JPH11246937A - Steel with low yield point, excellent in toughness, and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a steel with low yield point, excellent in toughness, for use in an energy absorbing device for buildings at the time of earthquake by limiting the relation among C, N, and Ti equivalent, reducing effective C quantity, and limiting microstructure to a proper range. SOLUTION: The steel has a composition containing, by weight, <=0.1% C, <=0.4% Si, <=1.5% Mn, <=0.025% P, <=0.015% S, <=0.06% Al, and <=0.006% N, also containing one or >=2 elements among Ti, Nb, and V so that the relation among the Ti equivalent, defined by Ti(eq.)=Ti+Nb/2+V, and respective amounts of C and N satisfies -0.02%<=C-[Ti(eq.)-3.4N]/4<=0.01%, and having the balance iron with inevitable impurities and also has a structure where the fraction of ferritic structure of microstructure is regulated to >=95%. This steel is produced by casting a slab, subjecting the resultant cast slab, directly or after reheating up to <=1,250 deg.C, to hot rolling at >=750 deg.C, and then performing tempering or normalizing treatment, if necessary.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a low yield point steel having excellent toughness, which is suitable for a steel for an energy absorbing device for securing an input energy to a building mainly due to an earthquake to a specific portion and securing seismic performance. Things.

[0002]

2. Description of the Related Art Conventionally, seismic design is intended to absorb energy by plasticizing a pillar or beam structure during a large earthquake, thereby preventing collapse of buildings and preventing human damage. It is a very rational design method that can keep construction costs relatively low while preserving prevention as a major premise.

On the other hand, with the recent development of seismic design technology,
Development and commercialization of vibration control and seismic isolation structures are progressing, and the energy input to the building due to the earthquake is absorbed by a specific part (energy absorbing device) to ensure seismic performance, while at the same time damaging columns and beams, which are the main structures. Attention has been focused on design techniques to prevent this.

[0004] Low yield point steels are used for such energy absorbing devices. The principle is that the yield point is lower than that of ordinary column and beam structural materials, so that it yields early in the event of an earthquake, and suppresses the vibration response by converting the vibration energy due to the earthquake into plastic energy.

In order to reduce the yield point, Japanese Patent Application Laid-Open No. 3-31
As disclosed in Japanese Patent Application Laid-Open No. 467/467, it is close to pure iron with almost no added element.
214442, JP-A-5-320760,
As disclosed in Japanese Patent Application Laid-Open No. 5-320761, a component close to pure iron is subjected to normalizing at a higher temperature. All of these have a drawback that they are inferior in low-temperature toughness due to lowering the yield point by forming coarse ferrite. In addition, in any case, C needs to be 0.005% or less,
There is a problem that the load on the steelmaking process is high and there are almost no added elements, but the cost is not always advantageous.

[0006]

SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide a low-yield-point steel having excellent low-temperature toughness without extremely reducing C, and to provide a method for producing the same.

[0007]

SUMMARY OF THE INVENTION The present invention reduces the effective C by appropriately restricting the relationship between C, N, and Ti equivalents without significantly reducing the C to reduce the effective C. It has a structure and achieves excellent low-temperature toughness while being a coarse-grained ferrite for lowering the yield point.

The gist of the present invention is as follows.

(1) By weight%, C: 0.1% or less, S
i: 0.4 or less, Mn: 1.5% or less, P: 0.025
% Or less, S: 0.015% or less, Al: 0.06% or less, N: 0.006% or less, Ti (eq.) = Ti + Nb / 2 + V, Ti equivalent, C and N amounts. -0.02% ≦ C- [Ti (eq.)-3.4N] / 4
At least one of Ti, Nb and V is contained so as to satisfy ≦ 0.01%, the balance being iron and unavoidable impurities, and the microstructure of ferrite having a microstructure fraction of 95%.
% Low yield point steel with excellent toughness, characterized in that it is not less than 10%.

(2) By weight%, C: 0.1% or less, S
i: 0.4 or less, Mn: 1.5% or less, P: 0.025
% Or less, S: 0.015% or less, Al: 0.06% or less, N: 0.006% or less, Ti (eq.) = Ti + Nb / 2 + V, Ti equivalent, C and N amounts. -0.02% ≦ C- [Ti (eq.)-3.4N] / 4
A steel containing one or more of Ti, Nb, and V so as to satisfy ≦ 0.01%, and the balance consisting of iron and unavoidable impurities, is cast directly or at a temperature of 1250 ° C. or less after casting. After reheating, finish hot rolling at a temperature of 750 ° C or higher,
A method for producing a low yield point steel having excellent toughness, which is then allowed to cool.

(3) By weight%, C: 0.1% or less, S
i: 0.4 or less, Mn: 1.5% or less, P: 0.025
% Or less, S: 0.015% or less, Al: 0.06% or less, N: 0.006% or less, Ti (eq.) = Ti + Nb / 2 + V, Ti equivalent, C and N amounts. -0.02% ≦ C- [Ti (eq.)-3.4N] / 4
A steel containing one or more of Ti, Nb, and V so as to satisfy ≦ 0.01%, and the balance consisting of iron and unavoidable impurities, is cast directly or at a temperature of 1250 ° C. or less after casting. A method for producing a low yield point steel having excellent toughness, characterized in that after reheating, hot rolling is performed at a temperature of 750 ° C. or higher and tempering is performed at a temperature of ₁ point or lower.

(4) By weight%, C: 0.1% or less, S
i: 0.4 or less, Mn: 1.5% or less, P: 0.025
% Or less, S: 0.015% or less, Al: 0.06% or less, N: 0.006% or less, Ti (eq.) = Ti + Nb / 2 + V, Ti equivalent, C and N amounts. -0.02% ≦ C- [Ti (eq.)-3.4N] / 4
A steel containing one or more of Ti, Nb, and V so as to satisfy ≦ 0.01%, and the balance consisting of iron and unavoidable impurities, is cast directly or at a temperature of 1250 ° C. or less after casting. A method for producing a low yield point steel having excellent toughness, comprising hot rolling at a temperature of 750 ° C. or more after reheating, and normalizing at a temperature of ₃ points or less of Ac.

According to the present invention, it has become possible to supply a large amount of steel material excellent in low-temperature toughness at low cost for use as an energy absorbing device for buildings.

[0014]

BEST MODE FOR CARRYING OUT THE INVENTION The reason for limiting the steel composition according to the present invention will be described.

C has a significant effect on the formation of a hard second phase such as pearlite, which deteriorates toughness and increases strength, and is preferably as low as possible in the steel of the present invention, and the lower limit is not particularly limited. On the other hand, the upper limit is T
By properly maintaining the relationship between i and N, the effective C
The amount can be reduced, and 0.1% in relation to the amount of Ti and N
Up to acceptable.

[0016] Si is an element contained in the deoxidized upper steel, but if added in a large amount, the weldability and HAZ toughness deteriorate, so the upper limit was limited to 0.4%. Deoxidation of steel is Ti, Al
Alone is sufficiently possible, and the lower the better, from the viewpoints of HAZ toughness, hardenability, etc., and it is not always necessary to add.

Mn can be arbitrarily added according to the required strength level in order to increase the strength of the base material as a solid solution strength element. However, if the amount of Mn is too large, the hardenability increases and not only deteriorates the weldability and the HAZ toughness, but also promotes the center segregation of the continuous cast slab.
And

P is an impurity in the steel of the present invention,
Since a reduction in the amount of P tends to reduce grain boundary fracture in HAZ, a smaller amount is more preferable. If the content is large, the low-temperature toughness of the base material and the welded portion is deteriorated, so the upper limit is 0.02.
5%.

S, like P, is an impurity in the steel of the present invention, and is preferably as small as possible from the viewpoint of the low-temperature toughness of the base material. If the content is large, the low-temperature toughness of the base material and the welded portion is deteriorated, so the upper limit was made 0.015%.

Al is an element generally contained in the deoxidized upper steel, but the deoxidation is sufficient with only Si or Ti, and the lower limit is not limited in the steel of the present invention. But A
When the amount of l increases, not only does the cleanliness of the steel deteriorate, but also the toughness of the weld metal deteriorates, so the upper limit was made 0.06%.

Ti, Nb and V are all indispensable elements in the present invention for fixing C and reducing the effective C amount. The three can be expressed as Ti (eq.) = Ti + Nb / 2 + V as Ti amount = Ti equivalent (Ti (eq.)) Stoichiometrically equivalent from the viewpoint of C fixation.

Therefore, the actual amount of addition is naturally limited by the relationship between C, N, and Ti equivalents described later. At that time, Ti, Nb, and V may be added alone or in combination of two or more.

Although N is contained in steel as an unavoidable impurity, Ti, Nb and V for fixing C are consumed as nitrides, so the upper limit is limited to 0.006%.

After limiting the individual components of the steel as described above, the relationship between the C, N, and Ti equivalents was set to -0.02% ≦ C- [Ti (eq.)-3.4N] / 4.
≦ 0.01%. C- [Ti (eq.)-3.4
N] / 4 represents the amount of C remaining stoichiometrically after Ti, Nb, and V fix C and N, and is the effective amount of C involved in the formation of a hard phase such as pearlite. Since the hard phase becomes a starting point of the occurrence of brittle fracture and degrades toughness, the effective C amount needs to be 0.01% or less. A negative effective C amount means that the Ti equivalent is excessive, and Ti, N
From the viewpoint of the cost of b and V and toughness deterioration due to coarse precipitates due to excessive addition, -0.02% or more. By limiting the effective C content to the above range, the microstructure inevitably has a ferrite structure fraction of 95% or more in a manufacturing method described later. In other words, high toughness cannot be achieved unless the hard phase formation is suppressed and the ferrite structure fraction is not less than 95%.

In order to limit the steel composition as described above and to control the microstructure of the steel in the present invention, the manufacturing method also needs to be limited.

Basically, according to the present invention, steel (slab or slab) having a limited composition is hot-rolled and then cooled.
At this time, heating is not necessarily required, and if the rolling temperature is secured, direct rolling may be performed after casting.
The rolling temperature can be easily secured by reheating to a temperature of 0 ° C. or less. The reason for setting the upper limit to 1250 ° C. is to consider energy saving because reheating at a high temperature beyond that causes an unnecessarily coarse structure and sufficient rolling temperature. Further, particularly when Ti is added, the final precipitation form of Ti may change and the toughness may be deteriorated. Therefore, the upper limit temperature at the time of reheating is limited to 1250 ° C. or less.

Hot rolling must be performed at a temperature of 750 ° C. or higher. This is because the transformation point (Ar ₃ ) is relatively high in the steel whose effective C amount is controlled within the range of the present invention, and the rolling temperature is 75%.
If the temperature is lower than 0 ° C., in the vicinity of the surface layer, α / γ dual-phase rolling may occur, and a processed ferrite may be generated. Worked ferrite, that is, two-phase rolling, may cause problems such as deterioration of toughness and anisotropy of various properties, and should be avoided as much as possible.

After rolling, the steel is allowed to cool to ensure a ferrite structure fraction of 95% or more and to obtain good toughness. However, if necessary, it is necessary to adjust the strength level to be obtained. After rolling, tempering at a temperature of less than Ac ₁ or A
There is no problem if normalizing at a temperature of c ₃ or more. In either case, the strain introduced during rolling or transformation is recovered, and there is no problem in toughness.
Even with these heat treatments, the structure fraction of ferrite is 9%.
5% or more can be sufficiently secured.

[0029]

EXAMPLES Steel plates (thickness: 9 to 40 mm) of various steel components are manufactured in the converter-continuous casting-thick plate process, and their descending yield points (for those without a yield point, 0.2% proof stress) , Tensile strength, V Charpy impact absorption energy at 0 ° C., and microstructure fraction of ferrite.

Table 1 shows the steel composition of the steel of the present invention together with the comparative steel, and Table 2 shows the microstructure and various properties of the steel sheet and the production process at that time.

The steel sheet according to the present invention (the steel of the present invention) has good characteristics.

On the other hand, the comparative steel not according to the present invention is:
Either property is inferior, that is, in terms of steel composition, comparative steels 9, 11, 12, and 13 have C, Ti equivalents (Ti (e
q. )), The relationship between the amounts of N is not appropriate, and C- [Ti (e
q. ) -3.4N] / 4 is 0.0 as defined by the present invention.
Since it exceeds 1%, the toughness is generally poor. In addition, since the comparative steel 9 has a high C content, the structure fraction of ferrite is smaller than the value specified by the present invention, and the strength is high. Comparative steel 1
0 is the value of C- [Ti (eq.)-3.4N] / 4 itself within the range specified by the present invention, and the structure fraction of ferrite is in accordance with the present invention. Since none of V is added, the toughness (vTrs) is poor.

[0033]

[Table 1]

[0034]

[Table 2]

[0035]

According to the present invention, low yield point steel having excellent toughness can be supplied at low cost for use as an energy absorbing device for buildings during earthquakes, and the safety of buildings during earthquakes can be further improved. became.

Claims (4)

[Claims] C .: 0.1% or less by weight, Si:
0.4 or less, Mn: 1.5% or less, P: 0.025% or less, S: 0.015% or less, Al: 0.06% or less,
N: 0.006% or less, and the relationship between the Ti equivalent defined as Ti (eq.) = Ti + Nb / 2 + V and the amounts of C and N is −0.02% ≦ C− [Ti (eq.) -3.4N] / 4
At least one of Ti, Nb and V is contained so as to satisfy ≦ 0.01%, the balance being iron and unavoidable impurities, and the microstructure of ferrite having a microstructure fraction of 95%.
% Low yield point steel with excellent toughness, characterized in that it is not less than 10%. 2. In% by weight, C: 0.1% or less, Si:
0.4 or less, Mn: 1.5% or less, P: 0.025% or less, S: 0.015% or less, Al: 0.06% or less,
N: 0.006% or less, and the relationship between the Ti equivalent defined as Ti (eq.) = Ti + Nb / 2 + V and the amounts of C and N is −0.02% ≦ C− [Ti (eq.) -3.4N] / 4
A steel containing one or more of Ti, Nb, and V so as to satisfy ≦ 0.01%, and the balance consisting of iron and unavoidable impurities, is cast directly or at a temperature of 1250 ° C. or less after casting. After reheating, finish hot rolling at a temperature of 750 ° C or higher,
A method for producing a low yield point steel having excellent toughness, which is then allowed to cool. 3. The method according to claim 1, wherein C: 0.1% or less, Si:
0.4 or less, Mn: 1.5% or less, P: 0.025% or less, S: 0.015% or less, Al: 0.06% or less,
N: 0.006% or less, and the relationship between the Ti equivalent defined as Ti (eq.) = Ti + Nb / 2 + V and the amounts of C and N is −0.02% ≦ C− [Ti (eq.) -3.4N] / 4
A steel containing one or more of Ti, Nb, and V so as to satisfy ≦ 0.01%, and the balance consisting of iron and unavoidable impurities, is cast directly or at a temperature of 1250 ° C. or less after casting. A method for producing a low yield point steel having excellent toughness, characterized in that after reheating, hot rolling is performed at a temperature of 750 ° C. or higher and tempering is performed at a temperature of ₁ point or lower. 4. C .: 0.1% or less by weight, Si:
0.4 or less, Mn: 1.5% or less, P: 0.025% or less, S: 0.015% or less, Al: 0.06% or less,
N: 0.006% or less, and the relationship between the Ti equivalent defined as Ti (eq.) = Ti + Nb / 2 + V and the amounts of C and N is −0.02% ≦ C− [Ti (eq.) -3.4N] / 4
A steel containing one or more of Ti, Nb, and V so as to satisfy ≦ 0.01%, and the balance consisting of iron and unavoidable impurities, is cast directly or at a temperature of 1250 ° C. or less after casting. A method for producing a low yield point steel having excellent toughness, comprising hot rolling at a temperature of 750 ° C. or more after reheating, and normalizing at a temperature of ₃ points or less of Ac.