JPH0949016A - Production of steel for machine structural use, excellent in machinability, cold forgeability, and fatigue characteristic after hardening and tempering - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce a steel for machine structural use, having machinability equal to or higher than that of the conventional Pb-added free cutting steel without deteriorating cold forgeability and excellent in fatigue strength characteristics. SOLUTION: A steel bloom, having a composition consisting of, by weight, 0.1-1.5% C, 0.5-2.0% Si, 0.1-2.0% Mn, 0.0003-0.0150% B, 0.005-0.1% Al, <=0.0030% O, <=0.020% p, <=0.035% S, 0.0015-0.0150% N, and the balance Fe with inevitable impurities, is heated to a temp. not lower than respective solid solution limit temps. of BN and AlN and hot-rolled. After cooling down to room temp., the steel is heated to 800-980 deg.C and hot-rolled, and this steel is then heated to 650-740 deg.C and held for >=5hr.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a machine structure useful as a material for machine parts used in automobiles, etc., which has simultaneously improved machinability, cold forgeability and fatigue strength characteristics after quenching and tempering. The present invention relates to a method for manufacturing steel.

[0002]

2. Description of the Related Art Mechanical structural steel is processed into a predetermined shape by cutting or cold forging, and then quenched and tempered to be used for mechanical parts such as industrial machines and automobiles. Therefore, this steel is required to have machinability, cold forgeability, and mechanical properties after quenching / tempering, particularly fatigue strength. Among them, as a means for improving machinability, a method of adding a free-cutting element such as Pb, S, Bi, P or the like alone or in combination to steel is general. In particular, Pb is often used because it has the effect of significantly improving machinability. However
Since Pb is an element harmful to the human body, it not only requires a large-scale exhaust facility in the manufacturing process of steel materials and the processing process of parts, but also has a great problem in recycling steel materials. On the other hand, these elements are harmful for improving the cold forgeability of steel materials. As described above, free-cutting property and cold forgeability are generally contradictory properties, but the mechanical structural steel must have these properties. In order to solve this problem, graphite steel has been proposed, for example, JP-A-51-57621, JP-A-03-140411, and JP-A-
There is a proposal described in 03-146618.

[0003]

However, according to the studies made by the inventors, these methods were not sufficient in the properties as a steel for machine structural use, particularly in the fatigue resistance. That is, according to the method described in JP-A-51-57621, a new quenching treatment before graphitization treatment is indispensable in order to improve the above-mentioned various properties by refining graphite grains. There was a problem. In addition,
The method described in 03-140411 publication is a method of combining a specific component composition and an annealing treatment, but the obtained graphite particle size is
It was as large as 28 to 35 μm, was inferior in cold workability, and had insufficient fatigue strength after quenching and tempering.

Therefore, an object of the present invention is to advantageously solve the above-mentioned problems of the prior art, particularly the problems of graphite steel, without impairing cold forgeability. It is an object of the present invention to propose a method for producing a steel for machine structural use, which can obtain machinability equal to or higher than that of a conventional Pb-added free-cutting steel and excellent fatigue strength characteristics without pretreatment such as quenching.

[0005]

The gist of the present invention is as follows. (1) C: 0.1-1.5 wt%, Si: 0.5-2.0 wt%, M
n: 0.1 to 2.0 wt%, B: 0.0003 to 0.0150 wt%, A
l: 0.005 to 0.1 wt%, O ≦ 0.0030 wt%, P ≦ 0.020 w
t%, S ≦ 0.035 wt%, N: 0.0015 to 0.0150 wt
%, With the balance being Fe and inevitable impurities, the steel slab is heated to a temperature above the solid solubility limit of BN and AlN, hot-rolled, and cooled to room temperature. Machinability, characterized by heating to a temperature range and hot rolling, then heating to a temperature range of 650 to 740 ° C and holding for 5 hours or more,
A method for manufacturing steel for machine structural use, which has excellent cold forgeability and fatigue properties after quenching and tempering.

(2) C: 0.1-1.5 wt%, Si: 0.5-
2.0 wt%, Mn: 0.1 to 2.0 wt%, B: 0.0003 to 0.01
50wt%, Al: 0.005-0.1wt%, O≤0.0030wt%, P
≦ 0.020 wt%, S ≦ 0.035 wt%, N: 0.0015〜
0.0150wt%, and Ni: 0.10-3.0wt%, C
u: 0.1-3.0 wt%, Co: 0.1-3.0 wt%, Mo: 0.1
~ 1.0 wt% V: 0.05-0.5 wt%, Nb: 0.005-0.05 wt%, Ti:
A steel piece containing one or more selected from 0.005 to 0.05 wt%, Zr: 0.005 to 0.2 wt% and REM: 0.0005 to 0.2 wt%, and the balance being Fe and inevitable impurities. Is hot-rolled by heating it to a temperature above the solid solubility limit of BN and AlN, cooling to room temperature, and then 800-980
Heated to a temperature range of ℃, hot-rolled, then 650-740 ℃
A method for producing a steel for machine structural use, which is excellent in machinability, cold forgeability, and fatigue characteristics after quenching and tempering, characterized by heating in the temperature range of 5 and holding for 5 hours or more.

(3) C: 0.1-1.5 wt%, Si: 0.5-
2.0 wt%, Mn: 0.1 to 2.0 wt%, B: 0.0003 to 0.01
50wt%, Al: 0.005-0.1wt%, O≤0.0030wt%, P
≦ 0.020 wt%, S ≦ 0.035 wt%, N: 0.0015〜
A steel slab containing 0.0150 wt% and the balance of Fe and inevitable impurities is heated to a temperature above the solid solution limit of BN and AlN and hot-rolled, followed by a temperature range of 800 to 980 ° C. Machine structure with excellent machinability, cold forgeability, and fatigue characteristics after quenching and tempering, characterized in that it is hot-rolled from Steel manufacturing method.

(4) C: 0.1-1.5 wt%, Si: 0.5-
2.0 wt%, Mn: 0.1 to 2.0 wt%, B: 0.0003 to 0.01
50wt%, Al: 0.005-0.1wt%, O≤0.0030wt%, P
≦ 0.020 wt%, S ≦ 0.035 wt%, N: 0.0015〜
0.0150wt%, and Ni: 0.10-3.0wt%, C
u: 0.1-3.0 wt%, Co: 0.1-3.0 wt%, Mo: 0.1
~ 1.0 wt% V: 0.05-0.5 wt%, Nb: 0.005-0.05 wt%, Ti:
0.005 to 0.05 wt%, Zr: 0.005 to 0.2 wt% and REM
: A steel slab containing one or more selected from 0.0005 to 0.2 wt% and the balance being Fe and inevitable impurities is heated to a temperature above the solid solution limit of BN and AlN. Hot rolling, followed by hot rolling from a temperature range of 800 to 980 ° C, then heating to a temperature range of 650 to 740 ° C and holding for 5 hours or more, machinability, cold forgeability And a method for producing a mechanical structural steel having excellent fatigue properties after quenching and tempering.

[0009]

In order to solve the above-mentioned problems, the inventors examined a method for industrially producing a steel excellent in machinability, cold forgeability and fatigue strength characteristics. , Came to the following findings. If inclusions such as spherical cementite and MnS are present in the steel, the cold forgeability deteriorates because the voids generated from the interface between these inclusions and the matrix phase expand during the forging, leading to early fracture. To do. On the other hand, when C in the steel is graphitized, since graphite is extremely soft, it follows the deformation of the matrix phase during cold forging, the generation of voids from the graphite-matrix is suppressed, and fracture occurs. The amount of strain up to that point increases, and cold forgeability improves. On the other hand, machinability is improved by the presence of graphite, which acts as a lubricant during cutting and suppresses the temperature rise of the tool. Through these phenomena, the graphitization of C in steel makes it possible to simultaneously satisfy the contradictory properties of cold forgeability and machinability.

The inventors also investigated the effect of graphite grain size on machinability and cold forgeability. As a result, it was found that if the graphite grains were made finer, both machinability and cold forgeability could be further improved.
Although it is not clear about the mechanism by which both of the above characteristics are improved, it is considered to be due to the following reasons. First, regarding machinability, when graphite is present in the steel, a large strain acts in the shear region during cutting, so voids are generated from the interface between the graphite and the matrix,
This is connected to generate chips, but the same C
In the case of the amount, since the volume ratio of graphite is constant, the finer the graphite, the easier the voids are connected, and the machinability is improved. On the other hand, regarding the cold forgeability, it is considered that the cold forgeability is improved by increasing the critical strain amount in which voids are generated at the graphite-matrix interface as the graphite particle size becomes finer.

Furthermore, the following conclusions have been reached regarding the effect of graphite on fatigue strength characteristics. That is, the fatigue strength generally increases as the hardness of the steel material increases, but it is known that the fatigue strength is also affected by the size of non-metallic inclusions contained in the steel material. First, regarding the former, quenching and tempering treatment is performed in the secondary processing in order to secure the fatigue strength required for machine parts. In this case, the melting behavior of graphite particles depends strongly on the size of graphite. To do. This is because the dissolution of graphite is controlled by the self-diffusion of Fe to the place where the graphite disappears.
Therefore, when the graphite particles are coarse, the graphite is not sufficiently solid-solved by heating for a short time, and the hardness after quenching / tempering decreases, so that the fatigue strength decreases. In addition, since graphite is a type of non-metallic inclusion, when undissolved graphite is present due to the coarseness of graphite, this portion acts as the starting point of fatigue fracture and is predicted from the overall hardness. It further reduces fatigue strength. This tendency becomes more remarkable as the strength increases. From this, in order to increase the fatigue strength of the graphite steel after quenching and tempering, it is effective to make the graphite finer in a double sense. In the study by the present inventors, the size of the critical graphite that affects this fatigue strength is about 20 μm, and if it is larger than this, the dissolution of graphite does not proceed in a short time, and the origin of fatigue fracture Therefore, the fatigue strength decreases due to the effects of both of these.

As explained above, in order to improve the machinability, cold forgeability, and fatigue strength characteristics after quenching and tempering of steel for machine structural use, it is necessary to make the size of graphite grains smaller. It turned out to be advantageous. Therefore, the inventors further studied the chemical composition and the manufacturing method for specifically fulfilling such a request. The main points of the examination results are described below.

In order to finely disperse the graphite particles, it is necessary to form a large number of precipitates in the steel which will serve as nucleation sites for graphite crystallization. As a result of detailed examination of such precipitates, BN, AlN, TiN, ZrN, Nb (C, N), V (C,
It was found that N), (La, Ce) S, etc. are effective. Among them, BN works most effectively as a site for crystallization of graphite, and AlN also works effectively as a nucleus for crystallization of graphite. Then, they have found that when the BN and AlN are compounded, the action and effect are further enhanced.

In order to fully exert the graphite refining action of AlN and BN, it is not sufficient to simply add Al and B within the above-mentioned range of components, and certain specific hot rolling conditions and annealing. The conditions must be combined. That is, the first important condition is that BN and AlN are completely dissolved in the heating stage during hot rolling. This is because in the temperature range in which the precipitates in the steel cannot be completely dissolved, they coarsen and their number decreases, and as a result, the graphite particle size after graphitization is coarse and the number also significantly decreases. . On the other hand, if hot rolling is performed after raising the temperature of BN and AlN to a temperature where they can be completely dissolved, BN is formed during the cooling process after hot rolling and AlN is formed during the temperature rise during graphitization annealing. , Enables fine precipitation.

However, here, BN and Al once dissolved
Among N, BN precipitates extremely quickly in the cooling process after hot rolling, but AlN hardly precipitates in the cooling process of hot rolling, and AlN does not precipitate in the solid solution Al because the diffusion rate of Al is slow. Exists as. Therefore, in this state, AlN cannot be effectively used as a graphite nucleation site. Therefore, the second important condition is a treatment for promoting precipitation of AlN, that is, after the hot rolling (first hot rolling), after heating to a temperature range of 800 to 980 ° C, or 80
The second hot rolling is performed from the temperature range of 0 to 980 ° C.

That is, as described above, in the state of the first hot rolling, Al added into the steel does not precipitate as AlN and most of it is in solid solution. When heating (or holding) the γ region at a relatively low temperature from this state, part of the solid solution Al is finely precipitated as AlN. Since this precipitation temperature is relatively low, the growth rate of AlN is extremely slow, and the precipitated AlN is kept fine. Also this fine
The presence of AlN suppresses the growth of γ grains during the second rolling. On the other hand, the BN precipitated during the first hot rolling partly dissolves in the γ phase due to heating (or holding in the γ range) in the γ region during the second rolling, but at a relatively low temperature. Therefore, many of them remain as undissolved BN, and their growth is slow, so that a fine distribution can be maintained. However, even without using the effect of AlN, 2
The rolling temperature at the time of rolling is sufficiently low, and the rate of coarsening of the γ grain size after rolling is low. Therefore, even if the second rolling is performed after the first rolling, it is not always necessary to hold the temperature at the second rolling start temperature once, and after the first rolling is completed, the second rolling is started as soon as the temperature falls to a predetermined temperature. It is also possible to do so. By such hot rolling at a relatively low temperature, the matrix structure after the second rolling also becomes a fine ferrite-pearlite structure, and the grain boundaries between ferrite-ferrite or ferrite-cementite increase. Since these grain boundaries are more advantageous in diffusion of C than in the ferrite grains,
The increase in grain boundaries also has the effect of promoting the formation of graphite.

For this reason, by performing the second hot rolling after the first hot rolling, fine BN is obtained.
And AlN can coexist, and at the same time, fine ferrite-pearlite structure can be formed, so that fine graphite can be uniformly distributed, and the heat treatment time required for graphitization can be shortened. It will be possible.

Next, the composition of the steel in the present invention will be described. C: 0.1 to 1.5 wt% C is an essential component for forming a graphite phase. 0.1
If it is less than wt%, it will be difficult to secure the graphite phase necessary for ensuring machinability, so it is necessary to add 0.1 wt% or more, but if it exceeds 1.5 wt% during hot rolling. The deformation resistance of the hot rolled material decreases, the cracking of the hot rolled material and the occurrence of flaws increase, so 0.1 to 1.5 wt%
Limited to the range. The preferable content is 0.2 to 0.8.
wt%.

Si: 0.5 to 2.0 wt% Si is an element that does not form a solid solution in cementite in steel and makes the cementite unstable and promotes graphitization. If the content is less than 0.5 wt%, the effect is poor, while
Even if added in excess of 2.0 wt%, not only the effect of promoting graphitization reaches saturation, but also the temperature range in which the liquid phase is generated decreases and the appropriate temperature range during hot rolling narrows. It was limited to the range of%. The preferable content is 1.0 to 1.9 wt.
%.

Mn: 0.1-2.0 wt% Mn is an element which is effective not only for deoxidizing the steel but also for improving the hardenability and ensuring the strength of the steel. It forms a solid solution and inhibits graphitization. 0.1
Addition of less than wt% has no effect on deoxidation and contributes little to the improvement of strength, so addition of at least 0.1 wt% is necessary. However, addition of more than 2.0 wt% hinders graphitization, so it was limited to the range of 0.1 to 2.0 wt%. The preferable content is 0.2 to 0.8 wt%.

B: 0.0003 to 0.0150 wt% B combines with N in steel to form BN, which acts as a nucleation site for graphite, thereby promoting graphitization and refining graphite particles. It has an effect. It is also an important element in the present invention because it is a useful element for enhancing the hardenability of steel and ensuring the strength after quenching.
Addition of less than 0.0003 wt% has little effect on graphitization and improvement of hardenability, and addition of 0.0003 wt% or more is essential, but addition of more than 0.0150 wt% causes B to form a solid solution in cementite and cementite. On the contrary, the stabilization of graphite will hinder graphitization, so 0.0003 to 0.0150 wt%
Limited to the range. The preferred content is 0.0005-0.
It is 0035 wt%.

Al: 0.005-0.1 wt% Al is an element that reacts with N in steel to form AlN, which acts as a nucleation site for graphite, thereby promoting graphitization. Addition of less than 0.005 wt% has a small effect, and addition of at least 0.005 wt% or more is required. On the other hand, if added in excess of 0.1 wt%, a large amount of Al-based oxide will be generated in the casting process. This oxide not only serves as a starting point for fatigue fracture, but also forms extremely coarse graphite grains with this oxide as a nucleus. Further, since the Al-based oxide is hard, the machinability is deteriorated by abrading the tool during cutting. For these reasons, the amount of Al added is limited to the range of 0.005 to 0.1 wt%. The preferable content is 0.01 to 0.05 wt%.

O: 0.0030 wt% or less O forms an oxide-based non-metallic inclusion and reduces cold forgeability, machinability and fatigue strength, so it should be reduced as much as possible, but the upper limit is 0.0030. Allowed up to wt%. The preferable content is 0.0018 wt% or less.

P: 0.020 wt% or less P is an element that inhibits graphitization and deteriorates cold forgeability by embrittlement of the ferrite layer.
Further, by segregating at the grain boundaries during quenching and tempering to reduce the grain boundary strength, the resistance to the propagation of fatigue cracks is reduced and the fatigue strength is reduced. Therefore, it should be reduced as much as possible, but the upper limit is allowed up to 0.020 wt%. The preferable content is 0.010 wt% or less.

S: 0.035 wt% or less S forms MnS in the steel, which becomes a starting point of cracking during cold forging and deteriorates cold forgeability. In addition, MnS itself becomes the starting point of fatigue fracture and acts as a nucleus of crystallization of graphite to form coarse graphite, which has the effect of reducing fatigue strength and should be reduced as much as possible. However, the upper limit is 0.035wt%.
The preferable content is 0.010 wt% or less.

N: 0.0015 to 0.0150 wt% N combines with B to form BN, and this BN serves as a nucleus for crystallization of graphite, thereby significantly refining graphite particles and promoting graphitization. So it is an essential element. 0.0015wt%
Addition of less than BN does not form enough, while 0.0150wt%
%, The cracking of the slab is promoted during continuous casting, so the content was limited to 0.0015 to 0.0150 wt%. In addition,
The preferred content is 0.0015 to 0.0050 wt%.

In the present invention, if necessary, in addition to the above main components, the following: Ni, Cu, Co, Mo, V, Nb, Zr,
By adding one or more components selected from Ti and REM, in addition to promoting the action and effect of each of the main components listed above, it is possible to add and improve other various properties. It is possible to plan. The reasons for limiting the composition of these additive components will be described below.

Ni, Cu, Co: 0.1 to 3.0 wt% for each of these elements are all elements which promote graphitization and improve hardenability. Therefore, the hardenability can be improved without inhibiting the graphitization. If the amount of addition is less than 0.1 wt%, the effect of addition is small, while if the amount of addition exceeds 3.0 wt%, the effect is saturated, so the addition amount was limited to the range of 0.1 to 3.0 wt%. The preferable content is 0.5 to 2.5 wt%.

Mo: 0.1 to 1.0 wt% Mo is characterized in that it enhances hardenability and, at the same time, has a smaller distribution to cementite than Mn and Cr. Therefore, the hardenability of the steel material can be enhanced without inhibiting graphitization. Further, the steel material to which Mo is added has a large temper softening resistance, so that the hardness at the same tempering temperature can be improved, and therefore the fatigue strength is improved. Further, in the as-hot-rolled state because of its high hardenability, it is easy to form a bainite structure that forms fine graphite, and as a result, the dissolution of graphite during quenching can be completed in a short time. it can. For this reason, it is used when it is necessary to further improve the fatigue strength characteristics,
If the addition amount is less than 0.1 wt%, the effect is small, while
If added in excess of 1.0 wt%, graphitization is impaired and cold forgeability and machinability deteriorate, so it was limited to the range of 0.1-1.0 wt%. The preferred content is 0.1-0.3 wt.
%.

V: 0.05 to 0.5 wt% / Nb: 0.005 to 0.05
Although wt% V and Nb are both carbide forming elements, they do not form a solid solution in cementite, and do not hinder graphitization so much. In addition, it forms carbon / nitride and increases the strength by this precipitation strengthening action. Since both are elements that improve hardenability, they are suitable for use when it is necessary to improve fatigue strength. In the case of V, addition of less than 0.05 wt% has little effect on these, while addition of more than 0.5 wt% saturates the effect, so the addition was made within the range of 0.05 to 0.5 wt%. On the other hand, in the case of Nb, if the addition amount is less than 0.005 wt%, the above-mentioned effect is small, and if the addition amount exceeds 0.05 wt%, the effect is saturated, so the addition amount was made 0.005 to 0.05 wt%. The preferred content of each is 0.05-0.
3 wt% and 0.01-0.04 wt%.

Zr: 0.005-0.2 wt%, Ti: 0.005-0.05
wt% Zr and Ti together form carbon / nitride, and these act as nuclei for the crystallization of graphite, thereby refining the graphite particles, so when it is necessary to further refine the graphite particles. It is suitable to use. Further, by forming carbon / nitride, it is possible to make B effectively act on the hardenability during quenching. In order to exert such effects, it is necessary to add 0.005 wt% or more of both Zr and Ti. On the other hand, if Zr and Ti are added in excess of 0.2 wt% and 0.05 wt%, respectively, N for forming BN becomes insufficient, and as a result, the graphite particles become coarse and the graphitization time becomes extremely long. 0.005-0.2 wt% and 0.00 respectively
It was limited to the range of 5 to 0.05 wt%. The preferable contents are 0.05 to 0.2 wt% and 0.01 to 0.03 wt%, respectively.

REM: 0.0005 to 0.2 wt% REM, especially La and Ce are combined with S to form (La, Ce) S
Are formed, which serve as nuclei for graphitization to promote graphitization and miniaturize graphite particles. However, the amount is 0.0005
If it is less than wt%, the effect is poor, while if it exceeds 0.2 wt%, the effect saturates, so the range was limited to 0.0005 to 0.2 wt%. The preferable content is 0.005 to 0.03.
wt%.

Next, the hot rolling conditions and the annealing conditions for graphitization will be described. In the present invention, the heating temperature of the steel material during hot rolling is set to the solid solution limit of BN or AlN or higher.
This is because if the heating temperature during hot rolling does not reach this temperature, BN, which is the nucleus of graphite crystallization, does not form a solid solution in the steel and becomes coarse, resulting in coarse graphite during graphitization annealing after hot rolling. Produce grains. As a result, machinability, cold forgeability, and fatigue strength are reduced as described above. On the other hand, when BN and AlN are completely solid-soluted during heating before hot rolling, they are finely re-formed during the cooling process after hot rolling, the heat retention during the second rolling, and the heating during graphitization annealing. By precipitating and forming the nucleus of graphite crystallization, the graphite grains become finer, improving fatigue strength, machinability and cold forgeability. As described above, the heating temperature for achieving the complete solid solution of BN and AlN can be calculated by the following calculation of the solubility product. That is, log [Al] · [N] = − 7400 / T + 1.95 log [B] · [N] = − 13970 / T + 5.24 where [Al], [N] and [B] are Al, N The addition amounts of B and B, and T are absolute temperatures.

The finish rolling temperature during hot rolling and the cooling conditions thereafter are not particularly specified in the present invention, but the finish rolling temperature is preferably at least the recrystallization temperature of γ. This is because BN, which is the nucleus of graphite crystallization, is formed at the γ grain boundaries, but when the γ grains are refined by recrystallization, the BN distribution becomes more uniform.
Regarding the cooling rate, when this rate is extremely slow, 0.01 ° C / s is set because the precipitated BN coarsens graphite and causes machinability, cold forgeability and fatigue properties to decrease. It is desirable that the cooling rate is not lower than that.

The heating (or processing start) temperature in the second hot rolling must be 800 to 980 ° C. The reason for this is that at temperatures below 800 ° C, it does not completely become γ, so that partially solid-dissolved BN reprecipitates extremely unevenly, and in the final graphitized structure, the distribution of graphite particles becomes coarse and dense, and This is because the rolling resistance itself increases due to the increase in deformation resistance due to the reduction in rolling temperature. On the other hand, 98
If the temperature exceeds 0 ° C, the growth rate of precipitated AlN and undissolved BN will increase, and the γ grains will also become coarse.
And BN cannot be obtained, and fine graphite particles cannot be obtained. If the heating rate during the second hot rolling becomes extremely slow, undissolved BN becomes coarse, and the graphite after the graphitization treatment becomes coarse, making it difficult to achieve the intended purpose. It is desirable not to drop below 0.01 ℃ / sec.

Next, it is necessary to anneal under the conditions of heating for 5 hours or more after heating and raising the temperature in the temperature range of 650 to 740.degree. The reason for this is that at temperatures below 650 ° C, the graphitization reaction slows down, and the time required to complete graphitization becomes extremely long, while at temperatures above 740 ° C, a large amount of γ-phase is present in the steel. This is because graphitization does not proceed and the graphitization does not proceed. Also,
The holding time is set to 5 hours or more because graphitization that is sufficient to satisfy the machinability and cold forgeability does not proceed if this time is not reached. The preferred temperature range is 680-720 ° C.
It is.

[0037]

EXAMPLES The present invention will be described below with reference to examples. Steels having the chemical compositions shown in Table 1 were melted by a converter-continuous casting process to obtain blooms of 300 x 400 mm. In Table 1, steels A to L are steels whose composition complies with the method of the present invention, and steels M to P are comparative examples in which B, P, Al and Si are outside the range of the steel material of the method of the present invention. is there. Further, steel Q is a steel equivalent to JIS standard S30C, which has been conventionally used for mechanical structures, and steel R is a steel equivalent to S45C, which is an element that improves free-machining property and contains
Free-cutting steel with Pb and Pb added, Steel S is a Cr-Mo steel SC
This is an example of M435. The steel Q of S30C is a cold forged steel because of its excellent cold forgeability, and also S45C + (S, Ca, Pb)
Free-cutting steel R is used for applications that require high machinability due to its excellent machinability. Furthermore, steel S of SCM435 has excellent hardenability, mechanical properties after quenching and tempering, and rotational bending fatigue strength. It is used as a mechanical component that requires high fatigue strength because of its excellent durability.

[0038]

[Table 1]

These melted blooms were made into a 150 mm square by the first hot rolling and then 52 times by the second hot rolling.
It was rolled into a steel bar of mmφ and further subjected to graphitization annealing treatment in an annealing furnace. In the first hot rolling, solid solution temperatures of BN and AlN calculated from the compositional composition of steel were calculated, and the rolling temperature was set with this as a guide. In the case of performing the second hot rolling by cooling it to room temperature and then heating it to a predetermined temperature after the first hot rolling (No. 1, 3 to 6, 8,
9, 11, 13, 14, 16, 17, 23, 24, 2
6, 28 to 35, hereinafter abbreviated as "reheating rolling"),
When starting the second rolling from the predetermined temperature after the first hot rolling (No. 2, 10, 12, 15, 19, 2)
(1, 22, 25, 27, hereinafter abbreviated as "continuous rolling")
And went about. In addition, graphitization annealing (annealing),
It carried out until C in steel was almost completely graphitized. Table 2 collectively shows the solid solution temperatures of BN and AlN in each steel, rolling methods (reheating rolling / continuous rolling), first and second rolling conditions, and annealing conditions. In the reheating rolling, the cooling rate after the first rolling is 0.02-0.20 ℃ / se.
c The heating rate for the second rolling is 0.03 to 0.20 ℃ /
Each was done in the range of sec. In addition, regarding materials that have not sufficiently graphitized even after annealing for 200 hours or more,
At that point, the graphitization treatment was stopped. The symbol ** in the retention time column in Table 2 indicates that the graphitization treatment was interrupted.

[0040]

[Table 2]

The graphite particle size is 1000 to 1000 by an image analyzer after preparing an optical microscope sample from the annealed material.
The diameter of 2000 or more graphite particles was measured, and the average diameter was used. The as-annealed hardness was measured using a Vickers hardness meter. Cold forgeability is 15mmφ × 2 from the material after annealing
A 2.5 mml cylindrical test piece was prepared, a compression test was performed using a 300 t press, and the deformation resistance was calculated from the load during the test. Here, it is shown as the deformation resistance when the compression rate (height reduction rate) is set to 60%. Also, the presence or absence of cracks on the side surface of the test piece was confirmed, and the compressibility at which half of the tested test pieces cracked was defined as the limited compressibility and used as an index of deformability. The machinability test was performed using high-speed tool steel SKH4 at a cutting speed of 80 m / mi.
n. The tool life was evaluated as the time until the tool became uncuttable after the outer circumference turning under the condition of no lubrication. The characteristics after quenching and tempering are 15mmφ × 85mml made from the material, heated at 900 ° C × 30min, quenched in a water-soluble quenching solution, and then water-cooled at 500 ° C × 1h. Further, a tensile test piece of 8 mmφ was prepared and measured by a tensile test. In the rotary bending fatigue test, after carrying out the quenching and tempering treatment similar to the above, a 8 mmφ test piece was prepared, and the Ono-type rotary bending fatigue tester was used at 3600 rpm at room temperature.
Was carried out at a speed of. The results of these tests are also shown in Table 3.

[0042]

[Table 3]

Since the conventional steel could not be graphitized, it was carried out in accordance with the general working process, and the steel Q (S3
74 for steel equivalent to 0C) and steel R (equivalent to SCM435 steel)
After holding at 5 ° C. for 15 hours, spheroidizing annealing treatment of slow cooling was performed, and then each of the above tests was performed. For S45C-S-Ca-Pb steel, only the machinability was as-rolled, and other tests were 74
Spheroidized annealing was carried out by holding at 5 ° C for 15 hours and gradually cooling, and then performed. In the column of hardness after graphitization in Table 3, No. 3
The hardness after spheroidizing annealing is shown for 3 (Steel Q) and No. 35 (Steel S), and the as-rolled hardness is shown for No. 33 (Steel R).

As shown in Tables 2 and 3, the graphitization was completed in a short time when manufactured according to the method of the present invention, although it was slightly different for each steel type. On the other hand, when the hot rolling conditions are out of the range of the present invention (for example, No. 7 and No. 8), the annealing time is longer than that of the examples of the present invention (for example, No. 4 and No. 5). It was Further, the steel M having the B content outside the range of the present invention is steel B
The graphitization time is about 10 times longer than that of the above. Steels N and O whose P and Al are outside the scope of the present invention, respectively,
Graphitization time is 3 to 4 times longer than that of steel B. Further, the steel P having Si outside the scope of the present invention could not be graphitized.

As shown in the column of graphitized structure in the table, the graphite particle size (size) is any when the method of the present invention is adopted.
When it is out of the range of the present invention while it is less than 15 μm, the graphite particle size becomes extremely coarse up to about 55 μm. Also, the hardness and the deformation resistance during cold forging are
Although the influence of the graphite particle size is not observed, the critical compressibility and machinability (tool life during outer peripheral turning) during cold forging decrease as the graphite particle size becomes coarse. Further, when the component composition or the manufacturing conditions are out of the range of the present invention and the graphite particles are coarse, the mechanical properties after quenching and tempering are all deteriorated. This results in slower dissolution of graphite and lower hardenability, resulting in lower YS and TS, while residual graphite causes EL and
By lowering RA.

Next, cold forgeability, machinability and fatigue strength characteristics will be examined. Comparing the present invention example with the conventional example,
Deformation resistance and critical compressibility during cold forging are cold forged steel S3
Better than 0C steel. As for machinability, S45C-
Superior to Pb-Ca-S free-cutting steel. Also, fatigue strength
The method of the present invention is superior to SCM435. Even if the hot rolling condition and the annealing condition do not satisfy the present invention and only the component composition satisfies the present invention, the cold forgeability and the machinability under some conditions are equal to or higher than those of the conventional steel. Since the properties have been obtained, the hot rolling and annealing conditions need not necessarily be within the scope of the invention if only these properties are required. On the other hand, the fatigue strength, when the method of the present invention is applied, a fatigue strength of about 1.5 to 1.7 times the hardness is obtained, and a correlation with hardness is recognized, but outside the scope of the present invention and S45C- In the case of Pb-Ca-S steel, the fatigue strength does not increase in proportion to the hardness. This is because the undissolved graphite is S4 because the graphite particles are large in the case of outside the scope of the present invention.
In the case of 5C-Pb-Ca-S free-cutting steel, there are coarse non-metallic inclusions that improve machinability, and these act as starting points for fatigue fracture. In the present invention, it is not particularly necessary to add Ca, but when fatigue strength is not required, addition of Ca is effective for promoting graphitization and improving machinability.

[0047]

As described above, according to the present invention, the graphitization can be realized in a short time without the quenching treatment for the graphitization, and the obtained graphite particles are remarkably refined. it can. As a result, it is possible to obtain a steel material that has machinability equal to or higher than that of conventional Pb free-cutting steel without using Pb, and has excellent cold forgeability, mechanical properties after quenching and tempering, and fatigue strength. It will be possible to provide them, and it will greatly contribute to the manufacture of mechanical parts.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Akihiro Matsuzaki 1-chome, Mizushima Kawasaki-dori, Kurashiki-shi, Okayama Prefecture (no address) Inside the Mizushima Works, Kawasaki Steel Co., Ltd. (72) Shinichi Amano, Mizushima-kawasaki-dori, Kurashiki-shi, Okayama Prefecture 1 chome (without street number) Kawasaki Steel Works Mizushima Steel Works

Claims (4)

[Claims] 1. C: 0.1-1.5 wt%, Si: 0.5-2.0
wt%, Mn: 0.1 to 2.0 wt%, B: 0.0003 to 0.0150 wt%, Al: 0.005 to 0.1 wt%, O ≤ 0.0030 wt%, P ≤ 0.020 wt%, S ≤ 0.035 wt%, N: 0.0015 to 0.0150 A steel slab containing wt% and the balance being Fe and inevitable impurities is heated to a temperature above the solid solution limit of BN and AlN, hot-rolled, and cooled to room temperature.
It is heated in the temperature range of 800 to 980 ℃ and hot rolled, then 65
A method for producing a steel for machine structural use, which is excellent in machinability, cold forgeability, and fatigue properties after quenching and tempering, characterized by heating in a temperature range of 0 to 740 ° C and holding it for 5 hours or more. 2. C: 0.1-1.5 wt%, Si: 0.5-2.0
wt%, Mn: 0.1 to 2.0 wt%, B: 0.0003 to 0.0150 wt%, Al: 0.005 to 0.1 wt%, O ≤ 0.0030 wt%, P ≤ 0.020 wt%, S ≤ 0.035 wt%, N: 0.0015 to 0.0150 wt%, and Ni: 0.10-3.0
wt%, Cu: 0.1-3.0 wt%, Co: 0.1-3.0 wt%, Mo: 0.1-1.0 wt% V: 0.05-0.5 wt%, Nb: 0.005-0.05 wt%, Ti: 0.005-0.05 wt%, Zr: 0.005 to 0.2 wt% and REM: 0.0005 to 0.2 wt% One or more selected from, with the balance being Fe and inevitable impurities. 800 ~ 980 after heating to a temperature above the solid solubility limit and hot rolling, cooling to room temperature
Heated to a temperature range of ℃, hot-rolled, then 650-740 ℃
A method for producing a steel for machine structural use, which is excellent in machinability, cold forgeability, and fatigue characteristics after quenching and tempering, characterized by heating in the temperature range of 5 and holding for 5 hours or more. 3. C: 0.1-1.5 wt%, Si: 0.5-2.0
wt%, Mn: 0.1 to 2.0 wt%, B: 0.0003 to 0.0150 wt%, Al: 0.005 to 0.1 wt%, O ≤ 0.0030 wt%, P ≤ 0.020 wt%, S ≤ 0.035 wt%, N: 0.0015 to 0.0150 A steel slab containing wt% with the balance being Fe and inevitable impurities is heated to a temperature above the solid solution limit of BN and AlN and hot-rolled, followed by 800-98
Machinability, cold forgeability and fatigue after quenching / tempering, characterized by hot rolling from a temperature range of 0 ° C, then heating to a temperature range of 650 to 740 ° C and holding for 5 hours or more. A method for producing steel for machine structural use, which has excellent properties. 4. C: 0.1-1.5 wt%, Si: 0.5-2.0
wt%, Mn: 0.1 to 2.0 wt%, B: 0.0003 to 0.0150 wt%, Al: 0.005 to 0.1 wt%, O ≤ 0.0030 wt%, P ≤ 0.020 wt%, S ≤ 0.035 wt%, N: 0.0015 to 0.0150 wt%, and Ni: 0.10-3.0
wt%, Cu: 0.1-3.0 wt%, Co: 0.1-3.0 wt%, Mo: 0.1-1.0 wt% V: 0.05-0.5 wt%, Nb: 0.005-0.05 wt%, Ti: 0.005-0.05 wt%, Zr: 0.005 to 0.2 wt% and REM: 0.0005 to 0.2 wt% One or more selected from, with the balance being Fe and inevitable impurities. It is characterized in that it is heated to a temperature above the solid solubility limit and hot-rolled, then hot-rolled from a temperature range of 800 to 980 ° C, then heated to a temperature range of 650 to 740 ° C and held for 5 hours or more. A method for manufacturing a steel for machine structural use, which has excellent machinability, cold forgeability, and fatigue properties after quenching and tempering.