JP3844622B2 - Method for producing austenitic stainless steel bar wire - Google Patents

Description

【００２０】
まず、条件１）〜４）について「各ロールの１パスにおける表層加工率」を導出する。条件１）〜４）の断面減少率はそれぞれ約３３％，３３％，５０％，１１％であるが、同表層加工率は全く異なる。図３に示すように条件１）〜４）の表層加工率はそれぞれ約６７％，７５％，９０％，２２％である。またロールの接触回数はそれぞれ１．３，２．０，２．５，１．８である。従って、１回あたりの表層加工率は、
条件１） １−（１−０．６７）^1/1.3 ≒０．５７……５７％
条件２） １−（１−０．７５）^1/2 ＝０．５０……５０％
条件３） １−（１−０．９０）^1/2.5 ≒０．６０……６０％
条件４） １−（１−０．２２）^1/1.8 ≒０．１３……１３％ となる。
従って条件１）の場合、ＳＵＳ３０３の破断絞り値が５７％を超える１２００℃付近で傾斜圧延をすればよい。１３００℃以上では熱間加工性が却って悪化し割れが発生する。一方、２）の場合は５０％より大の温度範囲でよいから、１１００℃以上１３００℃以下の広い範囲で割れの問題なく圧延可能である。
条件３）は圧下率が大きいことからＳＵＳ３０３の圧延可能温度域が１２００℃付近と狭くなる。従って今回のような用途に使用する場合、傾斜圧延の加工率を従来の鋼管圧延に使用するような高圧下率で使用してもあまりメリットが無いことがわかる。
条件４）ではロール１回あたりの表層加工率が１５％を割り込むため、未再結晶で孔型圧延機に達し割れを生じる。
【００２１】
なお、ＳＵＳ３０４は熱間加工性が良好のため、同様の圧延を行っても条件１）〜４）全てで問題無く圧延出来る。但し。１３５０℃と高温では溶融脆化により熱間加工性が急に悪化し割れ疵を生じる。なお、表層加工率、平均ロール接触回数を変更するにはロール取り替え、調整の手間を要するので、圧延条件は難熱間加工性鋼種に合わせた方が効率的である。
【００２２】
【発明の効果】
本発明によると、オーステナイト系ステンレス鋼難熱間加工鋼種について、再結晶前処理を行わずに鋳造ままで直接圧延することが出来る。従って加熱は圧延のものだけで済み、更に分塊圧延等で発生する加工割れを研削する必要もなくなり、大幅なコスト，時間を省略し、飛躍的に圧延の効率を向上させることが出来る。
【図面の簡単な説明】
【図１】傾斜圧延直後の鋳片のメタルフローから表層加工率を求める方法を示した図である。
【図２】実施例に用いた圧延設備の工程図である。
【図３】実施例に用いた傾斜圧延機の４条件における表層からの深さと加工率の関係を示す図である。
【図４】実施例に用いたＳＵＳ３０４，ＳＵＳ３０３鋳片の温度と熱間加工性（破断絞り値）の関係を示す図である。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method of rolling a slab of austenitic stainless steel to produce a bar wire.
[0002]
[Prior art]
The production of stainless steel rods and rods is usually done by hot rolling a rolled material such as a slab and then hot rolling it with a continuous rolling mill with several or more continuous rolling mills. It is performed by molding 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 cracking. This is like free-cutting austenitic stainless steel (for example, SUS303) used for a polishing bar because of good machinability and high delta ferritic steel (for example, SUS309) used for welding rod because of its good weld crack resistance. In such a hot-workable austenitic stainless steel, hot-working cracks occur during rolling, and this becomes a flaw.
[0003]
As a conventional measure against this, the coarse crystals of the slab are recrystallized into fine grains by applying hot strain by split rolling or the like, and is often used for rolling after improving hot workability. . However, this method requires an extra step from casting to rolling and takes extra energy for reheating. Furthermore, it requires grinding to remove cracks generated in the corners and the like, resulting in a large yield loss.
[0004]
For this reason, measures have been devised in the past to eliminate ingot rolling. For example, Japanese Patent Laid-Open No. 2-8320 discloses a method for improving hot workability by applying a reduction of 20% or more before the steel material after casting is not cooled. If it is possible to omit the batch rolling (there is no description as to whether it can be omitted), it is certain that this is an effective measure that can omit the recrystallization pretreatment step and the energy of reheating. However, when there are a plurality of casting machines, it is necessary to provide a reduction machine for all of them, resulting in high equipment costs, and in the roll reduction and forging described in the specification, hot working cracks are generated as usual. The fact that a considerable amount of surface grinding is required to remove the material remains unchanged.
[0005]
On the other hand, a rolling method called inclined rolling has been devised and widely used for hole rolling. This is a method of rolling around a circular steel material while rotating the roll in a direction slightly inclined from the circumferential direction rather than the moving direction of the steel material, and in particular, in the production of seamless pipes, the number of 2Hi rolling mills is made one pass. It has been used as a high pressure mill. As a method of rolling a bar wire using the inclined rolling mill, Japanese Patent Application Laid-Open No. 5-277503 discloses an equipment row in which inclined rolling is carried out before the hole-type rolling in the bar wire rolling line. There is no indication of a strategy that can produce hot-worked steel grades. Japanese Patent Application Laid-Open No. 6-88128 discloses a method for obtaining a fine grain structure by rolling gradient rolling at a specific temperature and a specific rolling rate in a method for producing a seamless steel pipe. This patent relates to ordinary steel. In addition, it does not show a method for improving hot workability, or a method for producing a hard work material without difficulty.
[0006]
[Problems to be solved by the invention]
The present invention has been devised to solve the conventional problems as described above. In other words, among the austenitic stainless steels, rolling is carried out by an inclined rolling mill immediately before the hole rolling, and the rolling conditions such as the cross-section reduction rate, the roll angle and the rolling temperature are specified according to each steel type. A wide variety of hard-working steel grades can be rolled as a raw material without casting and recrystallizing pretreatment, and the time and energy previously spent for pretreatment such as ingot rolling can be saved. An object of the present invention is to provide a method for producing an austenitic stainless steel bar wire that can be used.
[0007]
[Means for Solving the Problems]
The present invention solves the above-mentioned problems, and the gist thereof is as follows.
(1) performs a slight rolling austenitic stainless steel slabs having a circular cross section in a heating after inclined rolling at slab heating step, then a method of manufacturing a rod wire by performing caliber rolling, the inclined Rolling is performed at a surface layer processing rate and average number of times of contact with the roll so that the processing rate per roll of the steel surface layer is 15% or more, and a high-temperature high-speed tensile test is performed to determine the relationship between the tensile temperature and the fracture drawing value. A method for producing an austenitic stainless steel bar wire, characterized by deriving a tensile temperature range in which the fracture drawing value exceeds the processing rate per roll of the steel material surface layer based on the results, and performing in that temperature range .
[0008]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, the present invention will be described in detail.
The inventors of the present invention have studied in detail the deformation behavior in tilt rolling and hole rolling. As a result, a detailed examination of the inclined rolling mill, which was previously only used as a “large rolling mill” when used in the production of seamless pipes, has the following three meanings: It was found to be an optimum rolling mill for rolling austenitic stainless steel, which is more suitable for avoiding hot work cracking and is hard workable. That is, by properly controlling the conditions, it is possible to add the processing strain required for recrystallization without causing cracks, so that a fine recrystallized structure can be created in-line, and as a result, no cracks will occur even in subsequent hole rolling. It can be rolled.
[0009]
The inclined rolling mill is superior to the hole rolling mill in the following three points. First of all, the strain distribution on the steel material is almost uniform. In the perforated rolling, the circumferential strain concentrates on the side surface of the steel that is not hit by the roll. Therefore, even if the average rolling reduction as seen from the reduction rate of the cross section is low, the side surface may exceed the crack limit strain and lead to cracking. On the other hand, in tilt rolling, since the method of giving strain is almost uniform, it is possible to give the rolling reduction to almost the same as the crack limit strain of steel.
[0010]
Second, the surface layer may be subjected to particularly high pressure. In the case of perforated rolling, the distribution of rolling strain in the depth direction is uniform, but in the case of inclined rolling, the surface layer is subjected to higher shearing due to the addition of shear strain, and it is a generation region from the center that is not related to surface cracks. The surface layer can be preferentially finely recrystallized. In other words, the rolling reduction necessary for obtaining a fine recrystallized structure may be smaller than that of the hole rolling.
[0011]
Thirdly, since each roll hits the surface several times even with one inclined rolling, it is actually a multi-pass rolling. Therefore, since the strain is almost eliminated by recrystallization between the first roll and the next roll, cracking can be avoided if it can withstand the strain for one pass of each roll, and the higher strain as a total. Can be given. In order to obtain the same effect in the hole-type rolling, it is necessary to connect a number of mills that are lightly reduced (and lightly further reduced than the inclined rolling for the first reason), and the equipment cost is enormous. Recrystallization in such a short time is a phenomenon peculiar to an austenite structure, and this method cannot be applied to ferritic stainless steel.
[0012]
In order to make the most of the characteristics of these inclined rollings, the present inventors have derived rolling conditions as shown below. In the case of manufacturing a conventional seamless pipe, the rolling reduction in the inclined rolling mill is as large as, for example, a cross-section reduction rate of 50%. However, in the method of the present invention, a minimum reduction ratio sufficient to recrystallize the surface layer is added, and this is utilized for rolling of a hot work 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, but cracking only affects the surface layer, so only the surface layer is recrystallized. Just do it. The inventors have found through various experiments that if the surface layer processing rate is 15% or more, this condition is sufficiently satisfied.
[0013]
The surface layer processing rate shown here is different from the normal rolling rate, that is, the cross-section reduction rate. As described above, in the case of inclined rolling, the surface layer is processed higher than the center portion. Therefore, the surface layer processing rate, which is a requirement of the present invention, is larger than the cross-section reduction rate that is the average processing rate of all cross sections. In addition, since even one mil of inclined rolling hits the roll several times, the processing rate per one pass of each roll must be obtained instead of the processing rate of the entire 1 mil of inclined rolling.
The above-mentioned “surface processing rate in one pass of each roll” can be obtained as follows. First, the surface layer processing rate can be derived by the method shown in FIG. 1 by taking out a slab immediately after rolling in a tilted rolling 1 mil and observing the metal flow of the structure. Next, the surface layer processing rate is converted into a surface layer processing rate per roll using the average number of rolls in contact with 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 inclined rolling, there is a case where a roll hits once on a certain surface and twice on a certain surface. In this case, the average number of rolls is taken. Therefore, the number of rolls may not be an integer.
[0014]
In the above-described inclined rolling, cracks may occur in some cases, which may become wrinkles. The wrinkles are much smaller than those obtained by hole rolling, but in order to avoid them, the processing rate must be suppressed to such an extent that cracks do not occur. In this regard, it is considered that the fracture drawing value in high-temperature high-speed tension at which hot workability can be measured most easily coincides with the processing rate at which cracking occurs in rolling, and based on this, the surface layer processing rate in one pass of each roll If it is made smaller than the fracture drawing value, cracking can be avoided. In contrast to the hole-type rolling, such a calculation is possible in the inclined rolling because the strain distribution is almost uniform.
[0015]
The fracture drawing value of each steel material changes with temperature. Therefore, the relationship between the tensile temperature and the fracture drawing value is obtained by the above test, and the gradient rolling is performed so that rolling is performed at a tensile temperature range in which the breaking drawing value exceeds the “surface layer processing rate in one pass of each roll” based on the result. What is necessary is just to prescribe | regulate processing temperature. In general, the hot workability of an austenitic stainless steel material is mostly improved as the temperature becomes higher, and it is necessary to perform rolling at a certain high temperature for a hard work material. However, if the temperature is too high, the hot workability deteriorates so as to resist even if recrystallization occurs due to melt embrittlement, so there is always an upper limit temperature.
[0016]
If the fracture drawing value is below the surface layer processing rate per roll at any temperature, change the rolling conditions to lower the surface layer processing rate or increase the average number of contact times of the roll, so that it does not fall below 15% and exceeds the drawing value. It is necessary to reduce the surface layer processing rate per roll within a range that does not exist. As the method, the average reduction rate, that is, the cross-section reduction rate may be lowered, or the same effect can be obtained even if the angle is changed so that the roll rotates more than the circumferential direction of the steel and the number of times the steel hits the roll is increased. I can do it.
[0017]
【Example】
Using the rolling equipment shown in FIG. 2, round slabs (diameter 170 mm) cast by continuous casting for austenitic stainless steels SUS304 and SUS303 are rolled while changing the processing conditions and temperature of the three-roll inclined rolling, and after rolling The state of the wrinkles was determined by visual evaluation. The following four conditions were tested as processing conditions.
1) Rolling a 170 mm diameter slab to 140 mm diameter.
2) As in 1), a 170 mm diameter slab is rolled to 140 mm diameter, but the angle with respect to the slab of the inclined rolling roll is reduced and adjusted so that the roll rotates more in the circumferential direction.
3) Rolling 170mm diameter to 120mm diameter at the same roll angle as 2).
4) Similarly rolled to 160 mm diameter.
The depth direction machining rate under these conditions is shown in FIG. The slab after tilt rolling was taken out and observed by observing the metal flow of the structure. From this result, the processing rate of 0.5 mm part of the surface layer was defined as each surface layer processing rate. Further, under the conditions 2) to 4), the roll is in contact with the entire surface of the slab 2 times or more and less than 3 times, but under the condition 1), there is a surface that does not contact twice. When calculating the average roll contact number,
1); 1.3 times, 2); 2.0 times, 3); 2.5 times, 4); 1.8 times.
Next, the fracture drawing value for each temperature of the SUS304 and SUS303 slabs was measured as follows. A test piece of 10φ × 120 was taken from the slab surface layer, and each test piece was rapidly heated and then pulled and broken at each temperature at a strain rate of 3 / sec. The area of the fracture portion was determined, and the fracture drawing value was determined by dividing the area before tension. FIG. 4 shows the measurement results of the fracture drawing value at each temperature.
[0018]
Table 1 shows the rolling conditions and the wrinkle results after rolling. Compared with SUS304, SUS303 with high S has extremely poor hot workability. Therefore, as shown in the comparative example, cracks are generated in SUS303 when rolling is performed using an as-cast slab in conventional hole rolling. On the other hand, when hole rolling is performed after tilt rolling, even if the SUS303 is cast as it is, it can be rolled without cracks if the conditions are well controlled.
[0019]
[Table 1]

[0020]
First, the “surface layer processing rate in one pass of each roll” is derived for the conditions 1) to 4). The cross-section reduction rates of conditions 1) to 4) are about 33%, 33%, 50%, and 11%, respectively, but the surface layer processing rates are completely different. As shown in FIG. 3, the surface layer processing rates under conditions 1) to 4) are about 67%, 75%, 90%, and 22%, respectively. The number of contact times of the rolls is 1.3, 2.0, 2.5, and 1.8, respectively. Therefore, the surface layer processing rate per time is
Condition 1) 1- (1−0.67) ^{1 / 1.3} ≒ 0.57 …… 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 condition 1), the rolling may be performed in the vicinity of 1200 ° C. where the fracture drawing value of SUS303 exceeds 57%. If it is 1300 ° C. or higher, the hot workability deteriorates and cracks occur. On the other hand, in the case of 2), since the temperature range may be greater than 50%, rolling is possible in the wide range from 1100 ° C. to 1300 ° C. without cracking problems.
In condition 3), the rolling reduction temperature range of SUS303 becomes narrow at around 1200 ° C. because the rolling reduction is large. Therefore, it can be seen that there is not much merit in using the processing rate of the inclined rolling at the high pressure reduction rate used in the conventional steel pipe rolling when it is used for such applications.
Under condition 4), the surface layer processing rate per roll falls below 15%, and therefore reaches the hole rolling mill without recrystallization and causes cracks.
[0021]
Since SUS304 has good hot workability, it can be rolled without any problems under all conditions 1) to 4) even if the same rolling is performed. However. At a high temperature of 1350 ° C., hot workability suddenly deteriorates due to melt embrittlement and cracks are generated. In order to change the surface layer processing rate and the average number of times of roll contact, it is necessary to replace and adjust the rolls. Therefore, it is more efficient to match the rolling conditions with the hard-working steel type.
[0022]
【The invention's effect】
According to the present invention, an austenitic stainless steel hard-working steel type can be directly rolled as cast without performing recrystallization pretreatment. Accordingly, heating is only required for rolling, and further, it is not necessary to grind the work cracks generated in the batch rolling, so that significant cost and time can be omitted and the rolling efficiency can be dramatically improved.
[Brief description of the drawings]
FIG. 1 is a view showing a method for determining a surface layer processing rate from a metal flow of a slab immediately after tilt rolling.
FIG. 2 is a process diagram of the rolling equipment used in the 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 (breaking drawing value).

Claims (1)

Perform mild rolled austenitic stainless steel slabs having a circular cross section in a heating after inclined rolling at slab heating step, then a method of manufacturing a rod wire by performing caliber rolling, the tilting rolling, As a result of performing the high-temperature high-speed tensile test to obtain the relationship between the tensile temperature and the squeezed drawing value while performing the surface layer processing rate and the average number of times of roll contact so that the processing rate per roll of the steel surface layer is 15% or more. A method for producing an austenitic stainless steel bar wire, characterized by deriving a tensile temperature range in which the fracture drawing value exceeds the working rate per roll of the steel material surface layer based on the temperature range, and performing in that temperature range .

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