KR19990039203A - Manufacturing Method of High-Strength Hot-Rolled Steel Sheet - Google Patents

Abstract

The present invention relates to a method of manufacturing a high-strength hot-rolled steel sheet, and its object is to provide a method of manufacturing a high-strength hot-rolled steel sheet by properly controlling a steel component, controlling a reduction rate and a finishing temperature during hot rolling, To obtain a high-strength hot-rolled steel sheet excellent in low temperature toughness with a tensile strength of 80 kg / mm < 2 > or more.

In order to achieve the above object, the present invention provides a ferritic stainless steel comprising, by weight, 0.03-0.10% of C, 0.01-0.50% of Si, 1.2-2.2% of Mn, % Of Mo, 0.2-0.7% of Mo, 0.1-0.5% of Ni, 0.02-0.10% of Nb, 0.02-0.10% of Ti, 0.01% or less of N and 0.001-0.005% of Ca, Hot rolling is performed in which a steel material made of an impurity is rolled at a temperature of 750 to 1000 占 폚 for 60% or more of the total reduction ratio (TRR) and a finish rolling temperature (FDT) at a temperature of 750 to 850 占 폚, The present invention relates to a method for producing a high-strength hot-rolled steel sheet in which the finish rolling temperature and the total rolling reduction ratio satisfy the relationship of " TRR ≥0.13 x FDT-32 ", hot rolling is performed, That is the point.

Description

Manufacturing Method of High-Strength Hot-Rolled Steel Sheet

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of manufacturing a high-strength hot-rolled steel sheet suitable for use in applications such as construction, a line pipe, and an offshore structure, and more particularly to a method of manufacturing a high-

Steel materials used for steel pipes and marine structures for transporting crude oil or gas used in Arctic or Siberian regions are required to have excellent properties due to severe environmental conditions. Especially, in order to prevent breakage at low temperature, low temperature toughness It is especially demanded. Accordingly, many attempts have been made all over the world to manufacture a high strength steel sheet having excellent low temperature toughness, and a large number of research results and patents have been proposed.

For example, in Japanese Patent Laid-Open Publication No. 53-119219, low-alloy high-strength steels are subjected to quenching and tempering to form microstructures composed of fine ferrite and bainite, can do. Japanese Unexamined Patent Publication No. 52-14486 proposes a method of lowering the coiling temperature to 250 占 폚 in order to produce a high tensile steel sheet excellent in low temperature toughness and Japanese Patent Laid-Open Publication No. 04-325626 discloses a method of improving the low temperature toughness And the like. In addition, Japanese Patent Laid-Open Publication No. 1-15320 discloses a control method of calcium, oxygen and sulfur for improving the low-temperature toughness.

Various methods have been proposed to obtain high strength at the same time as low temperature toughness. However, none of the above methods suggests a method of obtaining an extremely excellent low temperature toughness with a tensile strength of 80 kg / mm < 2 > Particularly, when the low temperature toughness is improved by the heat treatment, the manufacturing cost is higher than that in the case of only the hot rolling, and the production process is complicated when the coiling temperature is lowered or the cooling rate is controlled.

Therefore, in order to avoid such a problem, studies have been made to manufacture a high-strength hot-rolled steel sheet by performing only hot rolling. Among them, for example, Japanese Patent Application Laid-Open Nos. 52-107225, 52-14486, 325626. < / RTI >

The technical idea of the above-mentioned subclause 52-107225 is that the carbon content is 0.03-0.08%, Si <0.6%, Mn is 0.7-2.0%, P <0.010%, S <0.008%, Nb: 0.01-0.10% V: 0.01-0.15%, Mo <0.50%, Ni <1.0%, Cr <1.0% and Cu <1.0% is hot-rolled and finish-rolled at 850-600 ° C and rolled at 680-500 ° C It is characterized by drunkenness. However, this method uses a relatively large amount of alloying elements and at the same time, the tensile strength is only 60 kg / mm 2 or more, though the finish rolling temperature is lowered to about 850-600 ° C. Thus, the maximum effect of the alloying element addition and the adequate rolling process are insufficient Do.

Further, the small-52-14486 call the C: 0.01-0.20%, Si: 0.02-2.0 %, Mn: 0.5-2.5%, Mo: 0.1-2.5% and a degree of addition to the hot rolling, from more than Ar ₃ temperature of 15 ℃ / sec or more and winding it at about 250 ℃, the alloy component is comparatively simple. However, since the coiling temperature is very low at about 250 ℃, it is necessary to have a very large capacity of water cooling equipment and at the same time, Therefore, this is not a suitable method in the hot rolling process in which the material must be wound after hot rolling.

In the above-mentioned Japanese Patent Application No. 04-325626, steel containing 0.03-0.06% of C, 0.5% of Si, 1.3-1.6% of Mn, 0.02-0.06% of Nb, 0.002% of S and 0.0015-0.0050% In hot rolling at a temperature of about Ar ₃ -990 ° C, at least 60% or more of rolling is carried out, water-cooled at a cooling rate of 10 ° C / sec or more and wound in a temperature range of 500-600 ° C to exhibit excellent low temperature toughness . However, the tensile strength level is relatively low, the low temperature toughness is also low, and the hot rolling conditions are not clearly shown. Therefore, it can be seen that the effect of addition of the alloying elements and the optimization of the rolling conditions are not maximized and the economical efficiency of the manufacturing is somewhat poor

.

Accordingly, the present invention provides a needle-shaped ferrite structure in which precipitate particles are finely precipitated by appropriately controlling a steel component, controlling a reduction rate and a finishing temperature during hot rolling, and setting an appropriate coiling temperature, thereby obtaining a ferrite structure having a tensile strength of 80 kg / It is an object of the present invention to provide a method of manufacturing a high-tensile hot-rolled steel sheet excellent in toughness.

1 is a graph showing impact toughness according to changes in hot rolling conditions of the steel of the present invention

Fig. 2 is a graph showing changes in the toughness of DWTT according to changes in hot rolling conditions of the steel of the present invention

In general, it is known that precipitation strengthening in steel is excellent in the strength increase effect, but relatively lowers toughness (particularly low temperature toughness) of steel. However, in the case of precipitation for increasing the strength, the low-temperature toughness generally decreases, but the low-temperature toughness may be largely changed by the change of the microstructure due to the formation of precipitates and the distribution and size of the precipitate itself. Therefore, the present inventor has found a phenomenon in which the low temperature toughness largely changes with the change of the rolling process even with the same component, and has found a method of obtaining a high-tensile hot-rolled steel sheet excellent in strength due to precipitation and texture refinement as well as low temperature toughness .

The present invention, which is based on the above-mentioned viewpoints, is characterized in that it contains 0.03-0.10% of C, 0.01-0.50% of Si, 1.2-2.2% of Mn, 0.015% or less of P, 0.01-0.10% Mo, 0.2-0.7% Mo, 0.1-0.5% Ni, 0.02-0.10% Nb, 0.02-0.10% Ti, 0.01-0.0% N and 0.001-0.005% Ca, Hot rolling in which a steel material consisting of Fe and unavoidable impurities is rolled at a temperature of 750 to 1000 占 폚 in a total reduction ratio (TRR) of 60% or more and a finish rolling temperature (FDT) is carried out at a temperature of 750 to 850 占Wherein the finish rolling temperature and the total rolling reduction ratio satisfy the relation of " TRR ≥0.13 x FDT - 32 ", and then hot rolled, followed by quenching after rolling and winding at a temperature of 500 to 630 캜 .

Next, the reason for limiting the numerical value of the steel component composition of the present invention will be described.

The above C is an economical and effective element for strengthening the steel. However, when added too much, the weldability, formability and toughness are lowered. Therefore, the content is limited to 0.03-0.10% in the present invention. That is, when the addition amount is less than 0.03%, it is not economical because a relatively large amount of other alloying elements must be added in order to exhibit the same strength. When the addition amount is more than 0.10%, not only the weldability, moldability, In the rolling conditions such as the invention, the base structure is formed of needle-like ferrite, and ferrite-pearlite structure is formed from the structure, so that the strength is largely lowered or the upper bainite structure is formed and the toughness is lowered.

The Si is also required for deoxidizing molten steel and also exhibits an effect as a solid solution strengthening element, so it is necessary to add Si in the range of 0.01-0.50%. That is, when the addition amount is less than 0.01%, it is difficult to obtain a clear steel because it does not sufficiently deoxidize the molten steel. If the addition amount exceeds 0.5%, a red scale due to Si is formed during hot rolling, Is also undesirable.

The Mn should be added in an amount of 1.2% or more as an effective element for strengthening the steel, so that it is possible to exhibit high strength in addition to the effect of increasing the incombustibility. However, if it is added in excess of 2.2%, it is not preferable because the segregation part is greatly developed in the thickness of the cast steel during casting in the steel making process and the weldability of the final product is deteriorated.

P is an impurity element present in the steel, which causes center segregation and micro-segregation to occur in the steel to decrease the toughness of the steel. Therefore, if P is less than 0.015%, the adverse effect of P can be canceled The upper limit is set to 0.015%.

The S is also an impurity element present in the steel, and it is preferably combined with Mn or the like to form nonmetallic inclusions, thereby greatly reducing the toughness and strength of the steel. Particularly, in the present invention, Ca is added at the same time to reduce the non-metallic inclusions and to minimize the adverse effect of S by making the shape of the inclusions spherical, and the limit of S addition is set at 0.002%.

Since Al serves as a deoxidizing element to reduce impurities of steel and is effective in suppressing the coarsening of austenite grains, it is necessary to add at least 0.01%, but if it is added in an amount exceeding 0.1%, the inclusions are formed in a large amount, The upper limit was limited to 0.1%, thereby improving the cleanliness of the steel.

However, if the Mo content exceeds 0.5%, the toughness of the welded heat affected zone is lowered and the economic efficiency is lost. Therefore, the upper limit is set at 0.5%, and the value of 0.2 % Is added, it is difficult to form an acicular ferrite structure which is a microstructure excellent in low-temperature toughness under the conditions of the present invention, and at least 0.2% is added because the strength is low.

The Ni serves to improve the toughness of the steel and has an action to prevent cracking during the performance. When the Ni content is less than 0.1%, the hot brittleness suppression is insufficient. When the Ni content exceeds 0.5%, the economical effect , The range of the amount of Ni added was set to 0.1-0.5%.

Nb is a very useful element for refining the crystal grains, and at the same time, at least 0.02% should be added in order to greatly improve the strength of the steel. When the Nb content exceeds 0.10%, Nb carbonitride precipitates, 0.01-0.10% was added as the range of addition.

The Ti is an element which is very useful for refining the crystal grains and exists as TiN in the steel and has an effect of suppressing the growth of the grains during the heating process for hot rolling. Ti remaining in the reaction with nitrogen bonds with the carbon contained in the steel Precipitates of TiC are formed and the formation of TiC is very fine, which greatly improves the strength of the steel. Therefore, at least 0.02% of Ti should be added in order to obtain an effect of inhibiting the growth of austenite grain growth by TiN precipitation and an increase in strength by TiC formation, and when it is added in excess of 0.1%, the steel sheet is welded to the melting point As the TiN is reused due to heat generation, the toughness of the weld heat affected zone deteriorates, so the upper limit of Ti addition is 0.1%.

The reason for limiting the component of N is attributed to the above Ti addition. In general, N is dissolved in the steel and precipitates to increase the strength of the steel, which is much greater than carbon. However, on the other hand, it is known that the more the nitrogen is present in the steel, the lower the toughness of the steel, and it is a general tendency to reduce the nitrogen content as much as possible. However, in the present invention, a proper amount of nitrogen is allowed to react with Ti to form TiN and to inhibit crystal growth during reheating. However, since the part of Ti must react with carbon in the subsequent process without reacting with N, the upper limit of Ti is set to 0.01% or less.

Ca is an inclusion formation control element and improves low temperature toughness and ductility. In order to exhibit such an effect, at least 0.001% or more of addition is necessary, but even if it is added in excess of 0.005%, the effect is not improved.

Hereinafter, a method for manufacturing a hot-rolled steel sheet using the above-described steel will be described in detail.

Generally, the total rolling reduction is carried out at 70 to 99% at 700 to 1150 ° C. In the present invention, in the hot rolling of the steel having the above-mentioned composition, the total reduction ratio (TRR: total reduction ratio ) And the finish delivery temperature (FDT) in the range of 750-850, wherein the relationship between the hot rolling finishing temperature and the total rolling reduction is " TRR ≥ 0.13 x FDT-32 " Is hot rolled.

2. Description of the Related Art [0002] In general, a hot rolling process is divided into two types and is commercially produced. The hot rolling process for manufacturing a coil and the plate process for manufacturing a plate are the same as the process to which the present invention is applied. In the hot rolling process for producing coils, 6-7 rolling mills are continuously installed, so that the difference between the hot rolling finishing temperature and the finishing rolling starting temperature is usually not more than 200 ° C maximum. Therefore, even if the hot rolling finishing temperature is 850 캜 which is the maximum temperature of the present invention, the finish rolling starting temperature is 1050 캜.

In the present invention, Nb and Ti are added in combination, and the precipitation element is precipitated at the finish hot rolling temperature to suppress the recrystallization of austenite, thereby reducing the grain size and increasing the strength by precipitation strengthening. It is preferable to lower the hot rolling temperature as much as possible. Accordingly, in the present invention, the range of the finish hot rolling starting temperature and the finish temperature is set to 750-1000 ° C and the hot rolling finishing temperature is set in the range of 750-850 ° C for rolling in the non-recrystallized region of austenite. If the hot rolling finishing temperature is lower than 750 캜, the material may fall below the austenite / ferrite transformation temperature (Ar ₃ ) during hot rolling. In this case, the microstructure of the material may form a so- The lower limit of the temperature was set at 750 ° C. On the other hand, if the hot rolling finishing temperature is set to be higher than 850 ° C, the finish rolling starting temperature becomes high and recrystallization of austenite is caused. Recrystallization of austenite causes rapid grain growth immediately after heat treatment.

Therefore, in order to improve the toughness of the steel, it is preferable to lower the hot rolling finishing temperature. However, since the load to be applied to the rolling mill increases with the decrease of the rolling temperature, in the present invention, the total rolling reduction and finish rolling temperature , The relation between "TRR ≥ 0.13xFDT-32" was clarified, and an appropriate total reduction rate was set according to the change of finishing rolling temperature.

For example, the high tensile steel plate for a steel pipe generally has a thickness of about 7-22 mm, which is the thickest product in the hot rolling process. Therefore, the total rolling reduction may be less than 60%, but the thickness of the slab A steel sheet having excellent low temperature toughness can be produced by maintaining a total reduction ratio of at least 60%.

However, since the increase in the total reduction rate is impossible due to the rapid increase in the load applied to the rolling mill as described above, it is necessary to closely analyze and optimize the relationship between the rolling temperature and the total reduction rate.

In the present invention, after completion of hot rolling, quenching by water cooling or the like is performed on a run-out table to suppress growth of crystal grains of austenite and prevent coarsening of ferrite, thereby suppressing reduction in toughness. If the coiling temperature is higher than 630 DEG C, the microstructure is not formed into an acicular ferrite, but is changed into a ferrite-pearlite structure and a precipitate becomes coarse to greatly decrease the strength of the material. If the coiling temperature is lower than 500 DEG C The bainite transformation occurs and the strength is increased, but the toughness of the material is greatly lowered and the precipitation is also insufficient.

Hereinafter, the present invention will be described in more detail by way of examples.

Example 1

The inventive steels and comparative steels having chemical compositions as shown in Table 1 below were melted and made into slabs.

Steel grade Chemical composition (wt%) C Si Mn Mo Ni Nb Ti N Ca Comparative River A 0.02 0.24 1.55 0.30 0.18 0.097 0.001 0.0007 0.0022 Comparative Steel B 0.06 0.23 1.53 0.32 0.21 0.098 0.001 0.0040 0.0028 Comparative Steel C 0.07 0.28 1.60 0.01 0.79 0.044 0.025 0.0060 0.0023 Inventive Steel D 0.08 0.23 1.53 0.39 0.22 0.052 0.055 0.0045 0.0021 Invention steel E 0.07 0.25 1.55 0.33 0.20 0.079 0.033 0.0034 0.0022

The comparative steel A in Table 1 has a small carbon content of 0.02% and a very small Ti content of 0.001%. The comparative steel B has a very small Ti addition amount of 0.001% and the comparative steel C has a very small Mo content of 0.01% , And were selected as comparative steels.

The slabs having the above composition were hot rolled to obtain a plate material. At this time, the hot rolling finishing temperature was changed in the range of 750-900 ° C., and the cooling rate in the run-out table after hot rolling was the cooling flow rate in the normal field production process, s. The coiling temperature was varied depending on the type of steel and was controlled in the range of 500-690 ℃. That is, a hot-rolled steel sheet was produced under the conditions of finish rolling temperature (FDT), total rolling reduction (TRR) and coiling temperature (CT) as shown in Table 2 below.

Steel grade FDT (占 폚) The total reduction ratio (TRR),% CT (° C) Remarks Comparative Example 1 Comparative River A 820 75 550 Comparative Steel - Invention Rolling Comparative Example 2 Comparative Steel B 800 74 580 Comparative Steel - Invention Rolling Comparative Example 3 Comparative Steel C 810 76 510 Comparative Steel - Invention Rolling Comparative Example 4 Inventive Steel D 900 85 580 Invention steel - comparative rolling Comparative Example 5 Inventive Steel D 850 65 570 Invention steel - comparative rolling Comparative Example 6 Inventive Steel D 830 76 660 Invention steel - comparative rolling Honorable 7 Inventive Steel D 820 80 575 Invention steel - Invention rolling Honors 8 Inventive Steel D 780 72 600 Invention steel - Invention rolling Comparative Example 9 Invention steel E 900 85 560 Invention steel - comparative rolling Comparative Example 10 Invention steel E 800 60 530 Invention steel - comparative rolling Comparative Example 11 Invention steel E 850 80 690 Invention steel - comparative rolling Inventory 12 Invention steel E 850 80 600 Invention steel - Invention rolling Inventory 13 Invention steel E 800 73 560 Invention steel - Invention rolling

As shown in Table 2, the comparative examples (1) to (3) were hot rolled under the conditions of the present invention using comparative steels, while the other comparative examples and inventive examples were obtained by hot rolling . That is, in Comparative Examples (4) and (9), the hot rolling finish temperatures of the inventive steel D and the inventive steel E were higher than those of the present invention. Comparative Examples (5) and (10) (6) and (11) are higher than the coiling temperature in the range of the present invention.

The tensile properties, impact toughness and microstructure of the hot-rolled steel sheet thus obtained were measured and observed. The results are shown in Table 3 below. At this time, the tensile test was a standard test with a gauge distance of 50 mm, and the impact test was performed with a Charpy sub-size of 7.5 mm.

Yield strength (YS) (kg / ㎟) Tensile strength (TS) (kgf / mm2) Total elongation (t.E1) (%) Impact Toughness (vE _{-60 ° C} ) (Joules) Shock Transition Temperature (℃) Microstructure Comparative Example 1 64.8 70.7 25.9 95 -70 Needle ferrite Comparative Example 2 72.5 77.5 24.8 25 -25 Needle ferrite Comparative Example 3 67.3 74.2 24.5 65 -40 Ferrite + Bainite Comparative Example 4 67.3 88.5 19.4 55 -55 Needle ferrite Comparative Example 5 69.5 86.2 20.9 55 -65 Needle ferrite Comparative Example 6 63.2 78.4 24.9 70 -50 Ferrite + Pearlite Honorable 7 74.2 92.0 19.1 94 -75 Needle ferrite Honors 8 70.3 86.9 22.3 108 -100 Needle ferrite Comparative Example 9 68.4 87.7 23.2 50 -50 Needle ferrite Comparative Example 10 71.9 91.2 18.4 75 -60 Needle ferrite Comparative Example 11 62.9 77.2 25.7 65 -55 Ferrite + Pearlite Inventory 12 71.5 84.5 20.7 101 -85 Needle ferrite Inventory 13 73.4 85.8 21.2 105 -95 Needle ferrite

By analyzing the contents as shown in Table 3, the following results were obtained.

In the comparative example (1), other chemical compositions are similar to those of the present invention. However, since the addition amount of C and Ti is small, the tensile strength is small compared to the steel of the present invention even if rolling and cooling conditions are within the scope of the invention. It can be seen that the strengthening effect dramatically improves the strength of the material.

In Comparative Example (2), the comparative steel B having a small amount of Ti added was rolled in the range of the present invention, and the tensile strength was increased to 77.5 kg / mm 2. However, it is still below the target tensile strength value of 80 kg / mm &lt; 2 &gt; in the present invention, and the yield ratio (yield strength / tensile strength) is very high as 0.94 due to excessive precipitation hardening due to Nb alone, It can be seen that the temperature is only -25 ° C. From this, it can be seen that precipitation hardening by proper combination of Nb and Ti can maximize the strength without causing deterioration of impact toughness.

The comparative example (3) is a case in which the comparative steel C having a small Mo addition amount is rolled in the range of the present invention, and the microstructure is mixed with ferrite and bainite. Accordingly, the tensile strength is low, Transition temperature. If Mo is not added, the austenite / ferrite transformation temperature becomes high and the ingot strength decreases, and the strength of the material depends on the strengthening of the transformation. Therefore, in the case of high temperature winding, ferrite + pearlite structure is formed and the strength is low. Even if the strength is increased, the low temperature impact toughness is very bad.

Comparative Example (4) and Comparative Example (9) are cases in which the manufacturing conditions were changed using the inventive composite steel. Comparative Example (4) in which all the other rolling conditions in Inventive Steel D were in the scope of the present invention and only the finish rolling temperature was increased to 900 占 폚 and Comparative Example (9) in which only the finish rolling temperature was elevated to 900 占 폚 Because of the high finishing temperature, the grain size of the ferrite is coarsened, so the strength is high but the impact toughness is lowered and the impact transition temperature is only -50 to -55 ° C.

In Comparative Examples (5) and (10), the finish rolling temperature and the coiling temperature were in the range of the present invention, and the total rolling reduction was made to be smaller than the inventive conditions. The tensile strength and the yield strength were similar to those of the present invention However, since the cumulative rolling reduction in the non-recrystallized region is relatively small, the size of the needle-shaped ferrite is relatively large, the impact absorption energy at -60 ° C. is only 55 and 75 joules, and the impact transition temperature is also -60 to -65 ° C. .

Comparative Example (6) and Comparative Example (11) were produced only at the coiling temperature outside the range of the present invention. Since the coiling temperature was high, the microstructure was composed of ferrite + pearlite instead of acicular ferrite, The tensile strength is lower than that of the present invention, and the impact toughness is also low.

On the other hand, in the case of the inventive examples (7) (8) (12) and (13) using the inventive steel under the manufacturing conditions of the present invention, the yield strength was 70.3-74.2 kg / 92.0kg / ㎟, the impact strength at -60 ℃ is 94-108 joules, and the impact transition temperature is -75 ~ -100 ℃. These tendencies are all the same in the steel composition of the present invention.

In conclusion, it can be concluded that the change of the mechanical properties of the material due to the change of the rolling condition is due to the change of the microstructure and the change of the precipitate. First, microstructures are changed to bainite, needle-like ferrite, ferrite-pearlite, etc. These changes in microstructure lead to a decrease in strength and an improvement in impact toughness. Therefore, in order to maximize the strength and impact toughness of the material, the microstructure can be miniaturized and the size of the precipitate can be made appropriate.

Example 2

The steel sheet D shown in Example 1 was used to obtain a hot-rolled steel sheet while changing the hot rolling conditions as shown in the following (a), (b) and (c). (a) shows FDT: 780 DEG C, TRR: 72%, CT: 600 DEG C, (b) FDT: 820 DEG C, TRR: 80%, CT: 575 DEG C, %, CT: 580 占 폚.

The obtained hot-rolled steel sheet was subjected to an impact toughness test while changing the test temperature. As a result of measurement, the Charpy V notch impact absorption energy (vE) at -60 ° C was 108 J, (b) 94 J and (c) 55 J, and the V notch impact transition temperature (a) , (b) was -75 ° C, and (c) was -55 ° C. In addition, the impact absorption energy according to the test temperature was measured, and the results are shown in Fig. As shown in Fig. 1, when the relationship between the hot rolling finishing temperature and the total rolling reduction ratio satisfies the relation of " TRR? 0.13xFDT-32 " . &Lt; / RTI &gt;

FIG. 2 is a graph comparing the crack propagation degree of a material by DWTT (Drop Weight Tear Test), which is another test method showing impact toughness using the invention steel D as described above. (85% SA; Shear (85% SA)) in the case where the TRR is 75%, the FDT is 785 ° C, and the CT is 570 ° C and rolling according to the present invention satisfies the condition TRR? 0.13xFDT- area was -75 캜. On the contrary, (f) shows a case where the TRR is 65%, the FDT is 825 ° C and the CT is 580 ° C, and the condition TRR≥0.13xFDT-32 is not satisfied. Lt; RTI ID = 0.0 &gt; 45 C. &lt; / RTI &gt;

Thus, when rolled under the conditions of the present invention, high tensile strength steel having excellent low temperature toughness having a tensile strength of 80 kg / mm 2 or more and a impact transition temperature of -75 캜 or less can be produced.

As described above, according to the present invention, it is possible to form a structure of needle-like ferrite in which fine precipitates are distributed in a matrix by controlling the chemical composition of the steel and controlling the hot rolling and winding conditions. By controlling the rolling conditions, A high-strength hot-rolled steel sheet which can obtain a high strength due to the formation and precipitation of a high-strength hot-rolled steel sheet and exhibits an extremely excellent low-temperature toughness can be obtained. In addition, in manufacturing the steel sheet, relatively accurate hot rolling conditions can be used to improve the rolling productivity, and since the weldability is very excellent, it is possible to improve the yield and productivity at the time of manufacturing the parts, do.

Claims (1)

The steel sheet according to any one of claims 1 to 3, wherein the steel has a composition of C: 0.03-0.10%, Si: 0.01-0.50%, Mn: 1.2-2.2%, P: 0.015% A steel material containing 0.1-0.5% of Ni, 0.02-0.10% of Nb, 0.02-0.10% of Ti, 0.01% or less of N and 0.001-0.005% of Ca and the balance of Fe and unavoidable impurities is heated at 750-1000 ° C (TRR) is performed at a temperature of 750 to 850 DEG C, and the finish rolling temperature (FDT) is carried out at a temperature of 750 to 850 DEG C, wherein the finish rolling temperature and the total rolling reduction Is subjected to hot rolling after satisfying the relation of " TRR? 0.13 x FDT - 32 ", followed by quenching after rolling and winding at a temperature of 500-630 占 폚.