JP2016204716A - Manufacturing method of high carbon steel strip excellent in processability and heat treatment property - Google Patents

Abstract

PROBLEM TO BE SOLVED: To manufacture high carbon steel strip excellent in processability and heat treatment property at good efficiency.SOLUTION: A steel containing, by mass%, C:0.15 to 1.0% is hot rolled in an austenite range to prepare a steel strip S, winding the steel strip S at a temperature of 550°C or less to prepare a coil 2, then the coil 2 is unwound with heat treating the unwound steel strip S to 700°C or less at average heating rate of 50°C/s or more by using a heating device 1 again and the steel strip S is wound to prepare a coil 2'.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a method for producing a high carbon steel strip excellent in workability and heat treatment.

The high carbon steel strip is used as a material for automobile drive system parts and other machine parts.
These parts are manufactured using a high carbon steel strip as a raw material, through processing steps such as punching, bending, pressing and cutting, and quenching, tempering and other heat treatment steps. In order to improve the quality of parts, increase the efficiency of manufacturing parts, and reduce costs, soft and good workability and heat treatment characteristics that can easily provide strength after heat treatment are required.

Here, in order to obtain good workability, that is, a soft steel strip, the steel strip wound up in a coil shape after the end of hot rolling is usually subjected to box annealing.
Since the high carbon steel strip contains carbon in a high concentration, the steel strip after the hot rolling has a hard structure generally made of ferrite and pearlite. For this reason, in order to produce a steel strip that is soft and has good workability, usually, spheroidizing annealing is performed in which cementite is spheroidized to make it soft.

Since this spheroidizing annealing requires a long-time heat treatment, it is difficult to cope with the continuous annealing method, and coil box annealing is still employed.
In addition, since the continuous annealing furnace has a high equipment cost and is premised on continuous operation, coil box annealing is also adopted for small-lot products.

  Coil box annealing for the purpose of spheroidization is a radiant heating that heats the coil by heat transfer, that is, indirectly heated from the outside of the coil, so it takes a long time to heat the inside of the coil. There are drawbacks in that the heating cost is high and the temperature in the coil tends to be non-uniform.

  Here, as disclosed in Patent Documents 1 and 2, for example, many techniques for performing box annealing on high carbon steel have been developed for the purpose of improving workability.

JP 2011-012317 A JP 09-157758 A

As described above, the box annealing technique for imparting good workability (softening) to the high carbon steel strip has been developed in various ways.
However, in recent years, when manufacturing high-carbon steel strips, which are materials for automobile parts and machine parts, it is desired to further increase the strength and improve the manufacturability. However, technologies that satisfy all these requirements have not yet been developed. There is no current situation.
In other words, so far, heat treatment that can easily obtain strength after heat treatment has been studied from the viewpoint of steel components, but technology for improving heat treatment by a steel strip manufacturing method has not yet been studied. However, development of a technique capable of efficiently producing a high carbon steel strip that can achieve both workability and heat treatment is desired.

  This invention is made | formed in view of the said problem, and provides the technique which can manufacture the high carbon steel strip excellent in workability and heat processing property with high productivity.

  The gist of the present invention is as follows.

[1] Steel containing, by mass%, C: 0.15 to 1.0%, is hot rolled in the austenite region to form a steel strip, and then the steel strip is wound at a temperature of 550 ° C. or lower. Then, while unwinding the coil, the unrolled steel strip is reheated to 700 ° C. or less at an average heating rate of 50 ° C./s or more, and the steel strip is wound again. A high carbon steel strip manufacturing method with excellent workability and heat treatment.
[2] Production of a high carbon steel strip excellent in workability and heat treatment according to the above [1], wherein after the reheating treatment and the steel strip is wound up again, box annealing is performed. Method.
[3] When performing the reheating treatment, the distribution of the reheating temperature in the longitudinal direction of the steel strip so that the outer peripheral side of the coil wound after the reheating treatment has a higher temperature distribution than the inner peripheral side. The method for producing a high carbon steel strip excellent in workability and heat treatment properties according to the above [1] or [2].
[4] When performing the reheating treatment, when the steel strip after the reheating treatment is wound into a coil shape, the temperature at both ends in the width direction of the steel strip is higher than the temperature at the center of the width of the steel strip. A reheating temperature distribution is provided in the width direction of the steel strip so as to have a higher temperature distribution, and the workability and heat treatment properties according to any one of the above [1] to [3] Of excellent high carbon steel strip.

  According to the present invention, compared with the conventional annealing method, the time required for the heat treatment can be reduced in a short time and at a low cost, and a high carbon steel strip excellent in workability and heat treatment can be produced with high productivity. it can.

FIG. 1 is a schematic diagram showing an embodiment of reheating a steel strip in the present invention. FIG. 2 is a conceptual schematic diagram showing an embodiment in which box annealing is performed after reheating a steel strip in the present invention.

  Hereinafter, an embodiment of a method for producing a high carbon steel strip according to the present invention will be described with reference to the drawings. In the drawings, the shape, size, dimensions, etc. of the illustrated members are the dimensions of the actual members, etc. May be different.

FIG. 1 shows a process of the present embodiment in the case where a steel strip is once wound after the hot rolling process is completed, and then a steel strip coil is re-developed, reheated with a heating device 1 under predetermined conditions, and wound again. It is a conceptual schematic diagram to show.
As shown in FIG. 1, in this embodiment, C-containing steel is hot-rolled in an austenite region to form a steel strip S, and this steel strip S is used as a winding coil 2 at a temperature of 550 ° C. or lower. While the coil 2 is unwound by the unwinding device 3, the unrolled steel strip S is reheated to 700 ° C or less by the heating device 1 at an average heating rate of 50 ° C / s or more, and the steel strip S is taken up again. The device 4 is wound into a coil shape to form a coil 2 '.

  Hot rolling is performed in the austenite region, and after the hot rolling process is completed, when the steel strip is wound in a coil shape, the steel strip is cooled so that the winding temperature is 550 ° C or lower.

  The hot rolling in the austenite region is a condition for obtaining a structure mainly composed of bainite or martensite after winding. When ferrite is precipitated at a low hot rolling temperature, carbon is concentrated to austenite, and a structure (bainite or martensite structure) in which carbon is uniformly dispersed cannot be obtained.

A coiling temperature of 550 ° C. or lower prevents the formation of a structure in which the dispersion of carbon composed of ferrite and pearlite is not uniform, and a structure in which carbon mainly composed of bainite or martensite is uniformly dispersed (uniform structure) To get.
The coiling temperature is preferably as low as possible from the viewpoint of uniform structure. The structure in which carbon is uniformly dispersed does not concentrate the alloy element in the carbide, and the carbide is rapidly dissolved during the heat treatment after the part is formed. In particular, in heat treatment with a short heating time such as induction hardening, excellent heat treatment properties are exhibited.

  There is no particular limitation on the standing time until the coil is unwound and reheated after winding at 550 ° C. or less, but softening will fade even if reheated from a state where a large amount of untransformed austenite remains at a high temperature. It is preferable to unwind and reheat after the coil has been cooled to 300 ° C. or lower.

At the time of reheating the steel strip coil, the coil 2 is unwound by the unwinding device 3, and the unrolled steel strip S is heated by the heating device 1 at an average heating rate of 50 ° C./s or higher at 700 ° C. or lower. It is reheated to a temperature, and is wound again into a coiled coil 2 ′ by the winding device 4.
In order to make the heating device (reheating facility) 1 compact and to improve the productivity of reheating, it is preferable that the average heating rate during reheating is higher. If the average heating rate is less than 50 ° C./s, the heating zone length becomes long, or the productivity is lowered and it is not economical, so the average heating rate is set to 50 ° C./s or more. The upper limit of the average heating rate is not particularly limited and may be appropriately determined depending on the heating equipment used, but can be set to 500 ° C./s or less in terms of equipment capacity.
The heating device 1 is preferably an electric heating means such as energization heating or induction heating capable of rapid heating and uniform heating.

By this reheating process, the structure mainly composed of bainite and martensite is tempered, the steel strip is softened, and a steel material having a hardness suitable for press molding and forging is obtained.
When the reheating temperature exceeds 700 ° C., austenite appears, pearlite is precipitated by subsequent cooling, carbon non-uniformity occurs, and the heat treatment property decreases, so the upper limit of the reheating temperature is 700 ° C. The lower limit of the reheating temperature is not particularly limited, but is preferably 380 ° C. or higher from the viewpoint of sufficiently softening.

  When the reheating temperature is high (close to the upper limit temperature), tempering proceeds sufficiently before rewinding, so cooling may be performed before rewinding. It is possible to simplify the winding device 4 from the viewpoint of heat resistance by actively cooling after winding again before winding again.

In addition, if the temperature of the steel strip coil 2 'after rewinding is low, the steel strip coil 2' can be sent to the next step such as pickling without waiting for the steel strip coil 2 'to cool, and the coil stays between the steps. The amount can be reduced and the productivity is improved.
In this way, productivity is improved by lowering the temperature of the steel strip coil 2 ′ after rewinding, and increasing the reheating temperature increases the size of the heating equipment, so the reheating temperature is kept low. However, if the reheating temperature is low, it takes time to soften, and therefore the amount of tempering becomes too small when the coil 2 'is naturally cooled after rewinding.
In that case, it is preferable to perform box annealing (see FIG. 2) and to increase the tempering time while the coil 2 ′ remains at a high temperature after the reheating step and the winding coil 2 ′ again.
Even when box annealing is performed, the steel strip coil 2 'is preheated in advance, so unlike normal box annealing, there is no need to raise the temperature in the box annealing furnace, thus shortening the box annealing time. it can.

In the present invention, after reheating, the coil is cooled in a coiled state, so that the outer periphery of the coil is cooled more rapidly, and uneven tempering occurs in the longitudinal direction of the steel strip. There is a fear.
Therefore, if necessary, a reheating temperature distribution can be provided in the longitudinal direction of the steel strip, and the cooling condition of the steel strip in the finished winding state can be made uniform on the inner and outer circumferences of the coil.
That is, when reheating with the heating apparatus 1 while unwinding the coil 2, it is preferable to provide a temperature distribution so that the reheating temperature on the tail end side is higher than the front end side of the steel strip. By making the steel strip to which such a temperature distribution is provided as the winding coil 2 ′ again, the temperature deviation between the outer peripheral side and the inner peripheral side of the coil 2 ′ can be eliminated.

In addition, when the coil 2 'is used, both ends of the steel strip are easily cooled, and uneven tempering may occur in the width direction of the steel strip. Therefore, if necessary, the reheating temperature distribution in the width direction of the steel strip The cooling conditions in the steel strip width direction after forming the coil 2 ′ can be made uniform.
That is, when reheating with the heating device 1 while unwinding the coil 2, a temperature distribution is given so that the reheating temperature at both ends in the width direction of the steel strip is higher than the reheating temperature at the center in the width direction of the steel strip. It is preferable to do. By making the steel strip to which such a temperature distribution is provided as the winding coil 2 ′ again, the cooling condition in the width direction of the coil 2 ′ can be made uniform.

  Even in the case of spheroidizing annealing by conventional box annealing, the heating distribution and cooling distribution in the coil longitudinal direction and width direction affect the softening, but in the case of softening by tempering (reheating process) in the present invention, the temperature is particularly high. Since it is easily affected by the history, adjusting the reheating temperature distribution is important for obtaining a uniform tempering strength.

  The steel strip after hot rolling is generally subjected to removal of oxide scale by pickling. In the present invention, since surface oxidation or bluing occurs in the reheating step, it is preferable not to perform pickling after hot rolling but to perform pickling after reheating.

  When manufacturing a steel strip having a thin plate thickness, the steel strip manufactured according to the present invention may be further cold-rolled. In addition, in order to prevent oxidation scale in the heat treatment during component manufacturing, various hot dip plating such as hot dip galvanization, alloyed hot dip galvanization, hot dip aluminum plating, Zn, Zn-Ni, etc. Electroplating such as Zn-Fe may be performed. When heating is involved in hot dipping, the maximum heating temperature is preferably set to 700 ° C. or lower.

  Here, the steel strip which this invention makes object contains C 0.15-1%. When the amount of carbon is large, it becomes too hard if it is cooled to a coiling temperature of 550 ° C. or less after hot rolling, and winding becomes difficult or the steel strip breaks, so the upper limit of the C amount is 1%. And On the other hand, when the amount of carbon is small, the amount of carbide is also small, and the dissolution of carbide in the heating of the component heat treatment is rapid, so the effect of the present invention is small. The lower limit of the amount of C targeted by the present invention is 0.15%.

The other component elements of the steel strip targeted by the present invention are not particularly limited and may be appropriately determined as long as the effects of the present invention are not impaired. From the viewpoint of ensuring workability and heat treatment, Examples of the component composition are as follows.
In the following description, “%” represents mass% unless otherwise specified.

Si: 0.05-1.0%
Si acts as a deoxidizer, improves scale adhesion, and is an effective element for improving hardenability. If it is less than 0.05%, the scale easily peels off during hot rolling, so the lower limit is preferably made 0.05%. On the other hand, if it exceeds 1.0%, the Ac3 transformation point increases and the heat treatment property may be deteriorated, so the upper limit is preferably made 1.0%.

Mn: 0.2 to 3.0%
Mn acts as a deoxidizer and is an element effective for improving hardenability. If it is less than 0.2%, the effect of addition cannot be obtained, so the lower limit is preferably made 0.2%. On the other hand, if it exceeds 3.0%, the hot rolling load increases and the hardenability may be saturated, so the upper limit is preferably made 3.0%.

P: 0.005 to 0.10% or less P is a solid solution strengthening element and is an element effective for the strength of the steel strip. Since excessive inclusion inhibits toughness, the upper limit is preferably made 0.10%. Since reduction to less than 0.005% causes an increase in refining costs, the lower limit is preferably made 0.005%.

S: 0.0005 to 0.010%
Since S forms non-metallic inclusions and becomes a cause of hindering workability and toughness after heat treatment, the upper limit is preferably made 0.010%. Since reduction to less than 0.0005% causes a significant increase in refining costs, the lower limit is preferably made 0.0005%.

Al: 0.005-0.50%
Al is an element that acts as a deoxidizer. If it is less than 0.005%, the effect of addition cannot be obtained sufficiently, so the lower limit is preferably made 0.005%. On the other hand, if it exceeds 0.50%, the Ac3 transformation point increases and the heat treatment properties deteriorate, so the upper limit is preferably made 0.50%.

B: 0.0003 to 0.0050%
B is an element that improves hardenability and may be added as necessary depending on the use of the part. However, the effect is not sufficient at 0.0003% or less, so the lower limit is preferably made 0.0003%. . On the other hand, if it exceeds 0.0050%, the effect of addition is saturated, and precipitates are formed to deteriorate toughness. Further, there is a risk of surface cracking of the slab, so the upper limit is preferably made 0.0050%. .

In the present embodiment, in order to strengthen the mechanical properties of the steel strip, one or more of Cr, Ni, Cu, and Mo, one or more of Nb, V, Ti, and W, Ca and REM may be added.
In addition, when scrap is used as the raw material for the steel strip, one or more of Sn, Sb, and As may be inevitably mixed, but the effect of the present invention is sufficient if both are extremely small amounts. Does not disturb.
In addition, when scrap is used as a raw material for melting the steel strip, elements such as Zn and Zr are mixed as inevitable impurities. However, in the steel strip according to this embodiment, the effects and characteristics of the present invention are not impaired. In the range, mixing of the above elements is allowed. In addition, elements other than Zn, Zr and the like are allowed to be mixed as long as the characteristics of the steel sheet of the present invention are not impaired.

  In the present embodiment, the balance other than the above-described elements is substantially made of Fe, and a small amount of elements that do not impair the effects of the present invention, such as inevitable impurities, can be added.

  According to the manufacturing method of the high carbon steel strip according to the present invention described above, the time required for the heat treatment is short and low in cost as compared with the conventional annealing method (for example, box annealing), and the workability. High carbon steel strips with excellent heat treatment properties can be manufactured with high productivity. As a result, when processing the high carbon steel strip of the present invention, secondary processing such as press molding and forging can be easily performed, and heat treatment such as subsequent quenching is excellent, and formability and high strength are also achieved. Therefore, it is possible to provide a steel material that is suitable for automobile parts and the like that are required.

  EXAMPLES Next, although an Example demonstrates this invention still in detail, this invention is not limited to the conditions used in the following Examples.

Example 1
First, in mass%, C: 0.20%, Si: 0.25, Mn: 1.3%, P: 0.017, S: 0.003, Al: 0.04%, Ti: 0.027 , B: Steel containing 0.002% was melted in a laboratory, made into a steel plate having a thickness of 2.0 mm by hot rolling, and then reheated.

  In hot rolling, finish rolling was completed at 850 ° C., which is an austenite region, and then cooled to the winding temperature shown in Table 1 by water spray, and then gradually cooled in a furnace maintained at the winding temperature. In the cooling of the hot-rolled steel sheet, assuming that the steel strip coil is formed in the actual production line and then cooled in the air, the cooling rate of this slow cooling is 20 ° C / h, and the steel strip in actual production is The speed at which the coil is cooled in the atmosphere was simulated.

  The reheating treatment after the slow cooling described above was simulated using an electric heating device. Specifically, by directly energizing the cut plate, it was heated to the reheating temperature shown in Table 1 at an average heating rate of 100 ° C./s, and then gradually cooled in a furnace maintained at the reheating temperature. . The cooling rate of slow cooling was 20 ° C./h. In Test No. 1-1, no reheating treatment was performed.

The heat treatment property of the obtained steel sheet was evaluated.
The heat treatment property was evaluated using the Vickers hardness at the center of the thickness of the steel sheet after reheating with an electric heating type heating device and gradually cooling. Specifically, in the temperature history of heat treatment evaluation, the temperature was increased to 850 ° C. at a temperature increase rate of 100 ° C./s, and then cooled to room temperature at a cooling rate of 30 ° C./s to measure the Vickers hardness.
Table 1 shows the combination of the coiling temperature and the reheating temperature at which the above tests were performed, and the Vickers hardness before (after heat treatment evaluation) and after (after heat treatment evaluation) the heat treatment evaluation test.
In addition, evaluation of heat processing property was determined as follows.
<Before heat treatment evaluation>
A: HV <200
○: 200 ≦ HV <300
Δ: 300 ≦ HV <400
×: 400 <HV
<After heat treatment evaluation>
○: HV ≧ 400
×: 400> HV

Test number 1-1 in which reheating treatment was not performed is too hard before heat treatment evaluation and is not suitable for processing applications, but any of those subjected to reheating treatment has a Vickers hardness of less than 400 before heat treatment evaluation. It is softening.
The hardness after the heat treatment evaluation is the Vickers hardness after the heat treatment evaluation in Test Nos. 1-7 and 1-8 where the coiling temperature is higher than the range of the present invention and the Test No. 1-4 where the reheating temperature is higher than the range of the present invention Is less than 400 and the heat treatment property is not sufficiently improved.
Test numbers 1-2, 1-3, 1-5, and 1-6, in which the coiling temperature and the reheating temperature are within the range of the present invention, have low hardness before heat treatment evaluation, and hardness after heat treatment evaluation. Is high and is excellent in workability and heat treatment.

(Example 2)
First, in mass%, C: 0.20%, Si: 0.25, Mn: 1.3%, P: 0.017, S: 0.003, Al: 0.04%, Ti: 0.027 , B: Steel containing 0.002% was melted in a laboratory, made into a steel plate having a thickness of 2.0 mm by hot rolling, and then reheated.

  In hot rolling, finish rolling was completed at 850 ° C., which is an austenite region, and then cooled to the winding temperature shown in Table 2 by water spray, and then gradually cooled in a furnace maintained at the winding temperature. In the cooling of the hot-rolled steel sheet, assuming that the steel strip coil is formed in the actual production line and then cooled in the air, the cooling rate of this slow cooling is 20 ° C / h, and the steel strip in actual production is The speed at which the coil is cooled in the atmosphere was simulated.

  The reheating treatment after the slow cooling described above was simulated using an electric heating device. Specifically, by directly energizing the cut plate, it was heated to the reheating temperature shown in Table 2 at an average heating rate of 100 ° C./s. Thereafter, the steel sheet was moved to a furnace maintained at the reheating temperature, and gradually cooled after being maintained at the reheating temperature for 24 hours. Holding for 24 h simulated box annealing in actual production, and the cooling rate of slow cooling after holding for 24 h was 20 ° C./h.

The heat treatment property of the obtained steel sheet was evaluated.
The heat treatment was evaluated using the Vickers hardness at the center of the thickness of the steel sheet after heat treatment simulating box annealing with an electric heating type heating device. Specifically, in the temperature history of heat treatment evaluation, the temperature was increased to 850 ° C. at a temperature increase rate of 100 ° C./s, and then cooled to room temperature at a cooling rate of 30 ° C./s to measure the Vickers hardness.
Table 2 shows the combination of the coiling temperature and reheating temperature at which the above tests were performed, and the Vickers hardness before (after heat treatment evaluation) and after (after heat treatment evaluation) the heat treatment evaluation test.
The heat treatment was evaluated in the same manner as in Example 1.

In each of the present examples in which the holding for 24 h was performed after the reheating treatment, the Vickers hardness before the heat treatment evaluation was softened to less than 300, and a softer steel plate was produced than in Example 1 in which the holding for 24 h was not performed. is made of.
The hardness after heat treatment evaluation is the Vickers hardness after heat treatment evaluation in Test Nos. 2-7 and 2-6 where the reheating temperature is higher than the range of the present invention. Is less than 400 and the heat treatment property is not sufficiently improved.
Test numbers 2-1, 2-2, 2-4, and 2-5, in which the coiling temperature and the reheating temperature are within the range of the present invention, have low hardness before heat treatment evaluation, and hardness after heat treatment evaluation. Is high and is excellent in workability and heat treatment.

  According to the present invention, a high carbon steel strip excellent in workability can be produced with high productivity, secondary processing such as press molding and forging can be easily performed, and heat treatment such as subsequent quenching can be performed. It is excellent, and it is possible to provide a steel material suitable for automobile parts and the like that require formability and high strength.

1: Heating device 2, 2 ': Coil (steel strip coil)
3: Unwinding device 4: Winding device S: Steel strip

Claims (4)

  A steel containing C: 0.15 to 1.0% by mass is hot rolled in the austenite region to form a steel strip, and then this steel strip is used as a winding coil at a temperature of 550 ° C. or lower. The unrolled steel strip is reheated to 700 ° C. or less at an average heating rate of 50 ° C./s or more while unwinding the coil, and the workability is obtained by winding the steel strip again. And high carbon steel strip manufacturing method with excellent heat treatment.   The method for producing a high-carbon steel strip excellent in workability and heat-treatability according to claim 1, wherein box annealing is performed after the reheating treatment and the steel strip are wound up again.   When performing the reheating treatment, providing a reheating temperature distribution in the longitudinal direction of the steel strip so that the outer circumference side of the coil wound after the reheating treatment has a temperature distribution that is higher than the inner circumference side. The method for producing a high carbon steel strip excellent in workability and heat treatment properties according to claim 1 or 2.   When performing the reheating treatment, when the steel strip after the reheating treatment is wound in a coil shape, the temperature at both ends in the width direction of the steel strip becomes higher than the temperature at the center of the width of the steel strip. The high-carbon steel excellent in workability and heat-treatability according to any one of claims 1 to 3, wherein a reheating temperature distribution is provided in the width direction of the steel strip so as to have a temperature distribution. Manufacturing method of the belt.

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