Technical Field
[1] The present invention relates to a steel having resistance to corrosion by sulfuric
acid and hydrochloric acid used for a desulfurized duct, a gas gas heater (GGH), and an electrostatic precipitator (EP) in de-sulfurization facilities of a thermal power plant and a method for manufacturing the same, and more particularly, to a steel having resistance to corrosion by sulfuric acid and hydrochloric acid capable of enhancing resistance to corrosion by sulfuric acid and hydrochloric acid in a range of low temperature and low sulfuric acid concentration in de-sulfurization facilities to extend useful life of the facilities, and a method for manufacturing the same.
[2]
Background Art
[3] When a sulfur-containing fuel is burned, SOx is generated in an exhaust gas to
chemically bind with moisture therein, thereby producing sulfuric acid. In a case where the exhaust gas has a temperature lowered to about 160°C, i.e., a sulfuric acid dew point, the sulfuric acid condensed on a surface of the steel results in a severe corrosive environment. Moreover, a hydrochloric ion (CI) contained in the exhaust gas is condensed into hydrochloric acid together with sulfuric acid at a temperature of 80°C or less, thus deteriorating the corrosion environment. Given the design trend of environment-friendly facilities of domestic thermal power plants, the operational temperature tends to be lowered to increase efficiency in electrostatic precipitation and de-sufurization. This causes not only sulfuric acid but also hydrochloric acid to be condensed on the surface of the steel. This accordingly has led to a demand for a material improved in corrosion resistance to sulfuric acid and to hydrochloric acid as well.
[4] To produce a low-alloy corrosion-resistant steel used for the thermal power plants,
up to now, Cu and other corrosion resistant alloy elements are added in combination to develop a steel with superior corrosion resistance to sulfuric acid.
[5] However, a technology of improving corrosion resistance to sulfuric acid by adding
Cu-Co in combination only concerns corrosion resulting from sulfuric acid condensation. Thus, in this technology, corrosion resistance is rapidly weakened in a hydrochloric acid atmosphere.
[6] A steel having alloy elements such as Cu-Cr-(Ti, Nb, V, Mo) added therein exhibits
mechanical properties suitable for high-temperature facilities. However, this steel is degraded in corrosion resistance in a range of low temperature and low sulfuric acid concentration and in a hydrochloric acid atmosphere.
[7] A steel having alloy elements such as Cu-Sb-(Mo, Cr, Ni, Sn) added therein is
improved in corrosion resistance to hydrochloric acid. However, the steel with the expensive Mo added therein is increased in manufacturing costs and less resistant in a sulfuric acid condensation atmosphere.
[8]
Disclosure of Invention Technical Problem
[9] The present invention has been made to solve the foregoing problems of the prior
art and therefore an aspect of the present invention is to provide a steel having excellent resistance to corrosion by sulfuric acid and hydrochloric acid in a range of low temperature and low sulfuric acid concentration in de-sulfurization facilities.
[10]
Technical Solution
[11] According to an aspect of the invention, the invention provides a steel having
excellent resistance to corrosion by hydrochloric acid and sulfuric acid, the steel including, by weight%: greater than 0 to 0.15% C, greater than 0 to 1.0% Si, greater than 0 to 2.0% Mn, greater than 0 to 0.03% S, greater than 0 to 0.02% P, 0.01 to 0.1% Al, 0.2 to 1.0% Cu, 0.02 to 0.1% Co, 0.02 to 0.2% Sb, 0.02 to 0.15% Sn, 0.02 to 0.2% W, and the balance of Fe and unavoidable impurities.
[12] According to another aspect of the invention, the invention provides a method for
manufacturing a steel having excellent resistance to corrosion by hydrochloric acid and sulfuric acid, the method including: reheating a steel to 1100 to 1300°C, the steel including , by weight%: greater than 0 to 0.15% C, greater than 0 to 1.0% Si, greater than 0 to 2.0% Mn, greater than 0 to 0.03% S, greater than 0 to 0.02% P, 0.01 to 0.1% Al, 0.2 to 1.0% Cu, 0.02 to 0.1% Co, 0.02 to 0.2% Sb, 0.02 to 0.15% Sn, 0.02 to 0.2% W, and the balance of Fe and unavoidable impurities; finish hot-rolling the steel at 850 to 950°C; and coiling the steel at 560 to 660CC.
[13]
Advantageous Effects
[14] A steel is advantageously improved in corrosion resistance to sulfuric acid and hy-
drochloric acid in a range of low temperature and low sulfuric acid concentration.
[15]
Best Mode for Carrying Out the Invention
[16] Hereinafter, a description will be given of reasons for defining a content range of
elements according to an exemplary embodiment of the invention.
[17] Carbon (C) content is greater than 0 to 0.15wt%.
[18] C content exceeding 0.15wt% significantly degrades corrosion resistance to sulfuric
acid and welding characteristics, thus causing a steel to risk more defects and facilities employing the present invention to be reduced in useful life thereof. Thus, C may be added at 0.15wt% or less.
[19] Silicon (Si) content is greater than 0 to 1.0wt%.
[20] Si is mainly added to enhance strength of the steel. However, Si content exceeding
1.0wt% considerably deteriorates corrosion properties in a range of low temperature and low sulfuric acid concentration, and triggers red scale defects during hot rolling of the steel. Thus, Si may be added at 1.0wt% or less.
[21] Manganese (Mn) content is greater than 0 to 2.0wt%.
[22] Mn is typically added to precipitate sulfur S solved in the steel as manganese
sulfide to prevent red shortness resulting from the solved sulfur and satisfy desired mechanical properties. When Mn content exceeds 2.0wt%, its effect of enhancing strength is outweighed by disadvantages of impairing corrosion resistance to sulfur and combined corrosion resistance properties. Thus, Mn is added at up to 2.0wt%.
[23] Sulfur (S) content is greater than 0 to 0.03wt%.
[24] S may be added in as small an amount as possible. S content exceeding 0.03wt%
causes the steel to risk more defects resulting from hot shortness. Thus, sulfur may be added at up to 0.03wt%.
[25] Phosphor (P) content is greater than 0 to 0.02wt%.
[26] P content exceeding 0.02wt% ensures higher strength but considerably degrades
corrosion resistance to sulfur and combined corrosion resistance properties. Particularly, P at a crystal grain boundary deteriorates material characteristics and adversely affects surface quality. Thus, P may be added at up to 0.02wt%.
[27] Aluminum (Al) content is in a range of 0.01 to 0.1 wt%.
[28] Al is added to deoxidize the steel to suppress crack occurrence in a continuous
casting process during refining thereof. Al content less than 0.01wt% ensures less deoxidizing effect. On the other hand, Al greater than 0.1 wt% leads to more Al oxides, thereby causing the steel to risk more defects. Thus, Al content may be added at up to 0.1 wt%.
[29] Copper (Cu) content is in a range of 0.2 to 1.0wt%.
[30] Cu is necessarily added to increase corrosion resistance to sulfuric acid and
combined corrosion resistance properties. Cu, when added at 0.2wt% or more noticeably enhances corrosion resistance properties. Meanwhile, Cu content exceeding 1.0wt% brings about less increase in corrosion resistance properties, thus economically ineffective. Therefore, Cu content may be added at up to 1.0wt%.
[31] Cobalt (Co) content is in a range of 0.02 to 0.1wt%.
[32] Co, in addition to Cu, is a representative element for increasing corrosion resistance
resulting from sulfuric acid condensation. Co added in combination with Cu ensures more remarkable corrosion resistance to sulfuric acid than in a case where Cu is added alone. On the other hand, Co's effect on combined corrosion resistance properties is not significant compared to its effect on corrosion resistance to sulfuric acid. Nonetheless, Co is essentially added in view of its effect on corrosion resistance to sulfuric acid. Co content may range from 0.02 to 0.1wt%. Co content less than 0.02wt% ensures less effect and Co content exceeding 0.1 wt% brings about less improvement in corrosion resistance relative to its amount. Co content exceeding 0.1 wt% also greatly raises steel manufacturing costs.
[33] Antimony (Sb) content is in a range from 0.02 to 0.2wt%.
[34] Sb is effective for increasing corrosion resistance to sulfuric acid and combined
corrosion resistance properties. Sb forms a corrosion byproduct on a surface of the steel thereby to enhance corrosion resistance. Sb content less than 0.02wt% brings about less effect and a greater content of Sb increases corrosion resistance. However, Sb content exceeding 0.2wt% ensures little further effect resulting from an increase in the amount.
[35] Tin (Sn) content is in a range from 0.02 to 0.15wt%.
[36] Like Sb, Sn is effective for improving corrosion resistance to sulfuric acid and
combined corrosion resistance properties. Sn is remarkably contributive to increasing corrosion resistance to sulfuric acid. Sn content less than 0.02wt% brings about negligible effect. On the other hand, Sn content exceeding 0.15wt% does not ensure a big improvement in corrosion resistance but tends to impair rolling workability. That is, the steel may be fractured due to grain boundary precipitation of Sn during hot rolling.
[37] Tungsten (W) content is in a range from 0.02 to 0.2wt%.
[38] W is also added to improve corrosion resistance characteristics. W is effective for
enhancing combined corrosion resistance and ensuring corrosion resistance to sulfuric acid. W content may range from 0.02 to 0.2wt% due to similar reasons described regarding the Sb content.
[39] Hereinafter, relations of the present invention will be described.
[40] Two relations of the present invention define a content range of elements and kinds
of essential elements to achieve corrosion resistance to sulfuric acid and hydrochloric acid.
[41] First, relation [W(wt%)xSn(wt%)]/ Sb(wt%) < 0.2 defines a content range of W,
Sn, and Sb as essential elements to maximize their corrosion resistance effect. According to the relation, notably, Sb is relatively most effective among these
elements and thus it is more beneficial to increase Sb content in place of W, and Sn contents. That is, the relation denotes that it is desirable to lower a ratio of Sb to other elements, i.e., increase Sb content. This brings about corrosion resistance and also allows the steel to be manufactured at a low cost by adding low-priced Sb rather than high-priced W.
[42] Next, relation [W(wt%)xSn(wt%)]/ Sb(wt%) * 0 denotes that a multiplied or
divided value of each element is not zero. That is, to ensure that the value of the relation does not equal zero, no element should have zero content. Elements in the relation are essential for attaining corrosion resistance and thus required to be added. To satisfy corrosion resistance to sulfuric acid and hydrochloric acid, Sn mainly serves to increase corrosion resistance to sulfuric acid and W and Sb largely contribute to corrosion resistance to hydrochloric acid. Therefore, to attain combined corrosion resistance to sulfuric acid and hydrochloric acid, these elements need to be essentially added.
[43] Hereinafter, a method for manufacturing a steel having excellent resistance to
corrosion by sulfuric acid and hydrochloric acid will be described.
[44] The method includes: reheating a steel to 1100 to 1300°C, the steel including, by
weight%: greater than 0 to 0.15% C, greater than 0 to 1.0% Si, greater than 0 to 2.0% Mn, greater than 0 to 0.03% S, greater than 0 to 0.02% P, 0.01 to 0.1% Al, 0.2 to 1.0% Cu, 0.02 to 0.1% Co, 0.02 to 0.2% Sb, 0.02 to 0.15% Sn, 0.02 to 0.2% W, and the balance of Fe and unavoidable impurities, and content of the Sb, Sn, and W satisfies a relation of [W(wt%)*Sn(wt%)]/ Sb(wt%)<0.2 and [W(wt%)*Sn(wt%)]/ Sb(wt%) # 0; finish hot-rolling the steel at 850 to 950°C; and coiling the steel at 560 to 660°C.
[45] (1) A reheating temperature ranges from 1100 to 1300°C.
[46] A reheating temperature in a rolling process for manufacturing a steel sheet is set
such that a slab is maintained at a predetermined temperature for rolling to control structures in the slab and re-solve precipitates formed by the added elements. Therefore, the reheating temperature satisfies a desired hot rolling finishing temperature and involves a predetermined range in which columnar structures are eliminated in a continuous casting. The reheating temperature is typically set at 1100°C or more. Meanwhile, to allow the added elements to be re-solved, the temperature varies according to characteristics of the added elements. However, in a case where the added elements are expected to form precipitates hardly dissolved at a high temperature, the elements are heated at a temperature higher than a temperature for controlling the structures. W added according to the present invention is relatively stable, thus rendering precipitates thereof hardly re-solved at a high temperature. This requires the reheating temperature of the slab to be increased to 1300°C.
[47] (2) A finishing temperature ranges from 850 to 950°C.
[48] A finishing temperature is typically set higher than a temperature range where
austenite is transformed into ferrite. This allows the steel slab to be finish-rolled just above a transformation point temperature, thereby ensuring structures to be transformed into ferrite with uniform distribution. These uniform structures prevent local corrosion from arising from any difference in structures under the same corrosion condition and the finishing temperature may be set at 850°C or more to ensure uniform corrosion. The finishing temperature has an upper limit set such that scales generated on the surface of the steel at a high temperature do not occur too excessively. Thus, the finishing temperature is set to an upper limit of 950°C or less to inhibit surface defects resulting from scale formation.
[49] (3) A coiling temperature ranges from 560 to 660°C.
[50] A coiling process after finishing rolling involves a process where transformation
into ferrite is performed. A higher coiling temperature allows more crystal grains to grow, thereby rendering the steel material to be ductile. However, too low a coiling temperature for preventing the crystal grains from growing hardens the steel material due to formation of minute crystal grains. The steel undergoes oxidization at a surface layer during coiling, and thus too high a temperature leads to formation of scales, very likely to cause surface defects. Therefore, according to the present invention, the coiling temperature after rolling is set in such a range of 560 to 660°C that desired material characteristics are satisfied and surface defects are prevented.
[51]
Mode for the Invention
[52] Hereinafter, the present invention will be described in greater detail by way of
Example.
[53]
[54] Example
[55] Steel ingots were solved and manufactured to satisfy a content range noted in Table
1 below. The steel ingots were re-heated for 1 hour in a heating furnace of 1250°C, and then hot-rolled. The steel ingots were hot-rolling finished at a temperature of 870 to 890°C, and coiled at 620°C and had a final thickness of 3.2mm. In order to evaluate corrosion resistance to sulfuric acid, the hot-rolled samples were immersed in 70°C -50wt% sulfuric acid under a low temperature and low concentration condition for 6 hours and measured for decrease in corrosion. When it comes to combined corrosion resistance to sulfuric acid and hydrochloric acid, the steel ingots were immersed for 6 hours in a modified green death solution (16.9 Vol% sulfuric acid + 0.35 Vol% hydrochloric acid) which is most closely similar to actual corrosive environment of domestic low-temperature desulfurization facilities to measure decrease in corrosion,
-7-
in the same manner as corrosion resistance to sulfuric acid. The results are shown in
Table 2.
[56] Table 1
(Table Removed)

[57]
[58]
[59]
[60] Table 2
(Table Removed)

[61]
[62] Inventive steels 1 and 2 have chemical composition including Cu, Co, Sb, W, and
Sn, respectively. Of these, the Sb, Sn, and W contents satisfy relations of [W(wt%) x Sn(wt%)]/ Sb(wt%) < 0.2 and ([W(wt%) x Sn(wt%)]/ Sb(wt%) ≠ 0), showing superior corrosion resistance to sulfuric acid and combined corrosion resistance properties. In terms of combined corrosion resistance properties, Sb, Sn, and W contents play an essential role. That is, absence of even a single element thereof deteriorates corrosion resistance characteristics as shown by the immersion test of Inventive steels 1 and 2, and Comparative steels 5, 6, 7, and 9.
[63] Comparative steels 2 and 8 having Sb and Mo added therein exhibit good corrosion
resistance to sulfuric acid and combined corrosion resistance. However, Mo, when
added, raises manufacturing costs and is inferior in corrosion resistance characteristics
to a case where Sb, W, and Sn are added in combination.
[64] Comparative steels 3 and 4 have compositional difference in terms of presence of
Co. Comparative steel 3 without Co added, is noticeably degraded in corrosion
resistance to sulfuric acid. This indicates that Co should be added essentially in view of
corrosion resistance to sulfuric acid.
[65] Comparative steel 1 demonstrates inferior corrosion resistance to sulfuric acid and
combined corrosion resistance properties due to Ni addition. Thus, Ni should be
excluded from compositional design. [66] [67]

We Claims:-
[1] A steel having excellent resistance to corrosion by hydrochloric acid and sulfuric
acid, the steel comprising, by weight%:
greater than 0 to 0.15% C, greater than 0 to 1.0% Si, greater than 0 to 2.0% Mn, greater than 0 to 0.03% S, greater than 0 to 0.02% P, 0.01 to 0.1% Al, 0.2 to 1.0% Cu, 0.02 to 0.1% Co, 0.02 to 0.2% Sb, 0.02 to 0.15% Sn, 0.02 to 0.2% W, and the balance of Fe and unavoidable impurities.
[2] The steel of claim 1, wherein contents of the Sb, Sn, and W satisfy relations of
[W(wt%)xSn(wt%)]/ Sb(wt%) < 0.2 and [W(wt%)xSn(wt%)]/ Sb(wt%) * 0.
[3] A method for manufacturing a steel having excellent resistance to corrosion by
hydrochloric acid and sulfuric acid, the method comprising: reheating a steel to 1100 to 1300°C, the steel comprising, by weight%: greater than 0 to 0.15% C, greater than 0 to 1.0% Si, greater than 0 to 2.0% Mn, greater than 0 to 0.03% S, greater than 0 to 0.02% P, 0.01 to 0.1% Al, 0.2 to 1.0% Cu, 0.02 to 0.1% Co, 0.02 to 0.2% Sb, 0.02 to 0.15% Sn, 0.02 to 0.2% W, and the balance of Fe and unavoidable impurities, and contents of the Sb, Sn, and W satisfy relations of [W(wt%)xSn(wt%)]/ Sb(wt%) < 0.2 and [W(wt%)xSn(wt%)] / Sb(wt%) ≠ 0;
finish hot-rolling the steel at 850 to 950°C; and coiling the steel at 560 to 660°C.
[4] Steel having excellent resistance to corrosion substantially as herein
described with reference to forgoing examples.
[5] A method for manufacturing a steel having excellent resistance to corrosion
substantially as herein described with reference to forgoing examples.