JPS62199721A - Production of steel sheet or strip of ferritic stainless steel having good workability - Google Patents

Abstract

PURPOSE:To produce a sheet or strip of a ferritic stainless steel having good workability by starting rough rolling of the slab of the ferritic stainless steel at a specific temp. and subjecting the slab to many passes of rough rolling under adequate conditions. CONSTITUTION:The slab of the ferritic stainless steel having substantially the single phase structure of ferritic is subjected to hot rough rolling and hot finish rolling at a hot rolling temp. to obtain a hot rolled sheet. The rough rolling is executed in multiple passes and is started at the slab temp. in a 1,050-1,180 deg.C range. The pass of >=30% draft is executed at least once in the passes before and after the rough rolling until the sheet thickness is reduced down to 1/2 the initial slab thickness. The next pass of the rough rolling is executed after >=30sec of delay in succession to said pass. The hot rolled sheet is annealed upon ending of the hot rolling or, if necessary, the annealing of the hot rolling sheet is omitted and immediately the hot rolled sheet is subjected to one pass of cold rolling or >=2 passes of cold rolling including intermediate annealing. The generation of ridging in the press forming stage is thereby decreased and the sheet or strip of the ferritic stainless steel having excellent secondary working characteristic after deep drawing is obtd.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Field of Application] The present invention relates to a method for producing a steel plate or band made of ferritic stainless steel with excellent workability. More specifically, the present invention relates to a method for manufacturing a ferritic stainless steel plate or steel strip that causes less occurrence of welding and zinc during press forming and has excellent secondary workability after deep drawing.

[Conventional technology]

Ferritic stainless steel represented by 5US430 has characteristics such as no period cracking phenomenon after forming and low stress corrosion cracking susceptibility compared to austenitic stainless steel represented by 5US304. It is widely used as a durable consumable material because it is inexpensive because it does not contain any carbonaceous substances. However, ferritic stainless steels have inferior press formability and corrosion resistance compared to austenitic stainless steels, and moreover, during deep drawing, a unique wrinkle-like surface roughness called zinc often occurs. Furthermore, in some ferritic stainless steel types, embrittlement cracks called longitudinal cracks occur during secondary processing after large deep drawing.

Conventionally, in order to improve the corrosion resistance and press formability of ferritic stainless steel, reduction of C and N and Ti
Steels with relatively large amounts of carbonitride-forming elements such as Nb have been developed.1For example, JT, 5G4305 has 5U
It is specified as S430LX. However, the above-mentioned problems regarding zinc and vertical cracking have not yet been solved.

Although there have been many reports on the ridging phenomenon, the technology is not necessarily sufficient to eliminate it industrially. The cause of ridging is, for example, the Journal of the Japan Institute of Metals,
31 (1967), P, 519, it is thought to reside in the coarse ferrite band remaining in the center layer of the hot rolled sheet, and therefore.

In order to improve ridging, the main focus is on dividing and refining the coarse ferrite bands of the hot rolled sheet. To summarize the treatments for improving zinc in ferritic stainless steel in previously published reports and patents, (
1) Increasing the equiaxed quality of the slab 3 (2) Performing low 4L hot rolling, (3) Performing hot rolled plate annealing, etc.

Regarding the problem of vertical cracking that occurs during secondary processing after deep drawing of ferritic stainless steel, the present inventors have
In No. 0-168626, it was proposed to improve this from the viewpoint of steel composition. This is Aβ.

By appropriately regulating the content of Ti or Nb, the secondary processability is improved.

[Problem that the invention seeks to solve]

As mentioned above, ferritic stainless steel beams are accompanied by zinc problems and secondary workability problems.

A treatment method that simultaneously solves this problem and is industrially advantageous has not yet been completed. The present invention aims to solve this problem.

[Means to solve problems]

The present invention provides a method for producing a ferritic stainless steel plate or strip that solves the above problems.

In manufacturing a hot rolled sheet by rough hot rolling and finish hot rolling a slab of ferritic stainless steel that exhibits a substantially ferritic single phase structure at hot rolling temperature, and then cold rolling, 2. The rough rolling is carried out in passes, and the temperature of the slab at the start of this rough rolling is in the range of 1050°C to 1180°C.

In the first pass of rough rolling until the plate thickness becomes 1/2 of the initial slab, perform at least one pass with a reduction ratio of 30% or more, and after this pass, wait for a delay of 30 seconds or more before starting the next roughing pass. To perform rolling.

(1) After that, cold rolling is performed by one cold rolling including hot-rolled plate annealing or two or more cold rolling including intermediate annealing, or (2) In particular, strict longitudinal cracking resistance is required. If so, after hot rolling is completed.

Cold rolling is performed by omitting hot-rolled plate annealing and performing cold rolling once or twice or more including intermediate annealing.

The present invention provides a method for manufacturing a ferritic stainless steel plate or strip having excellent workability.

In the method of the present invention, substantially ferrite single-phase Mi is formed in the hot rolling process.
Any ferritic stainless steel that exhibits texture can be used. After the hot rolling process, it is also possible to carry out hot-rolled plate annealing and then normal cold rolling, but in the case of steel containing Ti and/or Nb, especially in the longitudinal direction during secondary processing. If cracking is a particularly severe requirement, it is possible to produce steel sheets or strips that do not have such longitudinal cracking problems by performing cold rolling after hot rolling using the above method and omitting hot-rolled sheet annealing. Obtainable.

It is particularly desirable to apply the method of the present invention to steels with good press formability. Steels that can advantageously achieve the object of the present invention include c: o, o 3% or less, Si: 0.75% or less, Mn: 0.40% or less, P: 0.04% or less.

s: 0.01% or less, Cr: 12. O%~22.0%,
Ni: 0.5% or less, N j 0.03% or less,
O: 0.01s% or less.

Sol, ^j! : 0.05% or less, and 0.04
% to 0.40% Ti or 0.10% to 0.80% Nb, and in some cases further contains 2% or less Mo or 1% or less Cu, with the balance There is steel that consists of iron and unavoidably mixed impurities.

The content of the method of the present invention will be specifically explained below.

Various causes of ridging can be considered, but as mentioned above, the present inventors believe that it is due to the coarse ferrite band remaining in the hot-rolled temporary center layer, and first focused on the hot rolling process in which the coarse ferrite band is formed. . Generally, in steels such as No. 5115430, in which austenite precipitates and forms two phases at high temperatures, the structure is refined due to effects such as dispersion of austenite precipitated during hot rolling, but up to the high temperatures targeted by the present invention ( In ferritic stainless steels, which are essentially a single phase of ferrite (at hot working temperatures), M by other means.
It is necessary to make the i-weave finer. The present inventors have conducted systematic research on this point and have already published "Tetsu to Hagane" 70 (1984).
), P2152, reported on the possibility of tissue division due to deformation zones. However, this was aimed at forged materials and did not reach the level of sufficient industrial technology.

Therefore, we collected columnar crystals, which are thought to be the cause of ridging, from the center of the thickness of actual ferritic stainless steel slabs, and based on these samples, we conducted various treatments to refine the structure. We conducted a number of experimental studies. In particular, since it is necessary to refine the structure in the hot rolling process to make columnar crystals finer, we investigated the changes in the structure of columnar crystals during the film thickness process. In addition, the effects on ridging were investigated in detail by varying conditions such as the chronological position of the holding time. As a result, Figure 1~
We were able to find an interesting relationship shown in Figure 4.

First, the test conditions under which the results shown in FIGS. 1 to 4 were obtained will be explained. The chemical composition values of the test steel are shown in Table 1. A
Steel No. 1 is a ferritic stainless steel containing Nb, and steel A2 is a ferritic stainless steel containing Ti. These steels were melted through a 40-ton electric furnace, a converter, and a vacuum degassing device, and were continuously cast into slabs using continuous casting equipment. Then, after macro-etching the cross section of this slab, a 4o1m
A rolling sample of F2 diameter x 100fi width x I11 length was taken. After heating this sample, the roll diameter was 330 m.
The sheets were hot-rolled to a thickness of 3.6 mm using a mφ hot rolling mill under various hot-rolling conditions as shown in Table 2. The hot rolling conditions are the temperature at the start of rolling (the temperature at which the slab is extracted from the heating furnace).

The retention time (delay time) from the 9th film pressure 1 pass to the second pass was used as a variation factor. The obtained hot-rolled sheet was heated at 1000°C x 1 for A+ steel.
For A2 steel, short-time annealing at 900°C x 1 minute was carried out, and cold-rolled twice and annealed twice to finally achieve 0.
．． A cold-rolled annealed plate with a thickness of 7 fl was used.

From the obtained cold-rolled annealed plate, a parallel portion 35 is formed parallel to the rolling direction.
A tensile test piece with a width of f1 and a length of 120 mm was taken.

After this was stretched by 20%, ridging that appeared on the surface was measured.

Rigging is measured by using a surface roughness meter to measure the center line average roughness R.
At the same time as measuring a, the appearance of the surface was judged.

These were evaluated on the following five levels. When judging the appearance, items with large pack rings were ranked down by one rank.

Rigging rating Center line average roughness Ra Appearance (Cu
toff straight 2.5mm) Good I Ra<2.2 pm
Rank l↑ 2 2.2≦Ra <2.8
p m Rank 2 No 5 4.5≦Ra
Rank 5 Figure 1 shows the relationship between the extraction temperature (rough rolling start temperature) and the presence/absence of delay after 1 pass for the ^1 steel in Table 1 and the rating. l
During the delay after the pass, the temperature was maintained at approximately the same temperature as the material temperature after the pass, and the delay time was 120 seconds in both cases.

As is clear from the results in FIG. 1, when the extraction temperature is lowered, the ridging is improved overall1, and when a delay is provided, the ridging is improved compared to the case without a delay. For example, in the case without 1 delay, there are cases where the extraction temperature was around 1100℃ and a ridging score of 2 or less was obtained, but on average the score was around 3, and there was a large variation and industrially stable characteristics could not be obtained. Even if the extraction temperature is further lowered, a sufficient and stable extraction score cannot be obtained. On the other hand, when a delay is provided, the extraction temperature is in the range of 1050 to 1180'C and the ridging rating is 2.
has been obtained stably. This level of ridging rating of 2 is a level that satisfies very strict requirements for ridging properties in materials for press molding.

FIG. 2 shows the relationship with path positioning in which a delay is provided. In Figure 2, the ○ mark indicates a delay of 30-120 seconds after 1 pass at each extraction temperature, and the Δ mark indicates a delay of 30-120 seconds at each extraction temperature after 3 passes.
A delay of 0 seconds is provided. Figure 3 illustrates the position of this delay. As can be seen from the results in FIG. 2, providing a delay in the pass before rough rolling improves the overall ridging score. 1050-1180℃
When a delay was provided after one pass at an extraction temperature of , a ridging score of 2 was obtained with the exception of 2 points. One of the items that did not receive a rating of 2 had an exceptional rolling reduction of 20%, the others had a rolling reduction of 30% or more, and the other one had an exceptional delay time. The delay time was 10 seconds, and the other delay times were 30 seconds or more.

From the above basic tests, in order to improve the soldering of ferritic stainless steel, the hot rolling start temperature in the hot rolling process (slab extraction from the heating furnace) should be set at 1050 to 11 degrees.
It is necessary to set the temperature in the range of 80°C, and to find a delay time of at least 30 seconds at the stage of rough rolling, and to take this delay at the time of an early pass in rough rolling. It has been found that it is better to set the rolling reduction ratio to % or more. In terms of actual operating conditions, the extraction temperature of the slab is set to 1050 during the rough rolling process.
"The temperature range is 0 to 1180℃, and the plate thickness is 1 of the initial slab thickness.
The rolling reduction rate is 30 in the first pass of rough rolling until it reaches 72.
% or more passes at least once and after this pass.

It is better to carry out the next pass rough rolling after a delay of 30 seconds or more.

The reason for such beneficial results can be found in Section 4.
Judging from the organizational photographs shown in Figures 7 to 7, the following can be considered. Figures 4 and 5 are from 840 dragons to 12
When rolling 3 passes to the dragon.

Figure 4 shows the extraction temperature at 1200°C, and Figure 5 shows the extraction temperature at 1100°C, with a delay of 120 seconds after each pass. The one extracted at 1200°C in Figure 4 has a coarse recovered ferrite structure even after a delay is applied.
The sample shown in FIG. 5, which was extracted at 1100° C. and subjected to a delay, has a recovered Mi texture (partially recrystallized) containing many deformed bands (which appear as diagonal stripes in the photograph of FIG. 5). That is, as the extraction temperature was lowered, a deformation zone was introduced during hot rolling and the U weave was significantly fragmented, and then by correcting the delay, recovery (partial recrystallization) progressed around this deformation zone. It suggests that. The use of this deformation zone to refine the structure during hot rolling, and the progress of recovery and recrystallization during the subsequent delay, allows the resulting structure to contribute to the improvement of ridging, which is the 6th point of the 9th process. It can be seen from the figure and the photograph in Figure 7.

Figures 6 and 7 show hot-rolled sheets prepared from the hot-rolled structures shown in Figures 4 and 5, respectively.
This is a photograph showing the state of recrystallization when water-cooled annealing was performed after soaking for 1 minute. In the hot-rolled annealed sheet shown in Figure 6, coarse unrecrystallized ferrite exists in the center of the sheet thickness;
It can be seen that the hot-rolled annealed plate shown in Figure 7 is sufficiently recrystallized up to the center of the plate thickness.

Zunawara 2 Under the hot rolling conditions according to the method of the present invention, a deformed band structure is introduced and recovered, and a fine hot rolled structure can be obtained, 2 which is thought to contribute to the improvement of ridging.

In this way, we were able to solve the conventional problem of ferritic stainless steel gluing, but the next problem that the inventors encountered was vertical cracking that occurs during secondary processing. These vertical cracks occur when secondary processing such as shape correction is performed after high-level primary drawing processing, and they tend to occur frequently especially in winter. Although the addition of Ti and Nb is beneficial for improving the workability and corrosion resistance of ferritic stainless steel, such longitudinal cracking is significantly promoted in steels containing Ti and Nb. In order to solve this problem, the present inventors conducted test research not only from the viewpoint of composition but also from the viewpoint of manufacturing conditions such as cold rolling distribution ratio and annealing conditions, but the hot rolling conditions for improving the rolling properties described above were adopted as they were.

It has been found that if the obtained hot rolled sheet is not annealed, that is, the hot rolled sheet annealing is omitted and conventional cold rolling is carried out, this longitudinal cracking transition temperature can be lowered by about 20°C. Figure 8 shows the results.

Figure 8 shows the two types of steel in Table 1, when hot rolled plate annealing is omitted (As Hot), when annealing is performed at 850°C for 1 hour (furnace cooling), and when annealing is performed at 950°C for 1 minute (air cooling). ), the vertical cracking transition temperature (to,
z) was investigated. The manufacturing conditions were the same except for the presence or absence of annealing and the conditions. In other words, hot rolling conditions,
The cold rolling conditions were the same as described in the section of the sliding test above. In addition, the longitudinal cracking transition temperature (To, z) was determined by an electric drop test method as shown in Figure 9. In this lightning test, the cold-rolled annealed sheet was subjected to step drawing with a drawing ratio of 3. :Outer diameter 2
Make a deep drawn cup with 7 rhymes, drop the ears and make the cup height 4.
A test cup 1 made up of two dragons is placed horizontally as shown in FIG. 9, and a weight 2 is dropped onto it to examine whether or not vertical cracks occur. At that time, the impact energy was changed by changing the falling height of 1 weight 2, and the test temperature was also changed. did.

As seen in the results in Figure 8, it is better to omit hot-rolled sheet annealing than to perform annealing at the longitudinal crack transition temperature (
ro, z) decreases, and the degree of decrease also reaches about 20°C. Therefore, if secondary workability is a particular problem, the above-mentioned gluing improvement measures should be adopted and 77%i
This means that temporary annealing can be omitted.

In this sense, ferritic stainless steels to which the method of the present invention can be advantageously applied are Ti, N, and Ti, which have improved workability and corrosion resistance.
It is a b-added steel, and in the present invention, c: o,
o3% or less, Si: 0.75% or less.

Mn: 0.40% or less, P: 0.04% or less, S: 0.01% or less, Cr: 12. O%~22.0%
, Ni: 0.5% or less.

N: 0.03% or less, O: 0.015% or less,
sol, Af: 0.05% or less, and 0.04
% ~ 0.40 Shiro Ti or 0.10% ~ 0.80%
containing one or two types of Nb.

In some cases, it is recommended to use a ferritic stainless steel that further contains 2% or less Mo or 1% or less Cu, and the balance is iron and unavoidably mixed impurities.

The reason for limiting the range of components of this steel is as follows.

C is an element harmful to formability and corrosion resistance.

Furthermore, increasing the C content increases the amounts of Ti and NbO, which in turn leads to a decrease in secondary workability, so the upper limit is set at 0.03%. Si is added as a deoxidizing agent, but if its content is high, the material will harden, so 0.75
% or less. Mn fixes S as MnS and improves hot workability, but on the other hand, MnS becomes a starting point for pitting corrosion and deteriorates corrosion resistance. Therefore, it is advantageous to strictly control S and keep the amount of Mn added low, and in this sense it is set to 0.40% or less. Since P has a negative effect on secondary processability, the lower the content, the better it is 0.04% or less.

Since S is harmful to corrosion resistance and is preferably lower, it is set to 0.01% or less. The lower limit of Cr is 12 to obtain a substantially single-phase ferrite structure at the hot rolling temperature and from the viewpoint of corrosion resistance.
．． The content should be 0% or more. On the other hand, if it exceeds 22%, the toughness of the hot rolled sheet will decrease, so the content should be in the range of 12.0 to 22.0%.

Ni is mixed in from auxiliary raw materials such as scrap.

If the bond exceeds 0.5%, the cost will increase, so 0.5%.
The following shall apply. Since N, together with C, is harmful to formability and corrosion resistance, the content should be 0.03% or less. ○ increases non-metallic inclusions in the steel, which is harmful to workability, but from the standpoint of secondary workability, A1
In order to limit oxygen consumption, it is necessary to regulate the upper limit of oxygen.

Toughness and forming properties 1 Oxygen content is 0.0 in terms of bending workability.
If it exceeds 15%, these characteristics will deteriorate, so 0.0
15% or less.

81 is added as a deoxidizing agent, but it is also useful for improving processability. However, it is also an element that promotes vertical cracking, and for this reason it is set at 0.05% or less. Ti is added for the purpose of improving processability and corrosion resistance, but it is added in an amount exceeding 0.40%. % crackability decreases significantly. Like Ti, Nb is also added for the purpose of improving processability and corrosion resistance.
Nb can be added up to a high content because it causes fewer vertical cracks than Ti, but if it exceeds 0.80%, the toughness of the hot rolled sheet will decrease, so it should be kept at 0.80% or less.

Typical examples in which the method of the present invention was implemented in actual operation are listed below.

Table 3 shows the chemical composition of the steel produced in Example. Slabs were manufactured from these steels by continuous casting, and real coils were manufactured under the conditions shown in Table 4. Rough rolling was carried out in 6 to 8 passes to reduce the rolling thickness from 200 fi to 25 fi, and rolling was carried out in 6 passes to reduce the rolling from 25 fi to 3.6 fi. In Table 4, the rolling reduction column shows the rolling reduction in the pass with the delay (the pass before the delay treatment). The obtained hot rolled sheet was continuously annealed at 1000° C. and cold rolled twice including intermediate annealing to produce a cold rolled annealed sheet having a thickness of 0.7 mm. The surface condition of the hot coil and the gluing score of the cold rolled annealed plate are also listed in Table 4. The ridging score is based on the criteria described in the main text.

From Table 4, all of the gluing scores of cold-rolled annealed sheets manufactured under the hot-rolling conditions specified in the present invention are 2 or less,
It can be seen that it exhibits excellent rinsing properties. On the other hand, No. 1 has a high extraction temperature, Magneto No. 2 has a low reduction rate in the pass just before the delay, and N113 has one pass with one delay. At No. 4, where the delay time is short, the sliding score is high and the effect as in the present invention is not obtained. Moreover, looking at the surface condition of the hot coil, when the extraction temperature is 1050° C. or lower as in No. 5, linear surface defects occur frequently. When this surface flaw occurs, polishing is required in the next step, which causes an increase in cost. Although Ik5 has a good peeling score of 2 or less, there is a problem with this surface flaw, and when implementing the present invention, the extraction temperature is 105
The temperature is preferably 0°C or higher.

Table 5 shows the mechanical properties and deep drawability of cold rolled annealed sheets depending on the presence or absence of hot rolled sheet annealing (with annealing means when the above-mentioned continuous annealing is performed, and when annealing heat means when the above mentioned continuous annealing is not performed). It shows the Lankford value r and the secondary processing cracking incidence [vertical cracking transition temperature (T6.z) described in the text]. As is clear from the results in Table 5, all steels have good elongation and Lankford values and exhibit excellent workability, but A4 steel with excessive Ti added has a high hardness when hot-rolled sheet annealing is performed. While the incidence of secondary processing cracks is high, when hot-rolled sheet annealing is omitted, the (To, z) value is below 0°C. Note that the gluing score of the cold-rolled annealed sheet obtained by omitting this hot-rolled sheet annealing was 2.

[Brief explanation of drawings]

Figure 1 is a diagram showing the relationship between the rough rolling start l1 degree (extraction temperature) and the presence or absence of delay after 1 pass in rough hot rolling of ferritic stainless steel, and the rolling rating, and Figure 2 is the relationship between the extraction temperature and the delay position π. FIG. 3 is a diagram illustrating the position of the delay in FIG. 2, and FIGS. 4 and 5 are for 3 passes of rolling from 40111 to 12 dragon. Fig. 4 shows the rolled Mi weave (hot rolled Mi weave after 3-pass rolling) when the extraction temperature is 1200°C, and Fig. 5 shows the extraction temperature at ttoo°C, with a delay of 120 seconds after each pass. The micrographs, Figures 6 and 7, show hot-rolled sheets prepared from the hot-rolled structures shown in Figures 4 and 5, respectively, soaked at 1000°C for 1 minute, and water-cooled annealed. Figure 8 is a diagram showing the relationship between the presence or absence of hot rolled sheet annealing and the longitudinal crack transition temperature T0.2, Figure 9 is the test in the longitudinal crack transition temperature test shown in Figure 8. It is a figure showing the relationship between a cup and a weight. 1. Test cup, 2. Weight.

Claims (4)

[Claims] (1) A slab of ferritic stainless steel that exhibits a ferritic single-phase structure at hot rolling temperature is roughly hot rolled and finished hot rolled to produce a hot rolled sheet, and then subjected to normal annealing and cold rolling. In a method for manufacturing a steel plate or steel strip of ferritic stainless steel, the rough rolling is performed in multiple passes, and the temperature of the slab at the start of the rough rolling is in the range of 1050°C to 1180°C, and the plate thickness is In the first pass of rough rolling until the initial slab thickness is reduced to 1/2, a pass with a reduction ratio of 30% or more is performed at least once, and after this pass, there is a delay of 30 seconds or more before the next pass of rough rolling. A method for producing a ferritic stainless steel plate or strip with good workability. (2) Ferritic stainless steel has C:0 in weight%.
．． 03% or less, Si: 0.75% or less, Mn: 0.40
% or less, P: 0.04% or less, S: 0.01% or less, C
r: 12.0% to 22.0%, Ni: 0.5% or less, N
: 0.03% or less, O: 0.015% or less, sol. A
l: 0.05% or less, and 0.04% to 0.40%
of Ti or 0.10% to 0.80% of Nb, and in some cases, further contains 2% or less of M.
2. The manufacturing method according to claim 1, wherein the steel contains 0 or 1% or less of Cu, with the balance consisting of iron and unavoidably mixed impurities. (3) A slab of ferritic stainless steel that exhibits a substantially ferritic single-phase structure at hot rolling temperature is rough hot rolled and finished hot rolled to produce a hot rolled sheet, and then cold rolled to produce a ferritic stainless steel slab. In the method for manufacturing a steel plate or steel strip, the rough rolling is performed in multiple passes, and the temperature of the slab at the start of the rough rolling is set at 1050°C to 1180°C.
In the first pass of rough rolling until the plate thickness becomes 1/2 of the initial slab thickness, perform at least one pass with a reduction ratio of 30% or more, and after this pass, for at least 30 seconds. After a delay, the next pass rough rolling is performed, and after hot rolling is completed, hot rolled sheet annealing is omitted and cold rolling is performed by cold rolling once or two or more times including intermediate annealing. thing. A method for manufacturing ferritic stainless steel plates or strips with excellent workability. (4) Ferritic stainless steel has C:0 in weight%.
．． 03% or less, Si: 0.75% or less, Mn: 0.40
% or less, P: 0.04% or less, S: 0.01% or less, C
r: 12.0% to 22.0%, Ni: 0.5% or less, N
: 0.03% or less, O: 0.015% or less, sol. A
l: 0.05% or less, and 0.04% to 0.40%
of Ti or 0.10% to 0.80% of Nb, and in some cases, further contains 2% or less of M.
4. The manufacturing method according to claim 3, wherein the steel contains 0 or 1% or less of Cu, with the remainder consisting of iron and unavoidably mixed impurities.