JPH03248750A - Method for coiling and recoiling steel strip - Google Patents

Abstract

PURPOSE:To produce a steel strip based on high productivity and economy by coiling and recoiling at the specific strain velocity after continuously casting a hot-rolling steel for working containing the specific wt.% of C, S, Mn, P, Si, Al and the specific ppm of O in the specific thickness. CONSTITUTION:The hot-rolling steel for working is constituted with wt.% of 0.001-0.3% C, <=0.02% S, 0.06-0.40% Mn, <=0.03% P, 0.001-2.00% Si, <=0.15% Al, <=150ppm O, further Mn/S >=20 and the balance of Fe with inevitable impurities. This steel is continuously cast to <=50mm thickness. After that, this is coiled and recoiled at <=0.2sec<-1> strain velocity. By this method, even if the severe coiling and recoiling are executed, the development of high temp. brittle cracking can be prevented.

Description

Detailed Description of the Invention <Industrial Application Field> The present invention provides high-temperature embrittlement even when a cast steel strip manufactured by continuous casting and having a thickness of 50 mm or less is wound and unwound at a strain rate of about 0.2 sec-'. The present invention relates to a highly productive and economical method for winding and unwinding without causing cracking.

<Prior art> In recent years, with the aim of improving productivity and economic efficiency by omitting the rough rolling process, a method has been developed in which thin cast steel strips with a thickness of 50 mm or less are manufactured by continuous casting and only finish rolling is performed. It is being put into practical use.

In this case, since economical efficiency is further improved if the cast steel strip becomes longer, a method of using a thin cast steel strip instead of a slab and winding it into a coil shape is being considered.

To do this, the solidified cast steel strip must be wound and rewound at a high temperature of 1000°C or higher, which is the temperature of casting.

When winding and unwinding are performed in this high temperature range, cracks may occur in the slab due to high temperature embrittlement.

Even in the direct rolling and hot charge rolling of slabs with a thickness of 200 mm or more produced by current continuous casting, cracking cracks occur during rolling of slabs, and methods to prevent this are as follows:
JP-A-55-77901, JP-A-55-8420
Publication No. 2, JP-A-55-109503, JP-A-5
There are proposals such as JP-A No. 6-33103, JP-A-56-41005, and JP-A-60-187625.

However, all of these methods are methods for preventing kneeling cracking during rolling, and the deformed form B (processing style) is different, and therefore the slab thickness, strain, strain rate, and temperature history are different during the winding and unwinding of one casting band. It cannot be directly applied to methods for preventing embrittlement cracking.

Specifically, first of all, cracks are particularly likely to occur in areas where tensile stress is applied, but in the case of rolling, basically only compressive stress acts and almost no tensile stress acts, whereas in rolling and unwinding. In this case, compressive stress acts on the inside and tensile stress acts on the outside, and the maximum tensile strain is about 7%.

Furthermore, the strain rate has a large influence on the occurrence of cracks, and in the case of rolling, it is about 5 sec-', whereas in the case of winding and unwinding, it is about 0.2 sec-' at the maximum, and the difference between the two is large.

Furthermore, since the cooling rate is considerably faster in continuous casting of thin haze steel strips than in conventional casting methods, the precipitation conditions of precipitates, which have an important influence on crack occurrence, are different.

Therefore, the stress state, strain application, strain rate, etc. are quite different between rolling and winding and unwinding, and the precipitation situation of precipitates also differs depending on the cooling rate of the slab. When winding and unwinding a steel strip, the conditions in which cracks occur are different, so the former method of preventing embrittlement cracking cannot be directly applied to the latter.

Regarding the winding and unwinding of a thin haze rope belt having a thickness of 50IIIll or less, which is the object of the present invention, Japanese Patent Application Laid-Open No. 63-27864
In Publication No. 7, w4'f with a thickness of about 20 to 80 mm
A method has been proposed for winding an IT slab at a bending strain rate of about 0.01 to 0.001 and a curvature that gives a strain of 2 to 6%.

However, this proposal does not specify the steel composition, and therefore, although there are a wide variety of applicable steel types, it does not take into account the fact that the occurrence of high-temperature embrittlement cracking differs depending on the steel composition.

For example, there are descriptions of C, Si, Mn, P, and S, but no description of the composition, action, or effect of 0.

Moreover, the Japanese Patent Application Laid-Open No. 63-278647 is
The strain rate on which the proposal for winding with a strain of 6% is based is 0.01 to 0.001 sec-', and in winding and unwinding at this level, the copper strip components of the above proposals are However, in order to wind and unwind a thin haze steel strip at this strain rate, it is necessary to increase the winding and unwinding diameter, slow down the winding and unwinding speed, or It becomes necessary to implement both, which inevitably increases productivity and
The economic efficiency deteriorates, and the practicality of manufacturing thin haze steel strips becomes much lower.

However, the proposal does not mention this countermeasure.

<Problems to be Solved by the Invention> The present invention is capable of reducing the temperature of a thin haze steel strip of 50 mm or less in thickness exceeding 0.01 sec, which rapidly cools from a high temperature range, even in steel types with different occurrence conditions of embrittlement cracking. 2 sec-'
The object of the present invention is to provide a highly productive and economical method for manufacturing one cast steel strip having a thickness of 50 gn or less, which enables winding and unwinding even in the following strain rate ranges.

Means for Solving the Problems> In order to achieve the above-mentioned problems, the present invention provides the following in weight%: C: 0.001 to 0.30% S: ≦0.02%M
n: 0.06 to 0.40% P: 60.0
3%Si: 0.001 to 2.00% Al:
≦0.150 %O:≦isoppm However
Hot-rolled steel for processing consisting of Mn/S≧20, Fe and other unavoidable components is continuously cast and solidified to a thickness of 50 g+■ or less to form a thin haze steel strip, and then wound and unwound at a strain rate of 0.2 sec-' or less. is the means.

In the present invention, the hot-rolled steel for processing refers to a steel having the above-mentioned composition, for example, which is widely used in the field where hot-rolled steel sheets for processing are normally used.

<Function> In order to achieve the above-mentioned means, the present inventors set the following in weight%: C: 0.001 to 0.30% S: ≦0.02% M
n: 0.06 to 0.40% P: 60.03%
Si: 0.001-0.25% Al: 50.1
Contains 50% and other Fe and unavoidable components, O: 8
Various experimental studies were repeated using each hot-rolled steel for processing B steel containing 0 ppm AfIi4 and O: 160 pp+s, and the findings shown in FIGS. 1 to 3 were obtained.

The relationship between Mn/S and the aperture value in Figure 1 shows the test results for a thin haze steel strip with C: 0.047 wt% class, and the relationship between Pil1 degree and aperture value shows the test result of a C: 0.27 wt% class. The relationship between concentration and reduction of area is the test result of thin cast steel strip with C: 0.047 wt% class and Mn/S≧20, and the solidification of the molten sample. The temperature history is the three examples shown in FIG. 3, and the points E and F are for the three examples shown in FIG.

The findings will be explained below.

The present inventors manufactured one cast copper strip with a thickness of 50 mm or less,
After passing through the temperature history of the example in Figure 3 using a part of it as a test piece, the radius of curvature was 30 cm, where cracking is likely to occur due to high temperature embrittlement.
Bending is performed under conditions of a strain rate of 0.2 sec-' or less,
In addition to investigating the occurrence of high-temperature embrittlement cracking, after melting the sample and undergoing the temperature history shown in Figure 3, the reduction of area value (cross-sectional area after the test when performing the tensile test / cross-sectional area before the test x 100)
(%)] to confirm the correspondence between high-temperature embrittlement cracking and the reduction of area, and the relationship shown in FIG. 2 was obtained.

The indication of the occurrence of high-temperature embrittlement in the figure is as follows.

No: No cracks.

Large: 1m≦Total crack length per 1m2 of steel plate Medium: 10C1
1<Total crack length per 1m2 of steel plate 1m small zTotal crack length per 112 steel plates≦l0CI The further melted samples were melted more quickly than in Examples 1 and 2 in the 3rd session, during actual continuous casting steel strip production. Figure 3 shows the cooling rate of
The component dependence of was confirmed, and the results shown in Figure 1 were obtained.

The present inventors have found that hot embrittlement cracking increases in the area where the reduction of area is less than 10%, and the reduction of area (
%) was found to be in the range of 10% or more, the high temperature embrittlement cracking was reduced.

Furthermore, the aperture value falls within the range of 10% or more when Mn/
In A steel with S of 20 or more, the occurrence of the above-mentioned high-temperature embrittlement cracking is no longer observed, but in BE with Mn/S of 20 or more, the reduction of area is 1.
It was found that the above-mentioned high temperature embrittlement cracking occurs when the temperature does not reach 0% or more.

The present inventors observed the fracture surface of high-temperature embrittlement cracks, and found that the cracks were
It was confirmed that this was caused by plate-shaped or fine spherical precipitates such as (Fe, Mn) S and (Fe, Mn) 0 being precipitated at the austenite grain interface.

Moreover, in the case of BE where 0 is 160 ppH, Mn/S is 20
Even with the above conditions, it was found that a large number of the above-mentioned precipitates precipitated at the grain interfaces, making it easy to become brittle.

As a result, we found that it is possible to prevent the above-mentioned high-temperature embrittlement cracking by precipitating Mn as a single MnS within grains to prevent grain boundary precipitation.However, when Mn/S is less than 20, precipitation occurs at grain boundaries (Fe, Kn) S amount increases or ○ in steel becomes 160
When Mn/S exceeds 20, the amount of (Fe, Mn) precipitated at grain boundaries increases and cracking cannot be prevented, and when Mn/S exceeds 20, the amount of (Fe, Mn) S precipitated at grain boundaries decreases. However, it has been found that unless the O content in the steel is reduced to 150 ppm or less, the amount of (Fe, Mn) precipitated at the grain boundaries is not reduced, and cracking cannot be prevented.

In addition, high carbon P has a large effect on late cracking, but
It has been found that at a carbon concentration of about 0.3% by weight or less, the reduction of area value is always in a region slightly larger than 10%, and there is no need to consider the effect on high temperature embrittlement at all.

Based on the above-mentioned knowledge, the present inventors have determined that: - C content of hot-rolled steel for processing used for boat fishing is 0.001 to 0.3% by weight;
By specifying S to 0.02% or less, 0 to 150 ppm or less, and Mn/S to 20 or more, a thin cast steel strip with a thickness of 50 m+a or less can be heated at a temperature of 0.01 sec -' in a high-speed cooling process.
They have found that high-temperature embrittlement cracking does not occur in continuously cast thin cast steel strips even when subjected to severe winding and unwinding at strain rates exceeding 0.2 seconds and below, and the object of the present invention has been achieved.

<Example> 1. Chemical composition of test steel Table 1 shows the chemical composition of the test steel.

However, ■～■ are codes indicating the steel type, ■～■ indicate the target steel types of the present invention, ■ to ■ indicate target steel types of comparative examples.

2° winding and unwinding conditions (1) S winding and unwinding coil weight: approx. 5 (to opening) (2) Winding and unwinding diameter (ugly) ■ Innermost diameter: 300 ■ Outer diameter: 1500 (3) Winding Rewind speed 5-15 m/win (4
) Strain rate 2 is shown in Table 2.

3. Conditions of occurrence of high-temperature embrittlement cracks Table 2 shows the conditions of occurrence of high-temperature embrittlement cracks when the temperature history is as in the example shown in FIG.

(Note) (1) Z indicates the occurrence of high-temperature embrittlement cracking, and E and F in the cough column indicate points E and F in the temperature history of the third example in Fig. 3.

(2) The indication of the occurrence of high temperature embrittlement is as follows.

Unrivaled.

Large: 1m≦Total crack length per steel plate 112 Medium: 10C1
1<total crack length per 1 m2 of steel plate 1+small:
Total crack length per 1 m2 of steel plate 5101 As shown in Table 2, steel types ■ to ■ that satisfy Mn/S≧20.0:≦150PP11 do not undergo high-temperature embrittlement cracking at all even at a strain rate of 0.2 seconds. There wasn't.

On the other hand, 0: satisfies ≦150ppm, but Mn/S≧20
For steel types (■) to (2) which do not satisfy the following conditions and steel type (2) which satisfies Mn/S≧20 but does not satisfy O:≦150 ppm, high-temperature embrittlement cracking occurred at a strain rate of 0.2 sec''.

In addition, the strain rate is 0.001 sec-' and 0.01 sec
-', high-temperature embrittlement cracking did not occur in steel types ■ and ■.

<Effects of the Invention> In producing a continuously cast steel strip with a thickness of 50 m+n or less by continuous casting, the present invention contains 0.001 to 0.03% of C, 0.03% or less of S (both weight%), and other Fe. and Mn/S of molten steel consisting of unavoidable components ≧20.0≦150pp
Since the casting is adjusted to +m, the strain rate is 0.01 sec.
Continuous casting of continuous cast steel strips with a thickness of 50 mm or less is based on high productivity and economy, with no high-temperature embrittlement cracking even after severe winding and unwinding of more than 0.2 seconds and less than 0.2 seconds. The industrial effect it brings to five industrial fields is extremely large.

[Brief explanation of drawings]

Figure 1 shows the relationship between Mn154/0 and the reduction of area (%), Figure 2 shows the relationship between the reduction of area (%) and the occurrence of surface cracking due to high temperature embrittlement, and Figure 3 shows the relationship between the reduction of area (%) and the occurrence of surface cracking due to high temperature embrittlement. The cooling temperature history of the continuously cast copper strip used in the experiments and examples is shown.

Claims (1)

[Claims] (1) In weight%, C: 0.001-0.30% S: ≦0.02% Mn:
0.06-0.40% P:≦0.03%Si:0.0
01~2.00% Al:≦0.150%O:≦150
ppm, but Mn/S≧20 After continuous casting of hot-rolled steel for processing consisting of Fe and other unavoidable components to a thickness of 50 mm or less, the method is characterized by winding and unwinding at a strain rate of 0.2 sec^-^1 or less. A method for winding and unwinding thin cast steel strips.