JP3367414B2 - High carbon cold rolled steel strip and method for producing the same - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high carbon material which is suitable as a material for automobile parts, chain parts, knitting needles and the like and which can obtain a cut face having a smooth shape during punching.
The present invention relates to a cold rolled steel strip and a method for manufacturing the same.

[0002]

2. Description of the Related Art High carbon steel strips used for automobile drive system parts, chain parts, knitting needles, etc. are processed and then heat treated such as quenching and austempering to impart high strength and high hardness properties. Has been used. The steel strip contains carbon and alloying elements depending on the intended strength and hardness. Since carbon in steel precipitates as hard cementite and its amount is large, it is difficult to work it as it is in hot rolling. For this reason, normally, after hot rolling, annealing is performed to make the cementite spherical and the workability is improved before use.

As a method of processing a high carbon steel strip, punching (shearing) using a die and a punch is a basic method. Figure 1
FIG. 1 is a conceptual diagram showing a cut surface of a punching process.
(A) is a front view of a cut surface, and FIG. (B) is its AA sectional view. When punching, as shown in Fig. 1,
Since the tool causes sagging (distortion) 1 caused by drawing the material in the vicinity of the cut portion and burrs (burrs) 4 and the like at the corner portion at the end of the punching process, the dimensional accuracy of the component is impaired.

It is said that sagging and burrs are more likely to occur as the ductility of steel and the n value increase. For example, when the cementite is spheroidized and annealed, if it is excessively softened, these defects become a problem. On the other hand, if annealing is insufficient and the steel strip is too hard, sagging and burrs can be suppressed, but wear of the punching die becomes remarkable, and the processing cost increases, which is not suitable for mass production. Therefore, the high carbon steel strip is annealed by hot rolling to spheroidize the cementite, and then cold rolled to make the hardness so that the entire length and width are in an appropriate hardness region. Often adjusted.

The cut surface of the cutting portion is usually in contact with the cutting edge and plastically deforms to smoothly cut the sheared surface 2.
And a fracture surface 3 that occurs when a material is separated by a crack.
It is composed of. A large number of microcracks are generated on the fracture surface 3 when the material is separated, so that the fracture surface 3 has a shape in which a large number of fine irregularities are present. When stress acts on a member, stress concentrates on these recesses and fatigue cracks are likely to occur. Therefore, the fatigue life of a member having many fracture surfaces becomes short. This is considered to be a serious problem along with dimensional defects. Therefore, there is a demand for a method of reducing the fracture surface 3 as much as possible and obtaining a cut surface composed of a smooth shear surface 2.

A precision punching method has been developed as a method for obtaining a smooth and defect-free cut surface. This method is a method in which hydrostatic pressure is mechanically applied to the material to suppress the occurrence of cracks, and is a method in which the step of cutting and scraping the fracture surface after shearing is omitted. However, adopting these methods requires specialized equipment and additional steps, and is not an easy method.

As described above, the sagging and burrs can be suppressed by adjusting the annealing conditions and the cold rolling conditions to control the ductility of the work material steel strip within an appropriate range. However, regarding the shape of the cut surface, it is difficult to obtain a good shape by the method of controlling the ductility in the optimum range.

Japanese Unexamined Patent Publication (Kokai) No. 59-28527 discloses a method for producing a cold rolled special strip steel in which spherical carbides are finely dispersed. In this method, after hot rolling so that the phase transformation of the steel is completely completed after hot rolling, the cold rolled hot rolled steel strip is repeatedly subjected to cold rolling and intermediate annealing at low temperature to remove carbides. The shape is made fine and the heat treatment property is improved.
However, in this invention, there is no mention of the shape of the cut face during shearing, and it is difficult to make the shape of the cut face stable and good even if the carbide is simply finely dispersed. .

[0009]

SUMMARY OF THE INVENTION It is an object of the present invention to provide a high carbon cold rolled steel strip which has good shearing workability and is premised on heat treatment such as quenching, and a method for producing the same. More specifically, it is an object of the present invention to provide a high-carbon cold-rolled steel strip and a method for producing the same, which does not cause shape defects such as sagging and can obtain a cut face having a smooth sheared surface.

[0010]

The gist of the present invention resides in a high-carbon cold-rolled steel strip having a smooth punched fracture surface as described in (1) below and a manufacturing method thereof as described in (2).

(1) In% by weight , C: 0.20 to 0.80
%, Si: 0.30% or less, Mn: 0.60 to 1.60
%, Sol. Al: 0.010 to 0.100%, N:
0.0020-0.0100%, Ca: 0-0.010
0%, the balance consisting of Fe and inevitable impurities, and sol. Al / N is in the range of 5 to 10, the spheroidization rate of cementite in the steel is 80% or more, the average grain size is 0.80 μm or less, and the tensile strength of the steel is 600.
~ 700 N / mm ² , tensile strength (N / mm ² ) × {10
High carbon cold rolled steel strip satisfying the relationship of 0-elongation (%)} of 50 × 10 ^{3 to} 65 × 10 ³ .

(2) The steel having the chemical composition described in (1) above is hot-rolled, wound at 500 to 680 ° C., pickled, and then cold-rolled at a reduction rate of 10 to 80% for the first time. perform rolling, was subjected to intermediate annealing at six hundred fifty to seven hundred and twenty-five ° C., have such to put the second cold rolling at a reduction rate of 5-25%, then the thermal treatment
The method for producing a high carbon cold-rolled steel strip according to the above (1), which is a product without any reason .

The present inventors have set the C content to 0.20 to 0.8 in order to obtain the necessary strength after heat treatment of a high carbon cold rolled steel strip.
A detailed study was conducted on a method for improving the punchability of a steel strip having a constant elongation of 0%.

As a result, in order to suppress the generation of a fracture surface on the cut surface, in addition to making the carbide fine spherical,
We have learned that it is important to control the strength and ductility of steel within appropriate ranges.

When the tensile strength of steel is low and the ductility is excessively large, the sag becomes large. As the tensile strength increases, sagging decreases and becomes better, but in order to improve the shape of the cut surface, the larger the ductility, the better. As the strength increases and the ductility decreases, the plastic deformability decreases, so it is presumed that it becomes difficult to obtain a sheared surface. Furthermore, it was also found that the preferable property balance described above can be efficiently obtained by appropriately controlling the chemical composition of steel, the cold rolling condition after hot rolling and the annealing condition. The present invention has been completed based on these findings.

[0016]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described in detail below. The chemical composition% shown below represents% by weight.

A. The chemical composition C: C of steel has the effects of enhancing the hardenability of steel and improving the wear resistance and fatigue strength after heat treatment. In order to secure wear resistance and fatigue strength after heat treatment, the tensile strength of steel after heat treatment is 100 kgf / mm ² or more (Hv: Vickers hardness:
300 or more), and 0.20% or more of C is contained to secure this strength. If the C content is excessively increased, the toughness after heat treatment is impaired. In order to obtain a tensile strength of 210 kgf / mm ² (600 in Vickers hardness) or less, which is a range in which toughness is obtained, the upper limit of the C content is 0.80%.

Si: Si is added as a deoxidizing agent together with Al when refining steel. However, if Si is excessively contained, Si oxide is generated during heat treatment, which may lower the fatigue strength. , Si content is 0.30% or less. In the present invention, sol. Al at least 0.01
Since it contains 0%, Si may not be contained.

Mn: Mn has the effect of increasing the softening resistance when the steel is heated, and can increase the tempering temperature and the austempering temperature, so that the hardenability and toughness can be improved. To obtain this effect, it is contained by 0.60% or more. However, if the content exceeds 1.60%, the steel excessively hardens, and pickling and cold rolling become difficult, so the upper limit of the Mn content is set to 1.60%.

Sol. Al: In order to secure an AlN precipitate that suppresses abnormal growth of austenite grains that are likely to occur during heating for heat treatment and prevents a decrease in toughness,
sol. Al is contained by 0.010% or more. On the other hand, s
ol. When the Al content exceeds 0.100%, not only the grain growth suppressing effect of austenite is impaired, but also oxides such as Al ₂ O ₃ increase, which causes the toughness and fatigue strength after heat treatment to be impaired. Therefore, sol. The upper limit of the Al content is 0.100%.

N: N combines with Al to form a nitride (AlN
Etc.) to suppress coarsening of austenite grains that may occur during heat treatment, prevent sagging and dimensional change before and after heat treatment, and improve toughness after heat treatment. N is 0.0 to obtain this effect.
020% or more is contained. On the other hand, the N content is 0.010
If it exceeds 0%, the elongation deteriorates and the workability is impaired.
The upper limit is 0.0100%. Furthermore, sol. Al
The ratio of the content to the N content (sol.Al/N) is 5 to 1
It is preferable that these elements are contained in the range of 0. If this ratio is less than 5 or exceeds 10, the austenite grain size may become coarse during the austempering treatment, which is not preferable.

Ca: Ca is not an essential element, but it has the effect of reducing the solid solution oxygen in the steel and reducing the amount of Al oxides. In addition, it also has a function of forming a Ca-based sulfide by combining with S as an unavoidable impurity, and rendering the harmful effects of S harmless. Ca may be contained to obtain these effects. To obtain these effects, Ca is 0.001
The content is preferably 0% or more. On the other hand, if Ca is contained excessively, the manufacturing cost becomes high, and the fatigue limit may decrease due to the increase of Ca-based oxides, and the fracture surface at the time of punching may increase due to the increase of Ca-based sulfides. , The upper limit of the content is preferably 0.0100%.

Chemical components other than the above are Fe and inevitable impurities.

B. Carbide Carbide (cementite) is preferably spherical because the ductility of the steel is improved and the workability is improved. If the spheroidization rate is 80% or more, the fracture surface during punching is suppressed, so the spheroidization rate of the cementite of the high carbon cold-rolled steel strip of the present invention is 80% or more. In the present invention, the cross section of the steel strip is observed with an optical microscope at a magnification of 2000, and carbides having an aspect ratio (long axis / short axis) of 5 or less are used as spheroidized cementite. As for the spheroidization rate, the area occupied by the above-mentioned spheroidized cementite can be measured by a method such as image processing.

If cementite with a large grain size is present, it may be crushed during punching, and a fracture surface may occur starting from this, so the average grain size of cementite is 0.80 μm or less. On the other hand, if the cementite is excessively refined, the tensile strength and hardness may be remarkably increased, and conversely, the fracture surface may increase at the cut face, so the lower limit of the average particle diameter of the cementite is preferably 0.50 μm.
This average particle diameter is an average value obtained by measuring the major axis of all the carbides detected with a scanning electron microscope or the like at a magnification of 2000 times after polishing and etching the cross section of the steel strip test piece.

C. Mechanical properties In order to improve the dimensional accuracy and durability of products such as chains, in addition to the shape control of cementite for suppressing the formation of the fracture surface of the cut surface during punching, as described in item b, Controlling mechanical properties is also important. When the tensile strength is low, deformation such as warpage or sagging occurs during punching, which impairs the external shape and dimensional accuracy, which is not preferable. To prevent these deformations even when the steel strip is thin,
Tensile strength (hereinafter also simply referred to as "TS") is 600 N /
It must be at least mm ² . However, the tensile strength is 70
If it exceeds 0 N / mm ² , the fracture surface increases at the cut surface and
The upper limit of the tensile strength is 700, because the wear of the mold becomes significant.
N / mm ² .

Further, in order to suppress the generation of a fracture surface,
It is necessary to ensure ductility according to the tensile strength. In particular, it is preferable to increase the ductility as the strength of the steel strip increases. In the present invention, as a steel strip that suppresses the fracture surface of the cut surface, the tensile strength (TS) and the total elongation (El) of a steel plate during processing measured with a No. 5 test piece specified in JIS Z 2201 are used. In between, the following equation, TS (N / mm ² ) × {100-El
(%)} (Hereinafter, also simply referred to as “strength-elongation balance”) satisfies the relation that the value is in the range of 50 × 10 ^{3 to} 65 × 10 ³ . This value is 50 ×
If it is less than 10 ³ , sagging occurs on the punched end face, which is not preferable. If it exceeds 65 × 10 ³ , a fractured surface is generated, which is not preferable.

D. Production method A preferred method for producing the steel strip of the present invention will be described below.

Steel having the chemical composition defined in the above item a is melted by a known method such as a converter or an electric furnace, cast into a cast piece by a known method such as continuous casting, and then hot rolled. To do. The hot rolling can be performed by known methods and conditions except that the winding temperature is within the following range.

If the coiling temperature after the hot rolling is excessively lowered, the steel becomes hard and the coiling work may become difficult, and the pickling and cold rolling may be hindered. For this reason,
The winding temperature is preferably 550 ° C or higher. More preferably, the temperature is 625 ° C. or higher in order to improve the cold rolling property. On the other hand, if it is wound in a temperature range exceeding 680 ° C., the cementite in the hot-rolled steel strip becomes coarse, and it becomes difficult to make the cementite fine even by cold rolling. Therefore, the winding temperature is preferably 680 ° C or lower.

After hot rolling, descaling is performed. The descaling method is arbitrary, and known methods such as pickling and shot blasting can be applied. After that, the first cold rolling (hereinafter, simply referred to as "primary cold rolling") is performed.

The reduction ratio in the primary cold rolling affects the spheroidization ratio and grain size of cementite in the subsequent annealing. Reduction rate is 1
When it is less than 0%, cementite is difficult to be spheroidized, and it becomes difficult to make the spheroidization rate 80% or more. On the other hand, if the rolling reduction exceeds 80%, cold rolling becomes difficult, which is not suitable for mass production. Further, if the rolling reduction is excessively increased, the carbide formed by hot rolling may be finely crushed, and during intermediate annealing, the carbide may aggregate and grow, which may cause coarse cementite, which is not preferable. For these reasons, the rolling reduction range is preferably 10 to 80%. The more preferred range is 30 to 70%.

It is preferable that the first cold rolled steel strip is subjected to intermediate annealing and then subjected to a second cold rolling (hereinafter simply referred to as "secondary cold rolling"). The intermediate annealing temperature is preferably 725 ° C. or lower so that cementite does not become coarse. If the intermediate annealing temperature is too low, the spheroidization of cementite will be insufficient, so the intermediate annealing temperature is preferably 650 ° C or higher. The soaking time for intermediate annealing is not specified, but
In order to suppress the particle size of cementite finely, it is preferable to set it within 24 hours.

If the reduction ratio in the secondary cold rolling is less than 5%, the total elongation of the steel strip is too high, and a fractured surface is likely to occur at the cut surface, which is not preferable. When the reduction ratio of the secondary cold rolling exceeds 25%, the tensile strength of the steel strip becomes high,
The total elongation becomes too low and a fracture surface is likely to occur. Therefore, the reduction rate in the secondary cold rolling is preferably in the range of 5 to 25%.

After the second cold rolling, the steel strip of the present invention can be subjected to a degreasing treatment or the like according to a conventional method, and can be directly subjected to punching work or the like. The punching process may be carried out according to a conventional method, but it is preferably carried out under the conditions which are usually carried out in order to obtain a good punching fracture surface, for example, the clearance between the die and the punch is appropriately selected.
After the processing is completed, heat treatment such as quenching, tempering, and austempering can be performed according to a conventional method, whereby good tensile strength and hardness can be obtained.

[0036]

EXAMPLES Example 1 Steels A to T having the chemical compositions shown in Table 1 were vacuum melted, cast, and then forged into steel pieces, heated at 1200 ° C. for 1 hour, and then rolled at a finishing temperature of 850 ° C. Thickness 2.5 mm, width 200 mm
After being wound at 625 ° C., it was cooled at a rate according to the normal cooling rate after winding. After cooling, it was pickled, descaled, and cold-rolled to a thickness of 1.2 mm (rolling ratio 5
2%) and 2 at 655 ° C. in an atmosphere of 100% by volume of hydrogen gas.
Annealed for 0 hours and cold-rolled to a thickness of 1.0 mm (reduction ratio 1
6.7%).

[0037]

[Table 1]

Test pieces for crystallographic structure investigation were taken from the central portion in the plate width direction of these steel strips, the cross-sections were polished, corroded with a Nital corrosive solution, and the corroded surface was enlarged at a magnification of 2000 times and photographed. Then, the average particle size and spheroidization rate of cementite were measured. In addition, JIS5
A tensile test piece was sampled and subjected to a tensile test to determine the strength-elongation balance.

The punchability was investigated by the following method. SKD
11 (Hv900) clearance is 0.5% of plate thickness
Using the mold, a disc having an outer diameter of 10 mm was punched at a punching speed of 20 mm / sec. The entire circumference of the punched end surface of the obtained disk was magnified 50 times with a magnifying glass and the state of the cut surface was observed, the size d of the sag was measured, and the ratio d /
When t was less than 0.1, it was evaluated as good (◯), and when t was 0.1 or more, it was evaluated as bad (x). The fracture surface was evaluated as good (∘) when no fracture surface was observed, and as poor (x) when there was any fracture surface. The evaluation results obtained are
It is shown in Table 1 together with the chemical composition.

As can be seen from the results in Table 1, all of the steels A to K having the chemical composition within the range specified by the present invention have no sagging, and the cut surface has no fracture surface, and good products can be obtained. It was On the other hand, steels L and M whose C content was too low
Steel N having an excessively low n content had a low tensile strength, a low strength-elongation balance, and a large sag. Steel M with too high C content and steel O with too high Mn content had fracture surfaces because the tensile strength was too high. Steel P is sol. Al
Since the content and N content were low, the cementite became coarse, and the total elongation became too large, causing sagging. Steel Q is sol. The spheroidization of cementite was insufficient due to the excessively high Al content and N content, and the fracture surface occurred due to the small total elongation. Steel R is sol. On the contrary, the steel S was sol. Since Al / N was too low, the crystal structure became a mixed grain structure in all cases, and the total elongation became non-uniform and a fracture surface was generated.

Steel T had too much Ca content, and therefore had many inclusions and fractured surfaces.

Example 2 Three kinds of steels shown in Table 2 each having a chemical composition within the range specified by the present invention were vacuum-melted, cast and forged to form a steel piece having a width of 200 mm.

[0043]

[Table 2]

After heating these steel slabs at 1200 ° C. for 1 hour, they were hot rolled at a finishing temperature of 850 ° C. and wound at various temperatures.
After that , it was cooled at a rate according to the normal cooling rate after winding. After these hot-rolled steel strips have been pickled, they are first cold-rolled at various reduction ratios, and then subjected to intermediate annealing and secondary cold-rolling to obtain 1.0-mm-thick cold-rolled steel strips. It was The average particle size, spheroidizing rate, mechanical properties and punchability of cementite of these cold rolled steel strips were investigated by the same method as described in Example 1. The winding temperature, rolling conditions, intermediate annealing conditions and performance evaluation results are summarized in Table 3.

[0045]

[Table 3]

As can be seen from the results shown in Table 3, in the case of trial Nos. 21 to 29 in which the average particle size and spheroidizing rate of cementite and the mechanical properties are within the ranges specified by the present invention, sagging and fracture surface are all observed. A good product was obtained. On the other hand, in the trial Nos. 30 to 32, the coiling temperature was too low and the primary cold rolling was difficult. Therefore, the reduction ratio had to be lowered, and the secondary cold rolling was performed to obtain a predetermined plate thickness. The tensile strength (TS) becomes excessive because the rolling reduction is too high, or the total elongation (E
1) becomes too low and the strength-elongation balance {TS × (1
00-El)} became high, and a fracture surface occurred. Trial number 3
In Nos. 3 to 35, the coiling temperature was too high, and the reduction ratio of the primary cold rolling was too high, resulting in coarse cementite.
Since the reduction ratio of the next cold rolling was too low, the ductility became excessive, the balance between strength and elongation became low, and sagging occurred. Trial number 3
In Nos. 6 to 38, since the coiling temperature and the intermediate annealing temperature were too high, the cementite became coarse and a fracture surface was generated. In addition, since the crystal structure was a mixed grain, the sag increased.

In Test Nos. 39 to 41, the reduction ratio of the primary cold rolling was too low, so that the spheroidization was insufficient and the tensile strength became too high, and a fracture surface was generated. In the trial Nos. 42 to 44, the intermediate annealing temperature was too low, so that spheroidization was insufficient, and the tensile strength was too high, so that a fracture surface was generated.

[0048]

EFFECTS OF THE INVENTION The high carbon steel strip of the present invention does not cause sagging during punching, and has a cut surface having a smooth sheared surface with no fracture surface. Therefore, if this is heat treated,
It is possible to obtain a machine structural part having high dimensional accuracy and high strength and excellent wear resistance without deterioration in fatigue strength. Further, according to the manufacturing method of the present invention, the steel strip of the present invention can be manufactured using ordinary manufacturing equipment, so that a good quality product can be supplied at low cost.

[Brief description of drawings]

1A and 1B are conceptual views showing a cut surface of a punching process, FIG. 1A is a front view of the cut surface, and FIG. 1B is a sectional view taken along line AA.

[Explanation of symbols]

1 ... sagging, 2 ... sheared surface, 3 ... fracture surface, 4 ...
.. Flies.

─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Akio Nagai 1850 Minato, Wakayama, Wakayama Sumitomo Metal Industries, Ltd. Wakayama Works (72) Inventor Hiroyuki Nakagawa 4-533 Kitahama, Chuo-ku, Osaka-shi, Osaka Sumitomo Metal Industries, Ltd. (56) Reference JP-A-5-9588 (JP, A) JP-A-59-28527 (JP, A) JP-A-9-157758 (JP, A) JP-A-8-337843 (JP, A) JP-A-8-176726 (JP, A) JP-A-9-316595 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB name) C22C 38/00-38 / 60

Claims (2)

(57) [Claims] 1. By weight% , C: 0.20 to 0.80%, S
i: 0.30% or less, Mn: 0.60 to 1.60%, s
ol. Al: 0.010 to 0.100%, N: 0.00
20-0.0100%, Ca: 0-0.0100%, the balance consisting of Fe and unavoidable impurities, and so
l. Al / N is in the range of 5 to 10, the spheroidization rate of cementite in steel is 80% or more, and the average grain size is 0.
80 μm or less, the tensile strength of steel is 600 to 700 N
/ Mm ² , high carbon cold-rolled steel strip satisfying the relation of tensile strength (N / mm ² ) × {100-elongation (%)} of 50 × 10 ^{3 to} 65 × 10 ³ . 2. A steel having the chemical composition according to claim 1,
After hot rolling, winding at 500 to 680 ° C., pickling,
The first cold rolling was performed at a reduction rate of 10 to 80%, and 6
After performing intermediate annealing at 50 to 725 ° C, a reduction rate of 5 to 25
Not such to put a second cold rolling, the subsequent heat treatment is performed in%
Method for producing a high-carbon cold-rolled steel strip according to claim 1, characterized in that the product without.