KR101095911B1 - Weldable ultra-high strength steel with excellent low-temperature toughness - Google Patents

Abstract

The present invention provides a weldable super high strength steel excellent in low temperature toughness manufactured by adding copper (Cu) and boron (B), which are environmentally-friendly low cost elements, to a low carbon-low alloy steel. The ultra high strength steel is% by weight, C: 0.05-0.1%, Si: 0.1-0.5%, Mn: 1.5-2.5%, Ni: 0.2-0.5%, Cu: 0.5-2.0%, Cr: 0.1-0.5% , Mo: 0.1-0.5%, Nb: 0.01-0.05%, V: 0.01-0.1%, Ti: 0.01-0.03%, Al: 0.05% or less, B: 0.0005-0.002%, the remainder is Fe and other unavoidable impurities Ceq (carbon equivalent) is 0.3 to 0.6, Pcm (welding crack susceptibility index) is 0.3 or less, characterized in that the volume fraction of the mixed structure of tempered martensite and tempered bainite is 90% or more. .

Description

Weldable ultra-high strength steel with excellent low-temperature toughness

The present invention relates to weldable ultra-high strength having excellent low-temperature toughness, and more particularly, to low-carbon-low alloy steels, which are environmentally friendly low-cost elements such as copper (Cu) and boron ( It is related to ultra high strength steel with the yield strength of 1.1GPa or more and low temperature impact energy of 200J or more by adding B).

At present, structural steels used in construction, shipbuilding, offshore structures, pressure vessels, steel pipes, or industrial machinery are becoming extremely high strength with excellent weldability and toughness in terms of economics and stability. In addition, as the usage environment of shipbuilding, offshore structural and steel pipe steels becomes more severe, the demand for excellent low temperature toughness is increasing to secure structural stability even at low temperatures below -20 ^° C.

Nickel-containing steels, which are widely used as structural steels at low temperatures (usually about 3.0% by weight Ni or more), have good low temperature toughness, but due to their relatively low tensile strength they can be subjected to double annealing heat treatments to withstand large loads or There is a disadvantage in that the cost is greatly increased due to the increase in the nickel content or increase in thickness.

Meanwhile, HY-80 and HY-100 steels, which are well known as representative high-strength high toughness steels, have high strength, but have a very low weldability due to the carbon content of 0.12-0.20% by weight and Ceq range of 0.8 to 0.9. Since then, high-strength low-alloy (HSLA) steels have been developed in various ways, such as reduction of carbon content, grain refinement by addition of Nb, increase of hardenability by addition of Mn, Ni, Cr, Mo, and copper precipitates. Through various methods such as precipitation strengthening, the weldability was not deteriorated and the strength was greatly improved.

However, since most of HSLA steels have low low temperature toughness, many expensive alloying elements such as Ni and Mo are added to obtain high strength with excellent low temperature toughness. There is a problem. As shown in US Pat. No. 6,264,760, US Pat. No. 5,876,521 and US Pat. No. 6,159,312, various microhistological approaches, such as lower bainite, tempered martensite, biphasic tissue, etc. Although HSLA steel, that is, super high strength steel for steel pipes, has been developed, there is still a large amount of expensive alloying elements added, and the maximum yield strength is only about 830 MPa (corresponding to API standard 120).

In addition, with the recent environmental problems such as resource recycling and global warming, the economic aspect of raw material costs and high oil prices has been highlighted, and engineering demands for manufacturing ultra-high strength steel in a more eco-friendly and economic way are increasing. Therefore, it is necessary to manufacture 1.1GPa or more ultra-high strength steel having excellent weldability and low temperature toughness in a more eco-friendly and economical way by minimizing expensive alloying elements by using eco-friendly low-cost elements such as copper or boron.

Therefore, the technical problem to be achieved by the present invention is to provide a weldable ultra-high strength steel having excellent low temperature toughness at low cost by adding copper and boron, which are eco-friendly low cost elements, in order to minimize expensive alloying elements.

Ultra high strength steel of the present invention for achieving the above object by weight, C: 0.05-0.1%, Si: 0.1-0.5%, Mn: 1.5-2.5%, Ni: 0.2-0.5%, Cu: 0.5-2.0 %, Cr: 0.1-0.5%, Mo: 0.1-0.5%, Nb: 0.01-0.05%, V: 0.01-0.1%, Ti: 0.01-0.03%, Al: 0.05% or less, B: 0.0005-0.002% and , The remainder consists of Fe and other unavoidable impurities, Ceq (carbon equivalent) of 0.3 to 0.6, Pcm (welding crack susceptibility index) of 0.3 or less, volume fraction of mixed tissue of tempered martensite and tempered bainite This is over 90%.

In addition, the ultra-high strength steel has an impact energy of 200J or more at room temperature and -20 ° C or more, yield strength of 1.1GPa or more, and in weight%, ECO alloy index (Mn% + 2Ni% + 0.5Cu% + 4Mo%) Is 4.5 or less.

According to the weldable ultra high strength steel having excellent low temperature toughness of the present invention, by adding 0.5-2.0% by weight of copper and an extremely small amount of boron to prepare an ultra high strength steel composed of a mixed structure of tempered martensite and tempered bainite, Ni As an economical method of minimizing expensive alloying elements such as Mo and Mo, it is possible to secure very high yield strength of 1.1GPa (above API standard 150) and high impact energy of 200J at -20 ^° C.

1 is a manufacturing process diagram schematically showing a method for manufacturing a weldable ultra high strength steel having excellent low temperature toughness according to the present invention.
Figure 2 is a graph showing the yield strength and low temperature impact energy of the weldable ultra high strength steel according to the present invention according to the tempering temperature.
3 is a transmission electron micrograph showing the microstructure of steel 2 tempered at 450 ^° C according to the present invention.

Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings. The embodiments described below may be modified in various other forms, and the scope of the present invention is not limited to the embodiments described below. Embodiments of the present invention are provided to more fully explain the present invention to those skilled in the art.

In the following embodiment of the present invention will be described by dividing the weldability ultra-high strength steel excellent in low temperature toughness and the method of manufacturing the same. The steel and its manufacturing method have the following main features.

The weldable ultra high strength steel having excellent low temperature toughness according to the present invention contains 0.5% to 2.0% of copper and a trace amount of boron, and other C, Si, Mn, Ni, Cr, Mo, Nb, V, Ti, And Fe including some or all of Al. At this time, the steel has a yield strength of 1.1 GPa (more than API standard 150) or more, and the absorbed energy measured by Charpy V-notch impact test at room temperature and -20 ^° C. is characterized in that 200J or more.

Copper is very difficult to remove because it is inevitably contained in the scrap during the operation of the electric furnace, but when it is useful to improve the mechanical properties, it is an excellent recyclability element. Boron is used in the manufacture of ultra high strength steel to replace expensive hardenable elements such as Ni, Mo, etc., because it greatly improves the hardenability of the steel even with a very small amount of 0.0005-0.002% by weight. Therefore, in the present invention, the ECO alloy index (in weight%, Mn% + 2Ni% + 0.5 Cu% +) expressed as Mn content weighted on the price of Mn, Ni, Cu, and Mo, which are expensive alloying elements, based on carbon steel When evaluating eco-friendly and economical performance using 4 Mo%), the ECO alloy index is 4.5 or less, and the values of Ceq (carbon equivalent) and Pcm (welding crack susceptibility index), which are widely used for industrially expressing weldability, We tried to manufacture ultra high strength steel with excellent weldability and low temperature toughness in an eco-friendly and low cost way.

The ultra high strength steel according to the present invention is prepared by reheating a slab containing 0.5-2.0% by weight of copper and trace amounts of boron, and then by controlled rolling, accelerated cooling, and tempering, thereby producing tempered martensite and bainite. Weldable ultra high strength steel with excellent low temperature toughness composed of 90% or more of the volume fraction of the mixed structure can be obtained.

<Lecture composition>

Steel of the present invention has the following composition, and the reason for numerical limitation according to each composition will be described together. In this case,% represents weight%, and the ECO alloy index, Ceq (carbon equivalent) and Pcm (welding crack sensitivity index) are defined as follows.

ECO Alloy Index = Mn% + 2Ni% + 0.5Cu% + 4Mo%

Ceq = C% + Mn% / 6 + (Cr% + Mo% + V%) / 5 + (Ni% + Cu%) / 15

Pcm = C% + Si% / 30 + (Mn% + Cu% + Cr%) / 20 + Ni% / 60 + Mo% / 15 + V% / 10 + 5B%.

The ECO alloy index represents Mn, Ni, Cu and Mo contents, which are expensive alloying elements, as Mn contents weighted at a price. The lower the alloy index, the more environmentally friendly and economic it is. Ceq and Pcm are widely used in the industry as indicators of weldability, and the lower these values, the better the weldability.

(1) Carbon (C): 0.05-0.1%

If the content of C is less than 0.05%, it is difficult to secure a yield strength of 900MPa or more as a mixed structure of tempered martensite and bainite, and if it is more than 0.1%, the toughness and weldability deteriorate.

(2) Silicon (Si): 0.01-0.5%

It is added for deoxidation and strength improvement, and when it is less than 0.01%, the deoxidation effect is insufficient, and when it is added more than 0.50%, toughness and weldability are deteriorated.

(3) Manganese (Mn): 1.5-2.5%

The low C content compensates for the reduced hardenability to promote the formation of martensite and bainite-based tissues. 1.5% or more is added to obtain high strength, and is limited to 2.5% or less in order to prevent degradation and segregation of toughness and weldability.

(4) Nickel (Ni): 0.2-0.5%

Although it is an effective element for improving strength and toughness, a large amount is added, so a small amount is added in the above range to reduce the harmful effect of copper on surface cracking during hot rolling.

(5) Copper (Cu): 0.5-2.0%

As an alloying element having an important characteristic of the present invention, 0.5% or more is added to improve strength and toughness, but when it is added a large amount, the surface crack easily occurs during hot rolling, and the weldability is lowered, so the amount of addition is limited to 2.0% or less. do.

(6) Chromium (Cr): 0.1-0.5%

0.1% or more is added to secure sufficient hardenability during quenching, and when it is added a lot, the toughness and weldability are lowered, so it is limited to 0.5% or less.

(7) Molybdenum (Mo): 0.1-0.5%

If it is added as an element to increase the hardenability, such as Cr, the cost is increased and the toughness and weldability are reduced, so it is limited to 0.5% or less. When tempering, carbide particles are formed, contributing to precipitation strengthening.

(8) Niobium (Nb): 0.01-0.05%

Precipitates as carbonitride during hot rolling to inhibit recrystallization, inhibit grain growth and refine austenite grains, thereby improving both strength and toughness. In 0.01% or less, the effect is very small, and when added more than 0.05%, toughness falls. It also increases the strength by forming precipitates upon cooling or tempering.

(9) Vanadium (V): 0.01-0.1%

At tempering or cooling after welding, carbides are formed, contributing to the increase in strength. If it is less than 0.01%, the effect is small, and if it is more than 0.10%, the toughness and weldability deteriorate.

(10) Titanium (Ti): 0.01-0.03%

When 0.01% or more is added, it forms a precipitate and contributes to the strength improvement, but when it is more than 0.03%, the precipitate coarsens and the toughness decreases.

(11) Aluminum (Al): 0.05% or less

When added as a deoxidizer like Si, when added more than 0.05% _to form a non-metal oxide Al ₂ O ₃ to reduce the toughness of the base material and the weld.

(12) Boron (B): 0.0005-0.002%

As an alloying element to have a significant feature of the present invention with the addition of a very small amount of copper is 0.0005-0.002%, even give a hardenability elements such as Ni, Cr, Mo, because it improves the hardenability Steel effectively 20 ^o C / sec or faster With accelerated cooling, more than 90% of martensite and bainite mixtures can be obtained. However, if more than 0.002% is added, the hardenability is rather reduced due to the formation of fragile particles such as Fe ₂₃ (C, B) ₆ .

(13) It is desirable to minimize the inevitable addition of phosphorus (P), sulfur (S), nitrogen (N) and the like.

In the weldable ultra high strength steel having excellent low temperature toughness according to the present invention, the ECO alloy index is economically less than 4.5, and considering the securing of strength and weldability simultaneously, Ceq is 0.3-0.6 and Pcm is 0.3 or less. desirable.

<Manufacturing Method>

1 is a manufacturing process diagram schematically showing a method for manufacturing a weldable ultra high strength steel having excellent low temperature toughness according to the present invention.

Referring to Figure 1, the manufacturing method of the weldable ultra-high strength steel excellent in low temperature toughness, in weight%, C: 0.05-0.1%, Si: 0.1-0.5%, Mn: 1.5-2.5%, Ni: 0.2-0.5%, Cu: 0.5-2.0%, Cr: 0.1-0.5%, Mo: 0.1-0.5%, Nb: 0.01-0.05%, V: 0.01-0.1%, Ti: 0.01-0.03%, Al: 0.05% or less, B: 0.0005-0.002% and the rest of the steel slab consisting of Fe and other unavoidable impurities is usually heated to at least 1,000 ^o C. This corresponds to the reheating step (S100). Thereafter, hot rolling is carried out at and below the temperature at which the austenite is recrystallized. This corresponds to the control rolling step (S200). The hot rolled steel is quenched to 300 ^o C or lower at a rate of 20 ^o C / sec or higher. This corresponds to the accelerated cooling step (S300). The cooled steel is then tempered for at least 10 minutes at temperatures between 400 ^° C. and 600 ^° C. This corresponds to the tempering step (S400).

In the reheating step, all carbides or carbonitrides such as (V, Nb) (C, N) in the slab of the steel are completely dissolved.

In the control rolling step, the slab of the reheated steel is hot rolled by applying a reduction of 40% or more at temperatures above and below the temperature at which austenite is recrystallized. Because austenite grains are refined before accelerated cooling, and defects such as dislocations and strain bands are formed inside the austenite to promote the transformation of austenite to ferrite, thereby reducing the crystallographic size of the final microstructure, thereby improving strength and toughness. To improve them all. All hot rolling is finished at temperatures above Ar3, with the austenite grain size being less than 10 µm.

The accelerated cooling step is the process of quenching the hot rolled steel to 300 ^o C or less at a rate of 20 ^o C / sec or more at a temperature of Ar 3 or higher. Martensite is mainly formed from austenite during this process, and the lower the quenching finishing temperature, the higher the volume fraction of martensite. Thereafter, bainite is additionally formed during the air cooling of the quenched steel to room temperature.

The tempering step is a process in which the accelerated cooled steel is heated at a temperature of 400-600 ^° C. for at least 10 minutes and then cooled. It consists of a mixed structure of martensite and bainite tempered during the process, providing a substantially uniform microstructure and properties compared to the previous step.

The microstructure of the steel produced according to the above process is a mixed structure of tempered martensite and bainite, the volume fraction of which is more than 90%.

According to the present invention, it is possible to minimize the addition of expensive alloying elements and to produce weldable super high strength steel having excellent low temperature toughness by mixing the tempered martensite and bainite containing 0.5-2.0 wt% of copper and trace amounts of boron. Is possible.

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

<Examples>

Table 1 shows the chemical composition, the ECO alloy index, the cooling rate and the cooling end temperature of the tempering material to which the embodiment of the present invention is applied and the non-tempering material compared with the tempering material. Here, the non-tempering material refers to steel that has not undergone a tempering process.

According to Table 1, steels 1 to 3 are 100 mm thick steels as described, and reheated at 1,150 ^o C for about 2 hours, and then 50-60% of the austenite is recrystallized at or below the temperature. Rolling was applied and hot rolled to a thickness of 15 mm. Subsequently, the hot-rolled sheet is cooled to a rate of 20 ^o C / sec or more at a temperature of Ar ₃ or higher to 300 ^o C or lower to prepare a non-tempered material, or the non-tempered material of 450 ^o C, 550 ^o C, 650 ^o C Tempering was performed for 30 minutes at temperature, respectively, to prepare three types of tempering materials.

On the other hand, yield strength, tensile strength, elongation, and absorption energy of standard Charpy V-notched impact specimens at room temperature and -20 ^° C. were measured as shown in Table 1, and the results are shown in Table 2. Table 2 compares the mechanical properties of the tempering material and the non-tempering material of Table 1.

(* Is carried out at -20 ^o C)

Figure 2 is a graph showing the yield strength and low-temperature impact energy of the weldable ultra high strength steel for the steels shown in Table 1 and Table 2 according to the tempering temperature.

As can be seen in Table 2 and FIG. 2, the non-tempered steels 1 to 3 (un-tempered materials) have a yield strength of 1.1 GPa or less, impact energy at room temperature and low temperature of 100 J or less, and both yield strength and low temperature toughness are poor. It was. Among the tempered steels (tempering materials) conforming to the composition range of the present invention, the steels tempered at 450 ^° C. and the steels 3 tempered at 550 ^° C. have an impact energy of at least 200J at room temperature and low temperature, and a yield strength of 1.1GPa. The ultra high strength excellent in low-temperature toughness is shown above. However, the steel 3 tempered at 450 ^o C and 650 ^o C has a yield strength of 1.1 GPa or more, but the impact energy at room temperature and low temperature is 200 J or less, and is tempered at 550 ^o C and 650 ^o C. The steel 2 has a shock energy of more than 200J at room temperature and low temperature, but its yield strength is less than 1.1GPa.

In addition, although the copper content is outside the composition range of the present invention at 0.5 wt% or less, the tempered steel 1 at 450-650 ^° C. has a high impact energy of 200J or more at room temperature and low temperature, but the yield strength does not reach 1.1 GPa. It was.

Therefore, according to the present invention, in order to secure excellent mechanical properties of yield strength of 1.1 GPa or more and impact energy of room temperature and low temperature of 200J or more, the content of Cu is 0.5-2.0% by weight, and between 400 ^o C and 600 ^o C It is necessary to prepare at an appropriate tempering temperature to form a mixed structure of tempered martensite and bainite.

3 is a transmission electron micrograph showing the microstructure of steel 2 tempered at 450 ^° C by an embodiment of the present invention. As shown in the picture, the tempered martensite and bainite with lower dislocation densities can be seen. In particular, according to the embodiment of the present invention it was confirmed that the volume fraction of the mixed structure of the tempered martensite and bainite is 90% or more.

While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it is to be understood that the invention is not limited to the disclosed exemplary embodiments, but many variations and modifications may be made without departing from the scope of the present invention. It is possible.

Claims (3)

By weight%, C: 0.05-0.1%, Si: 0.1-0.5%, Mn: 1.5-2.5%, Ni: 0.2-0.5%, Cu: 0.5-2.0%, Cr: 0.1-0.5%, Mo: 0.1- 0.5%, Nb: 0.01-0.05%, V: 0.01-0.1%, Ti: 0.01-0.03%, Al: 0.05% or less, B: 0.0005-0.002%, the remainder is composed of Fe and other unavoidable impurities, Ceq Weldability with excellent low temperature toughness, characterized in that (carbon equivalent) is 0.3 to 0.6, Pcm (welding crack susceptibility index) is 0.3 or less, and the volume fraction of the mixed structure of tempered martensite and tempered bainite is 90% or more. Ultra high strength steel. The weldable ultra high strength steel of claim 1, wherein the ultra high strength steel has an impact energy of 200 J or more at room temperature and -20 ° C and a yield strength of 1.1 GPa or more. The ultra-high strength steel of claim 1, wherein the ultra high strength steel has, by weight%, an ECO alloy index (Mn% + 2Ni% + 0.5Cu% + 4Mo%) of 4.5 or less.

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