JPH10102195A - Heat treated high tensile strength steel excellent in resistance to hot dip galvanizing crack, and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a heat treated high tensile strength steel, excellent in weldability, having a strength level regulated to 70-80kgf class by means of tempering, and also having superior resistance to hot dip galvanizing crack, and its production. SOLUTION: The heat treated high tensile strength steel, excellent in resistance to hot dip galvanizing crack, has a composition which contains, by weight, 0.08-0.12% C, <=0.25% Si, 0.8-1.6% Mn, 0.008-0.05% Nb, 0.02-0.1% V, 0.008-0.015% Ti, (0.0005+Ti/3.4) to 0.01% N, and <0.00016% B and in which the value of Z=Ceq+600×B, defined by Ceq=C+Mn/6+Si/24+Ni/40+Cr/5+Mo/4+V/14 and B content, is regulated to <=0.40.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a tempered high-strength steel excellent in hot-dip galvanizing crack resistance of a weld portion and a base material and having a strength of 70 to 80 kg or more and a method for producing the same.

[0002]

2. Description of the Related Art When galvanizing steel materials used for power transmission towers and bridges in order to impart weather resistance and corrosion resistance, cracks due to so-called molten metal embrittlement may occur. This phenomenon is called galvanization crack and greatly impairs the safety of the structure. From the findings so far, it has been found that galvanizing cracks occur in areas where tensile stress is present when immersed in a hot-dip galvanizing bath. Parts, holes for welding high strength bolts and welded joints. In particular, a welded joint is liable to cause plating cracks in the material in addition to thermal strain due to a welding heat cycle, and this problem is serious. Therefore, when there is a concern about galvanized cracks in the welded portion, a structure is manufactured by high-strength bolt bonding. As described above, galvanized cracks in the bolted portion are raised as the next concern.

[0003] In order to prevent these problems, many studies have been made on improving the hot-dip galvanized crack resistance of the heat affected zone and the base metal, and a new HT60 for steel materials for steel towers has been studied.
Although the basic solution was shown before the development committee was formed, the applied strength level of the current technology is only 60 kg.

On the other hand, large power transmission towers, which are installed mainly in mountainous areas, require enormous costs for transporting materials, so that even higher strength is required to reduce the weight. It has been difficult to prevent galvanizing cracks even at the drilled portion for force bolting, and it has been extremely difficult to prevent galvanizing cracking at the welded joint.

[0005]

SUMMARY OF THE INVENTION Under such circumstances,
Japanese Patent Publication No. 7-57883 discloses a technique which combines a direct quenching process and tempering at the time of galvanizing for 80 kg high strength steel. However, in this technique, the weldability is not good as easily inferred from the content of C reaching a maximum of 0.2%, and preheating is required to prevent low-temperature cracking during welding. In addition, since it is used for processing of the member in the as-quenched state where the strength exceeds 100 kg, in addition to problems such as low-temperature cracking due to gas cutting and difficulty in processing, residual stress introduced into the steel sheet during quenching There is a concern that galvanized cracks may occur at the bolt hole processing part.

The present invention has been made in view of the above, and has excellent weldability, and is subjected to tempering to remove residual stress at the time of quenching to thereby prevent galvanized cracks in a base material as in a bolted portion. It is an object of the present invention to provide a tempered steel material of 70-80 kg class which prevents the stress concentration portion and is excellent in hot-dip galvanizing cracking of a welded portion, and a method of manufacturing the same.

[0007]

Means for Solving the Problems Conventional tempered 80 kg class steel is made of Ceq = C + Mn / 6 + Si / 24 + Ni / 40 + Cr in order to secure base metal strength.
/ 50 Mo / 4 + V / 14 is provided in a high alloy component having a carbon equivalent of approximately 0.45% or more. As a result, Pcm = C +
Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + C
The weld crack susceptibility index defined by r / 20 + Mo / 15 + V / 10 + 5B exceeds 0.22%, and usually requires preheating of at least 50 ° C. or more during welding. Further, since the microstructure of the weld heat affected zone becomes a burnt-off structure having high susceptibility to galvanization cracking, it is considered impossible to galvanize a welded structure.

The means for improving the weldability is Pcm = C + S
i / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr
Pc defined by / 20 + Mo / 15 + V / 10 + 5B
This can be achieved by reducing m. However, reducing C in order to reduce the Pcm requires further increasing the Mn. In order to secure the base metal strength of an 80 kg class steel. Cu. Ni or M
It is necessary to add alloying elements such as o, which not only raises the cost but also acts in the direction of quenching the microstructure of the heat-affected zone of the weld, and tends to cause galvanization cracking.

In order to ensure the hot-dip galvanizing cracking resistance of the base material, the base material should have a homogeneous structure such as bainite instead of a heterogeneous structure such as ferrite-pearlite structure. is important. In the tempered 80 kg steel material to which the present invention is directed, uniform microstructure can be inevitably achieved from the viewpoint of securing strength. However, this alone is not sufficient, and as described above, if the strain (residual stress) introduced during quenching is concentrated in the bolted portion, galvanizing cracks may occur even in the base material.

[0010] From the above, it can be said that the challenge is how to balance the performance of a weld heat-affected zone having resistance to hot-dip galvanizing cracking and the strength as a 70-80 kg class steel while ensuring excellent weldability.

According to the present invention, a 70-80 kg class steel material having a low Ceq value and a low Pcm value and having excellent hot-dip galvanized cracking resistance in a weld heat-affected zone by making full use of the following knowledge is provided. Have been found to be available.

(1) Assuming that the direct quenching and tempering process is applied, the softening due to tempering can be drastically suppressed by adding Nb and V in combination.

(2) As a result, even a tempered steel material having a lower Ceq value than the conventional one can secure a strength of 70 to 80 kg class. However, for that purpose, the cooling rate at the time of direct quenching must be at least 25 ° C./sec at least at the center of the wall thickness.

(3) By setting the tempering temperature of the steel material according to (1) and (2) to 500 ° C. or higher, it is possible to remove residual stress in order to prevent galvanized cracks from occurring in bolted portions.

(4) As a means for securing weldability, it is necessary to avoid high alloying due to low C. Rather, it is necessary to add C to a low alloying range as long as the weldability is ensured to reduce the welding heat. Was found to be effective in ensuring the galvanized steel cracking susceptibility of the part. Specifically, Mn, Cu, Ni, Cr,
The upper limit of the addition of alloying elements such as Mo was found from the viewpoint of ensuring hot-dip galvanizing crack resistance of the heat affected zone.

(5) By actively adding Ti, the resistance to hot-dip galvanizing cracking of the heat affected zone can be improved. Also,
Addition of Ca or REM as needed can further improve the hot-dip galvanized crack resistance of the heat affected zone. However, in order to secure N necessary for the production of these carbonitrides for the purpose of adding Nb and V in a complex manner, N must be intentionally added.

That is, the present invention relates to (1) by weight of C:
0.08 to 0.12%, Si: 0.25% or less, Mn:
0.8 to 1.6%, Nb: 0.008 to 0.05%,
V: 0.02-0.1%, Ti: 0.008-0.01
At 5%, N: (0.0005 + Ti / 3.4) or more.
01% or less, B: contains less than 0.00016%, Ceq
= C + Mn / 6 + Si / 24 + Ni / 40 + Cr / 5 tens Mo / 4 + V / 14 and B content, Z = Ce
The tempered high-strength steel excellent in hot-dip galvanized cracking resistance with a value of q + 600 × B of 0.40 or less, (2) steel,
Further, Cu: 0.5% or less, Ni: 0.5% or less, C
r: 0.5% or less, Mo: 0.05% or less, Ca: 0.
004% or less, Zr: 0.001% or less, REM: 0.
A tempered high-strength steel excellent in hot-dip galvanizing crack resistance according to (1), containing 001% or less of -001% or more, (3), C: 0.08 to 0.12% by weight%, Si: 0.25% or less, Mn: 0.8-1.6%, Nb: 0.008-0.
05%, V: 0.02-0.1%, Ti: 0.008-
At 0.015%, N: (0.0005 + Ti / 3.4)
Not less than 0.01% and B: less than 0.00016%, Ceq = C + Mn / 6 + Si / 24 + Ni / 40 +
A steel slab having a value of Z = Ceq + 600 × B of 0.40 or less, defined by Cr / 50 Mo / 4 + V / 14 and B content, is heated to 1000 to 1250 ° C.
From the temperature of 3 transformation temperature or more, the center of the sheet thickness is cooled to room temperature at a cooling rate of 25 ° C./sec or more, and then 500 ° C. or more
Method for producing tempered high-strength steel excellent in hot-dip galvanizing crack resistance, characterized by performing tempering at a temperature equal to or lower than the cl transformation temperature. (4) Steel: Cu: 0.5% or less, Ni : 0.5
% Or less, Cr: 0.5% or less, Mo: 0.05% or less,
Ca: 0.004% or less, Zr: 0.001% or less, R
EM: The method for producing a tempered high-strength steel excellent in hot-dip galvanizing cracking resistance according to (3), which contains at least 0.001% or less of types.

[0018]

BEST MODE FOR CARRYING OUT THE INVENTION

C is added in an amount of 0.08% or more to ensure the strength of the base material without relying on the addition of a large amount of alloying elements. However, when added in an amount of 0.12% or more, the weldability and the galvanized crack resistance of the heat affected zone are impaired.

Si Si is added because it contributes to deoxidation during smelting and also has the function of increasing the strength of the base material. If it exceeds 0.25%, so-called galling will easily occur due to zinc plating, so the upper limit is made 0.25%, preferably 0.2%.

Mn Mn is added in an amount of 0.8% or more to ensure the strength of the base material. However, the addition of 1.6% or more impairs the hot-dip galvanized crack resistance and weldability of the heat affected zone, so the upper limit is set to 1.
6%, preferably 1.5%.

Nb, V Nb, V is added to precipitate these carbonitrides during tempering and to suppress softening. In the present invention in which a steel material with low Ceq is directly quenched at a high cooling rate after hot rolling, the effect of suppressing temper softening is extremely important.
Nb: 0.008% or more and V: 0.02% or more must be added in combination. However, since excessive addition deteriorates the toughness of the base metal and the welded joint and also tends to impair the weldability, Nb: 0.05% or less, preferably 0.035% or less, and V: 0.1% Less than 0.0
The compound is added in a range of 6% or less.

Incidentally, the precipitation hardening action of Nb is brought about by Nb dissolved in the heating stage during hot rolling. The amount of Nb capable of forming a solid solution derived from the heating temperature and the upper limits of the C and N contents employed in the present invention is approximately 0.05%. On the other hand, the solid solution temperature of the carbonitride of V is lower than that of Nb, and even if it is added beyond the above range, the entire amount is dissolved. But V
The contribution by precipitation hardening is small at 0.06% or more and leads to an increase in cost. That is, it is necessary to add Nb and V in combination as a method of effectively suppressing softening during tempering by minimizing adverse effects on weldability and toughness of the base material and the welded joint.

Ti Since Ti forms nitrides with N and reduces the hardenability of the heat affected zone by welding, thereby improving the hot-dip galvanizing cracking resistance, 0.008% or more is added. However, the upper limit is set to 0.015% because excessive Nb and V act in the direction of decreasing the necessary N when forming carbonitrides and degrade the toughness of the base material and the welded portion.

Although NN is contained as an impurity, it is an indispensable element for producing Nb, V and carbonitride. In addition, it produces Al and nitrides and contributes to grain refinement. Furthermore, since Ti and nitride are formed and the hardenability of the heat affected zone is reduced to improve the hot-dip galvanizing cracking resistance, 0.0005
+ Ti / 3.4 or more, preferably 0.001 + Ti / 3.
4 or more. However, since excessive N reduces the toughness of the base metal and the welded portion, the upper limit is limited to 0.01%, preferably 0.008%.

BB has an upper limit of 0.00016% as an impurity because it significantly deteriorates the hot-dip galvanized crack resistance of the heat affected zone.
Regulate to less than.

Cu, Ni, Cr, Mo Cu, Ni, Cr, Mo are supplementarily added to ensure the hardenability of the base material. However, in order to impair the hot-dip galvanized cracking resistance of the heat affected zone, the upper limits of Cu, Ni, and Cr are set to 0.5%, and Mo is set to 0.05%.

Ca, Zr, REM Ca, Zr, REM is obtained by adding Ti in combination with Ti.
It is added as necessary because it forms a systemic composite precipitate, reduces the hardenability of the heat affected zone and improves the hot-dip galvanizing cracking resistance. However, excessive addition lowers the toughness of the base metal and the welded joint, so the upper limit of Ca is 0.00
4%, preferably 0.003%, and 0.001% for Zr and REM.

P, SP, and S are all impurity elements. In order to obtain a sound base metal and a welded joint, it is desirable that both are regulated to 0.015% or less, preferably 0.01% or less.

Al Al is added for deoxidizing steel, and usually contains 0.005% or more. Also, about 0.01% is added in order to secure the base material toughness due to the refinement of the microstructure. However, excessive addition may cause scratches generated during rusting, so that the content is limited to 0.05% or less.

Z is an index indicating the hot-dip galvanizing resistance of the heat affected zone defined by ZZ = Ceq + 600 × B.

(However, Ceq = C + Mn / 6 + Si /
24 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14) By regulating the Z value to 0.40% or less, the zinc heat-cracked zinc-resistant test of the heat-affected zone of a 60 kg class steel (iron and steel Vol. .79 (1993), No. 9), the SLM-400 value obtained at about 50% or more can be secured.

The SLM-400 value required for a 60-kilometer steel for power transmission towers is 42% or more. However, since the strength level targeted by the present invention is as high as 70 to 80 kilometers, the residual strength of the welded portion is high. The stress can be higher than for 60 kilo-grade steel. Therefore, in the present invention, the SLM-400 value required for the hot-dip galvanized crack resistance of the heat affected zone is set to 50% or more.

Manufacturing Conditions The heating temperature is set to 1000 ° C. or higher to promote solid solution of Nb. However, extreme high-temperature heating causes a coarsening of the crystal structure and impairs the toughness of the base material, so the upper limit is set to 1250 ° C.

The hot rolling may be performed by a usual method. However, in order to further improve the toughness and hot-dip galvanizing crack resistance of the base material, it is preferable to include at least one pass or a plurality of passes having a rolling reduction of 10% or more.

After the completion of rolling, quenching is performed at a temperature not lower than the Ar3 transformation temperature. The cooling rate at the time of quenching is determined by Z
In order to secure a strength of 70 to 80 kg class after tempering with a steel material satisfying the above value, it must be at least 25 ° C./sec or more at the center of the wall thickness.

Tempering is performed to remove residual stress during quenching. 500 ° C or higher to prevent material change by hot-dip galvanizing heated to about 470 ° C
It is below the cl transformation point.

When the finished plate thickness is 25 mm or less under the above manufacturing conditions, the steel of the present invention exhibits a strength of 70 to 80 kg class and excellent hot-dip galvanizing crack resistance in the base metal and the welded portion.

[0038]

EXAMPLES Steel having the chemical composition shown in Table 1 was melted into steel ingots.
A steel sheet was manufactured under the conditions shown in Table 2. A predetermined test piece was sampled from each steel plate, and the strength and toughness of the base metal and S
LM measurement tensile test (iron and steel VOL.79 (199
3), No. According to 9), SLM-400 which is an index of hot-dip galvanizing crack resistance of the heat affected zone was determined. Also,
The y-type welding crack test based on JISZ3158
Using a SMAW ultra-low hydrogen type welding rod for 0 kg steel, ambient temperature 20 ° C, atmospheric humidity 60%, preheating 25 ° C
And weldability was evaluated. In addition, the steel plate was subjected to bolt hole processing and actually hot-dip galvanized, and the presence or absence of cracks was investigated.

Example No. 1 and 2 are a comparative example using steel No. A (Nb and V are smaller than the range of the present invention) and a steel No. B present invention. In each case, the Pcm value was as low as 0.17%, and the weldability was good. Also, the Z value (= Ceq + 6)
00 × B) is 0.40 or less, and the measured value of SLM-400 showing the hot-dip galvanized crack resistance of the heat affected zone is good. However, in Example No. 1 (comparison method) is N
b. Since V was not added in the range of the present invention, the plate thickness was 15 m.
When directly quenching a steel sheet of m, the strength as a 70-80 kilo class steel cannot be obtained despite the cooling rate at the center of the sheet thickness reaching about 50 ° C./sec.

Example No. No. 3 is an example using steel number C. No. 4 is a comparative example using steel number C. Example No. 1 manufactured by the reheating quenching / tempering process. In the case of No. 4, the intended strength of 80 kg class steel cannot be obtained.

Example No. 5 is a steel plate D having a thickness of 25 mm
In the example of the present invention in which 6 is a comparative example in which a steel sheet E having a thickness of 25 mm was manufactured (Z value exceeds the range of the present invention). No. 6 deviates from the chemical composition of the present invention and has a low C
The Pcm value is as low as 0.17%, which is within the range of the present invention, in order to secure the weldability by the formation of the alloy.
It is over 40%. For this reason, the hot-dip galvanizing crack resistance of the base material is good, but the measured value of SLM-400 is 39%.
And low.

Example No. 7 is an example of the present invention using a steel sheet F. Base metal properties, weldability, hot-dip galvanizing cracking resistance of base metal and hot-dip galvanizing cracking resistance of heat affected zone (SLM
(-400 actual measurement) Good performance was confirmed as an excellent 70-80 kg class steel in each case.

Example No. 8 is an example of the present invention based on steel number G in which Ca is added to steel number C. The base material properties, weldability and hot-dip galvanizing crack resistance of the base material are good, and SLM-40
0 measured values are the values of Example No. Good hot-dip galvanizing cracking resistance exceeding 3 was confirmed.

Example No. 9.10 uses steel number C,
In the case where tempering was omitted and the case where low-temperature tempering was performed, cracks occurred in the galvanizing steel cracking test.

Example No. 11 is tempered at a higher temperature and is 70 kg class steel.

In the above embodiment, the cooling rate at the time of direct quenching is about 50 ° C./sec when the sheet thickness is 15 mm and about 40 ° C./sec when the sheet thickness is 20 mm as a result of using water as a cooling medium. Second, the thickness is about 30 ° C./sec when the plate thickness is 25 mm, and both satisfy the requirement of the present invention of 25 ° C./sec or more.

[0047]

[Table 1]

[0048]

[Table 2]

[0049]

According to the present invention, Nb and V are added in a combined manner to suppress softening due to tempering, and to ensure a cooling rate of at least 25 ° C./sec at the center of the wall thickness at the time of direct quenching. ,
As a result, even a tempered steel having a lower Ceq value than the
A strength of up to 80 kg can be secured. As a result, it is possible to provide a tempered high-strength steel excellent in hot-dip galvanizing crack resistance and having a strength of 70 to 80 kg or more, and a method for producing the same.

Claims (4)

[Claims] (1) C: 0.08 to 0.12% by weight
Si: 0.25% or less, Mn: 0.8 to 1.6%, N
b: 0.008 to 0.05%, V: 0.02 to 0.1
%, Ti: 0.008 to 0.015%, and N: (0.0
005 + Ti / 3.4) or more and 0.01% or less, B: 0.
Ceq = C + Mn / 6 + Si / 24 + Ni / 40 + Cr
/ Ten Mo / 4 + V / 14 and B content, wherein Z = Ceq + 600 × B is 0.40 or less. Tempered high tensile strength excellent in hot-dip galvanizing crack resistance. steel. 2. The steel further comprises Cu: 0.5% or less,
i: 0.5% or less, Cr: 0.5% or less, Mo: 0.0
5% or less, Ca: 0.004% or less, Zr: 0.001
The tempered high-strength steel excellent in hot-dip galvanizing crack resistance according to claim 1, wherein the steel contains at least 1% or less and REM: 0.001% or less. 3. C: 0.08 to 0.12% by weight,
Si: 0.25% or less, Mn: 0.8 to 1.6%, N
b: 0.008 to 0.05%, V: 0.02 to 0.1
%, Ti: 0.008 to 0.015%, and N: (0.0
005 + Ti / 3.4) or more and 0.01% or less, B: 0.
Ceq = C + Mn / 6 + Si / 24 + Ni / 40 + Cr
A steel slab having a value of Z = Ceq + 600 × B of 0.40 or less is heated to 1000 to 1250 ° C., and is hot-rolled to an Ar3 transformation temperature or higher. From the temperature, the central part of the sheet thickness is cooled to a normal temperature at a cooling rate of 25 ° C./sec or more, and then is cooled to 500 ° C.
A method for producing a tempered high-strength steel excellent in hot-dip galvanizing crack resistance, characterized by performing tempering at a temperature equal to or lower than a transformation temperature. 4. The steel further comprises Cu: 0.5% or less;
i: 0.5% or less, Cr: 0.5% or less, Mo: 0.0
5% or less, Ca: 0.004% or less, Zr: 0.001
The method for producing a tempered high-strength steel excellent in hot-dip galvanizing crack resistance according to claim 3, wherein the steel contains at least one kind of REM: 0.001% or less.