JP2000178683A - Free-cutting non-heat treated steel excellent in toughness - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a free-cutting non-heat treated steel having machinability equal to or above that of Pb-added non-heat treated steel without the addition of Pb and having toughness equal to or above a certain level. SOLUTION: This steel has a compsn. contg., by mass, 0.30 to 0.55% C, 0.15 to 0.40% Si, 0.80 to 1.60% Mn, 0.005 to 0.050% P, <=0.040% S, 0.005 to 0.035% Al, 0.04 to 0.15% V, Cu+Ni+Cr+Mo: <=0.45%, 0.0050 to 0.0l50% B, 0.008 to 0.018% N, and the balance Fe with inevitable impurities, in which, also, B/N lies in the range of 0.5 to 1.5. To the above steel, 0.005 to 0.20% Ti is added, the content of N is controlled to 0.010 to 0.020%, B/(N-Ti/3.42): 0.5 to 1.5 is satisfied, and also, Cu+Ni+Cr+Mo is relaxed to <=0.60%. The content of S in the steel is increased to 0.04 to 0.08%, and Ca is added by 0.001 to 0.003%.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a non-heat treated steel used as a material for a crankshaft of an automobile part, various underbody parts, and the like. It relates to a free-cutting non-heat treated steel having machinability equal to or higher than that and a certain level of toughness.

[0002]

2. Description of the Related Art In recent years, the demand for non-heat treated steel has been increasing because it is possible to omit the heat treatment of quenching and tempering, and it has been used as a material for automobile parts such as crankshafts and various underbody parts. . At present, non-heat-treated steel to which Pb is added as a free-cutting element is most often produced and used in order to achieve free-cutting. However, there is a movement to suppress the use of Pb due to global environmental problems. To cope with this, Japanese Patent Application Laid-Open No. 9-25 / 1997
No. 539 discloses a non-Pb-free free-cutting non-heat treated steel (referred to as Prior Art 1). However, in the prior art 1, since the special element Nd is used,
It is disadvantageous in terms of cost reduction, and there is no description of the chip disposability, which is the greatest feature of Pb-added steel. Also, JP-A-1-219148 discloses that
A non-heat treated steel using BN as a free-cutting inclusion other than Pb is disclosed (referred to as Prior Art 2). However, as in Prior Art 1, there is no description of the chip disposability, which is the greatest feature of the Pb-added steel, and no description of the toughness indispensable to a certain level as the steel material.

[0003]

All of the prior arts described above are useful in terms of environmental measures in that they are component-based free-cutting steels to which Pb is not added. There is a problem in the evaluation of the chip disposability, which is an extremely important item in the property evaluation. Further, the prior art 1 has a problem in that the manufacturing cost is high, and the prior art 2 has a problem in securing toughness.

[0004] Therefore, in order to solve the above problems, the present invention presupposes the production of a free-cutting steel to which Pb is not added, and does not use a special and expensive additive element as a chemical component of the free-cutting steel. As a conventional free-cutting steel, within the range of commonly used component elements, it has excellent machinability including chip controllability, and is used for materials such as crankshafts for automobile parts and various underbody parts. It is an object of the present invention to develop a high-quality free-cutting non-heat-treated steel which maintains the toughness of a steel material characteristic to be provided as a free-cutting steel, particularly at a predetermined level.

[0005]

Means for Solving the Problems The inventors of the present invention have conducted intensive studies from the above-mentioned viewpoints, and as a result, by appropriately determining the chemical composition of steel, it has been found that the machinability can be ensured by the formation of BN precipitates and the formation of BN precipitates. The distribution is controlled and the toughness is ensured by Cu.
By limiting the total content of + Ni + Cr + Mo, or by further controlling the distribution of TiN precipitates and controlling this distribution, it is possible to produce a high-quality free-cutting non-heat treated steel capable of achieving the above object. it can. Furthermore, it has been found that a high-quality free-cutting non-heat treated steel with further improved machinability can be produced by the generation of Ca + S-based inclusions and their proper distribution.

That is, as a free-cutting non-heat treated steel, a steel having a basic component composition that satisfies predetermined mechanical properties, (a) a Ti-free steel, a B content and a N content, and
The content ratio of B / N is limited to an appropriate range, and Cu
+ Ni + Cr + Mo is designed to have an upper limit on the total content, and (ii) B,
Limiting each of the Ti and N content, and the content ratio of B / (effective N minus the N consumed for TiN formation with respect to BN precipitate formation) to within appropriate ranges; and , Cu + Ni + Cr + Mo in the above (a)
By designing a component composition with an upper limit that is less restrictive than above, it is possible to obtain a free-cutting non-heat-treated steel that secures toughness and has excellent machinability including chip disposability, as intended. I got the knowledge. Then, the machinability is further improved by an appropriate Ca + S-based inclusion distribution.

The present invention has been made based on the above findings, and the gist is as follows. The non-heat-treated free-cutting steel having excellent toughness according to claim 1 is: C: 0.30 to 0.55 mass%, Si: 0.15 to 0.40 mass%, Mn: 0.80 to 1.60 mass%, P: 0.005 to 0.050 mass%, S: 0.040 mass% or less, Al: 0.005 to 0.035 mass%, V: 0.04 to 0.15 mass%, Cu + Ni + Cr + Mo: 0.45 mass% or less, B : 0.0050 to 0.0150 mass%, and N: 0.008 to 0.018 mass%, the balance being Fe and inevitable impurities, and B / N: 0.5 to 1.5. It is characterized by being within the range.

[0008] The free-cutting non-heat treated steel having excellent toughness according to claim 2 is composed of: C: 0.30 to 0.55 mass%, Si: 0.15 to 0.40 mass%, Mn: 0.80 to 1. 60 mass%, P: 0.005 to 0.050 mass%, S: 0.040 mass% or less, Al: 0.005 to 0.035 mass% V: 0.04 to 0.15 mass%, Cu + Ni + Cr + Mo: 0.60 mass% or less , B: 0.0050 to 0.0150 mass%, Ti: 0.005 to 0.020 mass%, and N: 0.010 to 0.020 mass%, the balance being Fe and inevitable impurities, In addition, B / (N-Ti / 3.42) is characterized by being in the range of 0.5 to 1.5.

The non-heat-treated free-cutting steel having excellent toughness according to claim 2 is the invention according to claim 1 or 2, wherein S: 0.040 to 0.08 mass%, and Ca: 0.001. ０．００0.003 mass%.

[0010]

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a clamp for an automobile part.
To use as a material for shafts and various underbody parts
It is a suitable free-cutting non-heat treated steel.
Tensile strength: 800MPa or more, Aperture: 30% or more, and
Charpy impact test value: 30 J / cm ^TwoHave more than
It has excellent labor characteristics and machinability. Free cutting of this invention
Reasons for limiting the chemical composition of non-heat treated steel as described above
explain.

(1) C: 0.30 to 0.55 mass% C is an important element that has a great effect on the strength and machinability of steel, and when the C content is less than 0.30 mass%, sufficient strength is obtained. Can not be obtained. On the other hand, if the C content exceeds 0.55 mass%, the pearlite amount becomes too large, and the machinability deteriorates. Therefore, the C content is 0.30 to 0.55 mass
%.

(2) Si: 0.15 to 0.40 mass% Si is an element necessary for deoxidizing steel, and if the Si content is less than 0.15 mass%, a sufficient deoxidizing effect cannot be obtained.
On the other hand, if the Si content exceeds 0.40 mass%, the ferrite is hardened and the machinability deteriorates. Therefore, the Si content is limited to the range of 0.15 to 0.40 mass%.

(3) Mn: 0.80 to 1.60 mass% Mn is an element having a great effect on the strength and toughness of steel. When the Mn content is less than 0.80 mass%, sufficient strength can be obtained. Absent. On the other hand, if the Mn content exceeds 1.60 mass%, sufficient toughness cannot be obtained, and the machinability also decreases. Therefore, the Mn content is 0.80 to 1.60 mass%.
Within the range.

(4) P: 0.005 to 0.050 mass% P is an element that is unavoidably mixed into steel and contained. If the P content exceeds 0.050 mass%, sufficient toughness is obtained. Can not be obtained. However, if the P content is too low, the machinability tends to deteriorate. In addition, the cost of removing P increases as the P content decreases. Therefore, the P content is 0.005 to
It is limited to the range of 0.050 mass%.

(5) S: 0.040 mass% or less, or
S: 0.04 to 0.08 mass%, and Ca: 0.001
０．００0.003 mass% S is an element inevitably mixed in steel and contained. If the content exceeds 0.040 mass%, sufficient toughness cannot be obtained. Therefore, the S content is 0.040 mass
%.

On the other hand, when S is added in combination with Ca, Ca +
S-based inclusions are formed and exert an effect of improving the machinability of steel by an appropriate distribution. The effect of reducing the toughness of the Ca + S-based inclusions is not so large. In order to improve the machinability due to the generation of Ca + S-based inclusions, S is set to 0.04 mass%.
If it is less than 0.001% by mass, a sufficient effect cannot be exhibited. On the other hand, when S exceeds 0.08 mass% and Ca exceeds 0.003 mass%, the effect of improving the machinability is saturated, and the production cost is disadvantageously increased. Therefore,
When the machinability is further improved, S is set to 0.04 to 0.5.
Within a range of 08 mass% and 0.001 to 0.0
It is desirable to add a composite so as to be within the range of 03 mass%.

(6) Al: 0.005 to 0.035 mass
% Al is an element necessary for the deoxidation of steel. The effect of deoxidizing Al is insufficient if the content is less than 0.005 mass%, while the effect is insufficient even if it exceeds 0.035 mass%. Saturates. Therefore, the Al content is 0.005 to 0.0
Limited to the range of 35 mass%.

(7) V: 0.04 to 0.15 mass% V is a precipitation strengthening element, but the V content is 0.04 mass%.
%, Sufficient precipitation hardening cannot be obtained. On the other hand, if the content exceeds 0.15 mass%, the precipitates of V become coarse and the toughness decreases. Therefore, the V content is 0.0
Limit within the range of 4 to 0.15 mass%.

(8) Cu + Ni + Cr + Mo: 0.45
mass% or less (in the case of Ti-free steel) Cu + Ni + Cr + Mo: 0.60 mass% or less (in the case of Ti-added steel) Cu, Ni, Cr and Mo are inevitably mixed from the raw material scrap in the process of melting steel. In particular, since the scrap ratio in the main raw material charged in the electric furnace for steelmaking is high, it is necessary to limit the scrap quality in order to limit the amount of these tramp elements mixed. As described above, it is important to determine the allowable upper limit value of the total content of the above-mentioned playing cards from the viewpoint of reducing the manufacturing cost. On the other hand, according to tests by the present inventors, when the total content of these trump elements exceeds 0.45 mass%, the toughness is significantly deteriorated. Also, the machinability tends to deteriorate. Therefore, the content of Cu + Ni + Cr + Mo is 0.1%.
Limited to 45 mass% or less.

However, in the steel to which a predetermined amount of Ti is added as described later, the effect of improving the toughness is exhibited by the TiN precipitates, so that the content of Cu + Ni + Cr + Mo is set to 0.6.
The restriction can be relaxed to 0 mass% or less. Further, when the above restrictions are not relaxed, a free-cutting non-heat treated steel having more excellent toughness can be obtained.

(9) B: 0.0050 to 0.0150
mass% B is an element that plays an important role in improving the machinability in the present invention. That is, B is B that improves machinability.
It is an element required together with the presence of N to form N precipitates. However, if the B content is less than 0.0050 mass%, a sufficient amount of BN cannot be generated. On the other hand, if the B content exceeds 0.0150 mass%, the toughness decreases. Therefore, the B content is 0.0050 to 0.01.
It must be within the range of 50 mass%.

Here, in order for B having the above content to be precipitated as BN in an amount sufficient to exhibit the effect of improving the machinability, a predetermined amount of N which can be combined with B in accordance with the B content is determined. It is necessary that a rate exists. The N content required for this will be described in the section on the reason for limiting the N content and the section on the reason for limiting the B / N ratio.

(10) Ti: 0.005 to 0.020 ma
In the present invention, ss% Ti is the above-mentioned Cu + Ni + Cr
It is an element that plays an important role in improving the toughness by limiting the total content of + Mo to 0.45 mass% or less, and further improving the toughness.
That is, Ti is an element required together with N in order to form a TiN precipitate for improving toughness. But,
If the Ti content is less than 0.005 mass%, a sufficient amount of T
Since iN cannot be generated, the improvement in toughness is insufficient. On the other hand, if the Ti content exceeds 0.020 mass%, coarse TiN increases, and this becomes the starting point of fatigue fracture, so that the fatigue life tends to decrease and the machinability tends to deteriorate. Therefore, the Ti content is 0.005 to
It must be within the range of 0.020 mass%.

Here, in order for Ti of this content to be precipitated as TiN in an amount sufficient to exhibit the effect of improving toughness, a predetermined amount of N that can be combined with Ti in accordance with the Ti content is determined. Needs to be present. N required for this
Regarding the content rate, the section on the reason for limiting the N content rate and B /
The reason for limiting the (N-Ti / 3.42) ratio will be described.

(11) N: 0.008 to 0.018 mass
% Or less (in the case of Ti-free steel) N: 0.010 to 0.020 mass% or less (in the case of Ti-added steel) N combines with B to generate BN precipitates, thereby improving machinability, In a Ti-added steel, it is an important element that generates TiN precipitates and improves toughness.

However, in the case of (a) Ti-free steel, if the N content is less than 0.008 mass%, the amount of each of the above-mentioned precipitates is not sufficient, so that each effect is not sufficiently exhibited. On the other hand, when the N content exceeds 0.018 mass%,
Degrades toughness and fatigue properties. Therefore, the N content is limited to the range of 0.008 to 0.018 mass%.

On the other hand, in the case of (b) Ti-added steel, if the N content is less than 0.010 mass%, the amount of each of the above-mentioned precipitates is not sufficient, so that each effect is not sufficiently exhibited. On the other hand, if the N content exceeds 0.020 mass%, the toughness and fatigue properties deteriorate. Therefore, the N content is 0.1.
It is limited to the range of 010 to 0.020 mass%.

(12) B / N: 0.5 to 1.5 (in the case of Ti-free steel) B / (N-Ti / 3.42): 0.5 to 1.5 (in the case of Ti-added steel) In the case of Ti-free steel, B / N, which is the ratio of the B content to the N content, is a major factor that determines the amount of BN precipitates that are effective for improving machinability. On the other hand, in the case of Ti-added steel, N in the steel is Ti having a high affinity for N.
Consumed in binding with. Therefore, the effective content for producing BN is the effective N content obtained by subtracting the stoichiometric value of the N content consumed for producing TiN from the total N content, N-Ti /
{(Atomic weight of Ti) / (atomic weight of N)}, that is, NT
i / 3.42. Therefore, in the case of Ti-added steel, the ratio of the B content to the effective N content is B / (N−
Ti / 3.42) is a major factor that determines the amount of BN precipitates generated that is effective for improving machinability. When B / N or B / (N-Ti / 3.42) is less than 0.5, the amount of BN generated is not sufficiently ensured, so that the machinability is not sufficiently improved. In this case, hot workability deteriorates. Therefore, the contents of B, N, and Ti are within the ranges of the contents described above, and moreover, B / N or B / N
(N-Ti / 3.42) is limited to fall within the range of 0.5 to 1.5.

[0029]

Next, the present invention will be described in more detail with reference to examples. Inventive Examples Nos. 1 to 13 having a chemical composition within the scope of the present invention shown in Table 1, and Comparative Examples Nos. 14 to 27 and Reference Examples No. 28 having a chemical composition outside the scope of the present invention. ~ 29 were melted in a vacuum melting furnace and cast into steel ingots, and then each was hot-rolled into steel bars having a diameter of 80 mmφ. Furthermore, Examples Nos. 1 to 13 of the present invention and Comparative Examples Nos. 14 to 2
No. 7 and Reference Example No. 28 were heated to 1200 ° C., hot forged to a diameter of 70 mmφ, and air-cooled to room temperature. The steel equivalent to JIS S50C of Reference Example No. 29 was quenched and tempered after hot forging under the same conditions as described above.

[0030]

[Table 1]

The following tests were performed on the steel bars manufactured from the inventive examples, comparative examples and reference examples as described above. Tensile test: A JIS No. 4 test piece was collected and subjected to a tensile test.

Fatigue test: An Ono-type rotary bending test was performed. Cutting test: • Under the conditions shown in Table 2, the outer peripheral turning test was performed using two types of carbide tools (material P20) and high-speed tools (material SKH4), and the drilling test was performed using a high-speed drill (material SK).
H4).

Regarding the evaluation of the machinability, in the outer peripheral test using a carbide tool, the tool life was evaluated by the cutting time at which the lateral flank wear amount VB was 0.2 mm. The tool life was evaluated based on the time until it became impossible.

In the drill test, the total drilling depth is 100
The cutting speed at which cutting was impossible at 0 mm was determined and used as an index of tool life (drill life speed).

[0035]

[Table 2]

[0036]

[Table 3]

Table 3 shows the test results. As target values of the respective characteristic values, based on Pb-based non-heat-treated free-cutting steel of Reference Example No. 28, a tensile strength of 800 MPa or more, and a fatigue limit of 4
At least 00 MPa, a Charpy impact value of at least 30 J / cm ² , and a drill life speed of at least 55 m / min.

In each of the inventive examples Nos. 1 to 13, the test values satisfy the above-mentioned target values and have good characteristics.
On the other hand, Comparative Examples Nos. 14 to 27 and Reference Examples 28 to 27
In all cases, at least one of the test values does not satisfy the target value.

In Comparative Example No. 14, the tensile strength was lower than that of the inventive example because the C content was below the lower limit of the present invention. In Comparative Example No. 15, since the C content was equal to or higher than the upper limit of the present invention, the drawing and impact values were low, and the machinability was poor.

In Comparative Example No. 16, since the Si content was equal to or higher than the upper limit of the present invention, the drawing and impact values were low, and the machinability was poor. In Comparative Example No. 17, since the Mn content was equal to or less than the lower limit of the present invention, the tensile strength was lower than that of the present invention. In Comparative Example No. 18, since the Mn content was equal to or higher than the upper limit of the present invention, the drawing and impact values were low, and the machinability was poor.

In Comparative Example No. 19, Cu + Ni + Cr +
Since the Mo content is equal to or higher than the upper limit of the present invention, the impact value is low and the machinability is poor. In particular, Example No. 1 of the present invention and Example N of the present invention
Compared with o.3, the impact value is significantly reduced.

In Comparative Example No. 20, since the V content was below the lower limit of the present invention, the tensile strength and impact value were lower than those of the present invention. In Comparative Example No. 21, since the V content was equal to or higher than the upper limit of the present invention, the drawing and impact values were low, and the machinability was poor.

In Comparative Example No. 22, since the B content was less than the lower limit of the present invention, the machinability was poor. Comparative Example No. 2
In No. 3, since the B content was not less than the upper limit of the present invention, the impact value was low.

In Comparative Example No. 24, since the Ti content is equal to or higher than the upper limit of the present invention, the fatigue characteristics are deteriorated and the machinability is also poor. In Comparative Example No. 25, the impact value was lowered because the S content was equal to or higher than the upper limit of the present invention.

In Comparative Example No. 26, since the N content was less than the lower limit of the present invention, the machinability was poor. Comparative Example No. 2
In No. 7, since the N content was equal to or higher than the upper limit of the present invention, the impact value was reduced, and the fatigue characteristics were also poor.

The relationship between the drill life speed and the Charpy impact value was plotted in FIG. 1 based on the data obtained in the above test. In all of the examples of the present invention, both the drill life speed and the Charpy impact value satisfy the target values.

[0047]

As described above, according to the present invention,
Non-tempered steel used as a material for crankshafts of automobile parts and various underbody parts has a machinability equivalent to that of Pb-added non-heat-treated steel and a certain level of toughness without the addition of Pb. Since it is possible to obtain non-heat treated steel, it contributes to maintaining the global environment. In addition, it is a component element that has been used widely as a free-cutting steel in the past, without using special and expensive additional elements, and has excellent machinability including chip disposability, A high-quality free-cut non-heat treated steel having a toughness maintained at a desired predetermined level can be obtained. Providing such a free-cutting non-heat treated steel excellent in toughness brings about an industrially useful effect.

[Brief description of the drawings]

FIG. 1 is a graph showing a relationship between a drill life speed and a Charpy impact value obtained from test data of an example of the present invention and a comparative example.

Claims (3)

[Claims] 1. C: 0.30 to 0.55 mass%, Si: 0.15 to 0.40 mass%, Mn: 0.80 to 1.60 mass%, P: 0.005 to 0.050 mass%, S : 0.040 mass% or less, Al: 0.005 to 0.035 mass%, V: 0.04 to 0.15 mass%, Cu + Ni + Cr + Mo: 0.45 mass% or less, B: 0.0050 to 0.0150 mass%, and N: 0.008 to 0.018 mass%, the balance being Fe and unavoidable impurities, and B / N: within the range of 0.5 to 1.5. Tempered steel. 2. C: 0.30 to 0.55 mass%, Si: 0.15 to 0.40 mass%, Mn: 0.80 to 1.60 mass%, P: 0.005 to 0.050 mass%, S : 0.040 mass% or less, Al: 0.005 to 0.035 mass% V: 0.04 to 0.15 mass%, Cu + Ni + Cr + Mo: 0.60 mass% or less, B: 0.0050 to 0.0150 mass%, Ti: 0 0.005 to 0.020 mass%, and N: 0.010 to 0.020 mass%, the balance being Fe and inevitable impurities, and B / (N-Ti / 3.42): 0. A free-cutting non-heat treated steel, which is in the range of 0.5 to 1.5. 3. The free-cutting machine according to claim 1, wherein S: 0.04 to 0.08 mass% and Ca: 0.001 to 0.003 mass%. Non-heat treated steel.