KR20090070484A - High-strength and high-toughness thick steel plate and method for producing the same - Google Patents

Abstract

A high-strength and high-toughness thick steel sheet and a manufacturing method thereof are provided to obtain high toughness and strength while reducing the manufacturing cost by performing air cooling or cooling in the austenite partial recrystallization area. A high-strength and high-toughness thick steel sheet comprises C 0.04~0.1 weight%, Si 0.1~0.4 weight%, Mn 1.3~1.8 weight%, Mo 0.05~0.5 weight%, Cr less than 1.0 weight%, Ni less than 1.0 weight%, Ti 0.005~0.03 weight%, Nb 0.02~0.06 weight%, V less than 0.1 weight%, Cu less than 1.0 weight%, Ca less than 0.006 weight%, N 0.001~0.006 weight%, P less than 0.02 weight%, S less than 0.005 weight%, and the rest Fe and inevitable impurities. The internal structure of the steel sheet is composed of acicular ferrite 60%, polygonal ferrite less than 10%, and the rest bainite.

Description

High strength high toughness steel plate and its manufacturing method {HIGH-STRENGTH AND HIGH-TOUGHNESS THICK STEEL PLATE AND METHOD FOR PRODUCING THE SAME}

The present invention relates to a steel sheet used for the use of line pipes, building structures and marine structures and the like, and more particularly to a thick high-strength tough steel sheet and a method for producing the same.

In the case of pipelines, in order to increase operational efficiency, it is necessary to increase the amount of crude oil or gas that can be transported per unit time, which inevitably needs to improve the strength of steel.

It is essential to secure low temperature toughness due to the cooling of oil and gas mining areas. In the case of building structures and offshore structures, the demand for high strength and high toughness steels is increasing due to the increasing demand for large structures and the severity of operating conditions (operating temperature, connection structure, etc.).

Increasing the strength of a material, on the contrary, tends to reduce toughness. This tendency is because the alloying elements usually added play a contradictory role in inhibiting toughness even though they have a favorable effect on strength.

In order to solve this problem, it is possible to improve the strength and toughness by suppressing the adjustment of the element as much as possible, and to improve the toughness by reducing the internal grains, a method called TMCP (Thermo Mechanical Controlling Process), to form a hard structure. Many methods have been used to improve strength.

The TMCP method is a general term for the processing method for changing the physical properties of the steel sheet to the desired physical properties by applying thermal history at the same time as mechanical processing for the steel sheet, but is used in a very large number of forms, mainly rolling under strict conditions at a predetermined temperature It consists of a controlled rolling process and an accelerated cooling process that cools the steel plate at an appropriate range of cooling rates.

When using the TMCP method, there is an advantage in that the desired physical properties can be smoothly controlled to a certain extent in theory by forming fine grains in the steel sheet and controlling the structure in a desired form.

However, in order to produce a steel sheet having a desired strength through the accelerated cooling of the TMCP as described above, it is necessary to form a hard structure as in the prior art. Therefore, even in the case of steel sheet manufactured by the TMCP method, it is inevitable in the prior art that the toughness is in the tendency to decrease as the strength increases.

Therefore, in the field of high strength steel, efforts have been made to secure a means for securing low temperature toughness while continuously conducting research and development to increase the strength level of steel.

In order to improve the low temperature toughness in TMCP, the heating temperature is reduced, grain refinement through austenite recrystallization, and transformation of nucleation sites through rolling in the austenite uncrystallized region are utilized.

Usually, in order to roll in an austenite uncrystallized area | region after rolling in an austenite recrystallization area | region, air or cooling is performed in an austenite partial recrystallization area | region.

However, the austenite partial recrystallization occurs while the austenite grains refined and homogenized through rolling in the austenite recrystallization region pass through air cooling or cooling in the austenite partial recrystallization region. This encourages the disproportionation of austenite grains and adversely affects low temperature toughness. In addition, the production time is increased due to the time required for air cooling or room cooling, which may cause a rise in manufacturing cost.

The present invention is to solve the above problems of the prior art, to suppress the partial recrystallization occurring between the rolling in the austenitic recrystallization region and the austenite uncrystallized region to obtain a uniform austenite structure to obtain a high toughness characteristics In addition to having a low manufacturing cost of the steel sheet and to provide a method for manufacturing the same, the purpose is.

EMBODIMENT OF THE INVENTION Hereinafter, this invention is demonstrated.

In the present invention, C: 0.04 to 0.1%, Si: 0.1 to 0.4%, Mn: 1.3 to 1.8%, Mo: 0.05 to 0.5%, Cr: 1.0% or less, Ni: 1.0% or less, Ti: 0.005 to 0.03%, Nb: 0.02 to 0.06%, V: 0.1% or less, Cu: 1.0% or less, Ca: 0.006% or less, N: 0.001 to 0.006%, P: 0.02% or less, S: 0.005% or less, balance Fe and It contains other inevitable impurities, and the internal structure is composed of 60% or more acicular ferrite, 10% or less polygonal ferrite, and the remaining bainite. The present invention relates to a tough steel sheet.

Al may be added to the steel sheet in an additional 0.01 to 0.05% as needed.

In addition, the present invention by weight% C: 0.04 ~ 0.1%, Si: 0.1 ~ 0.4%, Mn: 1.3 ~ 1.8%, Mo: 0.05 ~ 0.5%, Cr: 1.0% or less, Ni: 1.0% or less, Ti: 0.005 to 0.03%, Nb: 0.02 to 0.06%, V: 0.1% or less, Cu: 1.0% or less, Ca: 0.006% or less, N: 0.001 to 0.006%, P: 0.02% or less, S: 0.005% or less, balance Heating the steel slab composed of Fe and other unavoidable impurities in the range of 1050 to 1180 ° C;

Rolling once or multi-step rolling at a reduction ratio of 20 to 80% in a temperature range of at least austenite recrystallization temperature;

At a cooling rate of 5 ~ 10 ℃ / sce at the austenite partial recrystallization temperature, the steel sheet is cooled to a temperature below the austenite microcrystallization temperature and finish rolling to a reduction ratio of 60 to 80% at a temperature below the austenite microcrystallization temperature Manufacturing a steel sheet; And

It relates to a method for producing a thick high-strength tough steel sheet, characterized in that it comprises the step of cooling the steel sheet to a temperature range of 400 ~ 600 ℃ at a cooling rate of 10 ~ 50 ℃ / sec.

Al may be added to the steel slab in an additional 0.01 to 0.05% as needed.

In addition, after cooling to the temperature range of 400 ~ 600 ℃ of the steel sheet it is effective to air-cooled or allowed to cool the steel sheet.

As described above, according to the present invention, the partial recrystallization occurring between the rolling in the austenite recrystallization region and the austenite microcrystallization region is suppressed to obtain a uniform austenite structure, thereby having high toughness and high strength. Steel sheet with low manufacturing cost can be provided.

Hereinafter, the present invention will be described in detail.

The inventors of the present invention have studied in depth the method for solving the problems of the prior art, it was confirmed that the low temperature toughness of the steel sheet can be significantly improved by maintaining the uniformity of the austenite grains in the austenite recrystallization region The present invention has been reached.

Hereinafter, the composition, internal structure and manufacturing method of the steel sheet of the present invention for achieving the above object will be described in detail.

(Composition of steel sheet)

In the present invention, the composition of the steel sheet as a target was selected as follows in order to have sufficient strength and welded part toughness.

C: 0.04 ~ 0.10 wt%

C is the most effective element for strengthening the matrix of metals and welds through solid solution strengthening, and can obtain a strengthening effect by precipitation hardening by formation of small cementite, V and Nb carbonitride and Mo carbide.

In addition, Nb carbonitride can simultaneously improve strength and low temperature toughness through grain refinement by inhibiting austenite recrystallization and preventing grain growth during hot rolling.

The C also serves to improve the hardenability, which is the ability to form a strong microstructure inside the steel sheet during cooling.

In general, when the content is less than 0.04% by weight, this reinforcing effect is not obtained, and when the content is more than 0.1% by weight, the toughness of the base metal and the heat affected zone including the low temperature crack after welding is degraded.

Si: 0.1 ~ 0.4 wt%

Si plays a role of deoxidizing molten steel by assisting Al and also has an effect as a solid solution strengthening element. When the Si is excessively added to 0.4 wt% or more, a red scale scale by Si is formed during rolling, and thus the steel sheet surface shape is very bad, and the weldability and toughness of the weld heat affected zone are greatly reduced.

Mn: 1.3 ~ 1.8% by weight

Mn is an effective element to solidify the steel to be added at least 1.3% by weight can exhibit high strength with the effect of increasing the hardenability. However, the addition of more than 1.8% by weight promotes central segregation and lowers toughness when casting slabs in the steelmaking process. In addition, the addition of excessive Mn excessively improves the curing ability, worsens the field weldability, thereby lowering the toughness of the weld heat affected zone.

Mo: 0.05 ~ 0.5 wt%

Mo improves the hardenability, especially when added together with Nb, inhibits austenite recrystallization and contributes to grain refinement. However, the addition of excessive Mo lowers the toughness of the weld heat affected zone during the field welding, so it should be maintained at 0.5% by weight or less, and considering the high cost, it is desirable to keep it at 0.10% by weight or less.

Cr: 1.0 wt% or less

Cr plays a role of improving the hardenability. However, the addition of excessive Cr causes low temperature cracks after welding in the field, thereby lowering the toughness of the base metal and the heat affected zone of the weld. Therefore, the Cr content should be kept below 1.0 wt%.

Ni: 1.0 wt% or less

Ni is an element that improves physical properties without losing on-site weldability and low temperature toughness in low carbon steels.

Compared with Mn and Mo, Ni forms less hard phases such as phase martensite, which lowers the low temperature toughness, and improves the toughness of the weld heat affected zone.

In addition, it suppresses the occurrence of surface cracking in Cu-added steel during continuous casting and hot rolling. However, Ni is an expensive element and excessive addition of Ni lowers the toughness of the weld heat affected zone.

Therefore, the content of Ni is preferably limited to 1.0% by weight or less.

Ti: 0.005 ~ 0.03 wt%

Ti contributes to grain refinement by forming fine Ti nitride (TiN) to suppress austenite grain coarsening during slab heating. In addition, TiN not only prevents grain coarsening in the weld heat affected zone, but also improves toughness by removing N in molten steel.

In order to sufficiently remove N, Ti must be at least 3.4 times the amount of N added.

Therefore, Ti is a very useful element to refine the strength and grains of the base metal and the weld heat affected zone, and is present as TiN in the steel to inhibit the growth of grains during heating for rolling. The solid solution in the steel combines with carbon to form precipitates of TiC, and the formation of TiC is very fine, greatly improving the strength of the steel.

When the amount of Al added is very small, Ti oxide is formed to act as a nucleation site of intragranular acicular ferrite in the weld heat affected zone.

Therefore, it is necessary to add at least 0.005% by weight or more in order to obtain the austenite grain growth inhibition effect by TiN precipitation and the strength increase by TiC formation.

On the other hand, when 0.03% by weight or more is added, coarsening of Ti nitride and hardening by Ti carbide are excessively harmful to low temperature toughness. Since the toughness of the affected portion deteriorates, the upper limit of Ti addition is made 0.03% by weight.

Nb: 0.02 ~ 0.06 wt%

Nb plays a role of simultaneously improving strength and toughness through grain refinement.

Nb carbonitride produced during hot rolling suppresses austenite recrystallization and prevents grain growth, thereby making fine austenite grains.

When added together with Mo, the austenite recrystallization is suppressed to increase the grain refining effect, the reinforcing effect through precipitation enhancement and hardenability is more pronounced. In order to obtain such an effect, it should be contained at least 0.02% by weight. However, when added in excess of 0.06% by weight, it is difficult to expect the effect to increase any more, and due to excessive precipitation of Nb carbonitride, the austenite microcrystallization temperature is too high. This adversely affects the weldability and the toughness of the weld heat affected zone.

V: 0.1 wt% or less

V plays a similar role to Nb, but the effect is somewhat weaker than Nb. However, the effect is greatly magnified when Nb and V are added together. However, considering the toughness and weldability of the weld heat affected zone, the upper limit thereof is made 0.1 wt%.

Al: 0.01 ~ 0.05 wt%

Al is generally added for the purpose of deoxidation of the steel. Further, not only the microstructure is fine but also the toughness of the heat affected zone is improved by removing N from the coarse grain region of the weld heat affected zone.

However, when contained in excess of 0.05% by weight, Al oxide (Al ₂ O ₃ ) is formed to reduce the toughness of the base metal and the heat affected zone. Since deoxidation can be carried out through addition of Ti and Si, Al is not necessarily added.

Cu: 0 ~ 1.0 wt%

Cu is an element to strengthen the base metal and the weld heat affected zone. However, excessively added Cu lowers the toughness and field weldability of the weld heat affected zone.

The content of Cu is preferably limited to 0 to 1.0% by weight.

Ca: 0.006 wt% or less

Ca is an element mainly added to control the shape of MnS inclusions and to improve low temperature toughness. However, excessive Ca addition causes a large amount of CaO-CaS to form and combine to form coarse inclusions, thereby degrading the cleanliness of the steel and damaging the field weldability.

Therefore, the Ca content is preferably limited to 0.006% by weight or less.

N: 0.001 ~ 0.006 wt%

N suppresses austenite grain growth during slab heating, and TiN precipitate suppresses austenite grain growth of the weld heat affected zone. Excessive N addition, however, promotes slab surface defects and, in the presence of solutes, reduces the toughness of the matrix and weld heat affected zones.

Therefore, the content of N is limited to 0.001 to 0.006% by weight.

P: less than 0.02% by weight

P is combined with Mn to form non-metallic inclusions, which causes the problem of embrittlement of steel. Therefore, it is necessary to actively reduce P. However, in order to reduce P to an extreme, the steelmaking process load is intensified and the above problem is greatly caused at 0.02% by weight or less. Since it is not, the upper limit is made into 0.02 weight%.

S: 0.005 wt% or less

S combines with Mn to form a non-metallic inclusion to embrittle steel and causes red-brittle brittleness. Like S, P is limited to an upper limit of 0.005% by weight in consideration of steelmaking process load.

(Internal organization)

As a steel sheet having the above-described component system, it is necessary to further limit the type and shape of the internal structure as preferable conditions for forming a steel sheet having excellent strength and excellent deformability.

Internal tissue is characterized in that it comprises 60% or more of acicular ferrite, less than 10% of polygonal ferrite and the remaining bainite.

In addition to the internal structure of the above form, some M & A (martensite austenite bonding phase) may be formed. Since the cause of the M & A toughness is inhibited, the content thereof may be 10% or less based on the area fraction.

As described above, the steel sheet having a component system and satisfying the internal structure conditions is a steel sheet which satisfies all properties desired in the present invention as having a yield strength of 500 MPa or more and Charpy impact absorption energy at -60 ° C. or more.

(Manufacturing method)

The most preferred method elicited by the present inventors for producing the steel which satisfies the object of the present invention as described above is described below.

In the manufacturing method of the present invention, after heating the slab in general, the heated slab is subjected to one or two or more multi-stage rollings in the austenite recrystallization zone, and then at a temperature of 5 to 10 ° C. at the temperature at which the austenitic partial recrystallization occurs. After cooling at the cooling rate of sec, after finishing two or more multi-stage rolling at a temperature lower than the austenite recrystallization temperature, the cooling is performed at a speed of 10 to 50 ° C / sec and the cooling is terminated at 400 to 600 ° C. Is done. It is preferable to air-cool or to cool a steel plate below the said cooling end temperature.

Hereinafter, detailed conditions of each step will be described.

Slab heating: 1050 ~ 1180 ℃

The heating process of the slab is a process of smoothly performing the subsequent rolling process and heating the steel to sufficiently obtain the properties of the target steel sheet, so the heating process should be performed within an appropriate temperature range according to the purpose.

What is important in the heating process is that not only the precipitated elements inside the steel sheet should be heated uniformly so as to be sufficiently dissolved, but also the maximum protection against excessive coarsening of grains due to too high a heating temperature is important.

If the heating temperature of the steel is less than 1050 ° C., Nb or V may not be re-used in the steel, making it difficult to achieve high strength of the steel sheet, and partial recrystallization may occur, causing austenite grains to be unevenly formed, making it difficult to toughen. When the temperature exceeds 1180 ° C., the austenite grains are excessively coarse, thereby providing a cause of increasing the grain size of the steel sheet, and as a result, the toughness of the steel sheet is extremely deteriorated. Therefore, it is preferable that the suitable heating temperature range is 1050 to 1180 ° C.

Rolling condition

In order for the steel sheet to have low temperature toughness, the austenite grains must be present in a fine size, which is possible by controlling the rolling temperature and the reduction ratio.

In the present invention, the rolling is preferably carried out in two temperature ranges, the recrystallization behavior is different in the two temperature ranges, it is preferable to set the conditions respectively. First, 20-80% of the initial slab thickness is subjected to one rolling or two or more multistep rolling in the austenite recrystallization region.

Rolling in the austenitic recrystallization area as described above has the effect of reducing the grain size through austenite recrystallization, in the case of multi-stage rolling to reduce the reduction rate and time of each step so that grain growth does not occur after austenite recrystallization Control.

Fine austenite grains formed by the above-described process serves to improve the toughness of the final plate. After rolling in the austenitic recrystallization zone, the slab is cooled at a cooling rate of 5-10 ° C./sec in the austenitic partial recrystallization zone.

This is to maintain the effect of the rolling carried out in the austenite recrystallization region.

If the cooling rate is less than 5 ℃ / sec austenite may be partially recrystallized, if it exceeds 10 ℃ / sec temperature deviation between the surface portion and the center may occur.

Thereafter, two or more multi-stage rollings are performed at or below the austenite uncrystallized temperature (T _nr ) region. At this time, 60 to 80% of the thickness of the slab that has been rolled in the recrystallization temperature range is rolled, and the rolling is finished at an Ar ₃ temperature (temperature of transformation from austenite to ferrite). Rolling between T _nr and Ar ₃ (the temperature of austenite-to-ferrite transformation) distorts the grains and develops dislocations within the grains, acting as nucleation sites that form needle-shaped ferrites during cooling. .

Cooling Speed: 10 ~ 50 ℃ / sec

Cooling rate is an important factor to improve the toughness and strength of the steel sheet. This is because the faster the cooling rate, the finer the grains of the internal structure of the steel sheet can be to improve the toughness, and the hard structure can be developed therein to improve the strength. If the cooling rate is less than 10 ° C / sec, coarse ferrites formed during cooling are mixed, which is disadvantageous in strength and toughness. Therefore, the cooling rate of the steel sheet after rolling should be at least 10 ℃ / sec to produce a steel sheet with improved toughness and strength.

On the contrary, when cooling at a cooling rate exceeding 50 ° C./sec, the steel sheet is subjected to the limitations of the cooling water amount control through the water cooling facility due to the characteristics of the steel sheet of the present invention. Warping occurs, resulting in poor shape control.

Cooling end: 400 ~ 600 ℃

In order to control the internal structure of the steel sheet, it is necessary to cool it to a temperature at which the effect of the cooling rate is sufficiently manifested. If the cooling stop temperature, which is the temperature at which the cooling stops, exceeds 600 ° C, it is difficult to sufficiently form fine grains and bainite phase inside the steel sheet, so the upper limit of the cooling stop temperature needs to be limited to 600 ° C.

However, when the cooling stop temperature is less than 400 ℃ not only saturation effect but also plate distortion due to over cooling can occur.

After cooling to the temperature range of 400-600 degreeC of the said steel plate, it is effective to air-cool or to cool a steel plate.

The rolling and cooling process of the present invention described above is shown in FIG. 1 with the conventional method.

1, curve A shows the conventional method, and curve B shows the method of this invention.

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

(Example)

A slab having a thickness of 300 mm was manufactured with the composition shown in Table 1 below, followed by heating-rolling-cooling under the manufacturing conditions shown in Table 2 to prepare a steel sheet having a thickness of 30 mm.

In the following Table 1, the invention steel A to invention steel D is a case of satisfying all the conditions of the present invention, and the comparative steel E to the invention steel I is a case out of the conditions of the present invention. Comparative steel E corresponds to a case where C is too low and comparative steel F corresponds to a case where C is excessively high. In addition, comparative steel G is the case where Mn is excessively high, and comparative steel H is the case where Ti is excessively high.

Inventive steels A1 to D1 shown in Table 2 below satisfy all of the alloy composition and manufacturing conditions of the present invention, and Comparative steels A2 to A7 represent alloy compositions of Inventive Steel A shown in Table 1 above, which satisfy the alloy composition of the present invention. It is a case where eggplant is not satisfied but the manufacturing conditions of the present invention. Comparative steels E1 to H1 are cases where the production conditions of the present invention are applied to slabs having alloy compositions of comparative steels E to H in Table 1.

In the following Table 2, in the case of the invention steel A1 to D1 is to satisfy all the conditions of the present invention, in the case of comparative steel A2 slab reheating temperature is excessively high, in the case of comparative steel A3 slab reheating temperature is When too low, in case of comparative steel A4, the cooling rate in the partial recrystallization zone is too low, in comparison steel A5, when the cooling rate is too low, in comparison steel A6, when the recrystallization reduction rate is too low, in comparison steel A7 The cooling stop temperature is too high.

Taking a portion of the steel sheet prepared as described above, the tensile test, Charpy (Charpy) impact test at -60 ℃ was performed to measure the tensile properties and impact absorption energy, the results are shown in Table 2 together.

division C Si Mn Mo Cr Ni Ti Nb V Al Cu Ca * N * P * S * Invention steel A 0.065 0.20 1.44 0.18 0.33 0.42 0.012 0.035 0.053 0.038 0.44 30 32 70 13 B 0.048 0.35 1.64 0.09 0.23 0.35 0.025 0.042 0.032 - 0.32 21 32 61 12 C 0.052 0.25 1.68 0.22 0.48 0.28 0.027 0.034 0.064 - 0.42 12 44 63 9 D 0.076 0.31 1.71 0.36 0.64 0.54 0.015 0.028 0.073 0.044 0.21 24 45 56 15 Comparative steel E 0.032 0.28 1.58 0.25 0.58 0.44 0.025 0.042 0.042 0.032 0.42 22 34 62 14 F 0.128 0.21 1.71 0.32 0.35 0.52 0.028 0.035 0.043 0.023 0.33 18 44 56 15 G 0.073 0.27 2.04 0.28 0.18 0.24 0.026 0.033 0.063 0.043 0.26 13 46 72 15 H 0.069 0.35 1.65 0.32 0.46 0.52 0.041 0.054 0.054 0.033 0.35 25 35 63 14

However, the content units of the * elements in the table is ppm, and the content units of the remaining elements are weight percent.

division Slab heating temperature (℃) Recrystallization rate reduction rate (%) Partial Recrystallization Station Cooling Rate (℃ / sec) Undetermined rolling reduction rate (%) Cooling rate (℃ / sec) Cooling stop temperature (℃) AF (%) PF (%) B (%) Yield strength (MPa) vE _{-60 ℃} (%) Invention steel A One 1156 64 8.2 72 12.8 452 72 7 21 525 361 B One 1126 69 5.8 68 27.8 421 82 8 10 510 420 C One 1142 62 6.4 74 16.5 555 83 8 9 523 396 D One 1162 58 7.2 76 17.6 456 78 8 14 545 338 Comparative steel A 2 1195 62 5.4 74 32.4 483 63 9 28 563 186 A 3 1034 60 7.5 75 26.4 434 68 9 23 423 395 A 4 1148 62 3.8 74 25.6 435 67 9 24 492 230 A 5 1152 58 7.5 76 8.7 456 52 34 14 454 138 A 6 1144 78 6.2 55 25.3 575 45 47 8 432 168 A 7 1133 57 6.3 77 35.6 638 76 24 - 463 246 E One 1134 60 7.4 75 26.5 423 72 10 18 421 450 F One 1155 70 8.8 67 18.5 432 76 8 16 593 124 G One 1144 57 7.2 77 19.4 553 73 8 19 514 81 H One 1154 58 8.8 76 16.8 472 64 8 28 525 56

Here, AF is a fraction of acicular ferrite, PF is a fraction of polygonal ferrite, B is a fraction of bainite, and vE _{-60 ° C} is Charpy impact absorption energy at -60 ° C.

As shown in Table 2, in the case of the invention steel having the steel composition and manufacturing conditions that are limited in the present invention, both the yield strength of 500MPa or more, and when looking at the fracture toughness, Charpy shock absorption at -60 ℃ It can be seen that energy represents a high value of 300J or more.

 In the case of Comparative Steel A2, the slab reheating temperature is excessively high, and the high solidity of the alloying element causing the solid solution strengthening effect is high, but the strength is sufficiently high, but the grain size of austenite when extracted from the furnace is poor. On the other hand, even after rolling in the austenite recrystallization region, the size of the austenite grains was not so small that the Charpy impact absorption energy was too low.

Comparative steel A3 shows that when the slab reheating temperature is too low, the crystal size of the initial austenite is fine when the furnace is extracted, and thus the Charpy impact absorption energy is high, but the strength of the alloy element is insufficient due to insufficient solidification effect.

In the case of comparative steel A4, the cooling rate in the partial recrystallization zone is too low. The uniform austenite grain at the austenite recrystallization temperature causes partial recrystallization during cooling, resulting in low uniformity of austenite grains, resulting in low Charpy impact absorption energy. It was.

Comparative steel A5 was too low in cooling rate to form needle-shaped ferrite properly and formed a large amount of polygonal ferrite, indicating low strength and low Charpy impact absorption energy.

Comparative steel A6 exhibited a poor value for both strength and Charpy impact absorption energy because the unrecrystallization reduction rate was too low, and there was a lack of nucleation sites that caused needle-like ferrite transformation during cooling.

Comparative steel A7 exhibited low yield strength and Charpy impact toughness due to excessive cooling stop temperature, which prevented formation of low temperature transformation phase.

On the other hand, Comparative steel E is a case where the C is too low, the toughness is excellent but the strength is very inferior, uniform elongation and work hardening index was very low.

Comparative steels F, G, and H were excessively high in C, Mn, and Ti, respectively, and the strength was satisfactory, but the Charpy impact absorption energy was poor.

Therefore, the effect of the manufacturing method which concerns on this invention was confirmed.

1 is a schematic view showing a rolling and cooling process of the conventional method (A) and the inventive method (B).

Claims (5)

By weight%, C: 0.04 to 0.1%, Si: 0.1 to 0.4%, Mn: 1.3 to 1.8%, Mo: 0.05 to 0.5%, Cr: 1.0% or less, Ni: 1.0% or less, Ti: 0.005 to 0.03% , Nb: 0.02 to 0.06%, V: 0.1% or less, Cu: 1.0% or less, Ca: 0.006% or less, N: 0.001 to 0.006%, P: 0.02% or less, S: 0.005% or less, balance Fe and other unavoidable Thick high strength toughness steel sheet, characterized in that it contains impurities, and the internal structure consists of 60% or more of acicular ferrite, 10% or less of polygonal ferrite, and the remaining bainite. The high strength and high toughness steel sheet according to claim 1, wherein Al is further contained in an amount of 0.01 to 0.05%. By weight%, C: 0.04 to 0.1%, Si: 0.1 to 0.4%, Mn: 1.3 to 1.8%, Mo: 0.05 to 0.5%, Cr: 1.0% or less, Ni: 1.0% or less, Ti: 0.005 to 0.03% , Nb: 0.02 to 0.06%, V: 0.1% or less, Cu: 1.0% or less, Ca: 0.006% or less, N: 0.001 to 0.006%, P: 0.02% or less, S: 0.005% or less, balance Fe and other unavoidable Heating the steel slab made of impurities contained in the range of 1050 to 1180 ° C; Rolling once or multi-step rolling at a reduction ratio of 20 to 80% in a temperature range of at least austenite recrystallization temperature; At a cooling rate of 5 ~ 10 ℃ / sce at the austenite partial recrystallization temperature, the steel sheet is cooled to a temperature below the austenite microcrystallization temperature and finish rolling to a reduction ratio of 60 to 80% at a temperature below the austenite microcrystallization temperature Manufacturing a steel sheet; And Method for producing a thick high strength tough steel sheet characterized in that it comprises the step of cooling the steel sheet to a temperature range of 400 ~ 600 ℃ at a cooling rate of 10 ~ 50 ℃ / sec The method for producing a thick high strength tough steel sheet according to claim 3, wherein the steel sheet further comprises 0.01 to 0.05% of Al. The method for manufacturing a thick high strength tough steel sheet according to claim 3 or 4, wherein the steel sheet is cooled by air or by cooling after cooling to a temperature range of 400 to 600 ° C of the steel sheet.

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