JP2008248290A - Low-yield point steel with excellent toughness for damper, and its manufacturing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide low-yield point steel for a damper as an energy absorbing device at a big earthquake and also to provide its manufacturing method. <P>SOLUTION: The steel has a composition containing 0.001 to 0.050% C, ≤0.80% Si, 0.1 to 2.0% Mn, ≤0.020% P, ≤0.015% S, Ti in an amount >4 times that of C and not larger than 0.30%,≤0.060% Al and ≤0.006% N and further containing, if necessary, one or more among Nb, V, B, Ni, Cu, Cr, Mo, Ca and REM each in a specific amount and also has a microstructure composed of ferrite single phase and shows no clear upper/lower yield point at a tensile test. A cast slab or steel slab, having the above components, is heated to 1,000 to 1,300°C and then subjected, after the completion of hot rolling at ≥900°C, to natural cooling or subjected, after hot rolling, to holding at 900 to 1,000°C for ≥10 min and then to natural cooling. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to steel for an energy absorbing device (damper) for absorbing energy input to a building caused by an earthquake in a specific part and ensuring seismic performance, and a method for manufacturing the same. Note that the main strength level is a low strength / low yield point steel having a yield strength of 150 to 250 MPa and a tensile strength of 250 to 450 MPa.

  The conventional seismic design of buildings is designed to absorb energy by making the structures of columns and beams plastic during a large earthquake, preventing the collapse of buildings and preventing human damage. It is a very rational design method that can keep the construction cost relatively low while assuming the main premise. On the other hand, with the development of seismic design technology in recent years, the development and practical application of vibration control and seismic isolation structures have progressed, and the energy input to the building due to the earthquake is absorbed by a specific part (energy absorption device (damper)), and seismic performance Attention has been paid to design technology that prevents damage to the pillars and beams, which are the main structures.

  For such an energy absorbing device (damper), low yield point steel is used. The principle is that the yield point is lower than that of a normal column or beam structural material, yielding at an early stage during an earthquake, and vibration response due to the earthquake is converted into plastic energy to suppress the vibration response.

  In order to achieve a low yield point, steel with a component close to pure iron with almost no added elements (see, for example, Patent Document 1), and in some cases, a component close to pure iron is normalized at a higher temperature. There are methods for processing (see, for example, Patent Documents 2 and 3). All of these have the disadvantage of being inferior in low-temperature toughness due to the low yield point by making coarse ferrite. In addition, in each case, the C amount needs to be 0.005% or less, and there is a problem that the load on the steelmaking process is high and there is almost no added element, but the cost is not necessarily advantageous.

  On the other hand, the inventors of the present invention previously described a low yield point steel and a method for producing the same, in which a substantial amount of C is reduced by appropriately adding a carbide-forming element without reducing the amount of C more than necessary. Invented (see, for example, Patent Documents 4 and 5). Narrow range control is required for yield strength in order to ensure plasticization at a specific site as a steel for dampers. With LY225 steel, which is a standard of the Japan Iron and Steel Federation, the yield strength of 205 to 245 MPa is only 40 MPa. ing.

  The inventors' previous invention has a yield point that is particularly clear in tensile tests, but in such steel materials, not only the material variations of the steel materials themselves, but also the responsiveness, rigidity, etc. of the tensile tester. However, the yield strength varies, and it was not always sufficient in terms of stable narrow range control.

JP-A-3-31467 Japanese Patent Laid-Open No. 5-214442 JP-A-5-320760 JP-A-10-183293 Japanese Patent No. 3411217

  The present invention eliminates the upper and lower yield points during a tensile test and eliminates the yield strength variation by properly adding Ti according to the amount of C without reducing the amount of C more than necessary. An object of the present invention is to provide a low-yield-point steel with reduced steel and a method for producing the same.

  The inventors of the present invention have made extensive studies to solve the above-mentioned problems. As a result of adding Ti slightly stoichiometrically with respect to the C content, the inventors have made IF (interstitial free), and set the yield point during the tensile test. The present invention was completed by finding that it disappears.

  The gist of the present invention is as follows.

(1) Steel composition is mass%,
C: 0.001% to 0.050%,
Si: 0.80% or less,
Mn: 0.1% or more and 2.0% or less,
P: 0.020% or less,
S: 0.015% or less,
Ti: more than 4 times the C content and 0.30% or less,
Al: 0.060% or less,
N: 0.006% or less, the balance being iron and inevitable impurities, the microstructure of the steel being a ferrite single phase, and a clear upper yield point and lower in the stress-strain curve during a tensile test Low yield point steel for dampers, characterized by having no yield point.

(2) Furthermore, in mass%,
Nb: 0.01% or more and 0.10% or less,
V: The low yield point steel for a damper according to the above (1), wherein either one or both elements are contained in a range of 0.01% to 0.10%.

(3) Furthermore, in mass%,
B: The low yield point steel for a damper according to the above (1) or (2), which is contained in a range of 0.0002% to 0.0030%.

(4) Furthermore, in mass%,
Ni: 0.05% or more and 0.50% or less,
Cu: 0.05% or more and 0.50% or less,
Cr: 0.05% or more and 0.50% or less,
Mo: One or more elements in a range of 0.05% or more and 0.50% or less are contained, and the low for a damper according to any one of (1) to (3) above Yield point steel.

(5) Furthermore, in mass%,
Ca: 0.0005% or more and 0.004% or less,
REM: For dampers according to any one of (1) to (4) above, wherein any one or two elements are contained in a range of 0.0005% to 0.008%. Low yield point steel.

  (6) The slab or steel slab comprising the steel composition described in any one of (1) to (5) above is heated to a temperature of 1000 to 1300 ° C, and hot rolling is performed at a temperature of 900 ° C or higher. The manufacturing method of the low yield point steel for dampers characterized by making the microstructure of steel into a ferrite single phase by finishing and cooling after that.

  (7) After hot rolling the slab or steel slab comprising the steel composition described in any one of the above (1) to (5), it is reheated to a temperature of 900 to 1000 ° C. A method for producing a low yield point steel for a damper, wherein the steel microstructure is made into a ferrite single phase by allowing it to cool after being retained for a long time.

  According to the present invention, a low yield point steel for a damper having a stable and low yield strength and excellent toughness can be supplied in a large amount and at a low cost. As a result, the use of the steel material for dampers can improve the earthquake resistance of the building and further enhance the safety.

The low yield point steel for dampers according to the present invention has a steel composition of mass%,
C: 0.001% to 0.050%,
Si: 0.80% or less,
Mn: 0.1% or more and 2.0% or less,
P: 0.020% or less,
S: 0.015% or less,
Ti: more than 4 times the C content and 0.30% or less,
Al: 0.060% or less,
N: 0.006% or less, the balance is made of iron and inevitable impurities, and if necessary, a specific amount of Nb, V, B, Ni, Cu, Cr, Mo, Ca, REM is one or two It contains more than seeds, the microstructure of the steel is a ferrite single phase, and has no clear upper and lower yield points in the stress-strain curve during the tensile test. .

  Hereinafter, the reasons for limiting the steel components, the microstructure of the steel, and the upper and lower yield points will be described.

  C has a great influence on the formation of a hard second phase such as pearlite, which deteriorates toughness and increases strength, and is added to completely fix C and convert to IF as described later. Also from the viewpoint of Ti amount, the lower the steel in the present invention, the better. However, considering the steelmaking cost for decarburization, the lower limit was made 0.001%. On the other hand, the upper limit is 0.050% because the appropriate addition amount of Ti for IF conversion by complete fixation of C is increased, which is not preferable from the viewpoint of cost and toughness of the welded portion. Limited to.

  Since Si is an element contained in steel from the top of deoxidation and also acts as a solid solution strengthening, it can be appropriately added depending on the required strength. However, too much addition deteriorates weldability and weld toughness, so the upper limit was limited to 0.80%. Deoxidation of steel can be sufficiently performed only with Ti and Al, and it is not always necessary to add them.

  Since Mn increases the strength of the base material as a solid solution strengthening element, it can be arbitrarily added depending on the required strength level. However, in order to stably secure the yield strength of 150 to 250 MPa and the tensile strength of 250 to 400 MPa that are the subject of the present invention, it is necessary to add at least 0.1%. However, if the amount of Mn is too large, not only the hardenability increases more than necessary and the weldability and weld toughness deteriorate, but also the center segregation of the continuously cast slab is promoted, so the upper limit was made 2.0%. .

  P is an impurity in the steel of the present invention, and a reduction in the amount of P tends to reduce the grain boundary fracture in the weld heat affected zone, so the smaller the amount, the better. If the content is large, the low temperature toughness of the base metal and the welded portion is deteriorated, so the upper limit was made 0.020%.

  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 metal and the welded portion is deteriorated, so the upper limit was made 0.015%.

  Ti should be said to be the greatest feature of the present invention, and the purpose is to completely fix it as a carbide (or carbonitride) of C in steel and to convert it to IF. The Ti carbide TiC has a stoichiometric mass ratio (Ti / C) of about 3.42, and in the present invention, even if considering the consumption of Ti as the Ti nitride TiN, C is still sufficient. It was limited to more than 4 times the content. The upper limit of 0.30% as an absolute value is limited from the viewpoint of the alloy cost in consideration of the fact that even if the upper limit of C amount is 0.050%, it may be more than 0.20%. Therefore, the upper limit is not necessarily limited.

  Al is an element generally used as a deoxidizer and contained in steel, but Si or Ti is sufficient for deoxidation, and the lower limit is not limited in the steel of the present invention. However, when the amount of Al increases, not only the cleanliness of the steel deteriorates but also the toughness of the weld metal deteriorates, so the upper limit was made 0.060%.

  N is contained in steel as an inevitable impurity, and the lower limit is not particularly limited. If the amount is too large, the above-described possibility of consuming Ti for C fixation as a nitride increases, so the upper limit was limited to 0.006%.

  Moreover, you may add any one or both of Nb or V to the low yield point steel for dampers which concerns on this invention.

  Nb and V are elements having a lower carbide forming ability than Ti, and have a complementary meaning to Ti in fixing C as a carbide. In order to surely enjoy the effect of addition of Nb and V, addition of at least 0.01% is necessary. On the other hand, too much addition deteriorates the toughness of the weld heat affected zone, so the upper limit is 0.10%. did.

  Further, B may be further added to the low yield point steel for dampers according to the present invention.

  B segregates at the ferrite grain boundary, strengthens the grain boundary, and has an effect of suppressing grain boundary fracture. Therefore, the ferrite single-phase low yield point steel that fixes C as in the present invention works more effectively as the grain size and strength decrease. According to the experiments of the present inventors, such an effect of B may be 0.0002%, and the effect is saturated even if it is added more than necessary. For this reason, in this invention, since it is the upper limit confirmed by experiment and the negative influence was not seen on steel material characteristics, it limited to the range of 0.0002 to 0.0030%. As a general test temperature of 0 ° C., if the grain size is not extremely coarse so that the grain size exceeds 100 μm, the Charpy absorbed energy is not drastically reduced even if the grain boundary breaks, so it is not an essential element. Although it selectively adds as needed, in this invention steel, adding is preferable.

  Further, one or more elements of Ni, Cu, Cr, and Mo may be added to the low yield point steel for dampers according to the present invention.

  The purpose of adding these elements to the basic components as needed is for strength adjustment.

  As will be described later, the steel of the present invention does not have clear upper and lower yield points in the stress-strain curve during the tensile test. In such a case, 0.2% offset proof stress is generally taken as the yield strength, but when the yield point disappears, this apparent yield strength (0.2% offset proof strength) is greatly reduced. The low yield point steel for dampers targeted by the present invention is not necessarily better as the yield strength is lower, but it is a general steel for building structures in order to ensure plasticization earlier than the structure of columns and beams. Although it is lower, it has a certain yield strength and its narrow range control is important. Therefore, the addition of Ni, Cu, Cr, and Mo should be selected and added as appropriate in consideration of the base material, weld heat affected zone toughness, alloy costs, etc., depending on the yield strength required in the design. become.

  If Ni is not added excessively, it improves the strength and toughness of the base material without adversely affecting the weldability and the toughness of the welded portion. In order to exert these effects, addition of at least 0.05% is essential. However, even though Ni is too much added, it is not preferable for weldability and is also a relatively expensive element, so the upper limit was kept at 0.50% in consideration of economics.

  Cu exhibits substantially the same effects and phenomena as Ni, with an upper limit of 0.50% in addition to deterioration of weldability, and excessive addition of Cu alone causes Cu-cracks during hot rolling, making it difficult to manufacture. Therefore, it is regulated. The lower limit should be the minimum amount for obtaining a substantial effect, and is 0.05%.

  Cr and Mo improve both the strength and toughness of the base material. In order to enjoy these effects, at least 0.05% of each is required when added. However, if the addition amount is too large, the base material, welded portion toughness and weldability are deteriorated, and economic efficiency is lost.

  Further, one or two elements of Ca or REM (rare earth element) may be added to the low yield point steel for damper according to the present invention.

  Ca and REM control the form of MnS and improve the low-temperature toughness and sheet thickness direction characteristics of the base material. In order to exert these effects, at least 0.0005% is required. However, too much addition adversely deteriorates the cleanliness of the steel, and not only the above-described property improvement effect is obtained, but rather deteriorates. Therefore, the upper limit of the addition amount is 0.004% for Ca and REM, respectively. Limited to 008%. Since Ca and REM have substantially the same effect, any one may be added in the above range, and two may be added.

  Next, the reasons for limiting the microstructure and the stress-strain curve during the tensile test will be described.

  The provision that the microstructure is a single phase of ferrite and that there are no clear upper and lower yield points in the stress-strain curve during the tensile test is the same as that of the present invention described above. If adjusted, the effect is inevitably obtained, which can be said to be the effect of the invention, but is intentionally limited to clarify the characteristics of the present invention.

  The microstructure of the ferrite single phase is indispensable for achieving the low strength that is the target of the present invention, that is, the low yield point steel. At the same time, in order to obtain excellent toughness under a coarse-grained structure for reducing strength, the presence of a structure containing hard cementite that can be a starting point of fracture is not preferable. For this reason, in the present invention, the microstructure is limited to the ferrite single phase.

  On the other hand, in the stress-strain curve during the tensile test, eliminating the yield point and not having clear upper and lower yield points stabilizes the yield strength in a narrow range as described above. Is necessary to control. In low yield point steel for dampers, narrow range control of yield strength leads to an increase in commercial value, and it is considered as one of the important components of the invention as a means for achieving it, and is limited according to the present invention.

  In order to stably obtain such a low yield point steel for a damper, it is extremely effective to limit the production conditions as in the present invention.

  The reason will be described below.

  Usually, in thick steel plates, toughening by refining the structure is a general direction, but the low yield point steel targeted by the present invention is rather the opposite direction, and the heating temperature may be high. . The upper limit is rather greater from the furnace side. In addition to the industrial production viewpoint, too high a heating temperature may damage the surface properties of the slab or steel slab, and thus the surface properties after rolling. In order to increase, the upper limit was limited to 1300 ° C. The lower limit temperature was set to 1000 ° C. from the viewpoint of securing the rolling temperature, which will be described later, and the final structure was refined more than necessary to prevent the strength from becoming excessive, and from the viewpoint of rolling efficiency. In terms of industrial mass production, the heating temperature range is preferably about 1100 to 1200 ° C.

  The slab or steel slab heated to the temperature range needs to be allowed to cool after hot rolling is finished at 900 ° C. or higher. When the rolling end temperature is lower than 900 ° C., the rolled structure becomes finer, and as a result, the final transformed structure also becomes finer, increasing the possibility of increasing the strength. In addition, even if hot rolling is finished at 900 ° C. or higher, if the subsequent cooling is higher than that of cooling, the transformation temperature may decrease, the final transformation structure may become fine particles, and the strength may increase. Will increase. For this reason, after finishing hot rolling at 900 degreeC or more, it limited to standing to cool.

  In addition, after rolling, reheating to a temperature range of 900 to 1000 ° C. has an effect of recovering strain and dislocation introduced by transformation during hot rolling and subsequent cooling, depending on the rolling temperature and thickness. Is more preferred as a low yield point steel. In order to reliably enjoy the effect of the reheating, it is necessary to stay in the temperature range for 10 minutes or more. In order to prevent the introduction of strain and dislocation during cooling, it must be allowed to cool after the reheating treatment. . In addition, since reheating treatment has the above-mentioned effect, when reheating treatment is performed, the heating temperature range of the steel slab or slab prior to the reheating treatment may be relaxed compared to the case where it is allowed to cool and manufacture after hot rolling. Although the upper limit is preferably the surface properties described above, the lower limit is preferably regulated equally from the viewpoint of rolling efficiency.

  Moreover, even if rolling temperature is less than 900 degreeC, the cooling after rolling does not need to be allowed to cool. Although the lower limit temperature of rolling is not particularly limited, it is preferable that the strain / dislocation to be recovered by reheating treatment is preferably not less than 800 ° C. so as not to be introduced more than necessary in rolling, and for the same reason, Needless to say, the cooling after rolling is preferably allowed to cool.

  Steel sheets of various steel components (thickness 6 to 50 mm) were produced by the converter-continuous casting-thick plate process, and the mechanical properties (strength and toughness) were investigated.

  Table 1 shows the steel components of the steel of the present invention together with the comparative steel, and Table 2 shows the production conditions and mechanical properties of the steel sheet.

  All the steel plates manufactured in accordance with the method of the present invention (present invention steel) have good characteristics. On the other hand, the specific mild steel not according to the present invention has an inadequate strength (yield strength) as a low yield point steel or is inferior in toughness.

  Since the comparative steel 13 has a high C amount and a Ti amount for fixing C is insufficient (four times or less than the C amount), the yield point appears and the yield strength becomes high, and the pearlite structure is generated and the toughness is generated. Also inferior. Although the comparative steel 14 is within the scope of the present invention when each of the C and Ti amounts is alone, the Ti amount is lower than the C amount (four times or less the C amount), so that the yield point appears and the structure containing cementite is present. Inferior toughness to produce. In this comparative example in which the yield point appeared, although the yield strength itself was low, it was confirmed that the variation in the upper yield point and the lower yield point was large in the tensile test using test pieces taken from the same steel plate. Since the comparative steel 15 has a high P content and the comparative steel 16 has a high S content, the toughness is inferior. Moreover, since the comparative steels 17 and 18 have high Si and Mn, respectively, it has been confirmed that the strength is slightly high, and at the same time, the toughness after applying the reproducible thermal cycle simulating the weld heat affected zone is confirmed.

  On the other hand, the comparative steels 19 to 22 have the same components as the steel 5 of the present invention that satisfies the steel composition range of the present invention, but the manufacturing method deviates from the scope of the present invention, so the characteristics are inferior. That is, the comparative steel 19 has a low heating temperature. As a result, the rolling temperature cannot be secured and the rolling end temperature is low, the comparative steel 12 has a low rolling end temperature, and the comparative steel 21 is accelerated and cooled after rolling. The structure becomes fine and the strength is high. The comparative steel 22 is also caused by a low rolling end temperature, but since the subsequent reheating temperature is low, the recovery of strain and dislocation is insufficient, the strength is high, and the toughness is inferior.

  From the above examples, it can be seen that according to the present invention, a low yield point steel for a damper having low yield strength and excellent toughness can be obtained.

Claims (7)

Steel composition is mass%,
C: 0.001% to 0.050%,
Si: 0.80% or less,
Mn: 0.1% or more and 2.0% or less,
P: 0.020% or less,
S: 0.015% or less,
Ti: more than 4 times the C content and 0.30% or less,
Al: 0.060% or less,
N: 0.006% or less, the balance being iron and inevitable impurities, the microstructure of the steel being a ferrite single phase, and a clear upper yield point and lower in the stress-strain curve during a tensile test Low yield point steel for dampers, characterized by having no yield point. In addition,
Nb: 0.01% or more and 0.10% or less,
The low yield point steel for a damper according to claim 1, wherein either one or both elements are contained in a range of V: 0.01% or more and 0.10% or less. In addition,
B: Low yield point steel for dampers according to claim 1 or 2, characterized in that it is contained in a range of 0.0002% to 0.0030%. In addition,
Ni: 0.05% or more and 0.50% or less,
Cu: 0.05% or more and 0.50% or less,
Cr: 0.05% or more and 0.50% or less,
The low yield for a damper according to any one of claims 1 to 3, wherein one or more elements are contained within a range of Mo: 0.05% to 0.50%. Spot steel. In addition,
Ca: 0.0005% or more and 0.004% or less,
REM: Any one or two elements are contained in the range of 0.0005% or more and 0.008% or less, and the low for dampers according to any one of claims 1 to 4 Yield point steel.   The slab or steel slab comprising the steel composition according to any one of claims 1 to 5 is heated to a temperature of 1000 to 1300 ° C, the hot rolling is finished at a temperature of 900 ° C or higher, and then allowed to cool. A method for producing a low yield point steel for a damper, characterized in that the microstructure of the steel is a ferrite single phase.   A slab or steel slab comprising the steel composition according to any one of claims 1 to 5 is hot-rolled and then reheated to a temperature of 900 to 1000 ° C and retained in the temperature range for 10 minutes or more. A method for producing a low yield point steel for a damper, wherein the microstructure of the steel is changed to a ferrite single phase by cooling.