JPH11350079A - Stainless steel excellent in hot workability - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a stainless steel excellent in cracking resistance at the time of hot working with the intention of improving manufacturing yield and reducing manufacturing costs. SOLUTION: This stainless steel has a composition consisting of, by weight, <=0.15% C, <=2% Si, <=3% Mn, <=0.07% P, <=0.003% S, <=0.005% O, 8-20% Ni, 15-20% Cr, <=0.3% N, 0.0002-0.001% B, and the balance Fe with inevitable impurities.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] The present invention relates to a stainless steel having excellent hot workability during production.

[0002]

2. Description of the Related Art It is well known that in recent years, with the progress of stainless steel making and rolling technology, a more efficient and less expensive method of manufacturing stainless steel has been developed, and the production yield has been improved. It is. In particular, the problem of cracking and flaw generation in the hot rolling process affects the surface refining load,
Since it greatly affects the manufacturing cost, various investigations have been made on aspects such as improvement of ductility at a processing temperature.

[0003] As design guidelines from the component side, S, P,
Reduction of O or the like or fixing in the form of a compound has been regarded as one of effective means from the viewpoint of improving hot ductility.
The technology that launched this as a component guide for continuous casting materials,
It is disclosed in JP-A-60-149748. Here, mainly S and O are reduced, and Ca, Ce, A
It is shown that by adding l, the amount of S and O segregating at the grain boundaries and the like is also reduced as much as possible, thereby improving the surface crack resistance particularly during rolling. Contrary to such means for cleaning grain boundaries, a method has been proposed in which the strength of the grain boundaries themselves is increased by adding an element which is apt to segregate at the grain boundaries to make them less likely to crack. A typical example of such elements is B. It has been. According to the technique disclosed in JP-A-63-157840, hot workability can be improved by adding B after limiting the amount of ferrite, and the same applies to JP-A-5-179405. Has the effect of adding B.

[0004]

However, the slab according to the technique disclosed in Japanese Patent Application Laid-Open No. 63-157840 may crack at the stage of slab rolling, and cannot be said to be a stable technique suitable for actual operation. This is evident from the results of test rolling on a production line.
Similarly, the method according to the technique of Japanese Patent No. 405 is also being evaluated that the effect of adding B lacks stability. That is, an object of the present invention is to provide a stainless steel having excellent crack resistance in hot working for the purpose of reducing the production yield and the production cost.

[0005]

In order to solve the above problems and achieve the object, the present invention uses the following means. (1) The stainless steel of the present invention has a C: 0.1% by weight.
5% or less, Si: 2% or less, Mn: 3% or less,
P: 0.07% or less, S: 0.003% or less, O:
0.005% or less, Ni: 8 to 20%, Cr: 15
~ 20%, N: 0.3% or less, B: 0.0002 ~
It is a stainless steel excellent in hot workability, characterized by containing 0.001% and the balance being Fe and unavoidable impurities. (2) In the stainless steel of the present invention, C: 0.15% or less, Si: 2% or less, M
n: 3% or less, P: 0.07% or less, S: 0.00
3% or less, O: 0.005% or less, Ni: 8 to 20
%, Cr: 15 to 19%, N: 0.3% or less, M
o: 1.5 to 4% and B: 0.0002 to 0.001%
And a balance of Fe and unavoidable impurities, and is excellent in hot workability.

(3) The stainless steel of the present invention further contains, by weight%, Al: 0.006 to 0.03% as a steel component.
And Ca: 0.01% or less, Mg: 0.0
Excellent in hot workability according to the above (1) or (2), characterized by containing one or more of 1% or less, Ti: 0.1% or less, and Zr: 0.2% or less. Stainless steel.

[0007]

BEST MODE FOR CARRYING OUT THE INVENTION The inventors of the present invention have studied the tendency of B to easily segregate at the grain boundary and increase the grain boundary strength by creep rupture tests of various stainless steels containing a small amount of B conventionally.
It has been repeatedly confirmed through a high-temperature tensile test and the like. From such knowledge, B should be effective for improving crack resistance during hot working. Nevertheless, the present inventors have conducted intensive studies on the cause of not providing a stable effect on the improvement of crack resistance in hot working.

As a result, it has been found that B certainly increases the grain boundary strength, but at the same time lowers the melting point in the vicinity of the grain boundary, thereby accelerating the melting of the steel when it is overheated. In other words, it was clarified that cracks generated in the stage of slab rolling at the time of actual production cause work cracks due to the loss of ductility at portions (grain boundaries) partially reaching the melting point.
In other words, as factors of cracking, the heating temperature of the furnace as a temperature factor, the temperature distribution in the slab depending on the slab dimensions, the effects of processing heat, etc., the degree of segregation as a material factor and the degree of melting point drop caused by local component fluctuation, All the influences such as the degree of local distortion are affected as processing factors.

Among them, the degree of local segregation or concentration as a material factor is about 10 times or more of the average composition in the case of B. Therefore, the amount of B as the average composition is 0.1%.
The addition of 0005% by weight should be the upper limit, and if it is less than this, B which has been conventionally confirmed by the present inventors has been used.
It was clearly confirmed that the effect of improving the grain boundary strength of the added steel was remarkable. In the prior art, when the amount of B added was as large as 0.001% or more, it was confirmed that cracks occurred during hot rolling depending on the slab heating conditions. is there.

Based on the above findings, the present inventors set the amount of B added to austenitic stainless steel (general-purpose 18Cr-8Ni stainless steel) within a certain range as long as the hot workability is not deteriorated. To control
A stainless steel excellent in hot workability was found, and the present invention was completed.

That is, the present invention can provide a stainless steel having excellent crack resistance in hot working for the purpose of reducing the production yield and the production cost by limiting the steel composition to the following range.

The reasons for adding and limiting the components of the present invention will be described below. (1) Component composition range C: 0.15% or less Although it is an element useful for improving the strength of steel, when added in excess of 0.15%, the hot ductility is significantly impaired. is there.

Si: 2% or less Effective in deoxidation and improves hot workability by reducing O in steel. However, if added in excess of 2%, flaws after rolling will occur frequently. The amount is not more than 2%.

Mn: 3% or less In stainless steel, it works effectively as an austenite stabilizing element, but if added in excess of 3%, ductility is reduced, so the amount of addition is 3% or less.

S: 0.003% or less S is the most harmful element to hot workability, and the lower the content, the better. If the content is 0.003% or less, hot ductility is acceptable, so the content is 0.003% or less.

P: 0.07% or less P is an element that impairs hot workability, and the lower the content, the better. If the content is 0.07% or less, hot ductility is acceptable, so the content is 0.07% or less.

O: 0.005% or less O is an element that impairs hot workability, and the lower the content, the better. If the content is 0.005% or less, hot ductility is acceptable, so the content is 0.005% or less.

Ni: 8 to 20% Ni is a strong element for stabilizing the austenite phase, and generally improves the ductility of steel. In the component system of interest, 8% or more must be added to stabilize the same phase. On the other hand, if added in excess of 20%, the same phase becomes excessively stable, the degree of harmful S, P, O and other grain boundary segregation becomes extremely large, resulting in impaired hot workability.
The addition amount is 8 to 20%.

Cr: 15-20%, 15-19% (Mo
1.5 to 4%) is a basic component of stainless steel, and is effective in improving oxidation resistance at high temperatures. Unless added in an amount of 15% or more, the oxide scale at the time of high-temperature heating remarkably develops inside along the grain boundaries, and as a result, cracking during hot working becomes remarkable. On the other hand, if more than 20% is added (when Mo is added 1.5 to 4%, 19
%), It is difficult to maintain the structure in an austenitic single phase, and cracks during processing are likely to occur at the interface with the precipitated ferrite phase.
Not less than 20% (19%).

N: 0.3% or less The austenite phase is stabilized by the addition. As in the case of Ni, if it is added in excess of 0.3%, the same phase becomes excessively stable, the degree of grain boundary segregation of harmful elements such as S, P and O becomes extremely large, and the hot workability is impaired. As a result, the addition amount is 0.3% or less.

Mo: 1.5 to 4% Mo is an element effective for improving the corrosion resistance of steel, and is in the range of 1.5% or more at which the effect is exhibited and 4% or less at which cracking during rolling due to self-segregation does not occur. , Selective addition is possible. B: 0.0002 to 0.001% As described above, the control of the addition amount is the main focus of the present invention. When the content is 0.0002% or more, the effect of strengthening the grain boundary is sufficiently exhibited. On the other hand, when the content exceeds 0.001%, a compound having a low melting point tends to be generated at the grain boundary,
Since cracks occur during rolling depending on the heating conditions, the content is limited to 0.0002 to 0.001%. Al: 0.006 to 0.03% Similar to Si, has an effect on deoxidation, and particularly, Ca, Mg, T
The effect is great when added simultaneously with i and Zr. In this case, the hot workability is improved through the reduction of O in the steel.
The effect is not sufficient unless it is added over 0.6%.
If the addition exceeds 3%, ground flaws after rolling will occur frequently.
006-0.03% are added simultaneously.

Ca ≦ 0.01%, Mg ≦ 0.01%, T
i ≦ 0.1%, Zr ≦ 0.2% Ca, Mg, Ti and Zr are all effective in deoxidation,
It improves hot workability by reducing O in steel, etc.
One or more types can be selectively added. 0.0 each
If added in excess of 1%, 0.01%, 0.1%, and 0.2%, ground flaws are generated and increased during hot working, so the added amount is Ca: 0.01% or less, Mg : 0.01% or less, T
i: 0.1% or less, Zr: 0.2% or less.

By adjusting to the above component composition range, stainless steel excellent in hot workability can be obtained. The manufacturing conditions are not particularly limited in the present invention. That is, the melting method of the stainless steel, the rolling method and the heat treatment method during the production of the steel sheet may be any conditions that are generally adopted. Hereinafter, examples of the present invention will be described to demonstrate the effects of the present invention.

[0024]

EXAMPLES Tables 1 to 3 show the chemical compositions and hot workability (surface cracks and surface defects during rolling) of the steels of the present invention (Nos. 1 to 29) and Tables 4 to 6 show the comparative steels (Nos. 30 to 53). The numbers and lengths of ear cracks are listed.

Each steel was melted in an experimental furnace in a vacuum, and the obtained steel ingot was subjected to the steps of hot-rolling, final rolling and annealing at 1250 ° C. to obtain a test steel. The bulk rolling was performed in three passes of large rolling under 20% or more, and the hot workability was evaluated by directly observing the surface / end face of the slab after rolling. That is, the number of hot cracks in the early stage of rolling (expressed as surface cracks) and the surface flaws including surface flaws (expressed as surface flaws and ground flaws) are counted with the center of the slab at the center, and the number of cracks or flaws per unit surface area is calculated. Evaluation was performed, and the maximum value of the edge crack length generated at both ends until the finished thickness (expressed as an edge crack) was measured as the depth from the edge.

According to the steel No. of the present invention, 1 to No. No. 29 has better hot workability, less ground flaws, and excellent rolling crack resistance as compared with a comparative steel described later. As a comparative object, B: Comparative steel No. 1 ppm or less. 43, 50 to 52, etc.

On the other hand, the comparative steel No. Comparative Steel No. 30 was excessive in C and was inferior in hot ductility. No. 36 lacks Ni, and the comparative steel No. No. 39 has an excessive amount of Cr, precipitates an excessive delta ferrite phase, and also has poor hot ductility. Comparative steel N
o. 40 has insufficient Al content and insufficient deoxidation,
All of the above are examples in which ear cracks after rolling are remarkable. Comparative steel No. The same applies to the case where Mo is excessive and precipitation of an intermetallic compound deteriorates hot workability as in the case of No. 45.

Further, the comparative steel No. 31, 35, 41, 4
Nos. 6, 47, 48, and 49 show significant generation of ground flaws by rolling under the influence of Si, O, Al, Ca, Mg, Ti, and Zr, which exceed the ranges specified in the present invention.

Further, the comparative steel No. Nos. 33 and 34 have lower grain boundary ductility due to the effects of excess P and S, respectively, leading to surface cracking at high temperatures. 44,
Nos. 52 and 53 had excess B, and Comparative Steel Nos. 43,5
Nos. 0 and 51 have insufficient B, and surface cracks frequently occur. Comparative steel No. 32, 37, 42, Mn, Ni, N
Exceeds the specified range, the Ni equivalent becomes too large, and the comparative steel No. In the case of No. 38, the Ni equivalent with respect to the Cr equivalent ratio becomes too large, and it is also found that surface cracks occur at high temperatures. FIG. 1 shows the relationship between the B content and the number of surface cracks generated. The trial production of the actual production line was carried out for 6 charges while corresponding to the above-mentioned laboratory examination, and the hot workability (manufacturability) was evaluated for three items of surface cracks, flaws, and edge cracks.
The results are summarized in Tables 7 to 9. Nos. In Tables 7-9. 54,
No. 55, No. No. 56 is the steel of the present invention. 57, N
o. 58, No. 59 is comparative steel.

According to the steel No. of the present invention, Nos. 54 to 56 include the Mo-added type and the non-added type, but all have surface cracks and flaws of 5 pieces / m ² or less, and edge cracks of 5 mm or less from the edge, at a level that can be easily removed. there were.

On the other hand, Comparative Steel No. No. 57 has a B content lower than the appropriate range, and the comparative steel No. No. 58, comparative steel No. Since No. 59 was larger than the proper range, all of them had many surface cracks or flaws, and the ear cracks also tended to be deep. FIG. 2 shows the relationship between the B content and the number of surface cracks generated.

As exemplified above, in order to obtain the hot workability intended in the present invention, the content of each alloy element which is a feature of the present invention is specified, and in particular, the B content is strictly controlled. It is understood that is necessary.

[0033]

[Table 1]

[0034]

[Table 2]

[0035]

[Table 3]

[0036]

[Table 4]

[0037]

[Table 5]

[0038]

[Table 6]

[0039]

[Table 7]

[0040]

[Table 8]

[0041]

[Table 9]

[0042]

As described above, according to the present invention,
By specifying the steel composition, a stainless steel excellent in hot workability can be provided, and an industrially useful effect is brought about through reduction of the production yield and the production cost.

[Brief description of the drawings]

FIG. 1 is a view showing the relationship between the B content and the number of surface cracks generated during hot rolling according to an example of the present invention.

FIG. 2 is a diagram showing the relationship between the B content and the number of surface cracks generated in a continuous casting material production line according to an example of the present invention.

Claims (3)

[Claims] (1) C: 0.15% or less by weight,
i: 2% or less, Mn: 3% or less, P: 0.07% or less, S: 0.003% or less, O: 0.005% or less, Ni: 8 to 20%, and Cr 15-20% and N:
Stainless steel excellent in hot workability, characterized by containing 0.3% or less and B: 0.0002 to 0.001%, the balance being Fe and unavoidable impurities. 2. C: 0.15% or less by weight,
i: 2% or less, Mn: 3% or less, P: 0.07% or less, S: 0.003% or less, O: 0.005% or less, Ni: 8 to 20%, and Cr : 15-19% and N:
0.3% or less, Mo: 1.5 to 4%, and B: 0.00
A stainless steel excellent in hot workability, characterized in that the content is from 0.2 to 0.001% and the balance is Fe and inevitable impurities. 3. The steel composition further comprises, in weight%, Al:
0.006 to 0.03%, and Ca: 0.0
3. The heat according to claim 1, wherein the heat contains one or more of 1% or less, Mg: 0.01% or less, Ti: 0.1% or less, and Zr: 0.2% or less. 4. Stainless steel with excellent workability.