KR930011626B1 - Steel for pipe - Google Patents

Abstract

The invention prevents the spherical corrosion of the steel tube by adjusting the alloy elements of it. The steel tube is manufactured by electrically welding a steel plate comprising (in wt.) 0.03-0.30 % C, 0.17-0.24 % Si, 0.30-1.6 % Mn, up to 0.025 % P, up to 0.005 % S, 0.010-0.060 % Al, 0.08-0.30 % Cu, 0.08-0.30 % Ni, 0.001-0.006 % Ca and balance Fe with inevitable impurities. The steel tube has high spherical corrosion resistance under a naked corrosion atmosphere.

Description

Base material for electric welded steel pipe with excellent corrosion resistance

1 is a schematic diagram of the salt spray test in the present invention.

2 is a principle test test principle of the present invention.

The present invention relates to a related art for steel pipes manufactured by electric welding, in particular, due to the change in metal structure and chemical composition of other welded parts and the base metal part in welding process (electric welding, high frequency electric welding, etc.) during the manufacturing process. Invented for the purpose of increasing product quality by providing alloy composition of steel pipes that can minimize the occurrence of grooved grooves (groove corrosion) in the situation where steel pipe manufactured in corrosive environment is exposed. The present invention relates to a base metal for electric welded steel pipe having excellent durability against corrosion. Generally, the chemical composition of the iron alloy for electric welding steel pipe for pipe is C 0.03-0.30%, Si 0.35% or less, Mn 0.30-1.60%, P 0.040% or less, S 0.040 or less, Al 0.010-0.080 Other unavoidable impurities and residual Fe When the steel pipe is manufactured, the heat treatment and the quenching are applied to the welding part, so that the base metal part and the welding part are different in metal structure and chemical composition. Unlike the base metal part, the welded part, which is changed to the metal microstructure and chemical composition, acts as an anode with respect to the base material part, so that a local deep groove is generated.

This type of corrosion is called Groove Corrosion.

In order to prevent such corrosion phenomenon, various types of methods, such as a heat treatment method for a welded part, a method of adding an alloying element, a surface treatment (galvanizing and antirust coating), and the like, have been proposed. In the heat treatment method, spherical corrosion can be somewhat suppressed by the change of metal structure, the removal of residual stress, and the reduction of natural potential difference, but it is not a fundamental countermeasure. Since it does not change and gives anticorrosion ability only to the surface layer, it also could not expect a fundamental spherical corrosion prevention effect.

Therefore, the present invention is to invent a new alloy for the electric welded steel pipe by the method of adjusting and adding the alloying element to the material of the welded steel pipe as proposed above to bring about the fundamental spherical corrosion inhibitory effect in the steel pipe as Referring to the alloy composition of the steel pipe determined by the embodiment in detail as follows.

The alloy composition of the steel pipe determined by the present invention and the alloy composition of the conventional steel pipe are shown in Table 1.

TABLE 1

As compared with the above Table 1 as the reason for the determination of each chemical component and content of the present invention to bring about a content change in the conventional steel pipe alloy composition

○ In case of Si, Al-Killed steel is mainly used as a deoxidizer, but Al-Si composite deoxidation steel is used for the purpose of improving corrosion resistance.In this case, residual Si content does not occur when hot dip galvanized in piping applications. The optimum content of 0.17-0.24% by weight was determined.

○ In the case of P, as in general steels, when P content is high, it is segregated at the grain boundary of metal and lowers the flatness of 90 ° and causes cracking in the welded part, so the maximum content is regulated to 0.025% by weight.

○ In the case of S plays a role in steel similar to the above P, but when the S content, such as general steel is high, the corrosion resistance and toughness is lowered, there is a lot of non-metallic inclusions of MnS generated in the weld of the electric welding tube of the present invention Lose.

At this time, MnS acts as a cathode in a corrosive environment, and S becomes an anode which is the starting point of spherical corrosion.

In other words, it was observed through the test that the more S (Sulfar) content, the more spherical corrosion is promoted.

Therefore, in the present invention, it was concluded that S should be regulated as much as possible, but it is regulated to 0.005% by weight or less in consideration of the current steelmaking technology level and economic feasibility.

In case of Ni and Cu, an appropriate content of 0.08-0.30% by weight was determined to reduce the natural potential difference and improve the corrosion resistance of the base metal part and the weld part in the welded steel pipe, which is a spherical corrosion mechanism.

○ In the case of Ca to reduce the content of S in the steel, in the present invention it is mobilized to reduce the S, the main key chemical component of spherical corrosion generation to the maximum. In addition, it was determined by confirming that the residual amount in the most suitable steel (하는데) to serve to lower the corrosion susceptibility index by spheroidizing non-metallic inclusions in the form of corrosion.

The range of alloy composition in the present invention was determined by the reasons identified through the various tests as described above, and the salt corrosion characteristics of the steel pipe by the alloy of the present invention and the steel pipe by the conventional alloy (see Table 1) were brine. Comparative tests were conducted by the spray test and the Potentiostat method.

A) Salt Spray Test Results

1) spherical corrosion resistance to the weld;

○ Test condition

Salt spray test was carried out according to the Korean Industrial Standard KS D9502-73 salt spray test method.Test conditions were NaCl solution 5%, PH 5.0-7.2, specific gravity 1.0268-1.0413, temperature 35 ℃ ± 2 ℃, spray amount 0.5-3.0 ml / Hr, conducted for 3 months. On the other hand, after the corrosion test was used as the index 나타내는 α: spherical corrosion susceptibility index 지수 as an index indicating how many times the weld portion corroded to the base material portion is expressed in the same manner as in FIG. That is, when α = 1 in FIG. 1, local corrosion does not occur in the weld at the corrosion depth h ₂ -θ of the weld, and a larger value of α means that corrosion tends to occur at the weld.

○ Test result

In the case of a welded steel pipe (hereinafter referred to as the product of the present invention) made of the alloy of the present invention, the spherical corrosion susceptibility index (α) was # α = 1 ＂, and no spherical corrosion occurred in the welded portion. In the case of a welded steel pipe made of a conventional alloy (hereinafter referred to as a conventional product), the spherical corrosion susceptibility index (α) α = 3.82 on average, indicating that spherical corrosion is frequently generated in the welded portion. As a result, it was confirmed that the welded steel pipe by the alloy of the present invention in the welded portion was excellent in durability at the endurance corrosion.

2) corrosive to the front of the mother tube;

Although the appearance pattern of the corrosion products of the present invention and the conventional products were the same, but the spherical corrosion of the welded parts was examined after removing these corrosion products, spherical corrosion occurred in the welded parts of the conventional products. Visual confirmation that no spherical corrosion occurred. In addition, the surface corrosion rate of the conventional product is 2.09mdd on the average, and 1.98mdd on the average of the present invention, and finally, the salt corrosion test showed that the corrosion resistance of the present invention was about 5%.

3) steel pipe corrosion life;

In view of the corrosion corrosion depth of the welded joint corrosion depth and the total corrosion depth of the welded pipe, the conventional product: 0.298mm / month for the conventional product, and 0.076mm / month for the present invention. It was confirmed that the corrosion life of the welded steel pipe by the alloy of the present invention is improved by three times or more than the welded steel pipe by the alloy.

B) Results tested by the Potentiostat method

The apparatus used for the potent shot test is shown in Figure 2, the experimental conditions were carried out in 30 ℃, 3% NaCl aqueous solution and the experimental period was 144 hours. Since the test solution turned black due to the dissolution of the sample and was contaminated with many products, the solution was replaced after 72 hours of continuous testing. A potential of -600 mV vs SCE (Saturated Calomel Electrode) was applied to the specimens using a potential test to promote corrosion, and the principle of the test apparatus was performed as shown in FIG.

○ Test result

Although spherical corrosion did not occur in the welded part of the product of the present invention (α = 1.0), it was confirmed that the product of the present invention was improved more than three times in corrosion resistance by the α of 3 or more in the conventional product. Here, it was confirmed that another comparative product (hot processed food) had α = 1.36, which was much more durable than the conventional product.

As can be seen from the various examples or the test as described above, using the iron alloy material of the alloy composition according to the present invention can be confirmed in Table 1 to produce a steel pipe by the electric welding (high frequency electric welding) processing method corrosion atmosphere It is possible to prevent piping accidents due to prolonged service life and spherical corrosion by being used as a piping material in a place exposed to exposure.

Claims (1)

C 0.03-0.30%, Si 0.17-0.24%, Mn 0.30-1.6%, P 0.025% or less, S 0.005 or less, Al 0.010-0.060, Cu 0.08-0.30%, Ni 0.08-0.30%, Ca 0.0010-0.0060% ( Weight percent) and other inevitable impurity and the residual amount of Fe excellent electrical corrosion resistance steel welding base material characterized in that the composition.