JPH08246094A - Building steel for low temperature use - Google Patents

Abstract

PURPOSE: To produce a building steel having stable low temp. toughness and low yield ratio at a low temp. and satisfying low temp. low yield ratio capable of new earthquake-resistant design. CONSTITUTION: This building steel for low temp. use is the one composed of an iron-base alloy having <=30ppm oxygen content, having a two phase structure of coarse-grained ferrite and bainite, having a compsn. contg., by weight, 0.04 to 0.18% C, 0.05 to O.40% Si, 0.6 to 1.7% Mn, 0.001 to 0.06% Al, <=30ppm N and <=30ppm O, furthermore contg. one or >= two kinds among 0.005 to 0.015% Ti, 0.005 to 0.04% Nb, 0.005 to 0.1%, V, 0.05 to 0.6% Cu, 0.05 to 0.6% Ni, 0.05 to 1.0% Cr and <=0.002% S, in which the content of P is regulated to 0.015% and S to <=0.002%, added with either of Ca by 0.5 to 2.0 in terms of Ca/S and 0-005 to 0.02% rare earth metals, and the balance Fe, low in HAZ crack sensitivity in ultrahigh heat input welding, and is excellent in low temp. toughness and satisfyies low yield ratio at a low temp.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a low-temperature building steel material used in a building such as a low-temperature warehouse where the environment temperature is not higher than room temperature in the building field designed by the new seismic design method.

[0002]

2. Description of the Related Art The seismic design method for buildings, which was revised in 1982, was changed to an elastic design in which the stress level in each part of the structure up to that point was kept within the yield point of the steel material, and after the steel material yielded. By utilizing the deformation capacity in the plastic range until the maximum strength is reached, the earthquake input energy is absorbed and the seismic safety of the building is ensured. From this, the steel materials for buildings to which the new seismic design method is applied are the yield ratio (YR value), which is a parameter expressing the deformation performance after yielding.
Low, that is, a low yield ratio is required.

A steel material of TS500 MPa class is subjected to hot rolling in a recrystallization region to coarsen the structure and secure a low yield ratio. Further, in the high-strength steel of TS600 MPa class or higher, by quenching from the two-phase region of ferrite-austenite, a low yield ratio is secured by forming a two-phase structure of ferrite and bainite or martensite.

Since buildings for offices and houses, so-called buildings, are used at room temperature, the above-mentioned new seismic design is also premised on room temperature. Therefore, the conventional low yield ratio steel also has a YR value of 80% or less or 75% at room temperature (0 to 30 ° C).
Manufactured as follows.

Among buildings, there is a building whose operating temperature is low (-20 ° C to -60 ° C), such as a low temperature warehouse. For example, a refrigerated warehouse for tuna is used at -55 ° C. In order to apply the new designing method to such buildings for low temperatures and to secure seismic safety, steel materials with low yield ratio at low temperature are required. However, although the conventional low yield ratio steel is premised on use at room temperature, the yield ratio at room temperature is shown, but the yield ratio at low temperature is not clear.

[0006]

Therefore, the present inventors have examined the tensile properties and toughness of conventional low yield ratio steels at low temperatures. Many low-yield ratio steels are coarse-grained in order to obtain a low-yield ratio as described above, and therefore have low low-temperature toughness,
For example, it was found that it cannot be used in a low temperature warehouse that uses -55 ° C. Japanese Patent Laid-Open No. 2-197522 and Japanese Patent Laid-Open No. 5-214040 have been reported as conventional techniques relating to a low yield ratio steel having excellent low temperature toughness. Although the inventions described in both publications show data applicable to a low temperature warehouse using −55 ° C. regarding toughness, YR at a low temperature is shown.
Values not shown. Therefore, when examining the low temperature tensile properties of the steel prototyped according to the invention described in both publications,
For example, it was found that the YR value was 80% or more at -55 ° C. Also, regarding toughness, not only a good value but also a large variation.

From the above, the problem to be solved by the present invention is that it has stable low temperature toughness and has low temperature (-20
The present invention provides a low-temperature low-yield ratio building steel material that enables a new seismic design exhibiting a low YR value (≤80%) at (C to -60C).

[0008]

In order to solve this problem, the present invention is an iron-based alloy having an oxygen content of 30 ppm or less, has a two-phase structure of coarse-grained ferrite and bainite, and has a low yield ratio at low temperature. It is a low temperature building steel material. Furthermore, in wt%,
C: 0.04-0.18%, Si: 0.05-0.40
%, Mn: 0.6-1.7%, Al: 0.001-0.
06%, N: ≤ 30 ppm, O: ≤ 30 ppm, the balance being Fe and unavoidable impurities, which is an excellent low temperature toughness and a low yielding ratio low temperature building steel material at low temperature. Furthermore, in addition to the above components, Ti: 0.005-0.015%, N
b: 0.005-0.04%, V: 0.005-0.1
%, Cu: 0.05-0.6%, Ni: 0.05-0.
6%, Cr: 0.05-1.0%, Mo: 0.02-
It is a building steel material for low temperature, which is excellent in low temperature toughness and has a low yield ratio at low temperature by adding one or more of 0.6%. Furthermore,
In addition to the above components, P: ≤ 0.015%, S: ≤ 0.
Restricted to 002%, Ca: Ca / S 0.5 or more 2.0
Below, REM (rare earth element): 0.005-0.02%
It is a building steel material for low temperature use, which is excellent in low temperature toughness with low HAZ crack susceptibility of ultra-high heat input welding and which has low yield ratio at low temperature.

[0009]

In the low temperature building steel material of the present invention, the metal structure of the iron-based alloy is limited to the two-phase structure of coarse-grained ferrite and bainite. The reason for this is that the two-phase structure of ferrite and bainite has a smaller increase in the YR value at low temperatures than the two-phase structure of ferrite and pearlite, and in particular coarse-grained ferrite has a greater tendency than fine-grained ferrite. Is. Here, the coarse-grained ferrite is ASTM grain size No. 11 or less.

In the low temperature building steel material of the present invention, the oxygen content of the iron-based alloy is set to 30 ppm or less. The reason for this is
This is because stable low temperature toughness can be obtained by limiting the oxygen amount within this range, and when the oxygen content exceeds 30 ppm, the low temperature toughness varies widely.

These findings have been found by the present inventors as a result of extensive studies on the relationship between the microstructure and the low temperature YR value (yield ratio). That is, the present inventors measured the YR value by temperature using the A1 steel plate (coarse grain ferrite + bainite), A2 steel plate (fine grain ferrite + bainite), and A3 steel plate (ferrite + pearlite) shown in Table 2. did. As shown in FIG. 1, the lower the tensile test temperature, the higher the YR value, but the ferrite + bainite structure has a lower increase than the ferrite + pearlite structure. It was also found that in the ferrite + bainite structure, the coarser the ferrite, the lower the YR value obtained at low temperature. The YR value <80% or less is achieved even at −100 ° C. by using a mixed structure of coarse-grained ferrite and bainite.

The inventors of the present invention have also used coarse steel (ASTM grain size No. = 9 to 11) and bainite mixed microstructure by using steel in which only oxygen is changed in the range of 19 to 44 ppm based on A steel. The low temperature toughness was investigated. As a result, as shown in FIG. 2, the low temperature toughness has a considerable variation.
The lower limit is governed by the oxygen content, and vE _-55 (minimum)> 10 by setting the oxygen content to 30 ppm or less.
It was found that stable toughness satisfying 0 J was obtained.
This is because by setting the oxygen content to 30 ppm or less, the oxides in the steel, which are the starting points of microcracks, were reduced and refined.

From the above, it has stable low temperature toughness and has a low YR value (≤8 at low temperature (-20 ° C to -60 ° C).
It was found that the requirement for a low-temperature low-yield ratio building steel material that enables a new seismic design exhibiting 0%) is that the oxygen content is 30 ppm or less and that it has the characteristics of a coarse-grained ferrite and bainite two-phase structure. . Quantitatively speaking about the organization, ASTM grain size No. A two-phase structure of ferrite of 9 to 11 and bainite of 40 to 70% in area ratio is desirable.

Next, in the present invention, C, Si, Mn,
The reason for adding Al and the amount of Al were limited as follows in order to obtain the required properties of the welded structural steel. C is the cheapest element and is an element effective for strengthening, but if it is added in excess of 0.18%, the weldability will be significantly reduced. 0.04%
If it is less than the above range, the strength is insufficient and the alloy is required to be added in a large amount, resulting in a high cost. Therefore, C is 0.04
% To 0.18% or less.

Si is an element necessary for strength of steel materials and preliminary deoxidation of molten steel. For preliminary deoxidation, addition of 0.05% or more is necessary. Excessive addition exceeding 0.4% deteriorates the toughness of the steel material and the weld HAZ toughness. Therefore, Si
The amount was limited to 0.05% or more and 0.4% or less.

Mn is a necessary element for ensuring the strength of the base material. If it is less than 0.6%, the material is thick and the strength is insufficient, so that it is necessary to add a large amount of alloying elements, resulting in high cost. Further, Mn is an element that tends to segregate in the center. 1.7
If added in excess of%, the center of the plate thickness becomes significantly brittle. Therefore, the range of Mn is limited to 0.6% or more and 1.7% or less.

Al is an element necessary for deoxidation. If the Al content is less than 0.001%, a sufficient deoxidizing effect cannot be expected. Also, if added in excess of 0.06%,
Scratches are likely to occur on the surface of continuously cast slabs. Therefore,
The amount of Al was limited to 0.001% or more and 0.06% or less.

N exists as solid solution N or nitride inclusions in the solid steel. Solid solution N and coarse nitride-based inclusions deteriorate the low temperature toughness of steel. When N is contained in excess of 30 ppm, solid solution N exists, and coarse nitrides (for example, TiN and NbN) are easily generated in the final solidified portion, and excellent low temperature toughness cannot be obtained. Therefore, the N content is 0.0
It was limited to 03% or less.

Nb, V, Cu, Ni, Cr and Mo are effective elements for increasing the strength. The reason for limiting the lower limit of the number of missing elements is Nb <0.005%, V <0.005%, Cu
<0.05%, Ni <0.05%, Cr <0.05%,
This is because when Mo <0.02%, no clear strength increasing effect is observed. The reason for limiting the upper limit is as follows.
Nb is Nb (CN), V is VC, and VC contributes to strengthening. However, addition of Nb exceeding 0.04% and addition of V exceeding 0.1% significantly increase the yield ratio. Will end up. Therefore, Nb is limited to 0.005% or more and 0.04% or less, and V is limited to 0.005% or more and 0.1% or less.

Cu, Ni, Cr and Mo contribute to higher strength through solid solution strengthening and hardenability improving effects. 0.6%
Addition of Cu in excess of 10 significantly increases the risk of Cu cracking. Ni is an expensive element, and from the viewpoint of cost,
The upper limit was set to 0.6%. Addition of Cr in excess of 1% and Mo in excess of 0.6% significantly deteriorates weldability. Therefore, Cu is 0.05% to 0.6% and Ni is 0.0%.
5% or more and 0.6% or less, Cr was limited to 0.05% or more and 1% or less, and Mo was limited to 0.02% or more and 0.6% or less.

Ti is an element that suppresses the coarsening of the structure of the welded HAZ portion of TiN and contributes to the improvement of HAZ toughness.
If Ti is added in an amount of less than 0.005%, the HAZ toughness improving effect is not exhibited. If added in excess of 0.015%, TiC precipitates during the cooling process of welding, resulting in deterioration of HAZ toughness.
Therefore, Ti is limited to 0.005% or more and 0.015% or less.

S segregates in the center, and MnS is formed at that portion. Since MnS stretches more than rolling, the stretched MnS is present in the central portion of the thickness of the steel sheet more than other portions. The application of the present application is for buildings, and most of them are used for buildings by assembling into box pillars by high heat input submerged arc welding (SAW). In high heat input submerged arc welding, since a large amount of bond type flux containing iron powder is used, the amount of hydrogen that penetrates into the steel is higher than in other welding methods, and cracks often occur in the heat affected zone. The starting point of cracking is the elongated MnS in the center of the plate thickness. Weld hydrogen accumulates at the interface between the elongated MnS and the base steel, causing hydrogen-induced cracking. When S exceeds 0.002%, MnS in the center of the plate thickness becomes large, and HAZ cracks are likely to occur in the boss-column joint. Therefore, the S content is limited to 0.002% or less.

P is also an element that tends to segregate in the center,
If the content exceeds 0.015%, the central portion of the plate thickness is significantly hardened. The above-mentioned HAZ crack starting from MnS is more likely to propagate as the surrounding hardens.
That is, P is 0.015 in order to suppress hydrogen cracking in the box column corner joint portion which is constructed by high heat input submerged arc welding.
Restricted to below%.

Ca and REM are H of the box column corner joint
It is added to suppress AZ cracking. As mentioned above, HA
The origin of Z cracking is elongated MnS, and cracking can be prevented by suppressing elongation. Ca and REM change sulfides in steel to CaS and REM-S, respectively, and they do not extend even when rolled. Ca / S: less than 0.5, R
If EM is less than 0.005%, sufficient suppression of sulfide elongation is not achieved. In addition, Ca / S: exceeds 2, REM:
Addition of more than 0.02% will cause clustered inclusions (Ca-A
lOS, REM-OS) is increased, which has the opposite effect of suppressing the above HAZ cracks. Therefore, Ca / S is limited to 0.5 or more and 2 or less and REM is limited to 0.005% or more and 0.02% or less.

[0025]

EXAMPLES Examples of the present invention will be described below. Table 1 shows the chemical composition of the test steel. Steel G, H, I, R, S is TS60
It is steel of the kilo class and the other is steel of the TS 50 kilo class. All were slabbed by continuous casting including a light reduction process.

The above steel was used as a steel plate under the manufacturing conditions shown in Table 2. Table 3 also shows the microstructure of the obtained steel sheet. The mixed structure of coarse-grained ferrite and bainite, which is a feature of the present invention, is [low temperature heating] + in TS50 grade steel.
It can be obtained by performing [high-temperature finish rolling] + [controlled cooling in which stronger cooling is performed in a lower temperature range]. In the TS60 kg class, [high temperature heating] + [high temperature finish rolling] + [direct quenching] +
[Two-phase region quenching] + [tempering] or [high temperature heating and quenching]
It is obtained by + [two-phase region quenching] + [tempering].

Table 4 shows the strength, toughness, and large heat input SAW of each steel sheet.
It shows the corner joint HAZ crack resistance. The tensile test piece is JIS No. 4 sampled in the C direction from 1 / 4t.
All the YS values are the values of the upper yield point, and the YR values are all the values of the upper yield point / TS. Charpy impact test piece is 1 /
It was collected in the L direction from 4t. In addition, vE-55 (av
e) and vE-55 (min) are the average value and the minimum value of N number 9.

For high heat input SAW square joint HAZ crack resistance, a half-box construction test is conducted on a specimen having the dimensions and shape shown in FIG. 3, ultrasonic flaw detection is carried out on the welded portion, and the crack occurrence state is measured. It was evaluated by In the figure, 1 is a corner welded portion by submerged arc welding (SAW welding), 2 is a web steel plate, 3 is a flange steel plate, 4 is a diaphragm,
Is an electroslag weld, and 6 is a metal plate for preventing welding leakage, and t is a plate thickness. The SAW angle welding by the half-box construction test is a one-layer welding of two electrodes, and the welding heat input is 150 kJ depending on the steel plate thickness.
(When the plate thickness is 16 mm) to 500 kJ (when the plate thickness is 60 mm). At that time, as the welding flux, a bond type flux containing iron powder was left for 3 hours in an environment of a temperature of 30 ° C. and a humidity of 80% to intentionally absorb moisture. The reason why the hygroscopic flux is used is to increase the amount of hydrogen invading during welding and to clearly evaluate the cracking susceptibility of the steel sheet due to welding hydrogen. After welding, leave it for 3 days, and make the weld flange corners indicated by U, S, and T by arrows in FIG. 3 JIS G
Ultrasonic flaw detection was performed according to 0901, and a crack profile was drawn as shown in FIG. In the figure, the shaded area is the echo generation portion 7 of the Δ defect and the X defect detected by ultrasonic flaw detection, and C ₁ , C ₂ and C ₃ are their lengths, that is,
The length in the weld length direction of the cracked portion is shown. The ratio (length%) of the sum of the crack lengths C ₁ , C ₂ , C ₃ to the weld length L was defined as HAZ crack ratio = (C ₁ + C ₂ + C ₃ + ...) / L. In this test, L = 700 mm
Is.

As shown in Table 4, the steel sheets of the present invention (A1, B1, B1) having a mixed structure of coarse-grained ferrite (ASTM No. 9-11) and bainite (bainite ratio 40-70%) with an oxygen content of 30 ppm or less. C1, D1, E1, F1,
G1, H1, I1, J1, K1, L1, M1, N1)
Has a YR value of 80% even at −55 ° C., and VE-55 (mi
n) has a toughness of 150 J or more. Ferrite +
The A2, B2 and F2 steel sheets, which have a pearlite structure, have vTs
Has a low toughness of -50 ° C or less. Fine-grained ferrite (AS
TM No. A3, C2, E2, G2, H2 steel sheet which is a mixed structure of +11) + bainite has a YR of -55 ° C.
The value exceeds 80%. Bainite single-phase structure E
Also in No. 3, the YR value at −55 ° C. exceeds 80%. Also,
Even if the structure is a mixed structure of coarse-grained ferrite and bainite, O1, P1, Q1, whose oxygen content exceeds 30 ppm
VE-55 (min) of R1, S1, T1 steel plate is 47J
Is below. Furthermore, S ≦ 20 ppm, P ≦ 0.0
The I1, J1, L1 steel plate added with Ca of 0.5% or more and 2.0% or less in 15% or less and the K1 steel plate added with REM of 0.005% or more and 0.02% or less are HA.
No Z cracking has occurred. Ca even if Ca is added
HAZ cracking has occurred in the T1 steel sheet to which N having an A / S of less than 0.5, N having an A / S of more than 2.0, and excessive REM is added.

[0030]

[Table 1]

[0031]

[Table 2]

[0032]

[Table 3]

[0033]

[Table 4]

[0034]

As is clear from the above results, the steel material according to the present invention has stable low temperature toughness and low temperature (-
Since it shows a low YR value (≤80%) at 20 ° C to -60 ° C), it enables new seismic design of building structures used at low temperatures. Therefore, the safety of the building is increased. In addition, the mass production of steel is possible, the price is low, the welding work is easy,
The construction period can be shortened and the construction cost can be reduced as a whole.

[Brief description of drawings]

FIG. 1 is a diagram showing a relationship between a tensile test temperature and a yield ratio (= upper yield point / tensile strength).

FIG. 2 is a graph showing the relationship between oxygen content and Charpy impact absorbed energy (vE-55) tested at −55 ° C.

FIG. 3 is an explanatory view of a shape of a half-box test body by large heat input SAW welding and an ultrasonic flaw detection test position.

FIG. 4 is a diagram for explaining the definition of HAZ crack ratio in a half-box construction test.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... SAW angle welding part, 2 ... Web steel plate, 3 ... Flange steel plate, 4 ... Diaphragm, 5 ... Electroslag diaphragm weld part, 6 ... Pad, 7 ... Delta defect and X defect echo generation part by ultrasonic flaw detection.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Saburo Tani, 1-2 Marunouchi, Chiyoda-ku, Tokyo Japan Steel Pipe Co., Ltd. (72) Hiroshi Ishikawa 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Date Main Steel Pipe Co., Ltd.

Claims (5)

[Claims] 1. A low-temperature building steel material which is an iron-based alloy having an oxygen content of 30 ppm or less, has a two-phase structure of coarse-grained ferrite and bainite, and satisfies a low yield ratio at low temperatures. 2. C: 0.04 to 0.18% by weight,
Si: 0.05 to 0.40%, Mn: 0.6 to 1.7
%, Al: 0.001 to 0.06%, N: ≦ 30 pp
m, O: ≤ 30 ppm, the balance being Fe and unavoidable impurities, which is excellent in low temperature toughness, has a low yield ratio at low temperatures, and is a low temperature building steel material. 3. C: 0.04 to 0.18% by weight,
Si: 0.05 to 0.40%, Mn: 0.6 to 1.7
%, Al: ≤ 0.07%, N ≤ 30 ppm, O: ≤ 30
In addition to ppm, Ti: 0.005 to 0.015%, N
b: 0.005-0.04%, V: 0.005-0.1
%, Cu: 0.05-0.6%, Ni: 0.05-0.
6%, Cr: 0.05 to 1.0%, Mo: 0.02
Of 0.6%, one or two or more, the balance being Fe and unavoidable impurities, which is excellent in low temperature toughness and has a low yield ratio at low temperature, and is a low temperature building steel material. 4. The steel material according to claim 2 or 3, further restricted to P: ≤ 0.015% and S: ≤ 0.002%, and C
a: 0.5 or more and 2.0 or less in Ca / S, REM: 0.0
A low-temperature building steel material to which at least one of 0.05 to 0.02% is added, which has excellent low-temperature toughness with low HAZ crack susceptibility in ultra-high heat input welding and has a low yield ratio at low temperatures. However, REM represents one or more rare earth elements. 5. The steel material according to any one of claims 1 to 4, wherein a yield ratio is 80% or less at a low temperature of -20 ° C to -60 ° C.