JPS5952207B2 - Manufacturing method of low yield ratio, high toughness, high tensile strength steel plate - Google Patents

Description

【発明の詳細な説明】 本発明は低温用高張力鋼の製造方法に係るものである。[Detailed description of the invention] The present invention relates to a method for manufacturing high-strength steel for low temperature use.

１ 近年クリーンエネルギー化に伴い天然ガスの輸送用
のラインパイプ、貯蔵用のタンク、貯蔵及び輸送用のタ
ンカー等の素材に厚肉低温用鋼材の開発の必要度が増加
してきた。1. In recent years, with the shift to clean energy, there has been an increasing need to develop thick-walled, low-temperature steel materials for materials such as line pipes for natural gas transportation, storage tanks, and tankers for storage and transportation.

ところでこれらの素材は強度が高く、低温靭性ノが優れ
ていることはもちろん必要なことであるが、一方構造物
に成形するときにもし、スプリングバックによって弾性
的に変形回復が生じ形状凍結性が不良であれば用途は著
しく限定される。By the way, it is of course necessary for these materials to have high strength and excellent low-temperature toughness, but on the other hand, when forming them into a structure, elastic deformation recovery occurs due to springback, resulting in shape fixability. If it is defective, its uses will be severely limited.

この形状凍結性は従来の知識で明らかのように、降１伏
強度と引張強さとの比（降伏強度÷引張強さ二以下降伏
比という）値が低いほど良好である。As is clear from conventional knowledge, this shape fixability is better as the ratio of yield strength to tensile strength (referred to as yield ratio of yield strength ÷ tensile strength 2 or less) is lower.

一般に高張力鋼材においては降伏比が０．７８以下であ
れば形状凍結性は良好である。In general, high tensile strength steel has good shape fixability if the yield ratio is 0.78 or less.

高張力鋼材においても一般に圧延まま材及びノ□ルマ（
ｎｏｒｍａｌｉｚｉｎｇ）材においては降伏比が０．７
８以下のことも多い。High-strength steel materials are generally as-rolled and rolled (
In normalizing) material, the yield ratio is 0.7.
It is often less than 8.

しかしこれらの材料では焼入れ焼戻しくＱｕｅｎｃｈ
ａｎｄ Ｔｅｍｐｅｒ）材のように高靭性及び高強度特
性が得られない。However, these materials cannot be quenched and tempered.
High toughness and high strength characteristics cannot be obtained as in the case of the (and Temper) materials.

一方焼入れ焼戻し材では高靭性は得られるが、降伏比は
０．８５程度かそれより高＜ ０．７８以下を得ること
は不可能に近い。On the other hand, although high toughness can be obtained with quenched and tempered materials, it is almost impossible to obtain a yield ratio of about 0.85 or higher < 0.78.

この理由はいろいろ考えられるが、従来の焼入れ焼戻し
処理では、焼戻しマルテンサイトが主体の組織であって
、焼戻し処理温度は当然Ａｃ１変態点以下であるからと
も考えられる。There are various possible reasons for this, but it is also believed that in the conventional quenching and tempering process, the structure is mainly composed of tempered martensite, and the tempering temperature is naturally below the Ac1 transformation point.

以上のように本発明の目的とする高靭性及び高張力特性
を維持しながら低降伏比の鋼を得ることは従来技術では
不可能に近いと言える。As described above, it can be said that it is nearly impossible to obtain steel with a low yield ratio while maintaining the high toughness and high tensile strength properties that are the objectives of the present invention using conventional techniques.

さて、本発明はかかる難題を一挙に解決し得るものと考
えられるものであり、その原理的骨子は以下のようであ
る。Now, the present invention is considered to be able to solve these problems all at once, and its principle outline is as follows.

Ｃ，Ｓｉ、Ｍｎ、Ａｌあるいはこれに付加すべき添加元
素に量的制限を行った鋼において通常の造。Normal construction for steel with quantitative restrictions on C, Si, Mn, Al, or other additive elements to be added.

塊・分塊法又は連続鋳造法でスラブとしたあと熱間圧延
に際し９５０℃以下の累積圧下率を３０％以上９０％以
下とることによりオーステナイト粒の微細化をはかる。The austenite grains are made finer by setting the cumulative reduction rate of 950° C. or less at 30% or more and 90% or less during hot rolling after forming a slab using the lump/blowing method or continuous casting method.

一方熱間圧延時に少くとも１回以上のクロス方向の圧延
（圧延スラブを鋼塊、或いは。On the other hand, during hot rolling, at least one cross-direction rolling (rolled slab to steel ingot, etc.)

鋼片製造時の鋳造方向に対して直角方向に圧延すること
を意味する）を施すことにより、成分とりわけ炭化物の
圧延方向の異方性を軽減することをはかり均一なオース
テナイト粒が得られる。By performing rolling (meaning rolling in a direction perpendicular to the casting direction during production of steel billets), uniform austenite grains can be obtained by reducing the anisotropy of components, especially carbides, in the rolling direction.

、鋼板の圧延直角方向の靭性向上に寄与する。, contributes to improving the toughness of the steel plate in the direction perpendicular to rolling.

この両方；の条件を組合わせることにより、次工程の熱
処理で最終的に均一微細な組織が高靭性及び低降伏比が
得られることを見出したものである。It has been discovered that by combining both of these conditions, a uniform fine structure with high toughness and a low yield ratio can be obtained in the heat treatment in the next step.

さて次に熱処理であるがこれはＡｅ３変態点以上に加熱
後通常の焼入れ（Ｑｕｅｎｃｈ）処理と同様に急冷して
焼；入れし、しかる後にＡＣ１変態点とＡＣ３変態点と
の間の温度域で加熱後空冷処理を施すものである。Now, next is heat treatment, which is heated to above the Ae3 transformation point, then rapidly cooled and quenched in the same way as normal quenching, and then heated to a temperature range between the AC1 transformation point and AC3 transformation point. After heating, air cooling is performed.

これにより、まず第１ステツプの焼入れ処理によりマル
テンサイト又はベイナイトの多く存在する組織となり、
次の第２ステツプのＡＣ１変態点と。As a result, the first step of quenching creates a structure with a large amount of martensite or bainite.
The AC1 transformation point of the next second step.

ＡＣ３変態点との間に再加熱後空冷することにより上記
マルテンサイト又はベイナイトの一部がフェライトの組
織となる。By air cooling after reheating to the AC3 transformation point, a part of the martensite or bainite becomes a ferrite structure.

なお、靭性の点でフェライトの割合は１５〜１７％が最
も好ましい。In addition, from the viewpoint of toughness, the most preferable ratio of ferrite is 15 to 17%.

この組織の代表的なものとして、第１図に示すようにマ
ルチ・ンサイト （６５％）とフェライト （３５％）
の微細な混合組織となり、降伏比は低く、かつ強度、靭
性とも高い鋼材が製造可能である。Typical examples of this structure are multi-site (65%) and ferrite (35%), as shown in Figure 1.
This results in a fine mixed structure, and it is possible to manufacture steel materials with a low yield ratio and high strength and toughness.

ここで注目すべき点は従来技術に存在するＡｃｍ変態点
とＡＣ３変態点との間のいわゆる２相域温度からの焼入
れ処理法との違いである。What should be noted here is the difference between the quenching process from the so-called two-phase region temperature between the Acm transformation point and the AC3 transformation point, which exists in the prior art.

この従来法では、たしかにマルテンサイトとフェライト
との混合組織は得られるが、これでは圧延後の比較的粗
大なフェライト粒が変態過程を経ずにそのまま残り、こ
のため低温靭性は十分に良好な値を示さない。Although this conventional method does produce a mixed structure of martensite and ferrite, relatively coarse ferrite grains remain as they are after rolling without going through the transformation process, so the low-temperature toughness remains at a sufficiently good value. does not indicate.

一方本発明法によれば、第１ステツプの焼入れ処理によ
り圧延後の粗大なフェライト粒も変態により微細なオー
ステナイト粒から微細なマルテンサイト等に変化して行
き、次の第２ステツプの再加熱処理と空冷により一部の
マルテンサイト等が微細なオーステナイトからＡＳＴＭ
粒度番号９番よりも微細なフェライト粒に変化するもの
で、結果的には第１図のように粒度番号１０．５番の非
常に微細なマルチ；ンサイトとフェライトとの混合組織
となり、従来法と異り結果的に著しく靭性が高められた
ものと考えられる。On the other hand, according to the method of the present invention, the coarse ferrite grains after rolling are transformed from fine austenite grains to fine martensite etc. by the quenching treatment in the first step, and then the reheating treatment in the second step is performed. and air cooling, some martensite etc. change from fine austenite to ASTM
The grain size changes to finer ferrite grains than grain size number 9, and as a result, as shown in Figure 1, a very fine mixed structure of mulch and ferrite with grain size number 10.5 is formed, which is different from the conventional method. It is thought that the toughness was significantly improved as a result.

なお、本発明の熱間圧延鋼材としての製品の組織をなる
べく細かくするには熱間圧延でオーステナイト未・再結
晶域での累積圧下を十分とる必要があり、９５０℃以下
での累積圧下率を３０％以上とする。In addition, in order to make the structure of the product as the hot-rolled steel material of the present invention as fine as possible, it is necessary to obtain a sufficient cumulative reduction in the non-austenite/recrystallized region during hot rolling, and the cumulative reduction rate at 950°C or less is required. 30% or more.

なお圧下率の上限は工業的に採用できる範囲とし、通常
９０％以下である。The upper limit of the rolling reduction is within an industrially applicable range, and is usually 90% or less.

また、仕上温度は本発明のクロス方向の圧延を□採用す
る圧延法においては通常、工業的に採用されている範囲
はＡＣ１変態点以上であり、本発明法の仕上温度もＡｃ
ｍ変態点以上とする。In addition, the finishing temperature in the rolling method of the present invention that employs cross-direction rolling is usually in the range of AC1 transformation point or higher, and the finishing temperature in the method of the present invention is also in the AC1 transformation point or higher.
m transformation point or higher.

以下本発明の特許請求の範囲の限定理由を述べる。The reasons for limiting the scope of the claims of the present invention will be described below.

Ｃは鋼の強度を高めるのに有効であり、最低０．０１％
は必要である。C is effective in increasing the strength of steel, with a minimum content of 0.01%
is necessary.

一方０．３０％超ではパーライトの生成が多く靭性の劣
下がみられる。On the other hand, if it exceeds 0.30%, pearlite is often formed and toughness is deteriorated.

なお０．１５以下であれば溶接性は大変向上する。Note that if it is 0.15 or less, weldability is greatly improved.

Ｓｉは鋼の脱酸の目的の他に、安定した強化元素；とし
て最低０．０１％は必要である。In addition to the purpose of deoxidizing steel, Si is a stable strengthening element; at least 0.01% is required.

一方１．０％超も含まれると靭性の劣下を招く。On the other hand, if the content exceeds 1.0%, the toughness will deteriorate.

なお０．６％以下であれば溶接性も良好である。Note that if it is 0.6% or less, weldability is also good.

Ｍｎは鋼の強化に必須な元素で最低０．５％は必要であ
る。Mn is an essential element for strengthening steel, and a minimum content of 0.5% is required.

一方３％超も含まれると靭性劣下が大１きい。On the other hand, if the content exceeds 3%, the toughness will deteriorate significantly.

ＡＩは鋼の脱酸に不可欠であり、最低０．００５％は必
要である。AI is essential for deoxidizing steel, and a minimum of 0.005% is required.

一方０．２％超も含まれるとＡ ｌ ２０３クラスター
によるヘゲ疵が多発する。On the other hand, if the content exceeds 0.2%, scratches due to Al 203 clusters will occur frequently.

。 Ｎｂ、 Ｖ、 Ｍｏ、 Ｔｉ、 Ｗ、 ＺｒからなるＡ
群の７元素は鋼の強化の他に加熱時のオースティト粒の
成長抑制に有効であり、その１種以上を計０．００５％
以上は必要である。. A consisting of Nb, V, Mo, Ti, W, Zr
In addition to strengthening steel, the seven elements in the group are effective in suppressing the growth of austite grains during heating, and one or more of them is added in a total of 0.005%.
The above is necessary.

一方０．８０％超も含まれると靭性劣下が大きい。On the other hand, if the content exceeds 0.80%, the toughness will deteriorate significantly.

稀土類元素、Ｃａ、 Ｍｇ、 １３からなるＢ群の元素
は酸素あるいは窒素あるいは硫黄と結合し、それらを微
細に分散させて靭性、とりわけ異分性を改善するのに有
効であり、その１種以上を計０．０００４％以上は必要
である。Rare earth elements, Ca, Mg, and group B elements consisting of A total of 0.0004% or more of the above is necessary.

一方０．１０％超ではそれらの元素の大型酸化物を形成
し、かえって靭性の劣下を招く。On the other hand, if it exceeds 0.10%, large oxides of those elements will be formed, leading to a decrease in toughness.

Ｃｕ、 Ｎｉ、 ＣｒからなるＣ群の元素は鋼の靭性の
劣下をさほど下げずに強度を向上させるのに有効であり
、その１種以上を計最低０．０５％は必要である。Group C elements consisting of Cu, Ni, and Cr are effective in improving the strength of steel without significantly reducing its toughness, and at least 0.05% of one or more of these elements is required in total.

一方１０％超も含まれると製品の表面性状を劣下させる
。On the other hand, if the content exceeds 10%, the surface quality of the product will deteriorate.

上記の元素の他にＳは不可避的に含有される不純物元素
であるが、好ましくは０．０１０％以下であれば靭性の
向上は顕著である。In addition to the above elements, S is an impurity element that is inevitably contained, but if it is preferably 0.010% or less, the toughness is significantly improved.

Ｐも不可避的に含まれる元素であるが、０．０２５％以
下であれば靭性の向上が認められ、０．０１５％以下で
は靭性の向上は顕著である。P is also an element that is inevitably included, but if it is 0.025% or less, the toughness is improved, and if it is 0.015% or less, the toughness is significantly improved.

次に製造工程上の限定理由を述べる。Next, we will discuss the reasons for the limitations in the manufacturing process.

まず鋼の焼入れ処理であるが、このときの加熱温度は完
全にオーステナイトにする必要があるのでＡＣ３変態点
以上である必要があり、一方、ＡＣ３変態点よりあまり
高い温度ではオーステナイト粒が粗大化しすぎて靭性の
向上は期待できないので上限はＡＣ３変態点＋１００℃
とする。First, the steel is quenched.The heating temperature at this time needs to be completely austenitic, so it needs to be higher than the AC3 transformation point.On the other hand, if the temperature is too high than the AC3 transformation point, the austenite grains will become too coarse. Since no improvement in toughness can be expected, the upper limit is AC3 transformation point + 100°C.
shall be.

次に再加熱処理の加熱温度であるが、本発明の１狙いは
前述のように一部マルテンサイト等の焼入れ組織を残し
、残りをフェライト粒に変態させるものなので、そのた
めの再加熱温度はＡＣ１変態点とＡＣ３変態点との間で
ある必要がある。Next, regarding the heating temperature of the reheating treatment, one aim of the present invention is to leave a part of the hardened structure such as martensite as mentioned above and transform the rest into ferrite grains, so the reheating temperature for that purpose is AC1. It needs to be between the transformation point and the AC3 transformation point.

この場合その範囲内では温度の高いほど強度が出やすく
、一方低いほど靭性が出やすい傾向もある。In this case, within this range, the higher the temperature, the easier the strength will be, while the lower the temperature, the easier the toughness will be.

しかし、強度、靭性及び降伏比との兼合からは（Ａｃ１
変態点＋５０℃）〜（Ａｃ３変態点−２０℃）が最適再
加熱の温度である。However, from the viewpoint of strength, toughness, and yield ratio (Ac1
The optimum reheating temperature is from (transformation point +50°C) to (Ac3 transformation point -20°C).

以下に本発明の実施例を示す。Examples of the present invention are shown below.

実施例 ｌ 第１表に示す化学成分の鋼を転炉（Ｂｌ、Ｂ２、Ｂ３は
電気炉）で出鋼し、一部（Ｅ、 Ｆ）は連続鋳造法で他
は造塊、分塊法でスラブとした後熱間圧延を行った。Example 1 Steel with the chemical composition shown in Table 1 was tapped in a converter (Bl, B2, and B3 are electric furnaces), and some (E and F) were cast using the continuous casting method, while the others were cast using the ingot making and blooming methods. After making it into a slab, hot rolling was performed.

熱延条件は第１表に示す。熱延板の板厚を第１表に示す
。The hot rolling conditions are shown in Table 1. Table 1 shows the thickness of the hot rolled sheets.

これを第１表に示す焼入れ処理の加熱温度及び再加熱処
理の加熱温度で熱処理を行った。This was heat treated at the heating temperature for quenching treatment and the heating temperature for reheating treatment shown in Table 1.

なおここで、Ａ２． Ｂ２、 Ｃ２，Ｂ２． Ｆは焼
入れ処理後６３０℃×３０分の焼戻し処理を行った。Here, A2. B2, C2, B2. F was tempered at 630° C. for 30 minutes after quenching.

これらの鋼の機械的性質、靭性を第１表に示す。Table 1 shows the mechanical properties and toughness of these steels.

ここで引張試験はＡＰＩ試験法、シャルピー衝撃試験は
２ｍｍＶノツチ試験片（板厚１０ｍｍ Ｘ １０ｍｍｆ
ｕｌｌ ５ｉｚｅ）、又ＤＷＴＴ （ＤｒｏｐＷｅｉｇ
ｈｔ Ｔｅａｒ Ｔｅ５ｔ）試験はＡＰＩ法で、かツ８
５％延性破面を示す試験温度つます８５％ＦＡＴＴ（Ｆ
ｒａｃｔｕｒｅ Ａｐｐｅａｒａｎｃｅ Ｔｒ
ａｎｓｉｔｉｏｎＴｅｍｐｅｒａｔｕｒｅ）でそれぞれ
表わした。Here, the tensile test is performed using the API test method, and the Charpy impact test is performed using a 2mm V notch test piece (plate thickness 10mm x 10mmf).
ll 5ize), also DWTT (DropWeig
ht Tear Te5t) test is API method,
The test temperature is 85% FATT (F
Rature Appearance Tr
AnsitionTemperature).

なお、機械試験、衝撃試験はいずれも圧延直角方向の値
である。Note that both the mechanical test and the impact test are values in the direction perpendicular to the rolling direction.

さて第１表に示すように本発明鋼は強度、靭性が優れる
ばかりでなく、とりわけ降伏比が低いのが特徴である。Now, as shown in Table 1, the steel of the present invention not only has excellent strength and toughness, but is also characterized by a particularly low yield ratio.

一方比較鋼では強度、靭性あるいは降伏比のいずれかが
劣っている。On the other hand, the comparative steels are inferior in either strength, toughness, or yield ratio.

なお、高強度冷延鋼板のように、強度及び降伏比を要求
される素材についても冷間圧延後の冷延板を本発明の熱
処理を施すことは有効であり、スプリングバックの小さ
い高強度冷延鋼板が得られる。It should be noted that it is also effective to apply the heat treatment of the present invention to cold-rolled sheets after cold rolling for materials that require high strength and yield ratio, such as high-strength cold-rolled steel sheets. A rolled steel plate is obtained.

実施例 ２ 第２表に示す化学成分及びＡＣ１変態点、ＡＣ３変態点
の鋼を転炉で出鋼し、通常の造塊、分塊法でスラブとし
た後熱延条件、つまりクロス方向の圧延開始温度及び９
５０℃以下の累積圧下率を第３表に示す値で熱延を行い
、しかる後に第３表に示す各種の熱処理を行った。Example 2 Steel with the chemical composition and AC1 transformation point and AC3 transformation point shown in Table 2 was tapped in a converter, made into a slab by normal ingot making and blooming methods, and then subjected to hot rolling conditions, that is, rolling in the cross direction. Starting temperature and 9
Hot rolling was performed at a cumulative reduction rate of 50° C. or lower as shown in Table 3, and then various heat treatments shown in Table 3 were performed.

製品の機械的性質、靭性及び製品組織中のフェライトの
割合及び粒度番号を第３表に示す。Table 3 shows the mechanical properties and toughness of the product, the proportion of ferrite in the product structure, and the particle size number.

゛第３表に示すように比較鋼においては、Ｋ２゜Ｍ
２のように焼入れ処理でＡｃ１変態点とＡＣ３変態点と
の間の温度領域のある温度から焼入れを行う従来法では
確かに降伏比が小さいが細粒化が不十分なため強度、靭
性共本発明鋼より劣る。゛As shown in Table 3, in the comparative steel, K2゜M
In the conventional method of quenching from a certain temperature in the temperature range between the Ac1 transformation point and AC3 transformation point as shown in 2, the yield ratio is certainly small, but the grain refinement is insufficient, so the strength and toughness are both poor. Inferior to invented steel.

なお、熱延時にクロス方向の圧延を行ない、Ｍ８又は９
５０℃以下の累積圧下率が３０％未満のＭ９は靭性が低
く、降伏比が高い。In addition, cross direction rolling is performed during hot rolling, and M8 or 9
M9 with a cumulative reduction ratio of less than 30% at 50° C. or less has low toughness and a high yield ratio.

又、冷間圧延後本発明の熱処理方法を行うに６も低降伏
比特性及び強度、靭性の優れた製品が得られている。Furthermore, by applying the heat treatment method of the present invention after cold rolling, products with low yield ratio characteristics and excellent strength and toughness were obtained.

【図面の簡単な説明】[Brief explanation of drawings]

図面は本発明の実施例の中の本発明鋼Ａ１の光学顕微鏡
組織写真（倍率５００倍）である。The drawing is an optical micrograph (magnification: 500 times) of the invention steel A1 in the examples of the invention.

Claims (1)

【特許請求の範囲】 Ｉ Ｃｏｏ、０１〜０．３０％ Ｓｉ ； ０．０１〜１．０％ Ｍｎ ； Ｑ、 ５〜３％ Ａｌ：０．００５〜０．２０％を含み残部鉄及び不可避
的不純物からなる鋼を鋼片とし、該鋼片を熱間圧延に際
し、９５０℃以下の累積圧下率を３０％以上９０％以下
とし、且つ全熱間圧延工程中に少くとも１回以上のクロ
ス方向の圧延を施して、鋼板に成形し、次にＡＣ３変態
点とＡＣ３変態点＋１００℃の間の温度で加熱後焼入れ
し、次にＡｃ１変態点とＡｃ３変態点との間の温度で加
熱後空冷することを特徴とする低降伏比、高靭性、高張
力鋼板の製造方法。 ２ Ｃ；０，０１〜０．３０％ Ｓｉ ： ０．０１〜１．０％ Ｍｎ ： Ｑ、 ５〜３％ Ａ ｌ ： ０，００５〜０．２０％を含み、更にＮｂ
、■、ＭＯｌＴｉ、 Ｗ、 ＺｒからなるＡ群の元素の
うち１種以上を計０．００５−０．８０％、稀土類元素
、Ｃａ、Ｍｇ、 １３からなるＢ群の元素の１種以上を
計０．０００４〜０．１０％、Ｃｕ、 Ｎｉ、 Ｃｒか
らなるＣ群ノ元素の１種以上を計０．０５〜ｌＯ％のう
ちいずれか１群以上を含み残部鉄及び不可避的不純物か
らなる鋼を鋼片とし、該鋼片を熱間圧延に際し、９５０
℃以下の累積圧下率を３０％以上９０％以下とし、且つ
全熱間圧延工程中に少くとも１回以上のクロス方向の圧
延を施して、鋼板に成形し、次にＡＣ３変態点とＡＣ３
変態点＋１００℃の間の温度で加熱後焼入れし、次にＡ
ｃｍ変態点とＡＣ３変態点との間の温度で；加熱後空冷
することを特徴とする低降伏比、高靭性、高張力鋼板の
製造方法。[Claims] I Coo, 01-0.30% Si; 0.01-1.0% Mn; Q, 5-3% Al: 0.005-0.20%, balance iron and unavoidable Steel consisting of impurities is used as a steel billet, and when the steel billet is hot rolled, the cumulative reduction rate at 950°C or less is 30% or more and 90% or less, and at least once in the cross direction during the whole hot rolling process. is rolled to form a steel plate, then heated at a temperature between AC3 transformation point and AC3 transformation point +100°C, then quenched, then heated at a temperature between Ac1 transformation point and Ac3 transformation point, and then air cooled. A method for manufacturing a steel plate with a low yield ratio, high toughness, and high tensile strength. 2C; 0.01-0.30% Si: 0.01-1.0% Mn: Q, 5-3% Al: 0.005-0.20%, further Nb
, ■, MOlTi, W, Zr, one or more of the elements of group A, total 0.005-0.80%, rare earth elements, Ca, Mg, one or more of the elements of group B, consisting of 13. A total of 0.0004 to 0.10%, one or more of the C group elements consisting of Cu, Ni, and Cr, and a total of 0.05 to 10%, and the balance consists of iron and inevitable impurities. When the steel is made into a steel billet and the steel billet is hot rolled, 950
The cumulative reduction rate below ℃ is 30% or more and 90% or less, and cross direction rolling is performed at least once during the entire hot rolling process to form a steel plate, and then AC3 transformation point and AC3
After heating at a temperature between the transformation point +100°C, quenching is performed, and then A
A method for producing a low yield ratio, high toughness, and high tensile strength steel plate, characterized by heating and then air cooling at a temperature between the cm transformation point and the AC3 transformation point.