JP3793294B2 - Method for producing 780 MPa class high-tensile steel with excellent galvanization resistance - Google Patents

Description

以上の条件下で、各種合金元素添加量の異なった鋼材により試験片を製作し、これを亜鉛浴中に浸漬した後、試験ビード止端部におけるわれ発生の有無を調査した。
【００３４】
実験結果をＣｅｑ（Ｚ）との関係で図２に示す。
【００３５】
図から明らかな通り、各種合金元素含有量が前記した限定成分範囲にあり、Ｃｅｑ（Ｚ）％が０．５８％以下であれば亜鉛めっきわれの発生を完全に防止できることが確認された。
【００３６】
次に、製造条件の限定理由について述べる。
【００３７】
以上述べた通り、亜鉛めっきわれ防止のためにはＣｅｑ（Ｚ）を０．５８％以下にすることが必要であるが、この条件を満たす鋼材は、従来の８０キロ級（７８０ＭＰａ級）鋼に比べ低成分鋼となるため、焼入れ処理による強度確保が必然となる。
【００３８】
本発明鋼では、通常の焼入れ、焼戻しだけでなく、焼戻し熱処理として、溶融亜鉛めっき時の加熱を利用した低温焼戻しにより、焼戻し熱処理が省略でき、より経済的な製造が可能となった。焼入れ温度は板内、板厚方向各部分が全てＡｒ３点以上となる９１０℃以上とし、また、加熱炉からは昇温後１２０分以内に取出し、さらに、焼入れ時の鋼材の表面と中心部との温度勾配を２℃／ｍｍ以内に維持することで、板内、板厚方向各部分の材質バラツキを小さく押さえることが可能となる。
【００３９】
次に、焼戻し温度としては、溶融亜鉛めっき時の加熱温度である４５０℃を下限とし、６５０℃超では強度が確保できないため、この温度を上限とする。
【００４０】
また、直接焼入れにより、再加熱の焼入れ熱処理が省略でき、より経済的であるが、直接焼入れ前の熱間圧延条件として、オーステナイト未再結晶域での圧下を行うと急激に圧延効率が低下するため、９００℃以上で圧延を終了する。また、圧延後３分以内に焼入れ、かつ、直接焼入れ時の鋼材の表面と中心部との温度勾配を２℃／ｍｍ以内に維持することで、板内、板厚方向各部分の材質バラツキを小さく押さえることが可能となる。さらに、焼入れ開始温度が７５０℃未満になると、強度を確保できないため、この値を下限とする。
【００４１】
本発明は主として、厚板、熱延鋼板についてであるが、山形鋼、Ｈ形鋼等の形鋼、線・棒鋼、鋼管等としても製造可能である。
【００４２】
【実施例】
以下実施例により本発明の効果を具体的に示す。
【００４３】
なお、耐亜鉛めっきわれ性は図１に示した試験方法によった。
【００４４】
表２に供試した鋼の組成、Ｃｅｑ（Ｚ）を，また、表３に製造条件、母材強度、靭性、及び耐亜鉛めっきわれ性評価試験結果をそれぞれ示す。
【００４５】
【表２】

【００４６】
【表３】

本発明鋼Ａ１〜Ａ４は、成分および圧延条件を全て満足しており、母材強度は８２２〜８５１ＭＰａ、靭性もｖＥ０で１７５Ｊ以上と良好であり、また、材質のバラツキも１０℃以下に抑えられ、さらに、全てＺｎめっきわれは発生しなかった（◎印）。
【００４７】
一方、比較例では、Ｂ１〜Ｂ５までは成分上は問題ないが、Ｂ１は直接焼入れが開始温度が低く、Ｂ２は圧延仕上げ〜焼入れ開始までの時間が３分を越えており、Ｂ３は温度勾配が２℃／ｍｍを越えている。また、Ｂ４は保持時間後１２０分を越え、Ｂ５は温度勾配が２℃／ｍｍを越え、Ｂ６はＣ％が多く、Ｂ７はＣｅｑ（Ｚ）が０．５８を越えている。この結果、Ｂ１は母材強度が６９３ＭＰａと低く、Ｂ２〜Ｂ５までは材質のバラツキが大きくなっており、Ｚｎめっきわれが発生する場合があった（○印）。Ｂ６とＢ７は靭性が低く、Ｚｎめっきわれが発生した。
【００４８】
本発明限定要件を満足する鋼は、構造用８０キロ級（７８０ＭＰａ級）鋼として充分な強度・靭性と優れた耐亜鉛めっきわれ性を有することが明らかである。
【００４９】
【発明の効果】
以上の説明から明らかなように、個々の合金元素添加量とこれらの総合的添加量を制限すると共に、製造条件を限定することにより、優れた耐亜鉛めっきわれ性を有する８０キロ級（７８０ＭＰａ級）高張力鋼の経済的な製造が可能である。
【００５０】
従って、本発明は産業上、大きな効果を有するものであるといえる。
【図面の簡単な説明】
【図１】 鋼材の亜鉛めっきわれ性評価の実験方法を示した図である。
【図２】 鋼材の亜鉛めっきわれ性評価の実験結果を示した図である。
【符号の説明】
１ 試験板
２ 試験ビード
３ 拘束ビード[0001]
BACKGROUND OF THE INVENTION
The present invention, steel structures are galvanized after welding (towers, bridges, buildings, etc.) relates to a manufacturing method of the steel used for, 780 MPa grade high-tensile particularly excellent in resistance to hot-dip galvanizing cracking characteristics of the weld The present invention relates to a method for manufacturing steel.
[0002]
[Prior art]
Conventionally, steel structures such as steel towers, bridges, and buildings are often hot dip galvanized from the viewpoint of corrosion resistance. However, in these structures, when the member is hot dip galvanized, cracks may occur mainly in the welded portion, and countermeasures for preventing the structure are required for safety of the structure.
[0003]
This crack is a phenomenon known as galvanized crack based on liquid metal embrittlement, and it is also empirically known in the industry that cracks are more likely to occur as the strength of the steel material increases. Several proposals have been made so far to prevent the galvanized crack.
[0004]
For example, Japanese Patent Laid-Open No. 59-50157 discloses a countermeasure by restricting the amount of S in steel to 0.030 to 0.060%, Japanese Patent Laid-Open No. 61-133363, Japanese Patent Laid-Open No. 61-231141. JP-A-62-50448 proposes a countermeasure by satisfying a specific relationship with the amount of alloying elements in steel.
[0005]
However, the proposed technology targets steel materials up to 60 kg class (590 MPa class) high-tensile steel, and the steel materials with higher strength are not necessarily sufficient to completely prevent plating. In particular, in the 80 kg class (780 MPa class) steel, there is no crack prevention technique, and a new technique is required.
[0006]
Thereafter, Japanese Patent Application Laid-Open No. 3-229817 proposed a method of manufacturing 80 kg class (780 MPa class) steel by restricting rolling conditions to a narrow range upon rolling and then directly quenching.
[0007]
However, this proposal has a drawback that it is a limited manufacturing method and has a narrow rolling condition range, a large variation in material in the plate and in the plate thickness direction, and a very low rolling efficiency.
[0008]
[Problems to be solved by the invention]
As described above, in steel structures that are hot dip galvanized for the purpose of rust prevention after welding assembly, zinc embrittlement may occur at the weld toes during plating, which is also prevented from the safety of the structure. Establishment of measures is desired.
[0009]
This zinc embrittlement is considered to be governed by the degree of zinc embrittlement susceptibility based on the structure factor in the vicinity of the weld toe, the welding residual stress acting on the part, and the thermal stress during plating.
[0010]
Conventionally, it has been known that the higher the strength of steel, the higher the alloying element content, and the accompanying increase in zinc embrittlement susceptibility is unavoidable. Further, the welding residual stress, which is a crack-dominating factor, is reduced by heating the welded portion, and it has been confirmed that the same phenomenon occurs in the plating process. However, since the magnitude of this welding residual stress is governed by the yield strength of the base metal, the higher the yield strength, the higher the residual welding stress remains during plating, and the easier the zinc embrittlement occurs.
[0011]
Factors mentioned above are all 80-kilogram (780 MPa class) is disadvantageous for steel 80 kilogram (780 MPa class) are believed to prevent zinc embrittlement cracking of steel is very difficult .
[0012]
In view of such a current situation, the present invention provides a production method with a high rolling efficiency and a wide production range of 80 kg class ( 780 MPa class) steel capable of completely preventing hot dip galvanization.
[0013]
[Means for Solving the Problems]
The present invention has been made to solve the above-mentioned problems, and the gist thereof is as follows.
[0014]
(1) In mass%,
C: 0.20% or less ,
Si: 0.35% or less ,
Mn: 1.7% or less ,
Al: 0.005-0.10%
In addition,
Including 1.0% or less in total of one or two of Cr and Mo ,
Furthermore, according to the requirements of strength toughness,
Cu: 1.0% or less ,
Ni: 1.0% or less ,
V: 0.2% or less ,
Nb: 0.05% or less ,
Ti: 0.03% or less containing 1 type or 2 types or more, comprising the balance Fe and impurities ,
Ceq (Z) = C + Si / 32 + Mn / 4 + Cu / 6 + Ni / 10 + Cr / 3.5 + Mo / 3.5 + V / 2 + Nb / 2 ≦ 0.58
When a steel material satisfying the above conditions is subjected to a reheating and quenching treatment after hot rolling , the steel material is taken out of the heating furnace within 120 minutes after the temperature rise due to the reheating, and a temperature gradient between the surface and the central portion of the steel plate is 2 ° C / maintained mm or less, quenching from 910 ° C. or more quenching temperature, the as quenched, or characterized by tempering at a temperature range of 450 to 650 ° C. if necessary, the resistance to galvanizing crack properties A method for producing an excellent 780 MPa class high strength steel.
[0015]
(2) In mass%,
C: 0.20% or less ,
Si: 0.35% or less ,
Mn: 1.7% or less ,
Al: 0.005-0.10%
In addition,
Including 1.0% or less in total of one or two of Cr and Mo ,
Furthermore, according to the requirements of strength toughness,
Cu: 1.0% or less ,
Ni: 1.0% or less ,
V: 0.2% or less ,
Nb: 0.05% or less ,
Ti: 0.03% or less containing 1 type or 2 types or more, comprising the balance Fe and impurities ,
Ceq (Z) = C + Si / 32 + Mn / 4 + Cu / 6 + Ni / 10 + Cr / 3.5 + Mo / 3.5 + V / 2 + Nb / 2 ≦ 0.58
After rolling a steel material satisfying the above conditions at 900 ° C. or higher, within 3 minutes after rolling, maintain the temperature gradient between the surface and the center of the steel sheet at 2 ° C./mm or lower and directly quench from a temperature range of 750 ° C. or higher. A method for producing a 780 MPa class high-strength steel having excellent galvanized cracking characteristics.
[0016]
As described above, the limitation requirements of the present invention are the individual contents of various alloy elements, the limitation of Ceq (Z)% by a combination thereof, and the limitation of the production conditions. The effects of the present invention are all these requirements. It can only be demonstrated when you are satisfied, and if you do not meet any of the requirements, it will not be effective.
[0017]
First, the reason why the individual alloy element contents are limited to the above ranges will be described.
[0018]
C is added to ensure strength, when it exceeds 0.20%, not only impair the toughness and weldability of the steel was made the upper limit 0.20% because significantly impair the cracking resistance galvanized.
[0019]
Si is added for securing the strength and deoxidation, but if it exceeds 0.35%, the toughness deteriorates and the soundness of the plated surface is impaired, so this was made the upper limit.
[0020]
Mn is added for securing strength, because significant loss of weldability and resistance to galvanizing crack resistance and addition of more than 1.7%, so this was made the upper limit.
[0021]
Cr and Mo are elements that are extremely effective for increasing the hardenability by adding a small amount and ensuring the strength. However, if the total of one or two types exceeds 1.0%, the resistance to galvanization resistance is significantly impaired, so this was made the upper limit.
[0022]
Al was limited to 0.005 to 0.100 percent in the range of usually used as a deoxidizing element.
[0023]
Cu, Ni, V, Nb, and Ti are elements added for the purpose of improving strength and toughness , respectively, but if added beyond the above-mentioned limited ranges, weldability and galvanizing resistance are impaired. The upper limit.
[0024]
In the present invention, as described above, the addition amount of each element is limited, and the total addition amount combining them exhibits its effect for the first time when a specific formula is satisfied. Explain with experimental results.
[0025]
This is the reason why the chemical composition of the steel used is limited by the formula of Ceq (Z) = C + Si / 32 + Mn / 4 + Cu / 6 + Ni / 10 + Cr / 3.5 + Mo / 3.5 + V / 2 + Nb / 2 ≦ 0.58 However, the carbon equivalent formula quantifies the effect of various alloy elements on the zinc embrittlement of the heat affected zone of the weld. The lower the value, the higher the zinc based on the structure factor in the vicinity of the weld start end. Brittleness is unlikely to occur.
[0026]
Therefore, it is desirable that the steel component has a low Ceq (Z) value within a range that satisfies the base material strength.
[0027]
The experimental method and experimental results obtained from this new finding are shown in FIGS.
[0028]
In FIG. 1, 1 is a test plate, 2 is a test bead, and 3 is a restraining bead for imparting residual stress to the test bead.
[0029]
In this experiment , after applying stress to the toe end of the test bead 2 with the restraint bead 3, the test piece was immersed in a zinc bath, and the resistance of the steel material was determined depending on whether or not galvanized cracking occurred at the toe end of the test bead. It evaluates the galvanization cracking property.
[0030]
In addition, galvanization will be easy to generate | occur | produce, so that the stress to provide will be high if it is the same steel material.
[0031]
According to this experimental method, it is possible to apply a residual stress corresponding to the yield strength at room temperature of the test plate in the vicinity of the test bead toe at a limit of 5 passes, so the number of bead in this experiment is All 5 passes.
[0032]
Table 1 shows the welding conditions for the test and restraint beads.
[0033]
[Table 1]

Under the above conditions, test pieces were made of steel materials having different amounts of various alloying elements, immersed in a zinc bath, and then examined for cracking at the test bead toe.
[0034]
The experimental results are shown in FIG. 2 in relation to Ceq (Z).
[0035]
As is apparent from the figure, it was confirmed that the occurrence of galvanized cracks can be completely prevented when the content of various alloy elements is in the limited component range described above and Ceq (Z)% is 0.58% or less.
[0036]
Next, the reasons for limiting the manufacturing conditions will be described.
[0037]
As described above, Ceq (Z) needs to be 0.58% or less in order to prevent galvanization cracking, but steel materials that satisfy this condition are the conventional 80 kg class (780 MPa class) steel. Compared to low-component steel, it is inevitable to ensure strength by quenching.
[0038]
In the steel according to the present invention, not only normal quenching and tempering but also tempering heat treatment can be omitted by low-temperature tempering using heating at the time of hot dip galvanizing, and more economical production becomes possible. The quenching temperature is 910 ° C. or more in which all the parts in the plate and in the plate thickness direction are at least Ar3 point. Also, the quenching temperature is taken out from the heating furnace within 120 minutes after the temperature rise. By keeping the temperature gradient within 2 ° C./mm, it is possible to reduce the material variation in each part in the plate and in the plate thickness direction.
[0039]
Next, as the tempering temperature, the lower limit is 450 ° C., which is the heating temperature at the time of hot dip galvanizing , and the strength cannot be ensured at temperatures exceeding 650 ° C., so this temperature is the upper limit.
[0040]
In addition, by direct quenching, the reheating quenching heat treatment can be omitted, and it is more economical, but as the hot rolling conditions before direct quenching, rolling in the austenite non-recrystallized region rapidly reduces the rolling efficiency. Therefore, the rolling is finished at 900 ° C. or higher. In addition, by maintaining the temperature gradient between the surface and the center of the steel material within 3 minutes after rolling and maintaining the temperature gradient within 2 ° C / mm during direct quenching, material variations in each part in the sheet thickness direction can be achieved. It becomes possible to hold down small. Furthermore, when the quenching start temperature is less than 750 ° C., the strength cannot be secured, so this value is set as the lower limit.
[0041]
The present invention mainly relates to thick plates and hot-rolled steel plates, but can also be manufactured as section steels such as angle steels and H-shaped steels, wires / bars, steel pipes and the like.
[0042]
【Example】
The effects of the present invention are specifically shown below by examples.
[0043]
In addition, the resistance to galvanizing was based on the test method shown in FIG.
[0044]
The composition of this was subjected steel in Table 2, Ceq a (Z), also shows produced in Table 3 condition, the base material strength, toughness, and resistance to galvanizing cracking resistance evaluation test results, respectively.
[0045]
[Table 2]

[0046]
[Table 3]

The invention steels A1 to A4 satisfy all the components and rolling conditions, the base material strength is 822 to 851 MPa, the toughness is vE0 as good as 175 J or more, and the material variation is also suppressed to 10 ° C. or less. In addition, no Zn plating cracks occurred (◎ marks).
[0047]
On the other hand, in the comparative example, B1 to B5 have no problem in terms of components, but B1 has a low start temperature for direct quenching, B2 has a time from rolling finish to start of quenching exceeding 3 minutes, and B3 has a temperature gradient. Exceeds 2 ° C./mm. Further, B4 exceeds 120 minutes after the holding time, B5 has a temperature gradient exceeding 2 ° C./mm, B6 has a large C%, and B7 has a Ceq (Z) exceeding 0.58. As a result, the base material strength of B1 is as low as 693 MPa, the material variation is large from B2 to B5, and Zn plating may occur (circle mark). B6 and B7 had low toughness, and Zn plating cracks occurred.
[0048]
It is apparent that steel satisfying the requirements of the present invention has sufficient strength and toughness and excellent galvanization resistance as structural 80 kg class (780 MPa class) steel.
[0049]
【The invention's effect】
As is clear from the above explanation, by limiting the addition amount of individual alloy elements and the total addition amount thereof, and by limiting the production conditions, the 80 kg class (780 MPa class) having excellent resistance to galvanization. ) Economical production of high strength steel is possible.
[0050]
Therefore, it can be said that the present invention has a great effect in the industry.
[Brief description of the drawings]
FIG. 1 is a diagram showing an experimental method for evaluating the galvanizing cracking property of a steel material.
FIG. 2 is a diagram showing experimental results of evaluation of galvanization cracking property of steel materials.
[Explanation of symbols]
1 Test plate 2 Test bead 3 Restraint bead

Claims (2)

% By mass
C: 0.20% or less ,
Si: 0.35% or less ,
Mn: 1.7% or less ,
Al: 0.005-0.10%
In addition,
Including 1.0% or less in total of one or two of Cr and Mo ,
Furthermore, according to the requirements of strength toughness,
Cu: 1.0% or less ,
Ni: 1.0% or less ,
V: 0.2% or less ,
Nb: 0.05% or less ,
Ti: 0.03% or less containing 1 type or 2 types or more, comprising the balance Fe and impurities ,
Ceq (Z) = C + Si / 32 + Mn / 4 + Cu / 6 + Ni / 10 + Cr / 3.5 + Mo / 3.5 + V / 2 + Nb / 2 ≦ 0.58
When a steel material satisfying the above conditions is subjected to a reheating and quenching treatment after hot rolling , the steel material is taken out of the heating furnace within 120 minutes after the temperature rise due to the reheating, and a temperature gradient between the surface and the central portion of the steel plate is 2 ° C / maintained mm or less, quenching from 910 ° C. or more quenching temperature, the as quenched, or characterized by tempering at a temperature range of 450 to 650 ° C. if necessary, the resistance to galvanizing crack properties A method for producing an excellent 780 MPa class high strength steel. % By mass
C: 0.20% or less ,
Si: 0.35% or less ,
Mn: 1.7% or less ,
Al: 0.005-0.10%
In addition,
Including 1.0% or less in total of one or two of Cr and Mo ,
Furthermore, according to the requirements of strength toughness,
Cu: 1.0% or less ,
Ni: 1.0% or less ,
V: 0.2% or less ,
Nb: 0.05% or less ,
Ti: 0.03% or less containing 1 type or 2 types or more, comprising the balance Fe and impurities ,
Ceq (Z) = C + Si / 32 + Mn / 4 + Cu / 6 + Ni / 10 + Cr / 3.5 + Mo / 3.5 + V / 2 + Nb / 2 ≦ 0.58
After rolling a steel material satisfying the above conditions at 900 ° C. or higher, within 3 minutes after rolling, maintain the temperature gradient between the surface and the center of the steel sheet at 2 ° C./mm or lower and directly quench from a temperature range of 750 ° C. or higher. method for producing characterized, 780 MPa grade high-tensile steel having excellent galvanized We characteristics to.

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