Comprehensive discussion on the research and development of high-strength bolt steel with delayed fracture resistance Hui Weijun Dong Wei Weng Yuqing Institute of Iron and Steel Research Institute of Metals 1 High-strength bolt steel development trend 1, the intensity level of strength snails is divided into 8.8 9.819 and 12.9 Grades, usually medium or medium carbon steel or medium carbon alloy steel, but also non-tempered forged boron steel double-phase steel or low-carbon martensitic steel with the development of various production sectors such as automobile machinery construction light industry, Higher requirements are placed on the manufacture of various types of fasteners such as bolts, screws, nuts, etc., such as in the high-performance of automobiles and the high-rise construction of lightweight building structures and the over-longing of bridges. The bolts put forward the requirements of high design stress and light weight. In this respect, the requirements of the automobile manufacturing industry are the strongest. The original automobile bolts, especially the engine bolts, have been difficult to meet the requirements of high stress of the automobile engine. The most effective measure is the high strength of bolt steel. In the United States, the bolt strength level used in automobiles is above 9.8. At present, some bolts for automobile construction machinery even require tensile strength greater than 1400 rivers, and the strength of the joints increases. Especially when the tensile strength exceeds 1200, the delayed fracture becomes very sudden. This is a low-intermediate tempering horse. Cylinder steel, 1 strength, the first major high-strength bolt encountered is a notched part, has a high notch sensitivity, and is prone to delayed fracture at the point where the notch is concentrated, such as the transition between the rod and the head or the root of the thread. Therefore, the delayed fracture of high-strength bolts is a very typical example, 2323! For example, in Japan, the bolt steel with a tensile strength exceeding 1200 rivers has been subjected to a passive avoidance method in the 967邶8186 repair, bolt-level, divided into 81 several clear-cut 131-class bolts after 1977, even at normal strength. 1. The national key basic research and development development planning blade funding project 01998061503 engineering machinery civil construction automobile construction machinery tensile strength MPa bolt strength level 21 bolts also occur delayed fracture. Called in 1979, the re-cultivation of Ting did not make the 1. Ding snail in 1981. Tiemaiqiao discontinued the use of graded screws. In the early 1980s, the GM’s Gongshan had delayed fractures on the two 12.9 bolts mounted on the bottom of the brakes. 27 traffic stresses occurred before and after. State 2 crack initiation starting point 8 rapid fracture, å»çŸœ tensile strength MPa fracture sensitivity impact accident, and finally replaced these two kinds of screw inspection on 640,000 cars in recent years, the demand growth, foreign countries, especially Japan and other countries High-strength bolt steel resistant to delayed fracture has been extensively researched and developed, and has developed a series of high-strength snail steels with excellent resistance to delayed fracture and excellent stress. For example, Sumitomo Metal's 1 series Kobe's super-strong steel plan and the high-performance structural steel metering iron 5 series, recently in Japan and Korea, put into the research and development of high-strength screw steel Development is one of the important lessons 45!
2 Characteristics of delayed fracture Delayed fracture refers to the material used in static stress. A phenomenon in which a sudden brittle fracture occurs after a certain period of time is a kind of environmental embrittlement that occurs when material environmental stress interacts. This type of delayed rupture phenomenon, which causes the deterioration of the village quality, is a major flash that hinders the high strength of steel for mechanical production. It can be roughly classified into the following two types of delayed fractures caused by hydrogen outside the main intrusion of external environment. Bolts used in bridges, etc., in long-term storms such as humid air and rain, and intrusion into the steel during the manufacturing process such as delayed fracture pickling and plating, the delayed fracture caused by hydrogen, such as plating bolts, after loading, after a few hours or Delayed fracture occurs after a short period of several days. The former is caused by corrosion during long-term exposure and intrusion of hydrogen generated by corrosion reaction at the corrosion pit. The latter is produced in a manufacturing process such as pickling and ruthenium plating. Intrusion into steel during processing, caused by stress concentration in the concentration of stress.
The actual steel is delayed in the environment, mainly quenched and tempered martensitic steel. It has the following characteristic 338 strength water, the sensitivity of delayed fracture is significant, and large delayed fracture usually occurs near room temperature, but from From room temperature to 100, nearby, with the increase of temperature, the sensitivity of delayed fracture increases and the difference between low temperature brittle fracture; macroscopically, delayed fracture is not accompanied by different points of large plastic deformation and creep fracture; The lower strain rate occurs at a different point from the fatigue fracture; it occurs at a stress much lower than the yield strength; at a temperature 350 at which low temperature temper brittleness occurs, the sensitivity of delayed fracture is greatest after tempering nearby; Dominance of grain boundary cracks 3 Research and development of high-strength bolt steel with delayed fracture resistance 3.1 Ways to improve the resistance to delayed fracture In the research of materials, the specific measures usually taken at the time of development of new steels at home and abroad are as follows 2,6 reduction The grain boundary hemiplegia reduces the phosphorus content of the impurity element as much as possible. Phosphorus reduces the grain boundary bond strength. The sulfur content sieve promotes the absorption of hydrogen in a corrosive environment while reducing the promotion of phosphorus. Sulfur co-segregated manganese contains antimony, which prevents the embrittlement and refinement of the product boundary 1 and adds elements such as human 1 to form dispersed carbonitrides to refine austenite grains and improve the tempering temperature. Adding elements with strong anti-tempering and softening ability, such as molybdenum and vanadium, can improve the tempering temperature of the tempering temperature to make the carbide spheroidize while keeping the strength constant, so as to avoid the embrittlement of the product boundary. The area, and the carbide can be finely packaged to adjust the alloying elements such as nickel to reduce the manganese content to obtain higher notch toughness.
Minimize the amount of hydrogen invaded by the steel surface, that is, add alloying elements that inhibit the formation of corrosion pits, such as elements to make invasive oxygen harmless, add appropriate amount of microalloys, etc., to form fine carbon halides before refining and quenching. In addition to austenite grains, it can also act as a trap for hydrogen, inhibit hydrogen diffusion and evenly distribute hydrogen. In addition, since the lower bainite structure has no carbide precipitation and a small effective grain size on the prior austenite grain boundaries, Good delayed fracture resistance 3 Therefore, the route is to obtain the multiphase structure of lower bainite and appropriate martensite retained austenite by isothermal treatment, using high strength and bainite structure of martensite structure. The good delayed fracture resistance to achieve good delayed fracture resistance under strength is a method of refining the microstructure and forming a large number of carbide preferential nucleation sites in the austenite grains by means of a magnetic field induction heat treatment method, and then passing through A suitable tempering treatment, such as induction magnetic field heat treatment, activates the preferential nucleation sites in the crystal to promote precipitation and refinement of the intracrystalline carbides. Reduces the elimination of carbides in the boundary and refines the martensite structure to obtain a fine martensite structure with no grain boundaries or less grain boundary carbides. Thus, high strength can be achieved without isothermal treatment. Good resistance to delayed fracture.
3.2 The development of new steel grades uses the above measures. High-strength snail steels with good resistance to delayed fracture have been successfully developed. For example, Sumitomo Kim of Japan, the 05 series of Kobe Steel's Qiao series of China Iron and Steel Research Institute's ADF series and other iU generations and the mid-1990s developed a new type of bolt-on steel for delayed fracture. Compared with Yang 501440, the tempering resistance of 052 and 03 steel is obviously improved. By adjusting the tempering temperature, the strength of the cathode can be adjusted to the intensity of the cathodic hydrogen transfer. The tensile test results show that at the same strength level, the steel is developed. Especially for children with high molybdenum content and vanadium content, due to the higher tempering temperature, the delayed fracture resistance is also significantly higher than that of human 052. This is because for human 033 steel, the carbide is uniform after high temperature tempering. Dispersed in the grain boundary product, so that the hydrogen of the precipitated material is evenly distributed, so as to delay the crystal growth of the fracture origin, and also improve the tenderness. The innocent scorpion of the gambling world, from the resistance, a new steel grade developed along the crystal break Nippon Steel 7 is similar to 053 steel, but the carbon and silicon test is lower and does not contain bismuth 42. He, steel has The basic strength of 5 tensile strength MPa was greatly improved, and the influence of common alloying elements on delayed fracture resistance was systematically studied. Compared with the regression analysis of the delayed fracture test results of 16 furnace steel, titanium can effectively improve the delayed fracture resistance of steel, and on this basis, the delayed fracture resistance of the 03 series of steel with good delayed fracture resistance is improved. KNDS23S is used for 1200 river, graded head bolts, mass production, and 033 car bolts.
The phosphorus and sulfur content in the series of strength bolt steels developed by Datong Special Steel is reduced to below 0.015 and 005, while reducing the silicon and manganese content, adding vanadium and increasing the molybdenum content, so that it will precipitate sub-hardening when tempered at high temperature. carbide. After tempering at 580, the tensile strength can reach 1400. The above-mentioned bending acceleration type delayed fracture test and bolt physical test prove that the delayed fracture resistance is good. The tough steel river 435440 in 13 is only 1200 rivers, 3 This level can only be achieved at the intensity level. The reason is that the tempering temperature is increased to spheroidize the cementite precipitated on the prior austenite grain boundary, and the grain boundary is added to the delayed fracture and the hydrogen is adsorbed. The hydrogen trap is used for the 1400 river! On the connecting rod bolts, the delayed fracture characteristics of the microalloying element 6 containing the high-strength bolt steel of the bainite structure are added by reducing the content of the sulfur-phosphorus, and the molybdenum containing + is added successfully. Almost all of the high-strength bolts that have been used are tempered martensitic steels. These steels are more sensitive to delayed fracture. Some studies have found that 3, the delay of steel with lower bainite structure is longer than that of tempered martensitic steel. The fracture resistance is better. However, there is a significant disadvantage in the lower body of the body. The bolt steel of the body structure is treated at a relatively low temperature of 300,375 to obtain a bainite structure = even if the steel is in the 1500 river, more than 3 high-carbon high-carbon steel is used in the laboratory to obtain 75 bainite +25 residual by isothermal treatment. The austenitic multiphase structure exhibits good resistance to delayed fracture at the strength level of 1750 and 3, and it is planned to produce a screw test in order to carry out the development of cold forging technology in the actual environment. The demand for forged bolt steel has increased significantly, and for this reason, low-cost low-carbon high-strength boron steel has been developed with the principle of designing a carbon source. Improve the cold deformation ability of steel, add a trace of boron to compensate for the loss of strength and transparency caused by carbon reduction. In addition, according to the need, we can also add appropriate amount, adding alloying elements, and improve the permeability. For example, Datong Special Steel 12 has recently developed a low-resistance fracture. Steel and mountains have reduced the phosphorus and sulfur content of the impurity elements and added fineness. The grain, thus the newly developed boron steel at 10001300 touch! The resistance to delayed fracture in the strength range is equivalent to or better than 535 steel. The steel has been applied to the new iron mouth of the car snail of (1) class. Recently developed to be similar to steel, the other steel grades of the manufacturer steel grade MPa Kobe Sumitomo Datong Boron Steel Nippon Steel Co., Ltd. Kobe Institute of Kobe Equipotential Treatment Pohang Isothermal Treatment 4 Conclusion Research institutions have used their own methods to conduct experiments, and this is an urgent need for high strength of bolts. In Japan and other countries, extensive research on high-strength bolt steel resistant to delayed fracture has been carried out, and a series of experimental methods for evaluating delayed fracture properties of delayed fracture-resistant systems have been developed. Each company and the quality are relatively compared and lack of mutuality. The lack of a unified delayed fracture experimental method has become the biggest obstacle to the practical application of delayed-breaking high-strength bolt steel. The scientific evaluation method of delayed fracture resistance is to study and develop the key link of delayed fracture strength snail steel. , smashed, times if¢O, degree レUレ, crisis, force, special, 1995, 441224, 26, 3 Songshan Jin Zuo.å’¦ ç ´ , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , ,
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