Patent Description:
A high-strength thick plate steel for low temperatures is comprised of a mixture structure of tempered martensite structure, retained austenite and tempered bainite structure, and since such a steel material should be able to be used as a cryogenic structural material during construction, cryogenic toughness and ductility are required.

On the other hand, as such, the high-strength structural steel for cryogenic use is required for excellent cryogenic toughness and ductility, and high strength hot-rolled steel manufactured through the related art normalizing treatment has a mixed structure of ferrite and pearlite. As an example thereof, the invention described in Patent Document <NUM> may be provided.

The patent document <NUM> discloses that the high-strength steel for 500MPa-class LPG is comprised of, in % by weight, C: <NUM>. <NUM> to <NUM>%, Si: <NUM> to <NUM>%, Mn: <NUM> to <NUM>%, P: <NUM> to <NUM>. <NUM>%, S: <NUM> to <NUM>%, Ti: more than <NUM>% to <NUM>% or less, Mo : <NUM> to <NUM>%, Ni: <NUM> to <NUM>%, a balance of Fe, and other unavoidable impurities, and it is characterized by the addition of Ni and Mo in the steel composition.

However, the invention described in the above publication has a problem that the cryogenic toughness and ductility of the steel material are insufficient even when Ni or the like is added, since the steel material is manufactured through general normalizing.

Therefore, in high-strength thick steel plates used for low temperature pressure vessels, ships, storage tanks, structural steels, and the like, there is a demand for the development of high-strength steel having excellent cryogenic toughness and ductility.

A low temperature pressure vessel steel plate having a tensile strength of <NUM> MPa or more and low temperature toughness is for example disclosed in <CIT>.

Therefore, to prevent the problems of the related art, an aspect of the present invention is to provide a steel plate for a low-temperature pressure vessel, in which a structure of a steel manufactured by controlling a cooling and heat treatment process is provided as a mixed structure of tempered bainite and tempered martensite, and thus a tensile strength of <NUM> MPa class may be secured, a method of manufacturing the same.

According to an aspect of the present invention, a steel plate for a low temperature pressure vessel having excellent cryogenic toughness and ductility comprises, in weight%, <NUM> to <NUM>% of C, <NUM> to <NUM>% of Si, <NUM> to <NUM>% of Mn, <NUM> to <NUM>% of Al, <NUM>% or less of P, <NUM>% or less of S, <NUM> to <NUM>% of Ni, <NUM> to <NUM>% of In, a balance of Fe, and unavoidable impurities. A steel microstructure of the steel plate is comprised of <NUM> to <NUM> area% of tempered bainite and a balance tempered martensite. The steel plate satisfies a tensile strength of <NUM> MPa class and may be stably used at a low temperature of -<NUM>.

The In may be contained in a range of <NUM> to <NUM> weight%.

According to another aspect of the present invention, a method of manufacturing a steel plate for a low temperature pressure vessel having excellent cryogenic toughness and ductility, includes:.

A steel microstructure obtained by the tempering is comprised of <NUM> to <NUM> area% of tempered bainite and a remainder of tempered martensite.

According to an exemplary embodiment of the present invention having the configuration as described above, provided is a steel plate for a low-temperature pressure vessel having excellent toughness and ductility, which may be stably used at a low temperature of about -<NUM> while satisfying the tensile strength of <NUM> MPa class.

First, a steel plate for a low-temperature pressure vessel having excellent low-temperature toughness and ductility according to an aspect of the present invention includes, in % by weight, C: <NUM> to <NUM>%, Si: <NUM> to <NUM>%, Mn: <NUM> to <NUM>%, Al: <NUM> to <NUM>%, P: <NUM>% or less, S: <NUM>% or less, Ni: <NUM> to <NUM>%, In: <NUM> to <NUM>%, balance Fe and unavoidable impurities. Detailed steel composition components and reasons for limiting the components are as follows, and in the following description of the steel composition components, % indicates weight%.

In the present invention, the C content in the steel plate is limited to the range of <NUM> to <NUM>%. If the C content is less than <NUM>%, the strength of the matrix itself is lowered, and if exceeding <NUM>%, the weldability of the steel plate is greatly impaired.

In the present invention, Si is a component added for the deoxidation effect, the solid solution strengthening effect, and the effect of increasing the impact transition temperature, and it is added at <NUM>% or more to obtain such an addition effect. However, if it is added in excess of <NUM>%, the weldability decreases and the oxide film is severely formed on the surface of the steel plate, and thus, the content is limited to <NUM> to <NUM>%.

In the present invention, since Mn forms MnS, which is an elongated non-metallic inclusion, together with S, lowering the room temperature elongation and low temperature toughness, the Mn content is limited to <NUM>% or less. However, if Mn is less than <NUM>% due to the nature of the components in the present invention, it is difficult to secure an appropriate strength, and thus, the amount of Mn is limited to be <NUM> to <NUM>%.

In the present invention, Al, along with Si, is one of the strong deoxidizing agents in the steelmaking process, and the effect thereof is insignificant if the Al content is less than <NUM>%, and if it is added in excess of <NUM>%, manufacturing costs may increase, and thus, the content is limited to <NUM> to <NUM>%.

In the present invention, P is an element that impairs low-temperature toughness, but it requires excessive cost to remove P in the steelmaking process, and thus, the P content is limited to be <NUM>% or less.

In the present invention, S is also an element that adversely affects low-temperature toughness along with P, but since it may take an excessive cost to remove S in the steelmaking process similarly to P, the S content is limited to be <NUM>% or less.

In the present invention, Ni is the most effective element for improving low-temperature toughness. However, if the amount thereof is less than <NUM>%, the low-temperature toughness decreases, and if the amount exceeds <NUM>%, manufacturing costs may increase. Therefore, Ni is added within the range of <NUM> to <NUM>%.

In the present invention, In is a low melting point metal and is an important element that increases ductility. However, if the amount is less than <NUM>%, the effect of the addition cannot be expected, and if it is added in excess of <NUM>%, it may appear as coarse precipitates during the continuous casting process and may impair the low-temperature toughness. Therefore, the In content is limited to <NUM> to <NUM>%.

More preferably, In may be added in the range of <NUM> to <NUM>%.

The steel plate of the present invention has a steel microstructure comprised of <NUM> to <NUM> area% of tempered bainite and the remainder of tempered martensite. If the tempered bainite fraction is less than <NUM>%, the amount of tempered martensite is excessive, and the low-temperature toughness of the steel plate may be deteriorated. On the other hand, if it exceeds <NUM>%, it may be difficult to secure the target strength of the steel plate.

The steel plate having the above-described steel composition component and microstructure has excellent ductility and low temperature toughness, as well as effectively maintaining a tensile strength of <NUM> MPa class.

Next, a method of manufacturing a steel plate for a low temperature pressure vessel having excellent ductility and low temperature toughness according to another aspect of the present invention will be described.

As a steel material for a pressure vessel according to another aspect of the present invention, a steel slab that satisfies the alloy composition proposed in the present invention may be manufactured through the processes of [reheating-hot rolling and cooling-heat treatment and cooling-tempering]. Hereinafter, the respective process conditions will be described in detail.

First, a steel slab satisfying the above-described alloy composition is heated to a temperature ranging from <NUM> to <NUM>. In this case, if the reheating temperature is less than <NUM>, it is difficult to dissolve solute atoms, whereas if the reheating temperature exceeds <NUM>, the austenite grain size becomes too coarse, impairing the properties of the steel, which is not preferable.

Subsequently, in the present invention, the reheated steel slab is hot-rolled to manufacture a hot-rolled steel plate. In this case, the hot rolling is performed at a reduction ratio of <NUM> to <NUM>% per pass.

If the reduction ratio per pass during the hot rolling is less than <NUM>%, there is a problem in that manufacturing costs may increase due to a decrease in rolling productivity. On the other hand, if the reduction ratio per pass during the hot rolling exceeds <NUM>%, it may cause a fatal adverse effect on the equipment by generating a load on the rolling mill, which is not preferable. The finish rolling is performed at a temperature of <NUM> or higher. Rolling to a temperature of less than <NUM> causes a load on the rolling mill, which is not preferable.

A process of primary cooling (water cooling) is performed at a cooling rate of <NUM> to <NUM>/sec within <NUM> seconds after hot rolling. If it exceeds <NUM> seconds before cooling after hot rolling, the temperature of the steel plate is excessively lowered, resulting in low hardenability, such that the required bainite + martensite structure may not be obtained. In addition, if the cooling rate is less than <NUM>/sec, a ferrite structure may be obtained, and to obtain a cooling rate exceeding <NUM>/sec, cooling equipment more than necessary is required, which are not preferable.

The primary cooled hot-rolled steel plate is subjected to heat treatment at a predetermined temperature for a predetermined time. In detail, the heat treatment is maintained for {(<NUM>×t)+(<NUM>-<NUM>)} minutes (where t is the thickness (unit: mm) of the steel plate) at a temperature ranging from <NUM> to <NUM>.

If the temperature during the heat treatment is less than <NUM>, it is difficult to perform re-solid solution of solute elements in solid solution, and thus, it may be difficult to secure the target strength, whereas if the temperature exceeds <NUM>, grain growth occurs, and thus, the low-temperature toughness may be deteriorated.

If the holding time during heat treatment in the above-described temperature range is less than {(<NUM>×t)+<NUM>} minutes, it is difficult to homogenize the structure, whereas if it exceeds {(<NUM>×t)+<NUM>} minutes, productivity is impaired, which is not preferable.

Then, in the present invention, secondary cooling (water cooling) of the heat-treated hot-rolled steel plate is performed to room temperature at a cooling rate of <NUM> to <NUM>/s.

If the cooling rate is less than <NUM>/s during the cooling, there is a concern that coarse ferrite grains may be generated, and if the cooling rate exceeds <NUM>/s, it may not be preferable because the economy is impaired by excessive cooling equipment.

Subsequently, in the present invention, the secondary cooled hot-rolled steel plate is tempered for {<NUM>×t + (<NUM>-<NUM>) } minutes [where t is the thickness (mm) of the steel material] at a temperature of <NUM> to <NUM>. If the temperature is less than <NUM> during the tempering treatment, it may be difficult to secure the target strength due to the difficulty of precipitation of fine precipitates. On the other hand, if the temperature exceeds <NUM>, the growth of the precipitate occurs and there is a concern that strength and low temperature toughness may be impaired.

If the retention time during the tempering treatment in the above-described temperature range is less than {(<NUM>×t)+<NUM>} minutes, it may be difficult to homogenize the structure, whereas if it exceeds {(<NUM>×t)+<NUM>} minutes, it is not preferable because productivity is impaired.

The steel microstructure obtained by the tempering process is comprised of <NUM> to <NUM> area fractions (%) of tempered bainite and the balance tempered martensite.

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

After preparing the respective steel slabs having the composition components illustrated in Table <NUM>, these steel slabs were respectively reheated at a temperature ranging from <NUM> to <NUM>. In addition, each of these reheated steel plates was hot-rolled at a reduction ratio of <NUM> to <NUM>% per pass, and at this time, the hot rolling end temperature was controlled as illustrated in Table <NUM>. Then, each of the hot-rolled steel plates was primarily cooled under the conditions of Table <NUM> within <NUM> seconds after hot-rolling, and then, was subjected to heat treatment under the conditions of Table <NUM>. Subsequently, the heat-treated hot-rolled steel plate was secondarily cooled to room temperature, and then, the secondary cooled steel plate was tempered under the conditions illustrated in Table <NUM>.

As described above, the yield strength, tensile strength, and low-temperature toughness were evaluated for the manufactured steel plates, and the results are also illustrated in Table <NUM> below. On the other hand, in Table <NUM> below, the low-temperature toughness is a result of evaluating with the Charpy impact energy value obtained by performing a Charpy impact test on a specimen having a V notch at -<NUM>. In addition, tensile tests for measuring tensile strength and yield strength were conducted in accordance with ASTM A20, A370 and E8.

-In Table <NUM>, IE is Inventive Example, CE is Comparative Example, A* is the hot rolling end temperature (°C), B* is the primary cooling (water cooling) rate (°C/s), C* is the heat treatment temperature (°C), D* is the heat treatment time (min. ), E* is the tempering temperature (°C), F* is the tempering time (hr), G* is the tempered bainite fraction (%), and H* is the -<NUM> impact toughness value (J).

As illustrated in Tables <NUM> and <NUM>, in the case of Inventive Examples <NUM>-<NUM> in which the steel composition components and manufacturing process conditions satisfy the scope of the present invention, after tempering treatment, <NUM>-<NUM> area% of tempered bainite and the remainder tempered martensite structure may be obtained, and thus, it can be seen that the yield strength and tensile strength are excellent by about <NUM> MPa and <NUM> MPa, respectively, compared to Comparative Examples <NUM>-<NUM>, and further, the elongation is excellent by <NUM>% or more, and the -<NUM> low-temperature toughness is also excellent by <NUM> J or more, compared to Comparative Examples <NUM>-<NUM>.

Meanwhile, in Comparative Examples <NUM> and <NUM> in which the steel composition component range proposed in the present invention is satisfied, but the manufacturing process conditions are out of the scope of the present invention, and in Comparative Examples <NUM> to <NUM> in which steel manufacturing process conditions are within the scope of the present invention, but the steel composition component is outside the scope of the present invention; it can be seen that it is difficult to secure a required microstructure and secure required physical properties.

In addition, in Comparative Examples <NUM> and <NUM>, in which not only the steel composition components but also the manufacturing process conditions are outside the scope of the present invention, it can be confirmed that it is difficult to secure a required microstructure and secure required physical properties.

Claim 1:
A steel plate for a low temperature pressure vessel having excellent cryogenic toughness and ductility, comprising:
in weight%, <NUM> to <NUM>% of C, <NUM> to <NUM>% of Si, <NUM> to <NUM>% of Mn, <NUM> to <NUM>% of Al, <NUM>% or less of P, <NUM>% or less of S, <NUM> to <NUM>% of Ni, <NUM> to <NUM>% of In, a balance of Fe, and unavoidable impurities,
wherein a steel microstructure is comprised of <NUM> to <NUM> area% of tempered bainite and a balance tempered martensite,
wherein the steel plate satisfies a tensile strength of <NUM> MPa class and may be stably used at a low temperature of -<NUM>.