GB2160221A

GB2160221A - Two phase stainless steel having improved impact characteristic

Info

Publication number: GB2160221A
Application number: GB08514402A
Authority: GB
Inventors: Hitoshi Oijima
Original assignee: Nippon Kokan Ltd
Current assignee: JFE Engineering Corp
Priority date: 1984-06-13
Filing date: 1985-06-07
Publication date: 1985-12-18
Also published as: FR2565999A1; DE3521101C2; DE3521101A1; FR2565999B1; GB2160221B; GB8514402D0; JPS61564A; SE464307B; SE8502916D0; SE8502916L

Abstract

In a ferrite-austenite two phase stainless steel, the quantity of soluble aluminum is made to be less than 0.02 wt. %, which improves impact strength. The steel is especially suitable to manufacture seamless steel pipes having improved toughness. A preferred composition comprises in weight %: C -0.03 Si 0.1-1.0 Mn 0.1-2.0 P -0.03 S -0.01 Ni 3-8 Cr 21-28 Mo 1-4 N 0.08-0.25 Sol. Al -0.02 Fe balance.

Description

SPECIFICATION Two phase stainless steel having improved impact characteristic This invention relates to two phase stainless steel having improved impact characteristic. and more particularly two phase stainless steel capable of improving toughness of seamless pipes.

Regarding two phase stainless steel there are various authorized regulations such as JIS (Japanese Industrial Standard) G 3459, ASTM A790, and DIN Nr. 14462, for example Steel maker's standards providing bases for these regulations are, for example, SAF 2205 (Sandvik Co.), AF 22 (Mannes man Co.), DP series (Sumitomo Kinzoku Co.), etc. In such two phase stainless steels, the object of improving the strength, corrosion resistant property, deoxidation, hot workability, and the nature of forming fine granules, and the aluminum content disclosed in various patent specifications are shown in the following table.

JPN Pat. Publication range of AQ Object Number (wt %) Patent pub. No, 4171/75 0.03 2.0 strength, corrosion resistant property Laid open patent spec. < 0.05 deoxidation No. 138421/1977 No. 138421/1977 1 0.05 deoxldatlon l No. 1439i2/1977 0.01 6.0 hot workability No. 143913 Laid open patent spec.

No. 12782/1974 1 .

I No. 46117/1977 | < 0.10 No. 33216/1977 9 0.01 0.50 strength No. 158256/1980 2 < 0.05 deoxidation, fine granule Any of the prior publications shown in the table does not contemplate improvement of the impact characteristics. Our invention shows that where the working temperature (rolling temperature) is high as in the manufacture of seamless pipes, of the order of 1 2501 350'C.

the impact value of the products after rolling decreases greatly. Moreover, subsequent working, shaping and heat treatment lack means for improving the impact characteristic so that in some cases, the impact characteristic is lower than ordinary carbon steel. In such a case, ordinarily required toughness value can not be attained.

It is an object of this invention to provide two phase stainless steel having improved impact characteristic.

According to this invention, there is provided ferrite austenite two phase stainless steel containing soluble aluminum of less than 0.02 wt % and having improved impact characteristic.

As is recognized in the art, aluminum has a strong deoxidation property as well as crystal granule adjusting property so that when aluminum is incorporated into ferrite ferrite austenite two phase stainless steel in the form of soluble aluminum in an amount of less than 0.02 wt %, a stable and high impact characteristic can be obtained. When the soluble aluminum content exceeds 0.02 wt % a reaction expressed by Al + N + AIN proceeds, so that a desired high impact property can not be obtained for seamless pipes subjected to complicated, severe and high temperature conditions. Even when the two phases stainless steel is subjected to a solution treatment after rolling, its impact characteristic can not be obtained.When the aluminum content is limited to be less than 0.02 wt %, the impact characteristic can be improved in an as rolled state, and after solution treatment followed by cold rolling.

In the accompanying drawing: Figure 1 is a graph showing the relation between the impact characteristic and the quantity of soluble aluminum of a solution treated stock: and Figure 2 is a graph showing the relation between the impact characteristic and the quantity of soluble aluminum of cold rolling products after the solution treatment.

The invention will now be described in detail. In this invention, the term two phase stainless steel means stainless steel in which the volume ratio of ferrite is 30 to 70% and the balance comprises austenite. More particularly, the two phase stainless steel of this invention preferably contains (all by wt. %) less than 0.03% C, 0.1 to 1.0% Si, 0.1 to 2.0% Mn, less than 0.03% P, less than 0.01% S, 3 to 8% Ni, 21 to 28% Cr, 1 to 4% Mo, 0.08 to 0.25% of N, the balance of iron and inherent impurities, for example, as well as less than 0.02% of soluble aluminium. If desired, either one or more of less than 2.5% Cu, less than 1.5% each of V and W, less than 1.0% each of Ti, Ta, Nb, and Zr, less than 0.01% each of Ca and Mg, less than 0.01% B, and less than 0.01% of REM may be added to the composition just described.

The preferred ranges of the components of the compositions described above are as follows.

C is an element which is inherently present in the manufacture of molten steel and has a property of promoting formation of the austenite phase, but when its content exceeds 0.03%, carbides may be formed which impair the corrosion resistant property and hot workability.

Accordingly, its preferred upper limit is set to 0.03%.

Si has a deoxidation function, and with higher than 0. 1% of Si, this function can be manifested. However, content of Si in excess of 1.0% may impair cold workability and weldability, so the preferred upper limit of Si is set to 1.0%.

Like Si, Mn acts as a deoxidation agent and combines with S to form MnS, which improves hot workability. However, with less than 0. 1% of Mn, these desirable properties may not be manifested. If the content of Mn exceeds 2%, it may not be possible to obtain the phase balance between ferrite and austenite, with the result that entire corrosion and local corrosion will proceed. Consequently, the preferred upper limit of Mn is set to 2%.

With less than 3% of Ni, it may not be possible to attain the desired phase balance between ferrite and austenite, thus degrading the corrosion resistant property. For this reason, the lower limit of Ni should preferably be higher than 3%. However, if Ni is incorporated in excess of 8%, the percentage of the austenite phase might become excessive, which would not only degrades the phase balance but also increase the manufacturing cost.

With less than 21% of Cr. sufficient corrosion resistant property and local corrosion resistant property might not be obtained and the phase balance between ferrite and austenite might exceed a desired range. However, when the content of Cr exceeds 28%, a third phase, i.e. the (s phase, tends to precipitate, which would not only embrittle the resulting stainless steel but also degrade the cold workability. Accordingly, the preferred range of Cr is from 21 to 28%.

With less than 1% of Mo, it may not be possible to prevent local corrosion, so its content should preferably be higher than 1%. However, incorporation of Mo in excess of 4% might result in embrittling and degration of workability. For this reason, the upper limit of Mo should preferably be 4%.

In order to adjust the phase balance between ferrite and austenite in a preferred range, more than 0.08% of N is necessary, which improves the corrosion resistant property as well as the local corrosion resistant property. When the nitrogen content exceeds 0.25%, gas bubbles tend to formed in steel to impair hot workability and corrosion resistant property. For this reason, the preferred upper limit of nitrogen is set to 0.25%.

P is an element inherently introduced into steel as an impurity, but when the amount of P exceeds 0.03% hot workability and weldability tend to be impaired, so the preferred upper limit of P is set to 0.03%.

S is also inevitably introduced into steel as an impurity but when its quantity exceeds 0.01% hot workability may be degraded and local corrosion may become remarkable, so its preferred upper limit is set to 0.01%.

Aluminum has a strong deoxidation function and crystal grain adjusting function but in ferrite austenite two phase type stainless steel, especially that utilized to manufacture seamless steel pipes, by incorporating less than 0.02% of soluble aluminum, a stable high impact characteristic can be obtained. Although the reason why the impact characteristic is degraded when the amount of soluble aluminum exceeds 0.02% is not yet clearly understood, as above described in the two phase stainless steel, since the content of nitrogen is considerably high, it is presumed that the reaction of Al + NeAIN proceeds to form aluminum nitride. In the manufacture of seamless steel pipes, the steel is subjected to much more complicated and severe workings than ordinary steel plates and, as it is necessary to use considerably higher rolling temperature, the impact characteristic of the seamless steel pipes are mainly determined by rolling steps. Accordingly, even when the pipes are subjected to solution treatment the impact characteristics would not be improved appreciably. According to this invention, the quantity of the soluble aluminum is selected to be in an optimum range for controlling the structure of the two phase stainless steel after high temperature hot rolling thereby improving the impact characteristic in any of the states after hot rolling, solution treatment and cold rolling.

In addition to the elements described above, as has been pointed above any one or more of Cu, W, V, Ti, Nb, Zr, Ta, Ca, Mg, B, and REM may be incorporated into the ferrite ferrite austenite two phase type stainless steel embodying the invention as will be described in detail in the following.

Although Cu is effective to improve acid resistant property as well as local corrosion resistant property, when its quantity exceeds 2.5%, hot workability and local corrosion resistant property are degraded.

W and V impart sea water resistant property to the steel, but when their quantities exceed 1.5%, workability and weldability are degraded. Ti, Nb, Zr, and Ta improve grain interface corrosion resistant property and hot workability, but the amount of incorporation of each of these elements exceeds 1.0%, toughness, ductility, and cold workability are degraded.

Each of Ca, Mg, B, and REM has a function of improving hot workability so that an amount exceeding 0.0005% can be incorporated. However, where much more of these elements is incorporated, a contaminant such as oxysulfide is formed which impairs the purity of steel and hot workability. Accordingly, their upper limit should be set to 0.01%.

Many samples of two phase stainless steel containing different quantities of soluble aluminum were prepared. The samples of the two phase stainless steel were rolled into seamless steel pipes and then these pipes were subjected to solution treatment. After the solution treatment, the pipes were cold worked to provide a yield strength of 100 KSi level, a Charpy impact test was performed at 0 C. Fig. 1 shows the relation between the amount of soluble aluminum and the impact characteristic (vex) after solution treatment, while Fig. 2 shows the relationship between the amount of soluble aluminum and the impact characteristic (vex) of steel subjected to cold working treatment followed by solution treatment.A longitudinal direction test was made for standard size test pieces of solution treated steel and a T direction test was made for half size test pieces (10 mm X 5 mm).

As can be noted from Figs. 1 and 2 when the aluminum content of the control steel is less than 0.02% defined by the invention the effect of composition is less, while the impact value is greatly influenced by the soluble aluminum and the impact value is greatly improved as the amount of the soluble aluminum decreases. But at 0.02% the effect saturates, and the quantity of soluble aluminum less than this value enables to obtain stable high impact value.

As above described, according to this invention, it is possible to greatly improve the impact characteristic of ferrite- austenite two phase stainless steel, thereby improving the toughness of seamless steel pipes made of the steel of this invention.

Claims

1. Two phase stainless steel of the ferrite-austenite type containing soluble aluminum of less than 0.02 wt. % and having improved impact characteristic.

2. Two phase stainless steel of the ferrite-austenite type containing less than 0.03 wt. % C, 0.1 to 1.0 wt. % Si, 0.1 to 2.0 wt. % Mn, less than 0.03 wt. % P, less than 0.01 wt. % S, 3 to 8 wt. % Ni, 21 to 28 wt. % Cr. 1 to 4 wt. % Mo, 0.08 to 0.25 wt. % N, less than 0.02 wt.

% Sol. Al, and the balance of iron and impurities.

3. Two phase stainless steel as claimed in claim 2, further containing one or more of the following elements in the following percentages: less than 2.5 wt. % of Cu, less than 1.5 wt. % each of V and W, less than 1.0 wt. % each of Ti, Ta, Nb, and Zr, less than 0.01 wt. % each of Ca and Mg, less than 0.01 wt. % of B, and less than 0.01 wt. % of rare earth metals.

4. Two phase stainless steel as claimed in any preceding claim, substantially as described herewith reference to the accompanying drawings.