JP2001001004A - Manufacture of austenitic stainless steel bar and wire - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for manufacturing bars and wires by which direct rolling is possible as it is cast without executing preprocessing for recrystallization, cost and time used for the conventional blooming or the like are omitted and the efficiency of the rolling is rapidly improved about hot hard-to-work kinds of steel of austenitic stainless steel. SOLUTION: This method is a method for manufacturing the bars and wires by executing slight rolling of a cast billet of austenitic stainless steel having a circular cross section by skew rolling after heating in a cast billet heating stage and, next, caliber rolling. In this manufacturing method of the bars and wires of austenitic stainless steel, the skew rolling is executed at the outer layer reduction ratio so that the reduction ratio of the outer layer of steel per roll is >=15% in the skew rolling and average number of roll contact.

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 rod or rod by rolling a cast piece of austenitic stainless steel.

[0002]

2. Description of the Related Art In the production of stainless steel rods, usually, a rolled material such as a slab is heated and then hot-rolled by a continuous rolling mill having several or more grooved rolling mills to form a square having a size of one hundred to several hundred mm. Alternatively, it is performed by molding a circular material to a diameter of several to several tens of mm. One of the problems when rolling austenitic stainless steel with this continuous rolling mill is a crack. This is a free-cutting austenitic stainless steel (eg, SU
S303) or a high delta ferritic steel (for example, SUS309) used for welding rods etc. because of its good resistance to welding cracking
In such an austenitic stainless steel which is difficult to hot work, hot working cracks are generated during rolling and these are flaws.

As a countermeasure against this, as a conventional countermeasure, the coarse crystals of the slab are recrystallized into fine grains by applying hot strain by slab rolling or the like to improve the hot workability before being subjected to rolling. Often. However, this method requires an extra step from casting to rolling, and requires extra energy for reheating. Further, grinding for removing cracks generated at corners and the like is required, and the yield loss is large.

[0004] Therefore, measures for omitting bulk rolling have been conventionally devised. For example, Japanese Patent Application Laid-Open No. 2-8320 discloses a method of improving hot workability by applying a reduction of 20% or more before the steel material is cooled after casting. If the slab rolling can be omitted by this (it is not described whether it can be omitted), it is surely an effective measure that can omit the energy of the recrystallization pretreatment step and the reheating. However, when there are a plurality of casting machines, a reduction machine must be provided for all of them, which increases equipment costs.In addition, roll reduction and forging described in the specification cause hot working cracks as usual, Still requires a significant amount of surface grinding.

On the other hand, a rolling method called inclined rolling has been devised and widely used for groove rolling. This is a method of rolling while rotating a roll around a circular steel material not in the moving direction of the steel material but in a direction slightly inclined from the circumferential direction. In particular, in the production of seamless pipes, etc., several 2Hi rolling mills are made into one pass. It has been used as a high pressure mill. Japanese Patent Laid-Open No. 5-277 discloses a method for rolling a rod or wire using the inclined rolling mill.
Japanese Patent Publication No. 503 discloses a line of equipment in which inclined rolling is performed at the preceding stage of grooved rolling in a bar and wire rod rolling line, but does not disclose a method for producing the above-described hot-worked steel without any scratches. Japanese Patent Application Laid-Open No. 6-88128 discloses a method for obtaining a fine-grained structure by rolling an inclined roll at a specific temperature and a specific rolling rate in a method for manufacturing a seamless steel pipe. This patent relates to ordinary steel. In addition, it does not disclose a method for improving hot workability or a method for producing a hot-worked material without any flaws.

[0006]

SUMMARY OF THE INVENTION The present invention has been devised to solve the above-mentioned conventional problems. That is, by performing rolling by an inclined rolling mill just before the channel rolling and by setting the rolling conditions such as the reduction rate of the cross-section, the roll angle, and the rolling temperature specific to each steel type, the austenitic stainless steel can be used. Various types of hot-worked steel can be rolled as casts without casting for recrystallization as a raw material, and the time and energy conventionally used for pre-treatment such as slab rolling can be omitted. It is an object of the present invention to provide a method for producing an austenitic stainless steel rod or wire that can be used.

[0007]

SUMMARY OF THE INVENTION The present invention solves the above-mentioned problems, and the gist thereof is as follows. (1) A method of manufacturing a rod or wire by heating a cast austenitic stainless steel slab having a circular cross section in a slab heating step, performing mild rolling by inclined rolling, and then performing groove rolling. A method for producing an austenitic stainless steel rod or wire, characterized in that inclined rolling is performed at a surface layer processing rate and an average number of roll contact times such that the processing rate per roll of a steel material surface in one roll is 15% or more. (2) A high-temperature high-speed tensile test was performed to determine the relationship between the tensile temperature and the fracture reduction value. Based on the results, a range of the tensile temperature at which the fracture reduction value exceeded the working rate per roll of the surface layer of the steel material was derived. The method for producing an austenitic stainless steel rod according to the above (1), wherein tilt rolling is performed in the temperature range.

[0008]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described in detail.
The present inventors have studied in detail the deformation behavior in inclined rolling and grooved rolling. As a result, when examining in detail the inclined rolling mill, which was conventionally only used as a "large rolling mill" when it was used for the production of seamless pipes, the hole rolling mill has the following three meanings. It has been found that the rolling mill is more suitable for avoiding hot working cracks and is most suitable for rolling austenitic stainless steel having low hot workability.
That is, by appropriately controlling the conditions, it is possible to apply the processing strain required for recrystallization without causing cracks, so that a fine recrystallized structure can be created in-line, and as a result, cracks do not occur even in subsequent hole rolling. Rolling becomes possible.

[0009] The inclined rolling mill is superior to the hole rolling mill in the following three points. First, the strain distribution in the steel material is close to uniform. In the groove rolling, circumferential strain is concentrated on the side of the steel material where the roll does not contact. Therefore, even if the average draft is low from the viewpoint of the cross-sectional reduction rate, the crack may exceed the critical strain on the side and cause cracking. On the other hand, in the inclined rolling, since the strain is applied almost uniformly, it is possible to give the rolling reduction almost equal to the critical strain of the steel material.

Second, the surface layer may be subjected to a particularly high pressure. Rolling rolls have a uniform rolling strain distribution in the depth direction, but inclined rolling has higher strain on the surface layer due to the addition of shear strain to the surface layer, and it is a region that is generated from the central part irrespective of surface cracking Fine recrystallization can be performed with priority on the surface layer. Conversely, the rolling reduction required to obtain a fine recrystallized structure may be smaller than that of slot rolling.

Thirdly, since each roll hits the surface several times even with one inclined rolling machine, it can be mentioned that multi-pass rolling is actually performed. For this reason, since the distortion has almost disappeared due to recrystallization between the first roll and the next roll, if it can withstand the distortion of one pass of each roll, it will be possible to avoid cracking, and as a result, a higher strain will be obtained in total. Can be given. In order to obtain the same effect in the groove rolling, it is necessary to connect a large number of mills under light pressure (and, for the first reason, under light pressure more than inclined rolling), and the equipment cost is enormous. Recrystallization in such a short time is a phenomenon peculiar to the austenitic structure, and this method cannot be applied to ferritic stainless steel.

In order to make the most of these characteristics of the inclined rolling, the present inventors have derived rolling conditions as described below. When manufacturing conventional seamless pipes,
The rolling reduction in the inclined rolling mill was as large as, for example, a cross-sectional reduction rate of 50%. However, in the method of the present invention, a minimum reduction ratio sufficient to recrystallize the surface layer is applied to roll the hot-worked material. Specifically, the processing rate is set as follows. First of all, recrystallization must be completed in one pass of each roll. For this purpose, recrystallization must be completed by dynamic recrystallization or static recrystallization between rolls in a very short time.However, since cracking is a problem only in the surface layer, only the surface layer is recrystallized. Just do it. The inventors have found from various experiments that a surface layer processing rate of 15% or more satisfies this condition sufficiently.

The surface layer processing rate shown here is different from the normal rolling rate, that is, the area reduction rate. As described above, in the case of inclined rolling, the surface layer is higher than the center. Therefore, the surface layer processing rate, which is a requirement of the present invention, is larger than the cross-sectional reduction rate, which is the average processing rate of all cross sections. In addition, since one roll of inclined rolling hits a roll several times, it is necessary to determine not the working rate of the whole inclined mill but the working rate per one pass of each roll. The “surface layer processing rate in one pass of each roll” can be obtained as follows. First, the surface layer processing rate can be derived by a method shown in FIG. 1 by taking out a slab immediately after rolling by 1-millimeter rolling and observing the metal flow of the structure. Next, the surface layer processing rate is converted into the surface layer processing rate per roll using the average number of rolls that contact each position during rolling. For this purpose, if the surface layer processing rate is ε and the number of rolls is N, 1- (1-ε) ^{1 / N} may be calculated.
In addition, in inclined rolling, once on the surface with the roll,
On some surfaces it may hit twice,
In that case, take the average of the number of rolls. Therefore, the number of rolls may not be an integer.

In the above-described inclined rolling, cracks may occur in some cases, which may be flaws. Although the flaws are much smaller than those caused by the groove rolling, in order to avoid such flaws, the working rate must be suppressed to such an extent that cracks do not occur. Regarding this, it is considered that the reduction ratio at high temperature and high speed tension, at which hot workability can be measured most easily, and the working ratio at which cracking occurs during rolling are almost the same,
On the basis of this, cracking can be avoided if the surface layer processing rate in one pass of each roll is made smaller than the breaking reduction value. Note that, unlike the grooved rolling, such a calculation is possible in the inclined rolling because the strain distribution is nearly uniform.

The breaking reduction value of each steel material changes with temperature. Therefore, the relationship between the tensile temperature and the breaking reduction value was determined by the above test, and based on the result, the inclined rolling was performed so that the rolling was performed in the range of the tensile temperature at which the breaking reduction value exceeded the “surface processing rate in one pass of each roll”. The processing temperature may be specified.
Generally, the hot workability of an austenitic stainless steel material generally improves as the temperature increases, and it is necessary to roll the hot-worked material at a somewhat high temperature. However, if the temperature is too high, even if recrystallization is performed due to melt embrittlement, hot workability deteriorates so much that it cannot be resisted, so that there is always an upper limit temperature.

If the breaking draw value is less than the surface working rate per roll at any temperature, the rolling conditions are changed to reduce the surface working rate or to increase the average number of roll contacts, so that the breaking draw value does not fall below 15%. It is necessary to reduce the surface layer processing rate per roll within a range not exceeding the value. The method may be to reduce the average draft or sectional reduction rate, or to change the angle so that the roll rotates more than the circumferential direction of the steel material and obtain the same effect even if the number of times the steel material hits the roll is increased. I can do it.

[0017]

EXAMPLE A round slab (170 mm in diameter) cast by continuous casting of austenitic stainless steels SUS304 and SUS303 using the rolling equipment shown in FIG.
Rolling by changing the processing conditions and temperature of roll inclined rolling,
The state of the scratch after rolling was determined by visual evaluation. The following four conditions were tested as processing conditions. 1) Roll a 170 mm diameter cast slab to a 140 mm diameter. 2) A 170 mm diameter slab is rolled to a 140 mm diameter in the same manner as in 1), but the angle of the inclined rolling roll with respect to the slab is reduced so that the roll rotates more in the circumferential direction. 3) Rolling 170 mm diameter to 120 mm diameter at the same roll angle as in 2). 4) Similarly rolled to 160mm diameter. FIG. 3 shows the working ratio in the depth direction under these conditions. It was derived by taking out the slab after inclined rolling and observing the metal flow of the structure. From these results, the processing rate of the surface layer of 0.5 mm was defined as each surface processing rate. Further, under the conditions 2) to 4), the roll makes contact with the entire surface of the slab twice or more and less than three times, but under the condition 1) there is a surface that does not contact twice. When calculating the average number of loin contacts, 1); 1.3 times, 2); 2.0 times, 3) 2.5 times,
4); 1.8 times. Next, SUS304, SUS
The breaking reduction value at each temperature of the 303 slab was measured as follows. A 10φ × 120 test piece was collected from the surface layer of the slab, and after rapid heating of each test piece, the strain rate was 3 at each temperature.
/ Sec. The area of the rupture portion was determined, and the area of the rupture was divided by the area before the tension to obtain the rupture aperture value. FIG. 4 shows the measurement results of the squeeze value at each temperature.

Table 1 shows the rolling conditions and the flaw results after rolling. SUS303 having a high S has extremely poor hot workability as compared with SUS304. Therefore, as shown in the comparative example, in the case of conventional groove rolling, the SUS
At 303, cracks occur. On the other hand, when the groove rolling is performed after the inclined rolling, if the conditions are well controlled, the SU
Even the cast slab of S303 can be rolled without cracks.

[0019]

[Table 1]

First, the "surface layer processing rate in one pass of each roll" is derived for the conditions 1) to 4). Condition 1)-
The cross-sectional reduction rates of 4) are about 33%, 33%, and 50%, respectively.
% And 11%, but the surface layer processing rate is completely different. FIG.
As shown in Table 1, the surface layer processing rates under the conditions 1) to 4) are about 67%, 75%, 90%, and 22%, respectively. The number of contact times of the rolls was 1.3, 2.0, 2.5, and 1.8, respectively. Therefore, the surface layer processing rate per operation is as follows: Condition 1) 1- (1−0.67) ^{1 /} 1.330.57... 57% Condition 2) 1− (1−0.75) ^1/2 = 0.50 ... 50% Condition 3) 1- (1-0.90) ^{1 / 2.5} ≒ 0.60 ... 60% Condition 4) 1- (1-0.22) ^{1 / 1.8} ≒ 0.13 ... ... 13%. Therefore, in the case of the condition 1), the SUS303 breaking aperture value is 5
The tilt rolling may be performed at about 1200 ° C. exceeding 7%.
If the temperature is higher than 1300 ° C., the hot workability is rather deteriorated and cracks occur. On the other hand, in the case of 2), the temperature range may be higher than 50%, so that rolling can be performed in a wide range from 1100 ° C. to 1300 ° C. without cracking. Condition 3) is that the rolling reduction temperature range of SUS303 is 1200 because the rolling reduction is large.
It becomes narrow at around ℃. Therefore, it can be understood that there is no merit in using the steel sheet in such a use as the present case even if the working rate of the inclined rolling is set at a high pressure reduction rate as used in the conventional steel pipe rolling. Under the condition 4), since the surface layer processing rate per one roll falls below 15%, it reaches the grooved rolling mill without being recrystallized and cracks occur.

Since SUS304 has good hot workability, even if the same rolling is carried out, it can be rolled without any problem under all of the conditions 1) to 4). However. At a high temperature of 1350 ° C., hot workability deteriorates rapidly due to melt embrittlement and cracks occur. In addition,
To change the surface layer processing rate and the average number of times of contact with the rolls, it is necessary to replace and adjust the rolls. Therefore, it is more efficient to set the rolling conditions according to the type of the hot workable steel.

[0022]

According to the present invention, it is possible to directly roll austenitic stainless steel hard-to-work hot-rolled steel as cast without performing recrystallization pretreatment. Therefore, only the heating is required for the rolling, and there is no need to grind the processing cracks generated in the slab rolling, etc., so that the cost and time can be greatly reduced, and the rolling efficiency can be dramatically improved.

[Brief description of the drawings]

FIG. 1 is a view showing a method of obtaining a surface layer processing rate from a metal flow of a slab immediately after inclined rolling.

FIG. 2 is a process diagram of a rolling facility used in Examples.

FIG. 3 is a diagram showing the relationship between the depth from the surface layer and the processing rate under four conditions of the inclined rolling mill used in the examples.

FIG. 4 is a diagram showing the relationship between the temperature of SUS304 and SUS303 slabs used in Examples and hot workability (fracture reduction value).

 ──────────────────────────────────────────────────の Continuing on the front page (72) Inventor Kyotaro Amano 3434 Shimada, Hikari-shi, Yamaguchi Prefecture Nippon Steel Corporation Hikari Works (72) Inventor Yoshitaka Nakamura 3434, Shimada, Hikari-shi, Hikari-shi, Yamaguchi New Japan (72) Inventor Hidekazu Nasu 3434, Shimada, Hikari-shi, Yamaguchi Prefecture Nippon Steel Corporation Hikari Works F-term (reference)

Claims (2)

[Claims] 1. A method for producing a rod or wire by heating a cast austenitic stainless steel slab having a circular cross section in a slab heating step, performing mild rolling by inclined rolling, and then performing groove rolling. A method for producing an austenitic stainless steel rod or wire, wherein the inclined rolling is performed at a surface layer processing rate and an average number of roll contact times such that a processing rate per roll of a steel material surface layer per roll in the inclined rolling is 15% or more. 2. A high-temperature high-speed tensile test is performed to determine the relationship between the tensile temperature and the rupture reduction value. Based on the results, a range of the tensile temperature at which the rupture reduction value exceeds the working rate per roll of the steel surface layer is derived. 2. The method for producing an austenitic stainless steel rod according to claim 1, wherein tilt rolling is performed in the temperature range.