JP4630221B2 - Method for producing soft and slow-aged surface-treated steel sheet or thin steel sheet for containers, and surface-treated steel sheet for containers - Google Patents

Description

  The present invention relates to a method for producing a surface-treated steel sheet or thin steel sheet for containers that is subjected to continuous annealing, and a surface-treated steel sheet for containers.

  Specifically, among steel plates for containers such as tinplate and TFS, steel plate Rockwell T hardness after baking at 210 ° C. for 30 minutes: HR30T is 46 to 60, and yield point elongation in the rolling direction: The present invention relates to a method for producing a soft, slow-aged surface-treated steel sheet or thin steel sheet for containers, and a surface-treated steel sheet for containers, wherein the YP-El value is 8% or less.

Continuous annealing is a manufacturing technique that can anneal a steel plate for containers in a short time of about 30 minutes, has high productivity, has less material variation than BAF annealing, and can easily make a good steel plate shape. On the other hand, continuous annealing is often applied to hard steel sheets with a tempering degree of T4 or higher, and BAF annealing is frequently used for soft steel sheets with a tempering degree of T3 or lower. There is a reality that is being done. In order to produce a surface-treated steel sheet for containers having a tempering degree T3 or less by continuous annealing, metallurgy of refining annealed recrystallized grains based on the heat pattern of continuous annealing, supersaturated residual of solid solution C, and solid solution N This is because it is necessary to eliminate the physical properties. Steel plates for deep drawing used for home appliances and automobile bodies have workability equivalent to steel plates for containers having a tempering degree T3 or less, and manufacturing technology by a continuous annealing method has already been disclosed. For example, JP-A-6-172869, JP-A-7-242959, and the like.
These are technologies for manufacturing deep-drawn cold-rolled steel sheets with a small amount of B, and the hot-rolling temperature of the previous process for continuous annealing: improving the properties of hot-rolled steel sheets by high-temperature FT or high-temperature CT treatment, and deep-drawing. To the steel sheet. That is, at present, it has been considered that a technique capable of producing a steel plate for containers having a tempering degree T3 or less by improving the continuous annealing method is not disclosed.

  In addition, there is an example in which a new manufacturing technique relating to recrystallized grain growth, reduction of solute N and solute C is disclosed by a continuous annealing method. For example, Japanese Patent Publication No. 63-54048 and Japanese Patent Publication No. 63-10213, the summary of which is a hot rolled heating temperature of 950 to 1100 ° C. using aluminum killed steel as a raw material, and a cutting temperature after hot rolling. Is limited to 620 to 710 ° C., the amount of dissolved N is suppressed to 10 ppm or less, the subsequent continuous annealing temperature is increased to 650 to 710 ° C., grain growth is attempted, and the overaging temperature after annealing is set to 300 to 500 ° C. By limiting the amount, the amount of solute C is reduced, and further, aging deterioration due to a small amount of solute N and solute C can be eliminated by optimizing temper rolling conditions (roll diameter or rolling force). .

  That is, although the metallurgical problem regarding the soft continuous annealing steel sheet should have been solved by a plurality of disclosed manufacturing techniques, there is still no fact that the continuous annealing method has been replaced with the BAF annealing method. The inventor has reconsidered why the new continuous annealing technique, which should have been completed, does not replace BAF annealing, and has realized that one major challenge remains. In other words, since the processing speed of the continuous annealing line has been steadily increased since its birth, there is a restriction on the size that can be manufactured as its negative aspect. Higher speed increases the superiority of the continuous annealing line over BAF annealing, and improvements in productivity and production efficiency are always required when a new continuous annealing line is constructed.

On the other hand, since such a high-speed and high-efficiency production line requires a long furnace, it is obvious that the steel plate tension control in the furnace becomes difficult as the line scale increases. The inconvenience of this tension control appears in the furnace meandering due to a drop in tension, cooling squeezing at the time of cooling (cooling buckle), high-temperature heating, heating throttling at the time of soaking (heat buckle), and the like. In order to resolve this problem, installation of a bridle roll in the furnace and improvement of the hearth roll profile were carried out as necessary, and the meandering improved. On the other hand, in the manufacture of soft continuous annealed steel sheets that require high temperatures, there is still concern about the occurrence of heat buckles and cooling buckles, which is a weak point of high-speed and long continuous annealing lines. In other words, heat buckles are more likely to occur as the soaking temperature is increased, and the higher the temperature annealing and the higher the speed of sheeting, the stronger the cooling and the generation of cooling buckles are promoted. It is becoming difficult to maintain the furnace environment. The present inventor considered that this limited the production size of the soft continuous annealing material, and further created the present situation where the BAF annealing method could not be eliminated.
JP-A-6-172869 JP-A-7-242959 Japanese Patent Publication No. 63-54048 Japanese Examined Patent Publication No. 63-10213

  The present invention aims to increase the high-temperature strength of the steel sheet by reviewing the high-temperature annealing that is conventionally used in the production of the surface-treated steel sheet for soft and slow-aged containers as described above, thereby reducing the in-furnace drawing of thin and wide materials. The object of the present invention is to provide a new continuous annealing operation technique capable of producing a soft surface-treated steel sheet or thin steel sheet for a container having a Rockwell T hardness: HR30T of 46 to 60 even if low temperature annealing is avoided.

  The present invention was devised to solve the above-described problems, and comprises heating, soaking, primary quenching, overaging treatment, and secondary quenching under the continuous annealing treatment conditions shown in FIG. 1 and Table 1. The process was developed focusing on the plate temperature of the soaking furnace and the time from the soaking furnace exit side to the primary quenching.

一般に均熱炉の板温度が低温であるほど炉内絞りが出難いとの経験則が知られている。本発明者は低炭素アルミキルド鋼板の連続焼鈍炉内における軟化挙動を確認し、この経験則を定量化する目的で表２に示す成分の熱間圧延材を条取りし試験片とし、高温強度調査を行なった。

In general, an empirical rule is known that the lower the plate temperature of a soaking furnace, the harder it is to draw out the furnace. The present inventor confirmed the softening behavior of the low carbon aluminum killed steel sheet in the continuous annealing furnace, and in order to quantify this empirical rule, the hot rolled material having the components shown in Table 2 was stripped and used as a test piece, and a high temperature strength investigation was conducted. Was done.

試験機は島津製作所製のインストロン型引張試験機で、試験片を円筒型電気炉で囲み冶具とともに再結晶温度域の６００℃から高温焼鈍に相当する７５０℃まで加熱した。加熱温度は試験片平行部の3箇所に取付けた熱電対により約±５℃に制御し、目標温度で２０分保定後、引張速度（歪み速度）５００ｍｍ／分で荷重を加えた。試験装置の主要部を図２に示し、測定した応力歪み曲線から連続焼鈍ライン通板時に鋼板が受ける歪みと同程度と考えられる０．５％歪み耐力を高温強度として求めた。その結果を図３に示す。この図より均熱温度６６０℃以下であれば鋼板強度は高めに維持されており、その強度は再結晶温度から６６０℃までほぼ同等であり、ヒートバックルへの抵抗性および冷却時の強度変化も少ないことからクーリングバックルに対する抵抗性もあることが推察される。すなわち低炭素アルミキルド鋼板において均熱温度６６０℃以下で焼鈍すれば連続焼鈍時の通板安定性は確保できるとの知見が得られた。

The tester was an Instron type tensile tester manufactured by Shimadzu Corporation. The test piece was surrounded by a cylindrical electric furnace and heated together with a jig from 600 ° C. in the recrystallization temperature range to 750 ° C. corresponding to high temperature annealing. The heating temperature was controlled to about ± 5 ° C. by thermocouples attached at three locations on the parallel part of the test piece. After holding at the target temperature for 20 minutes, a load was applied at a tensile rate (strain rate) of 500 mm / min. The main part of the test apparatus is shown in FIG. 2, and a 0.5% strain proof stress, which is considered to be the same level as the strain that the steel sheet receives during continuous annealing line passing, was obtained as a high temperature strength from the measured stress strain curve. The result is shown in FIG. From this figure, if the soaking temperature is 660 ° C. or less, the steel sheet strength is kept high, the strength is almost the same from the recrystallization temperature to 660 ° C., the resistance to the heat buckle and the strength change during cooling are also It is presumed that there is resistance to a cooling buckle because it is small. That is, it has been found that if a low carbon aluminum killed steel sheet is annealed at a soaking temperature of 660 ° C. or less, the sheet feeding stability during continuous annealing can be ensured.

  On the other hand, when the soaking temperature is lowered, the amount of C dissolved in equilibrium decreases, and the driving force for precipitating intragranular carbides is weakened in the overaging treatment after the primary cooling, so that the decrease in solute C progresses. There was a fear of disappearing. This inventor examined the cold rolled steel plate shown in Table 3 about the optimum conditions of the start of primary cooling prior to an overaging treatment: sheet temperature and time to start cooling as a test material. The conditions for continuous annealing are soaking plate temperature: 690 to 700 ° C., the time from soaking to the start of primary cooling is less than 1 second and 1 second or more, primary cooling start plate temperature: 500 to 700 ° C., primary cooling rate: 100 Secondary cooling was performed at a temperature of ˜120 ° C./second, a primary cooling end plate temperature: 390 to 405 ° C., an overaging treatment plate temperature: 390 to 410 ° C., and an overaging treatment time: 60 seconds or more to obtain a normal temperature annealed steel sheet. The strength of the driving force for precipitating intragranular carbides in the overaging treatment after primary cooling is determined by the amount of dissolved C remaining in the steel sheet after annealing, and the measurement is performed by an inverted hanging torsion pendulum type internal friction measuring device. It was. Since the internal friction coefficient peak value is proportional to the solid solution C amount, the absolute value of the solid solution amount can be determined from the proportional coefficient.

なお供試鋼板は微量Ｂが添加されており固溶Ｎは熱延時にほぼ析出していると考えられたため測定値は全て固溶Ｃ量として整理した。その結果を図４に示す。板温６９０〜７００℃の高温焼鈍であっても均熱出から一次冷却開始までの時間が１秒以上経過すると一次冷却開始温度が下がり、焼鈍後鋼板の固溶Ｃ残留量は多くなる。これは均熱処理時に溶解しマトリックス中に過飽和となった固溶Ｃが一次冷却までの待ち時間に析出、減少した結果、過時効処理での炭化物析出に働く駆動力を減じた結果と考えられる。また一次冷却開始温度が６２０℃以上あれば過時効処理による炭化物析出が効率よく進められる知見も得られた。これは均熱板温が６２０℃の低温であっても固溶Ｃが低減できる可能性を示すものと考えられる。

In addition, since the test steel plate was added with a very small amount of B, and it was considered that solute N was almost precipitated during hot rolling, all measured values were arranged as solute C amount. The result is shown in FIG. Even in the case of high-temperature annealing at a plate temperature of 690 to 700 ° C., if the time from soaking to the start of primary cooling elapses for 1 second or more, the primary cooling start temperature decreases, and the solid solution C residual amount of the steel plate after annealing increases. This is considered to be a result of reducing the driving force acting on carbide precipitation in the overaging treatment as a result of precipitation and reduction of the solid solution C dissolved during the soaking and supersaturated in the matrix during the waiting time until the primary cooling. Moreover, if the primary cooling start temperature was 620 ° C. or higher, it was found that carbide precipitation by the overaging treatment can proceed efficiently. This is considered to indicate the possibility that the solid solution C can be reduced even when the soaking plate temperature is as low as 620 ° C.

  That is, if the soaking plate temperature is set to 620 ° C. or higher and the time to primary rapid cooling is shortened to less than 1 second (hereinafter, abbreviated as rapid cooling immediately thereafter), and the primary cooling start plate temperature can be maintained at 620 ° C. or higher, low temperature annealing is possible. The present inventors have found that the amount of dissolved C can be reduced by annealing at high temperature. If the time from the soaking furnace exit side to the start of primary quenching is 0.9 seconds or less as a result obtained by the 620-660 ° C. low temperature annealing experiment shown in Table 6, Table 7 and Table 8 following the above experiment, It was confirmed that solid solution C precipitation was sufficiently performed in the overaging treatment, which is considered to be caused by the formation of intragranular carbide after the primary cooling immediately after maintaining the soaking plate temperature.

  In addition, by using a low-carbon aluminum killed steel with a small amount of B added to the steel sheet, the processing temperature in hot rolling can be widely selected, and the Rockwell T hardness (HR30T): 46- It was also found that the surface-treated steel sheet for containers can be produced with slow aging characteristics in a wide range of 60. The main purpose of the addition of B is to form BN precipitates during hot rolling (hot rolling), and this makes it possible to fix solid solution N effectively. In addition, since ferrite grains easily grow even by low temperature annealing due to BN precipitation in hot rolling, there is an advantage that softening and slow aging proceed. On the other hand, since addition of B is ineffective in fixing solute C, over-aging treatment is essential for suppressing strain aging after temper rolling with solute C. Immediately after the above-described rapid cooling, intragranular carbides are generated even at low temperature annealing, so that solid solution C reduction can be promoted without lowering the overaging treatment efficiency.

The present invention has been obtained by combining such new experimental findings, and the gist thereof is as follows.
(1) Mass%
C: 0.015-0.08% or less,
Si: 0.05% or less,
Mn: 0.05 to 0.6%,
P: 0.04% or less,
S: 0.02% or less,
sol. Al: 0.04% or less,
N: 0.006 or less,
B: 0.003 to 0.008% contained, the balance being Fe and steel of unavoidable impurities as a slab, hot rolled, descaled in pickling, cold rolled and cold rolled steel sheet None, soaking furnace plate temperature is above recrystallization temperature and below 660 ° C in continuous annealing, time from soaking furnace exit side to primary cooling start is 0.9 seconds or less, and plate temperature at primary cooling start is recrystallization Immediately after the temperature is set to 660 ° C. or less and the primary cooling rate is set to 100 ° C./second or more, the plate temperature is 300 to 450 ° C. and the secondary cooling is performed for 30 seconds or more in the overaging furnace. A method for producing a soft, slow-aged surface-treated steel sheet for steel or thin steel sheet, which is subjected to temper rolling at a rolling reduction of 0.5 to 3%.
(2) Steel plate Rockwell T hardness produced by the method described in (1) and baked at 210 ° C. for 30 minutes: HR30T is 46 to 60, and yield point elongation in the rolling direction: YP-El value A surface-treated steel sheet for soft and slow-aged containers, characterized in that is 8% or less.

  The low-carbon aluminum killed steel sheet to which the production method of the present invention is applied has a small distortion aging due to solute C and solute N, a tin with melt aging, or a can body after heat treatment such as a pre-coated steel sheet that adheres a film at a high temperature. Even in the applications to be molded, it is as slow-aging as the current high-temperature continuous annealed material, and it can produce beautiful can bodies due to its soft properties, and it is a thin wide steel plate with a plate thickness of 0.20 mm or less and a plate width of 900 mm or more. Even if it exists, it is a technique which has an industrially useful superior effect, such as being able to provide stable productivity without generating a heat buckle and a cooling buckle in a continuous annealing furnace.

  The individual production factors relating to the present invention are described in detail below.

  As the steel sheet of the present invention, a steel slab formed by continuous casting from molten steel produced in a normal melting furnace such as a converter or an electric furnace is used. The hot rolling of the slab can be performed by any of DR hot rolling performed immediately after casting, or CCR hot rolling after cooling and reheating in a hot rolling furnace, and the type is not limited. The components of the slab are C: 0.015-0.08% or less, Si: 0.05% or less, Mn: 0.05-0.6%, P: 0.04% or less, and S: 0.00% in mass%. 02% or less, sol. Al: 0.04% or less, N: 0.006% or less, B: 0.003 to 0.008%, the balance is composed of unavoidable impurities and Fe, with a particularly small amount of B added Is an important component of the present invention. The reason for limiting the slab component will be described in detail below. The lower the C, the more tend to soften. However, the lower limit is set to 0.015% because the solid solution C tends to remain after continuous annealing in an amount of less than 0.015% specifically to harden the matrix. On the other hand, as the amount increases, when continuous annealing is performed, the strength of the matrix increases and the crystal grains become finer, so excessive content must be avoided and the upper limit is set to 0.08%. In addition to increasing the strength of the matrix and making the crystal grains finer, Si also concentrates on the surface of the steel sheet and impairs the plating properties, so excessive content must be avoided and the upper limit was made 0.05%. Mn must be contained in an amount of 0.05% or more in order to prevent cracking during hot rolling induced by S inevitably mixed.

  On the other hand, an excessive content causes hardening, so the upper limit was made 0.6%. P is an element that refines crystal grains and increases the strength of the steel sheet, and segregates at the grain boundaries to deteriorate the corrosion resistance, so the upper limit was made 0.04%. Since S causes cracking during hot rolling as described in the section of Mn, the upper limit is set to 0.02%, and the amount thereof is made smaller than the lower limit of Mn amount, so that all S becomes MnS. sol. The smaller the amount of Al in relation to the B amount described later, the better the purpose of the present invention. That is, it is highly desirable to limit the initial addition amount to the amount added to the molten steel as a deoxidizer and removed as slag in steelmaking. There is no lower limit for Al. On the other hand, sol. When Al exceeds 0.04%, the precipitation of nitride in the hot-rolled coil is biased and the amount of precipitates is reduced, resulting in large variations in strength, ductility, etc., so the upper limit was made 0.04%. N forms a nitride with Al by forming a nitride with Al, and if excessively added, N aging occurs in the form of solute N, so the upper limit is made 0.006%. B is one of the important points of the present invention and is an element that forms a nitride like Al. On the other hand, since the action is greatly influenced by the amount of Al and the amount of N, sol. Unless it is used under the conditions of Al: 0.04% or less and N: 0.006% or less, the delayed aging effect of the present invention cannot be expected.

Details of the conditions for extracting the usefulness of B will be described below.
(1) For nitride formation by utilizing B, sol. Al is harmful. FIG. 5 shows this, and the test material used was a low carbon aluminum killed steel with a slight addition of 0.8 to 1.0 B / N as shown in Table 4, and the heating temperature in hot rolling: Steel sheet rolled at 1180 to 1250 ° C, finishing temperature: 870 to 895 ° C, cutting temperature: 600 to 680 ° C, pickled and cold-rolled, and recrystallized annealed at a soaking plate temperature of 650 to 700 ° C in continuous annealing. This is the knowledge. In the case of a steel sheet to which Al is added in an amount of 0.04% or more, the ratio of BN in the nitride is reduced, BN is difficult to precipitate, and the amount of precipitation of the entire nitride is also reduced.

      This indicates that as the Al content increases, solid solution N remains and N aging is likely to occur. In other words, the combination of Al and B is harmful for effective solute N precipitation, indicating that the formation of nitride by B alone is extremely desirable.

（２）Ｂの活用による窒化物形成にはＮ低減（０．００６％以下）が望ましい。表５に示 すＮ添加量の異なる２つの供試材を均熱板温：６６０℃で連続焼鈍を施し、低炭素 アルミキルド鋼の降伏強度に及ぼすＢ添加量効果を調査した結果を図６に示す。化 学量論的に全ＮをＢＮとする組成比であるＢ／Ｎ＝０．８が最も強度を低下させて おり、Ｂ添加されていないか、Ｂ添加が少ない場合、つまりＢＮの化学量論的組成 比０．８未満の場合には不完全なＮ固定によると推察される強度増加が認められ、 一方、Ｂ／Ｎ：０．８超になると過剰なＢによると推察される強度増加がある。こ の材質変化はＮ量との相互作用で生じていると考えられ、Ｎが多い場合には顕著だ が少ないと材質変化はゆるやかになる傾向が認められた。特にＮ：０．００６％以 下になるとＢ／Ｎ＝０．８のみにあった軟化点がＢ／Ｎ：０．５〜２．０に広がり 軟化範囲となって存在するようになる。この軟化範囲は固溶Ｎが少なく歪み時効を 弱める範囲でもある。この軟化範囲の発見はＮ量を低く抑え難い現状の冷延鋼板製 造技術にとって極めて有用な知見である。

(2) N reduction (0.006% or less) is desirable for nitride formation by utilizing B. Figure 6 shows the results of investigating the effect of B addition on the yield strength of low-carbon aluminum killed steel by subjecting two specimens with different N additions shown in Table 5 to continuous annealing at a soaking plate temperature of 660 ° C. Show. The stoichiometric ratio of B / N = 0.8, which is the composition ratio of total N as BN, is the most significant decrease in strength. When B is not added or when B is added less, that is, the BN stoichiometry. When the theoretical composition ratio is less than 0.8, an increase in strength presumed to be due to incomplete N fixation is observed. On the other hand, when B / N exceeds 0.8, an increase in strength presumed to be due to excessive B is observed. There is. This material change is thought to be caused by the interaction with the N content. When N is large, the change is noticeable, but when it is small, the material change tends to be gradual. In particular, when N is 0.006% or less, the softening point which is only at B / N = 0.8 spreads to B / N: 0.5 to 2.0 and becomes in the softening range. This softening range is also a range in which solid solution N is small and strain aging is weakened. The discovery of this softening range is extremely useful knowledge for the current cold-rolled steel sheet manufacturing technology that makes it difficult to keep the N content low.

Ｂ量上限はこの知見に基づき０．００３％（＝Ｂ／Ｎ：０．５）から０．０１２％（＝Ｂ／Ｎ：２．０）まで選択できるがＢ添加は合金コストアップを伴うので工業生産的な運用にはＢ／Ｎ：０．５〜１．３の範囲、即ち０．００３〜０．００８％が好ましい。一方、Ｂ下限はＢ／Ｎ：０．５によって定められるがその量が０．００３％未満になると効果が消失するので０．００３％を下限とした。
＜熱延鋼板の熱延条件＞
ＡｌＮの析出を抑制しＢＮ析出による全Ｎの固定を熱延の操業条件の狙いとする。熱延加熱温度はＭｎＳ析出物を微細多量に分散させ、かつ熱延仕上げ出口温度をＡｒ３変態点以上に維持できることが望ましいので１１００℃より高めとする。仕上げ出口温度がＡｒ３点以上においてはＢＮ析出がＡｌＮ析出に優先し、かつ高温での析出となって熱延板固溶Ｎはほとんどが窒化物として析出するためである。捲取り温度は５００〜７２０℃の範囲を選択できる。５００〜６５０℃の低温捲取りでは熱延コイルの温度バラツキが小さいことで均一材質を得やすいが、その温度が５００℃未満になると捲取り後の自己焼鈍効果が不十分となって冷間圧延性を阻害するため下限を５００℃とする。一方、高温の６５０超〜７２０℃捲取りではスキャベンジング効果によって焼鈍時の均一な粒成長が期待できる利点はあるが、加工性に有害なオーステナイト粒の生じさせないためには上限温度を７２０℃にする必要がある。さらに高温捲取りの弊害とされる材質バラツキはＢ添加により解消される。これは温度管理が厳密な熱間圧延時、即ちＡｒ３変態点以上の温度域でほとんどの固溶ＮがＢＮとして析出してしまうため、その後の捲取り工程で仮に温度バラツキがあってもＢＮ、ＡｌＮの析出がなく、材質バラツキが生じないことによる。５００〜７２０℃の低温〜高温捲取りで鋼板材質の均質性を維持できることはＢ添加鋼の有用な冶金効果である。
＜冷間圧延条件＞
連続焼鈍で再結晶後に均質整粒組織が安定して得られる圧延率を適用する。圧延率は低いほど再結晶組織を構成する結晶粒を粗大かつ混粒にする。従って８５％以上で、かつ冷間圧延機能力上限に相当する９５％以下を適用することが望ましい。
＜連続焼鈍条件＞
安定した炉内通板性を確保するには低温焼鈍が望ましいがコイル全体を再結晶させる温度以上で焼鈍しなければならない。微量Ｂ添加の低炭素アルミキルド鋼を冷延率８５％以上で冷間圧延した場合の再結晶温度は６２０℃であった。したがって焼鈍温度、特に均熱炉出側の鋼板温度は６２０℃以上でなければならない。一方、連続焼鈍温度が高くなるほど鋼板の高温強度が低下し薄手、幅広サイズほど炉内絞りが発生しやすくなる。したがって薄手、幅広な鋼板サイズを安定して通板する温度上限は６２０℃に近いほど望ましいが連続焼鈍操業上の加減速や鋼板サイズ構成などにより鋼板温度バラツキが最大４０℃変化すると見込まれるため均熱炉出側の鋼板温度上限を６６０℃と定めた。

Based on this finding, the upper limit of B amount can be selected from 0.003% (= B / N: 0.5) to 0.012% (= B / N: 2.0). For industrial production operation, B / N is preferably in the range of 0.5 to 1.3, that is, 0.003 to 0.008%. On the other hand, the lower limit of B is determined by B / N: 0.5, but when the amount is less than 0.003%, the effect disappears, so 0.003% was set as the lower limit.
<Hot-rolling conditions of hot-rolled steel sheet>
The aim of the hot rolling operation condition is to suppress the precipitation of AlN and fix all N by BN precipitation. The hot rolling heating temperature is preferably higher than 1100 ° C. because it is desirable to disperse MnS precipitates in a fine amount and to maintain the hot rolling finish outlet temperature at or above the Ar3 transformation point. This is because BN precipitation takes precedence over AlN precipitation when the finishing outlet temperature is at or above the Ar3 point, and the hot-rolled sheet solid solution N precipitates as nitrides at a high temperature. A range of 500 to 720 ° C. can be selected as the wrinkling temperature. In low-temperature cutting at 500 to 650 ° C., a uniform material can be easily obtained due to small temperature variation of the hot-rolled coil. However, when the temperature is less than 500 ° C., the self-annealing effect after cutting is insufficient and cold rolling is performed. In order to inhibit the property, the lower limit is set to 500 ° C. On the other hand, there is an advantage that uniform grain growth at the time of annealing can be expected by scavenging effect at a high temperature of over 650 ° C. to 720 ° C., but the upper limit temperature is 720 ° C. in order not to generate austenite grains harmful to workability. It is necessary to. Furthermore, the material variation which is an adverse effect of high temperature scraping can be eliminated by adding B. This is because, during hot rolling with strict temperature control, that is, most of the solid solution N precipitates as BN in the temperature range above the Ar3 transformation point, even if there is a temperature variation in the subsequent cutting step, BN, This is because there is no precipitation of AlN and no material variation occurs. It is a useful metallurgical effect of the B-added steel that the uniformity of the steel plate material can be maintained by the low-temperature to high-temperature cutting at 500 to 720 ° C.
<Cold rolling conditions>
The rolling rate at which a uniform sized structure is stably obtained after recrystallization by continuous annealing is applied. The lower the rolling rate, the larger and coarser the crystal grains constituting the recrystallized structure. Therefore, it is desirable to apply 85% or more and 95% or less corresponding to the upper limit of the cold rolling functional force.
<Continuous annealing conditions>
Low temperature annealing is desirable to ensure stable passage through the furnace, but it must be annealed at a temperature above the temperature at which the entire coil is recrystallized. The recrystallization temperature was 620 ° C. when the low carbon aluminum killed steel with a small amount of B was cold-rolled at a cold rolling rate of 85% or more. Therefore, the annealing temperature, particularly the steel plate temperature on the outlet side of the soaking furnace must be 620 ° C. or higher. On the other hand, the higher the continuous annealing temperature, the lower the high-temperature strength of the steel sheet, and the thinner and wider the size, the easier the furnace drawing occurs. Therefore, it is desirable that the upper temperature limit for thin and wide steel plate sizes to be stably passed is closer to 620 ° C, but the temperature variation in steel plate is expected to change by 40 ° C at maximum due to acceleration / deceleration during continuous annealing operation and steel plate size configuration. The upper limit of the temperature of the steel plate on the exit side of the furnace was set to 660 ° C.

The soaking time in the method of the present invention is not particularly limited, but is about 10 seconds to 5 minutes. In order to give the steel sheet soft and slow aging characteristics, it is important to reduce the solid solution C by overaging treatment. For this purpose, it is desirable to shorten the time from the discharge side of the soaking furnace to the start of primary cooling as much as possible. In low-temperature annealing at 660 ° C or less, this time is set to 0.9 seconds or less, so that C The primary cooling process is performed without impairing the supersaturated state. The aging property of a steel sheet that is immediately cooled to an overaging temperature at a cooling rate of 100 ° C./second or more and over-aged at a steel plate temperature of 300 to 450 ° C. for 30 seconds or more has characteristics that are not inferior to that of a high-temperature annealed material. ing. Immediately after cooling, not only is a large amount of carbides precipitated in the recrystallized grains to reduce solute C, but as an incidental effect, the carbides in the recrystallized grains become a dislocation source during temper rolling, and a slow aging is performed. It is also intended to proceed. If the overaging temperature is low, it takes a long time for precipitation of the solid solution C, so the lower limit is set to 300 ° C. On the other hand, the upper limit is set to 450 ° C. because the amount of C dissolved in equilibrium is large when the temperature becomes high. It is desirable that the retention time in this overaging temperature range be 30 seconds or longer. These low-temperature annealing conditions related to the present invention are most effective when the size is a plate thickness of 0.20 mm or less and a plate width of 900 mm or more.
<Conditioning rolling conditions>
In temper rolling, 0.5 to 3% of dry temper rolling can be applied to the steel sheet at a Rockwell T hardness (HR30T) of 46 to 60 after baking at 210 ° C. for 30 minutes. In the present invention, strain aging is harmful because it causes stretcher strain. Therefore, it is desirable that the solid solution N and the solid solution C are made harmless by fixing to the dislocations introduced by pressure regulation. Since this aging suppression effect is ineffective when the rolling reduction is less than 0.5%, the lower limit is set to 0.5%. On the other hand, the reduction rate of dry pressure adjustment, so-called skin pass rolling, is limited to 3% or more in the 2-stand tandem method, and this is the upper limit. The steel plate YP-El value after baking for 30 minutes at 210 ° C, which is the most severe paint baking condition applied at the time of can manufacturing, should be 8% or less by using the above-mentioned overaging treatment conditions and temper rolling together. Can do.
<Surface treatment conditions for steel sheet>
A typical steel sheet for surface treatment is a tin-plated steel sheet, but the steel sheet of the present invention can also be applied to an inexpensive TFS steel sheet. Furthermore, the present invention can also be applied to a steel plate using a single-layer or multi-layer film mainly composed of a polyester resin such as polyethylene terephthalate or polybutylene terephthalate obtained by press-bonding a plastic film to a TFS steel plate at 180 to 280 ° C. Preferably, the thickness of the plastic is 5 to 50 μm, and an adhesive such as an epoxy resin may be used for adhesion to the steel plate.

  The steel slab having the components shown in Table 6 was made into a surface-treated original plate by the hot rolling and continuous annealing conditions shown in Table 7, and in particular, the state of drawing in the furnace was confirmed in the continuous annealing line, and then a surface-treated steel plate was manufactured. Next, the steel plate was baked at 210 ° C. for 30 minutes, which is considered to be the most severe surface-treated steel sheet for containers, and the post-aging hardness (HR30T) and the post-aging YP-El value were investigated. 8. As is apparent from the results, according to the present invention, the hardness and YP-El value equivalent to those of conventional high-temperature annealed materials are obtained at low temperature annealing of 620 to 660 ° C., and the passage through the furnace is facilitated. The effect of was confirmed.

It is a figure which illustrates the processing conditions of continuous annealing in the present invention. It is a figure which shows the principal part of the test apparatus used for the Example of this invention. It is a figure which shows the result of having calculated | required 0.5% distortion proof stress of the steel plate of this invention as high temperature strength. It is a figure which shows the measurement result of the solid solution C amount of the steel plate of this invention. Nitride formation by utilizing B in the present invention and sol. It is a figure which shows the relationship with Al. It is a figure which shows the result of having investigated the B addition amount effect on the yield strength of the low carbon aluminum killed steel plate of this invention.

Claims (2)

mass%,
C: 0.015-0.08% or less,
Si: 0.05% or less,
Mn: 0.05 to 0.6%,
P: 0.04% or less,
S: 0.02% or less,
sol. Al: 0.04% or less,
N: 0.006 or less,
B: 0.003 to 0.008% contained, the balance being Fe and steel of unavoidable impurities as a slab, hot rolled, descaled in pickling, cold rolled and cold rolled steel sheet None, soaking furnace plate temperature is above recrystallization temperature and below 660 ° C in continuous annealing, time from soaking furnace exit side to primary cooling start is 0.9 seconds or less, and plate temperature at primary cooling start is recrystallization Immediately after the temperature is set to 660 ° C. or less and the primary cooling rate is set to 100 ° C./second or more, the plate temperature is 300 to 450 ° C. and the secondary cooling is performed for 30 seconds or more in the overaging furnace. A method for producing a soft, slow-aged surface-treated steel sheet for steel or thin steel sheet, which is subjected to temper rolling at a rolling reduction of 0.5 to 3%. Steel plate Rockwell T hardness produced by the method according to claim 1 and baked at 210 ° C for 30 minutes: HR30T is 46-60, and yield point elongation in the rolling direction: YP-El value is 8% A surface-treated steel sheet for a soft, slow-aged container characterized by:

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