JP2518873B2 - Steel plate for heat treatment - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial field of application) The present invention has good workability comparable to that of a low carbon steel sheet, and has hardenability comparable to that of a high carbon steel sheet, and after quenching-tempering treatment. The present invention intends to propose a steel sheet for heat treatment which is excellent in toughness.

Generally, a carbon steel material used after a heat treatment process such as a quenching-tempering process contains at least about 0.3% by weight of carbon.
Although it is referred to as high carbon steel, it is contained in the above (hereinafter simply expressed in%), and because such high carbon steel has high hardness and excellent strength and wear resistance, it is used in the fields of blades, springs, and various other mechanical parts. Widely used in.

In such application fields, steel sheets for heat treatment undergo various processes such as cutting, punching, drilling, and bending prior to the heat treatment, but steel with higher hardenability has higher strength and is difficult to process as described above. It is. In order to compensate for this, even if it is generally performed in advance softening treatment such as spheroidizing annealing, there is a limit to the degree of softening in the spheroidized cementite structure obtained by such treatment, It is difficult to obtain workability comparable to that of low carbon steel.

In other words, in the case of high carbon steel for heat treatment, the composition of the material is usually determined from the viewpoint of hardenability. Expecting good workability on a par with steel was an absolutely impossible order.

When using a high carbon steel in this way, for example, the restriction that processing of complex shapes is not possible, and the forming method and problems such as forming machines, and further increase in the number of processing steps and time as described above, The problem of manufacturing cost has arisen.

As another means for solving the above-mentioned difficulties in high carbon steel, for parts that require complicated forming, a low carbon steel with good workability is used as a material to be processed to a predetermined shape and then quenched. Although methods of carburizing or further nitriding have been proposed in order to secure the properties, such methods of carburizing and nitriding naturally involve an increase in man-hours and are economical. It goes without saying that it comes at a disadvantage.

(Prior Art) In Japanese Patent Application Laid-Open No. 60-52551, in order to further improve the workability of a carbon steel material without requiring extraordinarily troublesome processes and equipment and ensuring the required strength, it is necessary to improve the workability of steel. P
And forming a graphite phase by reducing the S content as much as P (%) × S (%) ≦ 10 × 10 ⁻⁶ to form a structure mainly composed of a ferrite phase and a graphite phase. The usefulness of is proposed.

In this case, C0.3% that should be suitable for use as heat treatment steel
Approximately 60 kgf / mm ² in the so-called high-carbon steel region Okei the tensile strength of the above, because it extends to 86kgf / mm ^2, the effect of the processability improvement is still not sufficient.

In addition, Japanese Patent Application Laid-Open No. 60-128245 discloses that workability is improved when a structure mainly composed of ferrite and graphite is used. However, this case is not intended for materials in the field used for quenching treatment, but is limited to structural materials that exclusively utilize the damping properties of the graphite phase in the structure. From the viewpoint of the above, since the larger the particle size of the graphite phase present in the structure, the better the structure is. The texture with grains is not suitable for the following reasons.

Generally, when steel is heated to the austenitizing temperature, the ease of dissolution of C in the steel into the austenite phase is lower in the case of graphite than in the case where C is in the cementite state, In particular, this tendency is stronger as the graphite particles are coarser. As described above, when the solubility of C in austenite is inferior, a predetermined quenching hardness cannot be obtained during quenching after austenitization, so that it cannot be used as a steel for heat treatment.

(Problems to be solved by the invention) In view of the above problems in heat treatment steel, in the case of cutting, punching, drilling and bending, it is as soft as a low carbon hole and has good workability. For heat treatment that not only possesses but also has heat treatment performance equivalent to normal high carbon steel when performing quenching-tempering treatment without performing troublesome treatment such as carburizing and nitrifying. It is an object of this invention to provide a steel sheet.

(Means for Solving Problems) The above-mentioned object is advantageously expressed by a configuration that outlines the following circumstances.

C: 0.30 to 1.20% Si: 0.30 to 2.00% Mn: 0.05 to 1.50% Al: 0.001 to 0.100% N: 0.0060% or less P: 0.020% or less S: 0.015% or less and B: 0.0005 to 0.0500%, balance Fe And has an unavoidable impurity composition, has a structure mainly composed of ferrite phase and fine graphite particles with a diameter of 10 μm or less, and has a tensile strength of 50 kgf / mm ² or less, excellent workability and hardenability, Further, a steel sheet for heat treatment (first invention), which is also excellent in toughness after quenching-tempering treatment.

C: 0.30 to 1.20% Si: 0.30 to 2.00% Mn: 0.05 to 1.50% Al: 0.001 to 0.100% N: 0.0060% or less P: 0.020% or less S: 0.015% or less and B: 0.0005 to 0.0500% Ti: 0.005 to It has a composition of 0.050% of balance Fe and unavoidable impurities, has a ferrite phase and a structure mainly composed of fine graphite particles with a diameter of 10 μm or less, and has a tensile strength of 50 kgf / mm ² or less, A steel plate for heat treatment, which is excellent in hardenability and excellent in toughness after quenching-tempering treatment (second invention).

C: 0.30 to 1.20% Si: 0.30 to 2.00% Mn: 0.05 to 1.50% Al: 0.001 to 0.100% N: 0.0060% or less P: 0.020% or less S: 0.015% or less B: 0.0005 to 0.0500% and La: + Ce: It has a composition of 0.002 to 0.050% of balance Fe and unavoidable impurities, has a structure mainly composed of ferrite phase and fine graphite particles with a diameter of 10 μm or less, and has a tensile strength of 50 kgf / mm ² or less. And a hardenability, and a toughness after quenching-tempering treatment are excellent.

C: 0.30 to 1.20% Si: 0.30 to 2.00% Mn: 0.05 to 1.50% Al: 0.001 to 0.100% N: 0.0060% or less P: 0.020% or less S: 0.015% or less B: 0.0005 to 0.0500% Ti: 0.005 to 0.050 % La + Ce: 0.002 to 0.050% and the balance is Fe and unavoidable impurities. It has a structure mainly composed of ferrite phase and fine graphite grains with a diameter of 10 μm or less, and the tensile strength is 50 kgf / mm ² or less. And a workability and hardenability, and a toughness after quenching-tempering treatment (4th invention).

Each of the first to fourth inventions has a material property that is as soft as a low-carbon steel and has good workability in processing, and has a good hardenability similar to a normal high-carbon steel in heat treatment. It is a steel sheet with excellent toughness, wear resistance and strength characteristics of the latter material, and in order to achieve such material characteristics, the microstructure of the steel has a structure in which fine graphite phase is uniformly dispersed in the farite phase (hereinafter (Referred to as ferrite / graphite structure), and adjusting the chemical composition of steel to obtain such a microstructure,
If necessary, in addition to the adjustment of the rolling conditions at the time of hot rolling, adjustment of the subsequent annealing conditions is performed.

Any of the heat-treated steel sheets listed above can be obtained in the form of hot-rolled sheet that has undergone an annealing treatment with a microstructure of ferrite-graphite structure after predetermined hot rolling. It can also be obtained in the form of a cold-rolled sheet that has been subjected to the above-mentioned annealing treatment after hot or cold rolling using a hot-rolled sheet as a raw material in the rolling temperature range of 500 ° C or less. In particular, since it has excellent workability during cold rolling, it is advantageous because it reduces the load on the cold rolling operation, but directly after the above hot rolling, it can be used for warm or cold rolling in the usual process. The cold-rolled sheet may be provided by subjecting it to an annealing treatment.

In the hot rolling, it is more preferable to advance the recrystallization and refining of the γ grains by making the γ grains as fine as possible in the hot rolling process, in order to uniformly disperse the fine graphite.

Warm or cold rolling finishes to a specified plate thickness with a reduction rate of 20% or more.

Annealing treatment condition is 500 ° C ~ 750 ° C, more preferably 650 ° C
Applicable by holding for ₁ to 200 hours between A1 transformation point.

(Operation) The reasons for limiting the numerical values in the above inventions will be described in detail below.

C is an indispensable element for securing hardenability, and is required to be 0.30% or more due to the demand for wear resistance, hardness, strength characteristics, etc. of various products manufactured using the above-described heat treatment steel sheet. . The reason for setting the upper limit to 1.20% is that not only does the hardenability become saturated with an amount of C exceeding this, but the amount of insoluble cementite or the graphite phase increases during austenitizing before quenching, and This is because the impact resistance is deteriorated.

Si is also an indispensable element for the following two reasons.

First of all, the steel base is strengthened by solid solution hardening, and after the quenching treatment, it is easy to obtain high strength in a range that cannot be achieved only by quenching and hardening with C, thereby improving wear resistance and increasing hardness. This is because it can be planned.

Secondly, to obtain a good fine graphite structure. That is, as described above, the as-hot-rolled microstructure of high-carbon steel is a structure containing ferrite and pearlite, or bainite in these, and is usually extremely high in strength, so that the formability is extremely poor. In order to improve this, the microstructure is changed to the desired ferrite-graphite structure by annealing, but Si is
It acts to facilitate the transformation of cementite into graphite grains and facilitates obtaining a ferrite-graphite structure after annealing, which contributes to softening and improvement of workability. In addition, Si also serves to improve hardenability by improving the solubility of graphite particles in austenite during heating before quenching.

Here, the mechanism by which Si promotes the conversion of cementite to graphite is as follows.

Since Si is a non-singtenizing element, it is difficult to dissolve in cementite in an equilibrium state, but when dissolved in a non-equilibrium state, cementite becomes extremely unstable. Ar ₁ after hot rolling
Since the rate of formation of cementite by transformation is very high, the composition of cementite thus produced inherits the composition ratio of the parent phase before transformation in a rich manner, and therefore contains excess Si above the equilibrium solubility. The higher the Si content of the base material, the more the amount of excess Si in the cementite increases, so that the degree of destabilization of the cementite increases and the conversion into graphite becomes easier.

In order to obtain the above-mentioned two effects, Si is 0.3
It is necessary to be 2.0% or less, and the reason for limiting the content to 2.0% or less is from the viewpoint of manufacturing cost, that is, the improvement of wear resistance related to solid solution hardening even when added in excess of 2.0%.
Also, the graphitization promoting effect during annealing is saturated, and the manufacturing cost only increases. Mn is an element that improves hardenability, and since it has a great effect of lowering the critical cooling rate in the quenching treatment process in particular, when the amount of Mn is increased, the cooling rate at the time of quenching from the viewpoint of quenching distortion prevention etc. It is possible to slow down, and it can be said that it is an effective element from this viewpoint, but on the other hand, Mn is easily dissolved in cementite, and when the amount is large, cementite is remarkably stabilized and graphitization is increased. If it exceeds 1.5%, such an adverse effect becomes remarkably large and the graphitization during annealing is delayed, making it difficult to obtain a desired ferrite-graphite structure, so the upper limit was made 1.5%. In addition, the lower limit of Mn was set to 0.05%,
This is because if S is low, the fixation of S as an impurity element becomes insufficient and hot brittleness is likely to occur.

Al is required as 0.001% or more in order to improve the cleanliness of steel as a deoxidizing element and to expect the effect of reducing solid-liquid N as AlN that inhibits graphitization, but this effect is 0.100%. If it exceeds, it will be saturated, so 0.001
The range is from ~ 0.100%.

N dissolves in the form of replacing C in cementite and has the effect of remarkably stabilizing it, so that it is difficult to obtain a ferrite-graphite structure during annealing, so 0.00
Must be less than 60%.

From the viewpoint of the effect of P on the transformation characteristics of steel and the segregation, the first reason is that it adversely affects both hardenability and workability, and P dissolves in trace amounts in cementite and It is not preferable for the second reason that it has the effect of stabilizing the formation of the ferrite-graphite structure during annealing because it has a stabilizing effect, and in order to avoid such an adverse effect of P, it must be 0.020% or less. However, it is not necessary to lower the value beyond economical efficiency, and may not be lower than about 0.002%.

Since S easily forms non-metallic inclusions, deteriorates workability, and also has an effect of inhibiting graphitization during annealing, S must be 0.015% or less. But S
For the same reason as for P, it does not have to be lower than about 0.0005%.

Since B is an element having a particularly important role, its action will be described in detail below. That is, one of its functions is that it is an element useful for improving the hardenability as conventionally known, and the other is that the graphitization is very effectively promoted and the obtained graphite particles are extremely effective. It has the function of refining, and the desired ferrite with fine and uniform dispersion of the graphite particles in the ferrite body.
This is useful for easily obtaining the graphite structure.

Needless to say, the former action of B is well known in the art, but the present inventors have uniquely found the latter action, and the mechanism thereof is as follows.

There are two important points in promoting graphitization. First, as mentioned above, cementite (Fe3
C) or M23C6 type iron carbide to increase the degree of anxiety, and secondly to facilitate the formation of graphitized nuclei.

B has an effect on any of these, and the mechanism for destabilizing the iron carbide is as follows.

Since B has a property as an interstitial element like C and N, it forms a solid solution by substituting with C in iron carbide to form Fe (CB) 3 or Fe2
3 (CB) 6 is produced. At this time, since B has a larger atomic radius than C and N, solid strain causes an increase in lattice strain of the iron carbide crystal lattice, which causes heat of the iron carbide. The degree of instability increases, and it becomes easier to convert to graphite by annealing. In addition to the above, B is also a strong nitride-forming element, so N, which has a strong cementite-stabilizing action already mentioned, is fixed as BN and its adverse effect is eliminated, thereby indirectly destabilizing the cementite. The effect of increasing is also added.

Next, regarding the action of forming graphitization nuclei of B, (1) when B reacts with N to form BN, and this BN acts as graphitization nuclei, (2) the Fe23 (CB) 6 itself decomposes. When acting as a nucleus at the same time, there are two ways.

The point to be considered here is that when such a nucleus is used, the final dispersed state of the graphite particles strongly depends on the distribution state of this initial nucleus, so that the final structure has a uniform graphite distribution. In order to obtain a microstructure, BN and Fe23 (C
B) It is desirable to control the distribution state of 6.

The state of microsegregation of B in the γ state is important for this control. For example, B is easily segregated to the old γ grain boundary as is generally known. Therefore, if the γ grain is coarse, the distribution of nuclei is large. This means that the state will be bent at a specific position, and the final state of distribution of graphite particles will also deteriorate. Regarding this, in order to homogenize B, which is easily segregated at the old γ grain boundaries, it is possible to control the hot rolling conditions so as to make the γ grains as fine as possible in the hot rolling process and to promote the recrystallization refinement of the γ grains by rolling. That's what I want.

This final improvement of the graphite grain distribution is also more advantageously achieved by the addition of Ti or La + Ce described below, the mechanism of which will be described later.

If the distribution state of B in the γ state is adjusted in this way, the distribution of the B precipitates as graphitization nuclei is improved, and as a result, these graphitization nuclei act effectively in the subsequent annealing. , A significant increase in graphitization rate, as well as the fineness of the resulting graphite particles,
Uniformity occurs.

In order to expect the above effects, B must be at least 0.0005% or more, and the addition of more than 0.0500% saturates the effects, but rather causes economic disadvantage.

Next, Ti in the second and fourth inventions has a stronger effect of fixing N than B. Therefore, if the amount of N is 0.0060% or less, the adverse effect of N can be almost eliminated by adding Ti. In that case, the amount of BN produced as graphitization nuclei is slightly reduced, but this can be sufficiently compensated by the above-described nucleation action of Fe23 (CB) 6. In addition to that, the graphite distribution state is further improved for the following reason. That is, as described above, since the core B is easily segregated in the old γ grain boundary, it is necessary to make the γ grain in the hot rolling process, which has already been touched, as fine as possible so as to make it uniform. Even if it is desirable to promote the recrystallization refinement of γ grains by rolling by controlling the conditions, even when there is a problem such as a limit to the degree of refinement due to this, or a restriction on hot rolling conditions Since the addition of Ti achieves effective miniaturization for both the desired γ grains during reheating and the recrystallized γ grains during hot rolling, the above problems can be easily resolved.

In order to exert this effect, Ti is required to be 0.005% or more, but even if added in excess of 0.050%, the effect is saturated, so 0.050% was made the upper limit.

Similarly to Ti, La and Ce in the third and fourth inventions also improve the final particle size and distribution of guarphite by improving the distribution of B in the γ state. That is, La and Ce form oxides and sulfides, but B easily causes micro-segregation around such precipitates of La and Ce. This means that the presence of the above-described precipitates of B that easily accumulate increases the microsegregation sites of B other than the γ grain boundaries, and as a result, the B microsegregation becomes uniform as a whole. It will be improved in the direction. Also, La and Ce are S
It also contributes to the improvement of workability due to the effect of fixing.

The amount of La + Ce added is required to be at least 0.005% or more in order to exert the above effect, and the effect is saturated when it exceeds 0.050%, so this is made the upper limit.

Next, through each invention for simultaneously satisfying workability and hardenability as described above, it is limited to have a ferrite-graphite structure, the reason for which will be explained below based on the research results of the present inventors. It is as follows.

Fig. 1 shows C: 0.60%, Si: 1.55%, Mn: 0.80%, Al: 0.021
%, N: 0.0018%, P: 0.008%, S: 0.001%, and B: 0.0025% of the composition of composition, using a small sample collected from an 8mm-thick hot-rolled steel strip, in various structures This is the result of investigating the tensile properties and the Charpy impact properties by changing the graphitization ratio of (1) and adjusting the structure consisting of ferrite and fine ferrite as the main components and the remaining C as spheroidized cementite.

FIG. 2 shows the difference in hardenability when the graphite particle diameters are different. The evaluation of this hardenability is that the graphitization rate is 80% or more, and when the average graphite particle diameter is variously varied, the heating holding time at 860 ° C is variously changed, and 50 ° C / sec after the holding. It shows the average hardness of the cross section when quenching was performed at the cooling rate.

According to the results of Fig. 1 and Fig. 2, (1) Tensile properties and impact properties depend on the graphitization ratio. When the graphitization ratio exceeds 80%, the tensile strength is low, and the elongation and impact properties are low. (2) On the other hand, the hardenability depends on the average particle size of graphite, and in the case of large graphite particles exceeding 10 μm, the heating time required for austenitizing is remarkably long (3) By increasing the graphitization ratio and adjusting the average grain size to less than 10 μm, the fine graphite has a structure in which it is mixed with ferrite, and it has been found that it has the characteristics of simultaneously satisfying workability and hardenability. .

Here, the range of the graphitizing annealing conditions is practically selected from the two viewpoints of obtaining an annealing structure most advantageous for workability under sufficient softening and reducing the annealing cost. For example, in low-temperature annealing such that the annealing temperature range is less than 500 ° C., the progress of softening is remarkably slow, and
If the temperature exceeds ℃, the ratio of austenite phase during annealing increases, and this portion remains as a pearlite phase during annealing, which is a cause of impairing the uniformity of the softened structure, which is not preferable. Therefore, an annealing temperature range of 500 to 750 ° C. is recommended, and an annealing time of about 1 to 200 hours is appropriate. However, a lower annealing temperature requires a longer time.

In terms of material, the optimal range of annealing temperature is 650
℃ in the range of to A ₁ transformation point, good material can be obtained in a short time annealing if specifically chosen temperature just below the A ₁ transformation point.

Moreover, after heating to a temperature to be as the annealing cycle for example once alpha + .gamma.2 phase temperature region, very slow cooling rate A ₁ Toka held in a temperature range below the transformation point, be employed in the methods annealing time reduction and The material can be improved.

Next, in the case where cold or warm rolling is performed prior to the graphitizing annealing, from the viewpoint of hardenability and cold workability, the microstructure comprising ferrite and fine uniform graphite grains is more efficiently used. It becomes a means to obtain. That is, by performing cold or warm rolling on a hot-rolled sheet using an appropriate amount of B, which acts as a graphitization nucleus in the chemical composition, prior to graphitization annealing, the graphitization nucleus effect becomes more remarkable during graphitization annealing. Since the graphitization rate is increased, the final microstructure in which the graphite particles are finely and uniformly distributed can be easily obtained.

In the graphitization process, graphite nucleation occurs first, followed by decomposition of cementite, solid solution of C into the base material, and diffusion of graphite into the graphite particles in this order. Among these, the nucleation process is a very important factor, and the more the nucleation sites are distributed more evenly, the more the final graphite grains become finer and more uniform. Further, if the number of nuclei increases, the diffusion distance of C after decomposition of cementite can be shortened, so that the graphite growth rate also increases. By adding B, the above-mentioned BN or Fe23
Precipitates such as (CB) 6 act as graphitization nuclei, but when cold or warm rolling is added thereto, the dislocation density in the vicinity of these precipitates increases remarkably on a microscopic scale. It will increase its nuclear action.

In addition to that, since a large number of point defects introduced by cold or warm rolling serve as nuclei sites, nucleation is further facilitated.

In addition, cementite is destabilized and easily decomposed by the above-mentioned rolling, and the dislocations introduced by the rolling act as a diffusion path of C dissolved in the base material, so that the growth rate of graphite is also increased. It is.

Further, the increase in dislocation density by cold or warm rolling as described above increases the number of ferrite grain recrystallization nuclei during annealing in the next step, so that the grain refinement of the ferrite phase base material after annealing is achieved. To be done. As a result, it contributes to improvement of toughness and strength-elongation balance. By stacking such composite effects, the effects become more apparent, and extremely effective and good results can be obtained.

In order to exert such an effect, a rolling reduction of 20% or more is required in a warm or cold rolling temperature range of 500 ° C. or less. If the rolling temperature exceeds 500 ° C, the number of dislocations and point defects that effectively act will be reduced due to strain recovery and recrystallization of the ferrite substrate after rolling, and the desired effect will not be fully exhibited. is there. On the other hand, if the rolling reduction is less than 20%, the effects of point defects and dislocations introduced by rolling are too small, so that the effect is hardly obtained.

Example 1 Using a steel whose chemical components are shown in Table 1, hot rolling was performed by a normal method to obtain a hot-rolled steel strip having a thickness of 8 mm, and then, the hot-rolled steel strip was subjected to predetermined annealing. Table 2 shows the results of annealing conditions, tensile properties after annealing, Charby properties, and hardness and toughness after quenching and tempering.

The tensile properties are the results of JIS No. 5 tensile test pieces with a thickness of 8 mm, and the hardness was heated at 850 ° C for 30 minutes, oil-quenched at a cooling rate of 70 ° C / sec, and then tempered at 250 ° C for 60 minutes. It is the result afterwards.

The state of the ferrite / graphite structure, which is the microstructural feature according to the present invention, is shown in FIG. 3 in comparison with a comparative steel for a representative example, and the tensile strength shown in Table 2 is shown in order to clarify the material feature according to the present invention. The relationship between the strength and the hardness after quenching-tempering is shown in FIG. 4, and the relationship between the tensile strength and the elongation is shown in FIG.

It can be seen from FIG. 3 that the invention steel has a structure in which fine graphite particles are additionally dispersed uniformly in the ferrite body. Further, from FIG. 4 and FIG. 5, according to the present invention, the tensile strength is 50 kgf / mm ² or less, and although the high carbon steel has the tensile properties, it has the same low strength and high ductility as the low carbon steel. Quenching-tempering hardness is comparable to that of
It can be seen that it has the same hardenability as the spheroidized cementite structure steel.

Furthermore, Table 2 shows that the invention steels have significantly better impact properties than the comparative spheroidized cementite structure steels, and this effect is particularly remarkable when compared with steels to which B alone is added.
It can be seen that it is more remarkable when Ce is added in combination.

Example 2 Hot-rolled steel strips were hot-rolled by a normal method using steels having the chemical compositions shown in Table 1, and subsequently the hot-rolled steel strips were pickled and subjected to various reduction temperatures and reductions. Rolling was performed at a constant rate or cold rolling, and then predetermined annealing was performed. Table 3 shows the annealing conditions and reduction temperature, reduction rate and performance of tensile test pieces after annealing for these, and hardness after heating at 850 ° C for 30 minutes, oil quenching at a cooling rate of 70 ° C / sec, and then at 250 ° C. 60min
The results after the tempering treatment of No. 2 are shown.

From Table 3, the invention steel has a tensile strength of 50 kgf / mm2 or less, and although it is a high carbon steel, the tensile properties show the same low strength and high ductility as low carbon steel, and the hardness after quenching-tempering is comparative. It can be seen that the steel has the same hardenability as that of the ferrite / spheroidized cementite structure steel of steel. In addition, the steel of the present invention is remarkably superior in impact properties to the comparative spheroidized cementite structure steel, and this effect is particularly remarkable when B and Ti or La + Ce are added in combination as compared with the steel containing B alone. It turns out that it is more remarkable.

(Effects of the Invention) According to the present invention, the high carbon steel for heat treatment, which was conventionally poor in workability, is as soft as low-pitched steel and has the mechanical properties as good workability, and is as good as conventional high carbon steel. A steel having both hardenability can be obtained. If the steel of the present invention is used for various machine parts such as blades, springs, wear-resistant parts, etc., the formability before heat treatment is remarkably improved, so that the processing steps can be simplified,
The molding shape can be complicated, and a great effect can be obtained in terms of process saving, labor saving, and cost saving.

[Brief description of drawings]

FIG. 1 is a graph showing the effect of the graphite ratio on the mechanical properties and hardenability, FIG. 2 is a graph showing the effect of the size of graphite particles on the hardenability, and FIG. 3 is an invention steel and a comparative steel. Fig. 4 is a micrograph comparing the microstructures of Fig. 4, Fig. 4 is a graph showing a comparison of tensile strength and hardness after quenching-tempering, and Fig. 5 is a relationship between tensile strength and elongation of the invention steel and the comparative steel. It is a graph which shows.

Claims (4)

(57) [Claims] 1. C: 0.30 to 1.20 wt% Si: 0.30 to 2.00 wt% Mn: 0.05 to 1.50 wt% Al: 0.001 to 0.100 wt% N: 0.0060 wt% or less P: 0.020 wt% or less S: 0.015 wt% The following and B: 0.0005 to 0.0500 wt% and the composition of the balance Fe and unavoidable impurities, having a structure mainly composed of ferrite phase and fine graphite particles with a diameter of 10 μm or less, tensile strength 50 kgf / mm ² or less A steel sheet for heat treatment, which is excellent in workability and hardenability and also has excellent toughness after quenching-tempering. 2. C: 0.30 to 1.20 wt% Si: 0.30 to 2.00 wt% Mn: 0.05 to 1.50 wt% Al: 0.001 to 0.100 wt% N: 0.0060 wt% or less P: 0.020 wt% or less S: 0.015 wt% Below, B: 0.0005 to 0.0500 wt% and Ti: 0.005 to 0.050 wt% are contained, the balance is Fe and the composition of unavoidable impurities, and it has a structure mainly composed of ferrite phase and fine graphite particles with a diameter of 10 μm or less. A steel sheet for heat treatment, having a strength of 50 kgf / mm ² or less, excellent workability and hardenability, and excellent toughness after quenching-tempering. 3. C: 0.30 to 1.20 wt% Si: 0.30 to 2.00 wt% Mn: 0.05 to 1.50 wt% Al: 0.001 to 0.100 wt% N: 0.0060 wt% or less P: 0.020 wt% or less S: 0.015 wt% or less B: 0.0005 to 0.0500 wt% and La: + Ce: 0.002 to 0.050 wt% and the balance is Fe and inevitable impurities, and the structure is mainly composed of ferrite phase and fine graphite particles with a diameter of 10 μm or less. And a tensile strength of 50 kgf / mm ² or less, excellent workability and hardenability, and excellent toughness after quenching-tempering treatment. 4. C: 0.30 to 1.20 wt% Si: 0.30 to 2.00 wt% Mn: 0.05 to 1.50 wt% Al: 0.001 to 0.100 wt% N: 0.0060 wt% or less P: 0.020 wt% or less S: 0.015 wt% And B: 0.0005 to 0.0500 wt% Ti: 0.005 to 0.050 wt% and La + Ce: 0.002 to 0.050 wt% and the balance is Fe and unavoidable impurities. A steel sheet for heat treatment, which has a structure of, a tensile strength of 50 kgf / mm ² or less, excellent workability and hardenability, and excellent toughness after quenching-tempering.