JP3271508B2 - Manufacturing method of low yield point structural steel sheet - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

TECHNICAL FIELD The present invention relates to a yield strength of 150 N / mm.
^{The present} invention relates to a method for producing a low yield point structural steel sheet having an extremely low yield strength of ² or less and sufficient toughness as a structural steel material. In particular, to ensure the stability of the building structure,
Although it is necessary to have a vibration damping structure that suppresses the response of the building structure to vibration disturbances such as earthquakes and winds, the present invention has a wide range of applications as a structural steel with high vibration absorption capacity, and is extremely advantageous in industry. The present invention relates to a method for manufacturing a low yield point structural steel sheet.

[0002]

2. Description of the Related Art Conventionally, in order to improve the safety of buildings and bridges, a material having a large energy absorption amount in a structural member during an earthquake, that is, a material having a low yield point has been desired. Various proposals have been made.

(1) The addition of a strengthening element is suppressed, and 0.5 to 4.0
A method for producing a structural steel having a low yield strength by adding crystal Si by weight to cause abnormal growth of crystal grains during a temperature rise process during a normalizing process after hot rolling (Japanese Patent Application Laid-Open No. 5-320761). Reference).

(2) The addition of a strengthening element is suppressed, and 0.4 to 4.0
Weight percent of Si and 0.10 to 3.0% of Al are added to cause abnormal growth of crystal grains during the normalization process after hot rolling, and to produce structural steel with low yield strength. Method (see JP-A-5-320762).

[0005]

Since the low yield point steels according to the prior art as described above all aim to lower the yield point by coarsening the crystal grains, the grain boundaries are weak and the toughness is slightly inferior. Therefore, there was a problem in application to building structures.

An object of the present invention, without containing Si, a tensile strength of 200 ~280N / mm ^2, yield strength 150 N / mm ²
To provide a method for producing a low yield point structural steel sheet having a small variation in the following range, an elongation of 40% or more, an impact value at 0 ° C of 47 J or more, and an excellent toughness level as a building structure. is there.

[0007]

In order to reduce the yield strength, it is effective to decrease the amount of various additional elements so as to approach a pure iron-based component and to make the crystal grains as coarse as possible.
However, in this case, grain boundary embrittlement occurs at the same time, and toughness deteriorates. This is because the strength of the grain boundary is lower than that inside the grain because the component is low and the crystal grain is coarse, stress concentration occurs at the grain boundary, and cracks are more likely to occur at the grain boundary.

As a result of various studies on the improvement of the toughness, the present inventors have found that the strength difference between the grain boundary and the grain is relatively reduced by reducing the strength inside the grain, and the crack from the grain boundary is reduced. It has been found that generation can be reduced.

That is, Si, which is a solid solution strengthening element in the grains, is not added, and C and N, which are solid solution strengthening elements, are further reduced by fixing as carbonitrides by adding a trace amount of Ti element. Strength can be reduced. As a result, the difference in strength between the grains and the grain boundaries is eliminated, and the generation of cracks from the grain boundaries due to stress concentration enables high toughness. Further, by adding Nb or B, the fixation of C and N can be further strengthened, and the above characteristics can be further enhanced.

In this way, the present inventors have added a trace amount of Ti or further a small amount of Nb or B to a pure iron-based material, performed appropriate finish rolling, performed hot rolling, and then performed a roughening heat treatment at an appropriate temperature. , The yield strength was adjusted to a predetermined low range, and it was found that excellent toughness was obtained, and the present invention was completed.

[0011] Here, the present invention relates to the following:
050% or less, Si: 0.02% or less, Mn: 0.01
0.30%, Al: 0.005 to 0.050%, N:
A slab having a steel composition containing 0.005% or less and Ti: 0.005 to 0.080%, the balance being unavoidable impurities is hot-rolled, and finish-rolled in a temperature range of 650 to 830 ° C. A method for producing a low yield point structural steel sheet, comprising performing normalizing treatment in a temperature range of 830 to 900 ° C. after performing. In another embodiment of the present invention, the steel composition further comprises Nb: 0.005 to 0.030%, B:
It may contain one or both of 0.0003 to 0.0030%.

[0012]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The reasons for limiting the chemical components in the present invention are as follows. Hereinafter, “%” representing the component content indicates “% by weight”.

C: 0.0050% or less C is an element for increasing the yield strength by strengthening the solid solution and fixing the dislocations, and it is necessary to suppress as much as possible to deteriorate the toughness. Therefore, the upper limit is 0.0050%. Preferably, 0.00
3 % or less.

Si: not more than 0.02% Si is an element that increases the strength of steel by solid solution strengthening, but does not need to be added within the strength range required in the present invention because it deteriorates toughness. That is, it may be substantially 0%. However, keeping the amount low increases the cost, so the upper limit was set to 0.02%. Preferably, it is 0.015% or less.

Mn: 0.01 to 0.30% Mn is an element for improving strength and toughness. C and Si
In order to obtain a tensile strength of 200 N / mm ² or more only by containing Mn without containing Mn, 0.01% or more is required. But 0.30
%, The yield strength exceeds 150 N / mm ² . Therefore,
The Mn content was set to 0.01 to 0.30%. Preferably, 0.05-0.
15%.

Al: 0.005 to 0.050% Al is an element necessary for deoxidation at the time of smelting. Since deoxidation by Si is not performed, if less than 0.005%, the deoxidizing effect is small.
If it exceeds 50%, the cost will increase. Preferably, 0.0
07 to 0.015.

N: not more than 0.005% N is an element that increases the yield strength by solid solution strengthening and the fixation of dislocations, and it is preferable that N is as small as possible because it deteriorates toughness. However, lowering the N leads to an increase in cost, so the allowable upper limit was made 0.005%. Preferably, 0.
003% or less.

Ti: 0.005 to 0.080% Ti forms carbonitrides and reduces solid solution C and N, thereby lowering intragranular strength and improving toughness. 0.005%
If less than 0.080%, the effect cannot be obtained because the amount of dissolved C and N is not sufficient, and if more than 0.080%, the amount of precipitates is very large and the precipitates are coarsened, and conversely, the toughness is deteriorated. Therefore, the Ti content is between 0.005 and 0.
080%. Preferably, it is 0.010-0.05%.

The basic components of the steel of the present invention are as described above, thereby sufficiently achieving the object. In a preferred embodiment, the toughness is improved by selectively adding Nb and B described below. More favorable results are obtained for the improvement.

Nb: 0.005 to 0.030% Nb, like Ti, forms carbonitrides and reduces solid solution C and N, thereby lowering intragranular strength and improving toughness. If it is less than 0.005%, the solute C and N are not sufficiently reduced, and the desired effect cannot be obtained. On the other hand, if it exceeds 0.030%, the amount of precipitates is very large, and the toughness is deteriorated due to coarsening. Therefore, the Nb content was made 0.005 to 0.030%. Preferably, 0.
010 to 0.020%.

B: 0.0003% to 0.0030% B forms nitride and reduces solid solution N, thereby lowering intragranular strength and improving toughness. If it is less than 0.0003%, the decrease in solid solution N is not sufficient, and the desired effect cannot be obtained. On the other hand, if it exceeds 0.0030%, the amount of precipitates is very large, and the precipitates become coarse to deteriorate the toughness. Therefore, the B content is 0.0003-0.
0030%. Preferably, it is 0.0005 to 0.0015%.

Next, the reason for defining the manufacturing method will be described. Hot finishing rolling temperature: 650 to 830 ° C The heating temperature to be performed prior to hot rolling is not particularly limited, and heating is generally performed at 1000 to 1300 ° C. Next, hot finish rolling is performed in the ferrite region, an appropriate amount of strain is introduced into the material, and this is normalized, heat-treated, and recrystallized, thereby obtaining a steel sheet with low yield strength and less variation. .

When the rolling finishing temperature is lower than 650 ° C., too much distortion is introduced, and when the temperature is higher than 830 ° C., the distortion introduced is small, and both are unfavorable because the yield strength increases. Preferably, it is 670-830 ° C.

Normalizing temperature: 830 to 900 ° C. Normalizing treatment for a strain-introduced material (normalized)
ing), the recrystallized grains are coarsened together with the recrystallization. The degree of coarsening at this time is 100 μ
m is sufficient. However, the normalizing temperature is 830
If the temperature is lower than 900C, the effect is not obtained. In the austenite region higher than 900 ° C, the grains are rather refined by transformation, and the yield strength is undesirably high. The heating time is not particularly limited, but generally, about 1 hour per 25.4 mm of plate thickness is sufficient.

[0025]

EXAMPLES Steel having the components shown in Table 1 was melted in a converter to form a continuous cast slab, heated to 1000 to 1250 ° C., then hot-rolled, and at the hot finishing temperature shown in Table 2, Hot rolling was performed to the thickness shown in Table 2.

The obtained hot-rolled steel sheet was subjected to a normalizing treatment at a heat treatment temperature shown in Table 2, air-cooled, and then, from the center of the heat-treated material, a tensile test piece (JIS No. 5 test piece), a 2 mm V notch Charpy impact test piece ( JIS No. 4 test piece) was taken, and the tensile strength, the yield strength, the elongation, and the absorbed energy at 0 ° C. were determined. Table 2 summarizes the results.

[0027]

[Table 1]

[0028]

[Table 2]

[0029]

As has been described in the foregoing, according to the present invention, the tensile strength of 200 ~280N / mm ^2, yield strength 150 N / mm ²
In the following range, the characteristics are small, the elongation is 40% or more, and the impact value at 0 ° C. is 47 J or more.
The industrial advantage of the present invention is particularly remarkable today, since a steel plate for a low yield point structure having excellent vibration damping properties against vibration disturbances such as earthquakes and winds is obtained, and there is great expectation for a vibration damping structure building.

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int. Cl. ⁷ , DB name) C21D 8/00-8/10 C22C 38/00-38/60

Claims (2)

(57) [Claims] C .: 0.0050% or less, Si: 0.02% or less, M
n: 0.01 to 0.30%, Al: 0.005 to 0.050%, N: 0.005% or less, Ti: 0.005 to 0.080%, balance of steel composition consisting of unavoidable impurities A low yield point structural steel sheet, which is obtained by hot rolling a slab having the following characteristics, performing finish rolling in a temperature range of 650 to 830 ° C, and then normalizing in a temperature range of 830 to 900 ° C. Manufacturing method. 2. The steel composition further comprises Nb: 0.005 to 0.03.
The method for producing a low yield point structural steel sheet according to claim 1, comprising one or both of 0% and B: 0.0003 to 0.0030%.