JP2005133155A - High strength and high toughness non-heat-treated bar steel, and its production method - Google Patents

High strength and high toughness non-heat-treated bar steel, and its production method Download PDF

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JP2005133155A
JP2005133155A JP2003370738A JP2003370738A JP2005133155A JP 2005133155 A JP2005133155 A JP 2005133155A JP 2003370738 A JP2003370738 A JP 2003370738A JP 2003370738 A JP2003370738 A JP 2003370738A JP 2005133155 A JP2005133155 A JP 2005133155A
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JP4171398B2 (en
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Toshio Murakami
俊夫 村上
Shigenobu Nanba
茂信 難波
Masao Toyama
雅雄 外山
Yoshiteru Fukuoka
義晃 福岡
Masami Somekawa
雅実 染川
Shoichi Ikeda
正一 池田
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Kobe Steel Ltd
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Abstract

<P>PROBLEM TO BE SOLVED: To provide high strength and high toughness non-heat-treated bar steel in which both of strength and toughness are increased, and the balance of the strength and toughness is excellent, and to provide its production method. <P>SOLUTION: The bar steel contains, by mass, 0.35 to 0.50% C, 0.1 to 0.6% Si, 0.5 to 1.5% Mn, 0.005 to 0.02% Al and 0.05 to 0.50% V, and the balance iron with inevitable impurities, and has a steel structure composed of a ferrite-pearlite dual phase structure in which the fraction of ferrite is 20 to 40%, the average lamellar spacing of pearlite is 0.05 to 0.20 μm, and the average grain size of the ferrite surrounded by large-angle grain boundaries having a crystal orientation difference of ≥15° is 2 to 10 μm. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

本発明は、強度と靱性をともに高くし、強度と靱性とのバランスに優れた高強度高靱性非調質棒鋼およびその製造方法に関するものである。   The present invention relates to a high-strength, high-toughness non-heat treated steel bar that has both high strength and toughness and excellent balance between strength and toughness, and a method for producing the same.

自動車を初めとする構造材料の鋼材(構造用鋼)として、フェライト−パーライト複相組織からなる棒鋼などが用いられている。これら棒鋼は、冷・温間鍛造された後に、主として高周波焼入れされ、各種CVJ部品やIQ歯車類として製造されている。   As steel materials (structural steel) for automobiles and other structural materials, steel bars made of a ferrite-pearlite double phase structure are used. These steel bars are cold-warm-forged and then induction hardened to produce various CVJ parts and IQ gears.

これらの棒鋼には、軽量化のために、一層の高強度化が求められている。しかし、一般的に、鋼を高強度化すると靱性の方が低下する。このため、フェライト−パーライト複相組織からなる棒鋼でも、高強度化して前記構造材料として適用するためには、少なくとも、従来使用していた鋼材と同等の製品靱性(冷・温間鍛造後の製品靱性)を確保する必要がある。   These steel bars are required to have higher strength in order to reduce weight. However, generally, when the strength of steel is increased, the toughness decreases. For this reason, in order to increase the strength of a steel bar made of a ferrite-pearlite multiphase structure and apply it as the structural material, at least the product toughness (the product after cold / warm forging) equivalent to that of the steel material used conventionally. Toughness) must be ensured.

従来から、鋼材の強化方法としては、固溶強化や、マルテンサイト等との複合組織化による第2相による強化、析出強化、結晶粒の微細化などが良く知られている。   Conventionally, as a steel material strengthening method, solid solution strengthening, strengthening by a second phase by forming a composite structure with martensite, precipitation strengthening, refinement of crystal grains, and the like are well known.

これらの強化方法の中でも、強度と靱性をともに高くし、強度−靱性バランスを良好にする方法としては、結晶粒の微細化が最も優れた方法である。この方法では、焼き入れ性を高めるNi,Cr等の高価な元素の添加を必要としないために、低コストで高強度鋼材の製造が可能とされている。   Among these strengthening methods, the refinement of crystal grains is the most excellent method for increasing both strength and toughness and improving the strength-toughness balance. This method does not require the addition of expensive elements such as Ni and Cr that enhance the hardenability, so that it is possible to produce a high-strength steel material at a low cost.

このため、フェライト−パーライト複相組織からなる棒鋼でも、従来から、例えば、仕上げ圧延温度を850〜950℃に制御することで、パーライトコロニーサイズを6番以上に微細化させ、強度−靱性バランスを向上させることが提案されている (特許文献1参照) 。   For this reason, even in a steel bar composed of a ferrite-pearlite multiphase structure, conventionally, for example, by controlling the finish rolling temperature to 850 to 950 ° C., the pearlite colony size is refined to 6 or more, and the strength-toughness balance is improved. It has been proposed to improve (see Patent Document 1).

また、旧γ粒径を5番以上に微細化させ、強度−靱性バランスを向上させることも提案されている (特許文献2、3参照) 。   It has also been proposed to refine the old γ grain size to 5 or more and improve the strength-toughness balance (see Patent Documents 2 and 3).

更に、鋼片を1050℃以上に加熱後、仕上げ圧延温度を650〜800℃に制御することで、フェライト分率をC含有量から定まる所定値以上で、フェライト粒度を10番以上に微細化させ、強度−靱性バランスを向上させることも提案されている (特許文献4参照) 。
特許1278456 公報(1〜3 頁) 特許161327号公報(1〜3 頁) 特許1845976 号公報(1〜3 頁) 特公平4 −25343 号公報(1〜3 頁)
Furthermore, after the steel slab is heated to 1050 ° C. or higher, the finish rolling temperature is controlled to 650 to 800 ° C., so that the ferrite fraction is refined to 10th or more with the ferrite fraction being a predetermined value or more determined from the C content. It has also been proposed to improve the strength-toughness balance (see Patent Document 4).
Japanese Patent No. 1278456 (1-3 pages) Japanese Patent No. 161327 (pages 1 to 3) Japanese Patent No. 1845976 (1-3 pages) Japanese Examined Patent Publication No. 4-25343 (pages 1 to 3)

また、一方で、強度と延性とのバランスを向上させるために、結晶方位差 (粒界の方位差角) が15°以上の大角粒界で囲まれたフェライトの平均粒径を例えば5μm以下に微細化させた、700MPa前後の引張強度の低炭素フェライト−パーライト複相組織鋼なども提案されている (特許文献5、6、7など参照) 。
特開平11-92855公報(1〜4 頁) 特開平11-315342 公報(1〜6 頁) 特開2000-309822 公報(1〜4 頁)
On the other hand, in order to improve the balance between strength and ductility, the average grain size of ferrite surrounded by large-angle grain boundaries whose crystal orientation difference (orientation angle of grain boundaries) is 15 ° or more is, for example, 5 μm or less. A refined low-carbon ferrite-pearlite double phase steel having a tensile strength of around 700 MPa has also been proposed (see Patent Documents 5, 6, 7, etc.).
JP-A-11-92855 (pages 1 to 4) Japanese Patent Laid-Open No. 11-315342 (pages 1-6) Japanese Patent Laid-Open No. 2000-309822 (pages 1 to 4)

しかし、これらの方法でも、C含有量が0.35〜0.50%の中炭素のフェライト−パーライト複相組織鋼として、例えば、最低でも引張強度で950MPa、靱性( uE20)で100J以上の、強度と靱性とのバランスに優れた高強度高靱性非調質鋼は得られていない。   However, even in these methods, as a medium-carbon ferrite-pearlite double phase structure steel having a C content of 0.35 to 0.50%, for example, at least 950 MPa in tensile strength and 100 J or more in toughness (uE20), A high-strength, high-toughness non-tempered steel excellent in balance between strength and toughness has not been obtained.

本発明はこのような課題を解決するためになされたものであって、強度と靱性をともに高くし、強度と靱性とのバランスに優れた高強度高靱性非調質棒鋼およびその製造方法を提供することである。   The present invention has been made to solve such a problem, and provides a high-strength, high-toughness non-heat treated steel bar having both high strength and toughness and excellent balance between strength and toughness, and a method for producing the same. It is to be.

この目的を達成するために、本発明の高強度高靱性非調質棒鋼の要旨は、質量%で、C:0.35〜0.50%、Si:0.1〜0.6%、Mn:0.5〜1.5%、Al:0.005〜0.02%、V:0.05〜0.50%を含み、残部鉄及び不可避的不純物からなり、鋼組織における、フェライト分率が20〜40%、パーライトの平均ラメラ間隔が0.05〜0.20μm、結晶方位差が15°以上の大角粒界で囲まれたフェライトの平均粒径が2〜10μmであるフェライト−パーライト複相組織からなることとする。   In order to achieve this object, the gist of the high-strength, high-toughness non-heat treated steel bar of the present invention is mass%, C: 0.35 to 0.50%, Si: 0.1 to 0.6%, Mn : 0.5 to 1.5%, Al: 0.005 to 0.02%, V: 0.05 to 0.50%, consisting of the remaining iron and inevitable impurities, the ferrite fraction in the steel structure Is 20 to 40%, the average lamella spacing of pearlite is 0.05 to 0.20 μm, and the average grain size of ferrite surrounded by large-angle grain boundaries with a crystal orientation difference of 15 ° or more is 2 to 10 μm. It shall consist of a phase organization.

また、この目的を達成するために、本発明の高強度高靱性非調質棒鋼の製造方法の要旨は、質量%で、C:0.35〜0.50%、Si:0.1〜0.6%、Mn:0.5〜1.5%、Al:0.005〜0.02%、V:0.05〜0.50%を含む鋼を、750〜900℃の開始温度で仕上げ圧延し、仕上げ圧延終了後10秒以内に平均冷却速度が10℃/秒以上で急冷を開始して500〜700℃の温度まで冷却し、その後、200℃まで平均冷却速度が0.1〜5℃/秒で冷却することとする。   Moreover, in order to achieve this object, the summary of the manufacturing method of the high-strength, high-toughness non-tempered steel bar of the present invention is mass%, C: 0.35-0.50%, Si: 0.1-0. Steel containing 1.6%, Mn: 0.5-1.5%, Al: 0.005-0.02%, V: 0.05-0.50% at a starting temperature of 750-900 ° C Within 10 seconds after finishing the finish rolling, rapid cooling is started at an average cooling rate of 10 ° C./sec or more to cool to a temperature of 500 to 700 ° C., and then the average cooling rate to 200 ° C. is 0.1 to 5 Cool at a rate of ° C / second.

本発明者らは、析出強化元素や固溶強化元素を含有させるとともに、引張強度で950MPa以上に高強度化させたフェライト−パーライト複相組織からなる中炭素鋼において、 uE20で100J以上に高靱性化させるためには、結晶方位差が15°以上の大角粒界で囲まれたフェライト(以下、有効フェライトとも言う)の平均粒径を制御することが重要であることを知見した。   The present inventors have included a precipitation strengthening element or a solid solution strengthening element, and in a medium carbon steel composed of a ferrite-pearlite multiphase structure whose tensile strength is increased to 950 MPa or more, the toughness of uJ20 is 100 J or more. In order to reduce the average grain size, it was found important to control the average grain size of ferrite surrounded by large-angle grain boundaries with a crystal orientation difference of 15 ° or more (hereinafter also referred to as effective ferrite).

即ち、この有効フェライトは、フェライト−パーライト複相組織からなる高強度化させた中炭素鋼において、靱性を向上させる支配因子である。したがって、有効フェライトの平均粒径を2〜10μmの範囲に制御および微細化させることで、上記高強度化した場合に、同時に、高靱性化することができ、強度−靱性バランスを向上させることができる。   That is, this effective ferrite is a controlling factor for improving toughness in a high-strength medium carbon steel composed of a ferrite-pearlite double phase structure. Accordingly, by controlling and refining the average particle diameter of effective ferrite in the range of 2 to 10 μm, when the strength is increased, the toughness can be increased at the same time, and the strength-toughness balance can be improved. it can.

因みに、この有効フェライトの平均粒径制御による高靱性化は、前記した700MPa前後の低強度の低炭素フェライト−パーライト複相組織鋼には顕著には見られない、950MPa以上に高強度化させた中炭素以上のフェライト−パーライト複相組織鋼に特有の現象である。   Incidentally, the increase in toughness by controlling the average particle diameter of the effective ferrite has been increased to 950 MPa or more, which is not remarkably observed in the low-strength low-carbon ferrite-pearlite double phase steel having a strength of around 700 MPa. This is a phenomenon peculiar to ferrite-pearlite double phase steel with medium carbon or higher.

(鋼組織)
本発明の高強度高靱性非調質棒鋼の組織の要件について、以下に図1を用いて順次説明する。図1はフェライト−パーライト複相組織の模式図であり、1個の旧γ粒を模式的に示している。図中、略6角形の一点鎖線が旧γ粒径(粒界)であり、この一点鎖線で囲まれた領域が旧γ粒である。この旧γ粒径に沿って、パーライトを囲む形でフェライト粒群が順次配列して析出している。一方、一点鎖線で囲まれた領域内がパーライトである。パーライト組織は、鋼をオーステナイト状態から冷却した時、共析変態によって得られる、フェライトとセメンタイトとが互いに層状に並んだ組織(ラメラ組織)である。
(Steel structure)
The requirements for the structure of the high strength and high toughness non-heat treated steel bar of the present invention will be sequentially described below with reference to FIG. FIG. 1 is a schematic diagram of a ferrite-pearlite multiphase structure, schematically showing one old γ grain. In the figure, a substantially hexagonal one-dot chain line is an old γ grain size (grain boundary), and a region surrounded by the one-dot chain line is an old γ grain. Along the old γ grain size, ferrite grain groups are sequentially arranged and precipitated so as to surround the pearlite. On the other hand, the area surrounded by the alternate long and short dash line is pearlite. The pearlite structure is a structure (lamellar structure) obtained by eutectoid transformation when steel is cooled from an austenite state and in which ferrite and cementite are arranged in layers.

このパーライト領域内において、種々の方向性を有する平行な斜線は、上記したセメンタイトとフェライトとが層状に並ぶラメラを示す。ここで、この平行な斜線間の間隔がパーライトのラメラ間隔であり、これら個々のラメラ間隔を平均化したものが平均ラメラ間隔である。更に、このラメラの向きが揃った領域(同じ方向性を有する平行な斜線で示す領域)がコロニーであり、点線で囲んだ領域が、フェライトの方位が揃った1個乃至複数のコロニーからなるノジュールである。図1では、1個〜4個のコロニーからなる合計4個のノジュールを示している。   In the pearlite region, parallel oblique lines having various directions indicate lamellae in which cementite and ferrite are arranged in layers. Here, the interval between the parallel oblique lines is the pearlite lamella interval, and the average of these individual lamella intervals is the average lamella interval. Further, a region where the lamellas are aligned (regions indicated by parallel diagonal lines having the same direction) is a colony, and a region surrounded by a dotted line is a nodule composed of one or a plurality of colonies where the orientation of ferrite is aligned. It is. FIG. 1 shows a total of four nodules composed of one to four colonies.

(有効フェライト)
このノジュールが本発明で言う、結晶方位差が15°以上の大角粒界で囲まれたフェライト(有効フェライト)である。更に、前記した、旧γ粒径に沿って配列して析出しているフェライトの内、結晶方位差が15°以上の大角粒界で囲まれたフェライトが、本発明で言う有効フェライトである。本発明では、これらの有効フェライトの平均粒径を2〜10μmとする。
(Effective ferrite)
This nodule is a ferrite (effective ferrite) surrounded by large-angle grain boundaries whose crystal orientation difference is 15 ° or more, as referred to in the present invention. Further, among the ferrites arranged and precipitated along the old γ grain size, the ferrite surrounded by the large-angle grain boundaries having a crystal orientation difference of 15 ° or more is an effective ferrite referred to in the present invention. In the present invention, the average particle diameter of these effective ferrites is 2 to 10 μm.

本発明では、これらの有効フェライトを上記範囲に微細化させることで、延性−脆性遷移温度が低下し、室温靱性・低温靱性が向上する。有効フェライトの平均粒径が2μm未満では、有効フェライトが微細化されすぎ、遷移温度が低下して、低温靱性は向上するが、室温靱性が低下し、高靱性化させることが困難となる。有効フェライトの平均粒径が10μmを越えた場合、例えば950MPa以上に高強度化した場合に、室温靱性・低温靱性の改善効果がともに不足し、例えば uE20で100J以上に高靱性化させることが困難となる。   In the present invention, by reducing these effective ferrites to the above range, the ductile-brittle transition temperature is lowered, and the room temperature toughness and the low temperature toughness are improved. If the average particle diameter of the effective ferrite is less than 2 μm, the effective ferrite is excessively refined, the transition temperature is lowered, and the low temperature toughness is improved, but the room temperature toughness is lowered and it becomes difficult to increase the toughness. When the average particle diameter of the effective ferrite exceeds 10 μm, for example, when the strength is increased to 950 MPa or more, both the room temperature toughness and the low temperature toughness are not sufficiently improved. For example, it is difficult to increase the toughness to 100 J or more with uE20. It becomes.

結晶方位差が15°以上の大角粒界の方位は、棒鋼の代表的な部位あるいは複数の部位における、0.1×0.1mmの数視野を1万倍のTEM(透過型電子顕微鏡)で観察し、1視野につき数百個のフェライト粒を電子線後方散乱回折(EBSD)法で測定することができる。この際、結晶方位差(フェライトの粒界の方位差角)は15°以上であるときを大角粒界とする。また、大角粒界が全粒界の70%以上を占めるとき、組織は大角粒界からなっているとする。大角粒界の割合が70%未満の時は、有効フェライトの微細化による強度上昇効果が十分ではない。   The orientation of a large-angle grain boundary with a crystal orientation difference of 15 ° or more is measured by a TEM (transmission electron microscope) with a 10,000 × magnification of several fields of 0.1 × 0.1 mm in a representative part or a plurality of parts of a steel bar. By observing, several hundred ferrite grains per field of view can be measured by the electron beam backscatter diffraction (EBSD) method. At this time, when the crystal orientation difference (orientation difference angle of the grain boundary of ferrite) is 15 ° or more, the large-angle grain boundary is defined. Further, when the large-angle grain boundary occupies 70% or more of the whole grain boundary, the structure is assumed to be composed of the large-angle grain boundary. When the ratio of large-angle grain boundaries is less than 70%, the effect of increasing the strength due to the refinement of effective ferrite is not sufficient.

なお、有効フェライトの平均粒径は、5000倍のSEM(走査型電子顕微鏡:JEOL社製 JSM-5410 )に、TSL 社製OIM TMを用いて、隣接した結晶粒と15度以上の方位差がある粒界を測定し、例えば、図1に一点鎖線で示す粒界を横断する一定長さ(例えば100μm)の直線lを引き、この直線lによって切断される前記粒界数で直線lの前記長さを除す、所謂切片法あるいは直線切断法により計測する。 The effective ferrite has an average grain size of 15 degrees or more from the adjacent crystal grains using an SEM (scanning electron microscope: JSM-5410 made by JEOL) and OIM TM made by TSL. A certain grain boundary is measured, and for example, a straight line l of a certain length (for example, 100 μm) crossing the grain boundary shown by a one-dot chain line in FIG. It is measured by the so-called intercept method or straight line cutting method, which removes the length.

この他、本発明非調質棒鋼の組織は、フェライト分率が20〜40%であるフェライト−パーライト複相組織からなる。フェライト単相の場合、あるいはフェライト分率が40%を超えた場合、例えば950MPa以上に高強度化することが困難となる。一方、パーライト分率の上昇にともなって、強度が向上するが、フェライト分率が20%未満では、言い換えるとパーライトの分率が80%を超えると、変形能の大きいフェライトの量が不足するため、例えば uE20で100J以上に高靱性化させることが困難となる。   In addition, the structure of the non-heat treated steel bar of the present invention is composed of a ferrite-pearlite double phase structure having a ferrite fraction of 20 to 40%. In the case of a ferrite single phase, or when the ferrite fraction exceeds 40%, it becomes difficult to increase the strength to, for example, 950 MPa or more. On the other hand, as the pearlite fraction increases, the strength improves. However, if the ferrite fraction is less than 20%, in other words, if the pearlite fraction exceeds 80%, the amount of ferrite having a large deformability is insufficient. For example, it becomes difficult to increase the toughness to 100 J or more with uE20.

本発明のフェライト分率は、5000倍のSEM(走査型電子顕微鏡:JEOL社製 JSM-5410 )を用いて3視野測定した。これを画像解析ソフト(MEDIA CYBERNETICS TM社製Image-Pro Prus)で、前記SEMで観察した視野における(前記図1で言う)旧γ粒径に沿って順次配列して析出している前記フェライト粒群の合計測定面積と、パーライト測定面積との合計面積に対する、前記フェライト粒群の合計面積の分率で表される。 The ferrite fraction of the present invention was measured in three fields using a 5000-times SEM (scanning electron microscope: JSM-5410 manufactured by JEOL). The ferrite grains deposited and arranged sequentially along the old γ grain size (referred to in FIG. 1) in the field of view observed by the SEM with image analysis software (Image-Pro Prus manufactured by MEDIA CYBERNETICS ). It is represented by a fraction of the total area of the ferrite grain group with respect to the total area of the total measurement area of the group and the pearlite measurement area.

また、本発明複相組織において、前記したパーライトの平均ラメラ間隔は0.05〜0.20μmとする。平均ラメラ間隔を微細化するほど強度は増すが、変態組織として得られる平均ラメラ間隔は0.05μm程度が限界であり、これを下限とする。一方、平均ラメラ間隔は0.20μmを超えた場合、例えば950MPa以上に高強度化することが困難となる。この平均ラメラ間隔も、前記SEMで観察した視野における個々のラメラ間隔を測定した上で平均化して測定される。   In the multiphase structure of the present invention, the average lamella spacing of the pearlite is 0.05 to 0.20 μm. Although the strength increases as the average lamella spacing becomes finer, the average lamella spacing obtained as a transformed tissue is limited to about 0.05 μm, and this is the lower limit. On the other hand, when the average lamella spacing exceeds 0.20 μm, it becomes difficult to increase the strength to, for example, 950 MPa or more. This average lamella interval is also measured by averaging the individual lamella intervals in the visual field observed with the SEM.

(鋼の化学成分組成)
以下に、上記本発明組織として、引張強度で950MPa以上に高強度化させた上で、 uE20で100J以上に高靱性化させ、構造用鋼としての特性を具備するために必要な、あるいは好ましい、非調質棒鋼の化学成分組成と、各元素の限定理由を説明する。
(Chemical composition of steel)
In the following, the structure of the present invention is necessary or preferable to increase the tensile strength to 950 MPa or more and toughness to 100 J or more with uE20 and to have the characteristics as a structural steel. The chemical composition of the non-tempered steel bar and the reasons for limitation of each element will be described.

本発明非調質棒鋼の基本的な化学成分組成は、構造用鋼としての特性を具備するために、質量%で、C:0.35〜0.50%、Si:0.1〜0.6%、Mn:0.5〜1.5%、Al:0.005〜0.02%、V:0.05〜0.50%を含み、残部鉄及び不可避的不純物からなることが必要である。そして、必要により、この基本成分組成に、更に、Cr:0.60%以下、Mo:0.5%以下、Ni:1%以下、Cu:1%以下、Ti:0.2%以下、Nb:0.10%以下、の1種または2種以上を含有させる。   The basic chemical component composition of the non-heat treated steel bar of the present invention is, in mass%, C: 0.35-0.50%, Si: 0.1-0. 6%, Mn: 0.5 to 1.5%, Al: 0.005 to 0.02%, V: 0.05 to 0.50%, and the balance iron and inevitable impurities are required. is there. And, if necessary, this basic component composition is further added to Cr: 0.60% or less, Mo: 0.5% or less, Ni: 1% or less, Cu: 1% or less, Ti: 0.2% or less, Nb : 0.10% or less of 1 type or 2 types or more.

(C:0.35〜0.50%)
Cは、経済的かつ有効な強化元素であり、フェライト分率を低下させ、強度を上昇させる効果がある。また、高周波焼入れ性を向上させる効果もある。この効果を十分に発揮させるにはCの含有量の下限が0.35%以上の中炭素鋼とする必要がある。しかし、Cの含有量が上限の0.50%を超えて高すぎると、フェライト分率が低くなり過ぎ、強度は上昇するが、靱性は低下する。したがって、C含有量は0.35〜0.50%の範囲とする。
(C: 0.35-0.50%)
C is an economical and effective strengthening element, and has the effect of decreasing the ferrite fraction and increasing the strength. It also has the effect of improving induction hardenability. In order to fully exhibit this effect, it is necessary to use medium carbon steel with a lower limit of the C content of 0.35% or more. However, if the C content exceeds the upper limit of 0.50% and is too high, the ferrite fraction becomes too low and the strength increases, but the toughness decreases. Therefore, the C content is in the range of 0.35 to 0.50%.

(Si:0.1〜0.6%)
Siは製鋼時の鋼の脱酸のために必要な元素である。また、固溶強化により強度を上昇させる効果もある。その含有量が0.1%未満と、下限の0.1%よりも少ない場合には、脱酸効果や強度向上効果が不十分となる。一方、上限の0.6%を超えて、含有量が多過ぎると、強度が高くなり過ぎ、靱性を低下させる結果となる。したがって、C含有量は0.1〜0.6%の範囲とする。
(Si: 0.1-0.6%)
Si is an element necessary for deoxidation of steel during steelmaking. There is also an effect of increasing the strength by solid solution strengthening. When the content is less than 0.1% and less than the lower limit of 0.1%, the deoxidation effect and the strength improvement effect are insufficient. On the other hand, if the content exceeds the upper limit of 0.6% and the content is too large, the strength becomes too high, resulting in a decrease in toughness. Therefore, the C content is in the range of 0.1 to 0.6%.

(Mn:0.5〜1.5%)
MnはSiと同様、脱酸剤として有用な元素であり、固溶強化により強度を上昇させる効果もある。また、変態温度が低下して、組織を微細化させる効果もある。その含有量が0.5%未満と、下限の0.5%より少ない場合には、これらの効果が無い。一方、Mnの上限の1.5%を超える過剰の含有は、強度が高くなり過ぎ、靱性を低下させる結果となる。また、空冷程度の冷却速度でも、ベイナイトが形成されるようになるため、靱性が低下する。したがって、Mn含有量は0.5〜1.5%の範囲とする。
(Mn: 0.5-1.5%)
Like Si, Mn is an element useful as a deoxidizer, and has an effect of increasing strength by solid solution strengthening. In addition, there is an effect that the transformation temperature is lowered and the structure is refined. When the content is less than 0.5% and less than the lower limit of 0.5%, these effects are not obtained. On the other hand, if the Mn content exceeds 1.5% of the upper limit, the strength becomes too high, resulting in a decrease in toughness. Further, bainite is formed even at a cooling rate of about air cooling, so that toughness is reduced. Therefore, the Mn content is in the range of 0.5 to 1.5%.

(Al:0.005〜0.02%)
Alは製鋼時の鋼の脱酸のために必要な元素である。また、固溶強化により強度を上昇させる効果もあり、AlNとして析出して、圧延時の組織の粗大化を抑制する効果もある。その含有量が0.005%未満と、下限の0.005%よりも少ない場合には、これらの効果が不十分となる。一方、上限の0.02%を超えて、含有量が多過ぎると、強度が高くなり過ぎ、靱性を低下させる結果となる。したがって、Al含有量は0.005〜0.02%の範囲とする。
(Al: 0.005 to 0.02%)
Al is an element necessary for deoxidation of steel during steelmaking. Moreover, it has the effect of increasing strength by solid solution strengthening, and also has the effect of precipitating as AlN and suppressing the coarsening of the structure during rolling. When the content is less than 0.005% and less than the lower limit of 0.005%, these effects are insufficient. On the other hand, if the content exceeds the upper limit of 0.02% and the content is too large, the strength becomes too high, resulting in a decrease in toughness. Therefore, the Al content is in the range of 0.005 to 0.02%.

(V:0.05〜0.50%)
Vは、VNとして析出して、析出強化により強度を上昇させる効果や、圧延時の組織の粗大化を抑制する効果がある。その含有量が0.05%未満と、下限の0.05%よりも少ない場合には、これらの効果が不十分となる。一方、上限の0.50%を超えて、含有量が多過ぎると、強度が高くなり過ぎ、靱性を低下させる結果となる。したがって、V含有量は0.05〜0.50%の範囲とする。
(V: 0.05-0.50%)
V precipitates as VN and has the effect of increasing strength by precipitation strengthening and the effect of suppressing the coarsening of the structure during rolling. When the content is less than 0.05% and less than the lower limit of 0.05%, these effects are insufficient. On the other hand, if the content exceeds the upper limit of 0.50% and the content is too large, the strength becomes too high, resulting in a decrease in toughness. Therefore, the V content is in the range of 0.05 to 0.50%.

(Cr:0.6%以下、Mo:0.5%以下、Ni:1%以下、Cu:1%以下、Ti:0.2%以下、Nb:0.10%以下、の1種または2種以上)
Cr、Mo、Ni、Cu、Ti、Nbは、固溶強化あるいは析出強化によって強度を向上させ、組織を微細化させる効果を有する点で同効元素である。したがって、これらの効果を発揮させるためには、これらの元素を1種または2種以上選択的に含有させる。
(Cr: 0.6% or less, Mo: 0.5% or less, Ni: 1% or less, Cu: 1% or less, Ti: 0.2% or less, Nb: 0.10% or less, or 1 or 2 More than species)
Cr, Mo, Ni, Cu, Ti, and Nb are synergistic elements in that they have the effect of improving the strength by solid solution strengthening or precipitation strengthening and refining the structure. Therefore, in order to exert these effects, one or more of these elements are selectively contained.

(Cr:0.60%以下)
Crは、固溶強化と、パーライトのラメラ間隔を微細化により、強度上昇効果がある。この様な作用を効果的に発揮させるためには、0.05%以上選択的に含有させる。一方、Cr量が多過ぎると、未溶解セメンタイトが生成しやすくなったり、変態終了時間が長くなり、マルテンサイトやベイナイトなどの過冷組織が生じるおそれがあるので、その上限を0.60%とする。
(Cr: 0.60% or less)
Cr has an effect of increasing strength by solid solution strengthening and by reducing the pearlite lamella spacing. In order to effectively exhibit such action, 0.05% or more is selectively contained. On the other hand, if the amount of Cr is too large, undissolved cementite is likely to be formed, or the end time of transformation becomes long, and a supercooled structure such as martensite or bainite may be generated, so the upper limit is 0.60%. To do.

(Mo:0.5%以下)
Moは、固溶強化により、強度上昇効果がある。この様な作用を効果的に発揮させるためには、0.05%以上選択的に含有させる。一方、Mo量が多過ぎると、強度が高くなり過ぎ、靱性を低下させる結果となるので、その上限を0.5%とする。
(Mo: 0.5% or less)
Mo has an effect of increasing strength by solid solution strengthening. In order to effectively exhibit such action, 0.05% or more is selectively contained. On the other hand, if the amount of Mo is too large, the strength becomes too high and the toughness is lowered, so the upper limit is made 0.5%.

(Ni:1%)
Niは、固溶強化により靱性を向上させる効果がある。この様な作用を効果的に発揮させるためには、0.05%以上選択的に含有させる。しかし、この効果は1%を超えて飽和し、かつNiは高価であるので、上限を1%とする。
(Ni: 1%)
Ni has an effect of improving toughness by solid solution strengthening. In order to effectively exhibit such action, 0.05% or more is selectively contained. However, since this effect exceeds 1% and Ni is expensive, the upper limit is made 1%.

(Cu:1%以下)
Cuは、固溶強化により靱性を向上させる効果がある。この様な作用を効果的に発揮させるためには、0.05%以上選択的に含有させる。しかし、一方、Cu量が多過ぎると、微細析出により、強度が高くなり過ぎ、却って靱性を低下させる結果となるので、その上限を1%とする。
(Cu: 1% or less)
Cu has the effect of improving toughness by solid solution strengthening. In order to effectively exhibit such action, 0.05% or more is selectively contained. On the other hand, if the amount of Cu is too large, the strength becomes too high due to fine precipitation, and on the contrary, the toughness is lowered, so the upper limit is made 1%.

(Ti:0.2%以下)
Tiは、析出強化により、強度上昇効果がある。この様な作用を効果的に発揮させるためには、0.05%以上選択的に含有させる。一方、Ti量が多過ぎると、強度が高くなり過ぎ、靱性を低下させる結果となるので、その上限を0.2%とする。
(Ti: 0.2% or less)
Ti has an effect of increasing strength by precipitation strengthening. In order to effectively exhibit such action, 0.05% or more is selectively contained. On the other hand, if the amount of Ti is too large, the strength becomes too high and the toughness is lowered, so the upper limit is made 0.2%.

(Nb:0.10%以下)
NbもTiと同様、析出強化により強度上昇効果がある。この様な作用を効果的に発揮させるためには、0.05%以上選択的に含有させる。一方、Nb量が多過ぎると、強度が高くなり過ぎ、靱性を低下させる結果となるので、その上限を0.10%とする。
(Nb: 0.10% or less)
Nb, like Ti, has an effect of increasing strength by precipitation strengthening. In order to effectively exhibit such action, 0.05% or more is selectively contained. On the other hand, if the amount of Nb is too large, the strength becomes too high and the toughness is lowered, so the upper limit is made 0.10%.

(製造方法)
次に、本発明の高強度高靱性非調質棒鋼の好ましい製造条件について以下に説明する。本発明では、上記化学組成成分からなる鋼を溶製して鋳片となし、鋳片を加熱後、棒鋼への仕上げ圧延を行い、仕上げ圧延終了後急冷および徐冷して棒鋼を製造する。なお、棒鋼への仕上げ熱間圧延までの、製鋼、鋳造時の鋳片のサイズ、凝固時の冷却速度、あるいは分塊圧延条件については、特に限定するものではなく、本発明の要件を満足すれば、常法によるいずれの方法や条件でもよい。
(Production method)
Next, preferable production conditions for the high strength and high toughness non-heat treated steel bar of the present invention will be described below. In the present invention, the steel having the above chemical composition components is melted to form a slab, the slab is heated, and then finish rolling to a bar steel is performed. The steelmaking, the size of the slab during casting, the cooling rate during solidification, or the condition of partial rolling until finish hot rolling to bar steel are not particularly limited, and satisfy the requirements of the present invention. For example, any method or condition according to a conventional method may be used.

(加熱温度)
仕上げ圧延に先立つ加熱温度は800〜1250℃の温度範囲、より好ましくは850〜1200℃の温度範囲とする。加熱温度が800℃未満、より厳しくは850℃未満では、仕上げ圧延開始温度が低くなり、圧延荷重が大きくなり過ぎる。また、加熱温度が1250℃を超えた場合、より厳しくは1200℃を超えた場合、製造コストが高くなるとともに、バ−ニングの危険性が増す。
(Heating temperature)
The heating temperature prior to finish rolling is set to a temperature range of 800 to 1250 ° C, more preferably a temperature range of 850 to 1200 ° C. When the heating temperature is less than 800 ° C., more strictly less than 850 ° C., the finish rolling start temperature becomes low and the rolling load becomes too large. Further, when the heating temperature exceeds 1250 ° C., more strictly, when it exceeds 1200 ° C., the manufacturing cost increases and the risk of burning increases.

(仕上げ圧延)
仕上げ圧延の開始温度は750〜900℃の温度範囲、より好ましくは800〜850℃の温度範囲とする。開始温度が750℃未満、より厳しくは800℃未満では、仕上げ圧延開始温度が低くなり、圧延荷重が大きくなり過ぎる。また、仕上げ圧延開始温度が900℃を超えた場合、より厳しくは850℃を超えた場合、仕上げ圧延後のオーステナイト粒が十分細かくならない。
(Finish rolling)
The starting temperature of finish rolling is set to a temperature range of 750 to 900 ° C, more preferably a temperature range of 800 to 850 ° C. If the start temperature is less than 750 ° C., more strictly less than 800 ° C., the finish rolling start temperature becomes low and the rolling load becomes too large. Further, when the finish rolling start temperature exceeds 900 ° C, more strictly, when it exceeds 850 ° C, the austenite grains after finish rolling are not sufficiently fine.

この仕上げ圧延終了後に10秒以内に急冷を開始する。仕上げ圧延終了から短時間で急冷を開始することで、オーステナイト粒の粗大化を防止する。急冷開始が仕上げ圧延終了後から10秒を超えた場合、オーステナイト粒が粗大化してしまう。   Rapid cooling is started within 10 seconds after the finish rolling. By starting quenching in a short time after finishing rolling, austenite grains are prevented from becoming coarse. If the start of rapid cooling exceeds 10 seconds after finishing rolling, the austenite grains become coarse.

仕上げ圧延終了後の平均冷却速度は10℃/秒以上の急冷とする。このような急冷とすることで、オーステナイト粒の粗大化を抑制し、変態温度を低下させる。変態温度が低い方がパーライトやフェライトの核生成頻度が上昇し、有効フェライトの粒径が前記規定した範囲に微細化する。またラメラ間隔が微細化することで強度が向上する。   The average cooling rate after finish rolling is a rapid cooling of 10 ° C./second or more. Such rapid cooling suppresses the coarsening of the austenite grains and lowers the transformation temperature. The lower the transformation temperature, the higher the nucleation frequency of pearlite and ferrite, and the effective ferrite particle size is refined within the specified range. Further, the strength is improved by reducing the lamella spacing.

この急冷の終了温度は500〜700℃の温度範囲、より好ましくは550〜650℃の温度範囲とする。急冷の終了温度が低い方がラメラ間隔が微細化することで強度が向上する。しかし、急冷の終了温度が500℃未満、より厳密には550℃未満の低温側となった場合、ベイナイトやマルテンサイトなどの過冷組織が形成されて靱性が低下する。一方、急冷の終了温度が700℃を超えた場合、より厳密には650℃を超えた場合、ラメラ間隔微細化の急冷効果が無くなる。   The quenching end temperature is set to a temperature range of 500 to 700 ° C, more preferably a temperature range of 550 to 650 ° C. The one where the end temperature of the quenching is lower improves the strength by making the lamella spacing finer. However, when the quenching end temperature is lower than 500 ° C., more strictly less than 550 ° C., a supercooled structure such as bainite or martensite is formed, and the toughness is lowered. On the other hand, when the quenching end temperature exceeds 700 ° C., more strictly, when it exceeds 650 ° C., the quenching effect of refining the lamella spacing is lost.

この急冷の終了後、200℃の温度まで、平均冷却速度が0.1〜5℃/秒で徐冷して棒鋼を製造する。上記急冷の終了後は、冷却速度は遅い方が良く、この徐冷によって、過冷組織の形成による靱性低下を防止する。平均冷却速度が0.1℃/秒未満では生産性が悪くなる。一方、平均冷却速度が5℃/秒を超えた場合、過冷組織の形成による靱性低下が生じる。また、過冷組織の形成は200℃の温度までであり、徐冷の温度は200℃までで良い。したがって、200℃以下の棒鋼の温度制御は自由である。   After the quenching is completed, the steel bar is manufactured by gradually cooling to a temperature of 200 ° C. at an average cooling rate of 0.1 to 5 ° C./second. After the rapid cooling is completed, the cooling rate should be low, and this slow cooling prevents toughness deterioration due to formation of the supercooled structure. When the average cooling rate is less than 0.1 ° C./second, the productivity is deteriorated. On the other hand, when the average cooling rate exceeds 5 ° C./second, toughness is reduced due to formation of a supercooled structure. Further, the formation of the supercooled tissue is up to a temperature of 200 ° C., and the temperature of the slow cooling may be up to 200 ° C. Therefore, the temperature control of steel bars below 200 ° C. is free.

以下に本発明の実施例を説明する。上記した鋼の化学成分組成や、加熱、仕上げ圧延の条件を種々変えて棒鋼を得、この棒鋼の組織調査と、強度−靱性のバランスの評価を行なった。   Examples of the present invention will be described below. A steel bar was obtained by variously changing the chemical composition of the steel, heating, and finish rolling conditions, and the structure of the steel bar was evaluated and the balance between strength and toughness was evaluated.

具体的には、表1に示す種々の化学成分組成からなる鋼を転炉にて溶製して鋳片となし、この鋳片を分塊圧延後に、155mm角のビレットに仕上げた。このビレットを加熱後、棒鋼への仕上げ圧延を行い、Φ30mmの棒鋼を製造した。これらの加熱、仕上げ圧延の条件を表2に示す。   Specifically, steels having various chemical composition compositions shown in Table 1 were melted in a converter to form a slab, and this slab was finished into a 155 mm square billet after partial rolling. After heating the billet, finish rolling into a steel bar was performed to produce a steel bar having a diameter of 30 mm. These heating and finish rolling conditions are shown in Table 2.

これら製造した棒鋼の組織として、フェライト分率(%)、パーライトの平均ラメラ間隔(μm)、結晶方位差が15°以上の大角粒界で囲まれたフェライトの平均粒径(μm)を各々測定した。これらの測定は各々前記した方法と要領で行なった。   As the structure of these steel bars, the ferrite fraction (%), the average lamella spacing of pearlite (μm), and the average grain size (μm) of ferrite surrounded by large-angle grain boundaries with a crystal orientation difference of 15 ° or more are measured. did. Each of these measurements was performed by the method and procedure described above.

また、これら製造した棒鋼の引張強度と靱性を測定した。引張強度(MPa)は、JIS4A号試験片を用いて引張試験を行なった。また、靱性(J)は、JIS3号試験片(2mmのUノッチ)を用いて、シャルピー衝撃試験を行なった。これらの結果を表2と図2とに示す。図2は表2の各発明例と比較例の引張強度と靱性を、縦軸:靱性、横軸:引張強度でプロットし、整理し直したものである。   Moreover, the tensile strength and toughness of these manufactured steel bars were measured. The tensile strength (MPa) was a tensile test using a JIS4A test piece. As for toughness (J), a Charpy impact test was performed using a JIS No. 3 test piece (2 mm U-notch). These results are shown in Table 2 and FIG. FIG. 2 plots the tensile strength and toughness of each invention example and comparative example in Table 2 with the vertical axis: toughness and the horizontal axis: tensile strength and rearranged.

表1、2および図2から明らかな通り、発明例1、12、13、14、17、18の各発明例は、各々本発明の範囲内の化学成分組成A、E、F、G、J、Kからなり、かつ、製造方法も、仕上げ圧延開始温度、仕上げ圧延終了後の急冷開始時間、急冷の際の平均冷却速度、急冷終了温度、その後の200℃まで平均冷却速度が各々好ましい範囲内である。   As is apparent from Tables 1 and 2 and FIG. 2, each of the inventive examples 1, 12, 13, 14, 17, and 18 is represented by chemical component compositions A, E, F, G, J within the scope of the present invention. , K, and the production method also includes finish rolling start temperature, quenching start time after finishing rolling, average cooling rate during quenching, quenching end temperature, and subsequent average cooling rate up to 200 ° C. It is.

この結果、棒鋼組織が過冷組織を有さない、フェライト−パーライト複相組織からなり、かつ、フェライト分率が20〜40%、パーライトの平均ラメラ間隔が0.05〜0.20μm、結晶方位差が15°以上の大角粒界で囲まれたフェライトの平均粒径が2〜10μmである。したがって、棒鋼の性能としても、950MPa以上の高強度であるとともに、100J以上の高靱性を有しており、強度−靱性のバランスに優れている。   As a result, the steel bar structure is composed of a ferrite-pearlite multiphase structure having no supercooled structure, the ferrite fraction is 20 to 40%, the average lamella spacing of pearlite is 0.05 to 0.20 μm, and the crystal orientation The average grain size of the ferrite surrounded by the large-angle grain boundaries having a difference of 15 ° or more is 2 to 10 μm. Accordingly, the steel bar has a high strength of 950 MPa or more and a high toughness of 100 J or more, and has an excellent balance between strength and toughness.

これに対して、比較例2〜8は、各々本発明の範囲内の化学成分組成であるAの発明鋼を用いているものの、製造方法が各々好ましい範囲から外れている。この結果、強度−靱性のバランスが発明例に比して著しく劣っている。   On the other hand, Comparative Examples 2 to 8 use the inventive steel of A which is a chemical component composition within the scope of the present invention, but the production methods are out of the preferred ranges. As a result, the balance between strength and toughness is significantly inferior to that of the inventive examples.

例えば、比較例2は、仕上げ圧延開始温度が950℃と高過ぎる。このため、フェライト分率が13%と小さくなり、有効フェライトの平均粒径が26μmと大きい。この結果、高強度の割に、靱性が低く、強度−靱性のバランスが発明例に比して著しく劣っている。   For example, in Comparative Example 2, the finish rolling start temperature is too high at 950 ° C. For this reason, the ferrite fraction is as small as 13%, and the average particle diameter of effective ferrite is as large as 26 μm. As a result, the toughness is low for the high strength, and the balance between strength and toughness is remarkably inferior to that of the inventive examples.

比較例3は、仕上げ圧延終了後の急冷開始時間が30秒と長過ぎる。このため、フェライト分率が18%と小さくなり、有効フェライトの平均粒径が20.2μmと大きい。この結果、高強度の割に、靱性が低く、強度−靱性のバランスが発明例に比して著しく劣っている。   In Comparative Example 3, the rapid cooling start time after finishing rolling is as long as 30 seconds. For this reason, the ferrite fraction is as small as 18%, and the average particle diameter of effective ferrite is as large as 20.2 μm. As a result, the toughness is low for the high strength, and the balance between strength and toughness is remarkably inferior to that of the inventive examples.

比較例4は、急冷の際の平均冷却速度が1℃/秒と遅過ぎる。このため、有効フェライトの平均粒径が25.4μmと大きく、パーライトの平均ラメラ間隔も0.231μmと大きい。この結果、強度も靱性もともに、強度−靱性のバランスが発明例に比して著しく劣っている。   In Comparative Example 4, the average cooling rate during the rapid cooling is too slow at 1 ° C./second. For this reason, the average particle diameter of effective ferrite is as large as 25.4 μm, and the average lamella spacing of pearlite is also as large as 0.231 μm. As a result, both strength and toughness are remarkably inferior in strength-toughness compared to the inventive examples.

比較例5は、急冷の際の平均冷却速度が7℃/秒と遅過ぎる。このため、パーライトの平均ラメラ間隔も0.215μmと大きい。この結果、高靱性の割に、強度が低く、強度−靱性のバランスが発明例に比して著しく劣っている。   In Comparative Example 5, the average cooling rate during the rapid cooling is too slow at 7 ° C./second. For this reason, the average lamella spacing of pearlite is as large as 0.215 μm. As a result, the strength is low for high toughness, and the balance between strength and toughness is significantly inferior to that of the inventive examples.

比較例6は、急冷終了温度が420℃と低過ぎる。このためベイナイトの過冷組織が形成されている。この結果、高強度の割に、靱性が低く、強度−靱性のバランスが発明例に比して著しく劣っている。   In Comparative Example 6, the quenching end temperature is too low at 420 ° C. For this reason, a supercooled structure of bainite is formed. As a result, the toughness is low for the high strength, and the balance between strength and toughness is remarkably inferior to that of the inventive examples.

比較例7は、急冷終了温度が730℃と高過ぎる。このため、有効フェライトの平均粒径が14.5μmと大きく、パーライトの平均ラメラ間隔も0.226μmと大きい。この結果、高靱性の割に、強度が低く、強度−靱性のバランスが発明例に比して著しく劣っている。   In Comparative Example 7, the quenching end temperature is too high at 730 ° C. For this reason, the average particle diameter of effective ferrite is as large as 14.5 μm, and the average lamella spacing of pearlite is also as large as 0.226 μm. As a result, the strength is low for high toughness, and the balance between strength and toughness is significantly inferior to that of the inventive examples.

比較例8は、200℃までの徐冷速度が20℃/秒と早過ぎる。このためベイナイトの過冷組織が形成されている。この結果、高強度の割に、靱性が低く、強度−靱性のバランスが発明例に比して著しく劣っている。   In Comparative Example 8, the slow cooling rate to 200 ° C. is too fast at 20 ° C./second. For this reason, a supercooled structure of bainite is formed. As a result, the toughness is low for the high strength, and the balance between strength and toughness is remarkably inferior to that of the inventive examples.

また、比較例9〜11、15、16は、各々本発明の範囲外の化学成分組成である各々B、C、D、H、Iの比較鋼を用いている。このため、製造方法は各々本発明の好ましい範囲を満たし、フェライト分率、パーライトの平均ラメラ間隔、有効フェライトの平均粒径など、組織的にも本発明要件を各々満たすものの、この結果、強度−靱性のバランスが発明例に比して著しく劣っている。   Further, Comparative Examples 9 to 11, 15, and 16 use comparative steels of B, C, D, H, and I, which are chemical component compositions outside the scope of the present invention. For this reason, each of the production methods satisfies the preferred range of the present invention, and the ferrite fraction, the average lamella spacing of pearlite, the average particle diameter of effective ferrite, etc. each satisfy the present invention requirements. The balance of toughness is remarkably inferior to that of the inventive examples.

例えば、比較例9の比較鋼BはSiが1.32%と上限を超えている。このため、靱性が低く、強度−靱性のバランスが発明例に比して著しく劣っている。   For example, in Comparative Steel B of Comparative Example 9, Si is 1.32% and exceeds the upper limit. For this reason, toughness is low and the balance between strength and toughness is remarkably inferior to that of the inventive examples.

比較例10の比較鋼Cは、Mnが2.31%と上限を超えている。このため、圧延後の冷却過程でベイナイトの過冷組織が形成されている。この結果、靱性が低く、強度−靱性のバランスが発明例に比して著しく劣っている。   In Comparative Steel C of Comparative Example 10, Mn is 2.31% and exceeds the upper limit. For this reason, a supercooled structure of bainite is formed in the cooling process after rolling. As a result, the toughness is low and the balance between strength and toughness is remarkably inferior to that of the inventive examples.

比較例11の比較鋼Dは、Crが1.25%と選択的に含有する場合の上限を超えている。このため、靱性が低く、強度−靱性のバランスが発明例に比して著しく劣っている。   The comparative steel D of Comparative Example 11 exceeds the upper limit when Cr is selectively contained as 1.25%. For this reason, toughness is low and the balance between strength and toughness is remarkably inferior to that of the inventive examples.

比較例15の比較鋼Hは、Vが0.010%と少な過ぎ、下限を下回っている。このため、靱性の割に強度が低く、強度−靱性のバランスが発明例に比して著しく劣っている。   In the comparative steel H of Comparative Example 15, V is too small as 0.010%, which is below the lower limit. For this reason, the strength is low for toughness, and the balance between strength and toughness is remarkably inferior to that of the inventive examples.

比較例16の比較鋼Iは、Vが1.2%と多過ぎ、上限を超えている。このため、強度の割に靱性が低く、強度−靱性のバランスが発明例に比して著しく劣っている。   In Comparative Steel I of Comparative Example 16, V is too high at 1.2% and exceeds the upper limit. For this reason, the toughness is low relative to the strength, and the balance between strength and toughness is remarkably inferior to that of the inventive examples.

更に、比較例19は、本発明の範囲内の化学成分組成であるLの発明鋼を用いているものの、仕上げ圧延の代わりに、熱処理として、焼入れ、焼戻しを行なったものである。この組織は焼戻しマルテンサイトとなっており、靱性の割に強度が低く、強度−靱性のバランスが発明例に比して著しく劣っている。   Furthermore, although the comparative example 19 uses the invention steel of L which is a chemical component composition within the scope of the present invention, it is subjected to quenching and tempering as heat treatment instead of finish rolling. This structure is tempered martensite, the strength is low for toughness, and the balance between strength and toughness is remarkably inferior to that of the inventive examples.

以上の結果から、本発明の、化学成分組成、組織、好ましい製造条件などの各要件の、強度−靱性のバランスに対する臨界的が意義が分かる。
From the above results, it can be understood that each requirement of the present invention, such as chemical component composition, structure, and preferable production conditions, is critical to the balance between strength and toughness.

以上説明したように、本発明によれば、強度と靱性をともに高くし、強度と靱性とのバランスに優れた高強度高靱性非調質棒鋼およびその製造方法を提供することができる。この結果、自動車を初めとする構造用鋼用途を拡大するものである。   As described above, according to the present invention, it is possible to provide a high-strength, high-toughness non-heat treated steel bar having both high strength and toughness and excellent balance between strength and toughness and a method for producing the same. As a result, structural steel applications including automobiles are expanded.

フェライト−パーライト複相組織の模式図である。It is a schematic diagram of a ferrite-pearlite double phase structure. 実施例表2の各発明例と比較例の引張強度と靱性との関係を示す説明図である。It is explanatory drawing which shows the relationship between the tensile strength of each invention example of Example Table 2, and a comparative example, and toughness.

Claims (3)

質量%で、C:0.35〜0.50%、Si:0.1〜0.6%、Mn:0.5〜1.5%、Al:0.005〜0.02%、V:0.05〜0.50%を含み、残部鉄及び不可避的不純物からなり、鋼組織における、フェライト分率が20〜40%、パーライトの平均ラメラ間隔が0.05〜0.20μm、結晶方位差が15°以上の大角粒界で囲まれたフェライトの平均粒径が2〜10μmであるフェライト−パーライト複相組織からなることを特徴とする高強度高靱性非調質棒鋼。   In mass%, C: 0.35-0.50%, Si: 0.1-0.6%, Mn: 0.5-1.5%, Al: 0.005-0.02%, V: 0.05 to 0.50%, consisting of the balance iron and inevitable impurities, in the steel structure, the ferrite fraction is 20 to 40%, the average lamella spacing of pearlite is 0.05 to 0.20 μm, the crystal orientation difference A high-strength, high-toughness, non-heat treated steel bar characterized by comprising a ferrite-pearlite double phase structure in which the average grain size of ferrite surrounded by large-angle grain boundaries of 15 ° or more is 2 to 10 μm. 前記極細高炭素鋼線が、質量%で、更に、Cr:0.60%以下、Mo:0.5%以下、Ni:1%以下、Cu:1%以下、Ti:0.2%以下、Nb:0.10%以下、の1種または2種以上を含有する請求項1記載の高強度高靱性非調質棒鋼。   The ultra fine high carbon steel wire is in mass%, further Cr: 0.60% or less, Mo: 0.5% or less, Ni: 1% or less, Cu: 1% or less, Ti: 0.2% or less, The high-strength, high-toughness non-heat treated steel bar according to claim 1, containing one or more of Nb: 0.10% or less. 質量%で、C:0.35〜0.50%、Si:0.1〜0.6%、Mn:0.5〜1.5%、Al:0.005〜0.02%、V:0.05〜0.50%を含む鋼を、750〜900℃の開始温度で仕上げ圧延し、仕上げ圧延終了後10秒以内に平均冷却速度が10℃/秒以上で急冷を開始して500〜700℃の温度まで冷却し、その後、200℃まで平均冷却速度が0.1〜5℃/秒で冷却することを特徴とする高強度高靱性非調質棒鋼の製造方法。
In mass%, C: 0.35-0.50%, Si: 0.1-0.6%, Mn: 0.5-1.5%, Al: 0.005-0.02%, V: Steel containing 0.05 to 0.50% is finish-rolled at a start temperature of 750 to 900 ° C., and is rapidly cooled at an average cooling rate of 10 ° C./second or more within 10 seconds after finishing rolling. A method for producing a high-strength, high-toughness, non-tempered steel bar characterized by cooling to a temperature of 700 ° C and then cooling to 200 ° C at an average cooling rate of 0.1 to 5 ° C / second.
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