JPH0530884B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Field of Application] The present invention relates to a method for manufacturing high-tensile, non-tempered bolts for use in automobiles and industrial machinery. [Conventional technology] In recent years, advances in cold working machines such as cold forging and improvements in cold working steel materials have been remarkable. A bolt is formed in between. In order to increase cold deformability and tool life, the steel materials used in these products are softened or annealed to spheroidize before cold forging, and then quenched and tempered after cold forming to achieve the specified strength. is normal.
However, when softening or spheroidizing annealing is performed before cold forging, and quenching and tempering are performed after cold forming, there is a drawback that the secondary processing step is long and the manufacturing cost is high. Therefore, in order to omit these spheroidizing annealing, quenching, and tempering, precipitation strengthening elements such as Ti, B, Nb, and V are added to low carbon steel, and forced air cooling (Stellmor) after wire rolling is applied. Therefore, a non-thermal wire material for bolts that increases strength and eliminates quenching and tempering after bolt forming (Japanese Patent Application Laid-Open No. 53-51121,
53-56121), and bolts that undergo isothermal transformation after hot rolling of low- and medium-carbon manganese steel wire rods, increase strength and toughness by controlling the wire drawing method, and omit quenching and tempering after bolt forming. Non-thermal tempered wire rods for use (Japanese Patent Publication No. 60-406, Japanese Patent Application Laid-Open No. 60-15622) are also being developed. However, these methods have a problem in that the deformation resistance during cold forging increases as the strength of the wire increases. For this reason, some methods utilize a type of Bauschinger effect by drawing wire before cold forging to reduce deformation resistance, but controlling the wire drawing method alone is not effective in reducing deformation resistance. is rare, and currently it has not been possible to achieve deformation resistance comparable to that of current spheroidized annealed wires. In addition, the strength variation within the coil is about 3 to 5 times larger than that of heat-treated tempered steel.
In order to meet the lower strength limit of the standard, the average strength must be further increased. Therefore, when manufacturing bolts from these wire rods, there is a problem that the life of cold working tools is significantly reduced, and the use of these wires has not progressed. [Problems to be Solved by the Invention] The present invention solves the problem of high-tensile bolts that have conventionally been manufactured by cold working after spheroidizing annealing and subsequent quenching and tempering, even if the spheroidizing annealing is omitted. The present invention provides a method for manufacturing a wire rod having sufficiently low deformation resistance during processing. [Means for solving the problem] In the process of manufacturing a bolt after cold drawing from a non-tempered wire material, the bolt is formed by deformation in the opposite compression direction after drawing, so a kind of Bauschinger occurs during this process. It is known that the deformation resistance is reduced. The present inventors not only controlled conventional wire drawing methods, but also conducted experimental research on various metallurgical factors such as composition, structure, and controlled cooling after hot rolling, and found that in order to maximize the Bauschinger effect, Rather than using the expensive precipitation-strengthening elements mentioned above, it is optimal to increase the amount of C and create a fine ferrite-pearlite structure with a low ferrite fraction by rapid cooling such as hot water bath cooling rather than a low-carbon bainitic structure. It was also confirmed that in wire rods made of these components, the Bauschinger effect increases as the area reduction rate during drawing increases. Therefore, it has been found that by effectively combining these factors, the deformation resistance during bolt forming can be significantly reduced. We also found that by cooling the hot-rolled low-carbon steel wire in a hot water bath, it was possible to significantly reduce strength variations within the coil, compared to conventional Stelmor coolant or off-line isothermal transformation treated wire. The present invention was made based on these findings, and includes, in weight%, C: 0.15% or more and 0.30% or less,
Si: 0.03% or more and 0.55% or less, Mn: 1.1% or more and 2.0%
A steel material containing the following, with the remainder being Fe and unavoidable impurities, is hot-rolled into a wire rod, wound into a coil, slowly cooled or allowed to cool for 3 to 10 seconds to make the austenite grain size uniform, and then placed in a hot water bath. The wire rod is cooled with a ferrite/pearlite structure, and the area reduction rate is 20.
This is a method for manufacturing a high-tensile non-temperature bolt with a tensile strength of 70 Kgf/mm ² or more, which is characterized by subjecting it to a drawing process of 50% or more. The reasons for limiting the chemical components and manufacturing conditions in the present invention will be explained below. It is well known that C has a very important effect on the strength and ductility of steel materials, but in order to obtain a strength of 70 Kgf/mm ² or more at a conventional drawing rate of 20% or more, the tensile strength must be at least 55 Kgf/mm 2. ² is required. It has been found that the Bauschinger effect can be increased by increasing the C content in steel with a ferrite-pearlitic structure. The reason for this is that the Bauschinger effect is influenced by the mobile dislocation density in ferrite, and increasing the C content reduces the ferrite fraction and increases the introduced dislocation density. Therefore, in order to enhance the Bauschinger effect and reduce deformation resistance, if C is less than 0.15%, this objective will not be achieved, and if it exceeds 0.30%, although the Bauschinger effect will increase, ductility will deteriorate and deformation will be reduced. Since the resistance would be excessive and the tool life would be shortened, it was set to 0.15% or more and 0.30% or less. In addition to being used for deoxidation, Si dissolves in iron and increases the yield point and tensile strength, but if it is less than 0.03%, the deoxidation effect is insufficient, and if it exceeds 0.55%, the tensile strength increases. However, compared to C, the Bauschinger effect tends to increase less, ductility deteriorates, and cold forgeability deteriorates, so 0.03% or more 0.55
% or less. In addition to Si deoxidation, Al, Al-
Si deoxidation is also frequently used, and Al is used for deoxidation and to prevent crystal grain coarsening during heat treatment.
It is desirable to add 0.010% to 0.060%. Mn significantly contributes to increasing strength, improves the microstructure of the wire, and facilitates cold forging. Mn
If it is less than 1.1%, the improvement in strength is insufficient. However, since Mn is not an element that increases the Bauschinger effect, excessive addition of more than 2.0% is not preferable because it significantly increases deformation resistance. Therefore 1.1%
It has been set at 2.0% or less. Other impurities are acceptable within the range normally present in this type of steel. Next, cooling after hot rolling will be described. The reason for slow cooling or cooling for 3 to 10 seconds after rolling is to equalize the wire temperature during this time, to equalize the austenite grain size that has become uneven due to rolling, and at the same time to adjust the scale for good peelability. This is what we do. If this time is too short, the effect cannot be obtained, and if it is too long, the crystal grains will become coarse and the scale will become thick and strong, which is not preferable. The first purpose of cooling the wire in a hot water bath is to increase the Bauschinger effect, which reduces deformation resistance during bolt forming. The purpose is to adjust the structure to a ferrite/pearlite structure with a low ratio. In the ferrite-pearlite structure, the Bauschinger effect is greatly affected by the cooling rate after rolling, and increases as the cooling rate increases. We found that the effect was significantly greater. The reason for this is that, as described above, the Bauschinger effect is influenced by the mobile dislocation density in ferrite, and increasing the cooling rate reduces the ferrite fraction and increases the introduced dislocation density. The second purpose is that in forced air cooling, differences in air volume occur at the overlapping parts of the wire rings, resulting in significantly larger strength variations than in tempered steel, whereas in hot water cooling, the overlapping parts of the rings are also covered with a steam film. Stable film boiling cooling keeps the cooling rate almost constant,
This is to make the strength variation extremely small. Note that the hot water according to the present invention has a temperature of 95°C or higher and 100°C or lower. The reason for this is that by cooling an austenite structure with a uniform grain size in hot water, a transformed structure of ferrite/pearlite with a uniform grain size can be obtained, and it is also possible to uniformly adhere a scale of an appropriate thickness to the wire surface. This is because the boiling film in hot water is stabilized and the uniformity of the cooling rate is improved. The starting temperature for hot water cooling is ferrite.
It is possible to start from 900℃, which is higher than the temperature at which pearlite transformation starts, but at about 700℃, which is just above the temperature at which pearlite transformation starts.
It may be used after quenching in a cooling trough to ℃.
The reason for cooling with boiling water after slow cooling or cooling for 3 to 10 seconds is to reduce the amount of ferrite, make the lamellar spacing finer, increase the strength, and also make the scale thinner and easier to peel off. In addition, if the temperature at which the water is withdrawn from the hot water bath is set to 400°C or higher and lower than the pearlite transformation temperature, precipitation of C and N dissolved in ferrite will proceed, thereby preventing a decrease in ductility due to strain aging after cold working. Effects can be obtained. Next, we will discuss the drawing process before bolt forming. This drawing process is performed to improve dimensional accuracy, and even for tempered materials, it is performed with an area reduction rate of around 10%. In the case of non-tempered wire rods, this process also has the effect of sufficiently work-hardening the strength of the hot-rolled wire rods imparted by controlled cooling to adjust the strength to a predetermined level. As is generally known, the Bauschinger effect increases as the drawing rate increases. This tendency is the same for ferrite-pearlite wires subjected to controlled cooling, and the reason for this is that the reverse stress in the ferrite increases. Therefore, in order to enhance the Bauschinger effect and reduce deformation resistance, the area reduction ratio is 20.
If it is less than 50%, this purpose will not be achieved, and if it exceeds 50%, the ductility will deteriorate and the drawing cost will increase, making it uneconomical. After that, shearing, bolt head shaping, and thread rolling are performed, and the final strength, elongation, and reduction can be improved by further performing bluing or baking treatment. In this case, heating at 500°C or less is desirable. Examples will be described below. [Example] Table 1 shows the chemical components of the test materials. B1~ in the table
B5 is a comparative example, and all others are steel materials according to the present invention. These steels are produced by forging or rolling after melting.
A 162 mm square steel billet was prepared and rolled into 9, 13, 15, and 18 mmφ wire rods shown in Table 2, and cooled in a hot water bath at 97°C to 99°C, normal cooling, or forced air cooling. The tensile test results are shown in Table 3. As shown in the table, the tensile strength of the material according to the present invention is high, and the elongation and reduction of area are also extremely good. In addition, the strength variation within the coil is very small, and the scale can be removed by pickling with hydrochloric acid.

【table】

【table】

[Table] Performance has also improved significantly. In particular, by lowering the winding temperature,
A3 and A4 cooled in a hot water bath in the temperature range of 700℃ to 400℃
It is possible to achieve higher strength and shorter scale removal time. However, B1 to B3, which are outside the scope of the present invention, have low apertures and large variations in strength. Moreover, it takes a long time to remove the scale, and the scale removal property is poor. B4 and B5 with forced air cooling also have high strength and narrowing, but the strength variation is large. Table 4 shows the tensile properties after drawing around 30%. However, the comparative example B3 is made from 13mmφ to 12mm after spheroidizing annealing, which is the same as the current manufacturing process.
Drawing was performed on mmφ (14.8%). The deformation resistance shown here was measured using each test steel wire before bolt forming, and it is known that the lower this value is, the longer the tool life during cold forging will be. Each test steel wire was cut off using a lathe to create a cylindrical specimen for upsetting [however, the upsetting ratio (height/diameter) was adjusted to 1.5], and the strain rate for the universal test was Upsetting is performed at 1/sec (however, the upsetting pressure plate is a restrained type carbide pressure plate with concentric grooves).
The deformation resistance during the processing was measured. The deformation resistance was determined at a logarithmic strain [1nH ₀ /H (where H ₀ and H represent the initial specimen length and the specimen length after upsetting, respectively)] 1.5, and the upsetting load was deformed. It is divided by the cross-sectional area of the subsequent test piece. According to this, all of the steels of the present invention have almost the same deformation resistance as wire-drawn B3 after spheroidizing annealing, which is a value that is sufficient to withstand actual hedding processing.
However, B1, B2, B4, and B5, which are outside the scope of the present invention, have high deformation resistance and significantly shorten tool life, so they are not suitable for practical use. Table 5 shows the tensile properties after forming this into a hexagonal bolt and subjecting it to bluing treatment at 350°C. However, only B3 was heated at 880°C, quenched, and then tempered at 500°C. The bolt tensile test used wedge tension at an angle of 10° to examine the strength and presence or absence of head popping. As a result, the bolt of the present invention has a tensile strength of 80 kg.
No head skipping at f/mm ² or higher, JISB1051 (1976)
It can be seen that it has good characteristics as an 8.8 class bolt/screw. Comparative examples B1 and B2 are tensile strength

[Table] *: Wire drawing after spheroidizing annealing

[Table] is low, and quenching and tempering can be performed at the same level as the conventional process.
B3 and forced air cooling B4 and B5 only, tensile strength 80Kg
f/mm ² or more. [Effects of the Invention] As described above, the bolt manufactured according to the present invention has deformation resistance during cold forging comparable to that of a softened or spheroidized annealed wire, although it uses a non-tempered wire. Therefore, the conventional spheroidizing annealing and quenching/tempering processes can be omitted without deteriorating the tool life during cold forging, resulting in significant savings. On the other hand, compared to conventional non-heat-treated bolts, tool life can be extended, tooling costs, which account for a large portion of bolt manufacturing costs, can be significantly reduced, and productivity can be improved by reducing the number of tool changes. Improvements have been made and the industrial effects are extremely significant.

Claims (1)

[Claims] A steel material containing, in 1% by weight, C: 0.15% or more and 0.30% or less, Si: 0.03% or more and 0.55% or less, Mn: 1.1% or more and 2.0% or less, and the balance is Fe and unavoidable impurities. , after hot rolling the wire rod, winding it into a coil,
After 3 to 10 seconds of slow cooling or cooling to make the austenite grain size uniform, the wire rod is cooled in a hot water bath to form a ferrite/parlato structure with an area reduction of 20% or more.
A method for producing high-tensile non-temperature bolts characterized by subjecting them to 50% or less drawing processing, followed by shearing, head forming, and thread rolling. 2. The method for manufacturing a high-tensile non-temperature bolt according to claim 1, wherein the starting temperature of cooling in a hot water bath is 700 to 900°C. 3 Cool in a hot water bath until the temperature at which it is withdrawn from the hot water bath is
The method for manufacturing a high-tensile non-temperature bolt according to claim 1 or 2, wherein the temperature is 400°C to below the pearlite transformation temperature.