KR20090122371A - Steel material having excellent high-temperature strength and toughness, and method for production thereof - Google Patents

Abstract

Disclosed is a steel material having excellent high-temperature properties and toughness. Specifically disclosed is a fire-resistant steel material which comprises the following components (by mass): C: 0.001-0.030%, Si: 0.05-0.50%, Mn: 0.40-2.00%, Nb: 0.03-0.50%, Ti: not less than 0.005 and less than 0.040%, and N: not less than 0.0008% and less than 0.0050%, in which the contents of P and S are limited to 0.03 mass% or less and 0.02 mass% or less, respectively, and in which the remainder comprises Fe and unavoidable impurities. In the steel material, a requirement represented by the following formula is satisfied: C-Nb/7.74 <= 0.004. The steel material contains a Ti-containing oxide having a particle diameter of 0.05 to 10 μm at a density of 30 to 300 particles/mm2.

Description

Steel material excellent in high temperature strength and toughness and its manufacturing method {STEEL MATERIAL HAVING EXCELLENT HIGH-TEMPERATURE STRENGTH AND TOUGHNESS, AND METHOD FOR PRODUCTION THEREOF}

TECHNICAL FIELD This invention relates to the steel material excellent in high temperature strength and toughness, and its manufacturing method.

The review of fireproof design was carried out by the Japan Ministry of Construction comprehensive project from the super high-rise of building, the advancement of building design technology, etc., and "New fireproof design method" was enacted in March, 1987. As a result, the limitation regarding the fireproof coating which sets the temperature of steel materials to 350 degrees C or less at the time of a fire is reviewed, and the appropriate fireproof coating method can be selected from the relationship between the high temperature strength of steel materials and the actual load of a building. Therefore, when the high temperature strength which satisfy | fills the design standard at 600 degreeC can be ensured, ie, the steel material with high high temperature strength in 600 degreeC is used, the fireproof coating was simplified and reduced.

To cope with these trends, the reinforcing mechanism of high temperature strength at 600 ° C of steel, (1) miniaturization of ferrite grain size, (2) solid solution strengthening by alloying elements, and (3) dispersion strengthening by hardening phase , (4) During precipitation strengthening by fine precipitates, refractory steels mainly using precipitation strengthening have been developed.

Conventionally, there have been many proposals for refractory steels which add Mo, Ti, Nb, etc., which contribute to precipitation strengthening, to secure high-temperature strength by carbides, nitrides, and the like. , Weldability deteriorated.

In response to these problems, hot rolled steel strips are proposed to secure high-temperature strength, improve toughness and weldability by reducing C and Mo, and controlling end temperature and winding temperature of hot rolling (examples) See, for example, Japanese Patent Application Laid-open No. Hei 7-300618).

However, this precipitates fine Mo and Nb carbides at the time of winding, and does not use solid solution Nb, so that the high temperature strength is not sufficient, and Ti is contained to weld heat affected zones (Heat Affected Zone, HAZ). Since the deposition of nitrides in the furnace is not suppressed, the toughness of the HAZ may be reduced.

In addition, steel sheets and steel pipes have been proposed in which C and Mo are reduced, high temperature altitude is increased by solid solution Nb, solid solution C and solid solution N are reduced, and the formability of cold working is secured (for example, Japanese Patent Application Laid-Open). 10-176237, Japanese Patent Application Laid-Open No. 2000-54061, and Japanese Patent Application Laid-Open No. 2000-282167. However, since these have high Ti / N, coarse TiN precipitates, and the fall of toughness of HAZ is especially concerned.

In addition, in order to secure high temperature strength, toughness and weldability, refractory steels using Mo and reducing solid solution and precipitation of Cu have also been proposed (see, for example, Japanese Patent Application Laid-Open No. 2002-115022). This is not to increase the high temperature strength by the solid solution Nb, but to lower the recrystallization temperature by adding Nb to atomize the crystal grains and to use the precipitation strengthening of Nb.

In addition, all the steel materials proposed by the patent document exemplified above did not consider reheat embrittlement in HAZ. Reheat embrittlement is high temperature embrittlement which is embrittled by precipitation of carbide and nitride when HAZ is heated to high temperature again.

In addition, the ultra-thick H-shaped steel mainly used as a pillar material for high-rise buildings is also low pressure drop and low cooling rate due to the increase in sheet thickness size, which is sufficient in comparison with relatively thin steels. In order to secure the strength in the prior art, it is necessary to add a large amount of alloying elements in order to secure the work heat treatment, and in this case, there has been a problem in which toughness decreases, weldability decreases, and the like.

The present invention provides a high temperature property and a base material and a HAZ including intrinsic heat embrittlement characteristics in a weld heat affected zone in a hot rolling state, that is, after hot rolling, without performing a heat treatment such as cold rolling, quenching, or tempering. It is to provide a steel material excellent in toughness and usable as a refractory steel or ultra-thick H-shaped steel, and a manufacturing method thereof.

The present invention restricts the contents of C and N, adds an appropriate amount of Nb, defines the relationship between C and Nb, and drags the solid solution Nb (the employed Nb is concentrated into lattice defects such as dislocations, so that a defect or dislocation High temperature strength) and fine Ti-based oxides for pinning of grain boundaries and formation of intragranular transformations, suppressing coarsening of HAZ, In order to improve the high temperature characteristics such as internal heat embrittlement, and to secure the toughness of the base metal and HAZ, the dissolved oxygen concentration in molten steel when Ti is added is adjusted to reduce the mechanical properties of Ti in the steel. It is the steel material which disperse | distributed fine oxide, and its manufacturing method.

The gist of the present invention is as follows.

(1) In mass%, C: 0.001% or more and 0.030% or less, Si: 0.05% or more and 0.50% or less, Mn: 0.40% or more and 2.00% or less, Nb: 0.03% or more and 0.50% or less, Ti: 0.005% or more and 0.040% or less It contains less than, N: 0.0008% or more and less than 0.0050%, is limited to P: 0.030% or less, S: 0.020% or less, the remainder contains Fe and unavoidable impurities, the content of C and Nb,

C-Nb / 7.74≤0.004

And having a Ti-based oxide having a particle diameter of 0.05 to 10 µm at a density of 30 to 300 particles / mm 2, wherein the steel has excellent high temperature characteristics and toughness.

(2) The steel material excellent in the high temperature characteristic and toughness as described in said (1) characterized by containing one or both of V: 0.10% or less and Mo: less than 0.10% by mass%.

(3) The steel material excellent in the high temperature characteristic and toughness as described in said (1) or (2) characterized by containing one or both of Zr: 0.03% or less and Hf: 0.01% or less by mass%.

(4) In mass%, it contains 1 type (s) or 2 or more types of Cr: 1.5% or less, Cu: 1.0% or less, Ni: 1.0% or less, The said any one of said (1)-(3) characterized by the above-mentioned. Steel materials excellent in the high temperature characteristics and toughness described.

(5) The mass (%), Mg: 0.005% or less, Al: 0.030% or less, REM: 0.01% or less, Ca: 0.005% or less of one or two or more characterized by containing (1) to ( Steel material excellent in the high temperature characteristic and toughness in any one of 4).

(6) The steel material excellent in the high temperature characteristic and toughness in any one of said (1)-(5) characterized by the product of the mass concentration product of Nb and C being 0.0015 or more.

(7) The steel material excellent in the high temperature characteristic and toughness in any one of said (1)-(6) characterized by the steel material being a refractory steel material.

(8) The steel material excellent in the high temperature characteristic and toughness in any one of said (1)-(6) characterized by the steel being extremely thick H-shaped steel with a flange thickness of 40 mm or more.

(9) After adjusting the dissolved oxygen to 0.003 to 0.015 mass% of the steel which consists of a component in any one of said (1)-(6), the steel piece obtained by adding Ti, solvent, and casting. Is heated to 1100 to 1350 ° C. and hot rolled to produce a steel having excellent high temperature characteristics and toughness.

(10) The method for producing a steel having excellent high temperature properties and toughness as described in the above (9), wherein the rolling reduction is performed at a cumulative reduction ratio of 1000% or less at 10% or more.

(11) After hot rolling, the temperature range from 800 deg. C to 500 deg. C is cooled at an average cooling rate of 0.1 to 10 deg. C / s. The high temperature property and toughness as described in the above (9) or (10) are excellent. Method of manufacturing steels.

According to the present invention, a steel material having sufficient room temperature strength and high temperature strength and excellent in toughness and intrinsic heat embrittlement properties of a base material and HAZ, in particular, refractory H-shaped steel or ultra-thick H-shaped steel is produced without cold working and tempering heat treatment. Thing or extremely thick H-shaped steel having a large plate thickness, for example, up to about 140 mm in flange thickness, can be manufactured while securing strength and toughness in a hot rolling state.

Among the steels, H-shaped steel produced by hot rolling is classified into parts of flanges, webs, and fillets from the shape thereof, and the rolling temperature history and the cooling rate are different according to the respective shapes. The steel having the composition of the present invention may be greatly changed, but the rolling finish temperature dependence and the cooling rate dependence on the strength and toughness are relatively small, and thus the variation of the material in the cross section of the H-shaped steel can be reduced. In addition, since the change in the material due to the thickness of the plate can be made small, particularly in a size with a large plate thickness such as ultra-thick H-shaped steel, it is possible to secure the strength and toughness and to reduce the variation in the cross-section of the H-shaped steel.

1 is a diagram showing the effect of C and Nb on the high temperature strength of steel materials.

2 is a diagram showing the effect of the number density distribution of Ti oxide on the toughness of HAZ of steel materials.

3 is a diagram showing the effect of the number density distribution of Ti oxide on the reheat embrittlement characteristics of steel materials.

4 is a diagram showing the effect of the relationship between the dissolved oxygen amount and the Ti amount before adding Ti to the density of the Ti-based oxide.

Fig. 5 is a schematic diagram of a shaped steel manufacturing process as an example of device arrangement for implementing the present invention method.

It is a figure which shows the cross-sectional shape of H-shaped steel and the collection position of a mechanical test piece.

MEANS TO SOLVE THE PROBLEM This inventor improves hardenability by addition of Nb, and produces | generates one or both of massive ferrite or bainite, and raises the strength and toughness at high temperature strength and normal temperature, and also obtains the steel material excellent in the intrinsic heat embrittlement characteristic. In addition, the drag effect of the solid solution Nb slowed the movement speed of the dislocation at high temperatures, thereby exhibiting resistance to softening at high temperatures, and securing the high temperature strength required as the refractory steel.

As a result, the following findings were obtained regarding the low C, the low N, and the addition of Ti in order to maximize the effect of Nb.

Low C and low N are effective for suppressing the production of polygonal ferrite and securing Nb in solid solution. NbC, which is a carbide of Nb, and NbN, which is a nitride, form nuclei of polygonal ferrite, and the solid solution Nb decreases due to their precipitation. Particularly, when a small amount of Nb carbides and nitrides are finely precipitated, they contribute to the improvement of strength due to precipitation strengthening, but when heated to high temperature after welding, NbC is a grain boundary (hereinafter referred to as γ grain boundary) of HAZ austenite. May precipitate and express reheat embrittlement.

Therefore, in order to secure the intrinsic heat embrittlement characteristic, it is very important to define the upper limits of the amount of C addition and the amount of N addition. Moreover, when carbon content exceeded 0.03%, the problem that the island-like martensite was produced partially and toughness fell remarkably was also proved.

In addition, when fine Ti-based oxides are dispersed in steel by controlled deoxidation with Ti, the grains are pinned and their growth is suppressed, resulting in a fine grain size. In particular, grain coarsening in thermal cycles such as 1400 ° C. heating and quenching as shown in HAZ can be prevented.

Thereby, it turned out that not only the HAZ toughness improves but also the high temperature embrittlement of HAZ is suppressed.

Based on the above findings, the present inventors also note that (1) the relationship between C and Nb and the high temperature strength of steel materials, and (2) Ti-based oxides when Ti is repeatedly deoxidized after adjusting dissolved oxygen by primary deoxidation. The effect of the particle size and water density distribution on the HAZ toughness and internal heat embrittlement properties was examined in detail.

This inventor is mass%, C: 0.001% or more and 0.030% or less, Si: 0.05% or more and 0.50% or less, Mn: 0.4% or more and 2.0% or less, Nb: 0.03% or more and 0.50% or less, Ti: 0.005% or more and 0.040 Dissolution at the time of adding Ti to a steel containing less than%, N: 0.0008% or more and less than 0.0050%, and limited to P: 0.03% or less and S: 0.02% or less, and the remaining portion contains Fe and unavoidable impurities. The steel piece obtained by changing the amount of oxygen, solvent, and casting was heated to 1100-1350 degreeC, the rolling reduction was carried out at 1000 degreeC or less to 30% or more, and hot-rolled, and the steel plate of 10-40 mm of sheet thickness was manufactured. .

The tensile test piece was extract | collected from the steel plate based on JISZ2201, the tension test at normal temperature was performed based on JISZ22241, and the tensile test at 600 degreeC was performed based on JISG0557. In addition, a small piece was taken from a steel sheet, heated to 1400 ° C. at a heating rate of 10 ° C./s for 1 s, and cooled to 10 s for the time required for cooling from 800 ° C. to 500 ° C. After performing a heat treatment (called HAZ reproduction heat treatment) to simulate the thermal history, the specimen was processed into a test piece and subjected to a Charpy impact test in accordance with JIS Z 2242. In addition, the particle diameter and density of Ti type oxide were measured using the scanning electron microscope.

FIG. 1: shows the relationship between content of C and Nb and high temperature strength, specifically, 0.2% yield strength (600 degreeCYS) in 600 degreeC with respect to C-Nb / 7.74. In FIG. 1, (circle) and (circle) are 600 degreeCYS of the steel materials whose tensile strength at normal temperature is 400 Mpa class, (square) is 600 degreeCYS of 490 MPa class steel materials.

From FIG. 1, when C-Nb / 7.74 becomes 0.004 or less, the 0.2% yield strength in 600 degreeC of the steel materials of normal temperature tensile strength of 400 Mpa class and 490 Mpa class exceeds a target value, and favorable high temperature strength is obtained. It can be seen that.

Figure 2 shows the effect of the water density distribution of Ti-based oxide having a particle diameter of 0.05 to 10㎛ in HAZ toughness in steel. 2 shows that in order to obtain favorable HAZ toughness, it is necessary to disperse | distribute Ti-type oxide whose particle diameter is 0.05-10 micrometers at the ratio of 30-300 piece / mm <2>.

In addition, using a tensile test piece of a round bar, HAZ heated to 1400 ° C. at a temperature rising rate of 10 ° C./s and held for 1 s, and cooled to 100 ° C. with a time required for cooling from 800 ° C. to 500 ° C. as 10 s. After the regeneration heat treatment was performed, the temperature was raised to 10 ° C / s and reheated to 600 ° C to measure the drawing value, that is, the reheat drawing.

As a result, as shown in Fig. 3, in the steel having excellent HAZ toughness in which the dispersion of the Ti-based oxide is in the above-described range, a good result of 30% or more of the reheat drawing is obtained, and it is confirmed that the intrinsic heat embrittlement characteristic is also excellent. It became.

4 shows the effect of the relationship between the amount of dissolved oxygen and the amount of Ti before adding Ti to the density of the Ti-based oxide. The numerical value of FIG. 4 is the density of Ti type oxide whose particle diameter is 0.05-10 micrometers. From FIG. 4, in order to obtain the steel material containing Ti-type oxide of 0.05-10 micrometers of particle diameter which has favorable HAZ toughness in the ratio of 30-300 piece / mm <2>, dissolved oxygen after primary deoxidation before Ti addition is made into mass%. It turns out that it is necessary to adjust to 0.003 to 0.015%, preferably 0.003 to 0.010%, and to make the content of Ti less than 0.005 to 0.040%, preferably 0.005 to 0.020%.

As mentioned above, especially in refractory shaped steels which do not contain B, after low-C and low-N, and further optimizing the relationship between C and Nb, the particle size and the number density of the Ti-based oxide, solid solution Nb is secured, Precipitation of carbides and nitrides at the γ grain boundary was suppressed, and it was found to be very effective in preventing reheat embrittlement.

In addition, as another advantage of the present component system, the balance between the elements contributing to steel strength and toughness while maintaining appropriate hardenability by solid solution Nb is very good, and the strength due to the cooling rate in the cooling process after heating and Since there is little dependence on toughness and there is very little variation in characteristics, when applied to a large sheet thickness, strength and toughness can be maintained at a high level in all parts, and it is known that it is a chemical component suitable for ultra thick H-beams. Could.

This invention based on the above knowledge is demonstrated in detail below. First, the Ti-based oxide will be described.

Particle diameter and density of Ti oxide:

The present invention is a refractory steel in which the grain coarsening of HAZ is suppressed by the effect of pinning, and the HAZ toughness and reheat embrittlement property are improved by using a finely dispersed Ti oxide. The lower limit of the particle size of the Ti-based oxide effective for this pinning is 0.05 µm or more. If the particle diameter of the Ti-based oxide is more than 10 µm, it becomes a starting point of fracture and inhibits toughness.

Moreover, 30-300 pieces / mm <2> is effective for the improvement of HAZ toughness and reheat embrittlement characteristic. If the density of the Ti-based oxide having a particle diameter of 0.05 to 10 µm is less than 30 pieces / mm 2, the effect of pinning is insufficient. On the other hand, when the density of the Ti-based oxide having a particle diameter of 0.05 to 10 µm exceeds 300 pieces / mm 2, propagation of cracks is promoted to impair toughness.

In addition, Ti-based oxides include composite oxides of TiO ₂ , Ti ₂ O ₃ , Si-based oxides such as SiO ₂ , and Al-based oxides such as Al ₂ O ₃ , sulfides such as MnS, and nitrides such as TiN. It is a general term of the oxide containing complex precipitated Ti.

The particle size and density of the Ti-based oxide can be measured using a scanning electron microscope (SEM). It is preferable to use SEM which has an energy dispersive X-ray analyzer for identification of Ti type oxide. Ti-based oxide is crystallized in a liquid phase and is not stretched even in hot rolling, and is observed as a spherical inclusion. When an energy dispersive X-ray analyzer is used, it can be confirmed that the spherical inclusions are oxides containing Ti.

By SEM, a density can be calculated by observing several fields of view, preferably 20 o'clock or more at 5000 to 10,000 times, counting the number of inclusions, and dividing by the area of the observation site. Incidentally, inclusions having a particle diameter of less than 0.05 µm or more than 10 µm do not contribute to the toughness improvement and are therefore ignored when calculating the density.

Dissolved oxygen amount before Ti addition:

In order to make a Ti-based oxide having a particle diameter of 0.05 to 10 µm and a density of 30 to 300 particles / mm 2 in steel, the amount of dissolved oxygen before adding Ti at the time of melting the steel is important. If the amount of dissolved oxygen before Ti addition is less than 0.003%, the particle size of the Ti-based oxide becomes small and the density decreases. On the other hand, when the amount of dissolved oxygen before Ti addition exceeds 0.015%, the particle diameter of the Ti-based oxide exceeds coarse to 10 µm, thereby inhibiting toughness. Therefore, the dissolved oxygen amount before adding Ti was made into 0.003 to 0.015% of range. When the steel is dissolved, the amount of dissolved oxygen can be 0.003 to 0.015% by deoxidation using Si and Mn as deoxidizers before adding Ti.

Next, the component of the fire resistant steel of this invention is demonstrated.

C is an element for reinforcing steel, and addition of 0.001% or more is necessary to obtain the strength required as structural steel. On the other hand, when more than 0.030% of C is added, coarse carbides are generated in the HAZ to reduce toughness and reheat brittleness, and further, island-like martensite is generated between the bainite phase laths, thereby decreasing the toughness of the base metal. Therefore, the lower limit of the amount of C was made 0.001% and the upper limit was made 0.030%. Moreover, it is preferable to set a minimum as 0.005% and an upper limit as 0.020% from a viewpoint of reheat brittleness and toughness.

Si is an important deoxidizer and an element which contributes to the improvement of strength in this invention. In order to make dissolved oxygen of molten steel before adding Ti into 0.003 to 0.015 mass%, and to secure the strength of a base material, 0.05% or more of Si addition is required. On the other hand, when Si amount exceeds 0.50%, oxide of a low melting point will be produced and scale peelability will deteriorate. Therefore, Si amount is made into 0.05% or more and 0.50% or less. Moreover, when Si amount exceeds 0.40%, the nonuniformity at the time of hot dip plating may arise, and aesthetics may be impaired. Therefore, it is preferable to make the upper limit of Si amount 0.40% or less.

Mn is an important deoxidizer in the present invention, and is an element that increases the hardenability and increases the amount of bainite structure to contribute to the improvement of strength and toughness. In order to make dissolved oxygen of molten steel before adding Ti into 0.003 to 0.015 mass%, and to secure the strength and toughness of a base material, 0.40% or more of addition is required. On the other hand, Mn is an element that tends to segregate to the center of the steel sheet when the steel sheet is produced in continuous casting. When Mn is added in excess of 2.00%, the hardenability of the segregation portion is excessively increased and the toughness deteriorates.

Therefore, Mn amount is made into 0.40% or more and 2.00% or less. In particular, when the amount of addition of reinforcing elements other than Mn is small, it is preferable to add 1.10% or more in order to secure strength by adding Mn.

Nb is added to secure solid solution Nb which is very important in the present invention. By securing the solid solution Nb, the hardenability can be increased to increase the room temperature strength, and the deformation resistance can be increased by the drag effect of the dislocation, thereby ensuring the strength even in the high temperature region. In order to secure the solid solution Nb which expresses such an effect, it is necessary to add 0.03% or more of Nb. On the other hand, when more than 0.50% of Nb was added, the HAZ toughness deteriorated, so the upper limit was made 0.50%. In order to further increase the high temperature strength, it is preferable to add Nb at least 0.10%.

In addition, Nb is a strong carbide forming element, and forms and precipitates excess C and NbC, so that the solid solution Nb decreases. Therefore, in order to secure solid solution Nb and improve high temperature strength,

C-Nb / 7.74≤0.004

It is necessary to satisfy. In addition, C and Nb are content of C and Nb here, respectively, and a unit is the mass%.

Since the lower limit of C-Nb / 7.74 can be obtained from the lower limit of C and the upper limit of Nb, it is not particularly defined.

The mass concentration product of Nb and C is an index of the amount of solid solution Nb, and in order to further improve the high temperature strength, it is preferable to set it to 0.0015 or more. The mass concentration product of Nb and C is a product of the content of Nb and C represented by mass%. The upper limit of the mass concentration product of Nb and C is not particularly specified since it is determined from the upper limit of the content of Nb and C.

Ti is an important element for forming a Ti-based oxide as described above. Moreover, it is an element which produces carbide and nitride, and it is easy to form stable TiN at high temperature. Since the formation of TiN can suppress precipitation of NbN, the addition of Ti is very effective for securing solid solution Nb. In order to acquire this effect, it is necessary to add Ti 0.005% or more. On the other hand, when Ti is added 0.040% or more, the Ti-based oxide and TiN are coarsened to impair toughness.

Therefore, Ti amount is made into 0.005% or more and less than 0.040%. From the viewpoint of securing the amount of the fine Ti oxide and improving the toughness, the upper limit is preferably 0.020%.

N is an impurity element for producing nitride. Reduction of N amount is effective for securing solid solution Nb, and makes an upper limit less than 0.0050%. It is preferable that content of N is as low a concentration as possible, but in order to make it less than 0.0008%, manufacturing cost increases. In addition, it is preferable to make the upper limit of N amount into 0.0045% from a viewpoint of securing toughness.

P and S are an impurity, and when it contains excessively, the welding crack and toughness fall by solidification segregation will arise. Therefore, P and S should be reduced as much as possible, and the upper limit of each content shall be 0.030% and 0.020%.

In the present invention, the properties can also be improved by appropriately adding V, Mo, Zr, Hf, Cr, Cu, Ni, Mg, Al, REM and / or Ca to the component system as needed. Hereinafter, the component which selectively adds them is demonstrated.

Although V is known as a precipitation strengthening element, in the present invention having a low C content, it contributes to the solidification of a solid solution. Even if V is added more than 0.10%, the effect is saturated and economical efficiency is also impaired. Therefore, the upper limit is preferably 0.10%.

Mo is an element contributing to the structure strengthening by solid solution strengthening and improvement of hardenability, and it is preferable to add Mo according to the target intensity | strength. However, if Mo or 0.10% or more is added, the economical efficiency is impaired, and the toughness and high temperature brittleness of the HAZ may be lowered. Therefore, the upper limit is preferably 0.10%.

Zr is an element that produces ZrN, which is a nitride that is more stable at a higher temperature than TiN. Compared with the case where Ti is added alone by the production of ZrN, it contributes more effectively to the reduction of the solid solution N in the steel, and the solid solution B and the solid solution Nb can be secured. When the Zr content exceeds 0.03%, coarse ZrN is formed in the molten steel before casting, thereby impairing the toughness at room temperature and the toughness of HAZ. Therefore, it is preferable that the concentration of Zr is made 0.03% or less. Moreover, in order to suppress precipitation of NbN and to prevent the fall of high temperature strength and drawing, addition of 0.005% or more is preferable.

Hf, like Ti, is an element that generates nitride and contributes to the reduction of solid solution N. However, when Hf exceeding 0.01% is added, the toughness of HAZ may fall. Therefore, it is preferable to make the upper limit of Hf into 0.01%.

Cr, Cu, and Ni are elements which contribute to the increase in strength by improving the hardenability. Since Cr and Cu may impair toughness when it adds excessively, it is preferable to make an upper limit into 1.5% and 1.0%, respectively. In addition, from an economical viewpoint, Ni preferably sets an upper limit to 1.0%.

Mg is a powerful deoxidation element and produces a stable Mg oxide at a high temperature, and does not dissolve in steel even when heated to a high temperature during welding, and has a function of pinning? Grains. Thereby, the structure of HAZ is refined | miniaturized and the fall of toughness is suppressed. However, when Mg exceeding 0.005% is added, Mg type oxide will coarsen and it will not contribute to pinning of (gamma) grain, and coarse oxide may be produced and a toughness may be impaired. Therefore, an upper limit shall be 0.005%. desirable.

Al is a strong deoxidizer and may be added in order to control the dissolved oxygen concentration after primary deoxidation to 0.003 to 0.015%. However, when more than 0.030% of Al is contained, island-like martensite may be formed to impair toughness, so the upper limit is made 0.030%. In view of toughness improvement, the upper limit is preferably 0.02%.

REM (rare earth elements) is oxidized and sulfided in steel to produce oxides and sulfides. These oxides and sulfides are stable at high temperatures and do not have a solid solution in steel even when heated to high temperatures during welding, and have a function of pinning grain boundaries. By this function, the structure of HAZ can be refined and the fall of toughness can be suppressed.

In order to acquire this effect, it is preferable to add content of the sum total of all the rare earth elements as 0.001% or more. On the other hand, when REM is added exceeding 0.01%, since the volume fraction of an oxide or a sulfide may increase and toughness may fall, it is preferable to make an upper limit into 0.01%.

Ca expresses the effect of suppressing elongation of the sulfide in the rolling direction in hot rolling by adding a small amount. Thereby, toughness improves and it contributes especially to the improvement of the Charpy value of a plate thickness direction. In order to acquire this effect, it is preferable to add Ca 0.001% or more. On the other hand, when Ca is added exceeding 0.005%, since the volume fraction of an oxide or a sulfide may increase and toughness may fall, it is preferable to set an upper limit to 0.005%.

Although the metal structure of the steel of this invention is not specifically limited, What is necessary is just to adjust content of the element which raises hardenability, and to comply with the requested intensity | strength. In order to increase the strength, it is preferable to increase the area ratio of one or both of the massive ferrite and bainite.

Massive ferrite is a structure in which austenite is diffused and transformed into ferrite of the same composition during cooling, and since the composition is the same before and after transformation, self diffusion of Fe atoms, ie, rearrangement of the lattice, rather than diffusion of C is a rate step. Therefore, since the massive ferrite is generated at a relatively fast transformation rate due to the short moving distance of atoms, the grain size is larger than that of polygonal ferrite, and the dislocation density is high.

Massive ferrite produced by such a mechanism differs from polygonal ferrite in the crystallographic observation by the optical microscope, but the shape is not different. Therefore, in order to distinguish these clearly, observation with a transmission electron microscope is necessary. In addition, bainite is a plate-like structure, and can be discriminated by mass ferrite and polygonal ferrite, and an optical microscope.

In addition to the massive ferrite, bainite and polygonal ferrite, a small amount of martensite, residual austenite and pearlite may be generated.

The production of massive ferrite and bainite is promoted by increasing the hardenability of the steel. Therefore, it is preferable to make Ceq which is a hardenability index into 0.05 or more. In addition, when Ceq is too high, since an intensity | strength may rise and a toughness may be impaired, it is more preferable to make an upper limit into 0.60 or less. Also,

Ceq = C + Si / 24 + Mn / 6 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14

C, Si, Mn, Ni, Cr, Mo, and V are content [mass%] of each element.

Next, a manufacturing method is demonstrated.

As above-mentioned, steel uses Si and Mn as a deoxidizer, adjusts the amount of dissolved oxygen before Ti addition, melt | dissolves, and casts it into a steel piece. In terms of productivity, continuous casting is preferred.

The obtained steel strip is formed into a steel sheet or a shaped steel by hot rolling and cooled. In addition, steel materials to which the present invention is concerned include rolled steel sheets, H-shaped steels, I-shaped steels, angular steels, grooved steels, and trapezoidal uneven thickness mountain steels.

Among them, H-shaped steel is particularly suitable for building materials requiring fire resistance and heat resistance embrittlement. Moreover, when using as a pillar material, the steel material of the magnitude | size of large plate thickness represented by ultra-thick H-shaped steel is suitable.

In order to obtain the steel of the present invention containing a Ti-based oxide having a particle diameter of 0.05 to 10 µm at a ratio of 30 to 300 particles / mm 2, adjustment of dissolved oxygen after primary deoxidation before addition of Ti is very important, and the amount of dissolved oxygen It is necessary to adjust to 0.003 to 0.015% by mass%. In order to produce Ti oxide, 0.003% or more of dissolved oxygen amount is required, and when it exceeds 0.015%, the particle size of Ti oxide becomes large, and the number whose particle diameter is 0.05-10 micrometers is not fully acquired. From this viewpoint, it is preferable that dissolved oxygen be 0.010% as an upper limit.

In order to manufacture steel materials by hot rolling, it is necessary to make the lower limit of the heating temperature of a steel piece into 1100 degreeC in order to make plastic deformation easy and to fully solidify Nb. In addition, when manufacturing a shaped steel by hot working, it is preferable to make heating temperature into 1200 degreeC or more, in order to make plastic deformation further easier. The upper limit of the heating temperature of the steel piece was 1350 degreeC from the performance and economy of a heating furnace. In order to refine the microstructure of steel, it is preferable to make the upper limit of the heating temperature of a steel piece into 1300 degreeC or less.

In hot rolling, it is preferable to make the cumulative reduction ratio in 1000 degrees C or less into 10% or more. Thereby, recrystallization in hot working can be promoted, the γ grain is atomized, and toughness and strength can be improved.

If the plate thickness of the product is less than 40 mm, the sheet thickness constraint of the material before rolling is less, and the strength can be improved by securing a cumulative reduction ratio of 1000 ° C. or lower at 30% or more. It is preferable to make the range of 30% or more.

Further, the hot working is completed in a temperature range in which the structure of the steel is an austenite single phase (referred to as γ single phase region), or when the volume fraction of ferrite produced by phase transformation is low, thereby significantly increasing the yield strength. Deterioration of mechanical properties such as lowering of toughness and occurrence of anisotropy of toughness can be avoided. Therefore, it is preferable to make the end temperature of hot rolling into 800 degreeC or more.

Moreover, after hot rolling, it is preferable to make the average cooling rate of the temperature range of 800-500 degreeC into 0.1-10 degreeC / s by control cooling. In order to further improve the strength and toughness of steel materials by controlled cooling after hot rolling, it is preferable to make the average cooling rate of the temperature range of 800-500 degreeC into 0.1 degreeC / s or more. On the other hand, when the average cooling rate in the temperature range of 800-500 degreeC exceeds 10 degree-C / s, the structure fraction of a bainite phase or martensite phase may rise and toughness may fall, so setting an upper limit to 10 degree-C / s desirable.

After the alloy was added to the molten steel dissolved in the converter, continuous casting was performed to prepare a 250-300 mm thick steel piece composed of the components shown in Table 1. In Table 1, the amount (mass%) of dissolved oxygen before adding Ti was also shown. In addition, the blank of Table 1 means that a selection element is no addition.

The obtained steel piece was hot-rolled on the conditions shown in Table 2, and it was set as H-shaped steel. 5 shows the shaped steel manufacturing process. The steel strip heated by the heating furnace 4 is rough-rolled by the roughing mill 5, and is then rolled by H-shaped steel by the universal rolling apparatus heat | difference which consists of the intermediate universal rolling mill 6 and the finishing universal rolling mill 8. It was. Water cooling between the rolling passes was performed by the water cooling apparatus 7 provided before and after the intermediate universal rolling mill 6, and spray cooling and reverse rolling of the flange outer surface were repeated. Cooling after hot rolling was performed by the cooling apparatus 9 provided in the rear surface of the finish universal rolling mill 8.

In addition, about the steel F, K, J, Z of Table 1, it hot-rolled also on the conditions of Table 3, and about EE, K, Z on the steel, it also hot-rolled on the conditions of Table 4.

In the obtained H-shaped steel, as shown in FIG. 6, 1/4 of a flange width full length B (referred to a flange) at the center portion 1 / 2t ₂ of the plate thickness t ₂ of the flange ₂ . Tensile test pieces were collected in accordance with JIS Z 2201 from the sites of and 1/2 (called fillets).

The tensile test at normal temperature was performed in accordance with JIS Z 2241, and the measurement of the 0.2% yield strength at 600 degreeC was performed based on JIS G 0567. In addition, the characteristics of these sites were determined because it was determined that each site was a representative site of the H-section steel, and that the average mechanical properties of the H-section steel and the variation in the cross-section could be exhibited.

Charpy impact test (Table 2-Table 4) extract | collected the small piece from the fillet and was performed at 0 degreeC based on JISZ2242 which is a typical test method.

When used as a refractory steel, reheating drawing (Table 2 to Table 4) of the regenerated weld heat affected zone (HAZ) is one of the important characteristics, and this evaluation is performed by reheating the weld heat cycle to the test steel and then heating it again. It carried out by the drawing value at the time of breaking and applying tensile stress at high temperature.

That is, after holding 1 second at 1400 degreeC to the tensile test piece of the round bar extract | collected from a flange, the welding heat cycle which cools to 100 degreeC by making the cooling time from 800 degreeC to 500 degreeC into 20 seconds, and also the state After heating to 600 degreeC at the temperature increase rate of 1 degree-C / sec, and hold | maintaining at 600 degreeC for 600 second, a tensile stress was applied and fractured at the stress increase rate of 0.5 Mpa / sec, and the drawing value was measured.

The toughness (Table 2) of the reproducible weld heat affected zone (HAZ) is similar to the reheat drawing, and the weld heat cycle is hysterized in the test steel, and then, the Charpy impact test is conducted at 0 ° C in accordance with JIS Z 2242, and the absorbed energy. Evaluated as. That is, after holding 1 second at 1400 degreeC, a V notch test piece is extract | collected from the small piece which heat-treated the heat cycle which welded to the 100 degreeC cooling time from 800 degreeC to 500 degreeC as 20 second, and the V-notch test piece was taken, and Charpy impact test Provided in.

There are two kinds of strength classes required for steel, one of which is a room temperature tensile strength specified by SM400 of 400 MPa class, and the other of which is a room temperature tensile strength of 490 MPa class defined by SM490. On the other hand, the ultra-thickness H-shaped steel is often mainly in accordance with the American ASTM standard, and is represented by dividing Grade 50 and Grade 65, which are representative strength classes.

In addition, the SM400 of the JIS standard, that is, the target of TS 400 MPa or higher class, the yield strength (YP) at room temperature is 235 MPa or more, preferably 355 MPa or less, and the tensile strength (TS) is 400-510 MPa, The target value of 0.2% yield strength PS in 600 degreeC is 157 Mpa or more.

The target of SM490, ie, TS 490 MPa or higher, is YP of 325 MPa or more, preferably 445 MPa or less, TS of 490 to 610 MPa, and PS of 217 MPa or more. In addition, in SM400 grade and SM490 grade, the target value of 0 degreeC impact absorption energy is 100 J or more, and the preferable upper limit of a yield ratio (YP / TS) is 0.80 or less.

In addition, the ASTM standard is YP 345 MPa or more, TS 450 MPa or more, Grade YP 450 MPa or more and TS 550 MPa or more in Grade 50. In addition, the toughness is in any case at 0 ° C. in terms of toughness. It is preferable that the impact absorption energy in a base material fillet part is 54J or more.

Regarding the characteristics of the reproduction HAZ, in any standard, the goal of reheat drawing is 30% or more, and the toughness goal is 27J or more. In particular, when evaluating as refractory steel, it is preferable that reheat drawing is 50% or more.

As shown in Table 2, as for the steels of manufacture Nos. 1 to 15 and 35 to 39 of the present invention, mechanical properties at room temperature and mechanical properties at high temperature are within a range of target values. In addition, a yield point is more than the lower limit of a JIS standard, and yield ratio (YP / TS) is 0.8 or less and exists in a preferable range. In addition, the Charpy impact value in 0 degreeC is the value more than a target value. Moreover, 30% or more of the reheat drawing of the reproduction welding heat affected zone is sufficiently satisfied.

On the other hand, in the steels of Production Nos. 16 to 22 and 40 to 42 as Comparative Examples, the density of the component, C-Nb / 7.74, and Ti-based oxide is outside the range of the present invention, and thus mechanical properties satisfying the target are not obtained. .

As shown in Table 3, in the case of H-shaped steel with a flange thickness of less than 40 mm, when the cumulative reduction ratio at 1000 ° C or lower is 30% or more, the mechanical characteristics are better than when the cumulative reduction ratio is less than 30%. .

In addition, in the case of extremely thick H-shaped steel with a flange thickness of 40 mm or more, as shown in manufacturing Nos. 43 to 48 as a representative example, the flange thickness is 90 to 125 mm, yielding is accompanied by an increase in the cumulative reduction ratio of 1000 ° C or less. Strength and tensile strength rise together, and when the cumulative reduction ratio is 10% or more, it becomes possible to more fully satisfy the strengths required as Grade 50 and Grade 65, respectively.

As shown in Table 4, when the thickness of the flange is less than 40 mm, the cooling rate between 800 to 500 ° C. is accelerated to 10 ° C./s by water cooling, and cooled to 0.1 ° C./s between 800 to 500 ° C. by cooling. It is possible to raise room temperature intensity | strength and high temperature intensity | strength rather than gradually cooling by the furnace.

In addition, about extremely thick H-shaped steel, as shown by the representative example in the case of size of 125 mm of flange thickness in manufacture No. 49-51, yield strength by accelerating-cooling 800-500 degreeC to 0.13 degree-C / s by water cooling, Tensile strength rises together, and it becomes possible to more fully satisfy the strength required as Grade65.

According to the present invention, a steel material having sufficient room temperature strength and high temperature strength and excellent in toughness and intrinsic heat embrittlement properties of a base material and HAZ, in particular, refractory H-shaped steel is produced without cold working and temper heat treatment, or plate In ultra-thick H-shaped steel having a large thickness, for example, up to 140 mm in flange thickness, it is possible to manufacture while securing strength and toughness in a hot rolling state, thereby significantly reducing construction costs and shortening of air. The cost reduction can be achieved, and industrial effects such as reliability improvement of large-sized buildings, securing safety, and economic efficiency are very remarkable.

Claims (11)

In mass%, C: 0.001% or more and 0.030% or less, Si: 0.05% or more and 0.50% or less, Mn: 0.40% or more and 2.00% or less, Nb: 0.03% or more and 0.50% or less, Ti: 0.005% or more and less than 0.040%, N: 0.0008% or more but less than 0.0050% Containing, P: 0.030% or less, S: 0.020% or less Limited to, the remainder comprising Fe and unavoidable impurities, The content of C and Nb is C-Nb / 7.74≤0.004 And having a Ti-based oxide having a particle diameter of 0.05 to 10 µm at a density of 30 to 300 particles / mm 2, wherein the steel has excellent high temperature characteristics and toughness. The method according to claim 1, wherein in mass%, V: 0.10% or less, Mo: less than 0.10% The steel material excellent in high temperature characteristic and toughness characterized by containing one or both of them. The mass% according to claim 1 or 2, Zr: 0.03% or less, Hf: 0.01% or less The steel material excellent in high temperature characteristic and toughness characterized by containing one or both of them. The mass% according to any one of claims 1 to 3, Cr: 1.5% or less, Cu: 1.0% or less, Ni: 1.0% or less The steel material excellent in high temperature characteristic and toughness characterized by containing 1 type (s) or 2 or more types. The mass% according to any one of claims 1 to 4, Mg: 0.005% or less, Al: 0.030% or less, REM: 0.01% or less, Ca: 0.005% or less The steel material excellent in high-temperature characteristics and toughness characterized by containing 1 type, 2 or more types, or one or both of them. The steel material according to any one of claims 1 to 5, wherein a product of a mass concentration of Nb and C is 0.0015 or more. The steel material according to any one of claims 1 to 6, wherein the steel material is a refractory steel material. The steel material according to any one of claims 1 to 6, wherein the steel is an ultra-thick H-shaped steel having a flange thickness of 40 mm or more. After adjusting the dissolved oxygen to 0.003 to 0.015 mass% of the steel which consists of a component of any one of Claims 1-6, Ti is added and a solvent is melted, and the steel piece obtained by casting is heated at 1100-1350 degreeC. Hot rolling is performed, The manufacturing method of the steel excellent in high temperature characteristic and toughness. 10. The method for producing a steel having excellent high temperature properties and toughness according to claim 9, wherein the rolling reduction is performed at a cumulative reduction ratio of 1000% or less at 10% or more. The method according to claim 9 or 10, after hot rolling, the temperature range from 800 ° C to 500 ° C is cooled at an average cooling rate of 0.1 to 10 ° C / s, characterized in that Manufacturing method.

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