JPS644580B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to an austenitic stainless steel with excellent hot workability, and in particular, an object of the present invention is to provide an austenitic stainless steel with sufficient hot workability to enable application of the Mannesmann drilling method in the manufacture of seamless steel pipes. . Seamless steel pipes made of austenitic stainless steel are usually manufactured by hot extrusion. As a manufacturing method for such seamless steel pipes, the so-called Mannesmann drilling method is superior to the hot extrusion method in terms of productivity, etc. However, this type of processing method involves severe deformation, so it is When applied to austenitic stainless steel, significant cracking often occurs on the inner and outer surfaces of the tube, so it is not generally applied. However, in recent years, the hot extrusion method has been used for sour gas oil country tubular goods or piping, etc. due to equipment limitations (the maximum outer diameter that can be manufactured even with relatively large 3000 TON class extrusion equipment is 230φ).
mm) The demand for large-diameter pipes and large-diameter long pipes, which are difficult to manufacture, is increasing, and for this reason, it has become necessary to manufacture these large-diameter pipes using the Mannesmann drilling method, which has excellent hot workability. The development of austenitic stainless steel was desired. The present invention was made in view of these circumstances, and includes (1) stabilization of the austenite phase by regulating austenite-forming elements, etc. within a specific range;
(2) extremely low S content, and (3) regulation of oxide-forming elements.
By combining these conditions, we succeeded in imparting excellent hot workability. Generally, in order to improve the hot workability of austenitic stainless steel, it is necessary to increase the amount of austenite-forming elements such as Ni, Mn, and N to decrease δ ferrite and stabilize the austenite phase, and to stabilize the austenite phase. It is said that it is effective to add an appropriate amount of a sulfide-forming element to reduce or fix S, but sufficient hot workability cannot be obtained by improving only these points. Therefore, it is difficult to stably apply the Mannesmann method. Based on the above, the present inventors investigated and found that not only austenite-forming elements such as Ni and Mn and S, but also O has an extremely large influence on the hot workability of austenitic stainless steel. It was discovered that hot workability is largely controlled in the extremely low S region, and as a result of further studies based on this knowledge, (1) impurity elements harmful to hot workability were found to be The content of a certain S is regulated to an extremely low S range of 0.002% or less, and in such an extremely low S range, (2) the amount of austenite-forming elements such as Ni and Mn is regulated within a predetermined range. In addition to stabilizing austenite, (3) Al, Ti,
It has been discovered that excellent hot workability can be obtained by regulating the amount of oxide-forming elements such as Si within a predetermined range. In other words, the basic features of the present invention are:
C: 0.2% or less, Mn: 5% or less, S: 0.002% or less, Ni: 5-45%, Cr: 13-27%, N: 0.05%
Contains the following, and Si: 2% or less, Al: 2% or less, Ti: 2% or less, Zr: 0.2% or less, Ca: 0.05%
Below, B: 0.05% or less, Ce: 0.05% or less, Mg:
Contains one or more of 0.05% or less and Mo: 5% or less, the balance consisting of iron and inevitable impurities,
The content of each of the above elements was adjusted to satisfy the following formulas (1) and (2). Δ[O]=6×%Al+8×%Ti+9×%Si+120×
%Zr+800×%B+1000×%Ca+1200×%Ce+
1000×%Mg−1200×% O =−2~12……(1) Δ[Ni]=%Ni+0.5×%Mn+30×%C+30×%
N+8.2−1.1×(%Cr+%Mo+1.5×%Si+2×%
Al+2×%Ti)0...(2) The reason for limiting the component composition of the present invention will be explained below. C is an austenite-forming element, and has the effect of improving hot workability in materials with unstable austenite phase components. However, if it exceeds 0.2%, the deformation resistance increases and carbides are formed, which adversely affects hot workability. Therefore, C is 0.2%
is the upper limit. Mn is also an austenite-forming element, and has the effect of improving hot workability in materials with unstable components in the austenite phase. However, if Mn is added in an amount exceeding 5%, hot workability is inhibited, and therefore the upper limit of Mn is set at 5%. S is an impurity element harmful to hot workability, and the smaller the amount, the better the hot workability is. However, the effect of improving hot workability due to a reduction in S is saturated at about 0.002%, and even if S is reduced further, no significant effect is observed. Therefore, the upper limit of the amount of S is set at 0.002%. Ni is a main component of austenitic stainless steel, and 5% or more is required to stabilize the austenite phase. However, if the content exceeds 45%, it is not preferable because it actually impairs hot workability. Therefore, the Ni amount should be in the range of 5 to 45%. Cr is also a major component of austenitic stainless steel, and 13% or more is required for heat resistance and corrosion resistance. However, since it is a ferrite-forming element
When added in excess of 27%, the austenite phase becomes unstable and hot workability is inhibited. Therefore, the Cr content is set to 13 to 27%. N is also an austenite-forming element, and has the effect of improving hot workability in materials with unstable austenite phase components. However, if added in a large amount, inclusions will be generated and the cleanliness will deteriorate, so it should be kept at 0.05% or less. Each of the elements Al, Ti, Si, Zr, Ca, B, Ce, and Mg is an oxide-forming element, and at least one of them is required as a deoxidizing agent. These elements exhibit the effect of improving hot workability by fixing oxygen, which is an impurity element harmful to hot workability. However, when the content of these elements becomes excessive, hot workability is adversely affected. Si,
As for Al and Ti, when they are added above a certain level, the deformation resistance of austenitic steel increases significantly, impairing hot workability. For this reason, each of these elements is allowed to be contained at an upper limit of 2%.
Furthermore, when their contents are excessive, Zr and B tend to segregate at grain boundaries, which impairs hot workability. For this reason, these elements contain 0.2% Zr,
B is contained up to 0.05% in each case.
If a large amount of Ca or Ce is contained, the remainder combined with S will exist in the steel in a free state, and if this amount is excessive, hot workability will be impaired. For this reason, each of these elements is allowed to be contained at an upper limit of 0.05%. Furthermore, when Mg is added in a large amount, it forms a low melting point eutectic of Mg-Ni, which deteriorates hot workability. Therefore, Mg is contained at an upper limit of 0.05%. Mo is an element that improves corrosion resistance and heat resistance, but adding a large amount of Mo increases deformation resistance and
Since it destabilizes the austenite phase and inhibits hot workability, it is added at an upper limit of 5%. Note that there is no problem as long as P is at most 0.03%, which is normally allowed as an impurity amount. Furthermore, in the present invention, in addition to regulating each component as described above, the content of each alloying element is also limited. That is, under the extremely low S region mentioned above,
The amount of each oxide-forming element Al, Ti, Si, Zr, B, Ca, Ce, and Mg is regulated in relation to the oxygen content O in the steel so that it satisfies the condition of the following formula (1). , this is a major feature of the present invention. Δ[O]=6×%Al+8×%Ti+9×%Si+120×
%Zr+800×%B+1000×%Ca+1200×%Ce+
1000 x % Mg - 1200 x % O = -2 ~ 12 ... (1) As will be clear from the explanation of the examples below, when Δ[O] is less than -2, the amount of oxide-forming elements is lower than that of steel. It is relatively insufficient in relation to the amount of O , and hot workability is not sufficient. Conversely, if Δ[O] exceeds 12, hot workability deteriorates. The reason for this is not necessarily clear, but it is important to note that the necessary and sufficient amount of oxide-forming elements is determined solely by the relationship with the oxygen content O , and that the content of each element itself exceeds its individual upper limit. At the very least, it is thought that this is due to the adverse effects caused by interactions between oxide-forming elements. Furthermore, in the present invention, the content of austenite-forming elements such as Ni, Mn, C, and N is regulated in relation to other elements so as to satisfy the condition of the following formula (2). Δ[Ni]=%Ni+0.5×%Mn+30×%C+30×%
N+8.2−1.1×(%Cr+%Mo+1.5×%Si+2×%
Al+2×%Ti)0...(2) In austenitic stainless steel, the factor that has the most significant effect on hot workability is the stability of δ ferrite, that is, austenite. As already explained, Ni, Mn, C, and N are austenite-forming elements and have the effect of stabilizing austenite, whereas Cr and Mo are ferrite-forming elements and have the effect of destabilizing austenite. Also, the oxide-forming element
Ti is also a ferrite-forming element. The present inventors have discovered that the stability of the austenite phase can be regulated by the value of Δ[Ni] defined by the above equation (2). That is, when Δ[Ni] is less than 0, the austenite phase is unstable and hot workability becomes insufficient. Next, features of the present invention will be explained with reference to examples. There are many reports on the improvement of hot workability of austenitic stainless steel, and it is said that the stability of the austenite phase related to δ ferrite is the most important for this type of material. Next, S is important, and it is known from experience that hot workability can be improved by adding an appropriate amount of sulfide-forming elements and fixing S. Naturally, by reducing S itself, hot workability can be improved. It is also known that processability can be improved. In recent years, there has been remarkable progress in desulfurization technology through refining.
In actual operation, a technology has been established that can reduce the S content to 0.002% or less, and this has led to some improvement in hot workability due to the extremely low S content. However, the effect of improving hot workability by reducing S is saturated at about 0.002%, and no further improvement by sulfide-forming elements can be expected. According to the studies of the present inventors, even if the austenite phase is stabilized by extremely lowering S and increasing the amount of austenite-forming elements, these measures alone will not result in a stable yield with few cracks. It has been found that performing a good Mannesmann drilling is extremely difficult. Therefore, the present inventors focused on oxygen, which is considered to be the next most important unavoidable impurity after S. It is generally believed that oxygen also inhibits hot workability, but the influence of oxide-forming elements on hot workability is not as clear as that of sulfide-forming elements. The reason for this is that it has been difficult to obtain molten raw materials with extremely low S, and most studies on hot workability have been conducted on components containing 0.002% or more of S. This is thought to be due to the fact that it was not clear. The present inventors investigated the influence of oxide-forming elements at extremely low S levels of 0.002% or less, and found that in the extremely low S range, the amount of oxygen in steel greatly controls hot workability. We were able to confirm that this was the case. The results of the study are shown in Table 1 and Figure 1. From these, it can be seen that in the extremely low S region of 0.002% or less, the influence of oxide-forming elements as well as the influence of Δ[Ni] clearly appears, and Δ[O]= 6×%Al+8×%Ti+9×%Si+120×
%Zr+800×%B+1000×%Ca+1200×%Ce+
Hot workability can be evaluated using the formula 1000 x % Mg - 1200 x % O.
It can be seen that the range of optimal addition amounts of Ca, B, Ce, and Mg can be determined. Here, it is known from experience that Mannesmann drilling is possible if the material has a breaking rotation number (torsion test) of 14 times or more at 1000℃, and from Figure 1, Δ[O]: -2 to 12 , Δ[Ni]
It can be seen that Mannesmann drilling is possible if the component satisfies the three conditions of 0.0% and 0.002% S.
On the other hand, it can be seen that it is difficult to apply the Mannesmann drilling method even if any one of the above three conditions is lacking.

【table】

[Table] According to the present invention described above, it has excellent hot workability that cannot be obtained with conventional austenitic stainless steel, and even when manufacturing seamless steel pipes, the material does not suffer from cracks or the like. It is possible to perform Mannesmann perforation without forming holes, which not only greatly improves the productivity of seamless steel pipes for oil country tubular goods, but also makes it possible to manufacture large-diameter pipes, etc., which are particularly difficult to manufacture using hot extrusion methods. can be rationally produced by the Mannesmann method, and is an invention with high industrial utility value.

[Brief explanation of drawings]

The drawings show Δ in Examples and Comparative Examples of the present invention.
This shows the relationship between the [O] value and hot workability (rotational speed at break).

Claims (1)

[Claims] 1 C: 0.2% or less, Mn: 5% or less, S: 0.002
% or less, Ni: 5-45%, Cr: 13-27%, N:
Contains 0.05% or less, and Si: 2% or less,
Al: 2% or less, Ti: 2% or less, Zr: 0.2% or less,
Contains one or more of Ca: 0.05% or less, B: 0.05% or less, Ce: 0.05% or less, Mg: 0.05% or less, and Mo: 5% or less, with the balance consisting of iron and inevitable impurities, and each of the above Austenitic stainless steel with excellent hot workability, made by adjusting the content of elements to satisfy formulas (1) and (2) below. Δ[O]=6×%Al+8×%Ti+9×%Si+120×
%Zr+800×%B+1000×%Ca+1200×%Ce+
1000×%Mg−1200×% O =−2~12……(1) Δ[Ni]=%Ni+0.5×%Mn+30×%C+30×%
N+8.2−1.1×(%Cr+%Mo+1.5×%Si+2×%
Al+2×%Ti)0...(2)