JP2002069592A - Cast stainless steel slab containing two phases of austenite and ferrite and having excellent hot workability - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a highly hot-workable cast stainless steel slab containing two phases of austenite and ferrite with which hot rolling of a continuously cast slab is made possible without using a slabbing stage while practically leaving the influence of ferrite content and S content out of consideration. SOLUTION: The cast stainless steel slab containing two phases of austenite and ferrite and having excellent hot workability is an as-cast slab of stainless steel having ferritic phases in austenitic phases and has a composition containing, by mass, <=0.05% Al, 0.005-0.1% Ti, 0.0005-0.01% Mg and 0.01-0.1% N and satisfying either or both of Ti×N>=8×10-4 and Al/Mg<=4. Moreover, in the region between the surface layer of the cast slab and a position at a depth of 10 mm from the surface layer, in the case of an as-cast state, the area ratio of the ferritic phases is 5-75% and the maximum length of an arbitrary ferritic phase is <=0.7 mm.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a slab which is used as a material for producing a stainless steel having two phases of austenite and ferrite, such as a high delta ferrite austenitic stainless steel used for a welding rod or the like. It is about.

[0002]

2. Description of the Related Art Stainless steels are classified into austenitic and ferritic based on phases at room temperature, and some of them have two phases of austenite and ferrite. An austenite and ferrite having almost equal amounts are classified as an austenite-ferrite system, and the austenite contains about 10% ferrite in S.
USY308, SUSY309, etc. are classified as austenitic.

[0003] If the weld has a structure containing a large amount of ferrite phase (so-called delta ferrite), welding cracks are unlikely to occur. Therefore, as a material for a welding rod, a stainless steel rod having a large amount of delta ferrite phase formed in a cast structure is used. Widely used. The amount of the ferrite phase (delta ferrite) of the weld metal is approximated by, for example, a relational expression with the following component (1), and a component system having a high δFe (cal) value, specifically, the above-mentioned SUSY309 or the like is used. It is considered to have good performance as a material for welding rods. δFe (cal) = 3 (Cr + Mo + 1.5Si) −2.8 {Ni + 0.5Mn + 30 (C + N)} − 19.8 (1)

[0004] However, steel slabs having such a composition contain a large amount of ferrite phase, and strain is concentrated on the phase boundary due to the difference in the high-temperature elongation of the ferrite and austenite phases. Therefore, even a continuous cast slab having a small cross section could not be directly hot-rolled. Therefore, conventionally, hot rolling was performed by subjecting a slab to slab rolling, grinding and removing surface flaws, and reheating.

[0005] As a countermeasure against these, conventionally, for example, Japanese Patent Publication No. Sho 5
As described in JP-A-6-25265 and JP-A-2-111852, in order to minimize the influence of S that segregates at the austenite-ferrite phase boundary at the grain boundary, S is reduced and S-fixing elements such as Ca and REM are added. Is widely practiced.
However, if these countermeasures are taken, the meltability of the molten metal at the time of welding and, consequently, the workability of welding deteriorate, so that it is difficult to apply the method to a material for a welding rod.

On the other hand, the present inventors have disclosed the invention described in Japanese Patent Application Laid-Open No. H11-256234 as a method for producing a continuous cast slab that can be hot-rolled without going through a slab rolling step. This manufacturing method is different from the above-mentioned δFe (cal), which indicates the amount of ferrite after heat treatment, by using the fact that the relationship between the amount of delta ferrite and the component is slightly different between that at the welded portion and that when the slab is subjected to heat treatment. The value of the estimation formula (hereinafter YI
This is a method in which the negative effect of the delta ferrite phase at the time of rolling is eliminated as much as possible by defining the slab to a certain value or less and subjecting the slab to a certain heat treatment before rolling.

[0007]

There is a demand for a stainless steel wire having a large amount of ferrite as a material for a welding rod. However, in order to avoid the occurrence of cracks during hot rolling in the past, it is necessary to omit slab rolling. could not. And the slab rolling process requires high cost, energy consumption for heating,
In addition, there was a problem that the number of production days increased.

According to the method disclosed in JP-A-11-256234, it is possible to omit the lumping step. However, the aforementioned δFe (cal) and Y
Although the I value does not correspond one-to-one, the YI value tends to increase when the value of δFe (cal) is increased in order to improve the welding characteristics, and the lumping process is performed with complete freedom in the components. It cannot be omitted.

Accordingly, the present invention relates to a stainless steel containing two phases of austenite-ferrite, such as a component-type austenitic stainless steel wire having a large amount of ferrite, which is preferable as a material for a welding rod. An object of the present invention is to provide a stainless steel slab with good hot workability, which enables hot rolling of a continuously cast slab without going through a slab rolling step, with almost no influence being taken into account.

[0010]

The gist of the present invention to achieve the above object is to provide a method of casting a slab from any surface to a depth of 10 mm from the surface of the slab just before heating the slab for hot working. If the area ratio of the ferrite phase is 5% or more and 75% or less and the maximum length of any ferrite phase is 0.7 mm or less, or if the area ratio of the ferrite phase is 2% or more and 75% or less after heat treatment, A stainless steel slab having an austenite-ferrite two phase excellent in hot workability, characterized in that an arbitrary ferrite phase has a maximum length of 0.4 mm or less.

To obtain this, from the viewpoint of nonmetallic inclusions, the maximum diameter of Ti-based nitride and Mg-based oxide is 0.05 to 2 mm at an arbitrary cross section to a depth of 10 mm from the surface layer of the slab.
It is only necessary that 200 μm / mm ² or more of composite non-metallic inclusions having a thickness of μm exist.
5% or less, Ti: 0.005 to 0.1%, Mg: 0.0
005 to 0.01%, N: 0.01 to 0.1%,
And (1) Ti × N ≧ 8 × 10 ⁻⁴ , (2) Al / Mg ≦
It suffices if one or both of the four conditions are satisfied.

[0012]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS As described above, the present inventors have studied various measures for reducing the amount of ferrite phase that hinders hot workability. There was naturally a limit in reducing the amount of ferrite in a piece. Therefore, as a result of various investigations to improve the hot workability while maintaining the high ferrite component system by controlling the morphology, not the amount of the ferrite phase, as shown below, despite the fact that the ferrite phase is high ferrite, A cast slab having good workability was obtained.

In a slab of a normal production method that is not a rapidly solidified metal or the like, the austenite phase exists in the form of columnar crystals, and the ferrite phase exists in the austenite phase in the form of a fish bone or a plane. In the case of such a structure, strain accumulates at the phase boundary,
If the limit is exceeded, cracks will occur. Therefore, it was considered that when the ferrite was made into a finely dispersed form, the strain at the phase boundary was dispersed and the hot workability was improved.

First, the reason for limitation in claim 1 will be described. FIG. 1 shows that S with a ferrite content of 10.5-12%
It shows the relationship between the maximum length of the ferrite phase and the rupture reduction at 1000 ° C. in the as-cast USY309 slab. As the maximum length of the ferrite phase becomes smaller, the hot workability improves.
It turns out that it becomes over.

The maximum length of the ferrite phase is calculated as follows. Depth 10 from the surface layer in the C section of the slab
After coloring with Murakami's reagent, an arbitrary 20 visual fields (total about 100 mm ² ) were observed with an optical microscope in the region up to mm.
As shown in FIG. 2, the maximum distance (arrow) was determined for each of the ferrite phase regions connected together (the boundary line with the austenite phase was closed), and the maximum value was defined as the maximum length. There may be a problem as to whether or not an arbitrary cross section can be represented by about 100 mm ² , but with the production method described below, the entire cross section has the same structure uniformly, so that the objective can be achieved industrially without any problem. .

The amount of ferrite is 5%
If less than, even if the ferrite phase is large, since the proportion of the whole is small, the hot workability is originally good, so
In the present invention, it is limited to 5% or more. Also, when the amount is more than 75%, there is no effect because the original hot workability is good, and it is out of the scope of the present invention. The amount of ferrite may be obtained by image analysis of the area occupied by ferrite at the time of the above-described structure observation, but may be measured by a commercially available ferrite meter.

Next, the reason for limitation of claim 2 will be described. In the case of a high ferrite material, the slab may be subjected to a heat treatment once to reduce the ferrite and then subjected to rolling. In this case, the necessary conditions slightly vary from the as-cast condition of the first aspect. As a result of the experiment, it was found that when the amount of ferrite was 2% or more and 75% or less and the maximum length of ferrite was 0.4 mm or less, the same effect as that of the as-cast product was obtained.

Further, the slab to be evaluated is a slab just before heating the slab for hot working. For example, when rolling is performed after the surface is removed by grinding, the cast slab after grinding is evaluated. When ferrite inside 10 mm from the surface layer is not finely dispersed, what happens is not evaluated in the method described later because the entire slab including the inside including 10 mm is finely dispersed. It has been found that even if cracks occur in the interior, they do not open to the surface layer and re-attach during hot rolling, and there is no need to evaluate the interior from 10 mm.

As one method of obtaining such a slab,
The present inventors have found that this can be achieved by dispersing nonmetallic inclusions in which a Ti-based nitride and a Mg-based oxide are combined. In addition, by adding a small amount of Ti and Mg in a predetermined component range as described later, even if casting is performed in a completely ordinary manner, such non-metallic inclusions can be dispersed in the cast slab, and ferrite can be obtained. The phase was finely dispersed.

When the maximum diameter of the nonmetallic inclusions is less than 0.05 μm, the effect of miniaturizing the ferrite is small, and when it exceeds 2.0 μm, other problems such as a decrease in corrosion resistance occur. The range was 0.05 to 2.0 μm. The effect is recognized at a distribution density of 200 / mm ² . In addition,
Among the Ti-based nitride and the Mg-based oxide, it is assumed that the largest components other than nitrogen and oxygen are Ti and Mg, respectively. To measure the component, size and distribution of these nonmetallic inclusions, an electron microscope and an energy dispersive spectroscopy (EDS) or X-ray
A method of investigating with a line microanalyzer (EPMA) or the like is considered.

The reason why these nonmetallic inclusions exhibit the ferrite refining effect is that Ti-based nitride (TiN) has a good lattice matching with ferrite, so that fine dispersion of TiN produces many ferrite solidification nuclei. As a result, it is estimated that ferrite is finely dispersed. However, even if the components are adjusted so as to produce only a large amount of TiN, the effect cannot be exhibited only by depositing coarse TiN. Mg-based oxides (MgO) serve as precipitation nuclei for TiN,
It is thought that it is effective for finely depositing TiN.

As a measure for producing these nonmetallic inclusions, it is sufficient to control the trace components within the following specified range, and the above-mentioned δFe (cal) is controlled by the amount of Cr, Ni or the like.
It is not particularly affected by the amount of ferrite contained in the slab in a shape along the shape.

Al: When Al is added in a large amount, an Al oxide is formed, which inhibits the formation of MgO.
Was set as the upper limit.

Ti: Ti forms Ti-based nitride, and Mg
Is an element capable of controlling the structure, which is an object of the present invention, when added in a composite with the alloy.
005% or more, so this was set as the lower limit. But,
If added in excess of 0.1%, problems such as manufacturability and workability arise, so 0.1% was made the upper limit.

Mg: Mg is an important element that enables the control of the structure, which is an object of the present invention, by forming an Mg-based oxide. 0.0005% exerts this effect, and this is defined as the lower limit. Even if added in a large amount, the effect is saturated and a problem such as a decrease in corrosion resistance is caused.
% As the upper limit.

N: N is an important element in forming a Ti-based nitride. 0.01% exerts this effect, and this is set as the lower limit. Further, if added in a large amount, it becomes hard and impairs the workability, so the upper limit was made 0.1%.

Further, when Ti and N are controlled to Ti × N ≧ 8 × 10 ^-4 , T exceeds the solid solution limit of TiN before solidification of molten steel, and T
Since iN precipitates, a fine dispersion effect of ferrite can be obtained. However, even below the solid solution limit of TiN, Al /
When Mg is 4 or less, fine MgO is generated and TiN is precipitated using the fine nucleus as a nucleus, so that the same effect can be obtained. If it exceeds 4, a coarse Al-Mg composite oxide is formed, and such an effect cannot be expected.
The ranges of Ti and Mg are as follows: Al is 0.07%, N is 0.02%.
Assuming that% is constant, it becomes as shown by oblique lines in FIG. However,
The boundary line of the range shown by the diagonal lines in FIG. 3 fluctuates as Al and N fluctuate.

[0028]

EXAMPLE For stainless steels of Nos. A to V having the chemical components shown in Table 1 and the balance being Fe and unavoidable impurities, a continuous cast slab having a diameter of 170 mmφ was prepared from 1200 stainless steel.
C., and hot-rolled to 5.5 mmφ in a continuous wire rod rolling line. Table 2 shows the results of evaluation of non-metallic inclusions and ferrite in the cast slabs for Nos. A to V, respectively. Each cast piece was evaluated by a microscope and an electron microscope at an arbitrary position from the surface layer to a depth of 10 mm for a total area of 100 mm ² .

As for the nonmetallic inclusions, a composition analysis was performed on any 20 non-metallic inclusions using an electron microscope and EDS, and the composition, shape,
Colors are added, and composite non-metallic inclusions of Mg-based oxide + Ti-based nitride are added, and the maximum diameter is 0.5 to 2
The number of nonmetallic inclusions of μm was counted. Since not all non-metallic inclusions were analyzed, not all non-metallic inclusions have the same composition, but the correspondence between the composition, shape, and color of the above-mentioned 20 was almost one. It is considered that this almost satisfies the required condition because the information is provided in one-to-one correspondence.

The maximum length of each grain of ferrite was calculated by a microscope using the method shown in FIG. 2, and the maximum value was calculated. Table 2 shows the condition of the flaw after rolling.
Indicated by Δ and ×. ○ is to the extent that there is no problem as a product, △
Can be remedied by grinding or can be partially used for products,
× indicates that the rolling was stopped because it could not be used at all or was cut during the rolling. Nos. A and B in the table are cast pieces of the same composition as they are at present, but No. A is a cast piece of the normal part, while No. B is a fragment of ferrite in the surface layer. The cast slab was extracted and evaluated. Also, among the evaluation materials, seven types of No. A, C, D, G, I, J, and L were subjected to a soaking treatment at 1200 ° C. for 20 hours.
Similar evaluations were made on the state of ferrite and the rolling results. The results are shown in Table 2 in each alphabet with '
Indicated by the symbol with.

First, among the 22 types of as-cast materials, Nos. B and I to M according to the present invention showed good rolling results.
Looking at the relationship between the state of ferrite and the rolling results for all of the 22 examples other than Nos. C and D, the area ratio was 7.2 to 14.5% for the O material and 6 for the ΔX material. .8-
Although no difference can be found with 10.5%, the maximum length of the ferrite phase is 0.09 to 0.7 mm for the ○ material and 0.73 for the Δ × material.
It can be seen that good characteristics are obtained when the maximum length is 0.7 mm or less, that is, 8.5 mm or less. For Nos. C and D, since the area ratio of the ferrite is out of the range of the present invention, the rolling result is good even if the maximum length of the ferrite phase is larger than 0.7 mm.

Unlike No. A, Nos. I to M have stable ferrite fragmentation over the entire length of the slab. Looking at the composition of the nonmetallic inclusions for these, almost all of the 20 evaluations were composite nonmetallic inclusions of Mg-based oxides and Ti-based nitrides. In Nos. I, K, and L of the examples of the present invention, there were some non-metallic single inclusions of Ti-based nitrides, which could be distinguished by the color of the electron micrograph. Therefore, when the number of the composite non-metallic inclusions having a maximum diameter of 0.5 to 2 μm was counted, as shown in Table 2, 245 to 6
It was 40 pieces / mm ² , which satisfied the prescribed 200 pieces / mm ² or more. As shown in FIG. 1, the hot workability of these materials exceeded 60% or more at 1000 ° C., and as a result, it is considered that rolling was possible without any problem.

On the other hand, a comparative example of N and Ti
In No. F to which only o.E and Ti were added, the nonmetallic inclusions were all Al, Cr, Mn oxides and Ti oxides, respectively, and moreover, the size almost exceeded 2 μm. Therefore, the maximum length of ferrite is 8.5 mm and 2.1 mm, respectively.
mm, and the hot workability was very poor. Comparative example
Nos. G and H are inadequate under the specified conditions for both Ti and Mg, and the number of composite non-metallic inclusions that are partially aimed at is small. As a result, the maximum length of ferrite is still large, and hot workability is high. It was short.

Further, for Nos. E to L and N to V, the rolling results (△ Δ ×) are arranged on the horizontal axis Ti and the vertical axis Mg in FIG. Of No. E, F, N to T in the figure, No. E,
F, O, P, Q, and R are out of the range indicated by the arrows in FIG. Among them, No. R alone has a good rolling result, but has extremely poor corrosion resistance. Since No. N was Al and No. T and S were N outside the range of the invention, the characteristics were deteriorated. No. T had good rolling results, but was hard due to a large amount of nitrogen, and the cold workability deteriorated.

With respect to Nos. G to L, U, and V shown in FIG. 4, the difference between ○ and Δ is not clear when only the Ti amount and the Mg amount are viewed, but are arranged by TixN and Al / Mg specified this time. Then, as shown in FIG.
J, K, and L were good, and Nos. G, H, and
It is clear that U and V are slightly defective.

With respect to the heat-treated material, the results of rolling were the same as before the heat treatment, and No. I ', J', and L 'were good. The maximum length of ferrite at that time is as shown in Table 2 and is 0.4 m
m or less, the hot workability is good. No.
Regarding C ′ and D ′, since the ferrite area ratio is out of the range of the present invention, the rolling result is good even if the maximum length of the ferrite phase is larger than 0.4 mm.

[0037]

[Table 1]

[0038]

[Table 2]

[0039]

According to the method of the present invention, a hot-workable stainless steel containing ferrite in austenite, such as austenitic stainless steel of a component type having a high ferrite content in a weld metal, which is preferable as a material for a welding rod. As for steel, a product can be manufactured industrially stably by hot rolling without going through a slab rolling step of a continuously cast slab. Therefore, the effects of reducing the production cost, the production time, and the energy consumption can be exhibited by omitting the bulk rolling.

[Brief description of the drawings]

FIG. 1 is a graph showing the relationship between the maximum length of ferrite and hot workability.

FIG. 2 is a diagram showing a method of obtaining a maximum length of ferrite.

FIG. 3 is a graph showing the relationship between the required Ti amount and the Mg amount in the method of the present invention.

FIG. 4 is a graph summarizing the relationship between the amounts of Ti and Mg and the rolling results for each test material of the example.

FIG. 5 shows Ti × N, Al /
It is the graph which put together the relationship between Mg and the rolling result.

 ──────────────────────────────────────────────────の Continued on the front page (72) Inventor Mayumi Okimori 3434 Shimada, Hikari-shi, Nippon Steel Corporation Inside the Hikari Works (72) Inventor Yoshinobu Tada 3434 Shimada, Hikari-shi, Hikari-shi New Japan (72) Inventor Hiroyuki Inoue 20-1 Shintomi, Futtsu City Nippon Steel Corporation Technology Development Division

Claims (4)

[Claims] 1. An as-cast slab of hot-rolled stainless steel having a ferrite phase in an austenite phase, wherein the area of the as-cast ferrite phase in a region from the surface layer of the slab to a depth of 10 mm. Rate is 5% or more 7
5% or less, and the maximum length of any ferrite phase is 0.7
mm, a stainless steel slab having an austenite-ferrite dual phase excellent in hot workability. 2. A slab obtained by heat-treating a slab of hot-rolled stainless steel having a ferrite phase in an austenite phase, wherein the area ratio of the ferrite phase in a region from the surface layer of the slab to a depth of 10 mm is reduced. 2% or more and 75% or less,
A stainless steel slab having an austenite-ferrite dual phase excellent in hot workability, wherein the maximum length of any ferrite phase is 0.4 mm or less. 3. The maximum diameter of Ti-based nitride and Mg-based oxide in the region from the surface layer of the slab to a depth of 10 mm.
Composite non-metallic inclusions is 200 / mm ² or more, characterized in that there claim 1 or 2 austenite has excellent hot workability according to the 05～2Myuemu - stainless steel slab having a ferrite two-phase. 4. In mass%, Al: 0.05% or less, Ti: 0.005 to 0.1%, Mg: 0.0005 to 0.01%, N: 0.01 to 0.1% And Ti × N ≧ 8 × 10−4, Al / Mg ≦ 4
2. The method according to claim 1, wherein one or both of the following conditions are satisfied.
4. A stainless steel slab having an austenite-ferrite dual phase excellent in hot workability according to any one of items 3 to 3.