KR880001356B1 - Low interstitial 29% chromium-48% molybdenun weldable ferrite stainless steel containing columbium or titanium - Google Patents

Abstract

Stainless steel (I) and method of making it are claimed. It comprises (by wt.) 25-35 (esp. 28.5-30.5)% Cr, 3.6-5.6 (esp. 3.75- 4.75)% Mo, less than 3% Ni, less than 2% Mn, less than 2% Si, less than 0.5% Al, less than 0.5% Cu, less than 0.05% P, less than 0.05% S, less than 0.01% C, less than 0.02% N, the sum of C and N being less than 0.025%, 0.05-0.5 (esp. 0.05-0.2)% Nb, and/or Ti, balance Fe. It is hot rolled, annealed, air cooled and then cold rolled to produce a steel with a charpy impact transition temp. of below O deg.F as cold rolled to a thickness of 0.062 ins. The Nb and/or Ti improve the toughness of the steel when the max. achievable cooling rate is limited.

Description

Low penetration 29% chromium-4% molybdenum ferritic stainless steel with weldable COB or Titanium

The present invention relates to a low ingressive, corrosion resistant, weldable ferrite stainless steel with improved toughness, which improves toughness even when the maximum achieved cooling is limited by the addition of a small amount of columbium or titanium. To a ferritic stainless steel which is low ingress, corrosion resistant and weldable.

Ferritic stainless steels must have good corrosion and corrosion resistance for use in certain chemical environments, such as power plants and pumps and paper mills exposed to seawater. Stainless steels containing 29% chromium and 4% molybdenum have high resistance to crevice corrosion. These steels have low levels of incorporation such as less than 0.025% carbon + nitrogen by weight in order to provide excellent malleability and intergranular corrosion resistance after welding. Requires total content.

Ferritic stainless steels of the present invention are used in the production of thin welded tubes, such as condensation tubes, as well as thick support products such as steel sheets, which are assembled by welding. The shape and size of the assembled device is usually incapable of the final heat treatment, and even though the final heat treatment can be performed, it must be rapidly cooled from the heat treatment temperature, thereby severely limiting the shape of the size of the assembled device. Moreover, the toughness of the alloy decreases with increasing thickness and with decreasing cooling rate. This is the title of "Toughness of Vacuum-Induced Molten High-Chromium Ferrite Stainless Steels" published in ASTM STP 706. This is already explained in Figure 12 in a paper published by Deberel. The decrease in toughness reduces the weldability and in some cases the steel sheet shows cracks during welding because of its size and cannot be quenched with water. When the product is quenched with water, due to the thickness of the product, the cooling rate can not be fast enough to obtain adequate toughness, and the product may show cracks during welding. Therefore, good toughness must be achieved by other means than by quenching with water.

Even if the final product is of thin thickness, conventional manufacturing methods require cooling of thicker slabs and bands during the process. The cooling rate of this thick portion is slow. Therefore, quenching with water can achieve cooling quickly but is impossible or impractical because of the size of the form.

As the thickness of the product increases, the toughness measured by the charpy impact transition temperature decreases. Toughness is the metal's ability to absorb energy by pre-rupture deformation and the transition temperature is the temperature at which 50% shear (tear) and 50% crack (break) occur from the impact.

The toughness of the 29% chromium-4% molybdenum alloy to which the present invention belongs is lower than that of low chromium alloys of the same carbon and nitrogen content because of the high alloy content. Therefore, the toughness of the 29% chromium-4% molybdenum alloy only improved the toughness of the alloy by quenching with water, which speeds cooling instead of late air cooling. In many cases, however, quenching with water is often impossible or impractical, and new methods are practically required with limited maximum cooling rates (the ratio of maximum cooling temperature per unit time). 29% Chromium-4% Molybdenum Ferritic Stainless Steel with a 0.25% Carbon + Nitrogen Maximum Content was already described in US Pat. No. 3,929,473. US Pat. No. 3,932,174 adds a small amount of other elements to achieve the same corrosion resistance of 3,929,473. These patents do not indicate the use of Columium or Titanium.

U.S. Patent No. 3,807,771 suggests adding 13-29 times of nitrogen or 0.065-0.363% colombium to 1% molybdenum steel to improve toughness and corrosion resistance under air-cooled conditions. Patent 3,957,544 addresses the addition of titanium and columbium according to the equation Ti% / 6 + Cb% / 8 = (C% + N%). The molybdenum content of the steel in these patents differs from the present invention with techniques that contain lower than the content of the invention.

The presence of titanium and columbium in steel decreases the steel's susceptibility to intergranular attack, but the weldability of the steel is bad unless the amount of intrusion is reduced. Molybdenum improves corrosion resistance under corrosion resistance, but according to U.S. Patent No. 4,119,765, if the presence of molybdenum is greater than 3.5% and combined with chromium, titanium, syracone or columbium, notch toughness is reduced under high weld conditions. . U.S. Patent No. 4,119,765 adds 2% -4.75% nickel to improve the weldability of steel. However, it is well known that Nickel's amount should be carefully controlled to improve odor toughness and acid corrosion resistance regardless of other properties.

Another reference can be found in Temus R. Ruta's paper, entitled “Corrosion Resistance and Economics of Ferritic Stainless Steels,” on July 24, pp. 24-29. Although described, no steel such as ferritic stainless steel of the present invention is described.

As mentioned above, the present invention is different from the description of the cited documents.

It is an object of the present invention to provide ferritic stainless steels with low intrusion, corrosion resistance and weldability that exhibit improved toughness at thicker sites and have high molybdenum content, with limited maximum achieved cooling rate.

It is a further object of the present invention to provide a low molten, corrosion resistant and weldable ferritic stainless steel of high molybdenum content exhibiting improved toughness by the addition of a small amount of columbium or titanium.

In particular, the present invention provides a low inductive ferritic stainless steel having internal resistance and weldability at room temperature. The improvement of the present invention is achieved by the addition of a limited amount of columbium or titanium to improve the toughness of ferritic stainless steel in a state where water quenching is impossible or impractical.

Ferritic stainless steels of the present invention have a weight of 25.0-35.0% chromium, 3.6-5.6% molybdenum, up to 3.0% nickel, up to 2.0% manganese, up to 2.0% silicon, up to 0.5% aluminum, up to 0.50% Copper, 0.050% or less phosphorus, 0.05% or less sulfur, 0.01% or less carbon less than 0.025% and at least one of 0.025% or less nitrogen, colombium or titanium is 0.05% -0.5% It consists of inevitable impurities and iron. At least one element of columbium and titanium is preferably present in an amount of 0.05-0.20%. The constituent elements other than the content of the columbium or the titanium are based on the conventional ferritic stainless steel, and the corrosion resistance or intergranular corrosion resistance, which is a basic characteristic of 29% chromium-4% molybdenum ferritic stainless steel, is chromium and molybdenum, and the like. Obtained by inductive element contents of carbon and nitrogen. In describing the conventionally preferred content for this purpose, it is preferable that chromium is present in an amount of 28.5-30.5% and molybdenum in an amount of 3.75-4.75%, and manganese and silicon are preferably present in an amount of 1.0% or less, and aluminum, copper, phosphorus And sulfur is 0.1% or less and carbon and nitrogen are preferably present in an amount of 0.008% and 0.016%, respectively.

The advantages of the inventive steel will become apparent from the following description of various forms of the invention.

EXAMPLE

A low inductive ferritic stainless steel having 25.0-35.0% chromium and 3.6-5.6% molybdenum content, in order to see the effect of the present invention, 0.05-0.20% of Colombium or Titanium is added to the steel composition up to Toughness was improved under conditions where the achievement cooling rate was limited.

Ingots from four samples are vacuum melted in the composition given in Table 1.

TABLE 1

Chemical composition (% by weight)

The ingot is heated to a temperature of 2050 ° F and hot rolled into a 0.140 inch thick strip. The hot rolled band is annealed to a temperature of 1850 ° F., quenched with water, and then cold stripped to 0.062 inch thick annealed at a temperature of 1850 ° F., quenched with water and then TIG welded.

Two total four charpy V-notch impact samples were taken from two 0.062 inch thick strips from the hot rolled bands. The samples were annealed at a temperature of 1850 ° F. and then quenched or quenched with water. The cooled samples were then tested for toughness and the test results are listed in Table 2.

TABLE 2

Shock Transition Temperature (° F)

Since the transition temperature decreases with increasing the content of columbium under air-cooled conditions, it is argued that colombium has an effect of improving toughness when gradually cooling the ferritic stainless steel of the present invention. Although the impact transition temperature of air cooled samples is slightly higher than that of water quenched samples, the impact transition temperature difference of 0.062 inch thick ferrite stainless steel strip of the present invention, which is quenched with water and quenched as shown in Table 2, is 100 ° F or less. On the other hand, the impact transition temperature difference of ferritic stainless steel having a range of conventional compositions (low content of columbium) is 240 ° F.

This phenomenon shows that toughness can be improved by adding colobium when cooling is not possible by water quenching or the cooling rate that can be taken off by quenching with thicker water than the thin air cooling rate is low. It can be seen that the impact transition temperature of a 0.062 inch steel sheet strip of the composition according to the invention is less than 0 ° F while the impact transition temperature of composition A in the conventional composition range is 130 ° F. It is extremely important that the impact transition temperature of the steel sheet strip be lower than room temperature during welding, so that cracking does not occur during welding, and the ferritic stainless steel of the composition within the conventional composition gives the required toughness characteristics of the final product when manufacturing the steel sheet. In order to reduce the impact transition temperature of the steel sheet below room temperature by quenching with water before cold rolling, there is an advantage that excellent toughness characteristics can be obtained by air cooling before cold rolling.

In addition, the impact transition temperature of the ferritic stainless steel of the composition of the present invention shown in Table 2 shows a correlation almost inversely proportional to the content of the columbium of the present invention in the composition of Table 1, so that the range of the composition of the composition in Table 1 can be significantly expanded. It can be understood.

Corrosion tests are performed on base metal samples heat-treated and air-cooled to 2250 ° F to mimic areas affected by heat by welding a thick thickness to a 0.062 inch thick welded strip. Samples of 1 inch × 2 inch were immersed in a boiling solution of ferric sulfate-50% sulfuric acid for 120 hours according to ASTM A 262 B for corrosion column and the corrosion rates are listed in Table 3.

TABLE 3

Corrosion rate in ferric sulfate-50% sulfuric acid

Intergranular corrosion

Test (ASTM A 262 Conduct B)

Additional test results of the mechanical properties under the welded conditions are shown in Table 4.

TABLE 4

Mechanical Properties of Welded Area of 0.062 Inch Thickness Strip

It will be apparent to those skilled in the art that the new principles of the present invention described in connection with particular embodiments as described above may have various other imitations and variations. Therefore, the invention should not be limited by the specific embodiments described, as long as they are within the scope of the claims.

Claims (7)

25.0% -35.0% chromium by weight, 3.6% -5.5% molybdenum, up to 3.0% nickel, up to 2.0% manganese, up to 2.0% silicon, up to 0.5% aluminum, up to 0.50% copper, 0.050% Less than 0.05% phosphorus, less than 0.05% sulfur, less than 0.025% 0.01% binomial carbon and at least one of less than 0.025% nitrogen, columbium or titanium is 0.05% -0.50% and the rest is inevitable impurities and Low-wearing, corrosion-resistant ferritic stainless steel composed of iron for improved toughness and weldability. The ferritic stainless steel with low toughness and corrosion resistance and improved toughness according to claim 1, wherein the content of chromium is 28.5-30.5%. The ferritic stainless steel with low toughness and corrosion resistance with improved toughness according to claim 1, characterized in that the molybdenum content is 3.75-4.75%. The ferritic stainless steel with improved toughness and corrosion resistance according to claim 1, characterized in that the content of at least one element of columbium and titanium is 0.05-0.20%. The ferritic stainless steel with low toughness and corrosion resistance with improved toughness according to claim 1, which contains 0.05-0.20% of columbium. Low lattice with improved toughness according to claim 1, characterized in that it contains 28.5-30.5% chromium, 3.75-4.75% molybdenum and 0.05-0.20% at least one element from colavium and titanium. Ferritic stainless steel with corrosion resistance and weldability. A method for producing an improved toughness ferritic stainless steel product that is hot rolled, annealed and cold rolled to strip thickness, comprising: 25.0% -35.0% chromium, 3.6% -5.5% molybdenum, 3.0% binary nickel, 2.0% or less manganese, 2.0% or less silicon, 0.5% or less aluminum, 0.50% or less copper, 0.050% or less phosphorus, 0.05% or less sulfur, sum less than 0.025%, 0.01% or less carbon and 0.025% At least one of the following nitrogen, colombium, or titanium is 0.05% -0.50%, and the remainder is composed of unavoidable impurities and iron to improve toughness and weldability. A method of producing toughened ferritic stainless steels by air cooling from temperature to cold to 0.062 inch thick after lowering the charpy impact transition temperature below 0 ° F.