JPH0774375B2 - Manufacturing method of thin free-cutting steel - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial field of application) The present invention relates to a method for producing thin plate free-cutting steel, particularly tensile strength of 30 to 50 kgf /
Free-cutting steel by pearlite dispersion, which is effective for products manufactured from a thin plate with a level of mm ² through a cutting process, and can also be applied to products manufactured through a cutting process + forming process from such a thin plate. Manufacturing method.

(Prior Art) Conventionally, rimmed steel from an undeoxidized ingot is often used for cutting applications.

Rimed steel has many inclusions in the steel, and the presence of such inclusions enhances machinability during cutting and weakens the bonding force between the structures in the steel, reducing wear of the cutting edge and further cutting. It is said that the powder is also easy to cut and has excellent machinability.

On the other hand, the killed steel having good inclusion cleanliness produced through a normal deoxidizing step generally has an alloy element having high machinability added to improve machinability. The main elements that enhance the machinability include Pb, S, P, Ca, Se, Te, Bi, and the like. By adding these elements alone or in combination,
The so-called machinability is improved by making the steel structure brittle and increasing the lubrication effect of the cutting blade by using non-metallic inclusions to extend the cutting life.

"96th and 97th Nishiyama Memorial Technology Course, Development of Cutting Steel Manufacturing Technology and Improvement of Quality" is known as a document introducing these.

(Problems to be Solved by the Invention) By the way, in the present steel manufacturing process, the ratio of the manufacturing process of continuous casting + hot rolling reaches 80 to 90%, and significant manufacturing rationalization is realized. There are few inclusions on the internal surface, and the uniformity of strength between lots is improved, so that today's demands can be fully satisfied in terms of materials.

However, so-called free-cutting steel is still manufactured by the slabbing method, and in order to manufacture rimmed steel for such purposes, many processes such as securing molds, caring for molds, installing large cranes, slabbing process, etc. Is unavoidable. further,
Rimed steel has excessive inclusions and severe segregation 2
It may break the sheets. Further, even from the same ingot, the strength is greatly different between the front end portion and the rear end portion, and it is often difficult to apply molding processing or the like.

Therefore, it is expected that the rimmed steel for free-cutting steel is manufactured by the continuous casting method to rationalize the manufacturing process and provide a stable product in terms of quality.

However, since ordinary continuous cast steel has less inclusions than killed steel and is not suitable for machinability, it is generally necessary to add a special element that improves machinability. Steels containing elements are difficult to apply to other applications. In other words, today's products tend to have a large variety of small lots and receive many small orders, so the remaining amount of casting tends to increase.

For example, when adding Pb (lead) element, etc., it is highly toxic,
It is dangerous because it affects the human body, and at the same time, installing protective equipment requires a very high cost. On the other hand, special alloy elements are expensive and are inevitably directly linked to cost increase.

An object of the present invention is to provide a method for producing a thin sheet free-cutting steel by a production process of continuous casting and hot rolling without adding a special element for imparting free-cutting property.

(Means for Solving the Problems) As a result of intensive studies by the present inventors in order to achieve the above object, as a result of improving the machinability, C, Mn, Si, S, Al,
Controlling 6 elements of P and hot rolling condition, tensile strength 30-50kg / m
The present invention has been completed by finding that it is necessary to set the perlite area ratio within 10 m ² within 10 m ² and the size per perlite within 5 to 30 μm.

That is, the present invention, by weight%, C: 0.05% or more and less than 0.20%, Si: 0.30% or less, Mn: 0.15 to 0.70%, sol.Al: 0.005 to 0.080%, S: 0.030% or less, P: 0.050 % Or less, more preferably N: 0.0150% or less, and continuously casting steel having a composition consisting essentially of Fe for the rest, and finishing temperature: 800 to 95
0 ° C., coiling temperature: hot rolling under the conditions of 500 to 700 ° C., a tensile strength size of 10-20% pearlite area ratio and per pearlite in 30~50kgf / mm ² is to tissue 5~30μ This is a method for producing thin plate free-cutting steel by pearlite dispersion.

Here, the thin plate free-cutting steel is not particularly limited in its dimensions as long as it is a thin plate manufactured by hot rolling, but as an example, 1.
It is a thin plate free-cutting steel with a thickness of 2 to 16.0 mm.

As described above, according to the present invention, the desired tensile strength and pearlite area ratio are controlled by adjusting the steel composition, finishing during hot rolling, and changing the winding temperature, and a free-cutting steel sheet of ferrite + pearlite structure is obtained. It can be manufactured.
Moreover, when it is manufactured by continuous casting, and further by so-called direct rolling, the profit is further improved.

In addition, as products manufactured from such thin plate free-cutting steel through cutting or cutting + forming, a retainer,
A yoke etc. are illustrated.

(Operation) Next, the reasons for limiting the constituent features of the present invention are as follows.

(1) Component range C is limited to the range of 0.05% or more and less than 0.20%, but this is added to control the pearlite composition and tensile strength. If it is less than 0.05%, the strength is insufficient and the ductility is increased. Therefore, the cutting resistance becomes large. On the other hand, if it is 0.20% or more, the strength becomes excessive, the cutting resistance becomes large, and the life of the cutting edge is shortened. Preferably, it is within the range of 0.06 to 0.18%. Si may be added at 0.30% or less. This is because by adding Si, the amount of alumina (Al ₂ O ₃ ) generated by Al added for deoxidation of molten steel can be reduced, and the wear of the cutting edge life can be suppressed. It tends to be finely dispersed. On the other hand, if it exceeds 0.30%, the effect is saturated. It is preferably 0.25% or less.

Mn is added in an amount of 0.15 to 0.70%, preferably 0.25 to 0.60%, for the purpose of controlling tensile strength and ensuring toughness. If it is less than 0.15%, the strength is insufficient and red hot brittleness due to S is observed. However, if it exceeds 0.70%, the toughness deteriorates.

The more S is made of non-metallic inclusions of MnS, the more advantageous it is from the viewpoint of cutting, but on the other hand, when it is used for pressing, there is a high risk of cracking due to inclusions, and 0.030%.
Is the upper limit.

Al only needs to be present in an amount necessary for deoxidation, and in general,
It is 0.005 to 0.080%, and if it is less than this, deoxidation is insufficient, and if it is more than this, Al ₂ O ₃ is generated and cutting failure occurs, and wear of the blade becomes more remarkable. Preferably 0.010-0.030
%.

P tends to finely disperse pearlite and is preferably added in a large amount. However, since the toughness of steel decreases, the upper limit is preferably 0.050%.

N is not particularly limited, but is preferably limited to 0.015% or less. N is also effective as an inclusion, but it is lowered in general applications because it deteriorates the aging property. Preferably, it is limited to 0.015% or less.

(2) Rolling finish strength and winding strength The tensile strength and the pearlite structure are limited according to the specifications of the intended use, but basically, the winding temperature is made higher, the cooling rate is slowed down, and ferrite + It has a pearlite structure and does not develop a disadvantageous bainite structure because it is too hard for machinability.

As with ordinary hot rolling, the finishing temperature is basically above the transformation point and above 800 ° C. The upper limit is set to 950 ° C, but if it is finished at a temperature higher than this, pearlite will grow and aggregate, and a fine pearlite dispersion structure will not be formed.

The finishing temperature is 800 to 950 ° C., preferably 830 to 900 ° C. By setting the finishing temperature in this range, rolling and pearlite dispersion control in the γ region are performed.

The winding temperature is basically adjusted to create a ferrite-pearlite structure. Below 500 ° C, some bainite is generated, so the lower limit was made 500 ° C. If it exceeds 700 ° C, grain growth proceeds and pearlite becomes coarse. Further, since there is a portion where the ferrite grains have an abnormal graining loss, the workability is deteriorated. Therefore, the upper limit is 700 ° C.

Thus, the winding temperature is 500 to 700 ° C, preferably 540 to 600 ° C. By limiting the content to such a range, the tensile strength is controlled, and the area ratio and size of the pearlite are controlled to print the homogenization within the structure, thereby forming a ferrite-perlite structure.

Thus, according to the present invention, the combination of the above steel composition and hot rolling controls tensile strength and pearlite area ratio,
Free-cutting steel with excellent machinability and excellent in punching and forming can be manufactured.

The free-cutting steel produced according to the present invention is limited to 10 to 20% pearlite in area ratio, but exceeds 20% and has too much pearlite (a large amount of C, high tensile strength) and a cutting property of a blade. The wear is fast and the adverse effect is produced.On the other hand, if the amount of pearlite is less than 10%, the material becomes too soft, and the cutting resistance is increased to make the built-up cutting edge during cutting, or the cutting edge is cut off and the cutting edge life is increased. It becomes extremely short.

Therefore, in the present invention, the pearlite area ratio is 10 to 20.
Limited to%.

The size per pearlite is limited to 5-30μ,
This is to ensure machinability comparable to that of rimmed steel.

In addition, the tensile strength is limited to 30 to 50 kgf / mm ² , which is to ensure sufficient machinability by suppressing excessive ductility and extending the cutting edge life due to hardness. As described above, the tensile strength is adjusted by the combination of the steel composition such as C content and the hot rolling conditions.

The present invention will be described more specifically by way of examples.

(Example) A steel having a composition shown in Table 1 was made into a continuously cast slab by a conventional continuous casting method, and hot rolling was performed under the same conditions as shown in Table 1 to obtain a thin steel plate having a thickness of 6.0 mm.

In the table, the pearlite area ratio is obtained by the pearlite ratio within a 100 mm ² field of view in an average composition portion in a 100 × microphotograph.

The thin steel sheet thus obtained was evaluated for machinability, and the evaluation procedure was as follows.

1. Burr height: Drill a 3mmφ hole and after a certain amount of work (eg 2500
After cutting each piece, the height of the burr attached to the outlet of the hole was calculated by averaging N 10 pieces.

Evaluation 0 to 0.30mm = 5 points 0.31 to 0.40mm = 4 points 0.41 to 0.50mm = 3 points 0.51 to 0.60mm = 2 points 0.61mm or more = 1 point 2. Roughness (Ra): Drill with a drill The cutting surface was measured in the direction of travel of the drill for a length of 4 mm. R at the start of drilling
The difference between a and Ra after cutting a certain amount was obtained.

Evaluation ◎ 5 points 2.01 μm or more ○ 4 points 1.51 to 2.0 μm △ 3 points 1.01 to 1.50 μm × 2 points 0.51 to 1.0 μm × × 1 point Within 0.5 μm 3. Wear: A new drill is prepared for each steel type and is constant After making a measuring hole, the drill blade edge was enlarged for comparative evaluation.

◎ There is almost no wear 5 points ○ Part of the cutting edge is missing 4 points △ Most of the cutting edge is missing 3 points × The cutting edge is shabby but still usable 2 points × × The entire cutting edge is shabby and unusable 1 point 4. Chip shape: Compare the shape of the chip at the start of cutting and the shape of the chip after cutting a certain amount.

1. Is there no seizure? 2. Is the chip winding shape constant or changing? 3. Is the chip long or short? ◎ 5 points Chip shape is constant ○ 4 points Chip shape is constant but long △ 3 points shape No. x 2 points Different shapes, long xx 1 points Different shapes, with seizure The results are summarized in Table 1 together with those of the comparative examples.

As can be seen from the results, according to the present invention, excellent machinability comparable to that of the conventional rimmed steel is secured.

Example 2 This example uses a series of steels with the compositions shown in Table 2
After heating to ℃, hot rolling, finishing temperature 890 ℃,
A hot rolled sheet having a sheet thickness of 6.0 mm was obtained at a winding temperature of 570 ° C.

Test pieces were cut out from these hot-rolled thin steel sheets and a machinability test was conducted. The test procedure was the same as in Example 1.

1 to 4 are graphs showing the results of the test of this example.

FIG. 1 shows the relationship between carbon content and flash height. The flash height indicates the amount of flash remaining at the portion where the cutting blade is exposed when cutting from a certain direction. The smaller the flash height, the better the machinability.

FIG. 2 is a graph showing the relationship between the size of each pearlite grain and the flash height.

In any case, if the allowable limit of the flash height is 0.50 mm, the size per pearlite grain is within the allowable limit of 5 μ or more and 30 μ or less, and it is clear that the machinability comparable to that of conventional rimmed steel can be secured.

FIG. 3 is a graph showing the influence of the pearlite area ratio and tensile strength on the amount of cutting wear. According to the present invention, it is understood that the free-cutting property comparable to that of the conventional rimmed steel can be secured.

(Effects of the Invention) Since the present invention is configured as described above, the following effects can be achieved.

(1) Since a steel plate with excellent machinability can be produced by the continuous casting method, there are the following merits.

Omission of various costs for ingot molds No need for large cranes No need for slabbing Continuous casting → Direct hot rolling can improve fuel consumption and labor saving (2) Merits for quality improvement The difference in strength level between the front and rear ends of the steel sheet is reduced. To do.

Reduction of surface flaws that occur from ingot to smashing Good cleanliness of inclusions (3) Low cost because it does not contain special alloying elements, and if not used or extra, the scope of application of allocation is expanded. .

(4) A steel sheet excellent in both machinability and formability can be obtained.

[Brief description of drawings]

FIG. 1 is an explanatory diagram of the relationship between the C amount and the flash height. FIG. 2 is an explanatory diagram of the relationship between the size of one pearlite grain and the flash height, and the flash height indicates the amount of flash remaining at the portion where the cutting blade comes out when cutting from a certain direction. The smaller the flash height, the better the machinability. FIG. 3 is an explanatory diagram showing a cutting blade wear condition as seen in the TS-pearlite area ratio.

Claims (1)

[Claims] 1. By weight%, C: 0.05% or more and less than 0.20%, Si: 0.30% or less, Mn: 0.15 to 0.70%, sol.Al: 0.005 to 0.080%, S: 0.030% or less, P: 0.050% Below, a balance of steel consisting essentially of Fe was continuously cast to a finishing temperature of 800-950.
℃, take-up temperature 500 ~ 700 ℃ hot rolling, the tensile strength
A method for producing thin plate free-cutting steel by pearlite dispersion, which has a structure in which the area ratio of pearlite is 30 to 50 kgf / mm ² and 10 to 20% and the size of each pearlite is 5 to 30 μm.