KR20000045513A - Method of manufacturing coatless weather resistance steel - Google Patents

Abstract

PURPOSE: A method of manufacturing weather resistance steel is provided to enhance weather resistance by maintaining the fine structure without growing Ca(OH)2 through rapid cooling and by changing the polygonal ferrite structure into deposition ferrite one. CONSTITUTION: A steel billet consists essentially of, by weight %: 0.01-0.15% of C, not more than 0.50% of Si, not more than 2.0% of Mn, not more than 0.020% of P, not more than 0.010% of S, 0.10-2.0% of Cu, 0.10-1.0% of Ni, 0.0010-0.0050% of Ca(OH)2 powder with a diameter less than 2 micrometers, Fe and incidental impurities. The steel billet is heated at a temperature of 1100-1250°C and hot-rolled with a temperature range of austenite structure. Finally, the steel billet is cooled at a speed of 6-15°C/sec at a temperature of 550-350°C.

Description

Manufacturing method of unpainted weathering steel

The present invention adds fine Ca (OH) ₂ (calcium hydroxide) powder to steelmaking and manufactures low-alloy steel that is accelerated and cooled during hot rolling to form a base-based atmosphere that is hard to cause rust in a wet corrosive environment. The present invention relates to a method for manufacturing uncoated weather resistant steel, and more specifically, to improve weather resistance of a matrix structure, Cu, Ni and fine Ca (OH) ₂ powders are included as a basic composition, and Cr, Mo, Ti, Nb, V After the steel ingot was optionally added and heated to a temperature range for hot rolling, immediately after finishing hot rolling in the temperature range of the austenitic structure of at least ₃ Ar, the temperature before the formation of the hardened martensite structure. The present invention relates to a method for producing an unpainted weather resistant steel having excellent weather resistance by accelerated cooling to a range, followed by air cooling or post-heat treatment after air cooling.

In the present invention, the Ar ₃ point is defined as a temperature point at which the ferrite structure is generated in the austenite structure by lowering the temperature again when the slab is heated to a high temperature in order to perform hot rolling.

In the prior art, by weight%, C: 0.01-0.15%, Si: 0.50% or less, Mn: 1.50% or less, P: 0.020% or less, S: 0.010% or less are contained as a basic composition, and the balance of Fe After manufacturing a steel ingot containing an inevitable impurity and heated to a temperature range of 1100-1250 ℃, after the hot rolling in the temperature range of the austenitic structure of Ar ₃ or more point to air-cooled to prepare a non-painted weather-resistant steel.

However, according to the prior art as described above, a large amount of rust is generated in a humid corrosive environment, and the weatherability is extremely poor. Therefore, there is a problem in using it as a weatherproof material, such as painting required when used in a bridge or a building structure.

The present invention has been made to solve the above problems, by adding fine Ca (OH) ₂ powder, accelerated cooling to maintain the fine structure without coarse growth of Ca (OH) ₂ of steel species and polygonal base (Granular) An object of the present invention is to provide a method of manufacturing a non-coated weather resistant steel having improved weather resistance by transforming it into a needle-like ferrite structure.

In order to achieve the above object, the production method of the non-painted weathering steel of the present invention is, in weight%, C: 0.01-0.15%, Si: 0.50% or less, Mn: 2.0% or less, P: 0.020% or less, S: 0.010% or less , Cu: 0.10-2.0%, Ni: 0.10-1.0%, Ca (OH) ₂ powder with a diameter of 2 μm or less in a 0.0010-0.0050% basic composition, in which a steel ingot containing a balance of Fe and unavoidable impurities After manufacturing and heating to a temperature range of 1100-1250 ° C, a cooling rate of 6-13 ° C / sec in the temperature range from 550-350 ° C immediately after finishing hot rolling in the temperature range of the austenitic structure of at least ₃ Ar It is a structure characterized by cooling a steel plate in the range.

In the present invention, Cr, Mo: 0.01-1.0%, Ti, Nb, V: may contain at least one kind at 0.01-0.10%.

Furthermore, after accelerated cooling, the post-heat treatment can be performed in the temperature range of 500-650 ° C.

Hereinafter, the present invention will be described in more detail with reference to the preferred embodiments.

In the present invention, by weight, basically C: 0.01-0.15%, Si: 0.50% or less, Mn: 2.0% or less, P: 0.020% or less, S: 0.010% or less, Al: 0.010-0.10% In order to improve weather resistance, Cu: 0.10-2.0%, Ni: 0.10-1.0% and Ca (OH) ₂ powder are included as a basic composition.

Particularly, the fine Ca (OH) ₂ powder contains 0.0010-0.0050% as a basic composition, and forms fine inclusions of 2 μm or less to increase the pH of the steel matrix to create a basic atmosphere, and at the same time to form fine needle-like ferrite tissue to provide weather resistance. Plays a crucial role in improving

The fine Ca (OH) ₂ powder suppresses acidic acid in the corrosive atmosphere contained in water, thereby forming a basic atmosphere having a pH of 12 or more, thereby reducing corrosion and improving weather resistance.

This powder is composed of 0.5-2.0㎛ average size before molten steel, but the average size of Ca (OH) ₂ powder is 0.05-0.1㎛ most effective when it is distributed in molten steel and becomes a product.

The Ca (OH) ₂ powder is surrounded by a tube, which can be made of steel or copper or aluminum but is most economical in the case of cold rolled steel.

Cr, Mo: 0.01-1.0%, Ti, Nb, V: 0.01-0.10% among alloying elements can be added without additives or optionally added as one or more components to improve the weather resistance of the steel matrix structure and to refine the grain size to improve the strength. have.

Steels containing the above elements are subjected to hot rolling and accelerated cooling to further improve weather resistance, and hot rolling to remove weather stress of the steel matrix structure to improve weather resistance. The high temperature rolling is heated to a temperature range of 1100-1250 ° C. to finish the hot rolling in the temperature range of the austenitic structure of at least Ar ₃ .

In addition, it is accelerated cooling to reduce the band-shaped pearlite structure and to form a needle-like uniform ferrite structure to improve weather resistance to the best. Accelerated cooling cools the steel plate at a cooling rate of 6-13 ° C / sec in the temperature range of 550-350 ° C to keep the Ca (OH) ₂ of the steel grade fine without growing it coarsely. The ferrite tissue is transformed into acicular ferrite tissue.

In addition, there may be a post-heat treatment depending on the intended use, in this case, air-cooled or heat-treated in the temperature range of 500-650 ℃ after air cooling to remove the thermal stress in the steel to further improve weather resistance.

Hereinafter, the reason for numerical limitation of the composition component of the steel ingot will be described.

Increasing the C content results in an increase in the amount of pearlite, which deteriorates weldability and workability, reduces weatherability, and causes a large difference in hardness and texture between the welded part and the base metal. Therefore, the C content was limited to 0.15% or less. On the other hand, if the C content is too small at 0.01% or less, the drop in strength is severe and undesirable, so the C content is set at 0.01% or more.

Si is limited to 0.50% or less because it improves weather resistance and deoxidizes in steel, but when 0.50% or more is added, it acts as a non-metallic inclusion in steel to impair cleanliness.

Mn is an effective element that deoxidizes in steel and improves weldability, hot workability and strength, but when it is added more than 2.0%, it forms a non-metallic inclusion such as MnS, which increases during hot rolling and inhibits hot workability and weldability. It was limited to.

When P is present in a large amount of 0.020% or more, the upper limit is limited to 0.020% because segregation at grain boundaries lowers grain cohesion and promotes central segregation to impair weather resistance.

Since S is an element that is very harmful to corrosion resistance, the smaller the content, the more effective it is, but it is impossible to remove it completely. It was limited to%.

Cu is an element that strengthens the matrix structure and suppresses the absolute amount of hydrogen in the steel, thereby improving the hydrogen stress corrosion cracking resistance in the welded portion and the base metal portion. Below 0.10%, the lower limit was 0.10% because of less corrosion resistance improvement effect. When the Cu content exceeded 2.0%, the surface defects were easily generated during hot rolling and the upper limit was set to 2.0% because it deteriorated weldability. Since the addition of Cu is effective only by adding 0.10% or more, the addition range of Cu was made into 0.10-2.0%.

Ni has the effect of suppressing the high temperature cracking of the steel sheet due to the addition of Cu in steel, and it is an element that greatly improves the corrosion resistance and low temperature toughness without significantly increasing the strength and hardness of the welded part and the base material part after hot rolling. The addition of more than 1.0% not only increases the resistance to hydrogen-containing organic corrosion corrosion in the atmosphere of corrosion, but is also expensive, so it is desirable to regulate it to 1.0% or less from an economical point of view.

Ca (OH) ₂ acts as a nucleus to form acicular ferrite tissue in the river in the form of fine inclusions. Also, since it forms basic atmosphere in wet corrosive environment, adding 0.0010% or less is ineffective, and adding 0.0050% or more will cause central segregation in steel, which will damage corrosion resistance and weldability. It was limited.

In addition, when the Ca (OH) ₂ is 2㎛ or more to act as a large non-metallic inclusions in the steel, rather than impairing weatherability and cleanliness, the particle size was limited to 2㎛ or less.

Cr is an element that increases the strength by increasing the deterioration resistance to the post-heat treatment after hot rolling.In addition, up to 1.0%, the weatherability is continuously increased, but at 1.0% or more, the effect is not only slow but also economically 1.0%. Since it is preferable to keep below, it was limited to 0.01-1.0%.

Mo is an element that improves the strength by increasing the deterioration resistance to the post-heating treatment after hot rolling, and suppresses the vacancy which is the starting point of hydrogen organic cracking destruction, but at 1.0% or more, Mo is a hard structure of the matrix structure. It was limited to%.

By adding a small amount of strengthening elements of V, Ti, and Nb, these components are added because the grain size of the matrix structure can be refined without increasing the carbon equivalent, thereby increasing the strength and improving the weather resistance. When added to 0.01% or less, there is no grain refinement effect, and when added to 0.10% or more, the micronization effect is saturated and no further effect is shown. In the present invention, the amounts of V, Ti, and Nb are limited to 0.01-0.10%, respectively.

In the present invention, a steel ingot having the above chemical component range is manufactured to produce steel having excellent weather resistance through heating, hot rolling and post-heat treatment processes.

First, it is heated to a temperature range of 1100-1250 ° C. for hot rolling, and then 6-15 ° C./at a temperature range of 550-350 ° C. immediately after finishing hot rolling in the temperature range of the austenitic structure having an Ar ₃ point or more. After cooling the steel plate at a cooling rate of sec, it is cooled by air or heat-treated to a temperature range of 500-650 ° C. after cooling.

Hereinafter, the reason for limitation on the hot rolling and post-heat treatment conditions of the present invention will be described.

In hot rolling and accelerated cooling of steel ingots within the compositional limits of the present invention, the hot rolling temperature should be heated to 1100 ° C. or higher to sufficiently dissolve the precipitates distributed in the steel to bring about the effect of increasing the strength by increasing the capacity and weather resistance. Since it improves, it heats to 1100 degreeC or more. In addition, when heated to 1250 ℃ or more, the grain coarsening phenomenon occurs and the surface condition is poor, so that the surface defects at the time of hot rolling, which is likely to lower the weather resistance is to be heated to 1250 ℃ or less.

If the hot-rolled test piece is hot-rolled for hot rolling at a temperature of 950 ° C or higher, grain coarsening cannot be expected. Therefore, an increase in strength due to grain refinement cannot be obtained. The hot rolling finish temperature was limited to a temperature range of 900-950 ° C. because it could not bring about the sufficient hardening effect due to the crystallization.

Then, the accelerated cooling is started immediately after finishing the hot rolling, and in the actual industry, the accelerated cooling is started at 15-30 ° C. from the hot rolling finish temperature. In the present invention, the accelerated cooling start temperature is equal to the hot rolling finish temperature. The temperature range is not limited in the present invention because it is determined directly and automatically.

Accelerated cooling rate is the most influential factor for stressed fabrics. When the accelerated cooling rate is 6 ℃ / sec or less, the strength is not sufficiently hardened. When the accelerated cooling rate is 15 ℃ / sec or more, the martensite hardened structure In the present invention, it is limited to 6-15 ° C./sec because it is generated and impairs hydrogen organic stress corrosion cracking.

The end temperature of accelerated cooling should be limited to the extent that complete ferrite structure with little austenite structure can be shown. Above 550 ℃, austenite structure remains, which is likely to be incomplete quenching. Effect is achieved. When the temperature is continuously lowered to 350 ° C or lower, excess water and time are consumed, and therefore, the present invention is limited to 550-350 ° C.

The purpose is to remove the processing thermal stress by heat-treating the specimens which have undergone accelerated cooling and air-cooling after hot rolling.If the heat treatment is performed at 500 ℃ or lower, the processing thermal stress is not completely removed. Since the coarsening phenomenon occurred and the strength decreased, the heat treatment temperature was limited to 500-650 ° C.

Since the heat treatment time does not affect the weather resistance more sensitively than the heat treatment temperature, and the heat treatment time varies depending on the thickness of the steel sheet, the heat treatment time is not limited to the heat treatment time.

The steel produced by the present invention can be used in bridges, building structures, etc., where weather resistance is required in the atmosphere, and high strength and extremely excellent weather resistance do not need to be painted.

Hereinafter, the present invention will be described in more detail with reference to Examples.

Example

In order to verify the limited range for the alloy element, hot rolling and heat treatment, a hot-rolled steel ingot was prepared by vacuum dissolving in a single 30 kg layer by a vacuum induction furnace. Chemical components at this time are as shown in Table 1. The ingot was heated at 1100-1250 ° C. for 2 hours, and then a 120 mm thick ingot was hot rolled under the conditions as shown in Table 2 below to obtain a final thickness of 12 mm to prepare a weather resistant test piece. After finishing zinc in 6 passes, the finishing temperature was 925 ° C and the oil was cooled immediately after rolling. The cumulative reduction rate at 1000 ° C or higher at hot rolling was 68% (at 1020 ° C).

Immediately after hot rolling, the rolling finish temperature, accelerated cooling start temperature, accelerated cooling end temperature, accelerated cooling rate and post-heat treatment temperature were changed, and then weather resistance test was performed. When the temperature was lowered by 20 ° C. from the rolling finish temperature after hot rolling, the point was taken as the starting point of the accelerated cooling start temperature.

The weather resistance test was performed by selecting the salt spray test, one of the weather resistance promotion tests, with the effect of alloying elements, hot rolling, and post-heat treatment. In this case, in accordance with ASTM B 117-64 and JIS Z 3136, the test pieces were corroded at 35 ° C. for 120 hours in 5% NaCl spray to evaluate weather resistance by the amount of rust generated on the surface of the test pieces.

Table 3 shows the results of weather resistance according to the change of manufacturing conditions for hot rolling and post heat treatment. In the example of the present invention, the change of heating temperature (invention examples 1 and 2), the change of rolling finish temperature (invention examples 3 and 4), the change in accelerated cooling start temperature (invention examples 5 and 6) and the change in accelerated cooling end temperature (Invention Examples 7 and 8), the change in the acceleration cooling rate (Invention Examples 9 and 10) and the change in the post-heat treatment temperature (Invention Examples 11 and 12) were all tested, and all showed good weather resistance test results.

Table 3 shows the results of weather resistance according to the change of manufacturing conditions for hot rolling and post heat treatment. In the method of the present invention, a change in heating temperature (method 1-2), a change in rolling finish temperature (method 3-4), a change in accelerated cooling start temperature (method 5-6), and a change in accelerated cooling end temperature (method 7-) 8) Good weather resistance test results were obtained from the change of accelerated cooling rate (method 9-10) and the change of post-heat treatment temperature (method 11-12).

division Chemical composition (% by weight) C Si Mn P S Cu Ni Cr Mo Nb V Ti Ca (OH) ₂ Comparative steel A 0.19 0.35 1.24 0.017 0.003 - - - - - - - - B 0.08 0.28 1.50 0.019 0.004 0.25 0.18 - - - - - - C 0.05 0.32 1.28 0.015 0.003 0.28 0.18 0.24 - 0.01 - - - Invention steel D 0.10 0.29 1.28 0.016 0.004 0.25 0.16 - - - - - 0.0015 E 0.05 0.28 1.82 0.008 0.003 0.22 0.17 0.34 - - - - 0.0010 F 0.05 0.34 1.54 0.012 0.004 0.21 0.14 0.24 - - - 0.015 0.0014 G 0.18 0.32 1.50 0.017 0.004 0.28 0.12 - - - 0.01 0.015 0.0018 H 0.18 0.35 1.25 0.015 0.003 0.25 0.14 0.43 - 0.01 - 0.015 0.0019 I 0.09 0.30 1.23 0.014 0.003 1.02 0.38 0.39 0.24 - - 0.015 0.0010 J 0.09 0.29 1.12 0.019 0.003 1.05 0.42 0.27 0.20 0.01 - - 0.0022 K 0.08 0.28 1.09 0.018 0.002 1.49 0.56 0.29 - 0.01 0.01 0.015 0.0020 L 0.12 0.35 0.75 0.015 0.004 1.50 0.70 0.32 0.28 0.01 - 0.014 0.0035 M 0.02 0.29 0.89 0.014 0.003 1.87 0.78 0.35 0.29 0.01 0.01 - 0.0038 N 0.04 0.33 1.01 0.018 0.003 1.85 0.80 0.50 0.27 0.01 0.01 0.015 0.0040

Rolling start temperature (℃) - 1150 1110 1070 1020 970 925 Thickness after rolling (mm) 120 100 78 58 38 21 12 Cumulative reduction rate (%) 0 16.7 35.0 51.7 88.3 82.5 90.0

division Manufacturing condition change Weatherability Test Results Steel grade Heating temperature (℃) Rolling Finish Temperature (℃) Accelerated cooling start temperature (℃) Accelerated cooling end temperature (℃) Accelerated Cooling Speed (℃ / sec) Post heat treatment temperature (℃) Comparative example One A 1150 925 Air cooling Air cooling One - × 2 A 1150 925 Air cooling Air cooling One 620 × 3 B 1200 925 905 450 20 - △ 4 C 1200 925 905 450 20 - △ Inventive Example One D 1100 925 905 500 10 - ○ 2 D 1250 925 906 500 10 - ○ 3 F 1150 950 908 500 10 - ○ 4 F 1150 925 905 500 10 - ○ 5 H 1150 950 926 500 10 - ○ 6 H 1150 925 890 500 10 570 ○ 7 J 1200 925 905 500 10 620 ○ 8 J 1200 925 905 500 10 640 ○ 9 K 1200 925 905 500 7 620 ○ 10 K 1200 925 905 500 13 620 ○ 11 D, EF, GH, I 1180 925 905 500 10 - ○ 12 J, KL, MN 1180 925 905 500 10 620 ○

Weathering test results; ○: less than 15% rust, △: less than 15-60%

×: Rust generation of 60-100%

In this embodiment, the effect of the present invention was examined by changing the content of the alloying element and the manufacturing conditions for rolling and heat treatment. As shown in the comparative example of Table 3, when the content of the alloying element was excessive or deficient, or When the rolling conditions and post-heat treatment conditions exceeded the manufacturing conditions of the present invention, the weather resistance was poor.

Therefore, the unpainted weathering steel produced by the present invention, containing fine Ca (OH) ₂ powder to form a fine inclusion of 2㎛ or less to create a fine needle-like ferrite structure to improve weather resistance, and improve the internal stress of the steel matrix Hot-rolled to improve weather resistance by removing, and to improve the weather resistance to the best by reducing the band-like pearlite structure and accelerating cooling to form needle-like uniform ferrite structure. It can be used in structures and the like, and its weather resistance is extremely excellent, so there is no need to paint.

Claims (3)

By weight%, C: 0.01-0.15%, Si: 0.50% or less, Mn: 2.0% or less, P: 0.020% or less, S: 0.010% or less, Cu: 0.10-2.0%, Ni: 0.10-1.0%, diameter A Ca (OH) ₂ powder having a thickness of 2 µm or less in a 0.0010-0.0050% basic composition, a steel ingot containing Fe and an unavoidable impurity in the remainder was prepared and heated to a temperature range of 1100-1250 ° C., followed by at least ₃ Ar points. A method for producing an unpainted weathering steel, characterized in that the steel sheet is cooled at a cooling rate of 6-15 ° C./sec in the temperature range from 550 to 350 ° C. immediately after finishing hot rolling in the temperature range of the austenitic structure. The method for producing uncoated weather resistant steel according to claim 1, further comprising at least one of Cr and Mo, respectively 0.01-1.0%, and Ti, Nb, and V, respectively 0.01-0.10%. The method of claim 1 or 2, wherein the post-heat treatment is performed in a temperature range of 500-650 DEG C after accelerated cooling to remove thermal stress in the steel.