JP3577890B2 - High-strength steel excellent in plating crack resistance and method of manufacturing the same - Google Patents

Description

【００３７】
【表２】

【００３８】
【発明の効果】
本発明にかかる高張力鋼は、耐めっき割れ性を損なうことなく、優れた低温靱性を示しており、寒冷地において溶融亜鉛めっきを施して送電鉄塔等の大型構造物に使用する高張力鋼として極めて重要である。また、圧延条件を限定し中心偏析を改善することで、母材靱性、耐めっき割れ性の向上がはかれた。さらに従来以上に合金元素を添加でき、安定製造をも可能となる。
【図面の簡単な説明】
【図１】実施例で耐めっき割れ性の測定に用いた試験片を示すもので、図１（ａ） 、（ｂ） はＳＬＭ４００を求めるのに使用した試験片のそれぞれ全体平面図、円柱切り欠き部の拡大説明図であり、図１（ｃ） はεＺｃ値を求めるのに使用した試験片形状の説明図である。
【図２】実施例でＳＬＭ４００を測定するための切り欠き丸棒引張試験における経過時間と熱応力サイクルとの関係を示すグラフである。
【符号の説明】
１：丸棒
２：円周切り欠き
４：試験片[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to galvanized in a cold district, a large structure, a method of manufacturing a high strength steel and its superior in low temperature toughness and resistance to plating cracking resistance which is suitable for example are used primarily transmission tower for such .
[0002]
[Prior art]
Conventionally, hot-dip galvanizing has been widely used for the purpose of preventing corrosion of steel materials. However, when the welded structure is immersed in a hot dip galvanizing bath, cracks may occur particularly in the weld heat affected zone (hereinafter referred to as HAZ) depending on the composition of the steel material and the manner of immersion in the galvanizing bath.
[0003]
Such cracks are referred to as plating cracks, and are simply referred to as “plating cracks” in this specification . This is thought to be because liquid metal such as zinc enters the grain boundaries of steel and causes embrittlement. For example, it is known that a large residual stress is liable to occur when immersed in a hot-dip galvanizing bath particularly in a state of being applied to HAZ.
For example, in a high-strength steel material for a steel tower limited to JIS G 3129, a calculation formula of a galvanizing crack susceptibility equivalent of a steel sheet is disclosed as follows.
[0004]
(Equivalent to galvanizing crack sensitivity)
= C + Si / 17 + Mn / 7.5 + Cu / 13 + Ni / 17 + Cr / 4.5 + Mo / 3 + V / 1.5 + Nb / 2 + Ti / 4.5 + 420B However, even when the stipulation of the hot-dip galvanizing crack susceptibility specified in JIS G 3129 is satisfied. When the welded structure is immersed in a hot-dip galvanizing bath, plating cracks may occur. Various methods have been devised to prevent such plating cracks, such as a plating immersion method and the shape of a welded structure. However, at present, these measures cannot be said to be sufficient measures.
[0005]
On the other hand, as a new plating crack prevention method, a method of improving the steel material itself has been proposed.
For example, C: 0.04 to 0.11%, Si: 0.05 to 0.30%, Mn: 0.90 to 1.70%, Nb: 0.01 to 0.08%, V: 0. 02-0.09%, Ti: 0.005 to 0.030%, Al: 0.005 to 0.050%,
At the same time, 93-8.8 × 103C (C-0.1) -63Si-38Mn-340V ≧ 42 is satisfied, and a high-strength low-alloy steel composed of the balance Fe and unavoidable impurities (Japanese Patent Publication No. 2-5814), C: 0.20% or less, Si: 0.03 to 0.35%, Mn: 1.8% or less, Al: 0.005 to 0.07%, and 0.0002 of B existing as an impurity. % Or less, the balance consisting of Fe and unavoidable impurities and the combination of these elements being expressed in Ceq. (B) = C + Mn / 10 + Si / 30 + Cr / 10 + Mo / 20 + V / 3 + Nb / 3 + Tl / 3 + 1 / 40000B ≦ 0.19
(Japanese Patent Publication No. Hei 6-86649) has been proposed.
[0006]
The purpose of limiting the steel component in this way is to suppress plating cracking susceptibility and prevent plating cracking. However, for steel produced industrially, since center segregation always exists, cracks may occur from the center segregation part when immersed in a hot dip galvanizing bath due to local increase in alloy concentration. .
[0008]
[Problems to be solved by the invention]
The use environment of high-strength steel materials for steel towers is becoming harsh, such as in the mountains in cold regions, and low-temperature toughness is required in addition to plating crack resistance.
[0009]
Regarding the low-temperature toughness, the high-strength low-alloy steel and high-strength steel disclosed in the above-mentioned Japanese Patent Publication No. 2-5814 and Japanese Patent Publication No. 6-86649 satisfy the plating crack resistance when used for steel towers. However, there is a problem that low-temperature toughness cannot be satisfied.
Further, there is no disclosure of improving the base material's resistance to plating cracking by reducing center segregation.
[0010]
Accordingly, an object of the present invention is to overcome the drawbacks of the prior art and to provide a high strength steel and a manufacturing method thereof that combines beauty low temperature toughness Oyo excellent plating cracking.
Specifically, the present invention is, tensile strength 570 N / mm ² or more (except in the case of 780N / mm ² or more), vTrs-50 ° C., joint vTrs-10 ° C., high tension satisfying the above IpushironZc2% The object is to provide steel and its manufacturing method.
[0011]
[Means for Solving the Problems]
The present inventors have conducted various tests and studies in order to satisfy the above object. As a result, while controlling the ratio of Ti to N contained in the steel to a predetermined value, controlling the center segregation degree by rolling conditions and refining crystal grains by specifying Ti / N, HAZ toughness and plating cracking It has been found that the low-temperature toughness can be improved by reducing the crystal grain size while ensuring a tensile strength of 590 N / mm ² or more without impairing the plating cracking resistance. Reached.
[0012]
Here, according to the invention, in weight%:
C: 0.04 to 0.10%, Si: 0.05 to 0.40%, Mn: 1.00 to 2.0%,
P: 0.025% or less, S: 0.010% or less, Al: 0.050% or less,
Ti: 0.030 to 0.050 %, Nb: 0.01 to 0.10%, N: 0.002 to 0.005%,
Further, by satisfying Ti / N: 6.0 to 20.0, generation of coarse TiN is suppressed, and precipitation of fine carbonitride is promoted. In particular, crystal growth in HAZ during welding is suppressed. , HAZ can be improved in toughness.
[0013]
Further, one or more of V: 0.10% or less, Cu: 1.0% or less, Ni: 1.0% or less, Cr: 1.0% or less, and Mo: 1.0% or less In this case as well, by satisfying Ti / N: 6.0 to 20.0, the formation of coarse TiN 2 is suppressed, and the precipitation of fine carbonitrides is promoted. Crystal growth in the HAZ during welding can be suppressed, and the toughness of the HAZ can be improved.
[0014]
Further, in order to improve the plating crack resistance, the slab having the above steel sheet composition is heated to a temperature in the range of 1050 to 1250 ° C., is subjected to finish rolling at a temperature of ₃ points or more of Ar, and is subsequently heated to a temperature of 550 ° C. or less. By performing water cooling to, center segregation can be reduced and plating crack resistance can be improved. In order to further improve the base material toughness, tempering at 450 ° C or less is performed.
[0015]
BEST MODE FOR CARRYING OUT THE INVENTION
Next, the reason for defining the steel composition and the production conditions in the present invention as described above will be described.
[0016]
C is an essential element for increasing the strength of steel, but if it is less than 0.04%, the required tensile strength of 590 N / mm ² or more cannot be secured, and 0.10% If it exceeds, not only the toughness of the base metal and the weld and the plating crack resistance of the HAZ are lowered, but also the degree of center segregation is deteriorated. Preferably, the upper limit is 0.08%.
[0017]
Si is an element necessary for securing the strength in addition to the deoxidizing action, but if it is less than 0.05%, its effect is not sufficient, and if it exceeds 0.40%, the toughness of the welded portion and the plating cracking resistance of the HAZ. Therefore, the content was set to 0.05 to 0.40% because the property deteriorated. Preferably it is 0.10 to 0.30%.
[0018]
Mn is an effective element in securing the strength and toughness of steel, but if it is less than 1.00%, its effect is not sufficient, and if it exceeds 2.0%, the toughness of the weld and the plating resistance of HAZ Since not only the cracking property is lowered but also the center segregation degree is adversely affected, the content is set to 1.00 to 2.0%.
[0019]
P is an element that lowers the toughness of the steel material, and the lower the better, the better.
S is an element that lowers the toughness of the steel material like P, and is preferably as low as possible.
[0020]
Al is added as a deoxidizing element, but it is preferable that the content be lower. If the content exceeds 0.050%, the effect is saturated, and inclusions increase to impair the cleanliness of the steel. 0.050% or less.
[0021]
Ti produces a fine carbonitride in steel by a synergistic effect of limiting the N content as described later, improves strength by precipitation hardening, and refines crystal grains to form a base material and improves the HAZ toughness, it is less than 0.030% or effect of improving the low temperature toughness is not exerted, and since rather deteriorates the toughness by curing excessive precipitation exceeds 0.050% 0. Was 030 to 0.050 percent.
[0022]
Nb forms fine carbonitrides in steel like Ti, improves the strength by precipitation hardening, and improves the toughness of the base metal and HAZ by refining the crystal grains, but less than 0.01% In this case, the effect is not sufficient, and if it exceeds 0.10%, the toughness is rather deteriorated due to excessive precipitation. Preferably it is 0.02-0.05%.
[0023]
By containing N at 0.002% or more simultaneously with Ti and making Ti / N 6.0 or more, precipitation of fine carbonitrides is promoted, and in particular, crystal grain growth in HAZ at the time of welding is suppressed. , HAZ is improved, but if the content exceeds 0.005% and Ti / N exceeds 20, coarse Ti / N is generated, and the effect of suppressing the crystal grain growth cannot be obtained. 0.002 to 0.005%, and Ti / N was 6 to 20. Preferably, the lower limit is 0.003%, the upper limit is 0.004%, and the Ti / N is also preferably 15 or less.
[0024]
In the present invention, a high-strength steel may be used in which one or more of V, Cu, Ni, Cr and Mo are further contained to further increase the strength of the steel. ◇
If V exceeds 0.1% and each of Cu, Ni, Cr and Mo exceeds 1.0%, the toughness of the base material and the welded portion and the plating crack resistance of the HAZ are reduced. %, Cu, Ni, Cr, and Mo were each set to 1.0% or less .
[0025]
In the present invention, the plating crack resistance of the welded portion is improved by further defining the rolling conditions of the steel material. The reasons for limiting the rolling conditions are described below.
First, as for the heating temperature, the target base material strength cannot be obtained at a temperature lower than 1050 ° C., and when the slab is heated to more than 1250 ° C., the base material toughness decreases due to an increase in the initial γ grains. It was specified below ° C.
[0026]
Next, as for the rolling conditions, the center segregation is reduced by finishing the rolling at a high temperature of ₃ or more Ar and immediately cooling with water. In order to prevent elements such as C and Mn that cause center segregation from diffusing and concentrating during ferrite transformation, rolling is completed at _three or more points of Ar and water cooling is started.
Subsequently, if water cooling is performed to a temperature of 550 ° C. or less, no crack occurs.
[0027]
If the cooling rate is less than 5 ° C./sec, not only is it difficult to secure the base metal strength, but also there is no effect of suppressing carbide precipitation by accelerated cooling, and the base material toughness and the plating crack resistance of the base material deteriorate, There is no diffusion suppression effect, the degree of center segregation also worsens, and the plating crack resistance of the welded part deteriorates. The upper limit is not particularly limited, but if it exceeds 30 ° C / sec, the base material toughness tends to deteriorate. Therefore, it is preferably 30 ° C / sec or less, and more preferably 20 ° C / sec or less from a practical viewpoint.
[0028]
The steel material subjected to the accelerated cooling can be used as it is as a steel material having excellent resistance to cracking of Zn plating. However, in order to prevent changes in mechanical properties such as tensile strength and toughness when this steel material is immersed in a Zn plating bath, it is necessary to perform tempering in advance at a temperature close to the temperature of the molten zinc bath (450 ° C). desirable. However, if the tempering temperature exceeds 450 ° C., carbides dissolved in the solid solution by the accelerated cooling before will precipitate, and the galvanization resistance will decrease. Therefore, when performing tempering, it is performed at a temperature of 450 ° C. or less.
[0029]
【Example】
Steel No. 1 having the chemical components shown in Table 1 was used. 50 kg of steel of 1 to 30 was melted in a vacuum melting furnace, and a steel plate having a tensile strength of approximately 590 to 690 N / mm ² and a thickness of 16 t was manufactured.
[0030]
From each steel plate, a test piece is cut out, and the tensile strength according to the metal material tensile test method specified in JIS Z 2241 and the energy transition temperature (hereinafter referred to as vTrs) determined by Charpy impact test as the low-temperature toughness specified in JIS Z 2242 are used as the metal material specified in JIS Z 2242. The impact test method was used.
[0031]
In addition, the plating crack resistance of the melted portion was evaluated using SLM400 and εZc which are usually used.
As shown in FIG. 1 (a), a round bar 1 having a diameter (D) of 10 mm and a total length of 170 mm was cut out from each of the above steel plates, and a high-frequency heating was performed at a heating temperature of 1400 ° C. and a cooling time of 800 to 500 ° C. for 8 seconds, as shown in FIG. After applying the reproducible heat cycle, a test piece having a circumferential notch 2 with an angle of 60 degrees and a depth of 2 mm as shown in FIG. 1B was used as a test piece. After the hot-dip galvanizing was applied to the cutout of the test piece, as shown in FIG. 2, the temperature was raised from room temperature to 470 ° C. in 30 seconds, and the temperature was increased to a predetermined tensile stress in 30 seconds from the start of the test. 0.5 kgf / mm ² / min to give a thermal stress cycle,
(Rupture stress during hot-dip galvanizing) / (rupture stress without hot-dip galvanizing) x 100 (%)
To obtain the relationship between the SLM defined by
[0032]
Regarding the evaluation of plating cracking resistance, there is a knowledge that plating cracking does not occur if the SLM at 400 seconds, ie, SLM400 ≧ 42%, in consideration of actual hot-dip galvanizing work (for example, iron and steel , Vol. 79 (1993) No. 9, P96). Table 2 shows the results.
[0033]
On the other hand, with respect to the εZc value, a heat cycle of cooling at 800 to 500 ° C. for 15 seconds was applied after holding at 1250 ° C. for 10 minutes to prepare a test piece 4 shown in FIG. 1 (c). The numbers in the figure represent dimensions (mm). The thickness was 7 mm. The prepared test piece was pulled in a zinc bath at 450 ° C., and a critical strain amount at which no crack was generated was determined, and an εZc value of 2.0% was targeted.
[0034]
As a result of the above test, as shown in Table 2, steel No. The steels of the present invention Nos. 1 to 17 all show 42% or more, which is said to be free from plating cracking, and have a vTrs of -62 ° C or less. The SLM400 of the comparative steels Nos. 18 to 30 each showed 42% or more, which is said to be free from plating cracking. It is clear that the low-temperature toughness is inferior to those of the inventive steels Nos. 1 to 17.
[0035]
In addition, when the rolling conditions were No. out of the invention, 31, 32 and No. With respect to Nos. 34 and 35, the base metal strength or toughness was poor. No. 33 was No. 33 due to the deterioration of the degree of center segregation. With regard to No. 35, the εZc value was unfavorable due to the influence of the precipitated carbide. In addition, Ti / N is No. out of the range of the present invention. 24, no. It can be seen that all of the steels Nos. 26 to 28 have poor εZc values.
[0036]
[Table 1]

[0037]
[Table 2]

[0038]
【The invention's effect】
The high-strength steel according to the present invention has excellent low-temperature toughness without impairing plating crack resistance, and is subjected to hot-dip galvanizing in a cold region and used as a high-tensile steel for large structures such as power transmission towers. Very important. In addition, by limiting the rolling conditions and improving the center segregation, the base material toughness and plating crack resistance were improved. Furthermore, alloy elements can be added more than before, and stable production can be achieved.
[Brief description of the drawings]
BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 shows test pieces used for measuring plating crack resistance in Examples, and FIGS. 1 (a) and 1 (b) are respectively an overall plan view and a cylinder cut of a test piece used for obtaining an SLM400. FIG. 1 (c) is an explanatory view of the shape of a test piece used for obtaining an εZc value.
FIG. 2 is a graph showing a relationship between an elapsed time and a thermal stress cycle in a notched round bar tensile test for measuring SLM400 in an example.
[Explanation of symbols]
1: Round bar 2: Circular notch 4: Test piece

Claims (4)

In weight percent,
C: 0.04 to 0.10%, Si: 0.05 to 0.40%, Mn: 1.00 to 2.0%,
P: 0.025% or less, S: 0.010% or less, Al: 0.050% or less,
Ti: 0.030 to 0.050%, Nb: 0.01 to 0.10%, N: 0.002 to 0.005%,
Ti / N: 6.0-20.0,
Excellent in low-temperature toughness and plating crack resistance for large structures such as power transmission towers with a tensile strength of 570 N / mm ² or more (excluding 780 N / mm ² or more) having a steel composition containing Tension steel. In weight percent,
C: 0.04 to 0.10%, Si: 0.05 to 0.40%, Mn: 1.00 to 2.0%,
P: 0.025% or less, S: 0.010% or less, Al: 0.050% or less,
Ti: 0.030 to 0.050%, Nb: 0.01 to 0.10%, N: 0.002 to 0.005%,
Ti / N: having a steel composition consisting of 6.0 to 20.0,
Further, V: 0.10% or less, Cu: 1.0% or less, Ni: 1.0% or less,
Cr: 1.0% or less, Mo: for transmission lines than 1.0% of one or tensile strength having a steel composition containing two or more 570N / mm ² or more (except in the case of 780N / mm ² or higher) High-strength steel with excellent low-temperature toughness and plating crack resistance for large structures such as steel towers. Slabs having the steel composition of 請 Motomeko 1, wherein heating to a temperature in the range of 1,050-1,250 ° C., subjected to finish rolling at Ar ₃ point or higher, further followed by at 5 ° C. / sec or more cooling rate , characterized by a water-cooled to a temperature of 550 ° C. or less, a tensile strength of 570N / mm ² or more (except in the case of 780N / mm ² or more), large structures such as towers for power lines method for producing a high strength steel excellent in low temperature toughness and resistance to plating cracking resistance for use in. Further After water cooling down to 550 ° C. temperature below, 450 ° C. beauty Oyo low temperature toughness for use in large structures, such as claim 3 transmission lines towers of wherein the performing tempering at a temperature of Manufacturing method of high tensile steel with excellent plating crack resistance .

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