KR101568530B1 - Thick steel sheet having excellent high temperature yield strength and low-temperature toughness, and method for manufacturing the same - Google Patents

Abstract

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel sheet used in pipes, pressure vessels, and the like, and more particularly, to a steel sheet having excellent high-temperature yield strength and low-temperature impact resistance.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel sheet having excellent high-temperature yield strength and low-temperature impact toughness, and a method for manufacturing the steel sheet. BACKGROUND ART [0002]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel sheet used in pipes, pressure vessels and the like, and more particularly to a steel sheet having excellent high-temperature yield strength and low-temperature impact toughness, and a method of manufacturing the steel sheet.

Process pipes and pressure vessels for plants often require strength assurance at high temperatures when they serve as steam transport and high temperature reactors. In particular, when extracting crude oil from oil sands, steam assisted gravity drainage (SAGD) Steam is transported at a high temperature (about 300 to 400 ° C), so that products having the same and similar strength are required to be guaranteed not only at normal temperature but also at high temperature. Accordingly, there is an increasing demand for a steel sheet for a steam transport pipe having a yield strength of 550 MPa.

In addition, since the steel sheet is often used in an extreme ground, it is sometimes required to secure strength at high temperature and to secure properties at low temperature impact toughness.

Since the steel sheets developed by the present technology have a remarkably low strength at high temperature compared to the strength at room temperature, they are guaranteed high temperature strength by using a product which has significantly improved the room temperature strength compared with the high temperature strength.

However, the use of materials having high room temperature strength is accompanied by a high-cost alloy and a complicated heat treatment process, and has a disadvantage in terms of heat resistance in low-temperature impact toughness and processability.

On the other hand, the conventional techniques proposed for the production of a steel material having excellent high-temperature strength are techniques for predicting precipitation hardening at a high temperature mainly by containing a large amount of Cr, Mo, V, Nb, etc. at high cost. Have been proposed.

For example, Patent Documents 1 and 2 disclose a method of securing high-temperature strength by adding a large amount of elements of Cr, Mo, and V in the Patent Documents 1 and 2. Patent Document 3 discloses a method in which Mo is added at 0.7 to 1.2% Quenching) -T (Tempering) treatment to ensure high-temperature strength.

However, the methods proposed in Patent Documents 1 to 3 have disadvantages in that they are economically disadvantageous in that a large amount of high-priced precipitation-type elements are added, followed by a separate heat treatment after hot rolling.

As another example, Patent Document 4 discloses a method for producing a steel material having a high temperature strength by forming carbonitride at a high temperature by Mo and Nb containing low carbon. In Patent Document 5, rolling is terminated at 800 ° C or higher And then water-cooled to 450 캜 or lower to produce a high tensile strength steel having a tensile strength of 780 MPa and excellent in high temperature strength.

However, these inventions only disclose a technique for securing a tensile strength at a high temperature of 600 캜 or more. Therefore, when the strength is required to be secured at about 300 to 400 캜, The present invention does not disclose any method that can reduce the difference in yield strength between the two.

Therefore, it is required to develop a material capable of ensuring low-temperature impact toughness while having a small difference in strength between high temperature and normal temperature.

Japanese Patent Application Laid-Open No. 2000-001735 Japanese Patent Application Laid-Open No. 2001-049387 Japanese Patent Application Laid-Open No. 2002-012939 Japanese Laid-Open Patent Publication No. 2004-043961 Japanese Laid-Open Patent Publication No. 2004-323933

One aspect of the present invention is to provide a steel sheet having excellent yield strength at a high temperature and having a small difference in strength at a high temperature and at a room temperature and at the same time having excellent low temperature impact toughness by optimizing steel components and manufacturing conditions, .

An aspect of the present invention is a method of manufacturing a semiconductor device, which comprises 0.03 to 0.1% of carbon (C), 0.1 to 0.5% of silicon (Si), 1.0 to 2.0% of manganese (Mn) (Al): not more than 0.06%, nitrogen: not more than 0.01%, chromium (Cr): 0.05 to 0.5%, molybdenum (Mo): 0.05 to 0.5% Nb): 0.005% to 0.10%, and titanium (Ti): 0.005 to 0.03%, the balance Fe and unavoidable impurities, and has a microstructure in which an area ratio of martensite of 1 to 5% Ferrite, and bainite, which is excellent in high-temperature yield strength and low-temperature impact toughness.

According to another aspect of the present invention, there is provided a method of manufacturing a steel slab, comprising the steps of: reheating a steel slab satisfying the above-described composition of the composition at 1100 to 1200 占 폚; Hot-rolling the reheated steel slab at a temperature of Ar 3 + 30 ° C to 900 ° C to produce a hot-rolled steel sheet; And cooling the hot-rolled steel sheet at a cooling rate of 10 to 70 ° C / s to terminate the cooling at 300 to 600 ° C. The present invention also provides a method of manufacturing a hot-rolled steel sheet having excellent high-temperature yield strength and low-temperature impact toughness.

INDUSTRIAL APPLICABILITY According to the present invention, it is possible to provide a post-steel sheet having a yield strength of not less than 550 MPa at a high temperature not only at room temperature but also at low temperature impact toughness.

Particularly, since the present invention has an excellent yield strength at 300 to 400 ° C, it can be suitably used as a material for use in an actual use environment of 300 to 400 ° C.

FIG. 1 is a graph showing the yield strength difference (yield strength at normal temperature-yield strength at high temperature) according to the MA area fraction.

Generally, as the temperature rises, the strength of the steel decreases. Therefore, the steel has a limitation in selecting the steel having high strength at room temperature in designing the applicable material in the industrial environment.

In order to solve such problems, conventionally, a large amount of precipitate forming elements such as Cr, Mo, V and Nb have been added to reduce the strength reduction at high temperature. However, this has not only increased the manufacturing cost by promoting the use of expensive elements, In most cases, it is only aimed at securing strength at a high temperature of 600 ° C or more, and there is no case where the difference in strength to the practical use environment, that is, about 300 to 400 ° C, is taken into consideration.

The present inventors have found that the decrease in the high temperature strength of the steel material is mainly due to a decrease in the yield strength and the decrease in the tensile strength at 300 to 400 ° C. is not greatly reduced. And there was no significant effect on the reduction phenomenon. It has also been found that the reduction in the yield strength in the environment of 300 to 400 ° C is reduced by a constant width irrespective of the activation of dislocation movement and the heat treatment history of the material.

Accordingly, the inventors of the present invention have found that by optimizing the composition and production conditions of a steel material, the steel has excellent yield strength at high temperature and is excellent in low-temperature impact toughness with a small difference in yield strength between room temperature and high temperature (300 to 400 ° C) It has been confirmed that a steel material having the above properties can be advantageously produced even at a low cost compared to existing technologies, and the present invention has been accomplished.

Particularly, the inventors of the present invention found that the reduction of the yield strength due to a rise in temperature is a thermal activation effect of dislocation movement. In order to overcome this, a microstructure that facilitates continuous yielding at room temperature, that is, a Martensite Austenite constituent (MA) It is characterized in that the yield strength at room temperature is low and the MA structure disappears at a temperature of 300 to 400 ° C to reduce the continuous yielding effect so that the difference in yield strength between room temperature and high temperature can be reduced.

At this time, if the fraction of the martensite (MA) is too high, there is a problem that the yield strength at room temperature and the toughness at low temperature are greatly deteriorated. Therefore, in order to obtain a proper fraction of MA structure, desirable.

Hereinafter, the present invention will be described in detail.

The steel sheet having excellent high-temperature yield strength and low-temperature impact toughness according to one aspect of the present invention is characterized by containing 0.03 to 0.1% of carbon (C), 0.1 to 0.5% of silicon (Si), 1.0 to 0.5% of manganese (Mn) (P): 0.03% or less, S: not more than 0.003%, aluminum (Al): not more than 0.06%, nitrogen (N): not more than 0.01%, chromium (Cr): 0.05 to 0.5% (Mo): 0.05 to 0.5%, and niobium (Nb): 0.005 to 0.10% and titanium (Ti): 0.005 to 0.03%.

Hereinafter, the reasons for restricting the composition of the alloy component in the present invention will be described in detail. Here, the content of each component means weight% unless otherwise specified.

C: 0.03 to 0.1%

Carbon (C) is an element that has the greatest influence on the physical properties of steel among the steel components, and is an element particularly advantageous for securing strength of steel. If the content of C is less than 0.03%, it is difficult to obtain strength and the weld heat affected zone (HAZ) is softened more than necessary. On the other hand, if the content exceeds 0.1%, the low temperature impact toughness of the steel sheet is deteriorated, . Therefore, in the present invention, the content of C is preferably limited to 0.03 to 0.1%.

Si: 0.1 to 0.5%

Silicon (Si) is an element that acts not only as a deoxidizer in the steelmaking process but also to increase the strength of steel. If the content of Si exceeds 0.5%, the impact toughness deteriorates and the weldability deteriorates. Since there is a problem of causing scale peeling at the time of rolling, it is preferable that the Si content is 0.5% or less. However, in order to reduce the content of Si to less than 0.1%, an excessive cost is incurred and the production cost is increased. Therefore, in the present invention, it is preferable to limit the Si content to 0.1 to 0.5%.

Mn: 1.0 to 2.0%

Manganese (Mn) is an element that improves the low temperature impact toughness by lowering the ferrite phase transformation temperature without impairing impact toughness. In order to obtain such an effect, it is preferable to add Mn at 1.0% or more. However, if the content exceeds 2.0%, center segregation occurs to lower the impact toughness, as well as increase the hardenability of steel and deteriorate weldability . Therefore, the content of Mn in the present invention is preferably limited to 1.0 to 2.0%.

P: 0.03% or less (including 0%)

Phosphorus (P) is an element which is inevitably added in the steel. When the content exceeds 0.03%, the weldability is significantly lowered, and impact toughness is deteriorated. Therefore, the content of P is preferably limited to 0.03% or less, and more preferably 0.01% or less in view of low-temperature impact toughness.

S: 0.003% or less (including 0%)

Sulfur (S) is an element which is inevitably added in the steel. When the content exceeds 0.003%, there is a problem that the ductility, impact toughness and weldability of the steel deteriorate. Therefore, it is preferable to limit the content of S to 0.003% or less, and S is particularly preferable to be limited to 0.002% or less, since S combines with Mn to form MnS inclusions to lower the impact resistance at low temperatures.

Al: 0.06% or less (excluding 0%)

Aluminum (Al) is an element that generally acts as a deoxidizer to remove oxygen by reacting with oxygen present in molten steel. Therefore, it is preferable to add Al to the steel so as to provide sufficient deoxidizing power. When the content exceeds 0.06%, a large amount of oxide inclusions is formed, which hinders impact toughness of the material. Therefore, in the present invention, the content of Al is preferably limited to 0.06% or less.

N: 0.01% or less (excluding 0%)

Nitrogen (N) inhibits the growth of austenite grains by forming nitrides with Al, Ti, Nb, V and the like to help improve toughness and strength of the steel. When the content exceeds 0.01% , And N in these employment states adversely affect toughness. Further, since it is difficult to completely remove N from the steel industrially, it is preferable to limit the N content to 0.01%, which is an allowable range in the manufacturing process, to the upper limit. Therefore, in the present invention, the content of N is preferably limited to 0.01% or less.

Cr: 0.05 to 0.5%

Chromium (Cr) is an element that is dissolved in austenite during reheating of slabs and serves to increase the incombustibility of steel. If the content of Cr is less than 0.05%, it is difficult to obtain the above-mentioned effect. On the other hand, if the content of Cr exceeds 0.5%, the effect of increasing the austenite inclusion property to form a structure for heating the impact toughness, have. Therefore, the content of Cr in the present invention is preferably limited to 0.05 to 0.5%.

Mo: 0.05 to 0.5%

Molybdenum (Mo) is an element which is similar to or more aggressive than Cr and plays a role in increasing the entrapment of the steel. If the content of Mo is less than 0.05%, it is difficult to obtain the above-mentioned effect. On the other hand, when the content of Mo exceeds 0.5%, the entrapment of austenite is increased to form a structure for heating impact toughness, May cause temper brittleness. Therefore, the Mo content in the present invention is preferably limited to 0.05 to 0.5%.

The present invention preferably includes one or two of Nb and Ti in addition to the above-mentioned components.

Nb: 0.005% to 0.10%

Niobium (Nb) is dissolved during the reheating of the slab and inhibits the growth of austenite grains during hot rolling, and is then precipitated to improve the strength of the steel. If the content of Nb is less than 0.005%, the effect of finely pulverizing austenite particles is insignificant. On the other hand, if the content exceeds 0.10%, there is a problem of deteriorating the low-temperature impact toughness of the material. Therefore, the content of Nb in the present invention is preferably limited to 0.005 to 0.10%.

Ti: 0.005 to 0.03%

Titanium (Ti) is an element effective for inhibiting the growth of austenite grains by depositing in the form of TiN in combination with N when reheating the slab. If the Ti content is less than 0.005%, the austenite grain refinement effect is insignificant, whereas if the Ti content is more than 0.03%, the low temperature impact toughness of the material deteriorates. Therefore, the content of Ti in the present invention is preferably limited to 0.005 to 0.03%.

In addition, the present invention may further include one or two of Cu and Ni.

Cu: 0.05 to 0.3%

Copper (Cu) is an element that has a solid solution strengthening effect and serves to increase the strength of the base structure. Particularly, it is an element which plays a role of improving the strength of a steel sheet by precipitation in a high temperature environment. In order to obtain such an effect, it is preferable to add Cu at 0.05% or more. However, if the content exceeds 0.3%, there is a problem that the steel sheet cracks during rolling. Therefore, the content of Cu in the present invention is preferably limited to 0.05 to 0.3%.

Ni: 0.05 to 0.5%

Nickel (Ni) is an element for solid solution strengthening which increases the strength of the steel sheet and does not significantly lower the impact resistance at low temperatures. In order to obtain the above-mentioned effect, it is preferable to add Ni at 0.05% or more. However, if the content exceeds 0.5%, the manufacturing cost of the material increases sharply and deteriorates the steel sheet surface characteristics. Therefore, the content of Ni in the present invention is preferably limited to 0.05 to 0.5%.

The remainder of the present invention is iron (Fe). However, in the ordinary steel manufacturing process, impurities which are not intended from the raw material or the surrounding environment may be inevitably incorporated, so that it can not be excluded. These impurities are not specifically mentioned in this specification, as they are known to any person skilled in the art of steel making.

The steel sheet of the present invention satisfying the above-mentioned composition is preferably a microstructure containing at least one of martensite on the surface of 1 to 5% of the area fraction and ferrite and bainite as the remainder.

The present invention provides a post-steel sheet excellent in both high-temperature yield strength and low-temperature impact toughness, and it is preferable to include one or two kinds of ferrite and bainite as the matrix. At this time, the bainite may include a part of acicular ferrite. If a hard tissue such as martensite is included in the base structure, the target strength can be ensured, but the low-temperature impact toughness is poor, which is undesirable.

Further, the present invention preferably includes a small amount of on-ground martensite (MA) in the matrix.

The MA is an unstable phase at room temperature. As the content of MA increases, the yield strength of the steel is lowered, so that the MA easily deforms even at low stress. Especially, the MA is easily decomposed by thermal activation at 300 to 400 ° C.

In the present invention, by containing such MA in an area of 1 to 5% by area in the matrix, it is intended to lower the softening of the yield strength at a high temperature, particularly 300 to 400 ° C. That is, the MA structure continuously breaks at room temperature and lowers the room temperature strength compared to the conventional steel. On the other hand, when the MA structure is decomposed at high temperatures, the continuous yielding effect is reduced and the strength reduction can be suppressed. If the fraction of MA is less than 1%, the effect of suppressing decrease in yield strength at a high temperature is insignificant. On the other hand, if the MA exceeds 5%, the impact resistance at low temperature due to MA is drastically deteriorated. Therefore, the fraction of MA in the present invention is preferably limited to 1 to 5%.

The steel sheet of the present invention having the above advantageous composition and microstructure can be easily manufactured using ordinary knowledge in the technical field of the present invention without undue repeated experimentation if the steel sheet is made by those skilled in the art have. However, in the present invention, a more advantageous manufacturing method found by the inventors of the present invention is proposed, for example, a method of manufacturing the steel sheet.

The steel sheet according to the present invention can be manufactured by a reheating-hot-rolling-cooling process of a steel slab satisfying the composition of the present invention, and the conditions of the respective processes will be described in detail below.

Steel slab reheating

The reheating of the steel slab is a step of heating to a high temperature for hot rolling to be carried out subsequently, and is preferably carried out in a temperature range of 1100 to 1200 ° C.

If the reheating temperature exceeds 1200 ° C., there is a problem that the austenite grains are coarsened to deteriorate the low-temperature impact toughness of steel. If the reheating temperature is lower than 1100 ° C., the inventoryability of the alloying element is lowered. Therefore, in the present invention, it is preferable that the steel slab is heated at a temperature of 1100 to 1200 ° C during reheating, and it is preferable to restrict the temperature to 1100 to 1160 ° C in order to obtain a favorable low-temperature impact toughness.

Hot rolling

The reheated steel slab may be hot-rolled to form a hot-rolled steel sheet, and the finish rolling is preferably performed at Ar 3 + 30 ° C to 900 ° C.

If the temperature during finish rolling is less than Ar3 + 30 占 폚, there is a problem that since the starting temperature of the subsequent cooling is a ferrite-austenite anomaly, a super-precursor ferrite is formed in the microstructure, and the possibility that the remaining austenite transforms into martensite There is a problem that the low-temperature impact toughness deteriorates. Further, when rolling is carried out in a ferrite-austenite anomaly, the pro-eutectoid ferrite is hardened by rolling and martensite is formed to lower the impact toughness of the steel. On the other hand, if the temperature exceeds 900 DEG C during the finish rolling, the rolling effect due to rolling in the austenite non-recrystallization region is undesirably reduced. Therefore, in the present invention, it is preferable to perform finish rolling at a temperature of Ar 3 + 30 ° C to 900 ° C, that is, in a single phase of austenite during hot rolling.

When the finish rolling is carried out in the above temperature range, the cumulative rolling reduction is preferably 66% or more.

If the cumulative rolling reduction is less than 66%, the grain size increases and low-temperature impact toughness is lowered, and the entrapment of coarse austenite is increased, so that a martensite phase tends to be easily formed. Therefore, the cumulative rolling reduction during rolling is preferably 66% or more, and more preferably 70% or more for more favorable low temperature impact toughness.

Cooling

It is preferable to manufacture the hot-rolled steel sheet having the desired microstructure by cooling the produced hot-rolled steel sheet. At this time, it is preferable that the cooling is started in the austenite single phase region, and the cooling is preferably carried out at a cooling rate of 10 to 70 ° C / s to 300 to 600 ° C.

The cooling start temperature at the time of cooling is preferably austenite single phase region, more preferably Ar 3 to 850 ° C. If the cooling start temperature is less than Ar 3, cooling is caused in the ferrite-austenite anomaly, so that pro-eutectoid ferrite and martensite can be easily produced, thereby deteriorating the impact resistance at low temperatures. The upper limit value of the cooling start temperature is a maximum value at which cooling can be started within the range of the finish rolling finish temperature.

It is preferable to perform cooling at a cooling rate of 10 to 70 DEG C / s while cooling is started in the above temperature range. If the cooling rate is less than 10 占 폚 / s, it is difficult to produce a steel sheet having a desired strength. Therefore, there is a problem that an amount of alloy element used increases to secure strength. On the other hand, when the cooling rate exceeds 70 ° C / s, the amount of martensite in the matrix tends to be excessively increased, excessively increasing the strength of the steel, and significantly reducing the impact resistance at low temperatures.

It is preferable that the cooling is finished at 300 to 600 ° C to obtain a MA phase of a desired fraction while ensuring the strength of the steel sheet to an appropriate level. If the cooling end temperature exceeds 600 캜, MA formation is not easy and the strength is lowered. On the other hand, if the cooling termination temperature is less than 300%, the base structure is transformed into martensite, resulting in an excessively high strength and low temperature impact toughness.

Since the steel sheet produced according to the method proposed by the present invention includes the ferrite or bainite as the main phase and contains the appropriate fraction of the on-road martensite (MA), it can be used not only to secure the strength at high temperature, The toughness can be ensured excellently.

In particular, the steel sheet of the present invention can have a yield strength of at least 550 MPa at 300 to 400 ° C and an effect of suppressing a decrease in yield strength at a high temperature of less than 100 MPa in yield strength difference between room temperature and high temperature have. At this time, the difference in yield strength between the normal temperature and the high temperature can be expressed by the following relational expression (1).

[Relation 1]

Yield strength difference (MPa) = (room temperature yield strength) - (high temperature yield strength)

(Here, the room temperature yield strength refers to the yield strength at 10 to 35 ° C, and the high temperature yield strength refers to the yield strength at 300 to 400 ° C.)

Hereinafter, the present invention will be described more specifically by way of examples. It should be noted, however, that the following examples are intended to illustrate the invention in more detail and not to limit the scope of the invention. The scope of the present invention is determined by the matters set forth in the claims and the matters reasonably inferred from them.

( Example )

Hot-rolled steel sheets were prepared by continuously casting molten steel having the composition shown in Table 1 below into steel slabs, and then hot-rolling and cooling the steel slabs under the respective conditions shown in Table 2 below.

The microstructure of each of the hot-rolled steel sheets thus prepared was observed, and the yield strength and impact resistance at low temperature were measured.

The microstructures were observed under an optical microscope. The tensile strengths of the microstructures were measured at room temperature and high temperature (350 ° C) after preparing tensile test specimens of JIS No. 5, and the results are shown in Table 3. The impact toughness was evaluated by Charpy impact test at -45 캜, and the results are shown in Table 3.

Steel grade Component composition (% by weight) division C Si Mn P S Al N Cr Mo Cu Ni Nb Ti One 0.05 0.21 1.65 0.008 0.0010 0.035 0.005 0.31 0.25 0.29 0.31 0.04 0.016 Inventive Steel 1 2 0.04 0.19 1.32 0.009 0.0015 0.037 0.008 0.28 0.08 0.28 - 0.06 - Invention river 2 3 0.07 0.32 1.59 0.008 0.0014 0.029 0.007 0.29 0.15 - 0.22 0.03 0.020 Invention steel 3 4 0.06 0.19 1.55 0.007 0.0008 0.041 0.005 0.38 0.31 0.13 0.41 - 0.015 Inventive Steel 4 5 0.05 0.27 1.50 0.006 0.0009 0.033 0.004 0.36 0.38 - - 0.04 0.011 Invention steel 5 6 0.12 0.22 1.20 0.008 0.0008 0.041 0.005 0.27 0.22 - - 0.02 0.013 Comparative River 1 7 0.07 0.25 2.30 0.008 0.0009 0.033 0.005 0.18 0.09 - - 0.02 0.010 Comparative River 2 8 0.06 0.22 1.45 0.033 0.0007 0.029 0.007 0.22 0.15 0.13 0.22 0.02 0.009 Comparative Steel 3 9 0.06 0.22 1.19 0.009 0.0040 0.022 0.006 0.31 0.10 - - 0.03 0.012 Comparative Steel 4 10 0.04 0.23 1.45 0.009 0.0008 0.070 0.008 0.29 0.09 0.16 0.33 0.04 0.012 Comparative Steel 5 11 0.06 0.19 1.38 0.008 0.0009 0.029 0.012 0.27 0.15 0.22 0.25 0.02 0.009 Comparative Steel 6 12 0.04 0.22 1.65 0.007 0.0011 0.038 0.007 0.55 0.22 0.09 0.20 0.04 0.013 Comparative Steel 7 13 0.05 0.25 1.44 0.008 0.0008 0.041 0.005 0.19 0.60 - - 0.02 0.011 Comparative Steel 8 14 0.06 0.18 1.25 0.008 0.0009 0.025 0.005 0.20 0.16 0.12 0.19 - - Comparative Steel 9 15 0.04 0.22 1.36 0.009 0.0008 0.018 0.007 0.22 0.17 - - 0.14 0.013 Comparative Steel 10 16 0.04 0.23 1.57 0.009 0.0012 0.023 0.007 0.28 0.22 - - 0.02 0.060 Comparative Steel 11

Steel grade
Ar3
(° C) Dog heating
Temperature (℃) Hot rolling condition Cooling conditions division
Finishing temperature
(° C) Cumulative reduction rate
(%) Initiation temperature
(° C) speed
(° C / s) Termination temperature
(° C) Inventive Steel 1 720.95 1122 780 66 730 22 485 Inventory 1 Invention river 2 781.75 1123 830 70 780 25 477 Inventory 2 Invention steel 3 738.60 1119 792 70 742 22 465 Inventory 3 Inventive Steel 4 717.70 1160 801 75 751 33 431 Honorable 4 Invention steel 5 744.65 1155 822 75 772 40 506 Inventory 5 Comparative River 1 761.10 1133 829 66 779 25 620 Comparative Example 1 Comparative River 2 700.35 1145 821 66 771 23 285 Comparative Example 2 Comparative Steel 3 751.35 1125 831 66 781 20 630 Comparative Example 3 Comparative Steel 4 789.50 1129 845 66 795 19 633 Comparative Example 4 Comparative Steel 5 754.65 1127 839 66 789 23 645 Comparative Example 5 Comparative Steel 6 752.75 1144 821 66 771 41 250 Comparative Example 6 Comparative Steel 7 732.90 1133 795 66 745 31 285 Comparative Example 7 Comparative Steel 8 734.40 1121 796 66 746 23 264 Comparative Example 8 Comparative Steel 9 768.70 1137 835 66 785 33 660 Comparative Example 9 Comparative Steel 10 777.85 1122 840 66 790 38 632 Comparative Example 10 Comparative Steel 11 756.15 1130 820 70 770 29 640 Comparative Example 11 Inventive Steel 1 720.95 1220 779 80 729 23 466 Comparative Example 12 Inventive Steel 1 720.95 1122 720 70 670 28 485 Comparative Example 13 Inventive Steel 1 720.95 1122 796 30 746 27 489 Comparative Example 14 Inventive Steel 1 720.95 1125 755 70 700 25 500 Comparative Example 15 Inventive Steel 1 720.95 1136 796 80 746 23 650 Comparative Example 16 Inventive Steel 1 720.95 1130 781 80 731 26 220 Comparative Example 17 Inventive Steel 1 720.95 1129 779 80 729 6 490 Comparative Example 18

(The value calculated from Ar3 = 910-310C-80Mn-20Cu-15Cr-55Ni-80Mo + (0.35 * (thickness-8) in Table 2)

As shown in Tables 1 and 2, Inventive Examples 1 to 5 all satisfy the composition and manufacturing conditions proposed in the present invention.

On the other hand, in Comparative Example 1, the content of C and the cooling termination temperature do not satisfy the range of the present invention. In Comparative Example 2, the content of Mn and the cooling termination temperature do not satisfy the ranges of the present invention, 6 are cases in which the content of P, S, Al and N and the cooling termination temperature do not satisfy the range of the present invention. In Comparative Examples 7 and 8, the content of Cr and Mo and the termination temperature of cooling did not satisfy the ranges of the present invention. In Comparative Examples 9 to 11, the content of one or both of Nb and Ti and the cooling termination And the temperature does not satisfy the range of the present invention.

In Comparative Examples 12 to 18, a steel slab (having the same composition as that of Inventive Example 1) satisfying the composition of the present invention was used. In the case where the production conditions were deviated from the present invention, the reheating temperature in Comparative Example 12, The rolling reduction temperature and the cooling start temperature of Comparative Example 14, the cumulative rolling reduction of the finish rolling, the cooling start temperature of Comparative Example 15, the cooling termination temperature of Comparative Examples 16 and 17, and the cooling rate of Comparative Example 18, If not.

division MA fraction
(area%) Room temperature yield strength
(MPa) High temperature yield strength
(MPa) Yield strength difference
(MPa) Impact toughness
(J, -45 C) Inventory 1 3.3 615 576 39 236 Inventory 2 2.8 597 565 32 248 Inventory 3 3.6 618 590 28 245 Honorable 4 4.0 620 594 26 222 Inventory 5 4.2 635 589 46 202 Comparative Example 1 0.1 480 375 105 127 Comparative Example 2 0.9 698 556 142 18 Comparative Example 3 0.3 495 389 106 41 Comparative Example 4 0.2 476 360 116 150 Comparative Example 5 0.2 475 366 109 183 Comparative Example 6 0.8 707 561 146 27 Comparative Example 7 0.7 706 585 121 24 Comparative Example 8 2.3 739 655 84 22 Comparative Example 9 0.2 436 330 106 98 Comparative Example 10 0.1 495 362 133 87 Comparative Example 11 0.1 485 361 124 88 Comparative Example 12 2.7 571 520 51 74 Comparative Example 13 0.3 516 408 108 116 Comparative Example 14 2.3 630 585 45 26 Comparative Example 15 0.1 532 400 132 136 Comparative Example 16 0.2 485 366 119 233 Comparative Example 17 0.8 725 598 127 33 Comparative Example 18 0.1 455 316 139 229

As shown in Table 3, Inventive Examples 1 to 5 according to the present invention include MA structure in the microstructure in an amount of 1 to 5% by area, and have a yield strength at room temperature and high temperature of 550 MPa or more, I can confirm that it is excellent. In addition, a high strength can be secured even at a high temperature such that the difference in yield strength between the normal temperature and the high temperature is 100 MPa or less.

On the other hand, in Comparative Example 1, it can be seen that the MA fraction is less than 1% by area and the room temperature yield strength does not satisfy 550 MPa, and the yield strength difference between room temperature and high temperature is large due to the low MA fraction. It can be confirmed that the impact strength is low compared to the inventive examples.

In Comparative Example 2, when Mn was added excessively, the yield strength at room temperature was high. However, since the difference in yield strength between room temperature and high temperature exceeded 100 MPa, the decrease in strength occurred at high temperature and the low temperature impact toughness was also improved .

In Comparative Examples 3 to 7, the MA fraction was less than 1% by area, and the difference in yield strength between room temperature and high temperature exceeded 100 MPa.

In the case of Comparative Example 8, although the MA structure was sufficiently formed, it can be confirmed that the yield strength was too high and the low-temperature impact toughness was heated due to the excessive addition of Mo and the low cooling end temperature. This is because martensite is formed in a microstructure as cooling is terminated at less than 300 캜.

In Comparative Examples 9 to 11, the MA fraction was less than 1% by area, and the yield strength difference between the room temperature and the high temperature was large, and at the same time, the yield strength was less than 550 MPa.

Comparative Examples 12 to 18 are steel types satisfying the composition of the composition proposed in the present invention. However, it can be confirmed that the production conditions are deviated from the present invention, and in Comparative Example 12 in which the reheating temperature is too high, , The finish rolling temperature and the cooling start temperature were outside the present invention, it was confirmed that the MA fraction was less than 1% by area and the yield strength did not satisfy 550 MPa. In Comparative Example 14, it can be confirmed that the cumulative reduction ratio at the time of finish rolling is higher than that of the present invention, and the low temperature impact toughness is very high. In Comparative Example 15, it was confirmed that the cooling start temperature was not within the range of the present invention and the yield strength was less than 550 MPa, and the low-temperature impact toughness was heat-treated. In Comparative Example 16 and Comparative Example 17, it was confirmed that the cooling end temperature was a steel in which the MA fraction was less than 1% by area and the difference in yield strength between room temperature and high temperature was large, and in Comparative Example 17, have. In Comparative Example 18, when the cooling rate deviates from the present invention, it is also confirmed that the MA fraction is less than 1% by area and the yield strength is less than 550 MPa.

FIG. 1 is a graph showing the relationship between the MA area fraction and the yield strength difference (yield strength at normal temperature-yield strength at high temperature) of the inventive and comparative examples.

As shown in FIG. 1, it can be confirmed that the difference in yield strength can be reduced when MA is contained in a proper fraction.

Claims (8)

(P): 0.03% or less, sulfur (S): 0.003% or less, carbon (C): 0.03 to 0.1%, silicon (Si): 0.1 to 0.5%, manganese (Al): not more than 0.06% (excluding 0%), nitrogen (N): not more than 0.01% (excluding 0%), chromium (Cr): 0.05 to 0.5%, molybdenum (Mo) %, At least one of niobium (Nb): 0.005 to 0.10%, and titanium (Ti): 0.005 to 0.03%, the balance Fe and unavoidable impurities,
Wherein the microstructure comprises 1 to 5% of area martensite and at least one of ferrite and bainite as the remainder, and having excellent high-temperature yield strength and low-temperature impact toughness.
The method according to claim 1,
Wherein the steel sheet further comprises one or two of copper (Cu): 0.05 to 0.3% and nickel (Ni): 0.05 to 0.5% by weight, and has excellent high temperature yield strength and low temperature impact toughness.
The method according to claim 1,
Wherein the steel sheet is excellent in high-temperature yield strength and low-temperature impact toughness having a yield strength difference of 100 MPa or less as expressed by the following relational expression (1).
[Relation 1]
Yield strength difference (MPa) = (room temperature yield strength) - (high temperature yield strength)
(Here, the room temperature yield strength refers to the yield strength at 10 to 35 ° C, and the high temperature yield strength refers to the yield strength at 300 to 400 ° C.)
The method according to claim 1,
Wherein the steel sheet has a yield strength of 550 MPa or higher at 300 to 400 DEG C, and has excellent high-temperature yield strength and low-temperature impact toughness.
(P): 0.03% or less, sulfur (S): 0.003% or less, carbon (C): 0.03 to 0.1%, silicon (Si): 0.1 to 0.5%, manganese (Al): not more than 0.06% (excluding 0%), nitrogen (N): not more than 0.01% (excluding 0%), chromium (Cr): 0.05 to 0.5%, molybdenum (Mo) Reheating the steel slab containing the steel slab containing at least one of 0.005% to 0.10% of niobium and 0.005 to 0.03% of titanium (Ti), the remainder Fe and unavoidable impurities at 1100 to 1200 ° C;
Hot-rolling the reheated steel slab at a cumulative reduction ratio of at least 66% at Ar 3 + 30 ° C to 900 ° C to produce a hot-rolled steel sheet; And
Cooling the hot-rolled steel sheet at a cooling rate of 10 to 70 ° C / s to terminate cooling at 300 to 600 ° C
And a low-temperature impact toughness.
delete 6. The method of claim 5,
Wherein said cooling is started at a temperature of Ar 3 ~ 850 ° C which is an austenite single phase region, and which is excellent in low temperature impact toughness.
The method of claim 5, wherein
Wherein the steel slab further comprises one or two of copper (Cu): 0.05 to 0.3% and nickel (Ni): 0.05 to 0.5% in weight%, and the steel slab has a high- Gt;

Images