JPH0526850B2 - - Google Patents

Description

[Detailed description of the invention]

(Industrial Application Field) The present invention relates to a manufacturing method that makes it possible to omit softening annealing of cold-worked rods and wires. (Prior Art) Generally, mechanical structural steel of S30C to S60C class is used for parts such as outer race for automobiles. However, this class of steel is extremely hard as hot-rolled, so prior to cold working such as forward extrusion or backward extrusion, annealing treatment is performed to soften tools to extend the life of tools such as molds and dies. There is. After cold working, surface hardening is performed using high frequency waves or the like to improve fatigue strength. In the above process, the softening annealing treatment requires heating at high temperatures and for a long time, which requires a large amount of energy, which also increases heat treatment costs and reduces productivity. is expected to be developed. Traditional methods for softening hot-rolled steel include a method of slowly cooling it immediately after rolling using a heat retention furnace or cooling cover, or a method of softening hot-rolled steel materials, or a method of softening hot-rolled steel materials by slowly cooling the material immediately after rolling, or a method of softening hot-rolled steel materials.
As described in Publication No. 58235, heating and rolling temperatures are controlled to a low level to refine austenite grains, reduce hardenability, and cause ferrite and pearlite transformation to occur during cooling after rolling. method is being carried out. However, these methods are effective for steel materials containing large amounts of alloying elements that generate extremely hard structures such as martensite or bainite even when left to cool in the atmosphere, but they do not produce ferrite and pearlite structures. It was impossible to significantly reduce the hardness of mechanical structural steels of the S30C to S60C class. (Problems to be Solved by the Invention) The present invention aims to improve deformation resistance to the point where cold working is possible even if the softening annealing is omitted for high carbon mechanical structural steel, which has conventionally been cold worked after softening annealing. The present invention provides a method for manufacturing high carbon rods and wires that can reduce the amount of carbon used. (Means for Solving the Problems) The gist of the present invention is, in weight %, C: 0.30 to 0.60%, Si: 0.10% or less, Mn: 0.20 to 0.50%, Cr: 0.50% or less, S: 0.002 to Using steel containing 0.050%, Al: 0.01 to 0.10%, B: 0.0005 to 0.0100%, Ti: 0.005 to 0.100%, with the balance consisting of Fe and impurities, finishing rolling was completed at 650 to 750 °C, after that
A method for producing a high carbon rod and wire rod for cold forging, characterized by cooling at a rate of 0.2 to 1.5°C/sec, and in terms of weight %, C: 0.30 to 0.60%, Si: 0.10% or less, Mn: Contains 0.20 to 0.50%, Cr: 0.50% or less, S: 0.002 to 0.050%, Al: 0.01 to 0.10%, B: 0.0005 to 0.0100%, Ti: 0.005 to 0.100%, Nb: 0.005 to 0.300%, and the remainder Finish rolling is completed at 650-750℃ using steel consisting of Fe and impurities, and then
This is a method for producing high carbon rods and wires for cold forging, characterized by cooling at a rate of 0.2 to 1.5°C/sec. (Function) Next, the reason for determining the composition of the steel used in the present invention will be described. C is an element necessary to increase the strength of the rod and wire rod after rolling, increase the surface strength through induction hardening, and improve fatigue resistance. If it is less than 0.30%, the specified strength and fatigue resistance cannot be obtained. . Furthermore, since the amount of ferrite increases significantly, the degree of softening, which is the objective of the present application, is reduced, and the need for it is also reduced. If the C content exceeds 0.60%, the strength will become too high and it will be impossible to omit softening annealing.
It was set at 0.30-0.60%. Si dissolves in ferrite and increases the strength of ferrite and reduces cold forging properties, so the smaller the amount, the better.
However, Si has a deoxidizing effect, and is added to the upper limit of 0.10% in order to slightly assist in deoxidizing Al. Like Si, Mn reduces cold forgeability, so it is better to have less Mn. However, since S added to improve machinability causes hot embrittlement, Mn addition is essential to prevent this. In addition, since Si is significantly reduced, it also has the role of a deoxidizing agent, and is specified at 0.20 to 0.50%. Cr is an important element that improves hardenability, but it also has the effect of increasing the pearlite transformation temperature, resulting in coarser lamella spacing and lower hardness, but if it exceeds 0.50%, the strength increases. If it becomes too much, the deformation resistance will increase, so please set an upper limit.
It was set at 0.50%. Since S combines with Mn to form MnS and reduces the deformability during cold working, it is better to have less S, so the lower limit is the amount that can be manufactured in a normal converter.
It was set as 0.002%. On the other hand, S is an essential element for improving machinability, and it is better to increase the amount to improve machinability, but if it exceeds 0.050%, the deformability will decrease significantly, so the amount of S should be 0.002%. ~0.050%. In addition to being an important deoxidizing agent, Al is an element that prevents coarsening of austenite grains and improves deformability and toughness. For this, a minimum of 0.01%
Moreover, if it exceeds 0.10%, alumina-based inclusions will increase, the cleanliness of the steel will deteriorate, and the deformability and toughness will deteriorate, so the upper limit was set at 0.10%. By adding B in combination with Ti, which will be described later, the hardenability is significantly increased and it becomes possible to reduce the amount of elements such as Mn and Cr. B also has the property of completing pearlite transformation in a shorter time when steel is cooled from a high temperature. However, the amount of B
If it is less than 0.0005%, the above effect will be small, and
If it exceeds 0.0100%, coarse B compounds will precipitate at the grain boundaries and inside the grains, which will not only reduce hardenability but also cause embrittlement, so it is limited to 0.0005 to 0.0100%. Ti captures N in the steel in the form of TiN and fully exhibits the softening function of B, and for this purpose, a minimum content of 0.005% or more is required in relation to the amount of N.
Moreover, if it exceeds 0.10%, cleanliness deteriorates and toughness deteriorates, so this was set as the upper limit. In addition, if you want to further improve the toughness, it is effective to add Nb, and the amount is 0.005 to 0.300.
% range. The reason is that by increasing the recrystallization temperature of austenite during the rolling process, the ferrite and pearlite grains after transformation are made finer and the toughness is improved. Next, the reason why the finish rolling temperature was specified at 650 to 750°C will be described below. Steel that satisfies the above-mentioned compositional requirements is a steel type that can be finish rolled at a temperature exceeding 750°C and then has a relatively soft ferrite and pearlite structure even if it is allowed to cool in the atmosphere. However, in this case, the deformation resistance is still high, and the rolled material cannot be cold forged as is. In contrast to this
By finishing rolling at 750℃ or less, B
In addition to strengthening the high temperature of pearlite transformation and making pearlite softer, precipitation of ferrite before pearlite transformation is also promoted and the ferrite fraction increases. As a result, the deformation resistance was significantly reduced, and it was discovered that the rolled material could be cold forged as is. Based on this new knowledge, the finish rolling temperature is
The temperature was limited to 750℃ or less. In addition, the finish rolling temperature was set at 650℃.
If the rolling was carried out at a lower temperature than 650°C, processing strain during rolling would remain and deformation resistance would increase, so the lower limit temperature was set at 650°C. Furthermore, the reason why the lower limit of the cooling rate after rolling was set at 0.2℃/sec is that even if the cooling rate is lower than this, a very large softening effect cannot be expected. If rolling is performed at the above-mentioned finish rolling temperature using steel that satisfies the above requirements, a soft material can be obtained without special slow cooling, so the cooling rate was set at 1.5°C/sec, which corresponds to air cooling of a 10 mmφ material. (Example) Next, an example will be described. Table 1 shows the chemical composition of the test materials. C content is 0.45%, 0.49% and 0.55 as sample materials
% steel (a, b, c steel), 120φ billet with chemical composition compatible with mechanical structural carbon steel S35C (d steel), S40C (e steel) and S48C (f steel) was used as a comparison material. there was.

[Table] Table 2 also shows the heating temperature and finish rolling temperature of the 120φ billet.

[Table] For each steel, the heating temperature was adjusted to 1075°C and 955°C, and the finish rolling temperature was carried out at two levels: method A (925°C) and method B (740°C), and rolled into a 30 mm round steel bar. Cooling after finish rolling is 0.9 for each steel.
It was cooled at a rate of °C/sec. Next, in order to investigate the cold forgeability of the rolled material thus produced, tests were conducted on tensile strength and deformation resistance during compression. A JIS No. 4 test piece was used for the tensile test, and a test piece of 8 mm (diameter) x 12 (height) was used for the deformation resistance test.
A sample was taken from the pipe and used for testing. Table 3 shows the mechanical test results.

[Table] 0.45C steel (a steel), 0.49C steel (b
The deformation resistance of steel) and 0.55C steel (c steel) is 102.5Kgf/mm ² , 106.5Kgf/mm ² and 111.3Kg, respectively.
f/ ^mm2 . On the other hand, when finish rolling is carried out at a low temperature of 750°C or less using Method B, the deformation resistance is 93.7Kgf/mm ² , 96.4Kgf/mm ² and 99.6Kgf/mm ² , respectively, which is the same as the tensile strength. Together with the reduction, the value has decreased significantly, and the value that allows cold forging (100Kg
f/mm ² or less). On the other hand, in the case of comparison material, S35C steel with low C content (d
(steel), deformation resistance is low even with conventional manufacturing methods.
It is about 90.8Kgf/mm ² and can be cold forged.
However, with S40C steel (e steel) and S48C (f steel),
Even when rolling was completed at 740° C. using method A, the deformation resistance remained at least 100 Kgf/mm ² and did not decrease to the extent that cold forging was possible. Moreover, FIG. 1 shows the microstructures of rolled materials manufactured by method A and method B for steel b and steel f, respectively. The lower the finishing temperature for both steel b and steel f,
Both ferrite and pearlite grains are refined, but the ferrite fraction is much higher in Steel B, and the lamellar spacing of pearlite is also larger. This is the reason why the deformation resistance and tensile strength of the steel according to the invention are significantly reduced. The ferrite fraction is expressed by measuring the amount occupied by the ferrite structure mixed with the pearlite structure using an image analysis device equipped with an optical microscope function (product name: LUZEX500 manufactured by Nippon Regulator Co., Ltd.). (Effects of the Invention) As described above, the present invention provides a method for manufacturing high carbon rods and wires that can be cold worked even if the conventional softening annealing is omitted, and is an invention with great economic effects. be.

[Brief explanation of the drawing]

Figure 1 shows a steel (b steel) that meets the compositional requirements of the present invention and a comparative steel (f steel) when the finish rolling temperature is changed.
1 is a microscopic structure photograph showing the microstructure of steel).

Claims (1)

[Claims] 1% by weight: C: 0.30 to 0.60%, Si: 0.10% or less, Mn: 0.20 to 0.50%, Cr: 0.50% or less, S: 0.002 to 0.050%, Al: 0.01 to 0.10%. Using steel containing B: 0.0005 to 0.0100%, Ti: 0.005 to 0.100%, and the balance consisting of Fe and impurities, finish rolling was completed at 650 to 750 °C, and then
A method for producing high carbon rods and wires for cold forging, characterized by cooling at a rate of 0.2 to 1.5°C/sec. 2 In weight%, C: 0.30-0.60%, Si: 0.10% or less, Mn: 0.20-0.50%, Cr: 0.50% or less, S: 0.002-0.050%, Al: 0.01-0.10%, B: 0.0005-0.0100 %, Ti: 0.005-0.100%, Nb: 0.005-0.300%, with the balance consisting of Fe and impurities, finishing rolling was completed at 650-750°C, and then
A method for producing high carbon rods and wires for cold forging, characterized by cooling at a rate of 0.2 to 1.5°C/sec.