JP3297788B2 - High carbon thin steel sheet excellent in hole expandability and secondary workability 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 applied to a high-strength machine structural part by performing a heat treatment such as quenching and tempering after forming into a required shape by press working such as punching, bending and drawing. And a method for producing the same.

[0002]

2. Description of the Related Art High carbon thin steel sheets that are punched, bent, stretched, drawn, pressed, etc., formed into a desired shape, and then tempered by heat treatment such as quenching and tempering are used as raw materials. Is required to have good workability and to easily obtain required performance during tempering. Generally, a steel sheet in an annealed state is softer and has a higher elongation as the carbon content is lower, and the workability is good. However, the carbon content cannot be reduced to secure the strength after the heat treatment. For this reason, when large processing is required for a high carbon thin steel sheet, spheroidizing annealing is performed to improve elongation and workability. In addition, users who process parts using high carbon steel sheets can simplify the processing process, press parts with more complex shapes by pressing, or extend the life of processing tools. In order to reduce cost by using the same carbon content, there is a demand for softer and more workable thin steel sheets.

On the other hand, as a method for improving workability by making the cementite softer than spheroidizing cementite with the same carbon content, there is graphitization of carbon in steel. This is because the state of carbon present in the steel is softer than that of hard cementite and softer than that of hard cementite, and the workability is better.

[0004] In order to sufficiently graphitize a thin steel sheet having the same carbon content, the amount of elements that promote graphitization such as Si, Al and Ni is increased, and cementite such as Mn, Cr and Mo is stabilized to graphitize. After minimizing the number of elements that hinder the heat treatment, cold working is performed if necessary, and annealing is performed for as long as possible at a temperature as high as the A ₁ transformation point or lower. In addition to these, in order to further facilitate graphitization, hot rolling conditions and annealing conditions are improved, and fine precipitates such as BN and Al ₂ O ₃ which become precipitation sites for graphitization are used. It has been proposed to do so.

[0005] Graphitization of carbon in steel certainly lowers the hardness as a high-carbon thin steel sheet and improves workability, but has some problems in practical use.

First, high carbon steel sheets are premised on tempering by quenching and tempering after forming, but Mn, Cr, Mo and the like, which are commonly used as hardenability improving elements, hinder graphitization and are often added. Can not. On the other hand, Si, which is a graphitization-promoting element, also improves quenching properties, but its addition is limited because it hardens the base and deteriorates the surface properties.
Has the same effect as Si, except that it hardly relates to hardenability. Ni is also an element preferable for graphitization and hardenability, but is expensive, and its effect is small unless the addition amount is large.

[0007] Japanese Patent Application Laid-Open No. 4-14011 discloses an example of a component which can be easily and practically graphitized in view of the characteristics of such alloy components. This is C: 0.
32 to 1.3%, Si: 0.41 to 1.5%, Mn: 0.04 to 0.4
%, Al: 0.02 to 1.00%, and if necessary, Ni: 1%
Hereinafter, steel containing a small amount of Mo, V, Ti, Zr, B or the like is annealed in a ferrite region after hot rolling or cold rolling, thereby almost completely graphitizing cementite.
In this case, the C content is larger than the general carbon steel material for mechanical structure, and the Mn content is low and the Si and Al are high, but the Mn content is low and the hardenability is insufficient, and the Si content is high. However, even if it is graphitized, its strength will not decrease and its surface properties will not be good, so its use will be limited.

Next, in order to obtain a quenching hardness commensurate with the carbon content of the steel at the time of tempering of the formed product, when heated to the austenite region, the graphitized carbon is sufficiently re-dissolved in the base material. I have to do it. Heat treatment for graphitization in the production of thin steel sheets, that is, holding at a high temperature for a long time in the ferrite region, causes cementite to coarsen, and the resulting graphite to also coarsen. Since the coarsened graphite particles take a long time to form a solid solution, they cannot be sufficiently solid-dissolved in a usual heating time for quenching, resulting in insufficient hardness during quenching. As a countermeasure, for example, in JP-A-64-25946, the finishing temperature of hot rolling is set to a low temperature of 800 ° C. or less and 500 ° C. or more, thereby minimizing graphite precipitation in the subsequent process and facilitating re-solid solution. The quenching hardness is not reduced.

In the example of JP-A-64-25946, compared with the example of JP-A-4-14011, the main component Si
Is approximately equivalent to 0.2 to 2.00%, whereas Mn is
Higher 0.2-1.5% and lower Al 0.001-0.15%. Graphitization is expected to be difficult, but lowering the finishing temperature of hot rolling promotes graphitization in subsequent processes. Let me.

Japanese Patent Application Laid-Open No. 2-107742 discloses that the hot rolling conditions are set as low as possible to promote the graphitization and the graphite grains are refined, and that the cold rolling is effective. . In this publication example, C is 0.15 to 0.1 as a main component.
1.5%, Mn is as wide as 0.05-1.00%, Si is 0.49%
There is no particular reference to the Al content. In addition to these main elements, composite addition of B and N for promoting graphitization, addition of Ni, Co, Cu, etc., and C for improving hardenability
The addition of r and Mo indicates that it is particularly important to have a structure in which the graphite particles at the time of graphitization are distributed at 100 particles / mm ² or more in the ferrite phase.

As described above, in order to improve the workability of high carbon steel by graphitization and maintain the heat treatment characteristics of the product after working, not only the selection of the component system but also the finishing temperature by hot rolling. Severe hot rolling conditions are required, such as reducing the temperature as much as possible and increasing the degree of work at that temperature.

[0012] In each of the examples described above, the workability becomes better as the graphitization progresses.
When a graphitized high-carbon thin steel sheet is applied to actual parts, the effect of softening by graphitization is clear, and general press workability such as bending, drawing, and overhanging is improved. However, in processing in which local elongation is dominant, such as hole expansion, or in secondary processing in which the side wall of the cup after drawing is further processed, the workability has not been improved to the extent of softening. Judging from hardness alone, the more the graphitization proceeds, the softer it becomes, but the hole expandability and secondary workability tend to worsen. The hole expanding process and the secondary process are processes frequently applied when a thin steel plate is pressed and used.

A similar result is disclosed in Japanese Patent Application Laid-Open No. 4-31546. This is because when graphite and spheroidized cementite coexist, graphitizing 1 to 80% and making the remaining spheroidized cementite more excellent in workability and hardenability than graphitizing all carbon. The product is obtained, that is, excessive graphitization impairs workability.

In this case, C: 0.10 to 0.8%, Mn: 0.05
-1.0% or 0.05-3.0%, and Al: 0.003
1.0% and B: at least one of 0.0003 to 0.010%, and if necessary, Si: 3% or less, Ni: 3% or less and C
u: Use steel containing 1% or less of 1% or less, 700 ~
After heating and holding at 900 ° C for 1 minute or more, annealing (box annealing) is performed for 1 hour or more in a temperature range of 500 to less than 700 ° C.

In so-called box annealing, which is usually performed with a thin steel sheet and is heated from the outside in a state of being wound on a coil, the temperature variation among the parts in the coil is large when the temperature is raised. Increase the heat time to deal with this. Therefore, if the above-described two-stage temperature annealing is performed in box annealing, the temperature variation between the coil portions or the difference in the temperature history is greatly increased, so that uniform performance over the entire length of the coil is achieved. In order to achieve this, careful management will be required.

As described above, it is known that graphitizing cementite from a high carbon thin steel sheet softens cementite and improves workability. However, in actual production, if the graphitization is to be facilitated on the component side, the material becomes too hard to obtain a sufficient effect or the hardenability after processing is deteriorated. In order to graphitize without changing the components, it is necessary to strictly control production conditions such as hot rolling conditions and annealing conditions. In addition, if graphitization is excessively performed, the material becomes soft, but the hole expandability and the secondary workability are deteriorated. Therefore, it must be controlled in an appropriate range.

As described above, a high carbon thin steel sheet having excellent workability and excellent properties after tempering, that is, cementite is moderately graphitized by paying particular attention to hole expandability and secondary workability. High carbon steel sheet with sufficient hardenability,
It is difficult to say that this is provided, and it is difficult to say that the production method thereof is well established.

[0018]

SUMMARY OF THE INVENTION The present invention addresses the problems associated with the realization of graphitization as described above, with respect to hot rolling and annealing, which are carried out under particularly demanding manufacturing conditions. Accordingly, the present invention provides a high-carbon thin steel sheet which achieves both graphitization and hardenability, and which is particularly excellent in workability such as hole expandability and secondary workability, and a method for producing the steel sheet.

[0019]

The present inventor investigated the press formability of high carbon thin steel sheets by changing the degree of graphitization of carbon in steel in various ways. Although it becomes soft, in the case of hole expansion processing or secondary processing in which further processing is performed on the wall after molding the cup, if it is too soft, the processing limit may be adversely deteriorated, and there is an appropriate graphitization range I knew

If the graphitization is insufficient, the hardness does not naturally decrease. Then, as the graphitization progresses, the hardness decreases and the elongation increases, and the workability improves. However, if the hardness is excessively increased, the hole expandability and the workability are increased even though the hardness decreases. Subsequent workability deteriorates.
This is because, in the case of severe processing involving local ductility such as hole expansion, cracks originating from graphite particles were observed, and it was considered that excessive graphite precipitation caused deterioration in workability.

First, in order to clearly understand the influence of graphitization, a method of extracting and quantifying graphite by acid dissolution in chemical analysis, and polishing the sample cross section with an optical microscope or a scanning electron microscope to observe the distribution of graphite particles. And the degree of graphitization were evaluated. As a result, the progress of graphitization by the evaluation of chemical analysis and the increase of graphite particles in the microscopic observation of the cross section show almost the same tendency, and the latter is more favorable with respect to workability, especially regarding the goodness of hole expanding workability. It turns out that there is a good response.

For the production of thin steel sheets, related equipment such as hot rolling equipment, descaling equipment such as pickling, cold rolling equipment, annealing equipment, and temper rolling equipment are used. Regarding the graphitization for the purpose of improving the workability of high-carbon thin steel sheets, consider the processing conditions of each process as much as possible to the extent that they do not place an excessive burden on those facilities, investigated. As a result, it was found that the combination of the addition of B and the addition of appropriate amounts of N, Al, and Cr was important.

As described above, it is effective to perform the hot rolling at a finishing temperature as low as possible to promote the graphitization. However, a decrease in the finishing temperature has many problems in hot rolling work such as deterioration of the plate shape and occurrence of surface flaws. Therefore, assuming that annealing is performed after hot rolling under normal rolling conditions, the use of nitride as graphite precipitation nuclei to promote graphitization was considered, and the result of adding N after adding B was considered. The effect of promoting graphitization was recognized.

In addition, when a large amount of Al, which is effective for graphitization, was added, graphitization was further promoted.

In this case, the precipitation of AlN is also increased due to the increased Al content in addition to the increased N content.
It is possible that not only BN but also AlN effectively acted to promote graphitization. In addition, by adding a large amount of Al, an effect of improving the hardenability in the heat treatment after molding was also recognized. This was considered to be because when graphite was re-dissolved by heating in the heat treatment, BN was decomposed in the steel and N was bonded to Al, and solute B effective for improving hardenability was increased.

In the target steel sheet, graphitization, which generally hardly occurs, can be easily caused by controlling the components and pre-annealing treatment conditions. It must be managed within a certain range. For graphitization requiring long-term soaking, a so-called box-shaped coil annealing furnace is usually used.In this case, when the coil becomes large, the heat history of the hottest point on the outer periphery of the coil and the coldest point inside the coil greatly differ. In addition, there is a risk that the progress of graphitization may vary greatly depending on the location of the coil.

In view of this, a method was studied in which the degree of graphitization could be obtained as uniformly as possible over the entire length and width with a coil having a heavy weight exceeding 10 tons when such a box annealing furnace was used. As a result, it was found that by adding a small amount of Cr, graphitization in a portion where the soaking time of the hottest portion of the coil was long was suppressed.

As described above, it can be understood that a steel sheet having excellent workability, particularly excellent hole expandability and secondary workability, and excellent hardenability can be obtained by limiting the components and the distribution of graphitized or graphite particles. Was. Furthermore, although the effects and interaction mechanisms of each of these elements have not always been fully clarified, regarding the control of graphitization or graphite particle distribution, by clarifying the addition range of each component and the control limits of the process conditions, Thus, the present invention was completed.

That is, the present invention relating to a high carbon thin steel sheet is as follows.

(1) As a steel component, C: 0.20 by weight
~ 0.60%, Si: 0.05 ~ 0.50%, Mn: 0.05 ~ 0.30%,
P: 0.02% or less, S: 0.01% or less, Cr: 0.02 to 0.05
%, Al: 0.05 to 0.50%, N: 0.003 to 0.015%, B: 5
~ 50 ppm, Ni: 2.0% or less, Ca: 0.01% or less,
The balance is substantially composed of Fe and unavoidable impurities, and in addition, 10 to 50% of the C content is graphitized, and the steel structure of the cross section is made up of graphite particles having a size of 3 μm or more by C weight% × 1.
A thin steel sheet excellent in hole expandability and secondary workability, characterized by being a ferrite phase in which spheroidized cementite is dispersed, containing not less than ² pieces / mm ^{2 and not} more than C weight% × 10 ³ pieces / mm ² .

(2) When the form, thickness, etc. of the thin steel sheet is a hot-rolled steel sheet, the manufacturing method is to hot-roll the steel containing each of the above components at a finishing temperature of 750 to 900 ° C. , 400-65
It is rolled in a temperature range of 0 ° C to form a hot rolled coil, and after descaling, it is annealed in a temperature range of 600 to 720 ° C. 10 to 50% of the C content is graphitized, and the cross section is Steel structure of spheroidized cementite and graphite particles of 3 μm or more in C weight% × 10 ² particles / mm ² or more C weight% × 10 ³
Hole / mm ² or less
And a thin steel sheet with excellent secondary workability.

(3) If the thin steel sheet is a cold-rolled steel sheet having a smaller thickness and excellent sheet thickness accuracy, the manufacturing method is as described in the above (1).
After forming a hot-rolled coil under the same hot rolling conditions and winding conditions as in (2), a steel having the same composition as in (2) is subjected to 40-80% cold rolling and annealing at 600-720 ° C. Characteristic: 10 to 50% of the C content is graphitized, and the steel structure of the cross section is spheroidized cementite and graphite particles of 3 μm or more in C weight% × 10 ² / mm ² or more C weight% It is a thin steel sheet which is a ferrite phase containing not more than × 10 ³ pieces / mm ² and has excellent hole expandability and secondary workability.

(4) In the case of a cold-rolled steel sheet having a thinner steel sheet with a smaller thickness and excellent sheet thickness accuracy, and particularly good drawability, the method of manufacturing the steel sheet is the same as that of the above (1). ) And the same hot rolling conditions as in the hot rolled coil under the winding conditions, after descaling, annealing in a temperature range of 600 to 720 ° C, and then 40 to 80
%, Characterized by being annealed at 600 to 720 ° C., wherein 10 to 50% of the C content is graphitized, and the steel structure of the cross section is spheroidized cementite and 3 μm in size. The above graphite particles are C weight% × 10 ² particles / mm ² or more C weight% × 1
0 is ^three / mm ² or less inclusive ferrite phase, hole expandability
And a thin steel sheet having excellent secondary workability.

[0034]

In the present invention, the reasons for limiting the range of graphitization and the distribution of graphite particles in the steel sheet, the alloy composition, and the manufacturing conditions thereof are as follows.

(1) Graphitization and Graphite Particles Graphitization significantly increases the softening and elongation of the tensile test after the amount of graphite exceeds 10% of the total amount of carbon contained in the steel sheet. is there. As the graphitization further progresses, the hardness decreases and the elongation increases. However, when examining the hole expandability and the secondary workability, if the graphite exceeds 50% of the total carbon content, the graphite deteriorates. In particular, the distribution of graphite particles affects the hole expandability and secondary workability,
Those having a graphitization ratio of 10 to 50% of the total amount of carbon and a cross-sectional microscopic observation of the precipitated graphite particles having a diameter of 3 µm or more are C weight% × 10 ² particles / mm ² or more. C weight%
Excellent results are obtained in the range of × 10 ³ pieces / mm ² or less.

If the graphitization is 10% or less, it is insufficient to improve the workability. However, if it exceeds 50%, in the case of severe processing involving local ductility such as hole expansion, the frequency of occurrence of cracks starting from graphite particles increases, and the processing limit decreases. Even if the degree of graphitization is the same, the reason why excellent results in hole expandability and secondary workability can be obtained when a suitable amount of graphite particles of 3 μm or more is related to the origin of cracking It seems, but not clear. On the other hand, during the graphitization annealing, the spheroidization of the precipitated cementite also proceeds at the same time. Therefore, the size of the graphite particles is also related to the degree of spheroidization, and the presence of graphite particles having a size of 3 μm or more in C weight% × 10 ² particles / mm ² or more indicates that the spheroidization of cementite is progressing. The workability may have improved.

There are many graphite particles having a size of 3 μm or less, but when they are small, there is almost no effect on workability. If it exceeds 20 μm, cracks may occur at least, but under the production conditions of the present invention, such large graphite particles do not occur.

As described above, since the steel sheet is a high carbon steel sheet having excellent hole expandability and secondary workability, it has a C content of 10%.
5050% is graphitized, and the steel structure of the cross section is spheroidized cementite and graphite particles of 3 μm or more in C weight% ×
The ferrite phase contains 10 ² / mm ² or more and C weight% × 10 ³ / mm ² or less.

The function of each component and the reason for limiting the range will be described below.

(2) C: 0.20 to 0.60% From the viewpoint of workability alone, the lower the C, the better. However, at least 0.20% or more is necessary for tempering to a required strength by heat treatment such as quenching and tempering. On the other hand, considering the tendency of graphitization and the need for the wear resistance of the final product, the larger the C, the better. However, in the present invention, to improve workability, and to compensate for the decrease in hardenability by lowering Mn by adding B as described below, if C is increased, the effect of improving the hardenability of B is lost. 0.60%.

(3) Si: 0.05 to 0.50% Since it is a component which promotes graphitization, it is preferably present in an amount of 0.05% or more. However, since the substrate becomes hard and the surface properties of the product deteriorate, the upper limit is 0.50%. I do.

(4) Mn: 0.05 to 0.30% In order to prevent hot embrittlement by S and hardenability, it is an essential element in steel to a certain extent. I want to go. If it exceeds 0.30%, graphitization is inhibited, and if it is less than 0.05%, S harm cannot be prevented.

(5) P: 0.02% or less P is an element that greatly inhibits graphitization, and the smaller the better, the better. The lower limit of the effect of the inhibition is not more than 0.02%, preferably not more than 0.005%.

(6) S: 0.01% or less Not only inhibits graphitization, but also deteriorates toughness after tempering. Therefore, the smaller the S, the better, and the upper limit is made 0.01%, but preferably 0.003% or less.

(7) Cr: 0.02 to 0.05% Cr is an element that inhibits graphitization, but can be effectively used when excessive graphitization is to be suppressed as in the present invention. In particular, in the case of box annealing of a coil, there is a variation in temperature history depending on a part in the coil, and it is effective to make the progress of graphitization as uniform as possible over the entire length of the coil. 0.02%
If it is less than the above, there is no effect, and if it is too much, graphitization does not occur, so the upper limit is made 0.05%.

(8) Al: 0.05-0.50% Al is necessary as a deoxidizing agent at the time of casting, and usually 0.01% or more is added to prevent surface flaws of high carbon steel slabs. However, since it also has the effect of promoting graphitization and improving the hardenability in combination with the addition of B, in the present invention, it is added in a larger amount than a normal deoxidizing agent, and the content is made 0.05% or more. However, if the content is too large, the base material becomes hard or the toughness of the product after heat treatment deteriorates. Therefore, the maximum content is limited to 0.50%.

(9) N: 0.003 to 0.015% Graphite is promoted by coexistence with B, and graphite particles are finely dispersed. In order to obtain a large accelerating effect even under ordinary hot rolling conditions, a minimum of 0.003% is required, and when Al is increased as in the present invention, it is preferably 0.005% or more. 0.015
If it exceeds%, elongation will be poor.

(10) B: 5 to 50 ppm B is added for the effect of promoting graphitization and ensuring the hardenability during tempering. If it is less than 5 ppm, there is no effect, and if it exceeds 50 ppm, the steel becomes brittle. Since the effect is reduced because N is increased, the lower limit is preferably large. Since the effect is saturated even if it is increased, the range of 10 to 30 ppm is preferable.

(11) Ni: 2.0% or less Graphitization is promoted and hardenability is improved, but need not be added. Since the substrate is not hardened as much as Si, if the hardenability is insufficient due to the shape of the molded part for final use, it is added as necessary. Since the effect is not so large, it is desirable to add 0.1% or more when adding. As the hardness increases, the limit is 2.0%.

(12) Ca: 0.01% or less It is not necessary to add Ca, but it has the effect of promoting graphitization and changes the form of sulfide to improve workability and toughness. Take advantage of. In that case,
The content is preferably 0.001% or more. However, 0.01%
Above this, inclusions increase.

Next, the conditions of the manufacturing process of the high carbon thin steel sheet of the present invention will be described.

(13) Hot Rolling Conditions The production of the steel sheet of the present invention can be carried out by the three methods as described above. The steps up to the step of performing descaling such as washing to form a coil are common to all methods.

First, the hot rolling and winding conditions are as follows. When the finishing temperature is 750 to 900 ° C. and the winding temperature is 400 to 650 ° C., the graphitization and graphite within the scope of the present invention are most effectively achieved in the subsequent steps. The distribution of the particles is obtained. Finishing temperature is 7
Below 50 ° C, there is a risk that graphitization will proceed too much,
When the temperature exceeds ℃, the graphite particles become too coarse. Winding temperature
If it is lower than 400 ° C, the hot rolled coil becomes too hard, and if it is higher than 650 ° C, graphitization is delayed. Therefore, the finishing temperature of hot rolling should be 750-900 ° C and the winding temperature should be 400-650 ° C.

(14) Annealing condition of hot rolled sheet The hot rolled sheet is annealed after removing scale by pickling or the like to prevent decarburization from the surface. If the temperature is lower than 600 ° C., the graphitization takes too long. If the temperature exceeds 720 ° C., the austenite phase may appear at a temperature exceeding the transformation temperature depending on the steel composition, and the graphitization is inhibited. The heating time at the annealing temperature is not particularly limited, but if it is less than 4 hours, it is insufficient, and if it exceeds 48 hours, there is a risk that graphitization becomes excessive.

(15) Reduction of Cold Rolling and Annealing after Rolling Cold rolling is applied when the thickness is required to be thin and strict. The rolling reduction of the cold rolling and the annealing conditions after the rolling slightly differ depending on whether or not the hot-rolled sheet annealing for graphitization has been performed in advance.

(A) When there is no hot-rolled sheet annealing Cementite in the hot-rolled sheet is broken and dispersed by cold rolling, and graphitization and spheroidization of the cementite are performed by subsequent annealing. In this case, if the rolling reduction is less than 40%, there is no effect in promoting graphitization due to insufficient processing, and rolling exceeding 80% causes cracks or becomes too hard to be rolled.

Annealing after rolling is performed at 600 ° C. or more and 720 ° C. or less. If the temperature is lower than 600 ° C, the graphitization does not proceed sufficiently. If the temperature exceeds 720 ° C, the austenite phase may appear beyond the transformation temperature, and the graphitization is inhibited. In this case, the annealing time is not particularly limited, but is preferably 4 to 48 hours as a condition for obtaining appropriate graphitization.

Therefore, when there is no hot rolled sheet annealing, the rolling reduction of the cold rolling is 40 to 80%, and the annealing temperature is 600 to 720 ° C.

(B) In the case of hot-rolled sheet annealing After the target graphitization was performed by hot-rolled sheet annealing,
Cold rolling is particularly necessary to improve drawability.
This is to develop a texture preferable for deep drawing during annealing. In cold rolling with a rolling reduction of less than 40%, development of texture is insufficient, and rolling in excess of 80% hardens and is difficult.

If the annealing temperature is too low or too high, the development of the texture becomes incomplete, so that the annealing temperature is 630 to 780 ° C. The annealing time is not particularly limited as long as the recrystallization texture of the annealing is sufficiently developed and the graphite does not re-dissolve in solution. The method of annealing may be a continuous annealing method or a box annealing method.

[0061]

【Example】

[Example 1] Steels of steel types 1 to 6 having compositions different in Cr content shown in the upper part of Table 1 were melted and hot-rolled into thin steel sheets having a thickness of 3.0 mm. In this case, hot rolling conditions were such that a slab having a thickness of 200 mm was soaked at a heating temperature of 1250 ° C. for 1 hour, and a finishing temperature of 850 ° C. and a winding temperature of 500 ° C. After descaling, annealing was performed at 680 ° C. for 36 hours.

After annealing, the graphitization rate was evaluated by chemical analysis, and the distribution of graphite particles was examined by microscopic observation of the plate cross section. Graphite particles with a size of 3 μm or more were measured. Table 1 also shows the results of the graphitization of each steel.

[0063]

[Table 1]

For evaluation of workability, hole expandability and secondary workability were investigated. The hole expandability is determined by expanding a 10 mm diameter cut hole (d ₀ ) with a conical punch having a vertex angle of 60 ° and measuring the critical hole diameter (d) at which no cracks occur around the hole. _{(d-d 0) / d} 0 × 100 (%) was determined holes spread rate as.

Regarding the secondary workability, when the side wall is worked after squeezing into a cup shape in actual molding, the ductility is poor and the material often cracks. Therefore, as a simple method, the reduction rate is 20
% Cold rolling, then 90 ° to the rolling direction
A JIS No. 13 B tensile test piece was cut out and subjected to a tensile test to measure elongation. There is a good correspondence between this secondary working elongation and the actual forming limit.

FIG. 1 shows the relationship between the amount of Cr and the number of graphite particles, the hole expansion ratio, and the elongation after secondary working. The larger the hole expansion ratio and the secondary working elongation, the better the workability. From this figure, the number of precipitated graphite particles increases when the Cr content is small,
It can be seen that the Cr content decreases as the Cr content increases. On the other hand, there is an optimum amount of graphite deposition for workability, and the graphite is deteriorated if it is too large or too small.

[Example 2] Steels of steel types 7 to 12 having different C contents shown in the lower part of Table 1 were melted and made into slabs having a thickness of 200 mm, and the finishing temperature was 850 ° C and the winding temperature was 500 ° C. Hot rolling was performed to form a thin steel plate having a thickness of 3 mm. After descaling, 6
After annealing at 80 ° C. for 36 hours, graphitization was evaluated in the same manner as in Example 1. The results are also shown in the lower part of Table 1.

Next, the hole expansion ratio and secondary working elongation were measured in the same manner as in Example 1. FIG. 2 shows the relationship between the amount of C, the number of graphite particles, and workability. As the amount of C increases, the number of graphite precipitation particles increases, and the hole expandability and the secondary workability deteriorate. The workability is better as the C content is lower, but when the C content is lower, the required hardness cannot be obtained even when quenched, deviating from the object of the present invention. If the amount of C is too large, even if it is graphitized, the hole spreading property and the secondary workability are not improved.

[Example 3] Steels shown in Table 2 in which various components were changed were melted under the same conditions as in Example 1 except that the C content was 0.33 to 0.43% and the Cr content was 0.02 to 0.04%. After hot rolling and descaling, annealing was performed, and the precipitation state of graphite particles, a tensile test, a hole expansion ratio, and a secondary working elongation were investigated.
Table 3 shows the results. As is clear from these results, when the chemical composition is within the range of the present invention, the graphitization ratio and the number of graphite grains easily fall within the target ranges of the present invention, and a steel sheet having excellent workability is obtained. Can be Components outside the range of the present invention have poor workability, and result in inferior secondary working elongation even if the hole expansion rate is high.

[0070]

[Table 2]

[0071]

[Table 3]

Example 4 Steel 3 shown in Table 1 of Example 1
Was used to investigate the effect of manufacturing conditions on hot-rolled thin steel sheets. In this case, the standard manufacturing conditions are as follows: slab heating temperature 1250 ° C, finishing temperature 850 ° C, winding temperature 500 ° C, annealing
36 hours at 680 ° C.

The effects of the finishing temperature, the winding temperature, and the annealing temperature on the number of graphite particles having a size of 3 μm or more, the hole expansion rate, and the secondary elongation rate are shown in FIGS. 3, 4, and 5, respectively.

In this case, the steps before and after the conditions other than the conditions under which the influence was investigated are set to standard conditions.

From this, it can be seen that under conditions where the number of graphite particles is properly controlled, a steel sheet exhibiting excellent workability can be obtained.

Example 5 Steel 3 shown in Table 1 of Example 1
The effect of the rolling reduction of the cold rolling in the following two types of processes was investigated using the thin plate after hot rolling.

(A) After hot rolling, cold rolling and annealing for graphitization.

(B) After graphitizing annealing in a hot rolled sheet,
Cold rolled and annealed.

The graphitizing annealing was performed at 680 ° C. for 24 hours after cold rolling and after hot rolling. Also,
Annealing after cold rolling of (b) was performed at 680 ° C. for 12 hours.
The change in the number of graphite grains of 3 μm or more, the hole expansion rate, and the secondary elongation rate due to the rolling reduction in cold rolling are shown in FIG.
FIG. 7 shows the case (b). As can be seen from FIG. 6, graphitization is promoted as the rolling reduction increases, but there is an appropriate range for obtaining an optimal graphite particle distribution.

In the case of FIG. 7, since the graphitization is performed before the cold rolling, even if the rolling reduction is low, the distribution of the graphite grains falls within an appropriate range, and the hole expansion rate and the secondary working elongation rate are also low. Good. However, when the rolling reduction is low, the target drawability is insufficient, and if the drawability is not required, it is not necessary to adopt this process. On the other hand, if the rolling reduction is too high, the graphitization after the cold rolling proceeds, and the graphitization becomes more than necessary.

[0081]

The thin steel sheet of the present invention is soft at the time of forming because the carbon in the steel is appropriately graphitized,
It is a steel sheet that can obtain required strength by heat treatment after processing, and is particularly excellent in hole expandability and secondary workability. Further, in hot rolling and annealing of this steel sheet, a desired thin steel sheet can be manufactured without requiring particularly severe conditions.

[Brief description of the drawings]

FIG. 1 shows an example of the influence of the amount of Cr on the number of graphite particles in a steel, the hole expansion ratio as an index of workability, and the elongation after working indicating secondary workability of a steel sheet annealed after hot rolling. FIG.

FIG. 2 is a diagram showing an example of the influence of the C amount.

FIG. 3 is a diagram showing an example of the influence of the annealing temperature after hot rolling.

FIG. 4 is a diagram showing an example of the influence of the finishing temperature of hot rolling.

FIG. 5 is a view showing an example of the influence of the winding temperature of hot rolling.

FIG. 6 shows the number of graphite particles in the steel, the hole expansion ratio as an index of workability, and the cold rolling reduction with respect to the elongation after working indicating secondary workability of the steel sheet subjected to cold rolling and annealing after hot rolling. It is a figure showing an example of the influence of a rate.

FIG. 7 shows the number of graphite particles in the steel, the hole expansion rate as an index of workability, and the elongation after working showing secondary workability of the steel sheet annealed after hot rolling and then subjected to cold rolling and further annealing. It is a figure which shows the example of the influence of the cold rolling reduction rate with respect to.

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-2-107742 (JP, A) JP-A-1-25946 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) C22C 38/00-38/60 C21D 8/00-8/04 C21D 9/46-9/48

Claims (4)

(57) [Claims] (1) C: 0.20 to 0.60% by weight, Si: 0.
05-0.50%, Mn: 0.05-0.30%, P: 0.02% or less,
S: 0.01% or less, Cr: 0.02-0.05%, Al: 0.05-0.
50%, N: 0.003 to 0.015%, B: 5 to 50 ppm, Ni:
2.0% or less, Ca: 0.01% or less, the balance being substantially Fe
And inevitable impurities. In addition, 10 to 50% of the C content is graphitized, and the steel structure of the cross section is large.
Graphite particles of 3 μm or more are C weight% × 10 ² particles / mm ² or more C
A hole expansion characterized by being a ferrite phase in which spheroidized cementite is dispersed, containing not more than 10% by weight × 10 ³ / mm ^2.
Thin steel sheet with excellent workability and secondary workability. 2. C: 0.20 to 0.60% by weight, Si: 0.
05-0.50%, Mn: 0.05-0.30%, P: 0.02% or less,
S: 0.01% or less, Cr: 0.02-0.05%, Al: 0.05-0.
50%, N: 0.003 to 0.015%, B: 5 to 50 ppm, Ni:
2.0% or less, Ca: 0.01% or less, the balance being substantially Fe
And steel consisting of unavoidable impurities at a finishing temperature of 750 to 90
C content, characterized by hot rolling in the range of 0 ° C, winding in the temperature range of 400 to 650 ° C, and descaling and annealing in the temperature range of 600 to 720 ° C. 10 to 50% of the graphitized,
The steel structure of the cross section shows graphite particles with a size of 3 μm or more
Wherein wt% × 10 ² cells / mm ² or more C wt% × 10 ³ cells / mm ² or less, and is dispersed ferrite phase spheroidal cementite, hole expandability and secondary formability excellent hot-rolled thin steel sheet Manufacturing method. 3. The weight ratio of C: 0.20 to 0.60%, Si: 0.
05-0.50%, Mn: 0.05-0.30%, P: 0.02% or less,
S: 0.01% or less, Cr: 0.02-0.05%, Al: 0.05-0.
50%, N: 0.003 to 0.015%, B: 5 to 50 ppm, Ni:
2.0% or less, Ca: 0.01% or less, the balance being substantially Fe
And steel consisting of unavoidable impurities at a finishing temperature of 750 to 90
Hot rolling in the range of 0 ° C, winding in the temperature range of 400 to 650 ° C, cold rolling at a reduction of 40 to 80%, and then 600 to 7
Annealed at 20 ° C, characterized by a C content of 10 to 50
% Of the graphite particles, and the steel structure of the cross section shows that graphite particles having a size of 3 μm or more are C weight% × 10 ² particles / mm ² or more C weight% × 1
0 A method for producing a cold-rolled thin steel sheet containing not more than ³ / mm ² and being a ferrite phase in which spheroidized cementite is dispersed, and which has excellent hole expandability and secondary workability. 4. C: 0.20 to 0.60% by weight, Si: 0.
05-0.50%, Mn: 0.05-0.30%, P: 0.02% or less,
S: 0.01% or less, Cr: 0.02-0.05%, Al: 0.05-0.
50%, N: 0.003 to 0.015%, B: 5 to 50 ppm, Ni:
2.0% or less, Ca: 0.01% or less, the balance being substantially Fe
And steel consisting of unavoidable impurities at a finishing temperature of 750 to 90
Hot rolling in the range of 0 ° C, winding in the temperature range of 400 to 650 ° C, annealing after the descaling in the temperature range of 600 to 720 ° C, and then cold rolling with a reduction of 40 to 80% After annealing at 600-720 ° C, 10-50% of C content is graphitized, and the steel structure of the cross section has a size
Graphite particles of 3 μm or more are C weight% × 10 ² particles / mm ² or more C
Wherein wt% × 10 ³ cells / mm ² or less, and is dispersed ferrite phase spheroidal cementite, hole expandability and secondary
A method for manufacturing cold rolled thin steel sheets with excellent workability.