JP3385365B2 - Medium carbon steel with dispersed fine graphite structure - Google Patents

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention has high strength, good workability and machinability, can shorten the time required for graphitization, and, if necessary, can increase the graphitization heat treatment temperature. The present invention relates to a medium carbon steel having a dispersed fine spheroidal graphite structure capable of lowering the temperature.

[0002]

2. Description of the Related Art Conventionally, lead free-cutting steel has been used as a steel material having free-cutting properties, but since Pb is contained, environmental pollution problems have been closely watched and its use has recently been restricted. Came. For these reasons, the development of new steel materials that do not cause environmental problems, which is an alternative to lead free-cutting steel, is underway. Among these, graphite-dispersed materials are known as free-cutting steel materials. Graphite steel, which has been considered in the past, has improved machinability, but mechanical properties such as strength and workability. Since it is inferior, it cannot be said to be a material having sufficient properties as free-cutting steel. Therefore, development of graphite steel in which fine graphite having free-cutting properties and good mechanical properties is uniformly dispersed is expected.

It is generally known that graphitization is likely to proceed in high carbon steel having a hypereutectoid composition. In particular, it has been reported that a graphitization is remarkably promoted when 1% or more of graphite is added (Data 1: Tomio Sato, 1st person, “Study of Graphite Steel (1st Report): Effect on Graphite Steel”) Effect of Si "
(See The Japan Institute of Metals, 1956, 20: 5-9). Similarly, for high carbon steel, quenching, cold working or A
There is also a report that addition of 1, Si, Ni, Ti, Zr, B, etc. promotes graphitization (Data 2: Naomichi Yamanaka 1
Masterpiece "Consideration on graphitization mechanism of high carbon steel" Iron and steel 1
962, No. 8, pp. 946-953). In this document, the effect of addition of Ti is that the amount of solid solution of Ti with respect to cementite is very small and the special carbide TiC is easily formed.
It has been explained that the effect of denitrification by i appears significantly and promotes graphitization. However, all of these examples are high carbon steels, and in order to obtain strength and good workability, medium carbon steels having a C concentration of 1.0% or less are required. Therefore, the above examples are not satisfactory as free-cutting steel, and none of them are suitable. And
On the contrary, in the medium carbon steel, especially in the hypoeutectoid steel, the above graphitization is difficult, and the behavior of graphitization by heat treatment or alloy addition is not well known at present.

As shown in Document 1 above, Si is added as an element that promotes graphitization. However, if the amount of Si added is large, the ductility is significantly reduced due to solid solution hardening.
It is considered desirable to limit the addition to 5% or less. Under such circumstances, a report explaining the graphitization phenomenon by co-adding Si and Ni or Si and Co to hypoeutectoid steel has been made (Data 3: Shuichi Sueyoshi et al., “Graphitization of hypoeutectoid steel”). Phenomena and Effects of Alloying Elements on These "Journal of Japan Institute of Metals 1
979, 43, 333-339). However, the material containing Ni or Co is inferior in recyclability, and it cannot be said to be an effective means considering that such an additional element is a relatively expensive material, and it is stable and has good reproducibility. It cannot be said that the manufacturing technology of hypoeutectoid steel in which is dispersed is established.

It is known that in high carbon steel, graphitization proceeds when cold working is performed before graphitization.
As one of such methods, there is a report that hypoeutectoid steel is used to make it as soft and ductile as low carbon steel (Document 4: ATUKI OKAMOTO "Graphite Formation in
High-Purity Cold-Rolled Carbon Steels '' METALLURGI
CAL TRANSACTIONS A, VOLUME 20A, OCTOBER 1989, P.191
7-1925). In this method, cold rolling is performed until cementite is divided, and the divided gaps (voids) are considered to be sites for graphitization. However, this example has a problem that it requires severe cold working of 20% or more in a state where cold workability is poor in order to divide cementite, and it cannot be said to be a stable manufacturing method.

[0006] Further, a method of adding B, Al and rare earth metals (REM) such as La and Ce to 0.53% C steel to promote graphitization has been proposed (Data 5: "I. Effect of B, Al and REM on Graphitization Behavior of 53% C Steel "CAMP-ISIJ, Vol. 8, 19
1995, p. 1378). According to this method, it was reported that the average particle size of graphite could be 2.7 μm, so it seems that there is some effect, but in order to realize the largest number of graphite in this technology To BA
It requires multiple additions of 1 and the single addition requires data 5
FIG. As shown in No. 1, the number of graphite produced is not so large. It is not easy to control a small amount of additional elements in the production of steel, and the effect thereof is large in complex addition, so that it is not practical. A common problem with the above graphitization techniques is that the graphitization is at a high temperature and the treatment time is extremely long. 700 ° for complete graphitization
C requires a long time of about 10 to 50 hours, which significantly reduces the production efficiency.

[0007]

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention
INDUSTRIAL APPLICABILITY The present invention makes fine ZrC function as a nucleation site for graphite, has good reproducibility, high productivity, high strength, good machinability and machinability, and shortens the time required for graphitization. To obtain a medium carbon steel having a dispersed fine spheroidal graphite structure that can be converted into a carbon.

[0008]

The present invention includes: Substantially Si 0.1-1.5%, C 1.0% or less,
Zr 0.01-0.5% and Fe are contained (however, B
(Except positive addition of N and N) medium carbon steel with fine Z
1. Medium carbon steel with dispersed fine spheroidal graphite structure characterized in that rC functions as a nucleation site for graphite. 2. Medium carbon steel according to the above 1, characterized in that it has a structure in which graphite having an average size of 1 μm to 20 μm is dispersed in a matrix. 1 above, which has a structure in which cementite disappears and graphite is dispersed in a ferrite matrix.
Alternatively, the medium carbon steel according to the item 2 is provided. In addition, the medium carbon steel of the present invention is positively added with B and N.
Not a thing. That is, B and
It is a medium carbon steel without N. Further, the medium carbon steel of the present invention,
Substantially Si 0.1-1.5%, C 1.0% or less, Zr
Medium carbon steel composition set containing 0.01 to 0.5% and Fe
Is included in the composition as medium carbon steel.
Ingredients necessarily included.

[0009]

BEST MODE FOR CARRYING OUT THE INVENTION In the present invention, in order to prevent embrittlement due to solid solution of Si, the Si concentration is set to 1.5% (all in the present specification means mass%) or less. In addition, considering the recyclability of steel materials, minute amounts of elements that do not pose a problem for recycling are added, and fine graphite is deposited on medium carbon steel without harsh processing, and at the same time, the time required for graphitization is reduced. It will be shortened. Usually, non-metallic inclusions present in steel materials often adversely affect the properties of the material, so research is being conducted to remove them, but after solidification of fine inclusions with a size of 1 μm or less, The research centered on its use as a nucleation site for solid phase transformation of. The present invention selects Zr, which is one of the elements having the strongest tendency to form carbides in Fe, and uses ZrC as a graphite formation site.

The inventors of the present invention have previously added Ti to a material having a composition of hypoeutectoid steel, and have a dispersed fine graphite structure that allows fine TiC to function as a nucleation site of graphite, and a production thereof. Proposed a method (Japanese Patent Application No. 10-06790)
0). According to this, a dramatic improvement was recognized in the formation of the graphite microstructure. The present invention has an effect equal to or higher than that of the steel having the fine graphite structure by the addition of Ti. In particular, Zr used in the present invention has a large bond with C, and the solubility product of ZrC in the matrix is T.
It is smaller than iC, and ZrC can be precipitated with a smaller amount of addition. Further, when Zr is added, the graphitization rate is high, which is a feature that it is extremely advantageous in terms of energy consumption of heat treatment. As described above, the present invention has an excellent feature that a small amount of Zr is added and a finer graphite structure can be rapidly formed. The present invention uses medium carbon steel having a C concentration of 0.3% to 1.0 as a basic component. The Zr concentration is required to be 0.01% as a minimum amount for the precipitation of ZrC, and the upper limit is set to 0.5% as an amount that prevents coarsening of graphite and does not reduce the amount extremely. That is, the medium carbon steel of the present invention is Zr.
It contains 0.01 to 0.5%. The Si concentration needs to be at least 0.1% from the viewpoint of promoting graphitization, and the upper limit is 1.5% in order to improve cold workability and machinability.

In the present invention, a raw material having such a medium carbon steel composition is melted to form an ingot, and then, as shown in FIGS.
The medium carbon steel having the dispersed fine spheroidal graphite structure is manufactured through the three kinds of manufacturing processes (heat treatment history). Pattern 1 in FIG. 1 is obtained by melting a raw material of medium carbon steel composition into an ingot, followed by hot rolling, and then air cooling and heat treating at a temperature of 740 ° C. or lower for 0.5 to 10 hours to form graphite. A medium carbon steel having a dispersed fine spherical graphite structure was produced by subjecting it to graphitization treatment for growth. Pattern 2 in FIG. 2 is obtained by melting a material of medium carbon steel composition into an ingot, followed by hot rolling, and air-cooling this at 750 ° C. to 1300 ° C.
Heat treatment (solution treatment) for 10 hours, then air cooling, further heat treatment at a temperature of 740 ° C. or less for 0.5 to 10 hours, graphitization treatment for growing graphite, and provision of dispersed fine spherical graphite structure Manufactured medium carbon steel. Figure 3
Pattern 3 of No. 3 is produced by melting a material of medium carbon steel composition into an ingot, followed by hot rolling, and air-cooling this at 750 ° C to
Heat treatment at 1300 ° C for 0.5-10 hours (solution treatment)
Then, this is water-quenched to precipitate fine ZrC in the matrix, and after this water-quenching, heat treatment is performed at a temperature of 740 ° C. or lower for 0.5 to 10 hours to grow graphite using ZrC as a nucleation site. A medium carbon steel having a dispersed fine spheroidal graphite structure was produced by the treatment.

As described above, at a temperature of 740 ° C. or lower for 0.5 to 10 hours (in the case of heat treatment at a lower temperature, 0.
Heat treatment is performed for 5 to 100 hours, and graphitization is performed to grow graphite using ZrC as a nucleation site. As a result, cementite disappears, and a medium carbon steel having a structure in which 30 to 6500 graphite / mm ² having an average size of 1 μm to 20 μm are dispersed in a ferrite matrix is obtained. The size and number of graphite vary depending on the composition of Zr and C and the heat treatment conditions. Therefore, the size and number of dispersed graphite are controlled by adjusting these. In particular, after heat treatment (solution treatment) at 750 ° C. to 1300 ° C. for 0.5 to 10 hours and then water quenching, nucleation of graphite is remarkably generated as compared with others, and it is effective for refining graphite. In addition, the graphitization treatment is 450 ° to 7
When it is carried out at a temperature of 00 ° C, the growth of more finely divided graphite is observed. Graphite having an average size of 1 μm to 20 μm is 30 to 30 in a ferrite matrix of medium carbon steel.
Those having a structure in which 6500 pieces / mm ^{2 are} dispersed have optimum properties for improving cold workability and machinability and strength.

[0013]

EXAMPLES AND COMPARATIVE EXAMPLES Next, the present invention will be described by way of Examples in comparison with Comparative Examples. The samples used in the present invention had the basic composition excluding Zr of Fe-1% Si-0.
5% C (mass%, all mass% are shown in this specification). Further, 0.05 to 0.15% Zr was added to this basic composition so as to precipitate ZrC, and each was melted in a magnesia crucible using a high frequency melting furnace. In addition, regarding each component used, Fe (99.99)
%), Si (99.999%), C (99.999%)
And Ferro Zr was used as the material. Diameter 20mm after melting
Each of the rod-shaped ingots of
It was hot rolled with C to a thickness of 10 mm. Further, a 10 mm square sample for heat treatment was cut out from these. In the present invention, the following three types of heat treatments were performed. Schematic diagrams of these thermal histories are shown in FIGS. FIG. 1 is a schematic diagram of a thermal history when graphitizing at 450 ° C. to 700 ° C. after hot rolling (air cooling) (Pattern 1). FIG. 2 shows that a hot-rolled (air-cooled) material is austenitized (solution-treated) at 800 ° C., air-cooled, and then 450 ° C. to 700 ° C.
It is a schematic diagram of a thermal history when graphitized at ° C (Pattern 2). Figure 3 shows 800 of hot rolled (air cooled) material
FIG. 3 is a schematic diagram of a thermal history when water-quenching is performed after austenitization (solution treatment) at ° C and then graphitization is performed at 450 ° C to 700 ° C (Pattern 3).

After the above heat treatment, the structure was observed using an optical microscope and SEM. Before observing the SEM structure, the surface was corroded with 10% Nital after mirror polishing in advance. When observing graphite three-dimensionally, use the potentiostatic galvanic corrosion method, which is suitable for the corrosion method of non-metallic inclusions, and use 1% electrolytic solution.
Tetramethylammonium-10% acetylacetone-
Observation was carried out after corroding the surface with methyl alcohol.

(About air-cooled sample) Typical optical microscope images of the sample subjected to solution treatment and heat-treated at 700 ° C. for 50 hours in the history shown in pattern 2 are shown in FIGS.
FIG. 4A shows the case of only the basic composition of Fe-1% Si-0.5% C described above, and Zr is not added. FIG. 4 (b) has a composition of Fe-1% Si-0.5% C-0.05% Zr, and FIG. 4 (c) shows Fe-1% Si-0.5% C-0.1.
It has a composition of 5% Zr. In FIG. 4 (a) where Zr is not added, graphitization does not proceed, and the structure is such that cementite is dispersed in the ferrite matrix. Zr
When was not added, graphitization did not proceed even after heat treatment for 400 hours. Zr 0.05% each
4 (b) and 4 (c) in which 0.15% was added, graphitization proceeds in both cases. When 0.05% Zr is added, graphite with a diameter of about 5 μm is dispersed and 0.15% Zr
In the case of addition, it is observed that graphite having a diameter of about 15 μm is dispersed. Although not shown in the drawing when the heat treatment was performed in the pattern 1, the structure similar to that of FIGS.

Next, with respect to the sample to which 0.05% Zr was added, that is, the sample having the composition of Fe-1% Si-0.5% C-0.05% Zr, the heat treatment of pattern 2 was applied to the solution treatment. 5 (a), (b) and 6 (c) are optical microscope images when the graphitization heat treatment time at 700 ° C. was changed to 1 hour, 1.5 hours, 2 hours and 30 hours, respectively. , (D). As shown in FIG. 5 (a), some of the graphite has already started to nucleate one hour after the graphitization treatment, and as shown in FIG. 6 (c), the graphitization is completed within 2 hours. To be observed. After that, the structure is stable with almost no change even after heat treatment for a long time. Therefore, it is economically wasteful to heat the graphitization for a longer time than necessary (graphitization heat treatment) after the graphitization is completed.

Next, referring to FIGS. 7A to 7C, Fe-1% Si-
Regarding the sample having the composition of 0.5% C-0.05% Zr, the thermal history of pattern 2 was applied to the sample subjected to the graphitization heat treatment at 700 ° C. for 1.5 hours after the solution treatment, and the potentiostatic electrolytic corrosion method was used. Fig. 3 shows an SEM image in which the matrix was corroded by using and the graphite was three-dimensionally observed. Cubic ZrC of about 1 μm (slightly smaller than 1 μm) at almost the central position of FIGS. 7A to 7C can be seen, but graphite is observed to grow from this cubic ZrC. In the figure, it is 2 μm to 3.5 μm that exist in the vicinity of cubic ZrC.
A spherical material is graphite. Thus, it is clear from these figures that ZrC functions as a graphite nucleation site. Although not shown, when the Zr concentration increases (for example, 0.15% Zr), the graphite tends to coarsen. This is because Zr is an element that has a strong tendency to form carbides, so when the Zr concentration increases, it tends to suppress the diffusion of carbon atoms in the ferrite matrix and stabilize cementite, and therefore the core of graphite on ZrC is It is considered that the generation and dispersion were suppressed and the graphite was rather coarsened. Since such coarsening of graphite is against the purpose of producing dispersed fine graphite particles, it is necessary to avoid containing Zr more than necessary. Therefore, from the above, the upper limit of the Zr content is set to 0.5%.

Next, Fe-1% Si-0.5% C-0.
With respect to the sample having a composition of 05% Zr, two types of heat treatment (50-hour heat treatment) at a heat treatment temperature of 450 ° C. and a temperature of 550 ° C. after the solution treatment are performed according to the heat history of the pattern 2 and the progress of the graphitization is performed. Was observed. The results are shown in FIGS. 8 (a) and 8 (b), respectively. FIG. 8 (a) shows the case of heat treatment at 450 ° C. In this case, graphite is not confirmed. However, it is observed that graphitization is completed in the case of the heat treatment at 550 ° C. shown in FIG. From the above, it is shown that the addition of Zr promotes graphitization, and it is possible to sufficiently form dispersed fine graphite particles even at a lower temperature (about 500 ° C. from the above results). The fact that graphitization is possible at a low temperature in this way has a great effect in terms of cost.

(For water-quenched sample) Pattern 3
9 (a), 9 (b) and 9 (c) are the optical microscope images of the sample heat treated at 700 ° C. for 50 hours after the solution treatment. FIG. 9A shows a basic composition (Fe-1
% Si-0.5% C), FIG. 9 (b) shows Fe-1% S
In the case of the composition of i-0.5% C-0.05% Zr, FIG.
(c) is a composition of Fe-1% Si-0.5% C-0.15% Zr. In the case where Zr of the basic composition was not added, as shown in FIG. 9 (a), the structure was such that spherical cementite was finely dispersed, and no graphite was generated. On the other hand, as shown in FIGS.
When 05% Zr and 0.15% Zr are added, the structure is such that graphite of about 5 to 10 μm is finely dispersed in the ferrite matrix. Particularly, in the case of adding 0.15% Zr, fine graphite having a good shape (spherical) is uniformly dispersed, and has the same composition as that of the heat-treated pattern 2 shown in FIG. 4 (c). Nevertheless, the miniaturization of graphite is more remarkable. Usually, the matrix is transformed into martensite by quenching treatment, a large amount of lattice defects are introduced,
It is considered that diffusion of carbon atoms in the matrix is facilitated, but this facilitates diffusion of graphite in the vicinity of ZrC, which is a nucleation site of graphite, and a large number of nuclei of graphite are generated, resulting in a fine graphite structure. From the above results, the water quenching shown in pattern 3 is extremely effective for forming dispersed fine graphite particles.

Next, as shown in FIGS. 10 (a) and 10 (b), Fe-1% S
A sample of a medium carbon steel having a composition of i-0.5% C-0.05% Zr was subjected to solution treatment by the thermal history of pattern 3 and then 700
The micrograph of the structure at the time of changing the graphitization heat treatment time to 30 minutes and 1 hour at ° C is shown. As is clear from FIG. 10 (a), when the graphitization heat treatment time passed 30 minutes, the graphitization had already progressed considerably, and
As shown in (b), the graphitization is almost completed when the graphitization heat treatment time exceeds 1 hour. Compared to Fig. 5 which was not subjected to quenching treatment, although the sample was a hypoeutectoid steel sample of the same composition, the sample that had undergone the heat history of pattern 3 of water quenching had remarkably accelerated graphitization. I understand.

[0021]

As is apparent from the above description and the examples, the medium carbon steel of the present invention has a hot work temperature and time for graphitization controlled appropriately according to the Zr content. It is possible to precipitate uniformly finely dispersed graphite in the matrix by a method of performing heat treatment for graphitization immediately after rolling, or by performing solution treatment after hot rolling and then subjecting to air cooling and further heat treatment for graphitization. . In particular, when the Zr content is high and the graphitization heat treatment is performed at a high temperature, a dispersed fine spherical graphite structure can be obtained in a short time. Further, in this case, the graphitization heat treatment at a low temperature is also possible by adjusting the Zr concentration and the treatment time. further,
It has an excellent feature that fine ZrC can be precipitated in the matrix by quenching after solution treatment and dispersed fine graphite particles can be formed in an extremely short time. As described above, the present invention allows fine ZrC to function as a nucleation site of graphite, disperses fine graphite in a stable and reproducible manner, has good material recyclability, high productivity, and high strength. It is possible to obtain a medium carbon steel having a dispersed fine spheroidal graphite structure having workability and free-cutting property.

[Brief description of drawings]

FIG. 1 is a schematic diagram showing a heat history of graphitization treatment when the graphitization treatment is performed at 450 ° C. to 700 ° C. after hot rolling (air cooling) (pattern 1).

FIG. 2: Solution rolling after hot rolling (air cooling), then 45
It is a schematic diagram which shows the heat history of the graphitization process at the time of graphitizing process at 0 degreeC-700 degreeC (pattern 2).

FIG. 3 shows the heat history of precipitation treatment and graphitization treatment in the case of performing solution treatment after hot rolling (air cooling), water quenching, and then graphitization treatment at 450 ° C. to 700 ° C. (Pattern 3). It is a schematic diagram.

4 (a) to (c) are optical microscope images (photographs) of the structures of various samples subjected to graphitization heat treatment at 700 ° C. for 50 hours with a thermal history of pattern 2. FIG. (a) Sample Fe-1% Si-0.5% C, (b) Sample Fe-
1% Si-0.5% C-0.05% Zr, (c) Sample Fe
-1% Si-0.5% C-0.15% Zr

5 (a) (b) Sample Fe-1% Si-0.5% C-0.05% Zr with the thermal history of pattern 2 and at 700 ° C. for various heat treatment times (1 hour, 1 hour, 5 is an optical microscope image (photograph) of the tissue when treated for 5 hours.

(A) (b) Sample Fe-1% Si-0.5% C-0.05% Zr with thermal history of pattern 2 and at 700 ° C for various heat treatment times (2 hours, 30 hours). It is an optical microscope image (photograph) of the tissue at the time of processing.

7 (a) to (c) Sample Fe-1% Si-0.5% C-0.05% Zr was graphitized at 700 ° C. for 1.5 hours with thermal history of pattern 2. It is the SEM image (photograph) performed.

8 (a) and (b) Sample Fe-1% Si-0.5% C-0.05% Zr with a thermal history of pattern 2 and at a graphitization temperature of 450 ° C. and 5
It is an optical microscope image (photograph) which graphitized by 50 degreeC for 50 hours.

9 (a) to 9 (c) are optical microscope images (photographs) of the structures of various samples subjected to heat treatment for graphitization at 700 ° C. for 50 hours with the thermal history of pattern 3. (a) Sample Fe-1% Si-0.5% C, (b) Sample Fe-
1% Si-0.5% C-0.05% Zr, (c) Sample Fe
-1% Si-0.5% C-0.15% Zr

10 (a) (b) Microstructure of sample Fe-1% Si-0.5% C-0.05% Zr subjected to graphitization heat treatment at 700 ° C. with the thermal history of pattern 3. It is an optical microscope image (photograph) of.

─────────────────────────────────────────────────── ─── Continuation of front page (56) References JP-A-7-188846 (JP, A) JP-A-10-72639 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB name) C22C 38/00-38/60

Claims (3)

(57) [Claims] 1. Substantially 0.1 to 1.5% Si, C1.
0% or less, containing 0.01 to 0.5% Zr and Fe
(However, except positive addition of B and N) Medium carbon steel
Then , a medium carbon steel having a dispersed fine spheroidal graphite structure, in which fine ZrC functions as a nucleation site of graphite. 2. The medium carbon steel according to claim 1, which has a structure in which graphite having an average size of 1 μm to 20 μm is dispersed in a matrix. 3. The medium carbon steel according to claim 1 or 2, which has a structure in which cementite disappears and graphite is dispersed in a ferrite matrix.