KR20110006739A - High-strength steel sheet with excellent low temperature toughness and manufacturing method thereof - Google Patents

Abstract

PURPOSE: A high strength steel plate with superior low temperature toughness and a manufacturing method thereof are provided to manufacture a acicular ferrite and bainite structure without adding mo element. CONSTITUTION: A high strength steel plate with superior low temperature toughness comprises C 0.03~0.10 weight%, Si 0.1~0.4 weight%, N 0.001~0.006 weight%, Ti 0.005~0.03 weight%, Nb 0.02~0.10 weight%, Al 0.01~0.05 weight%, Ca less than 0.006 weight%, Ni less than 1.0 weight%,, Mn less than 1.8 weight%, and the impurity.

Description

High strength steel sheet with excellent low temperature toughness and its manufacturing method {HIGH-STRENGTH STEEL SHEET WITH EXCELLENT LOW TEMPERATURE TOUGHNESS AND MANUFACTURING METHOD THEREOF}

The present invention relates to a steel sheet used in line pipes, building structures, offshore structures, and the like, and more particularly, to a high strength steel sheet and a method of manufacturing the same, which are excellent in low temperature toughness and stably used even in harsh environments.

In order to increase the operational efficiency of the line pipe, it is necessary to increase the amount of crude oil or gas that can be transported per unit time, and to this end, it is necessary to secure a certain strength of steel. In addition, as crude oil and gas mining areas gradually move to cold districts, securing low temperature toughness is also required. In addition, in the case of building structures and offshore structures, the demand for high strength and toughness steels is gradually increasing due to the increase in demand for large structures and the severity of operating conditions (operating temperature, connection structure, etc.).

Conventionally, a technique for improving the hardness and strength of a steel sheet has been proposed by adding an element mainly improving hardenability to form a low temperature transformation phase upon cooling to improve strength, and thus, a low temperature transformation structure such as martensite When formed inside, the toughness of the steel sheet may be extremely poor due to the residual stress inside the steel sheet. In other words, the strength and toughness of the steel sheet is two conventionally incompatible physical properties, it was a general recognition that the toughness decreases as the strength of the steel sheet increases.

Since then, efforts have been made to provide high-strength steel by simultaneously securing the strength and toughness of the steel sheet, and this is the TMCP (Thermo Mechanical Controlling Process) method. The TMCP is a general term for a 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 variety of forms, mainly controlled rolling rolling under a predetermined temperature and strict conditions Process and accelerated cooling to cool the steel plate at a suitable range of cooling rates.

In the case of using the TMCP method, it is possible to smoothly control the desired physical properties to a certain extent by theoretically forming fine crystal grains in the steel sheet and controlling the structure in a desired form.

However, in order to manufacture a steel plate having a desired strength through accelerated cooling of TMCP, it is necessary to form a hard structure as in the prior art. Therefore, it was necessary to add an alloying element to improve the hardenability in order to form a low-temperature transformation structure, which is still a hard structure, such a hardenability improving element causes a problem of raising the manufacturing cost due to the high price. Therefore, in the field of high strength steel, research and development has been made to increase the strength level of steel, and efforts have been made to secure low temperature toughness.

In general, the rolling method by TMCP is largely divided into two. First, it is more reverse rolling method for performing the cooling after the rolling in the single-phase reverse rolling, and the temperature of Ar ₃ or less to conduct accelerated cooling after the rolling at a temperature in the temperature Ar ₃ or more, which transformation of ferrite from austenite. The single phase rolling method has the advantage of lowering the manufacturing cost due to less load on the rolling equipment and lower rolling time than the abnormal reverse rolling method, whereas the transformation structure can be formed during cooling to improve the strength. The addition of expensive alloying elements with excellent hardenability is required, and there is a manufacturing cost burden due to the addition of alloying elements. In addition, uneven transformation occurs inside the manufacturing plate during cooling, and thus the plate shape is badly manufactured. On the other hand, in the case of the abnormal reverse rolling method, since the transformation structure is formed during rolling, it is possible to add less hardenable elements, so that there is no problem of cost increase due to the addition of alloying elements, but the rolling temperature is low, which puts a load on the rolling equipment and the manufacturing time is long. Has the problem of rising.

In the past, various methods of manufacturing structural steels using TMCP have been proposed. After finishing rolling on Ar ₃ and performing accelerated cooling at about 150 to 500 ° C, bainite or martensite structure, which is a low temperature transformation state, is formed. Such a technique is a technique for manufacturing steel having a thickness. However, such a technique can produce polygonal ferrite according to the initial cooling rate, so it is not easy to derive a suitable cooling rate depending on the alloy composition. In addition, the rolling is performed directly up to Ar ₃ , which not only puts a load on the rolling equipment but also lengthens the rolling time, thereby increasing the manufacturing cost.

As another example, there is a technique including a process of additionally performing tempering below Ac ₁ transformation temperature (temperature at which ferrite transforms into austenite after heating) in order to secure sufficient low temperature toughness while using TMCP. However, since this technique must include a heating step in order to perform tempering after cooling the steel sheet again, there is still a problem of an increase in energy consumption for steel production and a cost increase due to an additional tempering process.

Therefore, there is a continuous demand for a method of manufacturing a breakthrough and stable steel that can solve such problems.

The present invention is to solve the above problems, it is an object of the present invention to provide a method for producing a steel sheet excellent in strength and low temperature toughness by not only adding a high-priced alloy element, but also shortening the rolling time.

The present invention, C: 0.03-0.10% by weight, Si: 0.1-0.4% by weight, Mn: 1.8% by weight or less, Ni: 1.0% by weight or less, Ti: 0.005-0.03% by weight, Nb: 0.02-0.10% by weight, Al: 0.01 to 0.05% by weight, Ca: 0.006% by weight or less, N: 0.001 to 0.006% by weight, P: 0.02% by weight or less, S: 0.005% by weight or less, balance Fe and other unavoidable impurities Provide a high strength steel sheet. The microstructure of the steel sheet has a needle-like ferrite and bainite structure as the main structure, and may have a phase-like austenite / martensite (M & A) structure in the second phase structure, and the grain size of the needle-like ferrite is 10 μm or less. (Excluding 0), the packet size of the bainite tissue may be limited to 5 μm or less (excluding 0). In addition, the in-phase austenite / martensite (M & A) structure may be 10% or less (excluding 0) based on the area fraction, the high strength steel sheet has a yield strength of 500 ~ 650MPa, Charpy (-40 ℃) Charpy) The impact absorption energy may be 300J or more.

Furthermore, the present invention is a heating step of heating at 1050 ~ 1180 ℃ for the steel slab of the composition, the first rolling step of rolling one or two or more times at a temperature of at least austenite recrystallization temperature (Tnr), Ar ₃ ~ T Preparation of a high strength steel sheet comprising a second rolling step of finishing rolling in one or two or more multi-stages in the _nr temperature range, an accelerated cooling step of cooling to stop cooling at 300 to 600 ° C., and an air or air cooling step Provide a method. In this case, the reduction rate of the first rolling step may be 20 to 80% and the reduction rate of the second rolling step may be 60 to 80%. The accelerated cooling step may include a first cooling step of cooling to a temperature range of Bs (bainite transformation start temperature) to Ar ₃ at a cooling rate of 30 to 60 ° C / sec, and 300 at a cooling rate of 10 to 30 ° C / sec. It may include a second cooling step of cooling to the range of ~ 600 ℃.

According to the method of manufacturing the steel sheet and the steel sheet according to the present invention, it is possible to effectively produce the needle-like ferrite and bainite structure without adding expensive elements such as Mo, structural steel that can secure high strength and high toughness characteristics Can be manufactured economically and efficiently.

1 is a comparison method of the conventional method of manufacturing the steel (A) and the production method of the invention steel (B) is a cooling method of suppressing the formation of polygonal ferrite due to the fast cooling speed of one step;
Figure 2 is an optical microscope observation photograph of the invention steel having a needle-like ferrite and bainite structure as the main structure

Hereinafter, the present invention will be described in detail.

The inventors of the present invention have studied in depth the method for solving the above-described problems of the prior art, and use a single phase reverse rolling method for shortening the manufacturing time and improving the strength of the steel sheet, but increasing the initial cooling rate. It confirmed that the structure which was excellent in intensity | strength and toughness by using it came to this invention.

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

(Component system)

In this invention, the composition of the steel plate made into the object was limited so that it might have sufficient strength and weld part toughness.

C: 0.03 ~ 0.10 wt%

C is the most effective element for strengthening the matrix of metals and welds through solid solution strengthening, and it is possible to obtain a strengthening effect by precipitation hardening by formation of small size 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. In addition, C improves the hardenability to form a strong microstructure inside the steel sheet during cooling. If the content of C is less than 0.03% by weight, this reinforcing effect cannot be obtained. On the other hand, when the content of C is added in excess of 0.1% by weight, the toughness of the base metal and the welded heat affected zone, including low temperature cracking after the spot welding, is reduced.

Si: 0.1 ~ 0.4 wt%

Since Si plays a role of deoxidizing molten steel by assisting Al and also has an effect as a solid solution strengthening element, at least 0.1 wt% is added. On the other hand, excessively adding more than 0.4% by weight of Si forms a red scale due to Si during rolling, so that the surface shape of the steel sheet is very low, and the development weldability and the toughness of the weld heat affected zone may be reduced. However, Al or Ti plays a role of deoxidation, so it is not necessary to add Si for deoxidation.

Mn: 1.8 wt% or less

Mn, when added as an effective element to strengthen the solid solution of the steel, has the effect of increasing the hardenability and can improve the strength. However, the addition of more than 1.8% by weight may promote central segregation and lower toughness in slab casting in the steelmaking process. In addition, the addition of excessive Mn excessively improves the curing ability, worsens on-site weldability, thereby lowering the toughness of the weld heat affected zone.

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 steel. In particular, compared with Mn and Mo, Ni forms less hard phases such as island 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 the addition of excessive Ni may rather lower the toughness of the weld heat affected zone, so the upper limit thereof is limited to 1.0% by weight.

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 serves to improve toughness by removing N in molten steel. In order to sufficiently remove N, Ti is preferably 3.4 times or more 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 the grains during the heating process for rolling, and also reacts with the nitrogen remaining in Ti. The solid solution in the steel is combined with carbon to form TiC precipitate. The TiC precipitate is very fine and greatly improves the strength of the steel. In particular, when the amount of Al added is very small, Ti forms Ti oxide and acts as a nucleation site of intragranular acicular ferrite in the weld heat affected zone. In order to obtain the effect of inhibiting austenite grain growth by TiN precipitation and increasing the strength by TiC formation, Ti must be added at least 0.005% by weight or more. On the other hand, if it exceeds 0.03% by weight, the coarsening of Ti nitride and the hardening by Ti carbide are excessive, so it is not very good at low temperature toughness. Since toughness may deteriorate, the upper limit of Ti addition shall be 0.03 weight%.

Nb: 0.02 ~ 0.10 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. In particular, Nb is known to suppress the austenite recrystallization when added with Mo increases the grain refining effect, reinforcement effect through enhanced precipitation and hardenability. In order to obtain such an effect, the present invention includes Nb of 0.02% by weight or more. In particular, since Nb can raise the austenite recrystallization temperature (T _nr ) to increase the rolling temperature, it is more preferably contained 0.035% by weight or more in order to lower the manufacturing cost. However, when added in excess of 0.10% by weight, it is difficult to expect an increase in effect any more, and due to excessive precipitation of Nb carbonitride, the austenite microcrystallization temperature is too high, resulting in an increase in material anisotropy and an increase in cost and weldability. And adversely affect the weld heat affected zone toughness.

Al: 0.01 ~ 0.05 wt%

Al is generally added for the purpose of deoxidation of the steel. In addition, since not only the microstructure is fine but also the toughness of the heat affected zone is improved by removing N in the coarse grain region of the weld heat affected zone, 0.01 wt% or more is added. However, when added in excess of 0.05% by weight, it is possible to form Al oxide (Al ₂ O ₃ ) to reduce the toughness of the base metal and the heat affected zone. In addition, since deoxidation can be carried out through addition of Ti and Si, Al is not necessarily added.

Ca: 0.006 wt% or less

Ca is mainly used as an element to control the shape of MnS inclusions and to improve low temperature toughness. However, excessive Ca addition may form a large amount of CaO-CaS and combine to form coarse inclusions, thereby limiting the cleanliness of the steel and limiting the upper limit to 0.006% by weight for field weldability.

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. However, excessive addition of N promotes slab surface defects and degrades the toughness of the known and welded heat affected zones in the presence of solute nitrogen.

P: less than 0.02% by weight

P is combined with Mn to form a non-metallic inclusion to cause the problem of embrittlement of the steel, so it is necessary to actively reduce it.However, in order to reduce P to an extreme, the steelmaking process load is intensified and the above problem occurs significantly 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.

Etc

In the present invention, instead of adding the alloying element to improve the cooling capacity is an invention that can overcome the hardenability at the cooling rate. Therefore, it is based on not adding Mo, Cr, V, etc. which are typical elements which improve hardenability. However, it is possible to add a trace amount of elements to improve the hardenability when it is difficult to implement the cooling rate proposed in the present invention due to the constraints of the equipment.

(Fine tissue)

As a steel plate 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 high strength, high toughness steel sheet having excellent plate shape.

That is, the microstructure in the steel sheet provided by the present invention has needle-like ferrite and bainite structure as the main structure, as shown in Fig. 2, and the phase austenite / martensite (M & A) as the second phase structure. It features. Here, the grain size of the needle-shaped ferrite and the packet size of bainite are key factors that have a great influence on impact toughness. In the present invention, the grain size of the needle-like ferrite is limited to 10 μm or less, and the packet size of bainite structure is limited to 5 μm or less.

The phase austenite / martensite (M & A), which is a phase 2 tissue other than the main structure, may be a cause of inhibiting toughness if its distribution amount is excessive, so the content is limited to 10% or less on an area fraction basis.

Steel sheet of the present invention having such a component system and a microstructure has a yield strength of 500 ~ 650MPa, Charpy shock absorption energy at -40 ℃ can exhibit 300J or more.

(Manufacturing method)

The method of manufacturing the steel sheet of the present invention is generally a step of heating a slab, rolling the heated slab once or two or more times in multiple stages in an austenite recrystallization zone, and twice at a temperature lower than the austenite recrystallization temperature. Finish rolling in a multi-step above, the step of cooling at a rate of 20 ~ 50 ℃ / sec and the step of finishing the cooling at 400 ~ 600 ℃. And below the said cooling completion temperature, a steel plate is air-cooled or cooled.

Hereinafter, detailed conditions of each step will be described.

Slab heating temperature: 1050 to 1180 ℃

The slab heating process 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 must be performed within an appropriate temperature range for 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 the heating temperature is required. If the heating temperature of the steel is less than 1050 ° C, Nb is not reusable in the steel, which makes it difficult to achieve high strength of the steel sheet, and partial recrystallization occurs, resulting in uneven formation of austenite grains, which makes it difficult to achieve high toughness. At such a temperature, austenite grains become excessively coarse, so that the grain size of the steel sheet increases and the toughness of the steel sheet is extremely deteriorated.

Rolling condition control

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. The rolling step of the present invention is characterized in that it is carried out in two temperature ranges. In addition, since the recrystallization behavior in each temperature range is different from each other, the conditions are also set separately.

(1) First rolling step: 20-80% rolling in the austenitic recrystallization zone

In the austenite recrystallization zone, 20 to 80% of the initial slab thickness is subjected to one rolling or two or more multistep rolling. Rolling in the austenitic recrystallization region has the effect of reducing the grain size through austenite recrystallization. In this multistage rolling process, the reduction ratio and time of each step must be well controlled to prevent grain growth after austenite recrystallization. do. Fine austenite grains formed by the above-described process serves to improve the toughness of the final plate.

(2) second rolling step: rolling at 60-80% above T _nr temperature and above Ar ₃ temperature

After the first rolling step, two or more multi-stage rollings are performed at or below the austenite recrystallization temperature (T _nr ) region. At this time, the slabs which have been rolled in the recrystallization temperature range are rolled to 60 to 80% of the thickness, and the rolling is finished at an Ar ₃ temperature (temperature of transformation from austenite to ferrite). Rolling between T _nr and Ar ₃ temperatures crushes the grains and develops dislocations within the grains, facilitating transformation into acicular ferrite and bainite structures upon cooling. As the end temperature of the rolling increases, the manufacturing time of the steel sheet is shortened, thereby lowering the manufacturing cost, which is possible when the initial cooling rate is high during accelerated cooling. This will be described in detail later in the description of the first stage cooling condition.

1st stage cooling rate: 30 ~ 60 ℃ / 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 to improve the strength. However, when the accelerated cooling is performed from the austenite region as in the present invention, since polygonal ferrite may be generated during cooling, the present invention is characterized in that the cooling rate is accelerated at the initial stage of cooling to suppress the generation of polygonal ferrite. do. If the initial cooling rate is 30 ℃ / sec or less can be generated polygonal ferrite can not secure strength and low temperature toughness. However, even if the cooling start temperature is high, if the one-step cooling speed is increased so as not to meet the section forming the polygonal ferrite, it is possible to form a mixed structure of acicular ferrite and bainite, which are microstructures required by the present invention. In other words, if the cooling speed is controlled quickly, the cooling start temperature can be increased, which means that the rolling can be performed at a high temperature, so that the load of the rolling equipment is low and the rolling time can be saved, thereby reducing the manufacturing cost. This method is shown in Figure 1, it can be seen that the formation of the polygonal ferrite is suppressed in the case (B) faster than the case of the slow cooling rate (A).

1st stage cooling end temperature: Bs ~ Ar ₃

The first stage cooling is terminated at a temperature below Ar ₃ , which is the temperature at which austenite transforms into ferrite, and above Bs, which is the start temperature of bainite transformation. More preferably, primary cooling is terminated within Bs + 10 degreeC in order to acquire needle-like ferrite and bainite structure stably.

2 stage cooling rate: 10 ~ 30 ℃ / sec

After completing the first stage of cooling, cool to 10 ~ 30 ℃ / sec to form needle-like ferrite and bainite structure. When cooling below 10 ° C / sec, the amount of retained austenite and M & A may be excessively increased, which may inhibit the strength and toughness. Therefore, the lower limit of the two-stage cooling rate is set to 10 ° C / sec. On the contrary, when cooling at a cooling rate exceeding 30 ° C./sec, the deformation of the steel sheet may occur due to the excessive amount of cooling water, resulting in poor shape control.

2 stage cooling end temperature: 300 ~ 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 cooling is stopped, is more than 600 ° C., since fine grains and bainite phases are hardly formed inside the steel sheet, the upper limit of the cooling stop temperature needs to be limited to 600 ° C. On the other hand, when the cooling stop temperature is less than 300 ℃ not only the effect is saturated but also plate distortion due to excessive cooling may occur. In addition, the impact toughness is hampered by excessive strength increase.

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

(Example)

To prepare a slab having a thickness of 300mm with the composition shown in Table 1, and then heat-rolled-cooled under the production conditions shown in Table 2 to prepare a steel sheet having a thickness of 30mm.

division C Si Mn Ni Ti Nb Al Ca * N * P * S * Tnr Ar ₃ Invention steel A 0.061 0.30 1.54 0.02 0.022 0.049 0.040 10 36 80 10 1015 774 B 0.048 0.25 1.65 0.05 0.015 0.043 0.022 11 42 71 13 984 768 C 0.052 0.27 1.38 0.07 0.024 0.036 0.021 12 34 60 9 954 787 D 0.037 0.32 1.72 0.04 0.017 0.029 0.024 14 46 76 15 891 766 Comparative steel E 0.025 0.18 1.52 0.41 0.026 0.032 0.030 12 38 65 12 959 766 F 0.122 0.26 1.72 0.32 0.018 0.045 0.041 18 42 76 15 1035 725 G 0.063 0.37 2.13 0.04 0.025 0.036 0.023 12 36 62 13 925 726 H 0.062 0.25 1.64 0.22 0.021 0.120 0.032 15 45 62 15 1407 755 I 0.053 0.21 1.58 0.31 0.024 0.015 0.028 13 39 62 14 885 758

(However, the unit of T _nr and Ar ₃ is ℃, the content unit of the * indicated element is ppm, the content unit of the remaining elements is weight%)

As can be seen from Table 1, the invention steel A to invention steel D satisfy all of the conditions of the present invention, and the comparative steel E to invention steel H correspond to cases that deviate from the conditions of the present invention. That is, comparative steel E corresponds to the case where C is too low and comparative steel F corresponds to the case where C is excessively high. In addition, comparative steel G is the case where Mn is excessively high, comparative steel H is the case where Nb is too high, and I is the case where Nb is too low.

division Slab
Heating temperature
(℃) Unresolved station
Rolling reduction
(%) Rolling end temperature
(℃) Stage 1
Cooling rate
(℃ / sec) Tier 2
Cooling rate
(℃ / sec) Cooling
Stop temperature
(℃) Invention steel A One 1136 76 933 64.3 10.9 472 B One 1124 74 918 62.6 11.8 521 C One 1152 68 879 45.5 15.4 557 D One 1172 74 812 57.1 22.3 426 Comparative steel A 2 1187 74 922 52.7 10.9 523 A 3 1013 76 915 46.8 12.4 483 A 4 1123 75 935 21.4 15.4 472 A 5 1118 74 920 38.5 7.8 485 A 6 1113 78 921 42.1 18.5 628 A 7 1134 77 931 38.5 22.2 334 E One 1127 75 820 56 18.4 533 F One 1150 76 930 58 21.7 482 G One 1132 78 790 62 18.4 513 H One 1152 69 960 58 16.8 522 I One 1136 66 810 35 17.6 489

Inventive steels A1 to D1 of Table 2 satisfy all of the alloy composition and manufacturing conditions of the present invention, and Comparative steels A2 to A7 have the alloy composition of Inventive Steel A of Table 1, which is a composition satisfying the alloy composition of the present invention. This is the case where the manufacturing conditions of the invention are not satisfied. 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 I in Table 1.

As can be seen from Table 2, the invention steels A1 to D1 satisfy the conditions of the present invention. Comparative steel A2 has excessively high slab heating temperature, comparative steel A3 has excessively high slab heating temperature, comparative steel A4 has too low one-stage cooling rate, and comparative steel A5 has too low two-stage cooling rate In this case, comparative steel A6 represents a case where the cooling end temperature is too high, and comparative steel A7 represents a case where the cooling end temperature is too low.

Using a slab having the composition of Table 1, a portion of the steel sheet manufactured under the manufacturing conditions of Table 2 was taken to measure the fraction of acicular ferrite and bainite, the grain size of acicular ferrite, and the packet size of bainite. As a result of the tensile test, tensile properties and impact absorption energy obtained by performing a Charpy impact test at -40 ° C are shown in Table 3 below. In Table 3 below, tensile properties and impact absorption energy mean test results in the vertical direction (the circumferential direction of the pipe) in the rolling direction.

division AF + B
(%) AF size
(Μm) B size
(Μm) Yield strength
(MPa) The tensile strength
(MPa) vE _{-40 ℃}
(J) Invention steel A One 95 6 3 525 624 371 B One 92 8 4 510 600 462 C One 93 7 4 523 617 406 D One 94 6 3 505 596 484 Comparative steel A 2 92 8 16 520 621 186 A 3 93 6 4 452 562 486 A 4 82 8 3 462 545 330 A 5 87 15 4 454 608 268 A 6 64 7 4 432 526 368 A 7 67 8 3 513 624 146 E One 76 12 8 421 520 488 F One 94 9 4 580 674 156 G One 93 7 15 514 615 124 H One 92 8 18 525 620 109 I One 94 7 25 486 575 78

* AF: Needle type ferrite, B: bainite

As can be seen from the results of Table 3, all of the inventive steels having the composition and manufacturing conditions that are limited in the present invention have a target strength value, and the shock absorption energy at -40 ° C also shows a high value of 300J or more. .

On the other hand, Comparative Steels A2 to A7, which satisfy the component system of the present invention but differ in manufacturing conditions, do not exhibit the physical properties of the present invention, respectively. That is, in the case of ① comparative steel A2, when the slab heating temperature is excessively high, the grain size of austenite when extracted from the heating furnace is coarse, and the austenite grains are not refined even after rolling in the austenite recrystallization region. However, this resulted in the impact absorption energy is lowered by increasing the bainite packet size after cooling. (2) Comparative steel A3 indicates that the slab heating temperature is too low and the strength is lowered due to insufficient solidifying effect by alloying elements. ③ Comparative steel A4 shows low strength because polygonal ferrite is formed because the first stage cooling rate is too low. ④ Comparative steel A5 has a low two-stage cooling rate, so that the needle-type ferrite and bainite structures are not properly formed, and the grain size of the needle-type ferrite and the packet size of bainite are coarse, resulting in low yield strength and impact absorption energy. Indicates. ⑤ Comparative steel A6 shows a low strength value because the cooling end temperature is too high to form needle-like ferrite and bainite structures. Finally, ⑥ comparative steel A7 is a case where the cooling end temperature is too low, the strength is high, but martensite, etc. is formed to show a low shock absorption energy value.

On the other hand, Comparative steel E was a case where C was too low, and was excellent in toughness but very inferior in strength. Comparative steels F, G, and H were excessively high in C, Mn, and Nb, respectively, and the strength was satisfactory, but the shock absorbing energy was insufficient. In particular, the comparative steel H with excessively high Nb caused the austenite uncrystallized temperature to rise to 1407 ° C., and thus the grain refinement effect due to the austenite recrystallization could not be obtained properly. Comparative steel I exhibited low impact absorption energy because Nb was too low to achieve austenite grain refinement.

Therefore, when looking at the results of this embodiment, the steel material satisfying the component system and manufacturing conditions of the present invention has a needle-like ferrite and bainite structure as the main structure (see Fig. 2 to 4) while showing excellent physical properties and cost and production efficiency You can see that it is excellent in terms of.

Claims (3)

C: 0.03 to 0.10 wt%, Si: 0.1 to 0.4 wt%, Mn: 1.8 wt% or less, Ni: 1.0 wt% or less, Ti: 0.005 to 0.03 wt%, Nb: 0.02 to 0.10 wt%, Al: 0.01 to 0.05 wt%, Ca: 0.006 wt% or less, N: 0.001 to 0.006 wt%, P: 0.02 wt% or less, S: 0.005 wt% or less, balance Fe and other unavoidable impurities,
Microstructure is a high-strength steel sheet having excellent low-temperature toughness of 90% or more in acicular ferrite and bainite structure and 10% or less (except zero) in austenite / martensite (M & A) structure.
The method according to claim 1,
The grain size of the needle-like ferrite is 10㎛ or less (excluding 0), the packet size of the bainite structure is 5㎛ or less (excluding 0) high strength steel sheet excellent in low temperature toughness.
C: 0.03 to 0.10 wt%, Si: 0.1 to 0.4 wt%, Mn: 1.8 wt% or less, Ni: 1.0 wt% or less, Ti: 0.005 to 0.03 wt%, Nb: 0.02 to 0.10 wt%, Al: 0.01 to 0.05 wt%, Ca: 0.006 wt% or less, N: 0.001 to 0.006 wt%, P: 0.02 wt% or less, S: 0.005 wt% or less, balance Fe and other unavoidable impurities,
High strength steel sheet with excellent low temperature toughness with yield strength of 500 ~ 650MPa and Charpy impact absorption energy at -40 ° C of 300J or more.

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