KR100957964B1 - Steel for a structure having excellent low temperature toughnetss, tensile strength and low yield ratio, of heat affected zone and manufacturing method for the same - Google Patents

Abstract

The present invention relates to a high-strength resistive ratio structural steel having excellent low temperature toughness and tensile strength of a welded heat affected zone and a method for manufacturing the same, and more particularly, after heating the steel for welding, even though the low temperature toughness and tensile strength of the steel are high. The low temperature toughness and tensile strength of the heat affected zone during welding are related to a high strength resistive structural structural steel and a method of manufacturing such steel has been solved.

Steel of the present invention is in weight%, C: 0.03 to 0.18%, Si: 0.01 to 0.8%, Mn: 0.3 to 2.5%, P: 0.02% or less, S: 0.01% or less, Al: 0.005 to 0.5%, Nb : 0.005 ~ 0.1%, V: 0.02 ~ 0.2%, Ti: 0.005 ~ 0.1%, N: 15 ~ 200ppm, balance Fe and inevitable impurities, 2.5 <(Ti + 0.2Nb) / N <5.5 It is characterized by having an internal structure consisting of ferrite having an average grain size of 5 µm or less and bainite or martensite having an average grain size of 10 µm or less.

Weld toughness, weld strength, structural steel, toughness, resistance ratio

Description

High-strength resistive-strength structural steel with excellent low temperature toughness and tensile strength of welded heat affected zone and manufacturing method thereof

The present invention relates to a high-strength structural steel having excellent low temperature toughness and tensile strength of the weld heat affected zone and a method of manufacturing the same. More specifically, although the low temperature toughness and tensile strength of the steel are high, the welding is performed after heating the steel for welding. The low-temperature toughness and tensile strength of the heat-affected portion subjected to time-related heat is related to a high-strength structural steel and a method of manufacturing the steel.

Structures such as buildings, bridges, pressure vessels, pipes and the like are often required to have high strength due to the large loads applied to the structures. In addition, since the total weight of the steel is continuously reduced due to the continuous demand for cost reduction input during the manufacture of the structure, the demand for increasing the strength of the steel has become an irresistible trend.

Generally, in order to increase the strength of the steel, an alloy component is added or a strong microstructure transformed at low temperature is used. However, this method reduces the low-temperature toughness of the steel, greatly deteriorating the safety of the structure. In order to solve this problem, the method proposed in the present invention is a method of increasing the strength and low temperature toughness simultaneously by miniaturizing the microstructure of the steel.

However, steels are usually made into structures through a process in which they are connected and attached to each other by welding. By welding, the connection part of the steel is partially dissolved, mixed with the welding material, and finally solidified. The connected steel reaches a melting temperature of about 1530 degrees. Therefore, during welding, the steel is transformed from the ferritic structure of the original steel at the highest temperature to austenite as the temperature increases, and to the delta ferrite phase near the weld line, and then martensite, bainite or Transform into a ferrite + pearlite structure. The larger the heat input of the weld, the thicker the steel, the slower the cooling rate. In the vicinity of the weld line, a softened portion made of ferrite and pearlite is produced. The size of the ferrite grains in the softening portion is coarser than that of the base steel (5 um or less), resulting in a decrease in strength and toughness. Therefore, when the coarsening of the weld heat affected zone occurs, the effect of improving the strength and low temperature toughness due to the refinement of the base steel material disappears.

 Therefore, the high strength steel used as the structure needs to be excellent in both high strength and low temperature toughness in the weld heat affected zone as well as high strength and low temperature toughness in the base metal.

As a invention for improving low-temperature toughness of steel materials, Japanese Unexamined Patent Application Publication No. 58-005967 is mentioned. This document describes a method of improving toughness by appropriately adjusting the steel components and rolling conditions. The low temperature toughness is secured by securing a fine ferrite grain size by adjusting the component system to an appropriate range and rolling in two phases. Was intended.

When the steel is rolled in the two-phase temperature range in which ferrite and austenite coexist, the ferrite produced by transformation becomes processed ferrite, which is refined by processing, so that the toughness of the steel can be improved. However, when the steel is simply rolled in two phases, it is difficult to obtain a sufficient structure refining effect, and there may be a problem such that rolling resistance is increased because rolling is performed in a low temperature range over the entire thickness of the steel.

The present invention is to solve the above-mentioned problems of the prior art, according to one aspect of the present invention, the structural resistive ratio steel material that can ensure the strength and toughness of the steel (base material), as well as the toughness and strength of the weld heat affected portion generated during welding And a method of manufacturing the same.

Steel material of the present invention for solving the problems of the present invention by weight%, C: 0.03 ~ 0.18%, Si: 0.01 ~ 0.8%, Mn: 0.3 ~ 2.5%, P: 0.02% or less, S: 0.01% or less , Al: 0.005 ~ 0.5%, Nb: 0.005 ~ 0.1%, V: 0.02 ~ 0.2%, Ti: 0.005 ~ 0.1%, N: 15 ~ 200ppm, balance Fe and inevitable impurities, 2.5 <(Ti It satisfies the condition of + 0.2Nb) / N <5.5, and has an internal structure composed of ferrite having an average grain size of 5 µm or less and bainite or martensite having an average grain size of 10 µm or less.

At this time, the area fraction of the ferrite is 70 ~ 90% and the area fraction of the bainite or martensite is preferably 10 ~ 30%.

In addition, the steel material is advantageously further comprising one or two or more selected from the group consisting of 0.05% to 0.05%, 0.01% to 2.0% Ni and 0.01% to 1.0% Cu.

Another aspect of the present invention is a method for producing the steel as a weight%, C: 0.03 ~ 0.18%, Si: 0.01 ~ 0.8%, Mn: 0.3 ~ 2.5%, P: 0.02% or less, S: 0.01% Or less, Al: 0.005 to 0.5%, Nb: 0.005 to 0.1%, V: 0.02 to 0.2%, Ti: 0.005 to 0.1%, N: 15 to 200 ppm, residual Fe and unavoidable impurities, and 2.5 <( Heating a steel slab having a composition satisfying the condition of Ti + 0.2Nb) / N <5.5 to a temperature range of 900 to 1300 ° C .; Rough rolling at a rolling reduction of at least 50%; Finishing the rolling in Ar3 ~ Ae3 but performing a finish rolling so that the reduction ratio in the Ar3 ~ Ae3 is 60% or more; And stopping the cooling at a temperature of 450 ° C. or lower after cooling at a cooling rate of 10 ° C./sec or more.

In this case, the steel slab preferably further comprises one or two or more selected from the group consisting of Cr: 0.05 to 1.0%, Ni: 0.01 to 2.0%, and Cu: 0.01 to 1.0%.

According to the present invention, it is possible to effectively provide a resistive welding ratio steel structure having excellent low temperature toughness and strength after welding.

Hereinafter, the present invention will be described in detail.

The inventors of the present invention have studied in order to solve the problems of the present invention, as a result, the steel material is obtained so as to obtain sufficient strength in the process of heating the steel to a high temperature in the state of sufficiently securing the low temperature toughness and strength of the steel base material The inventors have found that it is effective to add a component that improves the hardenability of and to add an element that is advantageous for securing low temperature toughness.

That is, the steel of the present invention is designed and implemented in the following steel design ideas.

(1) Low temperature toughness and strength of steel can be ensured by appropriately controlling the type and shape of the structure and controlling the component system to have sufficient strength.

(2) The low temperature toughness and strength of the weld heat affected zone can be achieved by adding a component having a so-called pinning effect that suppresses the growth of crystal grains even when cooled after welding, and controlling the components that can strengthen the steel material upon cooling.

Therefore, in the present invention, the structure of the steel is controlled by a structure composed of fine ferrite and bainite or martensite to have sufficient strength and toughness. First, the said structure suitable for achieving the objective of this invention is demonstrated.

First, it is necessary to refine the ferrite and the beitite or martensite structure. This is related to the tensile strength of the steel, in order to manufacture a steel having a tensile strength of 780MPa or more by using a low-low coal material as in the present invention, while controlling the alloying elements appropriately and the structure into a ferrite and beitite or martensite structure While it is important to fine the ferrite and bainite or martensite grains.

For this purpose, it is necessary to refine the ferrite crystal to an average size of 5 탆 or less. If the ferrite grains are larger than 5 mu m, it is difficult to obtain a tensile strength of 780 MPa or more in this alloy composition and structure. Therefore, the average grain size of the ferrite is limited to 5㎛ or less. In addition, the fine ferrite may have a property to block brittle crack propagation.

In addition, the average grain size of bainite or martensite is preferably 10 µm or less. Fine bainite or martensite is a hard structure, which not only improves the strength of the steel, but also minimizes the grain size, thereby effectively securing toughness.

In this case, it is preferable to limit the area fraction of ferrite in the final microstructure to 70 to 90%. If the ferrite fraction is less than 70%, it may be advantageous to improve the tensile strength, but the ferrite fraction is lowered and the second phase becomes relatively excessive. In this case, the ductility or moldability deteriorates, causing a problem in use, and when it exceeds 90%, the bainite or martensite is reduced to lower the tensile strength. Therefore, it is necessary to limit the area fraction of ferrite to 70-90%.

In addition, the area fraction of bainite or martensite in the final microstructure was limited to 10 to 30%. If the fraction of bainite or martensite exceeds 30%, the ductility or formability is excessively deteriorated, causing problems in use. If less than 10%, the tensile strength is lowered and the yield ratio is increased. Therefore, it is necessary to limit the fraction of bainite or martensite to 10-30%.

In addition, in the present invention, the structure consisting of ferrite and bainite or martensite is substantially made up of these structures, it should be noted that means that the total area fraction of these are 98% or more. The remainder refers to the structure inevitably formed during the rolling and cooling process.

As described above, when the two-phase or three-phase structure of ferrite, bainite, or martensite is used as the tissue, the strength characteristics of the two tissues are different, which is effective in implementing a resistance ratio.

Moreover, the component system of the steel material which has the above advantageous effects is demonstrated. The steel has a component system consisting of the following components and the remaining iron and inevitable impurities.

C: 0.03 to 0.18% (hereinafter, the content of each component means weight%)

In the present invention, the bainite or pearlite fraction of the base material and the weld heat affected zone is determined, and tensile strength and low temperature toughness are determined according to its size and fraction. If the content exceeds 0.18%, the tensile strength is greatly increased due to excessive bainite formation, but the low temperature toughness is significantly decreased. In addition, if it is less than 0.03%, the fraction of bainite is sharply reduced to cause a significant drop in tensile strength, it is limited to 0.03 ~ 0.18. In the case of a plate used as a steel structure for welding, the range of C is preferably 0.04 to 0.12% for weldability.

Si: 0.01 ~ 0.8%

Si is useful because of its strength-improving effect, but if it is more than 0.8%, low-temperature toughness decreases and weldability deteriorates. Moreover, when it becomes 0.01% or less, the deoxidation effect will become inadequate and it will limit to 0.01 to 0.8%. In addition, Si increases the stability of the MA (image martensite) generated in the weld heat affected zone to form a large number of MA, thereby lowering the low temperature toughness of the weld heat affected zone. Therefore, it is preferable to make the range of Si into 0.01 to 0.8%.

Mn: 0.3 ~ 2.5%

Mn is preferably contained at least 0.3% as a useful element to improve the strength by solid solution strengthening, but addition of more than 2.5% greatly reduces the toughness of the weld due to excessive increase in hardenability. Therefore, Mn is preferably included in the 0.3 ~ 2.5% range. In the case of a plate used as a steel structure for welding, it is more preferable that the range of Mn is 0.6 to 1.8% for strength and weldability.

P: 0.02% or less

Although P is an element that is advantageous in improving strength and corrosion resistance, it is advantageous to make it as low as possible because it is an element that greatly impairs impact toughness. Therefore, the upper limit thereof is preferably 0.02%.

S: 0.01% or less

Since S is an element which forms MnS or the like and greatly impairs the impact toughness, it is advantageous to make it as low as possible, so the upper limit thereof is preferably 0.01%.

Al: 0.005 ~ 0.5%

Since Al is an element that can be deoxidized at low cost, it is preferable to add 0.005% or more, but the addition of 0.5% or more causes nozzle clogging during continuous casting, so it is limited to 0.005 to 0.5%. In addition, since the dissolved Al promotes the formation of MA, the low temperature toughness of the weld heat affected zone is reduced by increasing the stability of the MA even with a small amount of Al to form a large amount of MA. As for the range of said Al, it is more preferable to set it as 0.01 to 0.05%.

Nb: 0.005 ~ 0.1%

Nb is an important element in the production of TMCP steel and precipitates in the form of NbC or NbCN to greatly improve the strength of the base metal and the welded portion. In addition, Nb dissolved in reheating at a high temperature suppresses the recrystallization of austenite and suppresses the transformation of ferrite or bainite, thereby making it possible to refine the structure. In addition, the present invention plays an important role of preventing the drop in strength by making the ferrite fine when the transformation from the softening region to the weld ferrite in the weld heat affected zone. Therefore, it is more advantageous to add Nb 0.01% or more. On the other hand, excessive addition of Nb promotes the formation of coarse MA, which is disadvantageous to low temperature toughness in the weld heat affected zone, and therefore it is preferable to add Nb below 0.1%.

V: 0.02 ~ 0.2%

V has a low solubility temperature compared with other alloying elements, and is effective in securing the properties of steel during welding because it has an effect of preventing the drop in strength by precipitating in the weld heat affected zone. Therefore, it is preferable to add said V 0.02% or more. However, when V is added excessively, toughness is lowered, so it is preferable to add it in the range of 0.02 to 0.2%.

Ti: 0.005 ~ 0.1%

Since Ti can suppress grain growth during reheating and greatly improve low-temperature toughness, more than 0.005% of Ti should be added in order for the effect to be expressed. There is a problem that is reduced. In addition, when the welding heat affected zone is heated up to 1300 degrees or more, it inhibits the growth of austenite grains and finally plays an important role in miniaturizing ferrite. Therefore, more preferable Ti content is limited to 0.005 to 0.03%.

N: 15 ~ 200ppm

N forms TiN precipitates when added with Ti to inhibit grain growth of austenite at high temperatures, but addition of more than necessary significantly reduces toughness while excess nitrogen is solubilized in the matrix to increase tensile strength. Therefore, it is necessary to control in the above range, and in the case of a plate used as a steel structure for welding, it is more preferable to set the range of N to 10 to 120 ppm for weldability.

2.5 <(Ti + 0.2Nb) / N <5.5

As described above, in order to suppress austenite grain growth at high temperature, the nitrogen is preferably combined with an element such as Ti or Nb to form TiNb (C, N) -based carbonitride. According to the ratio, the cube has a spherical shape and its size is changed from several nm to several hundred nm. When the (Ti + 0.2Nb) / N ratio is 2.5 or less, excess N which does not precipitate is present, and this excess N greatly inhibits the low temperature toughness of the base metal and the welded portion. In addition, when the value is 5.5 or more, the probability of coarsening of the precipitates to several hundred nm or more is greatly increased, causing partial brittle fracture, and thus low temperature toughness is also greatly reduced. According to the present inventors, it is preferable to keep (Ti + 0.2Nb) / N in the range of 2.5-5.5 as conditions suitable for formation of the said carbonitride. Here, Ti, Nb, N means the content (% by weight) of the corresponding element, respectively.

The above-described steel having the advantageous emphasis of the present invention can obtain a sufficient effect only by including the alloying elements in the above-described content range, but the characteristics such as the strength and toughness of the steel, the toughness and weldability of the weld heat affected zone, etc. In order to improve, it is more preferable to add the following alloying elements within an appropriate range. The following alloying elements may be added in one kind or two or more kinds together.

Cr: 0.05 ~ 1.0%

Since Cr has a great effect on increasing the strength by increasing the hardenability, addition of 0.05% or more is necessary to obtain the effect, and addition of 1.0% or more is preferably limited to 1.0% or less because it greatly reduces the weldability. In addition, it is more preferable to add in the range of 0.2 to 0.5% in order to obtain sufficient strength stable even at a relatively low cooling rate.

Ni: 0.01 ~ 2.0%

Ni is almost the only element that can simultaneously improve the strength and toughness of the base material, and in order to show the effect, more than 0.01% must be added. Since Ni is an expensive element, the addition of more than 2.0% lowers the economic efficiency. Also, weldability is degraded.

Cu: 0.01 ~ 1.0%

Since Cu is an element that can increase the strength while minimizing the toughness of the base metal, 0.01% or more must be added in order to exhibit the effect. Excessive addition greatly inhibits the product surface quality, so it is desirable to limit it to 1.0% or less. Do.

Therefore, the steel of the present invention is in weight%, C: 0.03 to 0.18%, Si: 0.01 to 0.8%, Mn: 0.3 to 2.5%, P: 0.02% or less, S: 0.01% or less, Al: 0.005 to 0.5% , Nb: 0.005 ~ 0.1%, V: 0.02 ~ 0.2%, Ti: 0.005 ~ 0.1%, N: 15 ~ 200ppm, balance Fe and inevitable impurities, 2.5 <(Ti + 0.2Nb) / N < It is characterized by the composition that satisfies the conditions of 5.5, and if necessary, further selected one or two or more selected from the group consisting of Cr: 0.05 ~ 1.0%, Ni: 0.01 ~ 2.0% and Cu: 0.01 ~ 1.0%. It may also include.

Steel having the above-described composition is easy to form a fine structure therein is effective to combine strength and toughness.

Hereinafter, one of the preferred methods for producing the steel having the above-described advantageous structure and components of the present invention will be described. According to the method of the present invention, after reheating and roughly rolling the steel, the steel is subjected to finish rolling at a temperature directly in the austenitic region to enter the austenite-ferrite two-phase region, and then to a cooling condition suitable for producing bainite or martensite. It consists of a process of cooling. Hereinafter, each process will be described in detail.

Reheating Temperature: 900 ~ 1300 ℃

Reheat the cast to provide sufficient ductility to the cast. The temperature in the reheating step is preferably 900 to 1300 ℃. In reheating the steel sheet of the present invention, the heating temperature is preferably set to 1000 ° C or higher, in order to solidify the carbonitride of Ti and / or Nb formed during casting. Moreover, in order to fully solidify the carbonitride of Ti and / or Nb, it is more preferable to heat to 1050 degreeC or more. However, when reheating excessively high temperature, austenite may coarsen, so the reheating temperature is preferably 1300 ° C or lower.

Rolling rate in the rough rolling stage: 50% or more

As for the total rolling reduction in the rough rolling stage which rolls a slab thickness beforehand to a suitable thickness range before finishing rolling, 50% or more is preferable. If the reduction ratio is not sufficient, it is not preferable because it is difficult to control the austenite average grain size (AGS) during finishing rolling to 50 µm or less suitable for producing fine grain steel in a subsequent process. That is, if the AGS just before finishing rolling exceeds 50 µm, the formation rate of the ferrite structure caused by the strain organic dynamic transformation during the subsequent hot processing is remarkably lowered and the place where the strain organic dynamic transformation ferrite is also very unevenly formed and finally mixed. Care should be taken because ferrite is formed, which can lead to deterioration of mechanical properties. However, the upper limit of the reduction ratio is not particularly limited. Of course, the upper limit of the reduction ratio is 100%, but the reduction ratio of the rough rolling step should be determined in consideration of the reduction ratio of the finishing rolling step and the thickness of the final material, which will be described later, so that the thickness range of the hot rolled sheet material targeted in the present invention It can be freely determined from.

In addition, the temperature of the rough rolling step is also not particularly limited. The reason is that the reheated slab is cooled roughly, because the temperature during the rough rolling does not involve metallurgical consideration in particular, but only when the rough rolling is terminated at a temperature higher than the temperature required for finish rolling described later.

After the rough rolling, finish rolling is performed.

Finish rolling finish temperature: Ae3 ~ Ar3

After the reheating step and the rough rolling step, the finishing rolling step is performed. In the finishing rolling step, so-called 'strain organic dynamic transformation' is used. By using the strain organic dynamic transformation phenomenon, fine ferrite can be generated. As a result, two-phase structure in which austenite crystal and fine ferrite are mixed is obtained in the finish rolling step.

The reason for this is as follows. As the temperature decreases, the steel usually enters the two-phase zone where ferrite and austenite coexist in the austenite zone. The temperature boundary is usually referred to as A3. The A3 is divided into three types because there is a supercooling or overheating phenomenon when the steel is cooled or heated. That is, in a static transformation that maintains steel for a very long time, austenite is transformed into ferrite + austenite (when cooling) or ferrite + austenite is transformed into austenite (when heating) at Ae3 temperature.

However, the transformation of the steel does not occur in the static state as described above, but occurs during the continuous cooling process or the continuous heating process, which inevitably occurs supercooling or overheating. The above supercooling or overheating generally depends on the composition of the steel. Therefore, the temperature of transformation from austenite to ferrite + austenite upon cooling becomes Ar3 lower than Ae3, and conversely, the temperature of transformation into ferrite + austenite upon heating becomes Ac3 higher than Ae3. Therefore, during cooling, even if the temperature of the steel reaches Ae3, the steel does not transform into ferrite. However, at this temperature, the two-phase region of ferrite + austenite becomes thermodynamically stable, so if a little driving force, that is, deformation, austenite is transformed into two phases of ferrite + austenite, and very fine ferrite is produced. . The finish rolling of this invention utilizes this phenomenon. Therefore, the finish rolling is preferably finished at the temperature between Ar3 ~ Ae3. However, the start temperature of the finish rolling does not need to be particularly limited, but considering the total reduction ratio and the like is preferably 1000 ° C. or less. When the finish rolling start temperature is 1000 ° C. or more, the range up to the Ar3 temperature, which is the end temperature of rolling, is too wide to stop rolling in the middle of the rolling and to wait for cooling, and the productivity is greatly deteriorated.

The Ae ₃ temperature according to the steel component can be obtained using basic laboratory research or a commercial thermodynamic database, and Ar ₃ temperature can also be sufficiently obtained through basic experiments. If there is no particular difficulty in obtaining the temperature to implement the present invention.

Rolling rate from Ar3 to Ae3: 60% or more

As described above, in order to promote ferrite transformation due to deformation, it is necessary to sufficiently reduce the pressure in the above temperature range where deformation organic dynamic transformation can occur during finish rolling. Therefore, it is preferable that the reduction ratio in the said temperature range is 60% or more. In addition, the reduction ratio is more preferably 70 to 90% in view of the load of equipment and sufficient miniaturization effect.

After undergoing the above process, the size of the fine ferrite grains (FGS, Ferrite Grain Size) in the internal tissue may be 5 μm or less, preferably 3 μm or less.

If the conditions of the finish hot rolling described above are not satisfied, a sufficient amount of dynamic transformation ferrite is not produced and the effect of promoting ferrite transformation is lowered in a subsequent cooling process.

Cooling rate: 10 ℃ / sec or more

After the finish rolling is finished, the steel undergoes a cooling process, and it is necessary to cool the steel at a sufficient speed so that austenite in the ferrite + austenite present after the finish rolling can be transformed into bainite or martensite. The cooling rate suitable for the component system of the steel made into object by this invention is 10 degreeC / sec or more. However, even if the cooling is as fast as possible to the quenching speed, which is the maximum speed that can be practically realized, the upper limit of the cooling rate does not need to be specifically determined because martensite to be preferably produced in the present invention may be formed inside the steel.

Cooling stop temperature (or winding temperature): 450 ℃ or less

Since the austenite present in the two phases must be sufficiently transformed into bainite or martensite, the cooling stop temperature or the coiling temperature is preferably set to 450 ° C. or lower, more preferably 400 ° C. or lower. Stopping cooling above this temperature is not good because it may produce undesirable tissues such as degenerated pearlite and the like which are not intended in the present invention.

In summary, the production method of the present invention comprises the steps of heating the steel slab of the above-described composition to a temperature range of 1000 ~ 1300 ℃; Rough rolling at a rolling reduction of at least 50%; Finishing the rolling in Ar3 ~ Ae3 but performing a finish rolling so that the reduction ratio in the Ar3 ~ Ae3 is 60% or more; And cooling at a temperature of 450 ° C. or lower after cooling at a cooling rate of 10 ° C./sec or higher.

Hereinafter, the present invention will be described in more detail with reference to Examples. However, it should be noted that the following examples are only intended to illustrate one embodiment of the present invention and are not intended to limit the scope of the present invention. This is because the scope of the present invention is determined by the matters described in the claims and the matters reasonably inferred therefrom.

(Example)

A steel slab having a component system as described in Table 1 was rolled and cooled under the conditions described in Table 2 to prepare a steel sheet having a product thickness of Table 2. In the following Table 1, the invention steel A to invention steel F shows steels satisfying the component range defined in the present invention, while comparative steel G is the case where the C content is excessive, and the comparative steel H is the V content defined in the present invention. The comparative steel I and the comparative steel J represent cases where (Ti + 0.2Nb) / N exceeds and falls short of the range defined by the present invention, respectively. In addition, in Table 1, the content unit of each component is weight%, in the case of N, the unit is expressed in ppm.

TABLE 1

Table 2 below describes rolling conditions for the inventive steels or comparative steels of Table 1 above, but the invention steels deviate from the invention examples 1 to 3 and the conditions corresponding to the conditions of the present invention irrespective of their types. Steels were manufactured according to phosphorus comparative examples 1 to 7, and in the case of comparative steel, steels were manufactured only according to inventive examples 1 to 3 corresponding to the conditions of the present invention. Here, in Comparative Example 1, when the slab heating temperature is too high, Comparative Example 2 is too low the rolling reduction rate in rough rolling, Comparative Example 3 is a low cumulative reduction of Ar3 ~ Ae3, Comparative Example 4 is the rolling end temperature Ar3 If less than, Comparative Example 5, the rolling end temperature exceeds Ae3, Comparative Example 6, the cooling rate is too slow, and Comparative Example 7 the conditions for the case where the cooling end temperature exceeds 450 ℃ irrespective of the steel type Set. However, in the case of Comparative Example 5, since rolling was not performed in Ar3 to Ae3, the cumulative reduction in Ar3 to Ae3 prescribed in Table 2 means a rolling reduction rate in finish rolling only in this case.

TABLE 2-1

Table 2-2

Structure observation and physical property evaluation as shown in Table 3 were performed about the steel material which has the composition of Table 1, and was rolled on the conditions of Table 2. In particular, the following welding operations were performed to observe the tensile strength (TS) and weld toughness (DBTT) of the weld. That is, the steel produced to the thickness as shown in Table 2 was formed into a pipe and then the joints were joined by submerged arc welding. At this time, the heat input of the welding is increased according to the thickness of the steel, but the heat input is adjusted so that the bonding of the shape as shown in the photograph of FIG. At this time, the weld heat affected zone of the pipe is heated to the highest melting point by welding and then cooled to room temperature again. In Table 3 below, DBTT is called a Ductile-Brittle Transition Temperature, and the lower the value, the better the toughness of the DBTT.

Table 3-1

Table 3-2

The particle size of the ferrite of Table 3 means the average particle size defined in the present application. As can be seen in Table 3, the steel rolled under the conditions defined by the present invention with the component system defined in the present invention contains fine ferrite and bainite in an appropriate ratio, and it is found that the fraction of the low-temperature structure is not excessively high. Can be. In the Table 3, bainite refers to a tissue containing some martensite in some cases. As a result, the tensile strength of the base material was 570 MPa or more and not only has an elongation of 30% or more, but also the ductile-brittle transition temperature of the base material and the welded part showed excellent results, and the strength of the welded part was also confirmed. However, in the case of Comparative Example 1 in which the slab heating temperature was too high, the toughness of the base material was drastically decreased due to the particle size of ferrite and bainite exceeding the range defined by the present invention. As a result of Comparative Example 2, the grain size of bainite was also large, and sufficient base metal toughness could not be secured. In addition, Comparative Example 3 was a case where the cumulative pressure drop of Ar3 ~ Ae3 is low, the particle size of the ferrite appeared larger than the range specified in the present invention, and as a result, the base material toughness was deteriorated. In Comparative Example 4, the rolling was carried out at a temperature below Ar3. In this case, the particle size, fraction, and pearlite size of the ferrite fall within the range defined by the present invention, but a large amount of work hardened ferrite was used instead of the raw polygonal ferrite. This resulted in a sharp drop in elongation. In Comparative Example 5, when the finish rolling was carried out at Ar 3 or more, the ferrite grain size exceeded the range defined by the present invention, and the strength and toughness of the base material were significantly reduced. In Comparative Example 6, the ferrite was considerably coarsened because the cooling rate was slow after rolling, and as a result, the elongation and toughness of the base material were lowered. In addition, Comparative Example 7 is the case where the cooling end temperature exceeds 450 ℃ accordingly the fraction of low-temperature structure, such as bainite and martensite is reduced and the fraction of pearlite is excessively increased, the yield ratio is significantly increased compared to other invention examples The results are shown.

In addition, Comparative steel G is a case where the manufacturing method meets the conditions specified in the present invention, but the C content of the component is excessive, the pearlite fraction is excessive, and thus the low temperature toughness is sharply decreased. As the case is lower than the range specified in the invention, the weld strength is reduced. Comparative steel I corresponds to the case where (Ti + 0.2 Nb) / N exceeds the range specified in the present invention, the toughness of the base material is poor, and the comparative steel K is in contrast to (Ti + 0.2 Nb) / N It falls under the range specified in the above, and the toughness of the base metal and the welded part was poor.

Figure 2 shows the structure of the weld heat affected zone of the steel sheet produced by the invention steel A. As can be seen in the photo, it was confirmed that the heating and cooling by welding, but the internal structure can be formed with a significantly finer by the advantageous component system provided in the present invention.

Therefore, the effect of this invention was confirmed.

1 is a photograph observing the appearance of the weld connection of the steel provided by the present invention with an optical microscope, and

Figure 2 is a photograph of the internal structure of the weld heat affected zone of the steel provided by the present invention observed with an optical microscope.

Claims (5)

By weight%, C: 0.03 to 0.18%, Si: 0.01 to 0.8%, Mn: 0.3 to 2.5%, P: 0.02% or less, S: 0.01% or less, Al: 0.005 to 0.5%, Nb: 0.005 to 0.1% , V: 0.02 to 0.2%, Ti: 0.005 to 0.1%, N: 15 to 200 ppm, having a component consisting of balance Fe and unavoidable impurities, satisfying the condition of 2.5 <(Ti + 0.2 Nb) / N <5.5, A high strength structural steel having excellent low temperature toughness and tensile strength, characterized by having an internal structure composed of ferrite having an average grain size of 5 μm or less and bainite or martensite having an average grain size of 10 μm or less. 2. The high strength structural steel having excellent low temperature toughness and tensile strength according to claim 1, wherein the area fraction of ferrite is 70 to 90% and the area fraction of bainite or martensite is 10 to 30%. . The welding according to claim 1 or 2, further comprising one or two or more selected from the group consisting of Cr: 0.05 to 1.0%, Ni: 0.01 to 2.0%, and Cu: 0.01 to 1.0%. High strength structural steel with excellent low temperature toughness and tensile strength. By weight%, C: 0.03 to 0.18%, Si: 0.01 to 0.8%, Mn: 0.3 to 2.5%, P: 0.02% or less, S: 0.01% or less, Al: 0.005 to 0.5%, Nb: 0.005 to 0.1% , V: 0.02 ~ 0.2%, Ti: 0.005 ~ 0.1%, N: 15 ~ 200ppm, balance Fe and inevitable impurities, and satisfy the conditions of 2.5 <(Ti + 0.2Nb) / N <5.5 Heating the steel slab to a temperature range of 900 ~ 1300 ℃; Rough rolling at a rolling reduction of at least 50%; Finishing the rolling in Ar3 ~ Ae3 but performing a finish rolling so that the reduction ratio in the Ar3 ~ Ae3 is 60% or more; And Stopping the cooling at a temperature of 450 ° C. or lower after cooling at a cooling rate of 10 ° C./sec or more; Method for producing a high strength structural steel material excellent in low temperature toughness and tensile strength of the weld heat affected zone. The welding method of claim 4, wherein the steel slab further comprises one or two or more selected from the group consisting of 0.05% to 0.05% of Ni, 0.01% to 2.0% of Ni, and 0.01% to 1.0% of Cu. Method for producing high strength structural steel with excellent low temperature toughness and tensile strength of heat affected zone.

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