JPH08165522A - Production of high carbon cold-rolled steel sheet in low cost - Google Patents

Abstract

PURPOSE: To provide a production method of a high carbon cold-rolled steel sheet in a low cost. CONSTITUTION: At the time of hot-rolling the steel composed of, by wt.% 0.30-0.90 C, <=0.80 Si, 0.30-1.50 Mn and as necessary, containing one or more kinds among <=1.20 Cr, <=0.50 Mo, <=0.0030 B and <=0.030 Ti and the balance Fe with inevitable impurities, the hot-rolled steel sheet obtd. by hot-rolling so as to satisfy <=850 deg.C finish temp., <=20 deg.C/sec average cooling speed at a run-out table and >=650 deg.C coiling temp. is successively cold-rolled and a box- annealing is executed.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a high carbon cold rolled steel sheet at low cost.

[0002]

2. Description of the Related Art Generally, chain parts, gear parts, clutch parts, buckles for seat belts, washers, etc. are made of high carbon cold-rolled steel sheet specified in JIS G3311, in view of requirements for wear resistance and impact resistance. It is manufactured by forming a steel plate into a desired shape and then hardening it by heat treatment such as quenching and tempering. From this, it is necessary that the high-carbon cold-rolled steel sheet, which is a raw material, is soft and easy to form before forming and that the required hardness is obtained by heat treatment. The hardness of the latter heat treatment is almost determined by the heat treatment conditions and the composition of the steel.

Further, high carbon cold-rolled steel sheets are often formed only by punching, and the material sheet thickness often becomes the product thickness. That is, since the dimensional accuracy of the product depends on the plate thickness accuracy of the material plate thickness, it is usual that the plate thickness accuracy in the width direction and the longitudinal direction of the material, surface flaws, etc. are strictly regulated. On the other hand, when a high carbon cold rolled steel sheet is manufactured, it becomes hard during cold rolling due to the high carbon content, and it is difficult to improve the flatness and the thickness accuracy of the steel sheet. Therefore, it is common to anneal the hot rolled coil to soften it after pickling. Further, cold rolling may be interrupted at a desired intermediate plate thickness, annealed, and cold rolled again. As described above, the high-carbon cold-rolled steel sheet has a problem in that it has more manufacturing steps and a higher manufacturing cost than the cold-rolled steel sheet for automobiles.

To solve this problem, Japanese Patent Laid-Open No. 49-49
Techniques such as Japanese Patent No. 91024 and Japanese Patent Laid-Open No. 62-284019 have been proposed. The technique described in JP-A-49-91024 reduces the hardness of ferrite by limiting the amount of Si to 0.10% or less, and presses after cold rolling and annealing even if spheroidizing annealing is omitted. It is to improve workability. However, since this technology is a component that deviates from the standard as mechanical structural steel, the applied products are limited. Further, the feature of the technique described in JP-A-62-284019 is that the cooling rate after hot rolling is 20 to 120 ° C.
The structure is refined by cooling the coil at an average cooling rate of / sec and winding the coil, and even if the coil is directly cold-rolled, ear cracking does not occur. However, in this technique, since the structure of the hot rolled coil is too fine, the hardness after cold rolling annealing is high,
There is a problem that the workability of the steel sheet is poor.

[0005]

The present invention has been made for the purpose of solving the problems in the production of the above-mentioned high carbon cold-rolled steel sheet, and specifically, a steel composition within the specifications of steel. Thus, a method of drastically improving the manufacturing cost by omitting the annealing process of the hot-rolled coil and the intermediate annealing process of cold rolling without degrading the plate thickness accuracy and flatness of the high carbon steel plate is provided. It is a thing.

[0006]

Means for Solving the Problems For the purpose of solving the above-mentioned problems, the present inventors have conducted various studies on steel composition, hot rolling conditions, cold rolling conditions, annealing conditions, and combinations thereof. It was found that the above problems can be solved by controlling the hot rolling temperature and the winding temperature regardless of the cold rolling and annealing conditions.

That is, the gist of the present invention is
% By weight, C: 0.30 to 0.90%, Si: 0.80
% Or less, including Mn: 0.30 to 1.50%, and if necessary, Cr: 1.20% or less, Mo: 0.50%
Hereinafter, when hot-rolling a steel containing one or two or more of B: 0.0030% or less and Ti: 0.03% or less, and the balance of Fe and unavoidable impurities, a finishing temperature of 85 is used.
The obtained hot rolled sheet was wound at a temperature of 650 ° C. or higher, and then cold-rolled after descaling,
It is a low-cost manufacturing method of a high-carbon cold-rolled steel sheet characterized by box annealing.

The constituent features of the present invention will be described below.
First, the present invention will be described in terms of steel composition. C is a high carbon steel plate that is finally hardened and tempered to adjust the hardness,
A minimum of 0.30% is necessary. If the amount of C is less than this,
Sufficient hardenability cannot be secured. On the other hand, the amount of carbon is 0.9
If it exceeds 0%, the steel sheet becomes hard even by the method of the present invention, so that it becomes difficult to perform cold rolling without annealing the hot-rolled / pickled sheet coil. Therefore, the upper limit of the amount of C is specified to be 0.90%.

Si is used as a deoxidizing material for molten steel. However, if the addition amount exceeds 0.80%, the steel plate becomes hard and the object of the present invention cannot be achieved. It is well known that Mn is an element that combines with S in steel to suppress hot embrittlement due to S and at the same time enhances hardenability of a high carbon steel sheet. Therefore, it is necessary to add at least 0.30%. On the other hand, when the amount of Mn added increases, the hardness of the steel sheet during rolling increases. Further, when the addition amount is increased, the molten steel is solidified, segregation is increased during the production of the slab, and the toughness after quenching and tempering is reduced. For this reason, the upper limit of the amount of Mn was specified as 1.50%.

In the present invention, in addition to the above basic components, one or more of Cr, Mo, B and Ti can be added, if desired. Cr is an element that not only enhances hardenability, but also hardens carbides after quenching and tempering to enhance wear resistance of the steel sheet. Therefore, when it is added, the upper limit is 1.2%. Cr content is 1.
If it exceeds 2%, the effect is saturated.

Mo is added up to 0.50% as an upper limit because it has a role of suppressing temper embrittlement, which is embrittled during tempering after quenching a steel sheet. Addition of more than 0.50% only raises the cost and saturates the material improvement effect. B is an element that enhances the hardenability and at the same time enhances the grain boundary strength of the steel sheet after quenching and tempering. Therefore, when it is used in applications requiring toughness and delayed fracture resistance, 0.0030
It is added within%.

Ti is added in order to bring out the effect of the addition of B. B segregates at the grain boundaries to increase the grain boundary strength,
Has the effect of enhancing hardenability. However, if Ti is not added, B is combined with N and the amount of B segregated at the grain boundaries is reduced. Before precipitation as BN, N is fixed with Ti and the amount of Ti is added within 0.030% in order to enhance the effect of B addition.

It is not necessary to specify S, P, N, etc. other than the above components, but since these elements lower the workability, toughness, etc. of the steel sheet, it is preferable to make them as low as possible. Molten steel having such a composition is melted by a normal melting method such as a converter or an electric furnace, and is made into a slab by continuous casting. The slab is hot-rolled into a hot-rolled coil, and the hot-rolling conditions at this time are important constituent elements. Experimental facts that found this condition will be described below.

In a vacuum melting furnace, C: 0.50%, Si: 0.
19%, Mn: 0.80%, Al: 0.01%, P:
0.010%, S: 0.010%, N: 0.0045%
After melting the steel of the composition of 1 to form a steel ingot,
It was hot rolled to a thickness of mm. Regarding the hot rolling conditions, the cooling rate after the final rolling was kept constant at 15 ° C./sec, and other conditions were changed. The steel sheet was pickled, cold-rolled, annealed at 690 ° C. for 10 hours, temper-rolled at 1.5%, and then the hardness of the steel sheet was investigated. The pickled plate is annealed at 690 ° C. for 10 hours, cold-rolled, and annealed at 690 ° C. for 10 hours.
An investigation was also conducted by the conventional method of performing temper rolling of 1.5%.

The relationship between the hardness of the obtained steel sheet and the hot rolling conditions is shown in FIG. In the figure, the solid line shows the winding temperature of 650 ° C or higher, and the broken line shows the winding temperature in the range of 600 to 630 ° C. The hardness of the product annealed before cold rolling is H
It was 74-78 in RB. As can be seen from the figure, the hardness of the steel sheet largely changes depending on the finishing temperature and the winding temperature.
That is, the finishing temperature is 850 ° C. or lower, and the winding temperature is 650.
When the combination is at or above 0 ° C, a soft steel sheet equivalent to that annealed before cold rolling can be obtained. If either the finishing temperature or the winding temperature is out of the range of the present invention, a stable and soft steel sheet cannot be obtained. By specifying the finishing temperature and the coiling temperature, it was found that a soft steel sheet can be obtained without annealing before cold rolling.

Further, in order to investigate the influence of cooling conditions, steel having the same composition as in the previous experiment was melted, and the finishing temperature was 800 ° C. and the coiling temperature was constant at 680 ° C. among the hot rolling conditions, and finish rolling was carried out. Hot rolling was performed by changing the cooling rate from winding to winding. The hot-rolled sheet thickness was kept constant at 3.2 mm, and the cold-rolling, annealing, and temper rolling conditions after hot-rolling were the same as in the case of FIG. The relationship between the hardness of the steel sheet after temper rolling and the cooling rate after hot rolling is shown in FIG. As is clear from the figure, the hardness of the steel plate greatly changes depending on the cooling rate, and the cooling rate is 20 ° C /
It can be seen that a soft steel plate is obtained when the time is less than a second.

From the above experimental facts, the hot rolling finishing temperature is set to 85.
It was specified that the temperature was 0 ° C or lower, the average cooling rate on the run-out table after finish rolling was 20 ° C / sec or lower, and the winding temperature was 650 ° C or higher. The hot-rolled coil thus produced is directly cold-rolled after descaling without annealing as in the prior art. Cold rolling rate is determined by product thickness,
In order to exert the effect of the present invention, it is desirable that the rolling reduction is 35% or more. In the conventional method, the steel plate becomes harder as the cold rolling rate increases, but in the method of the present invention, the effect is small and the steel sheet does not become so hard even if the cold rolling rate is increased.

In the cold rolling method in this case, the characteristics of the present invention are not impaired even in tandem or reverse rolling with one rolling mill. The roughness of the cold rolling roll is preferably Ra: about 0.5 μm in terms of the roughness of the steel plate so that the atmosphere gas flow to the surface of the steel plate during annealing is good. If the flow of gas to the surface of the steel sheet is made good, the temperature of the coil will be uniform, the variation in hardness of the steel sheet will be small, and seizure of the steel sheet will be eliminated.

Subsequently, the annealing is performed by box annealing. A higher annealing temperature is preferable because the steel plate hardness becomes softer. In the present invention, annealing is performed in the range of 670 to 730 ° C for 5 to 20 hours. The annealed coil has a hardness,
It is desirable to perform temper rolling of 1.5% or more in order to adjust the surface roughness, plate thickness and shape. Since the steel plate becomes too hard when the temper rolling rate is high, the temper rolling rate is preferably 5.0% or less.

The steel sheet manufactured in this manner is slit and provided to a product processing maker if necessary.

[0021]

EXAMPLE Steels having the chemical compositions shown in Table 1 were melted in a converter and continuously cast to form a slab having a thickness of 250 mm. The heating temperature of this slab was set to 1150 ° C., and the other conditions were the hot rolling conditions shown in Table 2 to obtain a hot rolled coil having a thickness of 3.2 mm. This coil was made into a cold-rolled steel sheet by the steps and conditions shown in Table 3 (continued from Table 2), and the surface hardness was measured. Table 3 together with the results
Shown in

[0022]

[Table 1]

[0023]

[Table 2]

[0024]

[Table 3]

Nos. In Tables 2 and 3 The letter of indicates the steel composition. A-1, B-1, C-1, D-1, E-1, F
-1, G-1 are examples within the scope of the present invention. A-2,
B-2, C-2, D-2, E-2, F-2, and G-2 are examples in which the hot rolling finishing temperature is outside the scope of the present invention, A-4, B-4, and
C-4, D-4, E-4, F-4, and G-4 are examples in which the cooling rate after hot rolling is outside the scope of the present invention. A-3, B-
3, C-3, D-3, E-3, F-3, and G-3 are examples of the conventional method in which annealing is performed before cold rolling. The hardness of the cold-rolled steel sheet is affected by the chemical composition of the steel, but in any steel, the hardness of -1 of A to G, which is an example within the scope of the present invention, becomes softer than the examples outside the scope of the present invention. Therefore, almost the same hardness as the conventional method in which an annealing step is performed before cold rolling can be obtained.
In any of the steels A to G whose hot rolling finish temperature is outside the range of the present invention, any steel is harder than the conventional method and the method of the present invention and sufficient formability is not obtained. Further, the examples in which the cooling rates of -4 of A to G are fast are hard, and sufficient formability is not obtained without annealing before cold rolling.

[0026]

As described in detail in the above embodiments, the annealing process before cold rolling can be omitted only when the relationships among the finishing temperature of hot rolling, the cooling rate immediately after that, and the coiling temperature are satisfied. It becomes possible to manufacture high carbon cold rolled steel sheet.

[Brief description of drawings]

FIG. 1 is a diagram showing the relationship between hardness and hot rolling conditions.

FIG. 2 is a diagram showing a relationship between hardness and a cooling rate after hot rolling.

Claims (2)

[Claims] 1. C: 0.30 to 0.90% by weight,
When hot rolling steel containing Si: 0.80% or less, Mn: 0.30 to 1.50%, and the balance Fe and unavoidable impurities, the finishing temperature was set to 850 ° C. or less, and the average in the runout table was set. A high-carbon cold-rolled steel sheet characterized in that the hot-rolled sheet obtained at a cooling rate of 20 ° C./second or less is wound at a temperature of 650 ° C. or more, subsequently descaled, cold-rolled, and annealed in a box. Low cost manufacturing method. 2. The steel composition further comprises Cr: 1.20%.
The high carbon cold-rolled steel sheet according to claim 1, wherein one or more of Mo: 0.50% or less, B: 0.0030% or less, and Ti: 0.03% or less is added. Low cost manufacturing method.