KR100584763B1 - Method for Manufacturing Fine Grain Ferritic Steel Having superior HAZ Toughness - Google Patents

Abstract

The present invention relates to a method for manufacturing ferrite fine grain steel used in welding structures such as construction, bridges, shipbuilding, offshore structures, steel pipes, line pipes, etc., and not only has excellent mechanical properties, but also can minimize the difference in toughness of the base material and the heat affected zone. It is an object of the present invention to provide a method for producing a ferritic fine grained steel having excellent weld heat affected zone toughness.

In the present invention, C: 0.02-0.18%, Si: 0.05-0.5%, Mn: 0.4-2.0%, Ti: 0.005-0.2%, Al: 0.005-0.1%, N: 0.005% or less, B: 0.0003 by weight. Preparing a steel slab of ordinary level that satisfies -0.01%, Nb: 0.005-0.1%, P: 0.03% or less, S: 0.003-0.05%, O: 0.005% or less and is composed of the remaining Fe and other impurities;

The steel slab prepared as described above was heated at a temperature of 1000-1150 ° C. for 100-180 minutes, whereby the N content of the steel became 0.008-0.03%, and the content of N, Ti, B, Al, and Nb satisfied the following relationship. Soaking to obtain; And

1.2≤Ti / N≤2.5, 10≤N / B≤40, 2.5≤Al / N≤7, 0.3≤Nb / N≤9,

7≤ (Ti + 2Al + 4B + Nb) / N1≤7

Maintain the austenite grain size of the steel slab treated as described above to 20㎛ or less and then cooled at a cooling rate in the range of 10 ~ 20 ℃ / sec at a rolling ratio of 50% or more in the 880 ~ 930 ℃ range of austenite uncrystallized Hot rolling, followed by cooling to a range of 740-780 ° C. at a cooling rate of 5-15 ° C./sec and rolling at a hot rolling ratio of 40% or more, followed by cooling to a room temperature of 10-20 ° C./sec. The manufacturing method of the ferrite fine grained steel which is excellent in the toughness of the weld heat-affected zone comprised by this is made into the summary.

TiN precipitate, Ti-Nb composite precipitate, austenite particle size, ferrite refinement, and sedimentation treatment

Description

Method for Manufacturing Fine Grain Ferritic Steel Having superior HAZ Toughness}

The present invention relates to a method for manufacturing ferrite fine grain steel used in welded structures, such as construction, bridges, shipbuilding, offshore structures, steel pipes, line pipes, etc. More specifically, fine TiN precipitates by immersion treatment on a conventional steel slab And finely distribute Ti-Nb composite precipitates in the base material and increase the fine ferrite phase fraction in the rolling process to improve the physical properties of the base material and also improve the toughness of the weld heat affected zone during high heat welding. will be.

Recently, steel materials used in accordance with the trend of high-rise buildings, structures are being replaced by thick materials. In order to weld such thick materials, high-efficiency welding is inevitable. As a technique for welding thickened steel materials, a high-pass heat submerged welding method and an electro-welding method capable of 1-pass welding are widely used.

In addition, in the field of shipbuilding and bridges, even when welding a steel plate having a plate thickness of 25 mm or more, the above-described high heat input welding method capable of one-pass welding is applied.

In general, in welding, the larger the amount of heat input, the larger the amount of welding passes, so that the number of welding passes decreases. In other words, increasing the amount of heat input in the welding will be able to widen the range of use. The range of high heat input currently used corresponds to approximately 100-200 kJ / cm, but in order to weld more thickened steel, that is, steel with a plate thickness of 50 mm or more, it is possible to have a super heat input range of 200-500 kJ / cm.

When the heat input is applied to the steel, the heat affected zone formed during welding, particularly the heat affected zone near the fusion boundary, is heated to a temperature close to the melting point by the amount of heat input. Accordingly, since the grains of the weld heat affected zone grow and coarse, and microstructures that are vulnerable to toughness such as upper pearlite and martensite are formed during the cooling process, the weld heat affected zone is the site where the toughness of the weld deteriorates most. Therefore, in order to secure the stability of the welded structure, it is necessary to suppress the growth of the austenite grains in the weld heat affected zone and to keep it fine. As a means to solve this problem, there is disclosed a technique for delaying grain growth of the weld heat affected zone during welding by appropriately distributing an oxide or Ti-based carbonitride, which is stable at a high temperature, to steel materials. For example, Japanese Patent Application Laid-Open No. 11-140582, No. 10-298708, No. 10-298706, No. 9-194990, No. 9-324238, No. 8-60292, (S) 60-245768, (Square) 5-186848, (S) 58-31065, (S) 61-79745, Journal of the Japan Welding Society, Vol. 52, No. 2, 49 and Japanese Patent Laid-Open And 64-15320.

Japanese Patent Laid-Open Publication No. 11-140582 is a representative technique using TiN precipitates. When 100 J / cm of heat input (maximum heating temperature of 1400 ° C) is applied, the impact resistance at 200 ° C is about 200J ( A structural steel material having a base material of about 300J) is disclosed. In this prior art, Ti / N was substantially managed at 4-12 so that TiN precipitates of 0.05 µm or less were 5.8 × 10 ³ pieces / mm 2 to 8.1 × 10 ⁴ pieces / mm 2, and TiN precipitates of 0.03 to 0.2 μm were 3.9 ×. by precipitation with 10 × 10 ^{4 3} /㎟~6.2 gae / ㎟ and by refining the ferrite to secure the toughness of the weld.

However, according to this prior art, when the high heat input welding of 100 kJ / cm is applied, the toughness of the base material and the heat affected zone is generally low (0 ° C.) and the highest impact toughness of the base material: 320J and the weld heat affected zone: 220J). As the toughness difference between the base material and the heat affected zone is about 100J, there is a limit in securing the reliability of the steel structure due to superheated welding of the thickened steel. In addition, as a method for securing the desired TiN precipitate, the slab is heated at a temperature above 1050 ° C. and rapidly cooled and then reheated for hot rolling. Becomes In addition, in the prior art, the slab surface cracks are more likely to occur because continuous nitrogen is cast into a slab of high nitrogen molten steel containing 0.005-0.02% of N. That is, since N is an austenite stabilizing element and austenite is maintained for a long time in the slag solidification process, impurity elements such as P and S may cause segregation in the uncoagulated portion, causing a slab surface crack.

Although many techniques have been known to improve the toughness of the weld heat affected zone during high heat input welding, the case of achieving high strength and high toughness of the base metal simultaneously while improving the toughness of the weld heat affected zone during long time heat welding at 1350 ℃ or higher. Has not been announced yet.

 The present invention finely and uniformly distributes a large amount of TiN precipitates and Ti-Nb complex precipitates through the immersion treatment on a steel slab of a normal level by increasing the fraction of fine ferrite in the base material while having excellent TiN precipitates and Ti-Nb composites having high temperature stability. By improving the number of precipitates to improve the mechanical properties of the base material, as well as effectively suppress the growth of austenite grains in the weld heat affected zone when the high heat input welding is applied, the weld heat influence can minimize the difference in toughness between the base and heat affected zone. It is an object of the present invention to provide a method for producing ferritic fine grain steel having excellent toughness.

EMBODIMENT OF THE INVENTION Hereinafter, this invention is demonstrated.

In the present invention, C: 0.02-0.18%, Si: 0.05-0.5%, Mn: 0.4-2.0%, Ti: 0.005-0.2%, Al: 0.005-0.1%, N: 0.005% or less, B: 0.0003 by weight. Preparing a steel slab of ordinary level that satisfies -0.01%, Nb: 0.005-0.1%, P: 0.03% or less, S: 0.003-0.05%, O: 0.005% or less and is composed of the remaining Fe and other impurities;

The steel slab prepared as described above was heated at a temperature of 1000-1150 ° C. for 100-180 minutes, whereby the N content of the steel became 0.008-0.03%, and the content of N, Ti, B, Al, and Nb satisfied the following relationship. Soaking to obtain; And

1.2≤Ti / N≤2.5, 10≤N / B≤40, 2.5≤Al / N≤7, 0.3≤Nb / N≤9,

7≤ (Ti + 2Al + 4B + Nb) / N≤17

Maintain the austenite grain size of the steel slab treated as described above to 20㎛ or less and then cooled at a cooling rate in the range of 10 ~ 20 ℃ / sec at a rolling ratio of 50% or more in the 880 ~ 930 ℃ range of austenite uncrystallized Hot rolling, followed by cooling to a range of 740-780 ° C. at a cooling rate of 5-15 ° C./sec and rolling at a hot rolling ratio of 40% or more, followed by cooling to a room temperature of 10-20 ° C./sec. It relates to a method for producing a ferritic fine grained steel excellent in the heat treatment zone toughness welded.

Hereinafter, the present invention will be described in detail.

In the present invention, the term " prior austenite " refers to austenite formed in a weld heat affected zone when high heat input welding is applied to steel materials (base materials), and is formed in a steel manufacturing process (hot rolling process). It is used for convenience to distinguish it from being austenite.

  The present inventors have studied a method to improve the toughness of the weld heat affected zone simultaneously with the strength and toughness of the steel (base metal) while preventing the surface slab cracks that may occur in ordinary high nitrogen steels. Instead of making slabs, uniformly distribute TiN precipitates and Ti-Nb complex precipitates having excellent high temperature stability through nitriding in the subsequent slab heating step and finely maintain the austenite grain size below 20 μm in the slab heating step. Fine ferrite grains can be obtained through controlled rolling and accelerated cooling, and the uniform TiN precipitates and Ti-Nb complex precipitates are uniformly distributed in these microstructures. It is confirmed that the toughness of the welded heat affected zone is greatly improved since it is suppressed to less than 탆. It was.

The present inventors who started from this point of view, the following method which can improve the toughness of a weld heat-affected zone also through the immersion process was able to derive.

[1] With the use of TiN precipitates and Ti-Nb composite precipitates, which were distributed by immersion treatment on low nitrogen steel slabs,

[2] In the heating stage of steel slab, the austenite grain size is kept below 20 μm, and it is pressed at the austenite unrecrystallized zone temperature to form strain bands in the stretched austenite grains. It is to secure ferrite grains. Also,

[3] In the welding of steel, the austenite of the welded heat affected zone is refined to less than 80㎛, and the ferrite formation fraction is increased from the austenite of the welded heat affected zone using TiN precipitate, Ti-Nb composite precipitate and BN and AlN precipitates. In particular, it promotes the transformation of polygonal or acicular ferrite in the austenite, thereby improving the toughness improvement effect.

These [1] [2] [3] are demonstrated in more detail.

 [1] TiN + NbN composite precipitates

The present inventors have focused on the fact that TiN precipitate loses the inhibitory effect of the austenite grain growth when the heat input welding is applied to the base material on which TiN precipitates are distributed, which is caused by diffusion of solid Ti atoms decomposed by welding heat. As a result of examining the characteristics of TiN precipitates according to the / N ratio, we found a new fact that in high nitrogen environment (low Ti / N ratio), the concentration of solid solution Ti and diffusion of atomic Ti atoms are reduced and the high temperature stability of TiN precipitate is improved. It became. More interestingly, even if steel slabs are made of low-quality nitrogenous steel of less than 0.005% with a low possibility of surface cracking, and then made into high-nitrogen steels by immersion in slab furnaces during the rolling process, the ratio of Ti / N is reduced. If it is managed in the range of 1.2 to 2.5, the amount of solid solution Ti is extremely reduced and the high temperature stability of TiN precipitate is increased, so that the fine TiN precipitates of 0.01 ~ 0.1㎛ size are distributed more than 1.0x10 ⁷ / mm2 at intervals of 0.5㎛ or less. The result was obtained.

In addition, in the present invention, when the heat input welding is applied to the steel on which the TiN precipitates are formed by the immersion treatment, the re-dissolution of the TiN precipitates can be prevented as the TiN precipitates are delayed to be re-used to the matrix. On the basis of this, when the TiN + NbN composite precipitate of NbN is properly wrapped around the TiN precipitate, even though it is heated to a temperature higher than 1350 ° C, the TiN precipitate distributed in the weld heat affected zone is significantly delayed in the matrix. I found out. In other words, NbN, which is preferentially reclaimed, is concentrated around TiN, affecting TiN decomposition and reusing rate to the base metal, and TiN effectively contributes to the suppression of the austenite grain growth, thereby significantly improving the toughness of the weld heat affected zone. It can be. The degree of distribution of NbN precipitates also affects the strength (or hardness) of the weld heat affected zone.

For this purpose, it is important to reduce the solubility product showing the stability of the TiN precipitate at high temperature while finely and uniformly distributing the TiN + NbN composite precipitate. The inventors have investigated the size and amount of TiN + NbN composite precipitates and their distributions according to the ratio of Ti and N (Ti / N) and the ratio of Nb / N, and the results indicate that Ti / N is 1.2 to 2.5 and Nb / N. When the ratio is 0.3-9, TiN precipitates and Ti-Nb composite precipitates having a size of 0.01-0.1 μm were precipitated at 1.0 × 10 ⁷ / mm 2 or more, and the intervals of the precipitates were confirmed to be 0.5 μm or less.

As described above, when the ratio of Ti / N is controlled to 2.5 or less (increasing the content of N) according to the present invention, the solubility area showing the high temperature stability of TiN is also lowered. Increasing the nitrogen content at the same Ti content combines all the dissolved Ti atoms with the nitrogen atom, increasing the fine TiN precipitation amount, so that the solubility product showing the stability of the precipitate at high temperature such as the weld heat affected zone is reduced. Therefore, in a high nitrogen environment, TiN precipitates are more stable than nitrogen-containing precipitates because the amount of solid solution Ti decreases. At this time, the important thing is that the presence of solid solution N due to high nitrogen can promote aging, so the ratio of N / B, Al / N, Nb / N, and overall management of these to precipitate N as BN, AlN, NbN It is to let.

[2] ferrite grain size management

According to the study of the present invention, in order to average the size of the old austenite in the welding heat affected zone to 80㎛, the size of the ferrite finely in the range of about 3 ~ 6㎛ in the base material structure of ferrite + pearlite with the management of precipitates It is important to do. In order to obtain a fine ferrite grain size, it is necessary to maintain the size of the austenite grain below 20 μm during the heating process of the steel slab. At this time, the ferrite grain refinement is performed with the austenitic grain refinement by the steel processing during the control rolling. It is necessary to promote fine ferrite nucleation by securing deformation bands in the inside, and is obtained by inhibiting growth of ferrite grains using precipitates and carbides (Fe ₃ C, NbC, etc.) in the accelerated cooling process.

[3] microstructure, welded heat affected zones

The facts of the present invention reveal that the toughness of the weld heat affected zone is not only the size of the former austenite grains when the base material is heated above about 1400 ° C., but also the amount and size of the ferrite (more than 70%) precipitated at the old austenite grain boundaries. (Less than 20㎛) And its shape has an important effect. In the present invention, the Ti / N ratio is 1.2-2.5, the Nb / N ratio is 0.3-9, and the ratio of (Ti + 2Al + 4B + Nb) / N is 7-17. And by using precipitates such as AlN, BN and the like to produce a large amount of fine ferrite in the mouth of the austenite, the ferrite is mostly polygonal (ferrite) and needle-like ferrite greatly improve the toughness of the heat affected zone.

Hereinafter, the components of the inventive steel and their manufacturing conditions will be described in detail.

The content of carbon (C) is preferably 0.02-0.18%.

When the content of carbon (C) is less than 0.02%, securing strength as structural steel is insufficient. In addition, when C exceeds 0.18%, microstructures susceptible to toughness such as upper pearlite, martensite, and degenerate pearlite during cooling are transformed to lower the low temperature impact toughness of the structural steel, and Increasing strength results in deterioration of toughness and generation of weld cracks.

The content of silicon (Si) is preferably limited to 0.05-0.5%.

If the silicon content is less than 0.05%, the deoxidation effect of the molten steel is insufficient and the corrosion resistance of the steel during steelmaking process is insufficient, and if the content exceeds 0.5%, the effect is saturated, and the quenchability during cooling after rolling is increased. It promotes transformation of island martensite and lowers low-temperature impact toughness.

The content of manganese (Mn) is preferably limited to 0.4-2.0%.

The Mn is effective in improving deoxidation, weldability, hot workability, and strength in steel, and precipitates in the form of MnS around Ti-based oxides, thereby affecting the formation of needle-shaped and polygonal ferrites effective for improving the toughness of the weld heat affected zone. Crazy Such Mn forms a solid solution to form a solid solution in the matrix structure to enhance the solid solution to secure the strength and toughness, for this purpose it is preferably contained 0.4% or more. However, in case of exceeding 2.0%, tissue heterogeneity caused by Mn segregation not only has a detrimental effect on the toughness of weld heat affected zone, but also causes macro segregation and micro segregation depending on the segregation mechanism during steel solidification. It promotes the formation of central segregation zone, which acts as a cause of creating low temperature transformation tissue in the center of the base material.

The content of aluminum (Al) is preferably limited to 0.005-0.1%.

Al is an element that helps Ti to form fine TiN precipitates by forming fine AlN precipitates in steel as well as elements necessary as a deoxidizer, and forming oxygen and Al oxides to prevent Ti from reacting with oxygen. For this purpose, it is desirable to add more than 0.005% of Al, but if it exceeds 0.1%, AlN is precipitated and the formation of Weidmanstatten ferrite and phase martensite, in which the remaining solid solution Al is vulnerable to toughness during cooling of the weld heat affected zone. To reduce the toughness of the high heat input welding heat affected zone.

  The content of titanium (Ti) is preferably limited to 0.005-0.2%.

Ti is indispensable in the present invention because Ti is combined with N to form a fine TiN precipitate that is stable at high temperature. It is preferable to add more than 0.005% of Ti in order to obtain such a fine TiN precipitation effect, but when it exceeds 0.2%, coarse TiN precipitates and Ti oxides are formed in molten steel, and thus it is impossible to suppress the growth of the austenite grains of the weld heat affected zone. Because it is not desirable.

The content of the boron (boron, B) is preferably limited to 0.0003-0.01%.

B is a very effective element for producing polygonal ferrite at grain boundaries as well as acicular ferrite having excellent toughness in grains. B forms a BN precipitate, which hinders the growth of the old austenite grains and forms Fe carbide in the grain boundary and in the mouth to promote ferrite transformation of acicular and polygonal toughness. If the B content is less than 0.0003%, such an effect cannot be expected, and if it exceeds 0.01%, the hardenability increases, which may cause hardening of the weld heat affected zone and low temperature cracking.

The content of nitrogen (N) is preferably limited to 0.008-0.03% after the immersion treatment.

The N is an indispensable element for forming precipitates such as TiN, AlN, BN, NbN, and VN. The N suppresses the growth of the austenite grains in the weld heat affected zone at the time of high heat input welding, and the TiN, AlN, BN, VN, Increase the amount of precipitates such as NbN. In particular, since the TiN and AlN precipitates have a remarkable effect on the size, precipitate spacing, precipitate distribution, complex precipitation frequency, and high temperature stability of the precipitate itself, the content is preferably set at 0.008% or more. However, if the nitrogen content exceeds 0.03%, the effect is saturated, and toughness decreases due to the increase of the solid solution nitrogen distribution in the weld heat affected zone, and it is mixed in the weld metal due to dilution during welding, which may cause the toughness of the weld metal. Can be.

In the present invention, N is managed in the steel slab to less than 0.005% of the low probability of occurrence of the surface cracks of the slab, and then made into high nitrogen steel of 0.008-0.03% through the immersion treatment in the slab reheating process.

The content of Nb is preferably limited to 0.005-0.1%.

The Nb is a useful element that combines with Ti to form a Ti-Nb composite precipitate to promote needle ferrite transformation in the weld heat affected zone. To form a fine Ti-Nb composite precipitate, Nb content of 0.005% or more is added. desirable. However, if the Nb content exceeds 0.1%, the low temperature transformation structure is increased in the weld heat affected zone, which adversely affects the mechanical properties.

The content of phosphorus (P) is preferably limited to 0.030% or less.

P is preferably as low as possible because it is an impurity element that promotes central segregation during rolling and hot cracking during welding. In order to improve the toughness of the base metal, the toughness of the weld heat affected zone, and to reduce the center segregation, it is recommended to manage it to 0.03% or less.

     The content of sulfur (S) is preferably set to 0.003-0.05%.

S is an element that improves the strength of the weld heat affected zone, and reacts with Cu to form CuS to improve strength (or hardness), and complex precipitation on TiN precipitate to improve high temperature stability of TiN precipitate. For this purpose, S should be contained 0.003% or more, but when it exceeds 0.05%, the effect is saturated and there is a possibility of promoting cracking under the surface of the slab in the slab during playing, and forming a low melting point compound such as FeS during welding. It is not preferable because it may cause welding hot cracking.

The content of oxygen (O) is preferably limited to 0.005% or less.

When the oxygen exceeds 0.005%, the Ti element is not preferable because Ti element is formed of Ti oxide in molten steel and does not form TiN precipitate. Also, inclusions such as coarse Fe oxide and Al oxide are formed, which is bad for the toughness of the base metal. It is not desirable because it affects.

In the present invention, the ratio of Ti / N of steel is 1.2-2.5, the ratio of N / B is 10-40, the ratio of Al / N is 2.5-7, the ratio of Nb / N 0.3-9, (Ti + 2Al + 4B + Nb) / It is preferable to impregnate nitrogen so that ratio of N may satisfy 7-17.

It is preferable to make ratio of said Ti / N into 1.2-2.5.

In the present invention, the Ti / N ratio is lowered to 2.5 or less, which has two advantages. First, it is possible to increase the amount of TiN, that is, the number of TiN precipitates. In other words, if the nitrogen content is increased at the same Ti content, all the Ti atoms dissolved in the cooling process combine with the nitrogen atom, thereby increasing the fine TiN precipitation. Second, TiN is stable at high temperatures. That is, since the solubility product which shows the stability of the precipitate at high temperature such as the weld heat affected zone becomes smaller, the precipitate such as high nitrogen TiN is more stable than the case where the nitrogen content is low. On the other hand, if the Ti / N ratio is higher than 2.5, coarse TiN is determined and no uniform distribution of TiN is obtained. Also, excess Ti remaining without precipitation into TiN is in solid solution, which is bad for the toughness of the weld heat affected zone. Affect If the Ti / N ratio is less than 1.2, the amount of solid solution nitrogen in the base metal increases, which is detrimental to the toughness of the weld heat.

It is preferable that the ratio of said N / B shall be 10-40.

In the present invention, if the N / B ratio is less than 10, the precipitation amount of BN that promotes the ferrite transformation of polygons at the old austenite grain boundary during the post-weld cooling process is insufficient, and when the N / B ratio is over 40, the effect is saturated and dissolved. This is because the amount of nitrogen is increased to lower the toughness of the weld heat affected zone.

It is preferable that the ratio of Al / N is 2.5-7.

In the present invention, when the Al / N ratio is less than 2.5, the AlN precipitates for inducing needle-like ferrite transformation are insufficient, and the amount of solid solution nitrogen in the weld heat affected zone may increase, resulting in a weld crack and an Al / N ratio of 7 If exceeded, the effect is saturated.

In addition, it is preferable that the ratio of Nb / N after a immersion process shall be 0.3-9.

In the present invention, when the Nb / N ratio is less than 0.3, it is difficult to secure an appropriate number and size of NbN precipitates deposited and distributed at the TiN precipitate boundary for improving the toughness of the weld heat affected zone. When the Nb / N ratio exceeds 9, the size of NbN precipitates deposited on the TiN precipitate boundary is coarsened, so that the number of NbN precipitates deposited on the TiN precipitate boundary is reduced, thereby reducing the ferrite phase fraction effective for the toughness of the weld heat affected zone. Let's do it.

It is preferable that ratio of said (Ti + 2Al + 4B + Nb) / N is 7-17.

In the present invention, when the ratio of (Ti + 2Al + 4B + Nb) / N is less than 7, the growth inhibition of the austenite grain growth of the weld heat affected zone, the formation of fine polygonal ferrite at the grain boundary, the amount of solid solution nitrogen, the needle shape in the grain and Insufficient size and distribution of TiN, AlN, BN and NbN precipitates for polygonal ferrite generation and control of tissue fraction, and the effect is saturated when (Ti + 2Al + 4B + Nb) / N exceeds 17 .

In the present invention, in order to further improve the mechanical properties in the steel composition as described above, one or more selected from the group consisting of Ni, Cu, V, Mo, and Cr is further added.

The content of nickel (Ni) is preferably limited to 0.1-3.0%.

Ni is an effective element which improves the strength and toughness of the base material by solid solution strengthening. In order to achieve this effect, the Ni content is preferably 0.1% or more, but when the content exceeds 3.0%, the hardenability is increased to reduce the toughness of the weld heat affected zone and the possibility of high temperature cracking in the weld heat affected zone and the weld metal. This is not desirable because there is.

 The content of copper (Cu) is preferably limited to 0.1-1.5%.

The Cu is an element to improve the strength of the weld heat affected zone, the CuS precipitate and the solid solution strengthening effect is not enough to show the strength improvement effect at less than 0.1%, the effect is saturated when more than 1.5%, rather the small portion of the weld heat affected zone It is not preferable because it increases the grain size and lowers the toughness and also dilutes the weld metal during welding, thereby lowering the toughness of the weld metal.

The content of the vanadium (V) is preferably limited to 0.01-0.10%.

V is an element that combines with N to form VN to promote ferrite formation in the weld heat affected zone, and VN precipitates alone or precipitates in TIN precipitates to promote ferrite transformation. In addition, V combines with C to form VC, which acts to inhibit ferrite grain growth after ferrite transformation. When the V content is less than 0.01%, it is difficult to obtain the ferrite transformation promoting effect in the weld heat affected zone because the VN deposition amount is small. On the other hand, exceeding 0.10% is not preferable because it causes toughness of the base metal and the welded heat affected zone (HAZ), improves the weld hardening property, and there is a risk of welding low temperature crack.

The content of chromium (Cr) is preferably made 0.05 to 1.0%.

The Cr increases the hardenability and also improves the strength. If the content is less than 0.05%, the strength cannot be obtained and if the content exceeds 1.0%, the base metal and the HAZ toughness deteriorate.

It is preferable that the addition amount of the said molybdenum (Mo) is 0.05-1.0%.

Mo is also an element that increases the hardenability and improves the strength. The content thereof is 0.05% or more for securing strength, but the upper limit is set to 1.0% similarly to Cr in order to suppress HAZ hardening and welding low temperature cracking.

In addition, in the present invention, one or two kinds of Ca and REM are further added to suppress grain growth of the former austenite in the weld heat affected zone during welding.

The amount of Ca added is preferably set to 0.0005 to 0.005%, and the content of REM is set to 0.005 to 0.05%.

The Ca and REM form an oxide having excellent high temperature stability to suppress the growth of the austenite grains when heated in the base material and to improve the toughness of the weld heat affected zone. In addition, Ca has the effect of controlling the coarse MnS shape during steelmaking. To this end, it is preferable to add more than 0.0005% of calcium (Ca) and more than 0.005% of REM. However, when Ca exceeds 0.005% or REM exceeds 0.05%, large inclusions and clusters are generated to generate cleanliness of the steel. Will hurt. As REM, 1 type, or 2 or more types, such as Ce, La, Y, and Hf, may be used, and any of the above effects can be obtained.

[Method of manufacturing welded structural steel]

[Slab manufacturing process]

Since molten steel of the present invention is a low level of ordinary nitrogen, the casting speed may be either high speed or low speed during continuous casting. In order to obtain good internal quality, it is preferable to make it into the range of 0.9-1.5 m / min.

[Slab Reheating Process (Neutization)]

In the present invention, by controlling the ratio of Ti and N through the nitriding treatment in the slab furnace, the amount of very fine TiN precipitates and Ti-Nb composite precipitates is increased and the amount of solid solution Ti in the weld heat affected zone during welding is reduced. Thereby maximally inhibiting Oswald ripening.

In addition to the fact that the nitriding effect in slab furnaces can make slabs from low-nitrogen molten steel, it is possible to fundamentally prevent the problems of playing surface cracks commonly found in high-nitrogen steels. have. The first is to increase the amount of fine TiN precipitates, and the second is to stabilize the fine precipitated TiN at high temperature. That is, by increasing the nitrogen content in the base material at the same Ti content through the nitriding treatment, all Ti atoms can be combined with the nitrogen atoms during the heat treatment in the slab furnace to increase the amount of fine TiN precipitates.

On the other hand, in the present invention, it is preferable to carry out the slab while heating the slab for 100-180 minutes under the setting of the above-mentioned denitrification treatment at 1000-1150 ° C, so that the nitrogen concentration of the slab is 0.008-0.03%. First, it is preferable to set the amount of nitrogen in the slab to 0.008-0.03%. In order to secure an appropriate amount of TiN precipitation in the slab, nitrogen must be contained in 0.008% or more, but when it exceeds 0.03%, it diffuses into the slab. This is because the amount of nitrogen deposited on the surface of the slab increases more than the amount of nitrogen precipitated with fine TiN, which causes hardening on the surface of the slab, which affects the subsequent rolling process.

In addition, the slab heating temperature is set to 1000-1150 ℃, the reason is that if the heating temperature is less than 1000 ℃, the driving force to diffuse the precipitated nitrogen is small, the number of fine TiN precipitates and Ti-Nb complex precipitates is reduced, In addition, since the heating time has to be increased in order to increase the number of TiN precipitates, there is a problem in that manufacturing cost increases. On the other hand, when the heating temperature is higher than 1150 ° C, the precipitate is decomposed or grown, which is not preferable because the austenite grains grow. On the other hand, when the slab heating time is less than 100 minutes, the sedimentation effect is not exerted, and when the heating time is longer than 180 minutes, not only the cost of the unworking industry increases but also the austenite grain growth in the slab causes the subsequent rolling process. It is not desirable because it affects.

When subjected to the nitriding treatment according to the present invention, the ratio of Ti / N in the slab is 1.2 to 2.5, the ratio of N / B is 10 to 40, the ratio of Al / N is 2.5 to 7, and the ratio of Nb / N is 0.3 It is preferable to immerse N so that it may become -17, and ratio of (Ti + 2Al + 4B + Nb) N will be 7-17.

 [Hot Rolling Process]

As above, the slab is heated to make the austenite grain size below 20 μm through the distribution of fine precipitates, and then the heated slab is in the range of 10 to 20 ° C./sec up to a temperature range of 880 to 930 ° C., which is an austenite uncrystallized region. Cool at speed and then hot roll at a rolling reduction of at least 50%.

If the rolling temperature is less than 880 ℃ because the ferrite coarsening occurs due to the rolling in the ferrite-austenite or higher region, it is not preferable, if the rolling temperature is above 930 ℃ it corresponds to the austenite recrystallization zone coarsening austenite Because it is not desirable. In addition, when the cooling rate is less than 10 ° C / sec austenite grain growth is not preferable, if it exceeds 20 ° C / sec it is difficult to secure the austenite uncrystallized region, it is difficult to secure fine ferrite grains during subsequent rolling to be. In addition, if the reduction ratio is less than 50%, the amount of strain bands formed in the austenite grains is small, which is undesirable for ferrite nucleation.

      After hot rolling as described above it is cooled to a temperature in the range of 740 ~ 780 ℃ at a cooling rate of 5-15 ℃ / sec range to roll at a rolling ratio of more than 40%. When the rolling temperature is less than 740 ° C., a large amount of coarse ferrite precipitates in the steel sheet before rolling and remains in a coarse stretched form after rolling, which has a detrimental effect on the refinement of ferrite. When the rolling temperature exceeds 780 ° C., deformation organic transformation does not occur, so it is difficult to obtain fine ferrite, and even if deformation organic transformation occurs, the grain size of ferrite is not preferable.

In addition, if the cooling rate is less than 5 ℃ / sec is not preferable because austenite grain growth occurs, if it exceeds 15 ℃ / sec is not preferable because it is difficult to secure a fine ferrite. It is preferable to make the reduction ratio at this time into 40% or more. If the reduction ratio is less than 40%, ferrite nucleation is lowered, which makes it difficult to secure fine ferrite.

After hot rolling as described above, it is acceleratedly cooled to room temperature at a cooling rate of 5 to 20 ° C / sec. If the cooling rate is less than 5 ℃ / sec it is difficult to suppress the growth of ferrite grains, and if the cooling rate is faster than 20 ℃ / sec is preferable because residual austenite or martensite is formed, which adversely affects the base material toughness Not.

[Microstructure of Steel]

In the present invention, after the hot rolling, the microstructure of the steel is made of ferrite + pearlite, and the size of the ferrite grains is preferably in the range of 3 to 6 μm. The reason is that the smaller the grain size of the ferrite, the higher the strength and the toughness can be secured at the same time.

In addition, the higher the phase ratio of the fine ferrite in the composite structure of ferrite + perlite, the toughness and elongation of the base material increases, but the fine ferrite is most preferably 80% or more.

[Distribution of precipitates (TiN precipitates and TiN + NbN composite precipitates)]

The base material of the present invention is to wind the TiN precipitates and Ti-Nb composite precipitate distribution per 1mm ² 1.0x10 ^seven or more in size 0.01-0.1㎛. If the precipitate size is less than 0.01㎛, it is easily re-used in the base metal during the high heat input welding, so that the effect of inhibiting austenite grain growth is insufficient, and if it exceeds 0.1㎛, the pinning (grain growth inhibition) effect on the austenite grain is small. It behaves like highly coarse nonmetallic inclusions and has a detrimental effect on mechanical properties. If the number of these fine precipitates is less than 1.0 × 10 ⁷ per 1 mm ² , it is difficult to control less than 80 μm, which is the critical austenite grain size of the weld heat-affected zone when welding more than a large heat input. Since these precipitates are more advantageous in suppressing the Ostwald ripening phenomenon in which the precipitates are coarse when uniformly distributed, it is preferable to control the interval of the TiN precipitates to 0.5 μm or less.

  Casting of the steel in the present invention can be produced by slab by continuous casting or mold casting. In this case, if the cooling rate is fast, it is advantageous to finely disperse the precipitates, and thus, continuous casting having a high cooling rate is preferable. For the same reason, slabs are advantageously thinner. In the hot rolling process of the slab, hot charge rolling and direct rolling may be applied according to a user's use, and various techniques such as control rolling and control cooling may be applied. In addition, heat treatment may be applied to improve the mechanical properties of the hot rolled sheet produced according to the present invention. However, even if the well-known techniques are applied to the present invention, it is natural that they are interpreted to be substantially within the technical scope of the present invention as a simple change of the present invention.

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

EXAMPLE

The steel grades having the composition as shown in Table 1 were prepared as steel slabs by a continuous casting method by melting them in a converter, and then, the steel slabs were immersed under the conditions shown in Table 3 below to obtain alloy element composition ratios of Table 2 below. Then, a hot rolled sheet was manufactured under the conditions shown in Table 3 below.

From the hot-rolled sheet as described above to take the test pieces for evaluating the mechanical properties of the base material to measure the mechanical properties, the results are shown in Table 4 below.

The test pieces were taken from the center of the plate thickness of the rolled material, the tensile test piece was taken in the rolling direction, and the Charpy impact specimen was taken in the direction perpendicular to the rolling direction.

  Tensile test piece was used KS standard (KS B 0801) No. 4 test piece and the tensile test was tested at a cross head speed (5 mm / min). The impact test piece was manufactured according to KS (KS B 0809) No. 3 test piece, and the notch direction was processed on the side of the rolling direction (L-T) in the case of the base material and in the welding line direction on the welding material. In addition, the size and number and intervals of precipitates and oxides that have a significant effect on the analysis of the microstructure after the cooling and the toughness of the weld affected zone were investigated, and the results are shown in Table 4 below.

The size, number and spacing of the precipitates and oxides were measured by a point counting method using an image analyzer and an electron microscope. At this time, the test surface was evaluated based on 100 mm 2.

In addition, the tissue properties of the base material, that is, ferrite grain size (FGS) and the ferrite phase fraction were investigated, and the results are shown in Table 4 below.

In addition, the austenitic grain size according to the maximum heating temperature of the weld heat affected zone, the weld heat affected zone microstructure (ferrite average grain size (FGS) and ferrite phase fraction) of 100 kJ / cm heat input, welded mechanical properties and impact toughness And the results are shown in Table 5 below.

The austenite grain size according to the maximum heating temperature of the weld heat affected zone is maintained at 140 ° C./sec until the maximum heating temperature (1200 to 1400 ° C.) using a reproducing welding simulator (simulator), and then maintained for 1 second. And quenched using He gas. The quenched specimens were ground and corroded to determine the austenite grain size at the highest heating temperature condition by KS (KS D 0205).

In other words, the weld heat-affected zone austenite grain size was measured by rapidly heating the maximum heating temperature to 1200, 1300, 1400 ℃ and maintained for 1 second, then quenched with helium gas and corroded grain boundaries. In addition, the impact toughness of the weld heat affected zone was evaluated by welding conditions corresponding to about 80 kJ / cm, 150 kJ / cm, and 250 kJ / cm corresponding to the actual heat input of the welding. After the cooling heat cycles of 60 seconds, 120 seconds, and 180 seconds were given a welding heat cycle, the surface of the test piece was polished, processed into an impact test piece, and evaluated by Charpy impact test at -40 ° C. .

As shown in Table 4, the number of precipitates (TiN and Ti-Nb composite precipitates) of the hot rolled material produced by the present invention has a range of 2.3 X 10 ⁸ / mm ² or more, whereas in the case of conventional steel It shows a range of 4.07 X 10 ⁶ / mm ² or less, which shows that the invention material has a fairly uniform and fine precipitate size compared to the conventional material, and the number thereof is also significantly increased. On the other hand, in the structure of the base steel of the present invention, the steel of the present invention has a ferrite grain size (FGS) of about 3 to 6 μm, which is very fine compared to the comparative material, and the base ferrite percentage of the steel of the present invention is all higher than 80%. It can be seen that it consists of a fraction.

On the other hand, as shown in Table 5, when looking at the austenite grain size at the maximum heating temperature 1400 ℃ conditions, such as the welding heat affected zone has a range of less than 70㎛ in the case of the present invention, in the case of conventional materials of about 180㎛ or more You can see that it has a very broad range. Therefore, in the present invention, it can be seen that the austenite grain suppression effect of the weld heat affected zone during welding is very excellent. In addition, the ferrite phase fraction of the steel of the present invention at a heat input of 100 kJ / cm is about 70% or more.

 As described above, the present invention is stable even at high temperature through the immersion treatment of a low-level nitrogenous slab of a normal level, and has excellent matrix material properties using the Ti-Nb composite precipitate together with fine TiN precipitates, and at the same time, excellent weld heat affected zone properties. In the development of weldable structural steel materials, TiN and Ti-NB composite precipitates are used to suppress the growth of austenitic grains affected by high heat input welding heat and to promote the transformation of polygonal and needle-like ferrites within the grains. There is a useful effect to provide ferrite fine grained steel (welding structural steel) that can secure the impact toughness at the same time.

Claims (3)

By weight%, C: 0.02-0.18%, Si: 0.05-0.5%, Mn: 0.4-2.0%, Ti: 0.005-0.2%, Al: 0.005-0.1%, N: 0.005% or less, B: 0.0003-0.01 Preparing a normal level steel slab satisfying%, Nb: 0.005-0.1%, P: 0.03% or less, S: 0.003-0.05%, O: 0.005% or less and composed of the remaining Fe and other impurities; The steel slab prepared as described above was heated at a temperature of 1000-1150 ° C. for 100-180 minutes, whereby the N content of the steel became 0.008-0.03%, and the content of N, Ti, B, Al, and Nb satisfied the following relationship. Soaking to obtain; And 1.2≤Ti / N≤2.5, 10≤N / B≤40, 2.5≤Al / N≤7, 0.3≤Nb / N≤9, 7≤ (Ti + 2Al + 4B + Nb) / N≤17 Maintain the austenite grain size of the steel slab treated as described above to 20㎛ or less and then cooled at a cooling rate in the range of 10 ~ 20 ℃ / sec at a rolling ratio of 50% or more in the 880 ~ 930 ℃ range of austenite uncrystallized Hot rolling, followed by cooling to a range of 740-780 ° C. at a cooling rate of 5-15 ° C./sec and rolling at a hot rolling ratio of 40% or more, followed by cooling to a room temperature of 10-20 ° C./sec. Method for producing ferritic fine grained steel with excellent toughness of weld heat affected zone The steel group of claim 1, wherein the steel comprises a weight% of Ni: 0.1 to 3.0%, Cu: 0.1 to 1.5%, V: 0.01 to 0.1%, Mo: 0.05 to 1.0%, and Cr: 0.05 to 1.0%. Method for producing ferritic fine grain steel excellent in toughness of the weld heat affected zone, characterized in that it contains one or two or more selected from. According to claim 1 or 2, wherein the steel contains one or two selected from the group consisting of Ca: 0.0005 ~ 0.005% and REM: 0.005 ~ 0.05% ferrite excellent in the heat resistance of the weld zone, characterized in that the toughness. Manufacturing method of fine grain steel