JPH0987805A - High carbon steel sheet and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce a Cr-containing steel sheet, in which the range of the amount of C from eutectoid to hyper-eutectoid is specified and which has superior precision punchability and excellent rolling fatigue resistance as well as wear resistance after refining. SOLUTION: This steel sheet is a high carbon steel sheet having a composition consisting of, by weight, 0.75-1.00% C, 0.05-0.35% Si, 0.10-0.60% Mn, <=0.02% P, <=0.01% S, 0.50-1.00% Cr, 0.50-2.00% Ni, 0.10% Al, <=0.0015% O, <=0.5% Mo, and the balance essentially Fe with inevitable alloy components and also having a ferritic structure in which cementite of 0.5-2.0μm average grain size is dispersed. This steel sheet can be produced by controlling hot rolling finishing temp. and coiling temp. and performing annealing, as spheroidizing annealing, at 730-800 deg.C or by performing cold working at >=20% after hot rolling and then applying annealing at 650-800 deg.C.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high carbon thin steel sheet containing Cr, which is used for blades, springs, clutches, bearings and the like.

[0002]

2. Description of the Related Art A bearing steel having a high C and a high Cr specified in JIS-G-4805 is specified as a steel that can withstand a large amount of contact pressure. Even for special steel strips or thin steel sheets similar to them, the C content is eutectoid (about 0.75%) for applications such as gears, sprockets, and blades, where flaking due to sliding wear and rolling fatigue is important.
To high eutectoid Cr equivalent to this bearing steel
Additive steel is applied.

When the material is a thin steel plate, especially for mild steel, the technique of press forming by combining various plastic deformations such as punching, bending, drawing, and overhanging has made great progress, and various complicated and precise techniques have been achieved. Shapes have come to be made without cutting. However, in the case of a thin steel sheet having a high C content as described above, the deformability of plastic working is low even in the annealed state, and it is not easy to apply press working which is a rational mass production method.

For this reason, in general, it is often cut to a desired shape, and after shaping, it is tempered to a required hardness by heat treatment such as quenching and tempering. In addition, a steel plate having a low C content may be press-worked and then carburized to increase the C content on the surface to obtain desired wear resistance and rolling fatigue resistance. Both the method of cutting and the method of carburizing after pressing are labor-intensive, but if the steel of the desired composition has a deformability that can withstand a certain degree of processing, pressing depending on the shape The law can be applied. For example, in the case of gears and sprockets, the tooth shape can be processed into the final shape by the precision punching method. Compared to normal punching, precision punching reduces the clearance between the punch and the die, clamps the flange to prevent the material from flowing into the die, and provides a punch for reverse restraint to keep the shear surface smooth. The punching is performed by increasing the water pressure. As a result, the fracture surface ratio at the punched end is 0%.
Therefore, an end surface equivalent to that obtained by cutting is obtained.

In the parts for such applications, the amounts of C and Cr, etc. are limited due to wear resistance and rolling fatigue resistance after tempering. Furthermore, as a texture after tempering, tempered martensite In addition, it is required that the spheroidized cementite of an appropriate size be uniformly distributed. Therefore, it is necessary to prepare the shape of the spheroidized cementite in the thin steel sheet as a processing material, and it is preferable that the steel sheet has the required workability.

[0006] It is a task required for the raw material to adjust the composition and structure in consideration of the end use and to improve the workability so as to withstand the processing required for it. It is hard to say that these requirements have been adequately addressed in thin plates.

[0007]

The object of the present invention is to provide a thin steel sheet containing Cr in which the amount of C is in the range of eutectoid to hypereutectoid (0.75 to 1.00%), which is excellent in precision punchability and is heat-treated. It is an object of the present invention to provide a steel sheet having good wear resistance and rolling fatigue resistance and a method for manufacturing the same.

[0008]

The present inventor relates to a hyper-eutectoid high carbon thin steel sheet containing Cr, which has a good precision punching property as a material for working and is particularly resistant to hardening after tempering. We investigated a manufacturing method for obtaining steel with excellent rolling fatigue properties for gears and bearing parts.

From the workability of a thin steel sheet, it is desirable that C and Cr are low, but if the application requires performance equivalent to that of a high C, high Cr bearing steel, C: 0.95 to 1.10%. Cr: 0.90 to 1.60%. However, in the case of thin-walled parts, the surface pressure is somewhat relaxed and the dimensional effect of heat treatment is small, so the allowable limit is C: 0.75 to
1.00%, Cr: 0.50 to 1.00% is sufficient, so for steels with this range of composition, the condition after which the performance after the final heat treatment and the workability before it, especially the precision punching property, can be compatible Various investigations were conducted.

Generally, from the viewpoint of the tool life of punching, it is preferable that the hardness of the material for processing is low, and for that reason, the spheroidized cementite is coarse, that is, the average spacing of the cementite particles is large. preferable. Further, in terms of elongation of the material, it is necessary that the cementite particles are appropriately dispersed in a proper size. In addition to these points, in precision punching, an end face crack that is perpendicular to the punching direction and is parallel to the plate surface is likely to occur. In order to prevent this end face cracking, a certain degree of ductility is required as a material, and since strain during punching concentrates on the sheared surface, it was presumed that a material with low work-hardening property is preferable. Therefore, as a result of investigating the factors of the material that affect the work hardening during high-speed deformation, it was found that addition of Ni to the cementite is also effective for alleviating the work hardening in addition to coarsening the cementite.

On the other hand, the distribution state of cementite in the steel that has been heat treated after forming is largely governed by the state before heat treatment, and furthermore, the rolling fatigue resistance of the steel is greatly affected, and coarse cementite is flared. Shorten the life until the occurrence of king. Therefore, the distribution of spheroidized cementite in such a high carbon thin steel sheet must be limited to a certain range in which the workability of the material and the performance after heat treatment are compatible.

A high carbon thin steel sheet having such a distribution of spheroidized cementite and the required performance is obtained by hot rolling and annealing a steel having a predetermined composition, or after hot rolling, cold rolling and annealing. The present invention has been completed by knowing that it can be obtained by a manufacturing method using an appropriate combination. The gist of the present invention is as follows.

(1) C: 0.75 to 1.00% by weight, S
i: 0.05 to 0.35%, Mn: 0.10 to 0.60%, P: 0.02% or less, S: 0.01% or less, Cr: 0.50 to 1.00%, Ni: 0.50
.About.2.00%, O: 0.0015% or less, Mo: 0.5% or less, the balance consisting essentially of Fe and unavoidable alloy components, and a ferrite structure in which spheroidized cementite having an average particle size of 0.5 to 2.0 .mu.m is dispersed. A high carbon thin steel sheet characterized by the above.

(2) A steel slab having the above composition is hot-rolled at a finishing temperature of 820 to 900 ° C. and wound in a temperature range of 550 to 650 ° C., after which temperature T (° C.) and time t (h) are Soak in the range shown by the following formula and formula, then
The method for producing a high carbon thin steel sheet according to (1), characterized in that annealing is performed at a cooling rate of 15 ° C / h or less up to a temperature in the range of 600 to 680 ° C.

730 ≤ T ≤ 800 ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ 800-T ≤ 167 × logt ≤ 1000-T ・ ・ ・ ・ (3) With a steel slab having the composition described in (1) Hot rolled coil
After cold rolling with a rolling reduction of 20% or more, temperature T (° C) and time t (h) are soaked within the range shown by the following formula and the above formula, and the cooling rate up to a temperature in the range of 600 to 680 ° C. To
Characterized by annealing at 15 ° C./h or less,
The method for producing a high carbon steel sheet according to (1).

650 ≦ T ≦ 800 ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ The steel of the present invention and the method for producing the same control the chemical composition and the distribution of cementite and have an effect on these. Within the range not to be excluded, the surface may be ground, cold rolled, or strain relief annealing may be performed in order to improve the plate thickness accuracy and surface quality of the steel sheet before the precision punching.

[0017]

BEST MODE FOR CARRYING OUT THE INVENTION The reasons for limiting the chemical composition, structure and manufacturing conditions in the thin steel sheet of the present invention will be explained below.

(A) Chemical composition and structure (1) C Content of C is 0.75% or more in order to sufficiently maintain the hardness, wear resistance and rolling fatigue resistance after heat treatment. However, if the amount increases, it will not be possible to suppress the occurrence of end face cracking in precision punching, so the upper limit is 1.00%.

(2) Si Since Si has a deoxidizing effect and is effective in reducing the oxygen content in the steel which impairs rolling fatigue resistance, it is contained at 0.05% or more. However, the upper limit is 0.35% because if the amount increases, the steel becomes hard and cracks easily occur on the end face during precision punching.
It is.

(3) Mn Mn has the effect of suppressing hot embrittlement due to S and increasing the tempering temperature to improve toughness and rolling fatigue resistance, so it is contained at 0.10% or more. However, excessive addition makes it impossible to sufficiently lower the hardness after spheroidizing annealing, so the content is made up to 0.60%.

(4) P P is segregated at the ferrite grain boundaries and the cementite-ferrite interface, causing end face cracking during punching.
In addition, the rolling fatigue resistance after conditioning is deteriorated, so the smaller the better, the better. The limit is 0.02% or less, but 0.015% or less is preferable as a limit that does not exert a significant effect.

(3) S S forms a non-metallic inclusion in the steel, becomes a starting point of cracks during processing, and lowers rolling fatigue resistance, so the smaller the better, the better. The limit is 0.01% or less, but 0.005% is preferable.
It is the following.

(4) Cr Cr is effective not only for improving hardenability but also for suppressing decomposition of cementite during quenching and heating, and for stably dispersing cementite in martensite. It is an effective component for improving rolling fatigue resistance. Therefore, 0.50% or more is added. However, if added excessively, the precision punchability is impaired, so the upper limit is made 1.00%.

(5) Ni Ni is effective for preventing end face fracture during precision punching,
Therefore, 0.50% or more is added. However, even if added excessively, the effect is saturated and the cost is increased, so the upper limit of the added amount is 2.00%.

(6) Al Al does not need to be added if it is sufficiently deoxidized by another method such as Si addition or molten steel vacuum treatment.
When added for the purpose of deoxidation, contain up to 0.10%. Even if added over 0.10%, the effect is saturated, and the steel tends to be hardened and the precision punchability deteriorates.

(6) O (oxygen) O forms non-metallic inclusions in steel similarly to S, and particularly reduces rolling fatigue resistance, so it is necessary to reduce it as much as possible, and the smaller the better. The limit is 0.0015% or less, which has no significant effect.

The steel of the present invention is not particularly limited, but may contain 0.10% or less of Al for the purpose of deoxidizing the steel.

(7) Mo Mo is not required to be added, but it is effective in suppressing end face cracking during precision punching and also effective in improving rolling fatigue resistance after tempering. To do. In order to obtain the effect, it is preferable to contain 0.1% or more, but if added in excess, conversely, the end face breakage during punching increases, so the upper limit of the content is made 0.5%.

(8) Particle Size of Cementite Particles In the steel of the present invention, the size of spheroidized cementite distributed in the steel is extremely important. If the cementite particles are too small, the rolling fatigue resistance after heat treatment under normal conditions is not good. It is considered that this is because the amount of solid solution C in the austenite became too large during the heating of the heat treatment, and residual austenite was generated during quenching. Cementite particles that do not deteriorate rolling fatigue resistance must have an average particle size of 0.5 μm or more.

Larger cementite particles are better for precision punching workability, especially for tool life. However, if it is too large, the frequency of end face cracking during punching tends to increase, and in addition, rolling fatigue life until flaking occurs decreases.

From these points, the average particle size of cementite should be 2.0 μm or less, preferably 1.0 μm.
It is the following.

As described above, the range of the average particle size of the cementite particles is set to 0.5 to 2.0 μm as a limit to satisfy both the workability of precision punching and the rolling fatigue resistance after heat treatment.

(B) Manufacturing conditions (1) Hot rolling As a manufacturing method for obtaining spheroidized cementite of the steel of the present invention, there are a method of pickling a hot rolled sheet and annealing as it is, and a method of annealing after cold rolling. There is. In the former case, the distribution of cementite in the hot-rolled sheet before annealing is greatly affected, so it is necessary to sufficiently control the hot rolling conditions. In this case, the finishing rolling temperature range is 820 to 900 ° C and the winding temperature range is 550 to 650 ° C.

The reason for limiting the finishing temperature is 820 ° C.
If it is below 900 ° C., coarse pro-eutectoid cementite will be generated, and it will become coarse cementite grains after annealing, which will easily cause end face cracking during precision punching.
This is because pearlite tends to remain even after the spheroidizing annealing, and the spheroidization rate decreases. In addition, the coiling temperature is restricted to the above range because the spheroidization rate after annealing becomes insufficient even when the finishing temperature is too high even when it exceeds 650 ° C, and when it falls below 550 ° C, the hot rolled sheet Is hardened and cracks occur in the coil during pickling.

(2) The form of cementite in the steel sheet after cold rolling and hot rolling is greatly affected by the hot rolling conditions, and even if it is spheroidized and annealed as it is, the distribution of the cementite particles necessary for the steel of the present invention is It may not be easy to obtain. In that case, eliminate the effect of hot rolling conditions,
In order to adjust the shape of cementite and obtain sufficient spheroidization, cold rolling is performed before annealing. If the reduction ratio of this cold rolling is at least 20%, this object can be achieved, and even if a larger reduction is applied, there is no problem, but since the material hardens and it becomes difficult to roll, it is naturally limited. Therefore, up to about 50% is appropriate. When cold rolling is performed before annealing, hot rolling conditions are not particularly limited,
When this hot rolling method is applied using the hot rolled material under the conditions shown in (1), a more uniform spheroidized cementite distribution can be realized.

(3) Annealing conditions Cementite in steel adjusted by hot rolling conditions or cementite in steel destroyed by cold rolling is
Annealing is performed under the following conditions in order to distribute the state of the invention steel. That is, when the soaking temperature is T (° C.) and the soaking time is t (h), it is within a range satisfying 800-T ≦ 167 logt ≦ 1000-T, where T is annealed after hot rolling. In the case of 730 ≤ T ≤ 800 ・ ・ ・ ・ ・ ・ ・ ・ When annealing after cold rolling, the range of 650 ≤ T ≤ 800 ・ ・ ・ ・ ・. After soaking, gradually cool to 680-600 ℃ at a cooling rate of 15 ℃ / h or less.

As described above, the temperature and time for soaking are regulated by the formulas. When the soaking temperature is low, it is necessary to take a sufficient amount of time, and when the soaking temperature is high, a short time is sufficient. This is because the effect does not change even if is lengthened. In the annealing after hot rolling, since the cementite is partly solid-dissolved to promote spheroidization, the lower limit temperature of soaking is Ac ₁ point or more, that is,
730 ℃ or higher. The minimum temperature is 650 ° C because cementite does not need to be re-dissolved in annealing after cold working.
It is. In either case, if the soaking temperature is too high, the amount of cementite re-dissolved increases and pearlite appears during cooling, so the upper limit temperature is set to 800 ° C.

In addition, cooling after soaking is performed in order to sufficiently spheroidize, reduce hardness as much as possible, and improve ductility.
Slowly cool from 680 to 600 ℃ at a cooling rate of 15 ℃ / h or less. Cooling in this temperature range or lower may be fast without any restriction. As described above, the reason why the lower limit of the temperature that requires slow cooling is wide is that when annealing is performed in a coil state, the temperature varies depending on the portion of the coil, and it is difficult to manage at a constant temperature. The annealing conditions of the present invention can be easily realized by carrying out annealing by the so-called box annealing method.

The above cold rolling and the subsequent annealing are usually carried out once after hot rolling to obtain a distribution of cementite particles of a desired size. However, if once, the required state can be obtained. If not, the steel having cementite particles having the size defined in the present invention can be reliably obtained by repeating the process twice or more.

[0040]

EXAMPLE Hot-rolled steel sheets were manufactured from 14 kinds of steel slabs A to N shown in Table 1, and after descaling, for some steels, 25
% Cold rolling and spheroidizing annealing was performed under the annealing conditions shown in Table 2. The cold rolling is shown in the table by dividing it into the primary cold pressure when the hot rolling is performed as it is and the secondary cold pressure when the cold rolling is performed after annealing the hot rolled steel sheet. A tensile test and a precision punching test were performed on the obtained steel sheet. The punching test is
A precision punching die was installed on a 100-ton hydraulic press, and square test pieces having corners with different curvatures were punched out in four stages shown in FIG. 1 to compare the minimum curvatures at which no breakage occurred at the corner end faces. The results are also shown in Table 2. Further, with respect to the steel sheet that was spheroidized and annealed, the rolling fatigue resistance at the time of quenching and tempering under the conditions shown in Table 3 was evaluated, and the results are also shown in the table. The rolling fatigue resistance at this time is 60 mm in diameter.
Using a thrust type rolling contact fatigue tester as shown in Fig. 2, the contact surface pressure: 560kgf / mm ² , rotation speed: 1860 rpm, lubricating oil of Nippon Oil # 60 spindle oil was used. The number of rotations until flaking occurred was measured under the conditions.
For this rolling fatigue resistance durability evaluation, 19 test pieces were tested, and these were arranged in the order of the number of revolutions until flaking occurred, and the probability of occurrence was 20%. The number of rotations is 16 when the probability of occurrence is 80%
The rotation speed of the second test piece was displayed. In both cases, the higher the number of rotations, the better the durability. Table 3 shows the results.

[0041]

[Table 1]

[0042]

[Table 2]

[0043]

[Table 3]

When the ferrite composition has a chemical composition defined by the present invention and an average grain size of 0.5 to 2.0 μm and cementite is dispersed, it is observed in Test Nos. 3, 4, 5, 8, 11, 18, and 19 and the like. As described above, the precision punching property after the spheroidizing annealing is excellent, and the end face crack does not occur even with a small radius of curvature. Also, test numbers 23, 24, 25, 31, 32, 34 and 35 in Table 3
It is shown that steel having an average grain size of cementite in such a range is steel having excellent rolling fatigue resistance after heat treatment. In particular, test numbers 3, 8, 9, 23, 2
From 4, 25 and the like, it can be seen that when the chemical composition and the average particle size of cementite are within the range of the present invention, it is possible to produce a high carbon thin steel sheet excellent in precision punching property and rolling fatigue resistance after heat treatment.

[0045]

EFFECT OF THE INVENTION The high carbon thin steel sheet of the present invention is excellent in precision punching property and excellent in rolling fatigue after quenching and tempering, and is conventionally carburized after press punching or heat treated after cutting. If applied to the manufacture of sprockets, blades, etc., the manufacturing process can be greatly improved.

[Brief description of drawings]

FIG. 1 is a drawing showing a test piece having corners with different curvatures, which is used for evaluation of precision punchability of a steel sheet.

FIG. 2 is a drawing that conceptually illustrates the structure of a thrust type rolling contact fatigue testing machine that performs a rolling fatigue test.

Claims (3)

[Claims] 1. A weight ratio of C: 0.75 to 1.00%, Si: 0.
05 to 0.35%, Mn: 0.10 to 0.60%, P: 0.02% or less,
S: 0.01% or less, Cr: 0.50 to 1.00%, Ni: 0.50 to 2.
00%, Al: 0.10% or less, O: 0.0015% or less, Mo:
A high carbon thin steel sheet containing 0.5% or less, the balance being substantially Fe and an unavoidable alloying component, and having a ferrite structure in which cementite having an average grain size of 0.5 to 2.0 μm is dispersed. 2. The steel having the chemical composition according to claim 1 is hot-rolled at a finishing temperature of 820 to 900 ° C. and wound in a temperature range of 550 to 650 ° C., and then the temperature T (° C.) and time are set. t
(H) is soaked within the range shown by the following formula and formula, and then the cooling rate to the temperature in the range of 600 to 680 ° C is
Characterized by annealing at 15 ° C./h or less,
The method for manufacturing a high carbon thin steel sheet according to claim 1. 730 ≤ T ≤ 800 ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ 800-T ≤ 167 × logt ≤ 1000-T ・ ・ ・ ・ 3. A steel hot-rolled coil having the chemical composition according to claim 1 is cold-rolled at a rolling reduction of 20% or more, and then temperature T (° C.) and time t (h) are calculated by the following formula and The method for producing a high carbon thin steel sheet according to claim 1, characterized in that soaking is performed in the range represented by the formula, and annealing is performed at a cooling rate up to a temperature in the range of 600 to 680 ° C of 15 ° C / h or less. . 650≤T≤800