KR102325473B1 - Steel material for earthquake-resistant structures and method of manufacturing the same - Google Patents

Abstract

The present invention relates to a steel material for an earthquake-resistant structure and a method for manufacturing the same.
One embodiment of the present invention is by weight%, C: 0.30 to 0.50%, Si: 0.5 to 1.0%, Mn: 0.5 to 1.0%, V: 0.10 to 0.30%, P: 0.03% or less (excluding 0%) , S: 0.03% or less (excluding 0%), Al: 0.005% or less (excluding 0%), N: 0.009 to 0.020%, the remainder including Fe and other unavoidable impurities, from the surface at D/4 The microstructure of the region up to 3D/4 provides a steel material for an earthquake-resistant structure containing 30 to 40 area % of ferrite and the remaining pearlite and a manufacturing method thereof.
(However, D is the diameter of the steel, and the unit is mm.)

Description

STEEL MATERIAL FOR EARTHQUAKE-RESISTANT STRUCTURES AND METHOD OF MANUFACTURING THE SAME

The present invention relates to a steel material for an earthquake-resistant structure and a method for manufacturing the same.

Anchor bolts are called foundation bolts and are structures that are buried in the concrete floor to fix the building support when building or installing machinery. Although the manufacturing method of the anchor bolt is different depending on the shape, the anchor bolt is usually manufactured through a wire rod rolling → cooling → wire drawing → rolling or bar rolling → cooling → rolling process. On the other hand, unlike the domestic construction law, in Japan, the use of earthquake-resistant anchor bolts is compulsory in preparation for earthquakes. do.

The reason that tensile strength and yield strength must satisfy certain criteria is that a yield ratio of 0.75 or less must be satisfied. When an earthquake occurs, the building moves left and right, and the steel structure undergoes plastic deformation at this time. However, if the yield ratio is too low, in the case of anchor bolts, since the fastening force itself is lowered, the length and thickness of the product are increased, and the ground depth is also increased, which is not preferable. The elongation is also in the same vein.

In places where earthquakes occur frequently, such as in Japan, the development of anchor bolts for seismic structures with yield shelves is required to shorten the construction period, economic feasibility, and improve lifespan. This is because, when the yield shelf is generated in the anchor bolt, the product length can be reduced and the thickness can be reduced, which is advantageous in terms of economy. 1 is an example of a stress-elongation graph. As shown in Figure 1, the yield shelf is non-uniform deformation occurs in the lower yielding after the upper yielding after passing through the elastic region, and it occurs because interstitial elements such as N and C are pinning to dislocations formed in the grains.

The reason why a yield shelf is required for a product is that it can improve the ability to withstand earthquakes. Figure 2 is a schematic view showing the degree of bending of the anchor bolt according to the size of the yield shelf. As shown in Figure 2, when the yield shelf is large, when the seismic wave reaches the building, the degree of bending is increased, so that it can be safer from an earthquake.

Therefore, in order to be used as an anchor bolt for earthquake resistance, it is necessary to satisfy the resistance yield ratio and to have a yield shelf.

One aspect of the present invention is to provide a steel material for an earthquake-resistant structure and a method for manufacturing the same, and more particularly, to provide a steel material for an earthquake-resistant structure having a resistance yield ratio and a yield shelf and a method for manufacturing the same.

One embodiment of the present invention is by weight%, C: 0.30 to 0.50%, Si: 0.5 to 1.0%, Mn: 0.5 to 1.0%, V: 0.10 to 0.30%, P: 0.03% or less (excluding 0%) , S: 0.03% or less (excluding 0%), Al: 0.005% or less (excluding 0%), N: 0.009 to 0.020%, the remainder including Fe and other unavoidable impurities, at D/4 from the surface The microstructure of the region up to 3D/4 provides a steel material for earthquake-resistant structures containing 30-40 area % of ferrite and the remainder of pearlite.

(However, D is the diameter of the steel, and the unit is mm.)

Another embodiment of the present invention is by weight%, C: 0.30 to 0.50%, Si: 0.5 to 1.0%, Mn: 0.5 to 1.0%, V: 0.10 to 0.30%, P: 0.03% or less (excluding 0%) , S: 0.03% or less (excluding 0%), Al: 0.005% or less (excluding 0%), N: 0.009 to 0.020%, balance Fe and other unavoidable impurities. heating; obtaining a wire rod by finishing rolling the heated billet at an exit temperature of 950 to 1100°C; And it provides a method of manufacturing a steel material for an earthquake-resistant structure comprising the step of cooling the region of 700 ~ 900 ℃ with respect to the wire at a cooling rate of 1 ℃ / s or less.

According to one aspect of the present invention, it is possible to provide a steel material for an earthquake-resistant structure having a resistance yield ratio and a yield shelf and a manufacturing method thereof, and thus can be preferably applied to a building structure requiring an earthquake-resistant design, and through this, the stability of the building can improve

1 is an example of a stress-elongation graph.
Figure 2 is a schematic view showing the degree of bending of the anchor bolt according to the size of the yield shelf.
3 is a microstructure photograph of Inventive Example 2 according to an embodiment of the present invention observed with an optical microscope.
4 is a stress-elongation graph of Inventive Example 2 and Comparative Example 1 according to an embodiment of the present invention.

Hereinafter, a steel material for an earthquake-resistant structure according to an embodiment of the present invention will be described. First, the alloy composition of the present invention will be described. However, the unit of the alloy composition described below is considered as weight % unless otherwise specified.

C: 0.30 to 0.50%

C is a major element for securing strength. C is a major element forming pearlite and exists as cementite. For every 0.1% increase in the C content, the strength increases by about 100 MPa. When the C content is less than 0.30%, the strength targeted by the present invention cannot be secured, and when it exceeds 0.50%, no elongation at the yield point occurs due to an increase in the pearlite fraction, or seismic resistance is secured as the elongation at the yield point becomes smaller can be difficult to do. Therefore, the content of C is preferably in the range of 0.30 to 0.50%. The lower limit of the C content is more preferably 0.32%, even more preferably 0.34%, and most preferably 0.36%. The upper limit of the C content is more preferably 0.48%, even more preferably 0.46%, and most preferably 0.44%.

Si: 0.5~1.0%

As a ferrite solid solution strengthening element, Si is an element that is solid solution in ferrite but also segregates at ferrite/cementite grain boundaries. The Si content increases with a yield strength of about 14 to 16 MPa for every 0.1% increase. When the Si content is less than 0.5%, it is difficult to secure the strength targeted by the present invention, and when it exceeds 1.0%, ductility deterioration occurs and scale peeling becomes difficult. Accordingly, the Si content is preferably in the range of 0.5 to 1.0%. The lower limit of the Si content is more preferably 0.6%, even more preferably 0.7%, and most preferably 0.75%. The upper limit of the Si content is more preferably 0.95%, even more preferably 0.9%, and most preferably 0.85%.

Mn: 0.5~1.0%

Mn is an austenite stabilizing element, has a deoxidation effect, and forms MnS by combining with S present in the steel to prevent red brittleness caused by sulfur. In addition, although the strength is slightly increased, it is added to ensure stable quenching. When the content of Mn is less than 0.5%, there is no effect of increasing the strength, and when it exceeds 1.0%, there is a problem in that a low-temperature structure is formed. Therefore, the Mn content is preferably in the range of 0.5 to 1.0%. The lower limit of the Mn content is more preferably 0.6%, more preferably 0.65%, and most preferably 0.7%. The upper limit of the Mn content is more preferably 0.9%, more preferably 0.85%, and most preferably 0.8%.

V: 0.10 to 0.30%

V combines with C present in the steel to form fine VC precipitates. When the content of V is less than 0.10%, the effect of increasing the strength by the fine VC precipitates is small, so it is not easy to secure physical properties such as strength targeted by the present invention. It is formed on the surface and greatly reduces ductility and toughness, which may cause fracture when stress is applied uniaxially. Therefore, the content of V is preferably in the range of 0.10 to 0.30%. The lower limit of the V content is more preferably 0.12%, even more preferably 0.14%, and most preferably 0.16%. The upper limit of the V content is more preferably 0.28%, even more preferably 0.26%, and most preferably 0.24%.

P: 0.03% or less (excluding 0%)

P is included as an impurity in steel, and its content is preferably as small as possible. However, if it is restricted to an extreme limit, the cost for removing impurities in the steelmaking process increases. Therefore, in the present invention, the P content is managed to 0.03% or less.

S: 0.03% or less (excluding 0%)

S is included as an impurity in steel, and its content is preferably as small as possible. However, if it is restricted to an extreme limit, the cost for removing impurities in the steelmaking process increases. Accordingly, in the present invention, the content of S is controlled to 0.03% or less.

Al: 0.005% or less (excluding 0%)

Since Al reacts strongly with O, it is used as a deoxidizer, and also combines with N in steel to form AlN, which serves to refine grains. In the present invention, since there must be a solid solution N in order to form the elongation at the yield point, the addition of Al, which has a strong bonding force, is suppressed as much as possible. Accordingly, in the present invention, the Al content is managed to 0.005% or less.

N: 0.009~0.020%

N is an element that greatly increases strength like C, and the strength increases by 100 MPa whenever its content increases by 0.1%. However, the most important thing is that the N makes it possible to manufacture the anchor bolt accompanied by the yield shelf proposed in the present invention. The N may be fixed to a dislocation (cottrell effect), in which case yielding elongation induce When the content of N is less than 0.009%, it is not possible to sufficiently secure the length of the yield shelf, and when it exceeds 0.020%, it is difficult to control the components due to nitrogen bubbling treatment, and also it may cause surface crack formation during playing. . Therefore, the content of N is preferably in the range of 0.009 to 0.020%. The lower limit of the N content is more preferably 0.01%, even more preferably 0.011%, and most preferably 0.012%. The upper limit of the N content is more preferably 0.019%, even more preferably 0.018%, and most preferably 0.017%.

The remaining component of the steel for earthquake-resistant structures of the present invention is iron (Fe). However, since unintended impurities from raw materials or the surrounding environment may inevitably be mixed in the normal steel manufacturing process, this cannot be excluded. Since these impurities are known to anyone skilled in the steel manufacturing process, all of them are not specifically mentioned in this specification.

In the steel material for earthquake-resistant structures according to an embodiment of the present invention, the microstructure of the region from D/4 to 3D/4 from the surface preferably includes 30 to 40 area% of ferrite and the remainder of pearlite. In this case, D is the diameter of the steel, and the unit means mm. When the ferrite fraction of the microstructure of the central region is less than 30 area %, there is a disadvantage that the yield shelf is not formed, and the yield ratio to exceed 40 area % is too low, so that it cannot be used as a product.

In the steel material for an earthquake-resistant structure according to an embodiment of the present invention, VC precipitates having an average size of 100 nm or less are preferably included in an area of 40% or more relative to the total VC precipitates formed in the steel. When the fraction of the VC precipitates having an average size of 100 nm or less is less than 40 area % of the total VC precipitates, it is difficult to secure high strength.

The steel of the present invention provided as described above has tensile strength: 750 MPa or more, yield strength: 490 MPa or more, yield ratio: 0.75 or less, elongation: 20% or more, and may have excellent strength, elongation and resistive yield ratio. In addition, the steel of the present invention can be preferably applied as an earthquake-resistant anchor bolt, etc., as the yield point elongation can occur up to a strain of 1.0% or more to ensure excellent earthquake resistance.

Hereinafter, a method for manufacturing a steel material for an earthquake-resistant structure according to an embodiment of the present invention will be described.

First, the billet having the above-described alloy composition is heated at 1000 ~ 1200 ℃. When the billet heating temperature is less than 1000 ℃, there is a disadvantage that coarse carbide remains, and when it exceeds 1200 ℃, there is a disadvantage that the ductility of the wire rod decreases because the austenite grain size increases. Therefore, the billet heating temperature is preferably in the range of 1000 ~ 1200 ℃. The lower limit of the billet heating temperature is more preferably 1030°C, even more preferably 1060°C, and most preferably 1090°C. The upper limit of the billet heating temperature is more preferably 1170°C, even more preferably 1140°C, and most preferably 1110°C.

When the billet is heated, the heating time may be 90 to 120 minutes. If the heating time is less than 90 minutes, coarse carbides may remain, and if it exceeds 120 minutes, austenite grains may be coarsened. Therefore, the heating time is preferably 90 to 120 minutes. Therefore, the heating time is preferably 90 to 120 minutes. The lower limit of the billet heating time is more preferably 95 minutes, even more preferably 98 minutes, and most preferably 100 minutes. The upper limit of the billet heating time is more preferably 115 minutes, even more preferably 112 minutes, and most preferably 110 minutes.

Thereafter, the heated billet is finished rolling so that the exit temperature is 950 to 1100° C. to obtain a wire rod. When the exit temperature during the finishing rolling is less than 950°C, there is a disadvantage in that the replacement cycle of the rolling equipment is increased by the rolling load, and when it exceeds 1100°C, there is a disadvantage in that coarse grains are formed. Therefore, the hot rolling temperature is preferably in the range of 950 ~ 1100 ℃. The lower limit of the rolling temperature is more preferably 970°C, even more preferably 990°C, and most preferably 1010°C. The upper limit of the hot rolling temperature is more preferably 1080°C, even more preferably 1070°C, and most preferably 1060°C.

Thereafter, a region of 700 to 900° C. with respect to the wire rod is cooled at a cooling rate of 1° C./s or less. The temperature range of 700 ~ 900 ℃ is a section in which fine VC precipitates are actively precipitated. Therefore, in the present invention, it is possible to obtain the effect of stably forming fine precipitates by cooling the 700 ~ 900 ℃ region at a cooling rate of 1 ℃ / s or less. If the cooling rate exceeds 1 °C / s, there is a disadvantage in that it is difficult to secure fine precipitates. The cooling rate is more preferably 0.9°C/s or less, more preferably 0.8°C/s or less, and most preferably 0.7°C/s or less.

In addition, in this invention, it does not specifically limit about the cooling rate after the said cooling process.

Hereinafter, the present invention will be described in more detail through examples. However, the following examples are only examples for explaining the present invention in more detail, and do not limit the scope of the present invention.

(Example)

After preparing a billet having an alloy composition shown in Table 1 ^{and having a size of 160 × 160 mm 2} , the billet was heated at 1180° C. for 100 minutes, and then, finish rolling was performed to prepare a wire rod. At this time, the finish rolling exit temperature was 980 ℃. Thereafter, the wire rod was cooled in a region of 700 to 900° C. at a cooling rate, followed by air cooling. After measuring the microstructure, the fraction of VC precipitates having an average size of 100 nm or less compared to the total precipitates, and mechanical properties of the thus-prepared wire rod, the results are shown in Table 2 below.

At this time, the microstructure was measured using an average value, assuming that the crystal grains were circular after securing a total of 10 sheets by photographing at a ×200-fold field of view using an optical microscope, and the VC precipitate fraction was obtained using a replica precipitate extraction method. A test piece was prepared, and an arbitrary 500 nm ² area was measured through a transmission electron microscope. Mechanical properties such as yield strength, tensile strength, and elongation at yield were measured through a tensile test.

division Alloy composition (wt%) C Si Mn P S Al V N Comparative Example 1 0.2 0.72 0.89 0.008 0.009 0.005 0 0.01 Comparative Example 2 0.4 0.73 0.9 0.01 0.011 0.003 0 0.011 Comparative Example 3 0.6 0.72 0.92 0.01 0.009 0.004 0 0.008 Comparative Example 4 0.4 0.71 0.91 0.01 0.009 0.004 0.07 0.011 Invention Example 1 0.4 0.71 0.91 0.01 0.009 0.004 0.1 0.01 Invention Example 2 0.4 0.71 0.91 0.01 0.009 0.004 0.15 0.012 Invention example 3 0.4 0.73 0.91 0.011 0.01 0.004 0.29 0.009 Comparative Example 5 0.4 0.72 0.89 0.011 0.011 0.004 0.42 0.01 Comparative Example 6 0.4 0.71 0.89 0.011 0.01 0.005 0 0.004 Invention Example 4 0.4 0.69 0.89 0.01 0.009 0.005 0.15 0.015 Comparative Example 7 0.41 0.72 0.9 0.011 0.01 0.004 0 0.021 Comparative Example 8 0.41 0.72 0.9 0.011 0.009 0.02 0.15 0.011

division yield strength
(MPa) tensile strength
(MPa) yield ratio elongation
(%) Yield Point Elongation
Whether it occurs Maximum elongation at yield point elongation
(%) microstructure
(area%) Fraction of VC precipitates having an average size of 100 nm or less compared to the total precipitates
(area%) Comparative Example 1 319 539 0.59 36.6 generation 1.00 54%F-P 0 Comparative Example 2 421 735 0.57 27.1 generation 1.10 32%F-P 0 Comparative Example 3 438 796 0.55 23.8 non-occurring 0 18%F-P 0 Comparative Example 4 475 732 0.65 26.7 generation 1.20 33%F-P 29 Invention Example 1 490 760 0.64 26.4 generation 1.10 35% F-P 41 Invention Example 2 518 790 0.66 25.2 generation 1.30 34%F-P 48 Invention example 3 571 872 0.65 24.2 generation 1.00 37%F-P 53 Comparative Example 5 fracture during tensile test Comparative Example 6 396 707 0.56 29.1 generation 0.30 34%F-P 0 Invention Example 4 585 869 0.67 25.1 generation 1.30 35% F-P 55 Comparative Example 7 390 720 0.54 15.2 generation 0.10 38%F-P 0 Comparative Example 8 490 769 0.64 25.2 generation 0.40 31%F-P 22 F: ferrite, P: pearlite

As can be seen from Tables 1 and 2, in the case of Examples 1 to 4, which satisfy the alloy composition and manufacturing conditions proposed by the present invention, not only satisfy the microstructure of the present invention, but also have excellent mechanical properties. It can be confirmed that, in particular, the yield point elongation occurred up to 1% or more.

In Comparative Examples 1 to 3, VC precipitates were not formed due to the non-addition of V, and it can be seen that, due to this, the strength desired by the present invention was not secured. In particular, Comparative Example 1 was less than the C content range of the present invention, and the strength targeted by the present invention could not be secured. It can be seen that he did not

In the case of Comparative Example 4, it was less than the V content range suggested by the present invention, and the strength increasing effect by the VC precipitate was small, so it was not possible to secure the strength targeted by the present invention, and in the case of Comparative Example 5, the present invention It can be confirmed that fracture occurred during the tensile test as the suggested V content range was exceeded.

In the case of Comparative Example 6, as the case where the N content range proposed by the present invention is less, the elongation at the yield point appears, but it can be confirmed that the elongation at the yield point is short. Comparative Example 7 is a case in which the N content range proposed by the present invention is exceeded, and it can be seen that the yield point elongation is also shown, but the yield point elongation length is short.

In the case of Comparative Example 8, the Al content range proposed by the present invention is exceeded, and the elongation at the yield point appears, but it can be confirmed that the elongation at the yield point is short.

3 is a microstructure photograph of Inventive Example 2 observed with an optical microscope. As can be seen from FIG. 3 , in the case of Inventive Example 2 that satisfies the conditions of the present invention, it can be confirmed that ferrite and pearlite of an appropriate fraction are formed.

4 is a stress-elongation graph of Inventive Example 2 and Comparative Example 1 according to an embodiment of the present invention. As can be seen from FIG. 4 , in the case of Inventive Example 2, it can be confirmed that the yield strength, yield strength and elongation were excellent as compared to Comparative Example 1, and the elongation at the yield point occurred.

Claims (6)

In wt%, C: 0.30 to 0.50%, Si: 0.5 to 1.0%, Mn: 0.5 to 1.0%, V: 0.10 to 0.30%, P: 0.03% or less (excluding 0%), S: 0.03% or less ( 0% excluded), Al: 0.005% or less (excluding 0%), N: 0.009~0.020%, the balance consists of Fe and other unavoidable impurities,
The microstructure of the region from D/4 to 3D/4 from the surface contains 30-40 area% of ferrite and the remainder of pearlite,
VC precipitates having an average size of 100 nm or less are included in 40 area% or more of the total VC precipitates,
Steel for seismic structures with an elongation of 20% or more.
(However, D is the diameter of the steel, and the unit is mm.)
delete The method according to claim 1,
The steel is tensile strength: 750 MPa or more, yield strength: 490 MPa or more, and yield ratio: 0.75 or less steel for earthquake-resistant structures.
The method according to claim 1,
The steel material for earthquake-resistant structures in which the yield point elongation occurs up to an elongation of 1.0% or more.
In wt%, C: 0.30 to 0.50%, Si: 0.5 to 1.0%, Mn: 0.5 to 1.0%, V: 0.10 to 0.30%, P: 0.03% or less (excluding 0%), S: 0.03% or less ( 0%), Al: 0.005% or less (excluding 0%), N: 0.009 to 0.020%, the balance Fe and other unavoidable impurities, heating the billet at 1000 ~ 1200 ℃;
obtaining a wire rod by finishing rolling the heated billet at an exit temperature of 950 to 1100°C; and
Comprising the step of cooling the region of 700 ~ 900 ℃ with respect to the wire rod at a cooling rate of 1 ℃ / s or less,
When heating, the heating time is a method of manufacturing a steel material for an earthquake-resistant structure of 90 to 120 minutes.
delete

Images