JP4600988B2 - High carbon steel plate with excellent machinability - Google Patents

Description

  The present invention relates to a high carbon steel plate excellent in machinability suitable as a material for machine parts, automobile parts and the like that are scheduled to be cut into a predetermined shape.

High carbon steel sheets are used in applications such as machine parts and automobile parts, but they are often formed into product shapes by cutting, so that excellent machinability is required. Conventionally, precipitation of graphite and addition of free-cutting elements such as Pb, Ca, Te, Bi, etc. have been adopted for improving the machinability of high carbon steel.
For example, after C: 0.30 to 0.60 mass% of carbon steel for mechanical structure or alloy steel for mechanical structure is heated to a temperature range of Ac ₁ to Ac ₃ , it is air-cooled at a cooling rate of 3 ° C./second or less. It is known that good machinability can be obtained (Patent Document 1). Ferrite + pearlite structure has been introduced as a metal structure effective for improving machinability, but details of ferrite + pearlite structure are unknown.
JP2003-306719

If the influence of the ferrite + pearlite structure on the machinability is clarified, not only can the machinability be improved, but also the addition of free-cutting elements that tend to adversely affect the environment can be omitted. Heat treatment necessary for graphite precipitation can also be omitted. Therefore, the present inventors investigated and examined various relationships between the ferrite + pearlite structure and machinability. As a result, it was clarified that the dispersion form of pearlite dispersed and precipitated in the mixed structure of ferrite + spherical cementite or ferrite + rod-like cementite has a great influence on machinability improvement.
The present invention has been completed on the basis of such elucidation, and by regulating the area ratio of pearlite dispersed and precipitated in the mixed structure and the lamellar spacing, it does not depend on the precipitation of graphite or the addition of free-cutting elements. It aims at providing the high carbon steel plate excellent in machinability.

The high carbon steel sheet of the present invention has C: 0.6 to 1.5 mass%, Si: 1.0 mass% or less, Mn: 0.1 to 2.0 mass%, S: 0.02 mass% or less, P: 0.03 mass% or less, Al: 0.005 to 0.20 mass%, Fe: Substantially remaining composition. If necessary, Ni: 2.5% by mass or less, Cr: 2.0% by mass or less, Mo: 1.0% by mass or less, and / or Ti: 0.07% by mass, Nb: One or two or more of 0.07% by mass or less, V: 0.4% by mass or less, and B: 0.07% by mass or less may be included.
The metal structure of the high carbon steel sheet is a mixed structure in which ferrite + spherical cementite or ferrite + rod-like cementite and pearlite having an area ratio of 40 to 80% and a lamellar spacing of 0.4 μm or more are dispersed.

Effects and embodiments of the invention

  The present inventors investigated and examined the influence of the metal structure on the machinability by variously changing the metal structure after annealing of the high carbon steel sheet. In the process of investigation and examination, in a state where a steel material having a predetermined composition and composition is annealed after hot rolling or cold rolling, pearlite area ratio: 40 to 80%, lamella spacing: 0.4 μm or more of pearlite and ferrite + spherical shape Alternatively, it has been found that excellent machinability is exhibited by adjusting to a mixed structure in which rod-like cementite is dispersed and precipitated.

The reason why the mixed structure affects the machinability improvement is considered as follows.
When a large shear deformation is applied to the primary shear area during cutting, stress concentrates at the pearlite / ferrite interface due to the difference in plastic deformability between pearlite and ferrite, and cracks occur. When the cutting further progresses and a large shearing process is applied, the crack propagates with pearlite which is inferior in plastic deformability compared to ferrite. As a result of continuous or simultaneous occurrence of crack propagation in the primary shear area, chips are easily cut, effectively reducing cutting resistance and improving tool wear and improving machinability.

In the hot-rolled structure of high carbon steel, it is generally difficult to obtain a mixed structure of pearlite and soft ferrite having an area ratio of 40 to 80% that is effective for improving machinability. When spheroidizing the cementite in the part, the precipitation area of the spherical cementite exhibits the same action as that of the soft ferrite, so that machinability can be improved. Even when cementite precipitates in a rod shape, the machinability is improved by the same mechanism because of the presence of ferrite in the periphery.
Even when the hot-rolled steel sheet is annealed at a temperature of A ₁ or less, it is possible to obtain a ferrite and pearlite mixed structure with a pearlite area ratio of 40 to 80%, but good cutting due to the influence of the pearlite lamella spacing. Sex cannot be obtained. In this case, by combining the heating and holding at a temperature higher than the point A ₁ and the control of the cooling rate, the machinability can be improved by setting the lamellar interval to 0.4 μm or more.

Hereinafter, the alloy component, content, metal structure and the like included in the high carbon steel targeted by the present invention will be described.
[Ingredients / Composition]
C: 0.6-1.5% by mass
In applications where high carbon steel sheets are cut into various parts and then heat treated, they are essential components for obtaining good machinability and required strength. C: When the content is 0.6% by mass or more, the influence of soft ferrite is suppressed, and machinability can be improved. However, if the C content exceeds 1.5% by mass, the adverse effect of hard cementite on tool wear becomes strong, and the machinability deteriorates. Preferably, the C content is selected in the range of 0.6 to 1.2% by mass.

Si: 1.0 mass% or less Although it is a component added as a deoxidizer in the steelmaking stage, it destabilizes cementite and exhibits a graphitization promoting action. If an excessive amount of Si is contained, the heat treatment property is hindered by the precipitation of graphite, so the upper limit was made 1.0 mass% (preferably 0.4 mass%).
Mn: 0.1 to 2.0% by mass
It is a necessary component for obtaining good hardenability, and MnS produced by reaction with S in steel contributes to improvement of machinability. The hardenability is improved by the addition of 0.1% by mass or more of Mn, but the excessive addition exceeding 2.0% by mass tends to deteriorate the toughness after the heat treatment. Preferably, the Mn content is selected in the range of 0.3 to 1.0 mass%.

S: 0.02% by mass or less Although it is a component that improves the machinability by becoming an MnS-based inclusion, if an excessive amount of S is contained, the strength and toughness after heat treatment deteriorate, so that it can be reduced as much as possible. preferable. In this component system, the adverse effect on strength and toughness is suppressed by setting the upper limit of the S content to 0.02 mass% (preferably 0.010 mass%).
-P: 0.03 mass% or less Although exhibiting the effect of improving the machinability to some extent, it is preferable to reduce it as much as possible because it is a component that deteriorates toughness after heat treatment. In this component system, the upper limit of the P content is limited. It was 0.03 mass% (preferably 0.015 mass%).

-Al: 0.005-0.20 mass%
It is a component added as a deoxidizer in the steelmaking stage. The deoxidation effect is seen at Al: 0.005% by mass or more, and a good deoxidation effect can be obtained according to the increase, but excessive addition exceeding 0.20% by mass decreases the cleanliness of the steel material, and the strength It also causes toughness deterioration. Preferably, the Al content is selected in the range of 0.010 to 0.100 mass%.
Ni: 2.5% by mass or less Ni is a component that delays pearlite transformation, and has the effect of increasing the pearlite lamella spacing by slowing the growth rate of pearlite. The addition effect of Ni is observed at 1.0% by mass or more, but excessive addition adversely affects the machinability. Moreover, it is a graphitization promoting element like Si, and hardenability is deteriorated due to precipitation of graphite. Therefore, when adding Ni, the upper limit is set to 2.5% by mass (preferably 1.8% by mass).

-Cr: 2.0 mass% or less It is a component which improves hardenability, and the effect of Cr addition is seen at 0.1 mass% or more. It also has the effect of stabilizing cementite and improving the strength of cementite by dissolving in cementite. However, excessive addition not only increases the cost of the steel material, but also promotes tool wear and deteriorates machinability due to the strengthening action of cementite. Therefore, when adding Cr, the upper limit is set to 2.0 mass% (preferably 1.0 mass%).
-Mo: 1.0 mass% or less It is a component which improves hardenability, and the effect of Mo addition is seen at 0.1 mass% or more. However, excessive addition not only increases the cost of the steel material, but also deteriorates the workability of the steel material due to hardening. Therefore, when adding Mo, an upper limit is made into 1.0 mass% (preferably 0.4 mass%).

Ti: 0.07% by mass or less Ti is a component added as necessary, and austenite grains are refined by the pinning action of precipitates formed by reaction with C, S, N in steel, and toughness after heat treatment To improve. It also has the effect of increasing the strength of the steel material by precipitation strengthening. Such an effect becomes significant when Ti is added in an amount of 0.02% by mass or more. However, excessive addition causes deterioration of hardenability and an increase in steel material cost, so the upper limit is set to 0.07% by mass (preferably 0.05. Mass%).
Nb: 0.07% by mass or less Nb is a component added as necessary, reacts with C in steel in the same way as Ti, refines austenite grains by the pinning action of the reaction product NbC, and toughness after heat treatment It is a precipitation hardening element that improves the strength of steel materials. Such an effect becomes remarkable when Nb is added in an amount of 0.02% by mass or more. However, excessive addition causes deterioration of hardenability and an increase in steel material cost, so the upper limit is set to 0.07% by mass (preferably 0.05. Mass%).

V: 0.4% by mass or less V is a component added as necessary, and precipitates as a carbonitride having a pinning action, contributing to refinement of austenite grains and consequently toughness after heat treatment. The effect of addition of V becomes remarkable at 0.1% by mass or more, but excessive addition causes deterioration of hardenability and an increase in steel material cost, so the upper limit is 0.4% by mass (preferably 0.3% by mass). And
-B: 0.007 mass% or less It is a component added as needed and improves hardenability. The hardenability is observed when 0.0003 mass% or more of B is added, and the hardenability improves as the amount of B increases. However, the effect of improving hardenability is saturated at 0.0008 mass%. Further, since excessive addition adversely affects hot workability, when B is added, the upper limit is made 0.007% by mass (preferably 0.005% by mass).

[Metal structure]
The high carbon steel sheet of the present invention has a mixed structure in which pearlite and ferrite + spherical cementite or ferrite + rod-like cementite having an area ratio of 40 to 80% and a lamellar spacing of 0.4 μm or more are dispersed and precipitated. As used herein, “mixed structure” refers to (1) a pearlite structure (area ratio: 40 to 80%, lamellar spacing: 0.4 μm or more), and (2) a structure in which spherical or rod-like cementite is precipitated in ferrite. Is a distributed organization.

-Perlite area ratio: 40-80%
The pearlite functions as a stress concentration source that promotes the generation of cracks in the primary shear area. As the pearlite area ratio increases, the influence of soft ferrite with a large spreadability is suppressed, and cutting resistance is reduced. Reduction of cutting resistance means a reduction in the amount of heat generated during cutting, and suppresses tool wear due to thermal wear and prolongs the tool life. Such an effect becomes remarkable when the area ratio of the precipitated pearlite is 40% or more.
On the other hand, when the area ratio of pearlite is less than 40%, the growth of crystal grains of the constituent cutting edge is promoted by heat generation during cutting, and the cutting resistance is remarkably changed due to repeated desorption of the constituent cutting edge in the cutting process. The fluctuation of the cutting resistance is a cause of so-called chattering and reduces the quality of the cutting surface. Further, the chipped cutting edge piece becomes a microchip, which promotes wear on the scooping surface and flank surface of the tool. As a result, when a steel sheet having a pearlite area ratio of less than 40% is cut, the cutting speed is inevitably reduced, and the work efficiency and the cutting cost are likely to increase.
However, as the pearlite area ratio increases, the effect of hard cementite appears strongly on the friction surface with the tool. That is, strong friction is generated between the scooping surface of the tool, the flank surface, and the work material during cutting, and the tool life is shortened due to mechanical wear.
For the above reasons, the pearlite area ratio is adjusted to a range of 40 to 80% (preferably 45 to 75%).

・ Perlite lamella spacing: 0.4 μm or more The detailed reason why the pearlite lamella spacing improves machinability is unknown, but it is speculated that the machinability improves with a lamellar spacing of 0.4 μm or more as follows. This can also be confirmed in examples described later.
The intensity of pearlite depends on the pearlite lamella spacing, and the pearlite strength increases as the lamella spacing decreases. Specifically, when the lamella spacing is less than 0.4 μm, the strength of pearlite becomes too high and the machinability deteriorates. When the lamella spacing is narrow, the pearlite / ferrite interface per unit volume increases. Therefore, when cutting pearlite, stress fluctuations generated at the interface between soft ferrite and hard cementite and tool wear due to crushed cementite microtips are promoted, and the tool life is shortened. Such a defect can be suppressed by setting the lamella interval to 0.4 μm or more (preferably 0.5 μm or more).

・ Annealing A At an annealing temperature of ₁ point or less, cementite is spheroidized, and the area ratio of the mixed structure of soft and strong ferrite + spherical or rod-like cementite is increased, or the lamellar spacing of residual pearlite generated by hot rolling is narrow. Machinability deteriorates due to high strength. However, when the heating temperature exceeds 800 ° C., undissolved cementite that becomes the core of the mixed structure decreases, it is difficult to obtain a mixed structure effective for improving machinability, and the area ratio of pearlite generated by annealing increases. The machinability deteriorates. Moreover, in an actual coil, annealing adhesion occurs, making industrialization difficult. For this reason, the annealing temperature is set in a temperature range exceeding A ₁ point and 800 ° C. or less.
In order to sufficiently dissolve cementite, the soaking time is set to 5 hours or more. When soaking is less than 5 hours, the dissolution of cementite is insufficient, the undissolved cementite increases to inhibit the formation of pearlite structure, and it becomes almost spherical cementite, so that good machinability cannot be obtained. However, increasing the soaking time increases the cost.
After soaking, it is preferable to cool at a cooling rate of 5 to 70 ° C./hour. At a cooling rate of less than 5 ° C./hour, cementite tends to be spheroidized, the area ratio of pearlite is reduced, and machinability is deteriorated. Conversely, at a cooling rate exceeding 70 ° C./hour, the area ratio of pearlite becomes too high and the machinability deteriorates.
・ Cold rolling The rolling ratio during cold rolling does not have a substantial effect on the machinability and is not subject to any particular restrictions. However, when the rolling rate exceeds 60%, the addition of a cold rolling process is increased, which tends to cause deterioration in manufacturability and increase in manufacturing cost.

  Steel materials having the components and compositions shown in Table 1 were melted and cast, and then hot rolled at a finishing temperature of 850 ° C. and a winding temperature of 600 ° C. After pickling the hot-rolled sheet, grinding or cold rolling was performed to produce a steel sheet having a thickness of 2.0 mm.

Each steel sheet is soaked at 680 to 800 ° C. for 10 hours, then cooled to 650 ° C. at a cooling rate of 5 to 70 ° C./hour, and further subjected to furnace cooling to room temperature to change the pearlite area ratio and lamella spacing. A sample was prepared.
Table 2 shows the effect of annealing conditions on the pearlite area ratio and lamella spacing.
Regarding the pearlite area ratio, the cross section of the test piece including the rolling direction and the plate thickness direction is mirror-polished and etched, and the cross section of the test piece is photographed at a magnification of 500 times using an optical microscope. The area ratio was measured.
Regarding the lamella spacing, similarly, the cross section of the specimen including the rolling direction and the plate thickness direction is mirror-polished and etched, and a portion with a narrow lamella spacing is selected while looking through an electron microscope, and 20 selected portions are photographed at a magnification of 2000 times. did. Ten pearlite having a narrow lamella interval was identified from 20 sheets, the lamella interval was measured, and the measured values were averaged to obtain the pearlite lamella interval.
All the steel types had a mixed structure of pearlite and ferrite + spherical cementite or ferrite + rod-like cementite, but the steel type L containing an excessive amount of Si was inferior in heat treatment properties because graphite was also precipitated.

Next, the tool life was investigated by a cutting test, and the machinability was evaluated from the tool life.
In the cutting test, an upright drilling machine equipped with a JIS B4301 general-purpose straight drill with a tip angle of 118 degrees, a cutting edge diameter of 10 mm, and a groove length of 95 mm was used. The piece was drilled. Drilling in the thickness direction from the surface of the test piece with a new drill was repeated, and the point at which cutting became impossible was determined as the tool life, and the machinability was evaluated from the number of through holes up to that point.

As seen in the test results of Tables 2 and 3, in the present invention example satisfying the pearlite area ratio: 40 to 80% and the lamella spacing: 0.4 μm or more, the tool life is reached even when the drilling is repeated 200 times or more. It can be seen that it is excellent in machinability.
On the other hand, in Test Nos. 1 and 2 having a pearlite area ratio of less than 40%, the influence of soft ferrite appeared strongly, cutting resistance increased, and the machinability was inferior. On the other hand, Test No. with a pearlite area ratio exceeding 80%. No. 10 was inferior in machinability because hard cementite greatly affected the friction surface with the tool.

Even if the pearlite area ratio and lamella spacing are adjusted to the ranges specified in the present invention, if the C content is insufficient as seen in Test No. 3, the influence of soft ferrite also appears strongly and the cutting resistance increases. Therefore, the machinability was inferior. When the pearlite area ratio in addition to the C content was out of the range defined in the present invention, as shown in Test No. 13, the influence of hard cementite appeared strongly and the machinability was poor.
Even in test No. 4 where the pearlite lamella spacing did not reach 0.4 μm, the tool life was short and the machinability was poor. The poor machinability is considered to be a result of the tool wear being promoted by the stress variation generated at the interface between the soft ferrite and the hard cementite and the crushed cementite microtips because the strength of the pearlite is too high.

In Test Nos. 9 and 17 in which both the area ratio of pearlite and the lamella spacing deviated from the ranges specified in the present invention, the influence of soft ferrite appeared strongly and the strength of pearlite was too high, so that the machinability was inferior.
Further, in Test No. 21 containing an excessive amount of Cr, tool wear was promoted by the strengthening action of cementite, resulting in poor machinability.
As is clear from the above comparison, it is a system with specified components and composition, and is composed of ferrite + spherical cementite or ferrite + rod cementite and pearlite structure, and by adjusting the pearlite area ratio and lamellar spacing, machinability It can be seen that a high-carbon steel sheet excellent in resistance can be obtained.

  As described above, in the specified component system, a mixed structure of ferrite + spherical cementite or ferrite + rod-like cementite in which pearlite adjusted to have an area ratio of 40 to 80% and a lamellar spacing of 0.4 μm or more is dispersed and By doing so, excellent machinability can be imparted to the high carbon steel sheet without depending on the precipitation of graphite or the addition of free-cutting elements. Therefore, when cutting a high carbon steel sheet into a product shape such as a machine part or an automobile part, the processing efficiency is improved, the tool life is extended, and the cost required for the cutting process is reduced.

Claims (3)

C: 0.6-1.5 mass%, Si: 1.0 mass% or less, Mn: 0.1-2.0 mass%, S: 0.02 mass% or less, P: 0.03 mass% or less , Al: 0.005 to 0.20% by mass, balance Fe and inevitable impurities , pearlite area ratio: 40-80%, pearlite lamellar spacing: 0.4 μm or more of pearlite dispersed and precipitated ferrite + A high carbon steel sheet excellent in machinability, characterized by having a mixed structure in which spherical cementite or ferrite + rod-like cementite is dispersed and precipitated. The high carbon steel sheet according to claim 1, further comprising one or more of Ni: 2.5 mass% or less, Cr: 2.0 mass% or less, and Mo: 1.0 mass% or less. Furthermore, Ti: 0.07 mass% or less, Nb: 0.07 mass% or less, V: 0.4 mass% or less, B: 0.07 mass% or less 1 type or 2 types or more are included. High carbon steel sheet.