JPH0112811B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for producing steel with excellent machinability.
More specifically, the purpose is to provide a method for manufacturing steel with excellent machinability that addresses the automation and high efficiency of the cutting process, which has recently seen remarkable progress, and also to provide a method for producing steel with excellent machinability. It is possible to simplify or omit the heat treatment (pre-heat treatment) that is normally applied to materials to improve machinability, and also to omit the post-heat treatment that is applied after cutting for some machined parts. Therefore, the present invention relates to a manufacturing method that aims to prevent dimensional changes due to post-heat treatment. Currently, a large amount of free-cutting steel is used in cutting processes in a wide range of fields, from automobile parts to precision machine parts. The reason for this is that steel is positioned as a steel that can fully utilize the capabilities of cutting machines and cutting tools. Particularly when automating or unmanned operation of a cutting machine, it is necessary to have excellent machinability, and in particular, it is important to have good chip disposability. Conventionally, in order to improve machinability, a preheat treatment is performed prior to cutting steel. For example, gears and shafts of automobiles are subjected to isothermal annealing or normalizing-tempering treatment. For example, this isothermal annealing process
It is carried out by holding at 880-920°C for 60 minutes and then at 600-700°C for 70 minutes, which has the problem of requiring a large amount of heat and a long time. Moreover, the cutting chips form ribbons with a relatively large curl radius.
The chip disposability was not sufficient, such as wrapping around the workpiece or cutting tool during cutting, resulting in poor workability. The present invention aims to eliminate these drawbacks, and its purpose is to create a steel with excellent machinability that requires less heat and does not cause chips to form into small pieces and wrap around the workpiece or cutting tool being cut. to provide manufacturing methods. As a result of intensive research aimed at achieving the above object, the inventors of the present invention have found that steel in which 10 to 75% martensite phase is uniformly mixed does not have the above drawbacks. When the proportion of martensitic phase in steel exceeds 75%, the remaining soft phase cannot absorb the amount of deformation in the shear zone, and some martensitic phase is also deformed. Therefore, the stress level for deformation increases, the heat generation value increases rapidly, and the chips become tougher and continuous in the form of a ribbon, which causes the disadvantage that they wrap around the workpiece or cutting tool being cut. On the other hand, the mixing ratio of martensitic phase in steel is
If it is less than 10%, strain concentration in the soft phase around the martensitic phase in the chip shear region becomes less active, the effect of generating microcracks in chips is significantly reduced, and the effect of improving the processing method is also reduced. do. As a method for forming a hard martensitic phase in the soft steel of the present invention, it can be easily produced by utilizing the transformation point specific to various steels. In other words, from a temperature above the Ar ₃ transformation point, during cooling
It can be produced by holding it in a temperature range from Ar ₃ to Ar ₁ transformation point for a certain period of time and then cooling it. In this case, by changing the holding time in the temperature range from the Ar ₃ transformation point to the Ar ₁ transformation point, the amount of ferrite phase produced can be changed, and by subsequent cooling, the desired proportion of martensite phase can be achieved. to be generated. The holding time may be determined with reference to the isothermal transformation diagram of the steel. If the cooling rate is slow during cooling after holding for a certain period of time, one to two phases of a pearlite phase and a bainite phase will be generated in addition to the martensite phase, so cooling with water or the like is preferable. The material obtained by the above method may be subjected to a tempering treatment if necessary. Softening the martensitic phase and bainite phase produced by such a method by tempering is effective from the viewpoint of tool wear under cutting conditions using high-speed steel tools and carbide tools. For this reason, it is effective to temper at a constant temperature range of 650°C or less. However, when tempering is performed at a temperature higher than 650°C, the hardness of the martensitic phase is significantly softened, and the effect of improving machinability is lost. On the other hand, cermets with high wear resistance and
Cutting using CBN tools does not require softening of the martensitic phase by tempering. When cutting steel containing a hard martensitic phase, strain concentrates on the soft ferrite, pearlite, or bainite phases in the chip shear region, and these phases become work hardened and microcracks are formed. Significantly becomes brittle. Therefore, it becomes chips that are easy to crush and have good processability. At this time, the shear zone is reduced due to the mixed martensite phase acting as an obstacle, thereby reducing the cutting force applied to the tool. According to the present invention, machinability evaluated in terms of chip control and cutting resistance can be improved.
The remarkable improvement in chip control makes it possible to use it as an alternative to free-cutting steel when automating the cutting process or promoting unmanned operation. The reduction in cutting resistance leads to an increase in the amount of cutting by the amount of the reduction, which can promote cutting efficiency. Furthermore, in the present invention, in order to simplify the pre-heat treatment of the cutting part material, for example, the cooling rate of the material after normal hot forging at 1000°C or higher is controlled and cooled, and if necessary, tempering treatment is performed. By simply doing this, you can obtain sufficient hardness for the parts. Therefore, post-heat treatment after cutting can be omitted, and there may be cases where there is no need to consider dimensional changes in the component caused by post-heat treatment. Example 1 Outer diameter 60mmφ, inner diameter 30mmφ, length 330mm
The SCM435 steel was lowered from the temperature range of Ar ₃ or higher to an intermediate range of Ar ₃ to _{Ar 1} using a salt bath, held at 700°C for varying holding times, and then water-cooled. The martensite content and hardness of the obtained steel were as follows.

【table】

[Table] Example 2 Instead of SCM435 steel in Example 1, SCM420
Steel was used, but the rest was the same. The amount of martensite and hardness obtained were as follows.

[Table] As described above, according to the method of the present invention, it is possible to omit or simplify the pre-heat treatment such as normally performed isothermal annealing, spheroidizing annealing, or annealing and tempering. This has the effect of requiring less heat and shortening the processing time. In addition, it is possible to obtain hardness comparable to, for example, when SCM435 steel is subjected to conventional quenching-tempering treatment. Furthermore, since the material can be obtained as a part material that is easy to cut, there is no need for some parts to undergo heat treatment such as quenching or tempering after cutting, and therefore dimensional changes due to such post-heat treatment can be avoided. It has a good effect even if it is not taken into account. The steel of the present invention has an increased chip shear angle (φ),
Even thin chips have a small curl radius and have excellent processing properties. Conventional knowledge has held that the harder steel is cut or the higher the cutting speed, the more thin, continuous chips with a large chip shear angle (φ) and poor processability are generated. However, in the steel of the present invention, which contains a certain range of martensitic phases, the chip control improves as the hardness increases, and this tendency does not change even in the high-speed cutting range. The machinability test results are shown in Figure 1. The samples shown in Tables 1 and 2 were used as work materials. The cutting conditions for this test were: cutting speed 150m/min;
Tool feed rate 0.25 mm/rotation, depth of cut 1.5 mm, chip breaker width 2.5 mm, carbide tool P10 type [0,
10, 6, 6, 15, 15, 0.3] was used. moreover,
Cutting speed 80m/min, tool feed rate 0.15, 0.25, 0.30,
Tests were conducted on chip control under a wide range of conditions using various combinations of 0.35mm/rotation and chip breaker widths of 2.0 and 3.0mm. In each case, as shown in Figure 1, there was a tendency for the chip shear angle (φ) to increase as the hardness increased. This shows that steel containing a martensitic phase has excellent processability even though it produces thin chips under a wide range of cutting conditions. Moreover, recently, CBN tools that can cut hardened steel with a hardness of up to Hv700 have become popular, but since they are processed at high speeds, continuous chips that are difficult to process are often generated. This can be avoided by using the steel of the present invention, and the CBN tool's durability can be further demonstrated. Figure 2 shows the cutting resistance component of steel containing martensitic phase, and at the same time, SCM420 steel was spheroidized annealed (held at 750℃ for 2 hours and then furnace cooled (cooling rate of 10℃/hour). )), ferrite/pearlite structure (two comparative samples in Table 2), ferrite/pearlite structure
Comparisons were also made with each workpiece material with a bainite structure (air-cooled from a hot-rolled state with a diameter of 75 mm). Cutting resistance is principal force (Fc), feed force (Fs), back force (Ft)
However, both of these three component forces decrease in the samples containing a mixed amount of martensite (six samples in Table 2). This means that in terms of the power and rigidity of the cutting machine and the deflection of the tool, when martensitic phase is present, the amount of cutting can be increased compared to steels with other structures, which increases the cutting process. It is linked to efficiency. For example, 100 in Figure 2
In the cutting speed range of m/min, the resultant cutting force ((Fc ² + Fs ² + Ft ² ) ^1/2 ) applied to the tool is 85 for the spheroidized material.
Kgf is 81 Kgf for ferrite/pearlite steel and 73 Kgf for ferrite/bainitic steel, but when martensitic phase is mixed, it is 63 Kgf, which is a 23% lower cutting force than that of ferrite/pearlite steel. The amount of cutting can be increased by Figure 3 shows the influence on tool wear and chip formation during cutting of steel containing a martensitic phase. Cutting conditions are cutting speed 100
m/min, tool feed rate 0.25mm/rotation, chip breaker width 2.5mm, carbide tool P10 [8, 10, 6, 6, 15,
15, 0.3], which shows the amount of _VB wear on the tool flank after cutting for 8 minutes. The tempering temperature of the workpiece material was around 300℃, which is known as the low-temperature tempering brittle range of the martensitic phase. As comparison materials, spheroidized steel and tempered martensitic single-phase steel are also shown. Hardness Hv145 and
The _VB wear amount of SCM420 and SCM435 steel in 150 is 0.04-0.05mm. On the other hand, hardness
The V _B wear amount of samples containing martensitic phase in the Hv range of 260 to 280 is 0.03 to 0.05 mm, which is almost less than that of the spheroidized material, which is evaluated to be preferable in terms of tool wear. It shows the same amount of wear. In addition, samples with hardness Hv250 and 325 that were tempered at 650℃ and 550℃ after normal quenching produced continuous ribbon-shaped chips that wrapped around the workpiece and tool, making it impossible to continue the tool wear test. .
(The chips shown here were cut artificially.) When cutting spheroidized SCM420 steel, chips that tend to wrap around the tool are often generated, and each time the wear test is interrupted and the cutting I had to get rid of the trash. Table 3 shows the results of a tool life test when steel containing a martensitic phase was cut using a high-speed steel tool. Cutting conditions: cutting speed 40m/min, tool feed rate
0.2mm/rotation, no cutting oil, tool SKH ₄ [8, 10,
6, 6, 15, 15, 0.4]. The influence of tempering temperature on tool life was shown using a sample containing 55-57% martensitic phase as the workpiece material. Furthermore, due to the shape of the work material, it was not possible to test the cutting time for more than 15 minutes.

[Table] From these results, it can be seen that the higher the tempering temperature is, the more desirable it is for the influence of tempering temperature on the life of high-speed steel tools. Furthermore, as a comparative material, when cutting a sample with hardness Hv250 that was tempered at 650℃ after normal quenching under the same conditions, the tool life time was 7 minutes and 40 seconds, which is the same hardness as shown in Table 3. Compared to the tool life of the sample with a tempering temperature of 500℃, the sample with a mixture of soft and hard martensitic phases is superior. Tool wear tests on carbide and high-speed steel allowed us to consider the process to obtain machined parts of the same hardness. In other words, in the conventional process, ○A hot forging → ○B preheat treatment → ○C cutting → ○D quenching
Tempering → ○Grinding... is the usual process. However, in the method of the present invention, ○A hot forging → ○B cooling rate control → ○C tempering → ○D cutting → ○H grinding... In some cases, it can be done as a process. The conventional "heat treatment of the material" is replaced with "controlling the cooling rate" and the "quenching/tempering" process after cutting can be omitted, so there is no dimensional change due to quenching/tempering of the cut parts, and grinding It has excellent effects such as reducing the margin or even omitting the "grinding" process for some parts.

[Brief explanation of drawings]

Figure 1 shows the cutting chip shapes of steel mixed with different amounts of martensite, and Figure 2 shows the cutting resistance components during cutting of steel mixed with martensite phase and steel mixed with other phases. Figure 3 shows the amount of tool wear and chip shape when cutting steel with different amounts of martensite mixed together.

Claims (1)

[Claims] 1. A steel containing 0.1 to 0.6% by weight of carbon, which is maintained at a temperature above the Ar ₃ transformation point and from the Ar ₃ transformation point to the Ar ₁ transformation point for a certain period of time, and then cooled. in steel
A method for producing steel with excellent machinability, which is characterized by the uniform generation of 10 to 75% martensitic phase. 2 After holding for a certain period of time in the temperature range from the Ar ₃ transformation point of steel containing _0.1 to 0.6% by weight of carbon to the Ar ₁ transformation point, it is cooled and incorporated into the steel.
A method for manufacturing steel with excellent machinability, which is characterized by uniformly generating a martensite phase of 10 to 75% and then tempering.