing batching, and controlling a batching temperature to be 580-650° C. There is also a Chinese patent Publication No. CN 103658178A, which invents a short-flow method for producing a high-strength thin strip steel. The invented strip steel has a yield strength (R eL ) larger than or equal to 550 MPa and a tensile strength (R m ) larger than or equal to 600 MPa. The strip steel includes following chemical components by mass percent: C: 0.02-0.15%, Si: 0.20-0.6%, Mn: 0.2-1.50%, P: 0.02-0.3%, S≤0.006%, Cr: 0.40-0.8%, Ni: 0.08-0.40%, Cu: 0.3-0.80%, Nb: 0.010-0.025%, Ti: 0.01-0.03%, Al: 0.01-0.06%, Re: 0.02-0.25%, and the balance of Fe and inevitable impurities. After smelting, a casting strip with a thickness of 1.0-2.0 mm is cast at a casting speed of 60-150 m/min; rolling is performed, and a finishing rolling temperature is controlled to be 850-1000° C.; atomization cooling is adopted at a cooling speed of 50-100° C./s, batching is performed, and a batching temperature is controlled to be 520-660° C. The tensile strength of the above two documents is very low, which cannot meet a demand of a high-end automobile body for ultra-high strength of 1900 MPa or more. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to a press hardening steel having a tensile strength of 1900 MPa or more and a production method thereof, which can meet requirements of automobile design for ultra-high-strength and can also successfully complete complex deformation with no springback after deformation and high dimensional accuracy of parts, so as to overcome the shortcomings in the prior art that a strength level is low and rigidity requirements of a user cannot be met. 
     Measures for achieving the foregoing objectives are taken as follows. 
     A press hardening steel is directly rolled through a medium and thin slab and has a tensile strength of 1900 MPa or more. The press hardening steel sheet has the chemical composition of: C: 0.31-0.40%, Si: 0.36-0.44%, Mn: 1.6-2.0%, P≤0.006%, S≤0.004%, Als: 0.015-0.060%, Cr: 0.36-0.49%, Ti: 0.036-0.045% or Nb: 0.036-0.045% or V: 0.036-0.045%, or a mixture of the above two or more in any ratio, B: 0.004-0.005%, Mo: 0.26-0.35%, Ni: 0.11-0.20%, N≤0.005%, and the balance of Fe and inevitable impurities, in terms of % by mass. A quenched microstructure is a full lath martensite. Mechanical properties are as follows: yield strength≥1300 MPa, tensile strength≥1900 MPa, and elongation A 80 mm ≥5%. 
     A method for producing the press hardening steel by the medium and thin slab casting and direct rolling and having the tensile strength of 1900 MPa or more is characterized by including following steps: 
     1) Hot melt desulphurizing, and controlling S≤0.002%, an exposed surface of the molten iron after slagging off being not lower than 96%. 
     2) Performing conventional electric furnace or converter smelting, and conventional refining. 
     3) Performing continuous casting, and controlling a degree of superheat of tundish molten steel to be 15-30° C., a thickness of a slab to be 61-150 mm, and the casting speed to be 3.0-5.0 m/min. 
     4) Performing descaling treatment before the slab enters a soaking furnace, and controlling a pressure of descaling water to be 300-400 bar. 
     5) Performing conventional soaking on the slab, and controlling inside of the soaking furnace in a weak oxidizing atmosphere, i.e. a residual oxygen content in the furnace being 0.5-5.0%. 
     6) Heating the slab, and controlling a temperature of the slab entering the furnace to be 800-1000° C. and a temperature of the slab leaving the furnace to be 1165-1195° C. 
     7) Performing high-pressure water descaling before entering a rolling mill, and controlling the pressure of the descaling water to be 280-420 bar. 
     8) Hot rolling, controlling a first pass reduction rate to be 40-50%, a second pass reduction rate to be 40-50% and a final pass reduction rate to be 10-16%, controlling a rolling speed to be 3-8 m/s, performing medium-pressure water descaling between a first pass and a second pass under the pressure of the descaling water of 200-280 bar, and controlling a finishing rolling temperature to be 850-890° C. 
     9) Cooling to a coiling temperature in a manner of laminar cooling, water curtain cooling or intensified cooling. 
     10) Performing coiling, and controlling the coiling temperature to be 585-615° C. 
     11) Performing austenitizing after uncoiling and blanking, controlling an austenitizing temperature to be 930-980° C., and holding for 6-15 min. 
     12) Die punching and deforming, and keeping a pressure for 6-9 s in a die. 
     13) Performing quenching, controlling a quenching cooling speed to be 50-100° C./s, and then naturally cooling to a room temperature. 
     It is characterized in that a rolling process of the medium and thin slab is carried out in a short-process production line in any one of rolling mill arrangement forms such as a 6F production line or a 1R+6F production line, or a 2R+6F production line, or a 7F production line, or a 3R+4F production line, or 2R+5F production line, or a 1R+5F production line. 
     Mechanism of each element and main process in the present invention 
     C: Carbon is a strong solution strengthening element, which plays a decisive role in the acquisition of ultra-high strength. The carbon content has a great influence on the microstructures and properties of the final product, but the content is too high, and it is easy to form a large amount of pearlite or bainite or martensite in the cooling process after finish rolling. The higher the content, the higher the strength, which results in a decrease in plasticity and difficulty in blanking before forming. Therefore, under the premise of ensuring heat treatment strengthening, the carbon content should not be too high. Therefore, the content is limited to a range of 0.31% to 0.40%. 
     Si: Silicon has a strong solution strengthening effect, which can improve the strength of steel. Furthermore, silicon can improve a hardenability of steel and reduce a volume change of austenite transforms into martensite, thus effectively controlling the production of quenching cracks. During low temperature tempering, a diffusion of carbon can be hindered, and the decomposition of martensite and the aggregation and growth of carbide are delayed, so that a hardness of steel decreases slowly during tempering, which significantly improves a tempering stability and strength of steel. Therefore, the content is limited to a range of 0.36% to 0.44%. 
     Mn: Manganese acts as a solution strengthening agent, and furthermore, it can remove FeO in steel and significantly improve the quality of steel. It can also form MnS with a high melting point with sulphide. In thermal processing, MnS has sufficient plasticity to prevent steel from hot shortness, reduce the harmful effects of sulphur, and improve the hot workability of steel. Manganese can reduce a phase change driving force, make a “C” curve shift to the right, improve the hardenability of steel, enlarge a γ phase region, and reduce the M s  point of steel, so it can be ensured that martensite is obtained at a suitable cooling speed. Therefore, the content is limited to a range of 1.6% to 2.0%. 
     Cr: Chromium can reduce the phase transformation driving force and also reduce the nucleation growth of carbides during phase transformation, so the hardenability of steel is improved. In addition, chromium can improve the tempering stability of steel. Therefore, the content is limited to a range of 0.36% to 0.49%. 
     B: Boron is an element that strongly enhances hardenability. The addition of trace amounts of boron to steel can significantly improve the hardenability of the steel. However, the content is lower than 0.0005%, or higher than 0.0050%, and the effect on improving hardenability is not obvious. Therefore, in order to consider the actual production and hardenability effects, the content is limited to a range of 0.004% to 0.005%. 
     Als: It deoxidizes in steel, it should be ensured that there is a certain amount of acid-soluble aluminium in the steel, otherwise it will not exert its effect, but too much aluminium will cause aluminium-based inclusions in the steel, which is not conducive to steel smelting and casting. Furthermore, the addition of an appropriate amount of aluminium in steel can eliminate the adverse effects of nitrogen and oxygen atoms on the properties of the steel. Therefore, the content is limited to a range of 0.015% to 0.060%. 
     P: Phosphorus is a harmful element in steel, which is liable to cause segregation in a centre of a slab. In the subsequent hot continuous rolling heating process, it tends to be segregated to a grain boundary, so that a brittleness of steel is significantly increased. Furthermore, based on cost considerations and without affecting the properties of the steel, the content is controlled to be 0.006% or less. 
     S: Sulphur is a very harmful element. Sulphur in steel is often present in the form of sulphides of manganese. This sulphide inclusion can deteriorate a toughness of the steel and cause anisotropy of properties. Therefore, it is necessary to control the sulphur content in the steel as low as possible. The sulphur content in the steel is controlled to be 0.004% or less based on consideration of manufacturing cost. 
     N: Nitrogen can be combined with titanium to form titanium nitride in titanium-added steel. This second phase precipitated at high temperature is beneficial for strengthening a matrix and improving a weldability of a steel plate. However, the nitrogen content is higher than 0.005%, and a solubility product of nitrogen and titanium is higher. At high temperature, a coarse titanium nitride is formed in the steel, which seriously damages the plasticity and toughness of the steel. In addition, the higher nitrogen content will increase the amount of micro-alloying elements required to stabilize the nitrogen element, thereby increasing the cost. Therefore, the content is controlled to be less than 0.005%. 
     Ti: Titanium is a strong C and N compound forming element. The purpose of adding Ti to steel is to fix the N element in the steel, but the excess Ti will combine with C to reduce the hardness and strength of martensite after quenching of the test steel. In addition, the addition of titanium contributes to the hardenability of steel. Therefore, the content is limited to a range of 0.036% to 0.045%. 
     Nb, V: Niobium and vanadium are also strong C and N compound forming elements, which can refine austenite grains. A small amount of niobium or vanadium can be added into steel to form a certain amount of niobium carbon and nitride, so that growth of the austenite grain is hindered, and therefore, a size of a martensite lath after quenching is small, and the strength of the steel is greatly improved. Therefore, the content is controlled between 0.036% and 0.045%. 
     Mo: Molybdenum can significantly improve the hardenability of steel, and a stacking fault energy of molybdenum is high. The addition of the molybdenum into steel can improve the low temperature plasticity and toughness of the steel. Therefore, the content is controlled between 0.26% and 0.35%. 
     Ni: Nickel is added to steel to increase the strength of the steel without significantly reducing its toughness. Furthermore, the processability and weldability of the steel can be improved. In addition, nickel can improve a corrosion resistance of steel, not only is acid-resistant, but also resists alkali and atmospheric corrosion. Therefore, the content is limited to a range of 0.11% to 0.20%. 
     The reason why the present invention adopts three times of descaling in the whole production process is that mill scale on a surface of a strip steel can be removed as much as possible by controlling the descaling pass and the appropriate descaling water pressure, thereby ensuring that the strip steel has a good surface quality. 
     Compared with the prior art, the present invention has high strength, short manufacturing process, good product surface quality, and high thickness precision, can meet the quality requirements of cold rolled products, and greatly saves energy consumption; in addition, compared with existing products directly rolled through medium and thin slabs, the strength is much higher than that of the existing products, which is of great significance for reducing the weight of automobiles. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a microstructure of a product according to the present invention. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     The present invention is described in detail below. 
     Table 1 is a list of chemical component values of various embodiments and comparative examples of the present invention. 
     Table 2 is a list of main process parameter of various embodiments and comparative examples of the present invention. 
     Table 3 is a list of property detection cases of various embodiments and comparative examples of the present invention. 
     In various embodiments of the present invention, production is performed according to following process: 
     1) Hot melt desulphurize, and control S≤0.002%, an exposed surface of the molten iron after slagging off being not lower than 96%. 
     2) Perform conventional electric furnace or converter smelting, and conventional refining. 
     3) Perform continuous casting, and control a degree of superheat of tundish molten steel to be 15-30° C., a thickness of a slab to be 61-150 mm, and the casting speed to be 3.0-5.0 m/min. 
     4) Perform descaling treatment before the slab enters a soaking furnace, and control a pressure of descaling water to be 300-400 bar. 
     5) Perform conventional soaking on the slab, and control inside of the soaking furnace in a weak oxidizing atmosphere, i.e. a residual oxygen content in the furnace being 0.5-5.0%. 
     6) Heat the slab, and control a temperature of the slab entering the furnace to be 800-1000° C. and a temperature of the slab leaving the furnace to be 1185-1215° C. 
     7) Perform high-pressure water descaling before entering a rolling mill, and control the pressure of the descaling water to be 280-420 bar. 
     8) Perform hot rolling, control a first pass reduction rate to be 40-50%, a second pass reduction rate to be 40-50% and a final pass reduction rate to be 10-16%, control a rolling speed to be 3-8 m/s, perform medium-pressure water descaling between a first pass and a second pass under the pressure of the descaling water of 200-280 bar, and control a finishing rolling temperature to be 860-900° C. 
     9) Cool to a coiling temperature in a manner of laminar cooling, water curtain cooling or intensified cooling. 
     10) Perform coiling, and control the coiling temperature to be 565-595° C. 
     11) Perform austenitizing after uncoiling and blanking, control an austenitizing temperature to be 930-980° C., and hold for 6-15 min. 
     12) Perform die punching and deforming, and keep a pressure for 6-9 s in a die. 
     13) Perform quenching, control a quenching cooling speed to be 50-100° C./s, and then naturally cool to a room temperature. 
     The rolling process of the medium and thin slab is carried out in a short-process production line in any one of rolling mill arrangement forms such as a 6F production line or a 1R+6F production line, or a 2R+6F production line, or a 7F production line, or a 3R+4F production line, or 2R+5F production line, or a 1R+5F production line. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Chemical component (wt. %) of various embodiments and comparative examples of the present invention 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                 Embodiment 
                 C 
                 Si 
                 Mn 
                 P 
                 S 
                 Als 
                 Cr 
                 Ti 
                 Nb 
                 V 
                 Mo 
                 Ni 
                 B 
                 N 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                 1 
                 0.37 
                 0.39 
                 1.91 
                 0.004 
                 0.003 
                 0.015 
                 0.38 
                 0.045 
                 — 
                 — 
                 0.27 
                 0.16 
                 0.0043 
                 0.003 
               
               
                 2 
                 0.38 
                 0.43 
                 1.72 
                 0.003 
                 0.002 
                 0.036 
                 0.48 
                 0.042 
                 0.036 
                 — 
                 0.26 
                 — 
                 0.0046 
                 0.002 
               
               
                 3 
                 0.39 
                 0.36 
                 1.60 
                 0.005 
                 0.003 
                 0.029 
                 0.47 
                 — 
                 0.045 
                 — 
                 0.30 
                 — 
                 0.0040 
                 0.004 
               
               
                 4 
                 0.31 
                 0.44 
                 1.86 
                 0.006 
                 0.004 
                 0.060 
                 0.49 
                 — 
                 0.044 
                 0.041 
                 0.29 
                 — 
                 0.0049 
                 0.005 
               
               
                 5 
                 0.35 
                 0.40 
                 1.95 
                 0.004 
                 0.001 
                 0.035 
                 0.36 
                 0.036 
                 — 
                 — 
                 0.35 
                 0.11 
                 0.0050 
                 0.004 
               
               
                 6 
                 0.32 
                 0.42 
                 2.00 
                 0.003 
                 0.002 
                 0.057 
                 0.46 
                 — 
                 — 
                 0.045 
                 0.34 
                 0.20 
                 0.0048 
                 0.002 
               
               
                 7 
                 0.40 
                 0.38 
                 1.75 
                 0.002 
                 0.002 
                 0.043 
                 0.42 
                 0.038 
                 — 
                 0.036 
                 0.32 
                 — 
                 0.0041 
                 0.003 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 List of main process parameter values of various embodiments and comparative examples of the present invention 
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                   
                 Temperature 
                   
                 Finish 
                   
                   
                   
                   
                   
               
               
                   
                 of slab 
                 Tapping 
                 rolling 
                 Coiling 
                 Austenitizing 
                 Temperature 
                 Quenching 
                 Pressure 
               
               
                   
                 into furnace 
                 temperature 
                 temperature 
                 temperature 
                 temperature 
                 holding time 
                 cooling speed 
                 keeping time 
               
               
                 Embodiment 
                 ° C. 
                 ° C. 
                 ° C. 
                 ° C. 
                 ° C. 
                 min 
                 ° C./s 
                 in dies 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 1 
                 926-940 
                 1202-1215 
                 886-900 
                 576-590 
                 950 
                 12 
                 70 
                 7.5 
               
               
                 2 
                 861-955 
                 1197-1210 
                 860-873 
                 571-585 
                 980 
                 10 
                 80 
                 6.6 
               
               
                 3 
                 845-858 
                 1199-1214 
                 882-898 
                 584-595 
                 930 
                 15 
                 60 
                 6.5 
               
               
                 4 
                 972-988 
                 1185-1198 
                 874-888 
                 567-586 
                 960 
                 6 
                 50 
                 8 
               
               
                 5 
                 800-811 
                 1190-1204 
                 867-885 
                 573-593 
                 950 
                 7 
                 100 
                 9 
               
               
                 6 
                  989-1000 
                 1186-1201 
                 884-897 
                 565-587 
                 970 
                 13 
                 90 
                 6 
               
               
                 7 
                 825-838 
                 1200-1213 
                 863-877 
                 570-581 
                 980 
                 9 
                 95 
                 7 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
             
               
                 TABLE 3 
               
             
            
               
                   
               
               
                 List of mechanical property cases of various embodiments and 
               
               
                 comparative examples of the present invention 
               
            
           
           
               
               
               
               
               
            
               
                   
                   
                 Yield 
                 Tensile 
                   
               
               
                   
                 Thickness 
                 strength R p0.2   
                 strength R m   
                 Elongation 
               
               
                 Component 
                 mm 
                 MPa 
                 MPa 
                 A 80 mm  % 
               
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 1 
                 5.0 
                 1360 
                 1960 
                 5.6 
               
               
                 2 
                 7.0 
                 1380 
                 1985 
                 5.1 
               
               
                 3 
                 2.1 
                 1430 
                 2055 
                 5.4 
               
               
                 4 
                 3.5 
                 1340 
                 1930 
                 6.3 
               
               
                 5 
                 4.5 
                 1345 
                 1920 
                 5.8 
               
               
                 6 
                 10.0 
                 1400 
                 2010 
                 5.4 
               
               
                 7 
                 9.0 
                 1425 
                 2020 
                 5.7 
               
               
                   
               
            
           
         
       
     
     As can be seen from Table 3, the present application successfully makes the strength of the inventive steel up to 1900 MPa through a short process, which is of great significance for promoting the development of lightweight automobiles. 
     The present specific implementation is merely exemplary and does not limit the implementation of the technical solutions of the present invention.