KR100401167B1 - Bainite-based high strength steel with excellent weld toughness and manufacturing method - Google Patents

Abstract

The present invention relates to a high-strength steel and a manufacturing method excellent in the toughness of the welded part, the composition of the steel by weight% C: 0.01% to 0.04%, Mn: 1.2% to 2.0%, Si: 0.1% to 0.3%, Al: 0.02% 0.06%, Ti: 0.01% to 0.03%, and Nb: 0.04% to 0.06%. V: 0.05 to 0.07%, Mo: 0.15 to 0.25%, sol_B: 0.001 to 0.002, S: 0.002% or less, P: 0.02% or less, N: 10 to 100 ppm, O: 0.01% to 0.05%, Ca: 10 to Improved productivity by heating slab of steel composed of 100ppm, remaining Fe and other unavoidable impurity elements in the temperature range of 1150 ~ 1250 ℃, finishing rolling at the temperature of 100 ℃ ~ 200 ℃ higher than transformation temperature, and then cooling by accelerated cooling method. The present invention relates to a method for producing a tensile strength 70 kgf / mm ² grade steel having particularly good low-temperature toughness of welded portions.

Description

Bainite-based high strength steel with excellent weld toughness and manufacturing method

The present invention relates to a bainite-based high strength steel having excellent weld toughness and a method of manufacturing the same, and more particularly, to fabricating a thick steel plate by high temperature rolling and rapid cooling, while maintaining a tensile strength of 70 kgf / mm ² or more at -27 ° C. The low temperature toughness of the welded portion of 350J / cm or more, as well as to improve the productivity of the bainite-based high-strength steel and a method for manufacturing the same.

Conventionally, high-strength structural steels requiring a tensile strength of 60kgf / mm ² or more alloy contain a large amount of C, Mn, and trace elements to increase the strength of the steel or to improve weldability during welding, which is a subsequent process in the processing of the steel. The method of compensating the content of the element as much as possible and thus reducing the strength through the control of the microstructure through rapid cooling has been mainly used.

On the other hand, in the case of ultra-low carbon type bainite alloy, a method of improving the strength and weldability by making the base structure into bainite through rapid cooling after rolling has been recently known (Korean Patent Application No. 1997-73716, No. 1997-62741). However, this method shows a level of tensile strength of about 60kgf / mm ² , and particularly has a disadvantage in that the weld part toughness is weak by containing a large amount of vanadium.

The present invention has been made to solve the above problems, to improve the toughness of the weld by excluding vanadium of the conventionally known alloys of ultra-low bainite steel, instead of maximizing the effect of increasing the hardenability using boron to increase the strength by, in to provide a weld zone toughness of high tensile strength of 70kgf / mm ² class bainite-based high strength steel and a method of manufacturing it is an object.

1 is a graph showing the yield strength and tensile strength of the present invention steel and comparative steel.

Figure 2 is a graph showing the heat affected zone toughness test results of the weld simulation specimens of the present invention steel and comparative steel.

According to the present invention, in order to achieve the above object, the bainite-based high strength steel having excellent weld toughness is 0.01% to 0.04% by weight, Mn: 1.2% to 2.0%, Si: 0.1% to 0.2%, and Al. : 0.02% to 0.06%, Ti: 0.01 to 0.03%, Nb: 0.04 to 0.06%. Mo: 0.15 to 0.25%, sol_B (Soluble Boron): 0.001 to 0.002%, S: 0.002% or less, P: 0.02% or less, N: 10 to 100 ppm, O: 0.01% to 0.05%, Ca: 10 to 100 ppm, It is characterized by consisting of the remaining Fe and other unavoidable impurities.

In addition, according to another embodiment of the present invention, a method for manufacturing bainite-based high strength steel having excellent weld toughness includes heating a slab of steel having the composition in a temperature range of 1150 ° C to 1250 ° C; After finishing rolling at a temperature of 100 ° C. to 200 ° C. higher than the transformation temperature Ar ₃ , rapid cooling is performed to obtain a dense bainite structure.

Hereinafter, embodiments of the present invention will be described in more detail.

According to the present invention, the bainite-based structural steel having excellent weld toughness and tensile strength of 70kgf / mm ² has a weight% of C: 0.01% to 0.04%, Mn: 1.2% to 2.0%, Si: 0.1% to 0.3%, Al: 0.02% to 0.06%, Ti: 0.01 to 0.03%, Nb: 0.04 to 0.06%. Mo: 0.15 to 0.25%, sol_B (Soluble Boron): 0.001 to 0.002%, S: 0.002% or less, P: 0.02% or less, N: 10 to 100 ppm, O: 0.01% to 0.05%, Ca: 10 to 100 ppm, The rest is made of Fe and other unavoidable impurities.

In addition, according to another embodiment of the present invention, the manufacturing method of the bainite-based high-strength steel with excellent weld toughness is heated to a slab of the steel is formed as described above in the temperature range of 1150 ~ 1250 ℃, 100 than the transformation temperature (Ar ₃ ) Rolling is complete | finished at the temperature of C-200 degreeC high, and it cools by the rapid cooling method.

Hereinafter, the reason for component limitation of the bainite-based high strength steel according to the embodiment of the present invention will be described.

If the content of C is small in the alloy component of the steel, the fraction of the second phase structure is lowered and the strength is lowered. When the C content is high, the strength of the steel is increased, but the impact toughness, especially low temperature toughness, is impaired and the weldability at the time of welding is lowered. Therefore, according to the present invention, the content of C is extremely low at less than 0.04% in order to make the base structure after rolling into bainite and suppress the formation of the second phase, but prevents the self strength of the bainike matrix from decreasing. In order to maintain the minimum content of C to 0.01% or more. Therefore, the content of C is limited to 0.01% to 0.04% of range.

Si is an element added as a deoxidizer during steelmaking, and is added in an amount of 0.1% or more to improve the solid solution strengthening effect and impact transition temperature, but when it is added in excess of 0.3%, an oxide film is severely formed on the surface of the steel sheet. Content is limited to 0.1 to 0.3% of range.

Since Mn forms MnS, a non-metallic inclusion drawn together with S, which lowers the normal temperature elongation and low temperature toughness, it is preferable to manage it at 2.0% or less. However, when Mn is less than 1.2%, bainite is formed during air cooling by reducing the hardenability. Since it is difficult to do it and it is difficult to secure strength, the content is limited to 1.2 to 2.0%.

Al is added as a deoxidizer during steelmaking, but when it is added in excess of 0.06%, Al ₂ O ₃ , a nonmetal oxide, is formed to lower impact toughness, and when it is less than 0.02%, deoxidation effect cannot be expected during steelmaking. Is limited to the range of 0.02% to 0.06%.

Ti is a major element that forms a TiN precipitate during the solidification process of steel and suppresses grain growth during heating of the ingot, and inhibits the growth of recrystallized grains during hot rolling, thereby playing a major role in grain refinement of steel. According to the present invention, the appropriate amount of Ti changes depending on the content of N, and the amount of TiN formed when the content of Ti is relatively small compared to the content of nitrogen (N), that is, the content of Ti is less than 0.01%. This amount is disadvantageous for miniaturizing the grains, whereas when excessively added in excess of 0.03%, TiN becomes coarse during heating, thereby reducing the effect of inhibiting grain growth. Therefore, content of Ti is limited to 0.01 to 0.03%.

Nb is an important element that is dissolved in austenite to increase the hardenability of austenite and precipitates as carbonitride [Nb (C, N)] matching with matrix (Matrix) to increase the strength of steel. According to the present invention, when producing a steel having excellent strength and low temperature toughness by forming bainite matrix, it is important to contain a large amount of Nb in order to enhance the hardenability, even if Nb is added in excess of 0.06% It exists in the solid solution state in a ferrite without augmenting an effect, and there exists a danger of reducing impact toughness, and can also reduce weldability. And, when Nb is added less than 0.04%, the effect of improving hardenability is reduced. Therefore, content of Nb is restrict | limited to 0.04%-0.06%.

B is known not only to increase the hardenability when it is employed in the matrix structure (sol_B: boron solid solution) but also to segregate at the grain boundary to enhance the binding force of the grain boundary. This is because sol_B makes a great contribution to improving low temperature toughness in steels with toughness fracture properties where fracture propagates along grain boundaries. That is, it is necessary to contain sol_B at 0.001% or more. However, when the content of sol_B exceeds 0.002%, the grain boundary becomes rather brittle. Therefore, the content of sol_B is limited to the range of 0.001% to 0.002%. The content of boron for maximizing the hardenability does not mean the amount of boron contained in the entire steel but the content of sol_B dissolved in the matrix structure. .

On the other hand, the content of boron solid solution (sol_B) is determined by the content of Ti and N, as shown in Equation 1 below. That is, since the content of Ti and N in the steelmaking process can be controlled by the existing technology, the amount of sol_B can be manufactured according to the composition range according to the following formula (1).

Mo is an essential element to make the matrix structure into bainite structure by enhancing the hardenability and suppressing ferrite transformation, but when it contains more than 0.25%, Mo-C precipitates are formed to embrittle grain boundaries, and the cooling ability by C is also lowered. If less than 0.15% is contained, it is difficult to secure hardenability. Therefore, the content of Mo is limited to the range of 0.15 to 0.25%.

P is an element that is particularly bad for impact toughness. The lower the content, the better. However, P is inevitable in steelmaking, so the content is limited to 0.02% or less so as not to adversely affect the physical properties.

S is present as a non-metallic inclusion of MnS and is elongated by hot rolling to promote anisotropy of steel sheet properties and to lower impact toughness. Therefore, the content is limited to 0.002% or less.

As other alloy components in addition to the alloy components, N, O and Ca are components commonly contained in the case of general structural steels, and their contents are in a conventional range, that is, N: 10 to 100 ppm, O: 0.01% to 0.05%, and Ca: Limited to 10 to 100 ppm.

Hereinafter, hot rolling conditions and cooling conditions of the bainite-based high strength steel according to the present invention will be described in more detail.

According to the present invention, after heating the slab of the steel composition as described above in the temperature range of 1150 ℃ 1250 ℃, hot rolling in the recrystallization region, hot rolling is transformed temperature (Ar ₃ ) + 100 ℃ ~ 200 ℃ The rolled sheet finished in the range is quenched at a cooling rate of 7 to 15 ° C / sec.

Before hot-rolling the slab of the low alloy steel is formed as described above is heated in the temperature range of 1150 ~ 1250 ℃, the reason is as follows.

It is important to make Nb exist in a dissolved state in austenite, and to refine the ferrite with the drop in transformation temperature caused by the increase in the hardenability by Nb, thereby improving strength and low temperature toughness. That is, in the slab state, Nb is combined with C to exist as carbide NbC. Therefore, when the slab is heated to 1150 ° C or higher before hot rolling, NbC must be dissolved and Nb exists in an atomic state, provided that the heating temperature is 1250 ° C. If exceeded, the austenite particles become too coarse and some delta ferrite (δ-ferrite) is formed in the steel, which degrades the properties of the steel sheet. Therefore, slab heating temperature is limited to the range of 1150 ~ 1250 ℃.

The ingot heated to the heating temperature is hot rolled in the recrystallization zone. The rolling finish temperature (T5) is closely related to low temperature toughness in addition to strength, and thus is an index to be particularly strictly managed. For example, in the case of the conventional rapid cooling steel, the rolling was finished at about ± 30 ° C based on the transformation temperature (Ar ₃ ) represented by the following formula (2). If the rolling end temperature (T5) is too high, the ductility, low temperature toughness, etc. is excellent, but the strength is lowered. If the rolling end temperature is too low, abnormal reverse rolling occurs, the stretched ferrite and perlite are present, and the pearlite band is formed. This is because the ductility and low temperature toughness are greatly reduced.

However, a feature of the present invention is also to provide a method for producing a steel material which improves productivity by about 25% by finishing rolling at a higher temperature, unlike the prior art. This is because it is possible to bainize the microstructure after the air cooling by maintaining the end temperature of the rolling at a temperature of 100 ℃ or more than the transformation temperature (Ar ₃ ) represented by the formula (2). On the other hand, when the rolling end temperature exceeds Ar ₃ + 200 ° C, the austenite becomes coarse and fineness of the particle size is prevented and the strength is lowered.

Therefore, in the present invention, the rolling end temperature is limited to a temperature range of about 100 ° C to 200 ° C higher than the Ar ₃ temperature.

After rolling, a cooling step of rapid cooling using water is performed. At this time, the cooling rate is maintained at 7 ℃ / sec or more to increase the hardenability so that the bainite structure can be formed. On the other hand, when the cooling rate exceeds 15 ° C / sec, martensite is produced and the toughness is lowered. Therefore, the cooling rate is maintained at 7 ~ 15 ℃ / sec.

Hereinafter, the present invention will be described in detail through examples.

(Example)

To produce a slab of alloy steel composition as shown in Table 1, the development steel A is a steel that satisfies the component range of the present invention, the existing steel B is a previously filed technology (Korean Patent Application No. 1997-73716) It is a component manufactured by.

As alloy components other than the following alloy components, Al, S, P, N, O and Ca are components commonly contained in the case of general structural steels, and their contents are in a conventional range, that is, Al: 0.02 to 0.06%, S: 0.002 % Or less, P: 0.02% or less, N: 10 to 100 ppm, O: 0.01% to 0.05%, and Ca: 10 to 100 ppm.

TABLE 1

The difference in composition between the inventive steel and the comparative steel is as follows.

That is, the invention steel does not contain V in comparison with the comparative steel in order to improve welded toughness. Instead, it increases the hardenability by increasing the amount of sol_B dissolved in the base to 10 ppm or more, thereby promoting bainite transformation.

The two steel grade slabs of [Table 1] were divided into two parts, and the rolling finish temperature was controlled by 950 ° C., 850 ° C., etc., and the rolled plate was rapidly cooled, and rolling conditions and cooling conditions are shown in the following [Table 2]. Shown in

TABLE 2

Tensile test results for the four kinds of rolled plates (A and B: component steel types and 950 ° C. and 850 ° C .: rolling end temperature) shown in [Table 2] are shown in FIG. 1.

Yield strength 52kgf / mm for existing steels, almost independent of rolling end temperature²Tensile strength 60kgf / mm²Intensity of 10kgf / mm in the case of invention steel²Yield strength 60kgf / mm above², Tensile Strength 70kgf / mm²It is understood that it is degree. And 70kgf / mm tensile strength²High-strength steel of the degree is a high-strength steel that can not be implemented in the existing non-heat treatment steel, the present invention has a peculiar effect of eliminating heat treatment and controlling the base structure to bainite to obtain high strength.

In addition, in the conventional ferritic + perlite steel, the strength decreases as the rolling end temperature increases, but in order to maintain the strength, the rolling end temperature has to be lowered to 800 ° C. or lower, but the bainite structured steel such as the present invention steel Since the strength is hardly influenced by the end temperature of rolling, it is very advantageous in terms of production cost since it has the advantage of finishing rolling at a high temperature.

Physical properties required for structural thick plates are strength and low temperature toughness. In particular, the thick plate is always welded in a subsequent process, mainly because the fracture of the structure occurs in the weld, the low temperature toughness of the weld portion of the thick plate is a major comparison item in evaluating the properties of the steel. There are two methods for evaluating the weld properties: a method of evaluating the low temperature toughness by collecting the heat affected zone after the actual welding and a comparison of the low temperature toughness of the heat affected zone of the test specimen which reproduces the welding heat cycle experimentally.

In the present example, the low temperature toughness of the weld heat affected zone when the weld heat was welded at a heat input amount of 42.5 kJ / cm was tested assuming a late arc arc welding method, which is mainly used for welding thick plates. In FIG. 2, the test value for the impact resistance of the inventive steel and the existing steel is shown with respect to the test temperature. Existing steels have a value of 300J / cm at room temperature, but the impact value rapidly decreases below 0 ℃, showing a value of about 50J / cm, while the inventive steel shows a high impact value of 350J / cm even at -27 ℃. Can be.

As described above, according to the present invention, by adjusting a small amount of alloying elements and using a high temperature rolling and rapid cooling method, it is possible to produce a steel that satisfies the tensile strength of 70kgf / mm ² grade, and excellent weld temperature low-temperature toughness of about 350 J / cm Has the effect.

Claims (3)

% By weight C: 0.01% to 0.04%, Mn: 1.2% to 2.0%, Si: 0.1% to 0.2%, Al: 0.02% to 0.06%, Ti: 0.01% to 0.03%, Nb: 0.04% to 0.06% . Mo: 0.15 to 0.25%, sol_B (Soluble Boron): 0.001% to 0.002%, S: 0.002% or less, P: 0.02% or less, N: 10 to 100 ppm, O: 0.01% to 0.05%, Ca: 10 to 100 ppm , Bainite-based high strength steel excellent in weldability, characterized in that the remaining Fe and other unavoidable impurities. % By weight C: 0.01% to 0.04%, Mn: 1.2% to 2.0%, Si: 0.1% to 0.2%, Al: 0.02% to 0.06%, Ti: 0.01% to 0.03%, Nb: 0.04% to 0.06% . Mo: 0.15 to 0.25%, sol_B (Soluble Boron): 0.001% to 0.002%, S: 0.002% or less, P: 0.02% or less, N: 10 to 100 ppm, O: 0.01% to 0.05%, Ca: 10 to 100 ppm In the manufacturing method of the bainite-based high-strength steel having excellent weld toughness made of the remaining Fe and other unavoidable impurities, Heating the slab of the composition in a temperature range of 1150 ° C to 1250 ° C; After the end of the rolling at a temperature higher than the transformation temperature (Ar ₃ ) 100 ℃ ~ 200 ℃ consists of a rapid cooling step, The transformation temperature (Ar ₃ ) is the following formula, Method for producing a bainite-based high strength steel excellent in weldability toughness, characterized in that represented by. The method of claim 2, The method of manufacturing bainite-based high strength steel having excellent weld toughness, characterized in that the cooling rate in the rapid cooling range 7 ℃ / sec ~ 15 ℃ / sec.