JPS61163247A - High alloy stainless steel excelling in hot workability as well as corrosion resistance - Google Patents

Abstract

PURPOSE:To improve corrosion resistance and hot workability by incorporating prescribed percentage of C, Si, Mn, P, Cr, Ni, N, Mo, S, O, Al, Ti, and Ca. CONSTITUTION:The titled stainless steel consists of, by weight, 0.005-0.3% C, <=5% Si, <=8% Mn, <=0.04% P, 15-35% Cr, 5-40% Ni, 0.01-0.5% N, 5.5-10% Mo, 0.01-0.1% Al and/or Ti, 0.001-0.008% Ca, <=30ppm S, <=50ppm O, and the balance Fe, and satisfies the following conditions: 3(Cr+1.5Si+ Mo)-28(Ni+0.5Mn+0.5Cu)-84(C+N)-19.8>-10% (where each component is represented by wt.%); and S+O-0.8Ca<=40 (where each component is represented by ppm).

Description

[Detailed description of the invention] (Industrial applications) The present invention relates to a high alloy stainless steel with excellent corrosion resistance and hot workability, and in particular to a high alloy stainless steel with excellent corrosion resistance to seawater. be.

(Prior Art) High-alloy stainless steel is used particularly when severe corrosion resistance, heat resistance, and oxidation resistance are required, and seawater-resistant stainless steel is likely to become increasingly important in the future. These alloys often contain large amounts of Cr HNi + Mo, Sl, etc., and N is a component that is desired to be actively utilized as an element that improves the strength and corrosion resistance of stainless steel.

(Problem that the invention attempts to solve) However, among these high alloys, especially Ni and N.

Alloys containing a large amount of Mo and the like have poor hot workability, and cracks or curved flaws occur during hot working, resulting in a decrease in yield. In particular, when high-alloy steel is continuously cast (hereinafter referred to as CC), cracks occur at the grain boundaries of dendrites during casting during the next step of hot rolling, making production impossible. Currently, there are many high-alloy steels that have not been tested.

On the other hand, for these high alloy steels, conversion to CC is particularly desired from the following points, and the effects associated with conversion to CC are extremely large.

1) Since high-alloy steel contains expensive alloying elements, it is particularly desirable to improve the yield, and in this respect, the CC method is strongly desired over the ingot-blubber rolling method.

2) In the production of high-alloy steel using the ingot/blubber method, hot workability is improved by soaking for a long time, but surface scale formation due to long soaking is difficult, and the formation of a Cr-free layer, etc. becomes large and deteriorates the corrosion resistance and oxidation resistance of the product surface. Therefore, for seawater-resistant stainless steel that requires crevice corrosion resistance, it is desirable to use CC with less chromium-free layer etc. on the surface.

(Means for solving the problem) The present inventors previously disclosed a method for improving hot workability in the steel ingot method in JP-A-49-135812, and
We have clarified the a treatment method, but as a result of various studies aimed at taking it one step further and converting it into CC, we have improved the hot workability of high-alloy steel CC slabs and have excellent corrosion resistance and seawater resistance. It has become clear that in order to obtain a product with a high temperature, it is necessary to strictly control the amounts of S and O in the alloy composition.

The present invention clarifies its regulatory limits and means.

That is, the gist of the present invention is as follows.

(1) Weight basis C0.005-0.3%, 81
5% or less, Mn 8% or less, P 0.04% or less, cr
15-35L%, Ni5-40%, N0.01-0.5
%, M0.5.5-10.0%, S is 30 ppm or less, 0 is 50 ppm or less.

One or two types of Al or aluminum at 0.01 to 0. i.

Contains 0.0% Ca. o o i ~0. o o s
In an alloy containing % gold and the balance essentially consisting of Fe and unavoidable impurities, δ-@) = 3 (Cr+1.5 Si + Mo)- expressed in weight percent of each component.
28(N i+0.5Mn+0.5c u )−84(
C+N)-19,8 is -10% or more, and each component is p
A high alloy stainless steel with excellent corrosion resistance and excellent hot workability, characterized in that (S+O-0.8Ca) expressed in pm is ≦40.

'(2) Weight/4- + 7 tote C0.005~0.3
%, 815% or less, Mn 8% or less, P 0.04% or less, 0115-35%, Nl 5-40%, N
0.01-0.5%, Mo 5.5-100%, Cu 3% or less, Nb 1% or less.

V1% or less, W2% or less, Zr 0.5% or less, S
Contains one or more of the following components: n0, 196 or less, 830 ppm or less, 050 ppm or less, Al
Alternatively, it contains 0.01 to 0.10 of one or two kinds of Ti, and further contains 0.001 to 0.0 of CIL. oos%, with the balance essentially consisting of Fe and unavoidable impurities, δa11$) = 3(Cr+1.5Si+Mo)-28( expressed as weight percent of each component)
Ni+0.5Mn+0.5cu)-84(C+N)-1
A high alloy with excellent corrosion resistance and excellent hot workability, characterized by 9,8 being 10% or more, and each component expressed in ppm (S + 0-0.8 Ca 'II ≦ 40) stainless steel.

The present invention will be explained in detail below.

In order to meet the stringent requirements such as corrosion resistance, corrosion resistance of 2%, and seawater resistance as mentioned above, Cr, Ni, M0. It is necessary to add large amounts of alloying elements such as Si, and N
The addition of is also extremely effective. Moreover, in order to produce large quantities of these high alloy steels by continuous casting in terms of yield and surface chromium-free layer, improving the hot workability of CC slabs is an extremely important issue. That is, it has been found that it is necessary to further improve the grain boundary ductility of the dendrite during casting, and stricter regulation of the alloy composition is required. In response to these requests, Japanese Patent Application Laid-open No. 49-135812
The method of deoxidizing and adding Ca was shown and further published in Japanese Patent Publication No. 1042/1983
No. 6 discloses Mg deoxidation and the addition of rare earth metals such as Ce. Furthermore, the alloy composition is
It is known to control the distance between the two as shown in Japanese Patent No. 61104.

The present inventors conducted detailed research with the aim of improving hot workability and seawater resistance of CC7M pieces, and found that they not only controlled the main components, but also deoxidized and desulfurized the seawater resistance and hot workability. This study revealed that the control of

In other words, many factors are related to the hot workability of high-alloy slabs, but the factors that have the greatest influence are the amounts of S, O, and Ca in the steel, and these have been determined from many experiments. It was found that each element affects hot workability in the form of [S10-0.8Ca:] expressed in ppm. The next most influential factor is N amount and M.

It was found that Nb, V, W, Cu ii, etc., as well as δ-←) defined by the following formula, also affected hot workability. δcnl1m)=3 (Cr-1-
1,5Si+Mo)-2,8(Ni-0.SMn+0
．． 5 Cu) -84 (e+N) -19,8 (In this case, each component is expressed in weight 1?-cents.) 1st
The figure shows a hot impact test piece taken from the surface of a slab, heated and then air cooled while changing the impact temperature (heating condition 1250℃, impact temperature 1200-10.00℃ each 50℃
The figure shows the results of evaluating hot workability based on the overall score.

Alloy (,) is 21 Cr -18Ni -0.3ON-
With a composition of 6,0Mo, its S, 0. It shows the relationship between Ca remission and hot workability. In the case of Korikishite (,) alloy, each element is expressed in ppm [S -1-0-0.8Ca)
(ppm) is 40 or less and has excellent hot workability. Of course, it is effective if a large amount of Ca is used, but if the amount of CaO is too large, corrosion resistance may be impaired, and the amount of Ca remaining is 0. '
001 to 0.008 chi.

Figure 9 shows the alloy 25Cr-(18~21)Nl-
(0.3-0.4) The influence of δ-@) of N 5.9Mo on hot workability is shown. As mentioned above, δ−@) is δdji)−3(Cr+1.58i+Mo)−
2,8(Ni+0.5Mn+0.5Cu)-84(C
+N)-19,8. In this case, for hot workability, a Greeple test piece was taken from the slab, and 950~
The tensile shear and minimum shrinkage values at 1250°C are used as indicators. If the reduction of area is 60% or more, hot workability is good.

In this alloy, when (S+O-0.8Ca)≦40 (region (B) in the figure) and δ-(%)>-10%, the hot workability is good. [S+O-0.8Ca] (ppm
) > 40 (region (A) in the figure), δ,,i+1) is set to 0.
It will be improved if it gets closer to , but the extent is still insufficient.

From the above experimental facts, the alloy composition (:S10-0.8
It is important to reduce Ca) υ, and in N-Cr-Ni-Mo alloys as seawater-resistant stainless steels, [S-
0-0.8 Ca) (ppm)≦40 is required. Furthermore, Nb.

Even when containing Cu, Sn, etc. [S 〇-0
．． 80a]≦4o. In addition to these conditions, δd@) = 3 (Cr + 1.5 S
i + Mo ) 2.8 (Ni-t-
0.5Mn-1-0.5Cu)-84(C+N)-1
9 and 8 are preferably larger, and δ-@)≧-10% is required. Here, the amount of S is reduced by reducing the amount of S in the alloy itself, and the content t is 30 ppm or less, preferably 15 ppm.
less than 0'! : is deoxidized with a deoxidizing element such as Al or Ti, and the total oxygen content is 50 ppm or less, preferably less than 40 ppm.

Furthermore, it is desirable to add Ca to fix sulfur and oxygen.

As a result of investigating the inclusion composition after casting of alloys in which these measures were implemented, it was found that oxides of MnS + Mn and Si were observed in the inclusions of alloys with poor hot workability and corrosion resistance, whereas hot workability was poor. In a good alloy, no sulfide is observed in the inclusions, and there is no S I +Mn in the oxide,
Only extremely stable oxides such as Al, Ti, and Ca were observed. These results show that there is no S in the dendrite grain boundaries after casting, and oxygen is fixed in the form of extremely stable oxides, resulting in improved grain boundary cleanliness and excellent ductility at high temperatures. This is thought to be linked to the improvement of high-temperature ductility.

Such changes in inclusions also greatly contributed to improving corrosion resistance.

The reason for limiting each component will be described below.

C: C is detrimental to the corrosion resistance of stainless steel, but is desirable in terms of strength. Therefore, it was set at 0.3% t. 0
．． If it exceeds 3%, corrosion resistance will be significantly deteriorated. The lower limit of 0.005% is determined by industrial economics.

SI: Sl increases the corrosion resistance and oxidation resistance of stainless steel. If the upper limit is 5%, the effect will be saturated and hot workability will deteriorate.

Mn: Mn increases the solid solubility of N but deteriorates corrosion resistance, so the upper limit was set at 8%. If it exceeds 8%, corrosion resistance and oxidation resistance will be impaired.

P: In terms of corrosion resistance and hot workability, less P is better, so it was set to 0.04% or less. If this value is exceeded, both characteristics deteriorate.

S: S is an important component for improving hot workability in the present invention,
The lower the better (30 ppm or less, preferably 15
Less than ppm. In particular, it is important to improve grain boundary ductility at high temperatures by lowering it to 0 as will be described later.

Also, in terms of corrosion resistance, the lower the better (30 ppm or less).

0:O is also an important component for improving hot workability in the present invention,
The lower the better (50 ppm or less, preferably 40 ppm or less)
Less than ppm. It is important to lower S and improve grain boundary ductility at high temperatures.

Cr: Cr is a basic component of stainless steel, and 15% or more is particularly effective, and the higher the amount, the higher the corrosion resistance and oxidation resistance.
If it exceeds 5 inches, it becomes expensive.

Ni: Along with Cr, Nl is a basic component of stainless steel and heat-resistant steel. If it is less than 5%, the corrosion resistance is insufficient, and the more it is, the more effective it is, but if it exceeds 40%, it becomes extremely expensive.

N: N increases the strength and corrosion resistance of stainless steel, and is particularly effective in seawater resistance, and is effective at 0.011 or more, but if it exceeds 0.5%, the solid solubility is exceeded and bubbles form.

Mo: Mo increases the corrosion resistance, especially the seawater resistance, of stainless steel, and the effect becomes noticeable at 5.5% to 10.0%. 5,5
If it is less than 10.0q6, the seawater resistance will be insufficient, and if it exceeds 10.0q6, the effect will be saturated and it will become expensive.

Cu: Cu increases the corrosion resistance of stainless steel and is selectively added in an amount of 3% or less depending on the application. If it exceeds 3 inches, hot workability deteriorates.

Nb: Together with N, Nb increases the strength of stainless steel.
Selectively add 1% or less depending on the application. If it exceeds 1%, hot workability deteriorates.

Al, Tl: Al+Ti is added in the range of 0.01 to 0.10% as a strong deoxidizing agent. 0.10
%, corrosion resistance deteriorates. AA and Ti are low S
It coexists with Ca in steel and fixes the value 0, preventing the appearance of oxides of Sl and Mn, significantly improving hot workability and corrosion resistance.

Ca: Ca is a powerful deoxidizing and desulfurizing agent from 0001 to
Add in a range of 0.008%. 0. If it exceeds ooss, corrosion resistance will deteriorate. Ca is Al1. YaT
It coexists with i, fixes S and O, prevents the formation of MnS, and contains N.
- Significantly improves hot workability of high Cr-Nl-Mo alloys and improves seawater resistance.

Sn: Sn improves the acid resistance of stainless steel and
Selectively add less than If this value is exceeded, hot workability deteriorates.

W: W improves the corrosion resistance of stainless steel, and is selectively added in an amount of 2 or less depending on the application. When it exceeds 2%, the effect is saturated.

V: V improves the corrosion resistance of stainless steel, and is selectively added in an amount of 1 g or less depending on the purpose. When it exceeds 1%, the effect is saturated.

Zr: Zr improves the corrosion resistance of stainless steel, and is selectively added in an amount of 0.5 or less depending on the application. When it exceeds 0.5%, the effect is saturated.

In addition to the above limitations on each element, the following two points are required. In other words, ■ In high N-containing Cr-Ni-Mo stainless steel,
In order to improve the hot workability and seawater resistance of the cast structure, it is expressed in ppm (SIO-0.8 Cal≦40 is required).

■ δ4 = 3 (C
r'+1.5Si+Mo) 2.8(Nl+0.5M
n+0.5Cu)-84(C+N)-19,8 represents the ratio of the amount of δ1o in the coagulated tissue, and δ1. appears, it reduces segregation to S and 00γ grain boundaries. Therefore J,
It is necessary to make nl(a) larger than -10%,
The action of δFe and the action of reducing [S+O-0.8Ca] show a synergistic effect and greatly improve hot workability.

Note that Nb, W, V, and Zr are not included in this formula.
etc. are δ ferrite-forming elements, but they are excluded from this formula because their influence is small within the range of addition amount of the present invention.

(Example) Examples of the present invention will be described below.

Table 1 shows the chemical composition of the present invention steel and comparative steel, which were melted by the electric furnace-AOD method and the electric furnace-VAC method, sufficiently desulfurized, and using Al, Ti, and Ca. Deoxidized. All of the steels of the present invention have S of 30 ppm or less, 0.050 ppm or less, [:S+O-0.8Ca) of 40 or less, and satisfy δ->-10%.

Comparative steel has high S and O (kl, Ca utilization is unsatisfactory and CS10-0.8Ca) exceeds 40 and δ
There are also ones that are 10 inches long.

These molten steels were cast into continuous cast slabs under normal conditions. Table 2 shows the results of hot rolling after normal rolling treatment, sorting into thick plate rolling and hot strip rolling, and hot rolling under normal stainless steel conditions. Compared to the comparative steel, the steel of the present invention did not cause any cracks or sag defects during hot rolling, proving the effectiveness of the present invention.

Furthermore, a crevice corrosion test was conducted using the product plate.

The critical crevice corrosion occurrence temperature (C, C, T
) was sought. The results are shown in Table 3. As a result, the normal 831
The temperature is approximately 10°C for 6L. C1C,T is high Cr-6%Mo
The temperature rises to about 60°C in the comparative steel, but the steel of the present invention, even with the same alloy composition, rises to about 70°C or more due to the effect of deoxidation treatment, proving the effect of the present invention.

(Effects of the Invention) According to the present invention, a seawater-resistant stainless steel containing high Cr, high Ni, high Mof, and high N can be produced by continuous casting, and cracks will not occur even if it is hot rolled with short heating. It can be manufactured easily and the yield υ is greatly improved.

Furthermore, according to the present invention, since the heating time is shorter than in the ingot/blosking method, the amount of surface scale produced is small, the resulting surface chromium-free layer is small, and the product surface has excellent corrosion resistance.

As described above, the effect of the improvement in high-temperature ductility achieved by the present invention is incomparable compared to conventional methods in terms of manufacturability and corrosion resistance, and the present invention is a monument in the industrial world. The benefits are extremely large.

[Brief explanation of drawings]

Figure 1 shows S, 0°Ca for hot workability of high alloy steel.
, Figure 2 shows the influence of C25Cr (18-21
)Ni-(0.3-0.4)N-5,9Mo) δ-(chi) and (
It is a figure showing the relationship with S+O-0.8Ca-0.3Ce) value. - To one 〇K to the power of power''

Claims (2)

[Claims] (1) C0.005-0.3% by weight percent, Si
5% or less, Mn 8% or less, P 0.04% or less, Cr15
~35%, Ni5~40%, N0.01~0.5%, M
O 5.5-10.0%, S 30 ppm or less, O 50
ppm or less, 0.0 of one or both of Al or Ti
Contains 1 to 0.10%, and further contains 0.001 to 0.00 Ca
8%, and the remainder substantially consists of Fe and unavoidable impurities, δcal (%) expressed as weight percent of each component = 3 (Cr + 1.5Si + Mo) - 2.
8(Ni+0.5Mn+0.5Cu)-84(C+N)
-19.8 is -10% or more, and each component is expressed in ppm [S+O-0.8Ca] (ppm) ≦40.It has excellent corrosion resistance and excellent hot workability. alloy stainless steel (2) C0.005-0.3% by weight percent, Si
5% or less, Mn 8% or less, P 0.04% or less, Cr15
~35%, Ni5~40%, N0.01~0.5%, M
o5.5 to 10.0% and further Cu3% or less, Nb1% or less, V1% or less, W2% or less, Zr0.5% or less, Sn
Contains 0.1% or less of one or more of each component, S
30ppm or less, O50ppm or less, Al or Ti
Contains 0.01 to 0.10% of one or two of the following, and further contains Ca
0.001 to 0.008%, and the remainder is substantially Fe.
In an alloy consisting of unavoidable impurities, δcal (%) expressed in weight percent of each component = 3 (Cr + 1.5 Si + Mo) - 2.
8(Ni+0.5Mn+0.5Cu)-84(C+N)
-19.8 is -10% or more, and each component is expressed in ppm [S+O-0.8Ca] (ppm) ≦40.It has excellent corrosion resistance and excellent hot workability. alloy stainless steel