JP2004169051A - Steel having excellent machinability, and production method therefor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a steel which has satisfactory machinability (a tool service life and machined surface roughness). <P>SOLUTION: The steel having excellent machinability comprises, by mass, 0.03 to 0.25% C and 0.05 to 1.0% S, and has a microstructure in which the area ratio occupied by pearlite particles with a particle diameter of >3 μm is ≤5%. <P>COPYRIGHT: (C)2004,JPO

Description

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【発明の効果】
以上述べたように、本発明は鋼のミクロ組織を制御することにより切削時の工具寿命と切削表面粗さ、切り屑処理性に優れた快削鋼を提供することが可能となる。
【図面の簡単な説明】
【図１】従来鋼のパーライトの大きさを示す図。
【図２】パーライト面積率と表面粗さの関係を示す図。
【図３】本発明による熱処理条件を示す図。
【図４】本発明による熱処理条件を示す図。
【図５】プランジ切削による実験方法を示す図。
【図６】本発明により得られる快削鋼のパーライトの大きさを示す図。[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to steel used for automobiles and general machines, and more particularly to steel having excellent tool life, excellent cutting surface roughness and excellent machinability with excellent chip controllability at the time of cutting, and a method for producing the same. is there.
[0002]
[Prior art]
General machines and automobiles are manufactured by combining various types of parts, and the parts are often manufactured through a cutting process from the viewpoint of required accuracy and manufacturing efficiency. At that time, cost reduction and improvement in production efficiency are required, and steel is also required to have improved machinability. In particular, conventionally, SUM23 and SUM24L have been developed with emphasis on machinability. It has been known that it is effective to add a machinability improving element such as S or Pb in order to improve machinability. However, some customers avoid using Pb as an environmental load, and the amount of Pb used is being reduced.
[0003]
Until now, when Pb is not added, a method has been used in which a soft inclusion is formed in a cutting environment such as MnS, as in S, to improve machinability, especially tool life. However, the life of a cutting tool is directly attracting attention because it directly affects the production efficiency and the like. Among the machinability, the technical difficulty is high in surface roughness, and the surface roughness is not It is difficult to make the surface roughness higher than that of conventional steel because it is affected by the essential properties of the cutting material. Since the surface roughness is directly linked to the performance of the part, the deterioration of the surface roughness causes a decrease in the performance of the part and an increase in the defective rate at the time of manufacturing the product, and is often regarded as more important than the tool life. In this sense, the conventional lead free-cutting steel is excellent, and is not only used for tool life but also excellent in surface roughness as compared with mere sulfur free-cutting steel.
[0004]
In the technology related to steel for improving the surface roughness, a free-cutting element such as Pb or Bi is generally added in many cases, but otherwise, for example, as shown in Patent Document 1, MnS inclusions may be added. What secures surface roughness by reducing the average size to 50 μm ² or less, characterized by having 0.20 to 1.0% of graphite having an average cross-sectional area of 5 to 30 μm ^{2 in} a ferrite matrix. Graphite free-cutting steel with excellent tool life and finished surface roughness can be seen. However, even with these methods, it is difficult to obtain a surface roughness higher than that of the conventional lead free-cutting steel, and the so-called low-carbon lead free-cutting steel SUM24L has been conventionally excellent in surface roughness. The reason is that the fine dispersion level of the inclusions in these regulations is only handling particles having an average diameter of about 3 μm, and the uniform dispersion is insufficient, so that the constituent cutting edge is easily generated, and the conventional lead It is estimated that the surface roughness cannot be improved as much as free cutting steel.
[0005]
[Patent Document 1]
JP-A-5-345951
[Problems to be solved by the invention]
The present invention improves both tool life and surface roughness while avoiding defects in rolling and hot forging by introducing technology from a completely different viewpoint from conventional knowledge in the range of low-carbon free-cutting steel. Another object of the present invention is to provide a steel having good machinability equal to or higher than that of the conventional low-carbon sulfur free-cutting steel SUM23, particularly, having a good surface roughness, and a method for producing the same.
[0007]
[Means for Solving the Problems]
Cutting is a breaking phenomenon that separates chips, and promoting it is one point. To obtain particularly good surface roughness, the matrix is embrittled to facilitate fracture and prolong tool life, while minimizing uniformity in steel to ensure micro-stable fracture. This caused a phenomenon and suppressed irregularities on the cutting surface. Specifically, paying attention to the distribution of pearlite in steel, C is uniformly dispersed as fine pearlite (strictly, cementite) in steel to cause stable fracture, thereby creating a cutting surface with less unevenness, Further, a manufacturing method for making this possible is provided.
[0008]
The present invention has been made based on the above findings, and the gist is as follows.
[0009]
(1) In terms of mass%, C: 0.03 to 0.2%, S: 0.03 to 1.0%, and the area ratio of pearlite particles having a particle size exceeding 1 μm in the microstructure is 5% or less. A steel with excellent machinability, characterized in that:
[0010]
(2) in mass% C: 0.03 to 0.2% S: in the course of cooling after hot rolling of steel containing 0.03 to 1.0%, from _{A 3} point or more temperature of the steel Machining characterized in that by cooling to 550 ° C. or lower at a cooling rate of 0.5 ° C./sec or higher, the area ratio of pearlite grains having a grain size of more than 1 μm to 5% or less in the steel microstructure. Method for producing steel with excellent heat resistance.
[0011]
(3) The method for producing steel excellent in machinability according to (2), wherein after the cooling is performed, a heating temperature for adjusting the hardness to be performed subsequently is limited to 750 ° C. or less.
[0012]
BEST MODE FOR CARRYING OUT THE INVENTION
First, the steel composition specified in the present invention will be described. In addition, all steel component compositions are mass%.
[0013]
C has a significant effect on machinability because it relates to the basic strength of the steel material and the amount of oxygen in the steel. If the strength is increased by adding a large amount of C, the machinability decreases, so the upper limit was made 0.2%. On the other hand, it is necessary to control the amount of oxygen to an appropriate amount in order to prevent the formation of hard oxides that reduce machinability and to suppress the adverse effects of dissolved oxygen at high temperatures such as pinholes during the solidification process. If the amount of C is simply reduced too much by blowing, not only the cost increases, but also a large amount of oxygen in the steel remains and causes problems such as pinholes. Therefore, the lower limit is set to 0.03% of C, which can easily prevent problems such as pinholes. Further, lowering the C further increases the ductility too much, and also shortens the drill life.
[0014]
S bonds with Mn and exists as MnS inclusions. MnS improves machinability, but elongated MnS is one of the causes of anisotropy during forging. Although coarse MnS should be avoided, it is preferable to add a large amount of MnS from the viewpoint of improving machinability, and it is preferable to finely disperse MnS. To improve machinability, 0.03% or more must be added. On the other hand, if it exceeds 1.0%, not only the generation of coarse MnS is inevitable, but also cracks occur during production due to deterioration of casting properties and hot deformation properties due to FeS or the like. .
[0015]
In addition, MnS means not only pure MnS but also an inclusion mainly containing MnS, and sulfides such as Fe, Ca, Ti, Zr, Mg and REM coexist with MnS by solid solution or bonding. And inclusions such as MnTe in which an element other than S forms a compound with Mn to form a compound with MnS to form a solid solution / bond and coexist with the MnS, or includes the above-mentioned inclusions precipitated with an oxide as a nucleus. In the chemical formula, Mn sulfide-based inclusions that can be expressed as (Mn, X) (S, Y) (here, X: an element other than Mn and a sulfide-forming element other than Y: S that binds to Mn) are collectively referred to. That's what they say.
[0016]
This steel type has a low C, and even if an alloy element is added at the level of a low alloy steel, the hardness is greatly increased and the machinability is not reduced. Rather, as alloying elements capable of improving hardenability and suppressing the generation of coarse pearlite, for example, Mn: 0.05 to 2%, Cr: 0.01 to 2%, V: 0.01 to 1.0%, Nb : 0.005 to 0.2%, Mo: 0.01 to 1.0%, W: 0.05 to 1.0%, Ni: 0.05 to 2.0%, Ti: 0.005 to 0% .2%, B: 0.0005 to 0.02%.
[0017]
Further, it can be combined with an oxide softening technology and a sulfide morphology control technology. Ca: 0.0002 to 0.01%, Zr: 0.0005 to 0.1%, Mg: 0.0003 to 0%. 0.01%, Al: 0.001 to 0.1%, Si: 0.01 to 0.5%, Te: 0.0003 to 0.2%, total-N: 0.001 to 0.02%, One or more of total-O: 0.0005 to 0.035% may be added.
[0018]
In order to further improve the machinability, P: 0.001 to 0.2%, Zn: 0.0005 to 0.5%, Sn: 0.005 to 2.0%, Cu: 0.01 to It is also possible to add one or more of 2.0%, Bi: 0.005 to 0.5%, and Pb: 0.01 to 0.5%.
[0019]
Next, the reason why the pearlite area ratio is set to 5% or less in the present invention will be described.
[0020]
The generation behavior of the constituent cutting edge on the tool greatly affects the cutting surface roughness. Originally, it is considered that mechanically above the cutting tool is the most severe environment for the material, and it is thought that the material is likely to be broken / separated. Constructed cutting edges result from strong adhesion between the materials and non-uniform texture of the work material. Therefore, it was considered important to increase the homogeneity of the microstructure of the material as much as possible. As a result, the present inventor has found that the pearlite distribution, which has been considered to have little relation to the above, is greatly related to the microstructure homogeneity.
[0021]
Here, the pearlite refers to a structure that is blackened by performing a nital etching on a mirror-polished surface. Strictly speaking, pearlite refers to a group composed of alternating ferrite and plate-like cementite, but looks like a single crystal grain with an optical microscope. Further, as shown in FIG. 1, in the production by ordinary rolling and cooling, the pearlite grains are deposited in a band shape (hereinafter referred to as a pearlite band). Since this pearlite has different mechanical properties from the single-phase ferrite of the matrix, it is considered that the deformation fracture near the cutting edge is made non-uniform, and further, the growth of the constituent cutting edge is promoted.
[0022]
Therefore, by adjusting the steel composition or the thermal history, a critical region in which pearlite particles having a particle size of 1 μm or more can be suppressed in the pearlite area ratio in the observation visual field of 4 mm ² to obtain good surface roughness was investigated. However, in order to suppress the deterioration of the surface roughness, it was found that the area ratio of the pearlite particles having a particle size of 1 μm or more was 5% or less. FIG. 2 shows the relationship between the pearlite area ratio and the surface roughness.
[0023]
As shown in FIG. 1, it can be seen that the free-cutting steel according to the present invention has an extremely small amount of this black-looking structure. Strictly speaking, in the present invention, tempered martensite or tempered bainite structure is obtained, and the possibility that carbide is not pearlite (in other words, striped structure of plate-like cementite and ferrite) but in the form of cementite grains cannot be denied. However, here, such iron-based carbides are collectively referred to as pearlite.
[0024]
Next, a method for producing free-cutting steel according to the present invention will be described.
[Heat history quenching: A 0.5 ° C / s from a temperature of ₃ points or more to 550 ° C or less]
In the present invention, as the heat history after hot rolling, it is important to from hot rolling after A ₃ point or higher temperature to 550 ° C. or less for cooling in a cooling rate higher than 0.5 ° C. / sec.
[0025]
Conventionally, rapid cooling of so-called low-carbon free-cutting steel has not been performed. Low-carbon free-cutting steel has a small C content, so that it hardly changes in hardness even when it is quenched. Therefore, it is considered that there was no influence on the strength / toughness due to the conventional “quenching and tempering”, and that it was obsessed with the stereotype that free cutting steel was unnecessary. However, when in pursuit of homogeneity of the material thinking going back to the nature of the cutting, freezing the movement in the steel C by quenching from A ₃ points, the formation of coarse cementite more pearlite occurring transformation during air cooling It suffices if it can be suppressed. In this case, since hardening by quenching is not the purpose, even if the quenched structure does not have a martensite structure, it is sufficient if the movement of C in the steel is frozen and formation of coarse cementite or pearlite can be prevented. To this end, it is necessary to cool at 0.5 ° C. / sec or faster from the _{A 3} point to 550 ° C. or less as shown in FIG. A cooling rate of 1 ° C./s or more is preferred, for example, when there are few hardenability improving elements. If the temperature after cooling exceeds 550 ° C. or the cooling rate is lower than 0.5 ° C./sec, coarse pearlite is generated. In general, it precipitates in a band shape and is often called a pearlite band. Naturally, if the alloying element is added in a large amount as in stainless steel, the pearlite band does not occur even if the cooling rate is lower than 0.5 ° C./sec. However, since general free-cutting steel is assumed here, It was defined as 0.5 ° C./sec.
[0026]
Next, in the present invention, the structure of the free-cutting steel can be further homogenized by performing a heat treatment that is maintained at a temperature of 750 ° C. or lower, following the above-described quenching treatment.
[0027]
In the actual manufacturing process, although the C content is small in order to further increase the stability of the product, it is preferable to reduce the hardness variation in steel. Therefore, by maintaining the material at a high temperature again, the variation in the material can be reduced. To initially suppress coarse pearlite is important to rapid cold to 550 ° C. or less no longer cause coarse pearlite from a temperature of more than _three points A. Then, as shown in FIG. 4, by maintaining the temperature at the predetermined temperature T ₂ ° C again, the hardness can be adjusted to satisfy the demand of the customer, and the hardness variation can be reduced. By heating and compensating to a temperature of 750 ° C. or less, the hardness is adjusted so as to satisfy the demands of customers.
[0028]
With respect to the holding temperature T ₂ ° C, the holding temperature and the holding time should be determined so that the hardness meets the demands of the consumer. However, when the retention temperature T ₂ ° C exceeds 750 ° C, transformation to austenite starts, and if the cooling rate at the time of re-cooling is low, a pearlite band is generated. Therefore, the retention temperature T ₂ ° C was set to 750 ° C or less. Furthermore, since secondary processing such as drawing is often performed in the post-process, it is preferable to adjust the temperature T ₂ ° C so that the hardness becomes suitable for handling in the post-process. The retention time is 3 minutes or less in terms of industrial production, and the hardness and the like are hardly changed as compared with the case where no retention is performed.
[0029]
Note industrial production due rolling or forging dimensions, to produce a nonuniform temperature within the steel, it should also be considered coercive constant time at a temperature T ₁ ° C. of 550 ° C. or less after quenching for coarse pearlite prevention . Preferably the temperature T ₁ ° C. below the 550 ° C. After quenching by retaining more than 5 minutes, regardless of the material size and segregation zone, can promote uniform ferrite transformation. By doing so, coarse pearlite or a pearlite band does not occur even if the temperature is subsequently raised to the retention temperature T ₂ ° C (≦ 750 ° C). Conversely, when the size after rolling or forging is large, the internal transformation is not completed if the holding time at 550 ° C. or less is shorter than 1 minute. Coarse pearlite and pearlite bands are formed.
[0030]
【Example】
The effects of the present invention will be described with reference to examples.
[0031]
Some of the test materials shown in Tables 1 and 2 were smelted in a 270 t converter, then continuously cast, slab-rolled into billets, and further rolled to φ50 mm. The others were melted and rolled in a 2t vacuum melting furnace. The drilling characteristics obtained by evaluating the machinability of the example materials shown in Table 1 based on the surface roughness and the drilling characteristics are indicators of tool life. Examples 2, 10, 13, and 17 correspond to claim 1, in which a so-called in-line heat treatment was carried out in which the steel sheet was rolled to a diameter of 20 mm and immediately put into a water tank at the rear end of the rolling line immediately before rolling and before cooling. After the quenching, no heat treatment was performed, and thus the holding temperature T1, the holding time L1, the holding temperature T2, and the holding time L2 are not shown. Examples 5,20,24,27,29 is an example which supplied the lead bath at 500 ° C. from _{A 3} point or higher. Further, the embodiment in which the holding time L1 is "600 minutes or more" is actually an embodiment in which the holding time L1 is left at the holding temperature T1 for 10 hours or more. In Comparative Examples after Example 34 shown in Table 2, the substantial normalizing process is not performed. Expressed according to this definition, it indicates that it is left to stand after being cooled to room temperature.
[0032]
The embodiment in which all of the holding temperature T1, the holding time L1, the holding temperature L2, and the holding time L2 are indicated is rolled to φ20 mm, and is directly injected into a water tank at the rear end of the rolling line immediately after rolling and before cooling. This is an example in which tempering is performed at a predetermined temperature after heat treatment.
[0033]
Here, the reason for specifying the retention time L1 is to show that the steel material is controlled to be sufficiently cooled to a temperature (550 ° C. or less) where pearlite is hardly generated, as shown in FIG. After cooling, by further controlling the holding temperature L2 and the holding time L2, the hardness of the steel material can be adjusted to a hardness suitable for cutting and wire drawing.
[0034]
As described above, the heat treatment described in the claims of the present invention was carried out by online and offline batch processing, although the method was different.
[0035]
The surface roughness was evaluated by so-called plunge cutting shown in Table 3 in which the tool shape was transferred by a parting-off tool. The outline of the experimental method is shown in FIG. In the experiment, the surface roughness when 200 grooves were processed was measured.
[0036]
Plunge cutting is a cutting method in which a groove is repeatedly machined by a parting-off tool as shown in FIG. 5, and in this embodiment, a stylus of a stylus type roughness meter is moved in a longitudinal direction of a test piece with respect to a bottom surface of the groove. Thereby, the roughness profile of the groove cross section was measured, and the surface roughness was evaluated.
[0037]
Table 4 shows the drilling test conditions used for tool life evaluation. The machinability was evaluated at the highest cutting speed (so-called LV1000) capable of cutting to a cumulative hole depth of 1000 mm.
[0038]
When only the heat treatment conditions were changed with similar chemical components, all of the inventive examples were superior to the comparative example in terms of drill tool life and good in surface roughness in plunge cutting.
[0039]
In the steel corresponding to claim 1, the tool life tends to be slightly reduced because the hardness is increased, but the tool life level is excellent and the tool life level is equivalent to the conventional one.
[0040]
In the example of the invention, coarse pearlite grains are not generally observed as shown in FIG. 6, and no pearlite band is observed. This is because even if the addition amounts of C, S, etc. are different, the order does not change. When elements such as Zn, Sn, B, etc. are added, the tool life is longer than that of the comparative steel having the same C, S contents. It turns out that it is excellent in surface roughness.
[0041]
[Table 1]

[0042]
[Table 2]

[0043]
[Table 3]

[0044]
[Table 4]

[0045]
[Table 5]

[0046]
[Table 6]

[0047]
【The invention's effect】
As described above, the present invention can provide a free-cutting steel excellent in tool life, cutting surface roughness, and chip controllability at the time of cutting by controlling the microstructure of the steel.
[Brief description of the drawings]
FIG. 1 is a view showing the size of pearlite of conventional steel.
FIG. 2 is a diagram showing a relationship between a pearlite area ratio and surface roughness.
FIG. 3 is a view showing heat treatment conditions according to the present invention.
FIG. 4 is a view showing heat treatment conditions according to the present invention.
FIG. 5 is a view showing an experimental method by plunge cutting.
FIG. 6 is a view showing the size of pearlite of free-cutting steel obtained by the present invention.

Claims (3)

% By mass, C: 0.03 to 0.2%, S: 0.03 to 1.0%, and the area ratio of pearlite grains having a particle size exceeding 1 μm in the microstructure is 5% or less. Steel with excellent machinability. By mass% C: 0.03 to 0.2% S: in the course of cooling after hot rolling of steel containing 0.03 to 1.0%, from 0.5 _{A 3} point or more temperature of the steel By cooling to 550 ° C. or lower at a cooling rate of not lower than 550 ° C./sec, the area ratio of pearlite grains having a grain size of more than 1 μm in the steel microstructure is reduced to 5% or less, and the machinability is excellent. Steel production method. The method for producing steel with excellent machinability according to claim 2, wherein a heating temperature for adjusting hardness, which is performed after the cooling, is limited to 750 ° C or less.

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