Patent Description:
Toughness of steel is a property, contrary to strength, and it is difficult to secure excellent levels of both the strength and the toughness.

In the related art, it has been attempted to simultaneously secure strength and toughness in high alloy steel materials, using heat treatments. However, there may be a problem of a cost increase due to the use of relatively expensive alloying elements, as well as defects in welding and cutting due to high alloying amounts.

In this regard, a heat control rolling technique for adjusting alloy elements and optimizing a microstructure by control of rolling and cooling conditions to secure toughness and strength has been developed and utilized (Patent Document <NUM>).

Meanwhile, when a thickness of a steel material is less than <NUM> (i.e. <NUM> thickness), the thickness is thin, and even when air cooling is carried out during cooling after rolling, a sufficient cooling rate may be achieved inside the steel material. However, when the thickness is <NUM> and over, internal latent heat is high such that the air cooling process may have a limitation in achieving a sufficient cooling rate.

For this reason, an accelerated cooling technique inducing microstructure refinement, while adjusting a cooling rate through water cooling during cooling after rolling, is utilized for general steel materials of <NUM> thickness and over.

However, for carrying out the above-mentioned accelerated cooling, a proper facility is required, and there is a disadvantage in which strict control is required because uneven cooling due to partial unstable operations may cause effects of non-flatness such as waviness, and others, during processing due to variations in residual internal stress.

Therefore, in manufacturing a thick steel having a thickness of <NUM> thickness and over, it is required to develop a method for stably securing product quality while significantly reducing facility investment.

(Patent Document <NUM>) <CIT>
<CIT>, <CIT> and <CIT> each disclose steel plates having compositions falling within the ranges, by weight %, <NUM> to <NUM>% of carbon (C), <NUM> to <NUM>% of manganese (Mn), <NUM>% or less of silicon (Si), <NUM>% or less of phosphorus (P), <NUM>% or less of sulfur (S), <NUM> to <NUM>% of niobium (Nb), <NUM> to <NUM>% of vanadium (V), a balance of iron (Fe) and inevitable impurities, wherein the thick steel plate optionally further comprises one or more of <NUM>% or less of Ni and <NUM>% or less of Cr; and <NUM>% or less of Ti.

An aim of the invention is to provide a thick steel plate having high-strength and high-toughness without carrying out accelerated cooling using water cooling, in the manufacturing process, by means of a Thermo-Mechanical Control Process (TMCP), of a thick steel having a thickness of <NUM> and over; and a method for manufacturing the same.

According to an aspect of the invention, a steel plate having high strength and high toughness includes by weight (%): <NUM> to <NUM>% of carbon (C), <NUM> to <NUM>% of manganese (Mn), <NUM>% or less of silicon (Si), <NUM>% or less of phosphorus (P), <NUM>% or less of sulfur (S), <NUM> to <NUM>% of niobium(Nb), <NUM> to <NUM>% of vanadium (V), a balance of iron (Fe) and inevitable impurities, wherein the thick steel plate optionally further comprises one or more of <NUM>% or less of Ni and <NUM>% or less of Cr; and <NUM>% or less of Ti, and has a microstructure composed of <NUM> to <NUM>% of ferrite and <NUM> to <NUM>% of pearlite by an area fraction, wherein a grain size of prior austenite measured according to ASTM E112 is ASTM grain size number of <NUM> or more, and a grain size of ferrite is ASTM grain size number of <NUM> or more, wherein the microstructure is observed using a microscope at a point of <NUM>/4t, where t is thickness in mm, wherein the thick steel plate has an impact toughness of 300J or more at -<NUM>, wherein impact toughness is evaluated using a Charpy V-Notch test with a proportional specimen of L<NUM>=<NUM>√S<NUM> for total thickness, where L<NUM> is an original gauge length, and S<NUM> is an original cross-sectional area; and wherein the thickness of the steel plate is <NUM> to <NUM>.

According to an aspect of the invention, a manufacturing method of the above steel plate having high strength and high toughness comprising steps of: reheating a steel slab including, by weight %, <NUM> to <NUM>% of carbon (C), <NUM> to <NUM>% of manganese (Mn), <NUM>% or less of silicon (Si), <NUM>% or less of phosphorus (P), <NUM>% or less of sulfur (S), <NUM> to <NUM>% of niobium (Nb), <NUM> to <NUM>% of vanadium (V), a balance of iron (Fe) and inevitable impurities, wherein the thick steel plate optionally further comprises one or more of <NUM>% or less of Ni and <NUM>% or less of Cr; and <NUM>% or less of Ti, at a temperature of <NUM> or higher; performing finish hot rolling on the reheated steel slab at a temperature within a range of <NUM> to <NUM> to prepare a hot-rolled steel plate having a thickness of <NUM> to <NUM>; and performing air cooling to room temperature after performing the finish hot rolling.

According to the invention, it is possible to provide a steel plate capable of stably ensuring impact toughness from <NUM> to -<NUM>.

As described above, there is an economically advantageous effect by providing a thick steel plate with high efficiency even after accelerated cooling is not performed during cooling after rolling.

The present inventors have conducted intensive research to provide a steel plate having a physical property equal to or more than that of a steel plate manufactured by a conventional method without carrying out a conventional water cooling process, in the manufacturing a thick steel having a thickness of <NUM> and over, by means of a Thermo-Mechanical Control Process (TMCP).

As a result, since alloy composition and manufacturing conditions are optimized, it has been confirmed that it is possible to manufacture a thick steel plate having desired physical properties even when air cooling is performed during cooling after rolling, thereby completing the present disclosure.

In particular, in order to overcome a cooling effect by not performing accelerated cooling, it is technically significant to excellently secure strength and toughness by utilizing V in a steel alloy composition while finely controlling a microstructure.

Hereinafter, the invention will be described in detail.

According to an aspect of the invention, a steel plate having high-strength and high-toughness comprises, by weight %: <NUM> to <NUM>% of carbon (C), <NUM> to <NUM>% of manganese (Mn), <NUM>% or less of silicon (Si), <NUM>% or less of phosphorus (P), <NUM>% or less of sulfur (S), <NUM> to <NUM>% of niobium (Nb), and <NUM> to <NUM>% of vanadium (V)and a balance of iron (Fe), wherein the thick steel plate optionally further comprises one or more of <NUM>% or less of Ni and <NUM>% or less of Cr; and <NUM>% or less of Ti and wherein the microstructure is in accordance with Claim <NUM> hereof.

Hereinafter, the reason why the alloy composition of the steel plate of the present disclosure is controlled as described above will be described in detail. In this case, the content of each element means weight % unless otherwise specified.

Carbon (C) is an essential element for strengthening of steel. However, when a content of C is excessive, a rolling load during rolling may increase due to increase of high-temperature strength, and instability of toughness at a cryogenic temperature of -<NUM> or less may be induced.

Meanwhile, when the content of C is less than <NUM>%, it is difficult to secure the strength required, and in order to control the content of C to less than <NUM>%, a decarburization process may be additionally required, which may lead to an increase in costs. On the other hand, when the content thereof exceeds <NUM>%, a rolling load may be increased and the rolling in a temperature range controlled in accordance with the invention may not be properly performed, and it may be difficult to control other elements favorable to the strengthening of steel, and the toughness may not be sufficiently obtained.

Therefore the content of C is controlled to <NUM> to <NUM>%.

Manganese (Mn) is an essential element for securing impact toughness of steel and controlling impurity elements such as S, but when manganese is added in excess with C, weldability may deteriorate.

As described above, the toughness of steel may be effectively secured by controlling the content of C, and in order to obtain high strength, the strength may be improved with Mn without adding C, such that impact toughness may be maintained.

Mn is contained in an amount of <NUM>% or more for the above-mentioned effect. However, when the content thereof exceeds <NUM>%, the weldability may deteriorate due to an excess of a carbon equivalent, and there is a problem in which toughness is lowered in only a portion of the thick steel plate and cracks generated due to segregation during casting may occur.

Therefore, in the invention, the content of Mn is controlled to be <NUM> to <NUM>%.

Silicon (Si) is a major element for killed steel, and is an element favorable for securing strength of steel by solid solution strengthening.

However when a content of Si exceeds <NUM>%, there is a problem that a load during rolling is increased and toughness of a welded portion during welding deteriorates with a base material (a thick steel plate itself).

Therefore the content of Si is controlled to be <NUM>% or less.

Phosphorus (P) is an element which is inevitably contained during manufacturing of steel, is an element which is liable to be segregated and easily forms a low-temperature microstructure and thus has a large influence on toughness degradation.

Therefore, it is preferable to control a content of P to be as low as possible. In the invention the content of P is controlled to be <NUM>% or less because there is no great difficulty in securing properties even when P is contained at a maximum of <NUM>%.

Sulfur (S) is an element which is inevitably contained (included) during manufacturing of steel. When a content of S is excessive, there is a problem that non-metallic inclusions are increased such that toughness deteriorates.

Therefore, it is preferable to control the content of S to be as low as possible. In the invention, the content of S is controlled to be <NUM>% or less because there is no great difficulty in securing properties even when S is contained at a maximum of <NUM>% at a maximum of <NUM>%.

Niobium (Nb) is an element favorable for maintaining a fine microstructure, during rolling, through high-temperature precipitation, and is an element favorable for securing strength and impact toughness. In particular the addition of Nb is required to stably obtain fine structure in addition to microstructure refinement secured by controlling a series of manufacturing conditions.

The content of Nb is determined by an amount of Nb dissolved by a temperature and time at reheating a slab for rolling, but the content exceeding <NUM>% is not preferable because it generally exceeds a solution range. Meanwhile, when the content of Nb is less than <NUM>% the precipitation amount is insufficient and the above-mentioned effect may not be sufficiently obtained, which is not preferable.

Therefore, in the invention the content of Nb is controlled to be <NUM> to <NUM>%.

Vanadium (V) is an element favorable for securing strength of steel. In particular, since the content of C is limited to secure impact toughness of steel and the content of Mn is limited to control a segregation effect, insufficient strength may be secured through the addition of the V without accelerated cooling, in addition to the limitations C and Mn. In addition, since V is precipitated at a low temperature region, there is an effect reducing the rolling load during rolling in a limited temperature range.

When the content of V exceeds <NUM>%, precipitates may be excessively formed and brittleness may be caused, which is not preferable. However, when the content of V is less than <NUM>%, an amount of precipitation is insufficient and the above-mentioned effect may not be sufficiently obtained, and thus it is not preferable.

Therefore the content of V is controlled to be to <NUM> to <NUM>%.

Meanwhile at least one or more of Ni and Cr may be further contained in an amount of <NUM>% or less, respectively for further improving properties of the steel plate satisfying the alloy composition described above, and Ti may further be contained in an amount of <NUM>% or less.

Nickel (Ni) and Chromium (Cr) may be added to secure strength of steel, and it is preferable to add in an amount of <NUM>% or less in consideration of carbon equivalent and the limitation of the elements essentially contained.

Titanium (Ti) may be added for surface quality control while adjusting the strength of the steel, but it is preferably added in an amount of <NUM>% or less in consideration of an influence of grain boundary brittleness due to precipitates when excessively added.

A remainder of the above-mentioned composition is iron (Fe). However, since impurities which are not intended from raw materials or surrounding environments is able to inevitably incorporated, in a manufacturing process in the related art, they may not be excluded. These impurities are not specifically mentioned in the present specification, as they are known to anyone in the skilled art.

The steel plate of the invention satisfying the alloy composition described above has a microstructure which includes ferrite and pearlite mixed structures.

More specifically, by including <NUM> to <NUM>% of ferrite and <NUM> to <NUM>% of pearlite by an area fraction a desired strength and impact toughness may be secured.

When the fraction of pearlite is excessive, the yield strength may be excessively increased as compared with the tensile strength.

As described above, in the thick steel plate of the invention including ferrite and pearlite mixed structures the grain size of ferrite is ASTM grain size number of <NUM> or more. When the grain size of ferrite is less than the ASTM grain size number of <NUM>, coarse grains are formed and the strength and toughness at a target level may not be secured.

The grain size of ferrite is influenced by a grain size of austenite. Thus, in the present disclosure, the grain size of prior austenite is ASTM grain size number of <NUM> or more. When the grain size of austenite is less than the ASTM grain size number of <NUM>, fine microstructure may not be obtained in a final product, and the desired properties may not be secured.

The thick steel plate of the invention satisfying both the alloy composition and the microstructure as described above, has a yield ratio (yield strength (MPa)/tensile strength (MPa))of <NUM> to <NUM>%, has excellent cryogenic impact toughness of 300J or more even at -<NUM>, and also has high strength.

The thick steel plate of the invention has a thickness of <NUM> to <NUM>.

Hereinafter, a manufacturing method for a thick steel plate having excellent cryogenic toughness, another aspect of the invention, will be described in detail.

In brief, according to the invention, the desired thick steel plate may be manufactured through [steel slab reheating-hot rolling-cooling] processes, and conditions for each step will be described in detail as below.

First, it is necessary to prepare a steel slab satisfying the alloy composition described above, and then reheat the steel slab at a temperature of <NUM> or higher.

The reheating process is to utilize a niobium compound formed during casting to perform microstructure refinement, and the reheating process is performed at a temperature of <NUM> or higher in order to disperse and finely precipitate Nb after re-dissolution.

When the temperature of reheating is less than <NUM>, dissolution does not occur properly and fine grains may not be induced, and it is difficult to secure the strength in a final steel material. In addition, it is difficult to control the grains due to the precipitates, such that only microstructure refinement obtained by controlling of rolling conditions to be described later may not obtain stable microstructure refinement and desired physical properties.

The reheated steel slab is hot-rolled according to the above-described method to manufacture a hot-rolled steel plate.

Finish hot rolling is performed at a temperature within a range of <NUM> to <NUM>.

When a temperature of performing the finish rolling is less than <NUM>, rolling at two phase regions is performed, and there is a problem that formation of pro-eutectoid structures and deformation during rolling cause unevenness of residual stress after rolling and cutting resulting in difficulty in controlling a shape. On the other hand, when the temperature exceeds <NUM>, recrystallization of austenite may lower the strength due to grain growth, which is not desirable.

When the shape is uneven after rolling, flatness should be secured by using a leveling facility, and there may be an additional residual stress on a plate due to the stress during cold leveling. Therefore, it is important to perform hot leveling in the view of removing residual stress, by performing hot finish rolling at a temperature within a range of <NUM> to <NUM>, a single-phase region, a temperature required for hot leveling may be secured, and a recovery temperature at which the stress may be removed even after the leveling may be secured, and in a further processing of a final product, it is possible to significantly reduce the possibility of unevenness in shape, or the like.

The hot-rolled steel plate manufactured according to the above-mentioned method is cooled to room temperature to prepare a final thick steel plate. In particular, air cooling is performed at the time of cooling.

The method of the invention is economically advantageous because it does not require a separate cooling facility by performing air cooling during cooling the hot-rolled steel plate, and even when air cooling is performed, all desired properties may be obtained.

Hereinafter, the invention will be described more specifically through embodiments. It should be noted, however, that the following embodiments are intended to illustrate the invention in more detail and not to limit the scope of the disclosure. The scope of the invention is determined by the claims.

A slab having an alloy composition illustrated in the following Table <NUM> was reheated at a temperature of <NUM> or higher, and then subjected to finish hot rolling and cooling under the conditions illustrated in the following Table <NUM> to prepare a final thick steel plate.

In this case, a thick steel plate having a thickness of <NUM> and a thickness of <NUM> was prepared for Inventive Steel <NUM>, respectively, and a thick steel plate having a thickness of <NUM> was respectively for Inventive Steel <NUM> and <NUM>, respectively. A thick steel plate having a thickness of <NUM> for Comparative Steel <NUM>, and a thick steel plate having a thickness of <NUM> and a thickness of <NUM> for Comparative Steel <NUM> and <NUM>, respectively was prepared.

Thereafter, with respect to each thick steel plate, microstructures were observed using a microscope at a point of <NUM>/4t (where, t is thickness(mm)), and tensile characteristics were evaluated by using proportional specimen of L<NUM>=<NUM>√S<NUM> (where, L<NUM> is an original gauge length, and S<NUM> is an original cross-sectional area) for the total thickness. The results are illustrated in Table <NUM> below.

In addition, Charpy V-Notch impact characteristics were evaluated for each thick steel plate, and the results thereof are illustrated in Table <NUM> below.

(In Table <NUM>, a remainder excluding a F fraction is P, where F is ferrite and P is pearlite.

As illustrated in the Table <NUM>, it can be confirmed that the thick steel plate of the invention may secure the same properties as those of steel (Comparative Steel <NUM>), which secures properties through water cooling after conventional rolling (grain size, yield ratio, and the like) even though an air cooling process was performed during cooling after rolling.

Meanwhile, Comparative Steel <NUM> illustrates that an increase in strength is insufficient, even though an addition amount of Nb is excessive. This is due to the fact that an effect of Nb does not sufficiently occur due to the limitation of the amount of solid solution even when the addition amount of Nb is increased.

In addition, as illustrated in Table <NUM>, it can be confirmed that impact transition does not occur up to -<NUM> in the thick steel plate of the invention.

Meanwhile, in the case of comparative steel <NUM>, a content of V in the steel alloy composition is excessive, and it can be confirmed that impact transition occurred near -<NUM> region.

In manufacturing the thick steel plate, an influence of an extraction temperature on the strength at the time of reheating slab was confirmed. Specifically, the slab of Inventive Steel <NUM> was heated to satisfy the respective extraction temperatures illustrated in Table <NUM>, and then subjected to finish hot rolling at a temperature of <NUM> to have a thickness of <NUM>, and then subjected to air cooling to room temperature to prepare respective thick steel plates.

Thereafter, the tensile characteristics of each of the above-mentioned thick steel plates were evaluated.

As illustrated in Table <NUM>, it can be confirmed that the strength is lowered as the extraction temperature is lowered. In particular, when the extraction temperature is <NUM>, it can be confirmed that the strength is lowered to be about <NUM> to <NUM> MPa compared with the case in which the extraction temperature is <NUM> and the yield ratio is also lowered to be less than <NUM>%.

As the extraction temperature is lowered, an Nb reuse effect, affecting the microstructure refinement, and the like, is reduced, which causes a decrease in strength and yield ratio under similar rolling conditions.

Claim 1:
A steel plate having high strength and high toughness comprising, by weight %:
<NUM> to <NUM>% of carbon (C), <NUM> to <NUM>% of manganese (Mn), <NUM>% or less of silicon (Si), <NUM>% or less of phosphorus (P), <NUM>% or less of sulfur (S), <NUM> to <NUM>% of niobium (Nb), <NUM> to <NUM>% of vanadium (V), a balance of iron (Fe) and inevitable impurities, wherein the thick steel plate optionally further comprises: one or more of <NUM>% or less of Ni and <NUM>% or less of Cr; and <NUM>% or less of Ti,
and having a microstructure composed of <NUM> to <NUM>% of ferrite and <NUM> to <NUM>% of pearlite by an area fraction,
wherein a grain size of prior austenite measured according to ASTM E112 is ASTM grain size number of <NUM> or more and a grain size of ferrite is ASTM grain size number of <NUM> or more,
wherein the microstructure is observed using a microscope at a point of <NUM>/4t, where, t is thickness in mm,
wherein the steel plate has an impact toughness of 300J or more at -<NUM>, wherein impact toughness is evaluated using a Charpy V-Notch test with a proportional specimen of L<NUM>=<NUM>√S<NUM> for total thickness, where L<NUM> is an original gauge length, and S<NUM> is an original cross-sectional area; and
wherein the thickness of the steel plate is <NUM> to <NUM>.