JP2002012937A - Structural steel low in yield strength and excellent in toughness, and its production method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a structural steel usable for a steel structure of building, and the like, having a yield strength of 14 kgf/mm2 or less (137 MPa or less) and excellent in toughness, and to provide its production method. SOLUTION: This structural steel low in yield strength and excellent in toughness, is composed of, by weight %, C of 0.005% or below, Si of 0.04% or below, Mn of 0.20% or below, P of 0.02% or below, S of 0.02% or below, Al of 0.015% or below, N of 0.004% or below, Ti of below 0.01-0.03% and Mg of 0.0005-0.0050% or below, and further one or two of Nb of below 0.002-0.01% and V of below 0.002-0.05%, and the balance Fe with inevitable impurities, wherein the total of solid soluble C and solid soluble N is 0.006% or below, and TiN composite particles, which are deposited in scale of 0.2 μm or less on MgO or MgO/Al2O3, are present in an average spacing between particles of 0.5 μm or less in this steel.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a structural steel having a low yield strength and excellent toughness, which is used for a steel structure such as a building, and a method for producing the same. In particular, the present invention relates to a structural steel having a yield strength of 14 kgf / mm ² or less (137 MPa or less) and excellent toughness, and a method for producing the same.

[0002]

2. Description of the Related Art When a building structure encounters an earthquake, the energy of the earthquake is absorbed by plastic deformation of members, and from the viewpoint of ensuring the soundness of the entire structure, the yield strength is low and brittle fracture occurs. There is a need for a material that can prevent the occurrence of phenomena.

Heretofore, several findings have been disclosed from such a viewpoint as a technique for producing steel having a low yield ratio and excellent toughness. For example, Japanese Patent Application Laid-Open No. 10-1832
No. 93, C: 0.010% or less, Si: 0.4
0% or less, Mn: 1.5% or less, Ti: 0.40% or less, N: 0.006% or less, and the relationship between the amounts of C, Ti, and N is -0.105≤C- (Ti-3 .4N) / 4 ≦ 0.
A low yield point steel which satisfies 010% and is subjected to normalizing treatment at a temperature of 900 to 960 ° C. after hot rolling and excellent in toughness, and a method for producing the same. Further, JP-A-10-245
No. 62 discloses that, at the above basic components, the slab is heated to a temperature of 1050 ° C. to 1200 ° C. and then heated to 850 ° C.
Hot rolling at the above temperature, and then 900 if necessary
A method of normalizing at a temperature of from 1000C to 1000C is disclosed in Japanese Unexamined Patent Publication No. Hei 10-298644, for a steel obtained by adding 0.005 to 0.015% of Nb to the above basic component. A technique is disclosed in which a slab is heated to 1050 to 1200 ° C, hot-rolled at a temperature of 850 ° C or higher, and then, if necessary, a normalizing treatment is performed at a temperature of 900 to 1000 ° C. It is shown that by defining the conditions of hot rolling and heat treatment, a structural steel having a low yield strength and a high elongation can be produced.

However, in the above-mentioned method, it is necessary to control the rolling temperature and the heat treatment conditions in order to obtain an improvement in toughness, and it is conceivable to use fine precipitates such as TiN for grain refinement. Have been. However, according to these methods, depending on the production method, not only solid solution N is generated to lower the toughness and the like of the base material, but also coarse precipitates are generated by the heat treatment and the like, and the toughness is also lowered. Are known.

[0005]

SUMMARY OF THE INVENTION An object of the present invention is to provide a steel having good toughness while keeping the yield strength low, and a method for producing the same.

[0006]

In order to reduce the yield point of steel, it is necessary to reduce alloying elements including C,
Heat treatment for sufficiently softening the ferrite after rolling is required. As in the present invention, the yield point is 14 kgf / mm ² or less (1
In order to stably produce an extremely low yield point of steel (37 MPa or less), the heat treatment conditions must be high temperature and long time, and there is a problem that toughness is reduced at the same time.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problems. In order to secure crystal grains having good toughness even after a high-temperature heat treatment performed to reduce the yield strength, the present invention has been made. Conducted various studies. As a result, according to the present invention, the crystal grains can be controlled by the composite particles with TiN precipitated on MgO or MgO.Al ₂ O ₃ , and at the same time, TiN is stably present even at a high temperature, and even after a long-time heat treatment. It consists of knowledge that toughness is good, and the gist is as follows.

(1) C: 0.005% or less, Si: 0.04% or less, Mn: 0.20% or less, P
: 0.02% or less, S: 0.02% or less, Al:
0.015% or less, N: 0.004% or less, Ti:
0.01 to less than 0.03%, Mg: 0.0005 to 0.5%.
0050 or less, and Nb: 0.002 to
Less than 0.01%, V: less than 0.002 to 0.05%,
One or two of which, the balance being Fe and inevitable impurities, the total of solid solution C and solid solution N is 0.006% or less, and MgO or MgO
T deposited on Al ₂ O ₃ with a size of 0.2 μm or less
A structural steel having low yield strength and excellent toughness, characterized in that iN composite particles are present in the steel at a distance of 0.5 μm or less at an average particle interval.

(2) B: 0.0002 to 0.0020
%. The structural steel according to (1), wherein the yield strength is low and the toughness is excellent.

(3) A slab or a slab having the same composition as the steel described in (1) or (2) is used
After heating to 50 ° C. and performing hot rolling so that the finishing temperature becomes 800 ° C. or higher, air-cooling after heating to 800 to 960 ° C., low yield strength, and excellent toughness Method of manufacturing structural steel.

(4) A slab or cast slab having the same composition as the steel described in (1) or (2) is used for 1050-12
After heating to 50 ° C. and performing hot rolling so that the finishing temperature is 800 ° C. or higher, in the process of heating to 800 to 960 ° C. and air cooling, maintaining the heat at 600 to 910 ° C. and air cooling. A method for producing a structural steel having low yield strength and excellent toughness.

(5) A steel slab or a cast slab having the same composition as the steel described in (1) or (2),
After heating to 50 ° C. and performing hot rolling so that the finishing temperature becomes 800 ° C. or higher, air cooling is performed after heating to 800 to 960 ° C., and further air cooling is performed after heating to 600 to 910 ° C. A method for producing a structural steel having a low yield strength and excellent toughness.

[0013]

DETAILED DESCRIPTION OF THE INVENTION Here, the total amount of solute C and solute N is measured by an internal friction method. In order to reduce the yield strength of structural steel, usually, in addition to reducing substitution-type solid solution hardening elements (Si, Mn, P, Cu, Ni, etc.), the amount of solute C and N is reduced. It is necessary to.
For this purpose, it is conventionally known that it is effective to contain carbon, nitride, and carbon generating elements such as Ti, Nb, and V, and to fix C and N as carbonitrides to reduce the amount of solute C and N. ing. In order to lower the yield strength, it is necessary to reduce the contents of C, N and to limit the contents of Ti, Nb, and V within an appropriate range. If the content of these elements is small, C and N in steel cannot be fixed sufficiently, and the amount of solid solution C and N increases, so that the yield strength cannot be kept low. Conversely, when these elements are contained in large amounts, solid solution C, N
Although the amount is sufficiently reduced and it is effective in lowering the yield strength, coarse and large amounts of carbonitride are formed, which hinders improvement in toughness.

As a means for solving this problem, the present inventors disperse fine oxides in steel, these act as precipitation nuclei of TiN, not only to prevent a decrease in toughness due to coarse carbides, but also to prevent high temperature. However, it has been found that fine TiN is prevented from dissolving and prevents excessive coarsening of crystal grains.

[0015] Based on these new findings, the amounts of alloying elements were limited for the following reasons. Since C increases the yield strength by solid solution hardening and dislocation fixing action, C is preferably lower. If C is contained in excess of 0.005%, it becomes difficult to lower the amount of solid solution C even if it contains a carbide forming element, yield strength cannot be suppressed low, and a large amount of coarse carbides is formed. Decrease elongation. Therefore, the upper limit is made 0.005%.

Since Si has a solid solution hardening ability, the lower the better, the better. If the content exceeds 0.04%, the yield strength cannot be suppressed low, so the upper limit was made 0.04%.

Since Mn has a solid solution hardening ability, the lower the better, the better. If the content exceeds 0.20%, the yield strength cannot be suppressed low, so the upper limit was made 0.20%.

Since P has a solid solution hardening ability and a tendency of embrittlement at grain boundaries, the lower the P, the better. If the content exceeds 0.02%, the yield strength cannot be suppressed low, and in addition, segregation at grain boundaries promotes grain boundary embrittlement, so the upper limit was made 0.02%.

Since S forms sulfides and lowers elongation, the lower the S, the better. If the content exceeds 0.02%, the elongation cannot be improved, so the upper limit was made 0.02%.

Al is a deoxidizing element, but if it exceeds 0.015%, an oxide containing Mg, which will be described later, will not be formed, so that the upper limit is set.

N, like C, increases the yield strength by solid solution hardening and dislocation fixation, so that N is preferably lower. If the N content exceeds 0.004%, it becomes difficult to lower the amount of solute N even if it contains a nitride-forming element, so that the yield strength cannot be kept low, and a large amount of coarse nitride is generated. And reduce elongation. Therefore, the upper limit is made 0.004%.

Ti is a necessary element for forming carbonitrides and suppressing the yield strength by lowering the amounts of solid solution C and N, and also acts as a deoxidizer. Therefore, if the content is less than 0.01%, the effect is not remarkable, so the lower limit is set to 0.01%. When the content is 0.03% or more, solid solution C and N are sufficiently reduced, and the yield strength is remarkably reduced. However, since coarse precipitates are formed, it is difficult to improve toughness. Therefore, the upper limit is set to less than 0.03%.

Mg is added as a deoxidizing agent.
When added in combination with i and Al, a fine oxide such as MgO or MgO.Al ₂ O ₃ is generated. It is clear from experiments by the inventors that these oxides act as precipitation nuclei for TiN. In order to improve the toughness of the base material, the oxide has an average particle diameter of not more than 0.2 μm and TiN. It is necessary that the distance between the composite average particles is dispersed in the steel at a distance of 0.5 μm or less. In order to achieve such a dispersion state of oxides, Ti, Al, and Mg are added while controlling dissolved oxygen in molten steel,
Can be achieved.

Incidentally, the dispersion state of the particles specified in the present invention can be determined by preparing an extraction replica and then using a transmission electron microscope (T
EM) at a magnification of about 10,000 to 50,000 times. In this case, the particles are identified by a composition analysis by energy dispersive X-ray spectroscopy (EDS) attached to the TEM in the discriminated inclusion in a visual field of about 1000 μm ² , and in some cases, the crystal is determined by electron diffraction. The structure may be analyzed.

With respect to the particles obtained as described above, the circle equivalent diameter was averaged as the average particle diameter for 30 or more observations, and the average particle size was determined. In each particle, the distance between the closest particles was measured at 20 or more, and the interval was defined as the average interparticle distance. Since Nb and V have an equivalent function as carbonitride forming elements, one or two of these elements can be used as selective elements.

Nb has an effect of suppressing the yield strength similarly to Ti. If the amount is less than 0.002%, the amount of dissolved C and N is not remarkably reduced, so the lower limit is made 0.002%. Conversely, 0.
When the content is 01% or more, the non-recrystallization region in hot rolling is widened, and therefore, when a steel sheet having a thickness of 10 mm or less, in which the rolling temperature is liable to decrease, is likely to be rolled in the non-recrystallization region. Therefore, the upper limit of the Nb content is set to less than 0.01%.

V also has the same effect as Nb. 0.00
If it is less than 2%, the amount of dissolved C and N is not remarkably reduced, so the lower limit is made 0.002%. V also has the effect of suppressing recrystallization similarly to Nb, but is not as remarkable as Nb. But,
When the content is 0.05% or more, recrystallization suppression becomes remarkable,
In particular, in a steel sheet having a sheet thickness of 10 mm or less, at which the rolling temperature tends to decrease, rolling in the non-recrystallized region is likely to occur, which also increases the yield point. Therefore, the upper limit of the V amount is set to less than 0.05%.

In the present invention, in order to keep the yield strength low, it is necessary to keep the amounts of solute C and solute N in the steel low. Since both elements have almost the same action, evaluation can be made based on the total amount of solid solution C and solid solution N. When this value exceeds 0.006%, the yield strength becomes remarkable due to the solid solution hardening and dislocation fixing effects of these elements. Therefore, the upper limit is made 0.006%. The total amount of solid solution C and solid solution N shall be measured by the internal friction method.

B segregates at the grain boundaries and increases the grain boundary strength. In particular, in steels having low solid solution C and N, the amount of grain boundary segregation of these elements decreases, and segregation of grain boundary embrittlement elements such as P is easily promoted. effective. If the B content is less than 0.0002%, the effect of strengthening the grain boundary cannot be obtained, so the lower limit is set. vice versa,
If the content exceeds 0.0020%, coarse BN precipitates are formed, which become the starting point of fracture and lower the toughness. Therefore, the upper limit was made 0.0020%. A is inevitable impurity
There are s, Sb, Sn and the like, but these segregate at the grain boundaries and cause a decrease in toughness. Therefore, the total amount of impurities is desirably 0.01% or less.

Next, the reasons for limiting the rolling and heat treatment conditions will be described below. When the heating temperature is lower than 1050 ° C., the particle size after the final heat treatment becomes small, and the yield strength increases. Also, Ti, Nb, V precipitated in the cooling process after casting.
Carbonitride remains coarse, which causes a decrease in toughness. Conversely, heating above 1250 ° C. increases the cost of heating and is industrially disadvantageous. Therefore, the range of the heating temperature is set to 1
500 to 1250 ° C.

When the rolling finishing temperature is less than 800 ° C.,
The lower limit was set to 800 ° C., because the ferrite grain size becomes smaller, the yield strength increases, and the crystal grains become excessively fine to increase the yield strength.

Next, the production method (3) is intended to cause recrystallization by heat treatment after hot rolling, or to form austenite once, and to form ferrite having a low dislocation density in the subsequent cooling process. is there. If the heat treatment heating temperature is lower than 800 ° C., recrystallization does not sufficiently proceed.
00 ° C is the lower limit. Conversely, if the temperature is higher than 960 ° C., the austenite grains become coarse, and the ferrite grains after transformation also become coarse, leading to a decrease in toughness. Therefore, the upper limit was set to 960 ° C.
The cooling after the heat treatment needs to be air-cooled to reduce the dislocation density. In a cooling method having a higher cooling rate than air cooling such as water cooling, dislocations generated during transformation remain, and the yield strength cannot be suppressed low.

In (4), as in (3), recrystallization ferrite is formed to reduce the yield strength.
By maintaining the temperature at 600 to 910 ° C. in the cooling process of the heat treatment at 60 ° C., recrystallization is promoted, and the yield strength can be reduced efficiently. If the heat retention temperature is lower than 600 ° C., the effect of promoting recrystallization is not significant. The heat treatment temperature prior to heat retention is 910.
Reverse transformation occurs at a temperature of not less than ℃, but if the holding during the subsequent cooling process is at 600 to 910 ℃, the recovery of the transformed ferrite proceeds, so that the yield strength can be significantly reduced. The method of heat retention does not matter, such as insertion into a constant temperature furnace.

In (5), after hot rolling, 800 to 960
After cooling to ℃, air-cooling and reheating at 600 to 910 ℃ reduce the dislocation density. The reason for limiting to this range is the same as above. It is desirable that the holding time in this temperature range be 10 to 60 minutes.

In (3) to (5), the cooling before the heat treatment after the hot rolling is not necessarily required, but the cooling may be performed to 150 ° C. or less in order to maintain the shape of the steel sheet surface. The structure of the object of the present invention may have a thickness or a wall thickness of 4 mm or more.
It does not depend on the cross-sectional shape of steel materials such as steel pipes.

[0036]

Examples of the present invention will be described below. Steel was melted by a converter, and a slab having a thickness of 240 mm was manufactured by continuous casting. Table 1 shows the chemical components of the steel materials. The table also shows the amounts of dissolved C and N measured by the internal friction method. They were subjected to hot rolling and heat treatment under the conditions shown in Table 2 to produce a steel sheet having a thickness of 25 mm. Thereafter, a tensile test and a Charpy test were performed, and an extracted replica was collected from the steel sheet. Oxides and precipitates of 2 μm or less were observed with an electron microscope, and the average particle diameter and average interval of 30 to 50 particles were measured. The results are also shown in Table 2.

Numbers 1 to 8 are invention steels, and 9 to 20 are comparative steels. All of the invention steels have a yield strength of 14 kgf / mm ² or less (137 MPa or less) and a high toughness of 200 J or more at −20 ° C. On the other hand, Comparative Steels 9 to 14 are examples in which the production conditions are out of the range of the present invention, although they are within the range of the component system of the present invention. That is, the comparative steel 9 has a high slab heating temperature prior to hot rolling, and has reduced toughness. The comparative steel 10 has a rolling finish temperature out of the invention range of 780 ° C. and a high yield point. . In Comparative Steels 11 and 12, the temperature of the heat treatment condition was deviated from the upper limit, and the yield point was kept low, but the toughness was lowered. Comparative steel 13 is an example in which a steel plate was water-cooled after heat treatment. In this case, the yield point is clearly higher. Comparative Example 14 is an example in which the holding during the heat treatment was performed on the low temperature side outside the range of the present invention,
In this case, the toughness is extremely low. Comparative steel 15 ~
Reference numeral 20 is an example produced outside the component range of the present invention. That is, the comparative steels 15 and 16 (steel E) have C
0.012% added, yield point 170MP
a or higher. Comparative steel 17 (steel F) is Mn
Is an example in which 0.48% is added. In this case, the yield point is high due to solid solution strengthening of Mn. Comparative steel 18 (steel G) is an example in which the addition amount of Mg is equal to or less than the lower limit in the range of the present invention. Comparative steel 19 (steel H) was one in which the Nb content was added outside the upper limit, and comparative steel 20 (steel I) was the one in which the N content was also outside the upper limit. In both cases, the yield point is as high as 150 MPa or more.

[0038]

[Table 1]

[0039]

[Table 2]

[0040]

As described above, according to the present invention,
Alloying elements including C and N, especially Ti, Nb, V
It is possible to reduce the amount of solid solution C and N in the steel and to suppress the yield strength by suppressing the amount of dissolved C and N in the steel, and to suppress the coarsening of the precipitates and improve the toughness by limiting the amount of Mg and adding an appropriate amount of Mg. Becomes Therefore, it is possible not only to provide a stable low yield strength as a vibration damping material when manufacturing a structure, but also to ensure safety against destruction at the time of an earthquake or the like.

 ────────────────────────────────────────────────── ─── Continued on the front page F term (reference) 4K032 AA01 AA04 AA16 AA21 AA22 AA27 AA29 AA31 AA35 AA36 BA01 CA02 CA03 CC03 CC04 CD05 CF02 CF03

Claims (5)

[Claims] (1) C: 0.005% or less in weight%, S:
i: 0.04% or less, Mn: 0.20% or less, P:
0.02% or less, S: 0.02% or less, Al: 0.0
15% or less, N: 0.004% or less, Ti: 0.01
Less than 0.03%, Mg: 0.0005 to 0.0050
The following are further contained, and Nb: 0.002 to 0.01
%, V: 0.002 to less than 0.05%, the steel containing one or two kinds and the balance consisting of Fe and unavoidable impurities has a total of solid solution C and solid solution N of 0.00
6% or less, and MgO or MgO.Al ₂
TiN composite particles precipitated at a size of 0.2 μm or less on O ₃ are present in steel at a distance of 0.5 μm or less at an average particle interval, and have a low yield strength and excellent toughness. Structural steel. 2. The structural steel having a low yield strength and excellent toughness according to claim 1, wherein the steel contains B: 0.0002 to 0.0020%. 3. A steel slab or a slab having the same composition as the steel according to claim 1 or 2 is heated to 1,050 to 1,250 ° C. and hot-rolled to a finishing temperature of 800 ° C. or higher. A method for producing a structural steel having a low yield strength and excellent toughness, comprising heating to 800 to 960 ° C. and then air cooling. 4. A slab or a slab having the same composition as the steel according to claim 1 or 2 is heated to 1,050 to 1,250 ° C. and hot-rolled to a finishing temperature of 800 ° C. or higher. A method for producing a structural steel having a low yield strength and excellent toughness, characterized in that, after heating to 800 to 960 ° C. and then air cooling, it is kept at 600 to 910 ° C. and then air cooled. 5. A slab or a slab having the same composition as the steel according to claim 1 or 2 is heated to 1,050 to 1,250 ° C. and hot-rolled to a finishing temperature of 800 ° C. or higher. A method for producing a structural steel having a low yield strength and excellent toughness, comprising heating to 800 to 960 ° C., air cooling, and further heating to 600 to 910 ° C. and air cooling.