KR20160078772A - The steel sheet having excellent heat affected zone toughness and method for manufacturing the same - Google Patents

Abstract

The present invention relates to a steel sheet for a structure used for a construction structure, a pressure container, a shipping structure, or the like and, more specifically, relates to a steel sheet having excellent welding heat affected zone toughness; and a method to manufacture the same. The steel sheet comprises: at least one type in a group including 0.16-0.20 wt% of carbon (C), 1.0-1.5 wt% of manganese (Mn), 0.25 wt% or less of silicon (Si), 0.005-0.5 wt% of aluminum (Al), 0.02 wt% or less of phosphorus (P), 0.01 wt% or less of sulfur (S), 0.005-0.02 wt% of titanium (Ti), 0.01 wt% or less of niobium (Nb), 0.006-0.01 wt% of nitrogen (N), 0.006 wt% or less of calcium (Ca), 2.0 wt% or less of nickel (Ni), 1.0 wt% or less of copper (Cu), 1.0 wt% or less of chromium (Cr), 1.0 wt% or less of molybdenum (Mo), 0.3 wt% or less of vanadium (V); and the remainder consisting of Fe and inevitable impurities.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel material having excellent toughness at weld heat affected zone,

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a structural steel used as a material for building structures, pressure vessels, ship structures and the like, and more particularly, to a steel material excellent in weld heat-affected portion toughness and a method for manufacturing the same.

In recent years, with the increasing trend of high-rise buildings and structures, the steel used as the material of these buildings has become larger, and they have been replaced by thicker steel plates.

In order to secure the stability of the welded structure, it is necessary to suppress the growth of the austenite grains of the welded heat affected zone (HAZ) to finely maintain the final transformed structure. .

Recently, as a means for solving this problem, there has been disclosed a technique of appropriately distributing a Ti-based carbonitride or the like, which is stable at a high temperature, on a steel material to delay the growth of grain growth in the weld heat affected zone during welding.

For example, Patent Document 1 discloses a structural steel having a impact toughness of about 200 J (base material is about 300 J) at 0 캜 when a heat input of 100 J / cm 2 (maximum heating temperature of 1400 캜) is applied as a typical technique using a precipitate of TiN In the above technique, the TiN precipitates having a TiN content of substantially 4-12 are deposited at a ratio of 5.8 × 10 ³ / mm ² to 8.1 × 10 ⁴ / mm ² with a TiN precipitate of 0.05 μm or less, and 0.03 to 0.2 μm, Is precipitated at 3.9 × 10 ³ / mm ² to 6.2 × 10 ⁴ / mm ² to make the ferrite finer to secure the toughness of the welded portion.

However, in Patent Document 1, there is a problem that cracks are generated on the surface of a slab during performance by forming excess carbon and nitride, and when a slab with many surface cracks is used to produce a heavy plate product, There is a problem in that there is a possibility that defective products such as surface repair or the like can not be manufactured due to the occurrence of problems such as edo cracks.

Japanese Patent Application Laid-Open No. 1999-140582

An aspect of the present invention is to provide a steel material excellent in strength and toughness of a base material and excellent in toughness of a weld heat affected zone after welding and a method for manufacturing the same.

An aspect of the present invention is a method of manufacturing a semiconductor device, comprising: 0.16 to 0.20% carbon, 1.0 to 1.5% manganese (Mn), 0.25% or less silicon (Si), 0.005 to 0.5% (P): not more than 0.02%, sulfur (S): not more than 0.01%, titanium (Ti): 0.005 to 0.02%, niobium (Nb): not more than 0.01% ): Not more than 0.006%, nickel (Ni): not more than 2.0%, copper (Cu): not more than 1.0%, chromium (Cr): not more than 1.0%, molybdenum (Mo): not more than 1.0%, and vanadium , And the balance Fe and other unavoidable impurities, and having a surface crack sensitivity index (Cs) defined by the following relational expression 1 of not more than 0.3. Lt; / RTI >

[Relation 1]

Cs = (71.4 x C ² ) - (30.3 x C) + 3.32

According to another aspect of the present invention, there is provided a method for manufacturing a steel slab, comprising the steps of: reheating a steel slab satisfying the above-described composition and composition relationships in a temperature range of 1050 to 1250 占 폚; Subjecting the reheated slab to hot rolling at 910 占 폚 or less to produce a hot-rolled steel sheet; And a step of air-cooling the hot-rolled steel sheet after the hot-rolling step.

INDUSTRIAL APPLICABILITY According to the present invention, it is possible to provide a steel material excellent in strength and toughness of a base material, and excellent in toughness at a weld heat affected zone at the time of large heat welding. In addition, since the steel material of the present invention has no defects such as surface cracking, it can be suitably applied as a structural steel material.

Figure 1 is a graphical representation of the relationship between the C content and the surface cracking susceptibility index (Cs) according to one embodiment of the present invention.
FIG. 2 is a graph showing the relationship between the Si content, the surface martensite (MA) fraction in the weld heat affected zone and the impact toughness (@ 0 C) of the weld heat affected zone according to an embodiment of the present invention.
3 is a graph illustrating the relationship between the hot rolling temperature and the substrate impact resistance according to an embodiment of the present invention.

The inventors of the present invention have conducted extensive research to fundamentally solve the problem of cracks and other defects on the surface of steel during the manufacture of a steel material for use in conventional structural steel. As a result, The present invention has been accomplished on the basis of confirming that it is possible to secure not only the toughness but also the weld heat affected zone having excellent toughness by controlling the microstructure of the weld heat affected zone at the time of welding.

Particularly, since the toughness of the weld heat affected zone (HAZ) can be ensured well during the large heat welding such as submerged arc welding, the present invention can be suitably applied as structural steel.

Hereinafter, the present invention will be described in detail.

The steel material having excellent toughness at the weld heat affected zone according to one aspect of the present invention is characterized by containing 0.16 to 0.20% of carbon (C), 1.0 to 1.5% of manganese (Mn), 0.25% or less of silicon (Si) (S): not more than 0.01%, titanium (Ti): not more than 0.005 to 0.02%, niobium (Nb): not more than 0.01%, nitrogen (N) ): Not more than 0.006%, not more than 0.006% of calcium (Ca), not more than 2.0% of nickel, not more than 1.0% of copper, not more than 1.0% of chromium and not more than 1.0% of molybdenum, Or less and vanadium (V): 0.3% or less.

Hereinafter, the reason for controlling the steel material composition of the present invention as described above will be described in detail. At this time, the content of each component means weight% unless otherwise specified.

C: 0.16 to 0.20%

Carbon (C) is the element that has the greatest influence on the slab solidification behavior, so it needs to be contained in the steel within an appropriate range.

If the content of C is less than 0.16% in the present invention, there is a problem that the coagulation layer strength is large at the time of phase transformation at the time of solidification of the slab, causing shrinkage and forming a non-uniform solidification layer to facilitate cracking on the surface of the slab, If it exceeds 0.20%, the carbon equivalent becomes too large, and the hardenability of the welded portion is greatly increased, so that the toughness of the welded portion is deteriorated.

Therefore, the content of C in the present invention is preferably limited to 0.16 to 0.20%.

Mn: 1.0 to 1.5%

Manganese (Mn) is an element useful for increasing the hardenability of steel and securing the strength of the steel sheet. In the present invention, however, it is necessary to appropriately limit its content in terms of ensuring toughness of the weld heat affected zone (HAZ).

In general, Mn tends to be segregated at the center of the thickness of the steel sheet, although the toughness of the weld heat affected zone is not greatly deteriorated. Since the Mn content is much higher than the average content in the region where Mn is segregated, There is a problem in that a brittle tissue that greatly damages the surface is easily generated. In view of this, in the present invention, Mn is preferably added in an amount of 1.5% or less. However, if the content is too low, it is difficult to secure the strength of the steel sheet. Therefore, the lower limit is preferably limited to 1.0%.

Therefore, the content of Mn in the present invention is preferably limited to 1.0 to 1.5%.

Si: not more than 0.25%

Silicon (Si) accelerates the morphology of martensite (MA) because it increases the strength of the steel sheet and inhibits the formation of cementite when the elements required for deoxidation of molten steel and the unstable austenite are decomposed, There is a problem that the toughness of the negative is largely lowered. In consideration of this, in the present invention, it is preferable to limit the content of Si to 0.25% or less, and if it exceeds 0.25%, coarse Si oxide is formed and brittle fracture may occur starting from such inclusions. Can not do it.

Therefore, in the present invention, it is preferable to limit the content of Si to 0.25% or less.

Al: 0.005-0.5%

Aluminum (Al) is an element capable of inexpensively deoxidizing molten steel, and is preferably added in an amount of 0.005% or more. However, when the content exceeds 0.5%, there is a problem that nozzle clogging occurs during continuous casting, and solidified Al can form a martensite on the welded portion, resulting in a decrease in the toughness of the welded portion.

Therefore, in the present invention, the content of Al is preferably limited to 0.005 to 0.5%.

P: not more than 0.02%

Phosphorus (P) is an element favorable for strength improvement and corrosion resistance, but since it is an element that greatly hinders impact toughness, it is advantageous to make it as low as possible, so that the upper limit is preferably 0.02%.

S: not more than 0.01%

Sulfur (S) forms MnS or the like and greatly inhibits the impact toughness. Therefore, it is possible to make the sulfur as low as possible. The upper limit is preferably 0.01%.

Ti: 0.005 to 0.02%

Titanium (Ti) combines with nitrogen (N) to form fine nitrides, thereby relaxing crystal graininess that may occur in the vicinity of a welding melt line, thereby suppressing the deterioration of toughness. At this time, if the Ti content is too low, the effect of suppressing the coarsening can not be fully exhibited due to the insufficient number of Ti nitrides. Therefore, if the Ti content is excessively excessive, It is preferable to limit the upper limit to 0.02%.

Therefore, the content of Ti in the present invention is preferably limited to 0.005 to 0.02%.

Nb: 0.01% or less

Niobium (Nb) is an effective element for increasing the strength of the steel, but it tends to greatly deteriorate the toughness of the weld heat affected zone, and therefore the content thereof should be appropriately limited. Particularly, Nb carbonitride precipitates at the austenite grain boundaries during the reverse transformation from the vicinity of the weld line to the austenite region, thereby lowering the toughness. This is remarkable when the content of Nb exceeds 0.01%. % Or less.

N: 0.006 to 0.01%

Nitrogen (N) combines with Ti described above to form fine nitrides to alleviate coarsening of crystal grains that may occur in the vicinity of the welding fusion wire, thereby preventing the decrease in toughness. In order to obtain the above effect, it is necessary to contain N in an amount of 0.006% or more. However, if the content is excessively large, there is a problem of significantly reducing toughness.

Therefore, the content of N in the present invention is preferably limited to 0.006 to 0.01%.

Ca: not more than 0.006%

Calcium (Ca) is mainly used as an element that controls the shape of the MnS inclusions and improves the low temperature toughness. If the content of Ca is excessive, a large amount of CaO-CaS is formed and bonded to form a coarse inclusion, which causes deterioration of cleanliness and in-situ weldability of the steel. Therefore, it is preferable to limit the Ca content to 0.006% or less in consideration of this.

In order to further improve the properties such as the strength and toughness of the steel material, it is necessary that the alloying elements described below are appropriately added to the steel material It is preferable to further include it within the range. At this time, only one of the following alloying elements may be added, or two or more alloying elements may be added.

Ni: not more than 2.0%

Nickel (Ni) is an almost unique element that can simultaneously improve the strength and toughness of a base material. However, since it is an expensive element, adding more than 2.0% is disadvantageous from the economical point of view and deteriorates in weldability. Can not do it.

Therefore, it is preferable to add Ni at 2.0% or less in addition.

Cu: not more than 1.0%

Copper (Cu) is an element capable of minimizing toughness deterioration of a base material and capable of improving the strength of a steel, and when it is added too much, the surface quality of the product is greatly deteriorated. .

Cr: not more than 1.0%

Chromium (Cr) has a great effect on strength improvement by increasing hardenability. However, if it is added too much, there is a problem that the weldability is greatly lowered. Therefore, it is preferable to limit the content to 1.0% or less.

Mo: 1.0% or less

Molybdenum (Mo) has the effect of greatly improving the hardenability by the addition of only a small amount, suppressing the formation of the ferrite phase, and greatly enhancing the strength. However, when the Mo content is excessively increased, the hardness of the welded portion is greatly increased, It is preferable to limit the content to 1.0% or less.

V: not more than 0.3%

Vanadium (V) has a lower temperature to be employed than other alloying elements and has an excellent effect of preventing the strength from dropping due to precipitation in the weld heat affected zone (HAZ). However, if the content is too high, vanadium , It is preferable to limit the content to 0.3% or less.

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

On the other hand, the steel material of the present invention satisfying the above-mentioned component composition preferably satisfies the surface crack sensitivity index (Cs) defined by the following relational expression 1 to be not more than 0.3.

[Relation 1]

Cs = (71.4 x C ² ) - (30.3 x C) + 3.32

As described above, the carbon (C) has the greatest influence on the slab solidification behavior. In the present invention, when the content of C is less than 0.16%, the surface crack sensitivity index (Cs) . That is, when the slab is solidified, the solidification layer strength is large at the time of occurrence of the phase transformation, causing shrinkage, and forming a non-uniform solidification layer, thereby facilitating cracks on the surface of the slab.

Therefore, in order to provide a steel material free from surface cracking, it is necessary to control the content of C so that the value (Cs) of the above-mentioned relational expression 1 satisfies 0.03 or less.

In the present invention as described above, it is preferable that the steel material has a ferrite-pearlite composite structure as a microstructure.

On the other hand, the steel material of the present invention is a steel material having improved sinterability compared to the conventional steel material, and a desired structure can be formed in the steel material without rapid water cooling or the like. However, when the hardness of the steel material is improved and the hard texture is easily formed, the low temperature toughness is often deteriorated. In the present invention, by specifying the preferable structure of the steel material, There is an effect of preventing deterioration of toughness characteristics.

Accordingly, the steel material of the present invention has a tensile strength of 500 MPa or more, a Charpy impact energy at 0 ° C of 150 J or more, and a martensite fraction of the microstructure of the weld heat affected zone (HAZ) % Or less, and excellent impact toughness at a temperature of 0 ° C and a Charpy impact energy of 100 J or more.

Hereinafter, a method of manufacturing a steel material according to the present invention will be described in detail. The following production method is a preferred example for producing the steel sheet of the present invention, but the present invention is not limited thereto.

The production method of the present invention roughly provides a method for producing a hot-rolled steel sheet by heating a steel slab satisfying the above-mentioned component system, homogenizing the steel slab, and then subjecting the steel slab to hot rolling and cooling. Hereinafter, detailed conditions for each step will be described.

Slab reheating temperature: 1050 ~ 1250 ℃

First, after preparing a steel slab satisfying the above-described composition and composition, it is preferable to reheat the steel slab in a temperature range of 1050 to 1250 ° C.

At this time, it is preferable that the reheating is performed at a temperature of 1050 ° C or higher, in order to solidify Ti and / or Nb carbonitride formed during casting. That is, in order to sufficiently solidify Ti and / or Nb carbonitride formed during casting, it is necessary to reheat at 1050 ° C or higher. However, if the reheating is performed at an excessively high temperature, the austenite may be coarsened. Therefore, the reheating temperature is preferably limited to 1250 ° C or less.

Hot finish Rolling temperature: 910 ℃ or less

It is preferable that the hot-rolled steel sheet is produced by subjecting the reheated steel slab to rough rolling under the usual conditions and then performing hot rolling at a predetermined temperature.

At this time, the hot rolling is a process for transforming the austenite structure into a heterogeneous microstructure, and when the hot rolling temperature exceeds 910 캜, a coarse texture is formed to deteriorate impact toughness.

Therefore, it is preferable that the hot rolling is performed at a temperature of 910 ° C or less, and more preferably, at a temperature range of 850 to 910 ° C. If the rolling finish temperature is less than 850 ° C, there is a problem that it becomes difficult to control the shape of the plate material.

It is preferable to cool the hot-rolled steel sheet obtained by the hot-rolling process. In this case, no separate water-cooling is required for cooling, which can be achieved through air cooling.

That is, as already mentioned, the steel material of the present invention is a steel material with improved incineration properties, and it is preferable to apply air cooling because a desired structure can be formed in the steel material even without rapid water cooling.

Hereinafter, the present invention will be described in more detail by way of examples. It should be noted, however, that the embodiments described below are for illustrating and embodying the present invention, and not for limiting the scope of the present invention. And the scope of the present invention is determined by the matters described in the claims and the matters reasonably deduced therefrom.

( Example )

Steel slabs having the composition shown in the following Table 1 were reheated at 1050 to 1250 占 폚, and then each steel slab was rolled under the conditions shown in Table 2, followed by final air cooling to prepare hot-rolled steel sheets.

The yield strength, tensile strength, yield ratio (YR) and impact toughness of each of the manufactured hot-rolled steel sheets were measured and are shown in Table 3 below.

At this time, the impact toughness was measured by Charpy V-notch impact test at 0 占 폚.

Impact toughness was measured in the same manner as described above with respect to the weld heat affected zone formed after submerged arc welding was performed on each steel sheet at 200 kJ / cm 2, and the results are shown in Table 3 below.

division Component composition (% by weight) Cs C Si Mn Al P S Ti Nb N * Ni Cu Cr Mo V Inventive Steel A 0.17 0.15 1.3 0.035 0.013 0.002 0.017 0.007 75 0.4 - 0.3 - - 0.23 Invention steel B 0.19 0.17 1.45 0.013 0.012 0.002 0.020 0.004 80 0.3 0.2 - 0.1 - 0.14 Inventive Steel C 0.18 0.18 1.2 0.013 0.013 0.002 0.018 0.005 85 0.21 - 0.5 - - 0.18 Inventive Steel D 0.165 0.18 1.35 0.013 0.013 0.002 0.019 0.003 90 - 0.22 - - - 0.26 Comparative River A 0.15 0.15 1.4 0.032 0.013 0.005 0.016 0.002 88 0.3 0.2 0.3 0.15 0.04 0.38 Comparative Steel B 0.172 0.40 1.3 0.024 0.013 0.005 0.015 0.004 70 - - 0.5 - - 0.22 Comparative Steel C 0.178 0.13 1.25 0.043 0.015 0.009 0.009 0.005 35 - - - 0.1 - 0.19

(In the above Table 1, * is expressed in ppm, and Ca contents in the inventive steels A to D and comparative steels A to C were measured as impurity levels and not shown. Value.)

division Rough rolling condition Hot finish rolling conditions Remarks Slab
Thickness (mm) Reheating
Extraction temperature (캜) Rough rolling
End temperature (캜) Rolling
Starting temperature (캜) Rolling
End temperature (캜) Inventive Steel A 244 1170 1090 935 895 Inventory 1 244 1175 1095 1010 970 Comparative Example 1 Invention steel B 250 1100 1020 872 832 Inventory 2 244 1185 1085 1005 965 Comparative Example 2 Inventive Steel C 250 1085 1005 920 880 Inventory 3 250 1800 1085 1020 980 Comparative Example 3 Inventive Steel D 244 1115 1035 855 815 Honorable 4 244 1195 1115 1095 1055 Comparative Example 4 Comparative River A 255 1115 1035 925 885 Comparative Example 5 Comparative Steel B 244 1195 1115 910 870 Comparative Example 6 Comparative Steel C 244 1105 1025 920 880 Comparative Example 7

division final
Product thickness (mm) Mechanical properties Shock Toughness (@ 0 ℃) surface
Poor crack YS TS YR Base material HAZ Inventory 1 70 396 528 75 161 137 Good Comparative Example 1 55 394 519 76 111 117 Good Inventory 2 75 416 540 77 203 129 Good Comparative Example 2 60 423 535 79 114 103 Good Inventory 3 70 395 534 74 171 125 Good Comparative Example 3 45 402 522 77 104 101 Good Honorable 4 75 399 525 76 214 125 Good Comparative Example 4 35 383 511 75 54 109 Good Comparative Example 5 65 397 516 77 167 137 Bad Comparative Example 6 65 413 529 78 177 51 Good Comparative Example 7 70 405 533 76 171 35 Good

(The yield ratio (YR) in Table 3 is the value obtained by yield strength (YS) / tensile strength (TS) * 100.

In addition, whether or not surface cracks in Table 3 are judged after visual observation of the surface of the final steel sheet.

As shown in Tables 1 to 3, when the composition, the composition and the manufacturing conditions proposed in the present invention are all satisfied, not only the strength and toughness of the base material are excellent, but also the toughness Can be ensured. Further, a steel material free from defects such as surface cracks can be produced.

On the other hand, it can be confirmed that the toughness of the base material is lowered in the case of Comparative Examples 1, 2, 3 and 4 in which the composition and the composition relationship satisfy the present invention but the hot finish rolling temperature is too high.

In addition, in the case of Comparative Example 5 in which the content of C was excessively large, there was a problem that a surface crack occurred.

In the case of Comparative Example 6 in which the content of Si is too large, it can be confirmed that the toughness of the weld heat affected zone is heated because of the increase of the surface martensite content in the weld heat affected zone by the excessively added Si.

Further, it can be confirmed that even in the case of Comparative Example 7 in which the content of N is insufficient, the toughness of the weld heat affected zone is heated. This is because the Ti-based precipitates are not sufficiently formed, It can not be secured.

The relationship between the C content and the surface cracking susceptibility index (Cs) is shown in FIG.

As shown in FIG. 1, in the present invention, the surface cracking susceptibility index (Cs) defined by the relational expression 1 is influenced by the C content. When the content of C is less than 0.16%, the value exceeds 0.3 It can be confirmed that it increases rapidly.

FIG. 2 shows the relation between the Si content and the fraction of the surface martensite (MA) in the weld heat affected zone and the impact toughness (@ 0 C) of the weld heat affected zone.

As shown in FIG. 2, when the content of Si exceeds 0.25%, the fraction of the martensite in the weld heat affected zone exceeds 3%, the impact toughness is less than 100 J and the toughness of the weld heat affected zone is lowered Can be confirmed.

The relationship between the hot finish rolling temperature and the base impact toughness is shown in Fig.

As shown in FIG. 3, when the hot rolling temperature exceeds 910 ° C, the microstructure of the base material becomes coarse, and the impact toughness is reduced to less than 150J.

Claims (6)

(Si): 0.25% or less; aluminum (Al): 0.005 to 0.5%; phosphorus (P): 0.02% (N): 0.006 to 0.01%, calcium (Ca): 0.006% or less, nickel (N) (1) selected from the group consisting of not more than 2.0% of Ni, not more than 1.0% of copper, not more than 1.0% of chromium (Cr), not more than 1.0% of molybdenum (Mo) Including the remainder Fe and other unavoidable impurities,
Wherein the surface crack sensitivity index (Cs) defined by the following relational expression 1 is 0.3 or less.

[Relation 1]
Cs = (71.4 x C ² ) - (30.3 x C) + 3.32
The method according to claim 1,
The steel material has a tensile strength of 500 MPa or more and a Charpy impact energy at 0 ° C of 150 J or more.
The method according to claim 1,
Wherein the steel material has a weld heat affected zone (HAZ) having a martensite fraction of not more than 3% at the time of welding.
The method according to claim 1,
Wherein the steel material has a weld heat affected zone (HAZ) having a Charpy impact energy of 100 J or more at 0 DEG C during welding.
The method according to claim 3 or 4,
Wherein the welding is submerged arc welding (SAW), and the welded heat affected zone toughness is excellent.
(Si): 0.25% or less; aluminum (Al): 0.005 to 0.5%; phosphorus (P): 0.02% (N): 0.006 to 0.01%, calcium (Ca): 0.006% or less, nickel (N) (1) selected from the group consisting of not more than 2.0% of Ni, not more than 1.0% of copper, not more than 1.0% of chromium (Cr), not more than 1.0% of molybdenum (Mo) Reheating the steel slab containing the remainder Fe and other unavoidable impurities, wherein the steel slab satisfies a surface crack sensitivity index (Cs) of 0.3 or less, defined by the following relational expression 1, in a temperature range of 1050 to 1250 캜;
Subjecting the reheated slab to hot rolling at 910 占 폚 or less to produce a hot-rolled steel sheet; And
After the hot rolling, air cooling
Wherein the welded heat affected zone toughness is excellent.

[Relation 1]
Cs = (71.4 x C ² ) - (30.3 x C) + 3.32

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