JPS6274053A - High-strength cold-rolled steel sheet having high hardenability - Google Patents
High-strength cold-rolled steel sheet having high hardenabilityInfo
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- JPS6274053A JPS6274053A JP21115985A JP21115985A JPS6274053A JP S6274053 A JPS6274053 A JP S6274053A JP 21115985 A JP21115985 A JP 21115985A JP 21115985 A JP21115985 A JP 21115985A JP S6274053 A JPS6274053 A JP S6274053A
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Abstract
Description
【発明の詳細な説明】 （産業上の利用分野） 本発明は高硬化性高強度冷延鋼板に関する。[Detailed description of the invention] (Industrial application field) The present invention relates to a highly hardenable, high strength cold rolled steel sheet.
（従来の技術）
近年、自動車の軽量化および衝突安全性を狙いとして軟
鋼板にかえて外板や内板に高強度鋼板が適用されつ＼あ
る。自動車用鋼板は深絞りなどの厳しい冷間加−［が行
なわれるため、成形性に優れた性質を有することが必要
である。この特性は一般に材料の高強度化によって箸し
く損なわれるため、成形性と強度を満足する鋼材を得る
ことは不可能であった。(Prior Art) In recent years, high-strength steel plates have been used for the outer and inner panels of automobiles in place of mild steel plates with the aim of reducing the weight and crash safety of automobiles. Steel sheets for automobiles are subjected to severe cold working such as deep drawing, so they must have excellent formability. Generally, this property is seriously impaired by increasing the strength of the material, so it has been impossible to obtain a steel material that satisfies formability and strength.
このような問題点を解決するための先行技術として、特
公昭５Ｂ−５０２９１号公報にはＮｂ 、Ｖ等の析出強
化元素を利用し、延性は十分ではないが強度の高い引張
強さ７０ Ｋｇｆ／ｓ１１′級の高強度鋼板、又特公昭
５９−５ｆｔ５４号公報には原子量でＴｉ／Ｃ＞１とし
て深絞り性のすぐれた軟質鋼板が、さらに又、特公昭６
０−１３４２０号公報にはＣｕ、Ｐを利用しテ３５〜４
０ Ｋｇｆ／ＩＩｔｓ’の引張強さと高い深絞り性が得
られる鋼がそれぞれ開示されている。As a prior art to solve such problems, Japanese Patent Publication No. 5B-50291 discloses a method using precipitation strengthening elements such as Nb and V to achieve a tensile strength of 70 Kgf/1, which is high in strength but not sufficiently ductile. S11' grade high-strength steel sheets, and JP-B No. 59-5ft54, a soft steel sheet with an atomic weight of Ti/C > 1 and excellent deep drawability;
Publication No. 0-13420 uses Cu and P to produce Te35-4.
Each of these steels is disclosed in which a tensile strength of 0 Kgf/IIts' and high deep drawability are obtained.
しかしながら１以上の例に示した従来技術ではいずれも
自動車用外板として要求されている高い深絞り性（指標
としてランクフォード値下）Ｔ≧１．５および５０ Ｋ
ｇｆ／１ｒｒ１１′以りの引張強さを兼ね備えた鋼材を
得ることは不可能であった。However, the conventional technologies shown in the above examples all have the high deep drawability (lower than Lankford value as an index) required for automobile outer panels T≧1.5 and 50 K.
It has been impossible to obtain a steel material with a tensile strength greater than gf/1rr11'.
一方、自動型部材の多くは必らずしもすべての部位につ
いて同等の性質を有する必要はない、たとえば、ある部
位は耐プント性の点から高い強度（または硬度）を必要
とするが、他の部分は主としてプレス成形性が要求され
従って延びやＹ値の高いことが必要である。又、自動車
に使われるような板厚の薄い鋼板の接合にはスポット溶
接が多用されているが、高強度鋼板のスポット溶接部の
疲労強度はＩ＆鋼板に比べて向」ニしないため、高強度
鋼板による軒賃化の障害となっていた。On the other hand, many parts of automatic molding parts do not necessarily need to have the same properties in all parts; for example, some parts require high strength (or hardness) from the point of view of Punt resistance, while others This part mainly requires press formability and therefore needs to have high elongation and Y value. In addition, spot welding is often used to join thin steel plates such as those used in automobiles, but the fatigue strength of the spot welded parts of high-strength steel plates is not as good as that of I& steel plates, so high-strength steel plates are often used. This was an obstacle to converting steel plates into rents.
以上のように、自動車用鋼板としては深絞り性にすぐれ
、目、つ硬化性およびスポット溶接部疲労強Ｉ■にすぐ
れている鋼材が必要であるが、従来の鋼では高い深絞り
性を確保することが最初に必要と考えられたため、極低
炭素鋼が主体であった。As mentioned above, steel sheets for automobiles need to have excellent deep drawability, eye hardenability, and spot weld fatigue strength I■, but conventional steels do not have high deep drawability. Ultra-low carbon steel was the main material because it was first thought necessary to
このため硬化性に劣るのみならず、溶接部は著しく粗粒
化するのでスポット溶接部の疲労強度も劣るのが実情で
あった。For this reason, not only is the hardenability inferior, but also the fatigue strength of the spot welds is inferior because the welds become extremely grainy.
（発明が解決しようとする問題点）
本発明は１〜記のような問題に鑑み、特に自動車用鋼板
用として、高い延性と高いＴ値を確保することによ−〕
で成形加Ｉ゛の際の深絞り性にすぐれ、Ｌ２かも成形加
■−後において焼入れ硬化処哩により高強度、就中スポ
ット溶接部の疲労強度を確保することが６丁能な成分組
成、の鋼を提供１．ようとするものである。(Problems to be Solved by the Invention) In view of the problems described in 1 to 1, the present invention has been developed by ensuring high ductility and high T value, especially for steel plates for automobiles.
A component composition that has excellent deep drawability during forming process I, and is capable of ensuring high strength through quench hardening after forming L2, especially fatigue strength at spot welds, Providing steel 1. This is what we are trying to do.
（問題点を解決するための手段）
即ち１．に発明の夛旨は
屯■で
Ｃ： ０．００６〜［＋、０３Ｌ Ｓ ｉ：ｏ、ｏ
ｌ 〜０．ｌｌＭｎ：Ｏ，１〜０．５Ｌ Ｐ ：
０．０８〜０．１２％Ｓ ： ０．０１％以下
−０，０２≦ｖ−（５，８６Ｇ ＋３．８ａＮ） ≦ｏ
、０３％Ａ ｌ：ｏ、０１５〜０．ｌＬ Ｎ ：
０．００１５〜０．０１％Ｂ ： ０．０００２−０．
００１５％、 ＲＦＭ：０．００１０％ 以下残部は
Ｆｅ及び不ｉｉｒ避的不純物からなることを特徴とする
高硬化性高強度冷延鋼板にある。(Means for solving the problem) That is, 1. The purpose of the invention is C: 0.006~[+,03L Si:o,o
l ~0. llMn:O, 1~0.5L P:
0.08~0.12%S: 0.01% or less -0,02≦v-(5,86G +3.8aN)≦o
, 03% Al:o, 015-0. lLN:
0.0015-0.01%B: 0.0002-0.
0.0015%, RFM: 0.0010% and the remainder is a highly hardenable, high-strength cold-rolled steel sheet characterized by being composed of Fe and unavoidable impurities.
以下に、本発明の詳細な説明する。The present invention will be explained in detail below.
（作用）
まず１最初に鋼の各成分についてその限定即山を述べる
。(Function) First, we will discuss the limitations of each component of steel.
Ｃは一般に鋼の強度を、情めるために有効な元素である
が、成形性に対して有害であり、両者を満足するように
決める必要がある。Ｃはｏ、ｏｏｅｘ未満では焼入れ時
に十分な硬度が得られない、またスポット溶接部の熱影
響部が著しく粗粒化して疲労強度を急激に低下させる。C is generally an effective element for improving the strength of steel, but it is harmful to formability, and must be determined to satisfy both. If C is less than o, ooex, sufficient hardness cannot be obtained during quenching, and the heat-affected zone of the spot weld becomes significantly coarse grained, resulting in a sharp decrease in fatigue strength.
深絞り性の指標である丁はＣ０，０３％超で低ドが著し
いため０．０３％以下とした。The carbon content, which is an index of deep drawability, was determined to be 0.03% or less because CO exceeds 0.03% and is extremely low.
次に、Ｓｌは脱酸および鋼の強化元素として有効である
が、深絞り性、スケール性状およびスポット溶接部疲労
強度から、出来るだけ低い方が良＜Ｏ，ｔＸ以下とした
。ＳｌがＯ，１％を超えると特に深絞り性の低下が大き
いためである。一方、０．０１２未満では脱酸および強
化元素としての効果がみられなくなるため０．０１％以
−ヒとする。Next, although Sl is effective as a deoxidizing and steel-strengthening element, from the viewpoint of deep drawability, scale properties, and spot weld fatigue strength, it was determined that it is better to be as low as possible <O, tX or less. This is because when the Sl content exceeds O, 1%, the deep drawability is particularly deteriorated. On the other hand, if it is less than 0.012, it will not be effective as a deoxidizing and reinforcing element, so it should be 0.01% or more.
また、Ｍｎは鋼の強化元素と１７で有効であり０．１ｚ
以１〜添加するが、深絞り性を茗しく損うので０．５ｚ
以ドとする。In addition, Mn is effective as a steel strengthening element and 0.1z
Add 1 or more, but since it seriously impairs deep drawability, 0.5z is added.
This is as follows.
さらに、Ｐは強化元素として有効であると１（にｆγ絞
り性の【気からも添加する心安があるが、０．０６２未
満ではこれらの効果が１〜分でないこと、一方０．１２
％を超えるとスポ−／ ト溶接部の疲労強度を箸しく低
下させるため０．０６〜０．１２％の範囲とする。Furthermore, it is said that P is effective as a reinforcing element, and there is no need to worry about adding it from the viewpoint of fγ squeezability.
If it exceeds 0.0%, the fatigue strength of the sport/sport weld will be significantly lowered, so it should be in the range of 0.06 to 0.12%.
一方、Ｓは不純物でありその含有には低い程望ましいが
、成形性の点から０．０１０％まで許容できるため０．
０１０１以下とした。On the other hand, since S is an impurity, it is desirable that its content be as low as possible, but from the viewpoint of moldability, up to 0.010% is acceptable;
0101 or less.
次に、■は本発明鋼にとって非常に重・要な成分であり
、Ｃ，！：Ｎとの関連で特定の関係を満足させることが
必要である。このような関係は次のような実験により求
められたものである。Next, ■ is a very important component for the steel of the present invention, and C,! : It is necessary to satisfy a specific relationship in relation to N. Such a relationship was determined through the following experiment.
即ち、第１図はＣＯ，０１１、Ｓ ｉ ０．０２％ 、
Ｍ ｎｏ、２％、 Ｐｏ、０８０％、 Ｓ ０．００
５ｔ、 Ｖ Ｏ〜Ｏ，１２％ 、 Ａ Ｉｏ、０２０％
、 ＮＯ，００２９％ 、 ８０．００１０％ テ残
部はＦｅ及び不可避的不純物からなる４、５ｍｓ厚さの
熱延鋼帯を１．ｏ＋＋ｍに冷間圧延して８５０℃Ｘ１分
の焼鈍を行なった後、１ｚの調質圧送を行なった冷ｋＥ
ｍ ？ｉＦのＶ％と深絞り指標である〒″値との関係
である。即ち、本発明鋼においては強い炭化物生成元素
であるＶの添加によってＣやＮを（ν化物としいわゆる
ｓｃａｖｅｎｇｉｎｇ効果によりｆ；４．１着してｒ伯
奢改ハするが、同図に見られるように過剰になるとその
効果はむしろ低下することがわかる。That is, FIG. 1 shows CO,011, S i 0.02%,
M no, 2%, Po, 080%, S 0.00
5t, VO~O, 12%, A Io, 020%
, NO, 0029%, 80.0010% The remainder consists of Fe and unavoidable impurities. After cold rolling to o++m and annealing at 850°C for 1 minute, cold kE was subjected to 1z temper pumping.
M? This is the relationship between V% of iF and 〒″ value, which is a deep drawing index.In other words, in the steel of the present invention, by adding V, which is a strong carbide-forming element, C and N (v) are converted to f; by the so-called scavenging effect. When it reaches 4.1, it improves, but as seen in the same figure, if it becomes excessive, its effect actually decreases.
なお、この場合ＶとＣ，Ｈの関係をＶ −（５，Ｅｉ６
Ｃ＋ ３．６６Ｃ＋３．６４Ｎ ）で整理したのは化学
当量的に添加したＶｚとＣとＮを炭窒化物として固着し
たｖｚとの差、即ち、地鉄中に固溶しているＶを特定す
るものであり、またＣの係数を５．８８．Ｈの係数を３
．６４と定めたのは生成する炭化物ηＣ３および窒化物
ｖＮニ見合つＶ、！＝ １．テ５０．９Ｘ４／１２Ｘ３
＝５．８８オよび５０．９／ １４＝ ３．６６Ｃ＋
３．６４より求めたものである。In this case, the relationship between V, C, and H is expressed as V − (5, Ei6
C+ 3.66C+3.64N) is the difference between Vz added chemically equivalently and vz fixed with C and N fixed as carbonitrides, that is, the V dissolved in the steel base is identified. and the coefficient of C is 5.88. The coefficient of H is 3
．． 64 was determined as V, which is commensurate with the generated carbide ηC3 and nitride vN! = 1. Te50.9X4/12X3
=5.88o and 50.9/14=3.66C+
3.64.
このようにＶは本発明の主体をなす元素であり、第１図
で示したように１．５以上の高いＴ値を得ルタメニハ、
Ｖ −（５，８６Ｃ＋ ３．６６Ｃ＋３．６４Ｎ ）が
−０，０２２以上のＶ添加が必要である。■を適正量を
超えて加えるとＴ値はかえって低下するため、■−（５
，６６Ｃ＋ ３．６６Ｃ＋３．６４Ｎ ）を０．０３％
以下とする必要がある。In this way, V is the main element of the present invention, and as shown in FIG.
It is necessary to add V so that V − (5,86C+3.66C+3.64N) is −0,022 or more. If ■ is added in excess of the appropriate amount, the T value will actually decrease, so ■ - (5
, 66C+ 3.66C+3.64N) by 0.03%
It is necessary to do the following.
また、ＡＩは脱酸のために必要な元素であり、０．０１
５％より少ないと脱酸が不十分で酸化物系介在物により
深絞り性を損う、また、 ０．１！より多いと介在物に
よってＴ値をかえって低下させるため０．０１５〜０．
１ｚの範囲とする。In addition, AI is an element necessary for deoxidation, and 0.01
If it is less than 5%, deoxidation is insufficient and deep drawability is impaired due to oxide inclusions, and 0.1! If the amount is more than 0.015 to 0.015, the T value will be lowered by inclusions.
The range is 1z.
さらに、Ｎは焼鈍時に生成するＡＩＮによってＴ値を向
上させるが、０．００１５Ｘ未満では焼鈍中に十分なＡ
ＩＮが生成できないこと、また０、０１＄超ではかえっ
てＴ値を低下させるため０．００１５〜０．０１１とし
た。Furthermore, N improves the T value by AIN generated during annealing, but if it is less than 0.0015X, sufficient A
The value was set at 0.0015 to 0.011 because IN cannot be generated and if it exceeds 0.01 $, the T value will decrease.
次にＢは焼入れ性を上げるために添加する。Next, B is added to improve hardenability.
０．００１５％超ではＢＮを生成して靭性及び７値を低
下させるが、一方、 ０．０００２Ｘ未満では焼入れに
対する効果がみられないため０．０００２〜Ｏ，０Ｏ１
５％の範囲で添加する。If it exceeds 0.0015%, BN will be generated and the toughness and 7 value will decrease. On the other hand, if it is less than 0.0002X, no effect on quenching will be seen, so 0.0002~O,0O1
Add in a range of 5%.
最後にＲＥＭは脱酸剤として、また鋼中硫化物の形状の
調整を行なわせるため、 ０．００１０％以下添加する
。 ０．０ＯＩＯ＄を超えて添加するとかえって成形性
を害するためである。Finally, REM is added in an amount of 0.0010% or less as a deoxidizer and to adjust the shape of sulfides in the steel. This is because adding more than 0.0 OIO$ will actually impair moldability.
次に、本発明鋼の製造条件の一例について示す。Next, an example of manufacturing conditions for the steel of the present invention will be shown.
製鋼・造塊条件は通常の工程を用いるが、Ｃや＋
Ｎを低くコトロールするためにＲ１（またはＤＨ脱ガス
装置を用いて脱ガス、脱炭処理を行なった後成分調整し
連続鋳造により鋼片とすることが有効である。The steelmaking and ingot making conditions are the same as normal processes, but in order to control C and +N to a low level, R1 (or DH degassing equipment is used to perform degassing and decarburization treatment, and then the composition is adjusted, and steel is made by continuous casting. It is effective to make it a piece.
また、熱延条件は炭ｅ窒化物を必要以上に溶解させない
ため、１０５０〜１１５０℃程度の低温加熱を行なって
熱延鋼帯とするのがよい、この熱間圧延の際、捲取温度
を６００℃以上の高温捲取とすれば、炭化物の析出と粗
大化を促進させることができるためＴ値の向上に有効で
ある。In addition, in order to avoid dissolving carbon-e-nitrides more than necessary, it is preferable to heat the hot-rolled steel strip at a low temperature of about 1050 to 1150°C. High-temperature winding at 600° C. or higher is effective in improving the T value because precipitation and coarsening of carbides can be promoted.
次に、冷間圧延は出来るだけＹ値を高くさせるためには
圧下率が高い方が好ましいので６０〜９５％で行なうの
がよい、さらに焼鈍は連続焼鈍炉においてＴ値を高くす
るため７５０〜８９０℃の高温焼鈍を行なうのが適当で
ある。すなわち、７５０℃未満では再結晶がｄれＴ値は
向上しない、また８９０℃を超えると変態を生じるため
Ｔ値は低下する。Next, in order to increase the Y value as high as possible, cold rolling is preferably carried out at a rolling reduction rate of 60 to 95%, and furthermore, annealing is performed in a continuous annealing furnace at a rolling reduction rate of 750 to 95% in order to increase the T value. It is appropriate to perform high temperature annealing at 890°C. That is, if the temperature is lower than 750°C, recrystallization will not occur and the T value will not improve, and if the temperature exceeds 890°C, transformation will occur and the T value will decrease.
以−ヒのようにして得られた鋼は、ＣやＮが炭・窒化物
として固着され、成形時には高い延性と７値を示すもの
であるが、その後プレス加工を行なった後、耐プント性
など強度の要求される部分を通常の加熱手段により鋼の
変態点以上に加熱した後急冷せしめることによりその部
分を焼き入れ硬化させることができる。また、この鋼は
スポット溶接部の疲労強度にもすぐれているので、従来
の析出強化型高強度鋼板のようなスポット溶接疲労強度
１の問題がないため重体セ量の軽量化に有効である。The steel obtained as described above has C and N fixed as carbon and nitrides, and exhibits high ductility and 7 value during forming, but after press working, it has poor Punt resistance. By heating a part where strength is required, such as a steel part, to a temperature above the transformation point of the steel using ordinary heating means and then rapidly cooling it, that part can be quenched and hardened. Furthermore, this steel has excellent fatigue strength at spot welds, so it is effective in reducing the weight of heavy steel because it does not have the problem of spot weld fatigue strength 1 unlike conventional precipitation-strengthened high-strength steel plates.
次に本発明の効果を実施例にてさらに具体的に説明する
。Next, the effects of the present invention will be explained in more detail with reference to Examples.
（実施Ｎ）
第１表に示す化学成分の鋼を溶製しＲＨで脱ガスを行な
い、連続鋳造により２００■の鋼片を製造した。これを
１１００℃に加熱して６■の熱延鋼帯としｆ（２０℃で
捲取った後、８５％の冷延圧下率によって１．０■の冷
延鋼板とした。この冷延板を８１０″ａＸ１分の焼鈍を
行なった後、ｌＯ℃／ｓｅｃで冷却、 ４００’ＣＸＳ
分の過時効処理を行ない、１ｚの：Ａ賀圧延を与えた。(Execution N) Steel having the chemical composition shown in Table 1 was melted, degassed with RH, and 200 square pieces of steel were manufactured by continuous casting. This was heated to 1100°C to form a 6-inch hot-rolled steel strip f (after rolling it at 20°C, a 1.0-inch cold-rolled steel plate was obtained by a cold rolling reduction of 85%. After annealing at 810"aX for 1 minute, cooling at 10°C/sec, 400'CXS
An over-aging treatment of 100 min was carried out to give 1z: Aga rolling.
これらの鋼板の特性値を第１表に併せて示す。本発明鋼
は伸び、Ｔ値にすぐれていることがわかる。The characteristic values of these steel plates are also shown in Table 1. It can be seen that the steel of the present invention has excellent elongation and T value.
次にこれらの鋼板の硬化性を調べるため８６０℃に加熱
した大気炉に装入、５秒保持した後Ｏ℃の氷水中に急冷
した。これらの鋼板の表面硬度をピンカース硬度計で測
定した結果を第１表に併せて示す０本発明鋼の硬化性が
すぐれていることがわかる。Next, in order to examine the hardenability of these steel plates, they were placed in an atmospheric furnace heated to 860°C, held for 5 seconds, and then rapidly cooled in ice water at 0°C. The surface hardness of these steel plates was measured using a Pinkers hardness tester. The results are also shown in Table 1. It can be seen that the hardenability of the steel of the present invention is excellent.
第１図はｖＱとＹ値との１″Ａ係を示す図である。 FIG. 1 is a diagram showing the 1″A relationship between vQ and Y value.
Claims (1)
％Ｍｎ：０．１〜０．５％、Ｐ：０．０６０〜０．１２
％Ｓ：０．０１％以下 −０．０２≦Ｖ−（５．６６Ｃ＋３．６４Ｎ）≦０．０
３％Ａｌ：０．０１５〜０．１％、Ｎ：０．００１５〜
０．０１％Ｂ：０．０００２〜０．００１５％、ＲＥＭ
：０．００１０％以下残部はＦｅ及び不可避的不純物か
らなることを特徴とする高硬化性高強度冷延鋼板。[Claims] C: 0.006 to 0.03%, Si: 0.01 to 0.1 in weight%
%Mn: 0.1-0.5%, P: 0.060-0.12
%S: 0.01% or less -0.02≦V-(5.66C+3.64N)≦0.0
3% Al: 0.015~0.1%, N: 0.0015~
0.01%B: 0.0002-0.0015%, REM
: 0.0010% or less, the remainder consisting of Fe and inevitable impurities.A highly hardenable, high strength cold rolled steel sheet.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| JP21115985A JPS6274053A (en) | 1985-09-26 | 1985-09-26 | High-strength cold-rolled steel sheet having high hardenability |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| JP21115985A JPS6274053A (en) | 1985-09-26 | 1985-09-26 | High-strength cold-rolled steel sheet having high hardenability |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| JPS6274053A true JPS6274053A (en) | 1987-04-04 |
Family
ID=16601370
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| JP21115985A Pending JPS6274053A (en) | 1985-09-26 | 1985-09-26 | High-strength cold-rolled steel sheet having high hardenability |
Country Status (1)
| Country | Link |
|---|---|
| JP (1) | JPS6274053A (en) |
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| WO2002044434A1 (en) * | 2000-11-28 | 2002-06-06 | Kawasaki Steel Corporation | Composite structure type high tensile strength steel plate, plated plate of composite structure type high tensile strength steel and method for their production |
-
1985
- 1985-09-26 JP JP21115985A patent/JPS6274053A/en active Pending
Cited By (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| WO2002044434A1 (en) * | 2000-11-28 | 2002-06-06 | Kawasaki Steel Corporation | Composite structure type high tensile strength steel plate, plated plate of composite structure type high tensile strength steel and method for their production |
| EP1338667A1 (en) * | 2000-11-28 | 2003-08-27 | Kawasaki Steel Corporation | Composite structure type high tensile strength steel plate, plated plate of composite structure type high tensile strength steel and method for their production |
| EP1338667A4 (en) * | 2000-11-28 | 2005-08-17 | Jfe Steel Corp | Composite structure type high tensile strength steel plate, plated plate of composite structure type high tensile strength steel and method for their production |
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