JP6237047B2 - High strength hot rolled steel sheet with excellent fatigue strength and method for producing the same - Google Patents
High strength hot rolled steel sheet with excellent fatigue strength and method for producing the same Download PDFInfo
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Description
本発明は、疲労強度に優れた高強度熱延鋼板、及びその製造方法に関するものである。 The present invention relates to a high-strength hot-rolled steel sheet having excellent fatigue strength and a method for producing the same.
近年、環境問題に端を発して自動車の燃費向上が望まれているが、それに向け自動車の軽量化が求められている。その為には、自動車用鋼板の板厚を低減する必要があるが、その課題となっているのは、疲労強度の改善である。軽量化の為鋼板の板厚を低減した場合、鋼材に加わる応力は増加し、疲労寿命は劣化する。そのため、より疲労寿命の高い鋼板の開発が望まれていた。 In recent years, there has been a demand for improvement in fuel efficiency of automobiles due to environmental problems, but there is a demand for weight reduction of automobiles. For that purpose, it is necessary to reduce the thickness of the steel plate for automobiles, but the problem is improvement of fatigue strength. When the thickness of the steel sheet is reduced for weight reduction, the stress applied to the steel material increases and the fatigue life deteriorates. Therefore, development of a steel plate with a higher fatigue life has been desired.
自動車の足回り部品として多用されている強度４４０ＭＰａ級の高強度熱延鋼板では、疲労限度比ＦＬ（疲労強度）／ＴＳ（引張強度）０．５以上が求められる。 A high strength hot rolled steel sheet having a strength of 440 MPa, which is widely used as an undercarriage part for automobiles, is required to have a fatigue limit ratio FL (fatigue strength) / TS (tensile strength) of 0.5 or more.
従来、疲労強度の改善に向けては、特許文献１に示されるようにミクロ組織をフェライト、マルテンサイトからなる複合組織とする、等の対策が取られていた。しかし、その場合、高価な合金を添加する必要が生じ、コスト増加を招いていた。 Conventionally, for improving the fatigue strength, as shown in Patent Document 1, measures such as making the microstructure a composite structure composed of ferrite and martensite have been taken. However, in that case, it is necessary to add an expensive alloy, resulting in an increase in cost.
また一方で、自動車用部品は多くの場合、プレス成形により部品形状に加工された後に用いられるため、優れたプレス成形性が必要とされる。プレス成形性の代表的な指標として、全伸びの値があり、多くの自動車用高強度熱延鋼板は、必要とされる全伸びの値が得られるように製造されている。上述の疲労強度の改善に際しては、対象の高強度熱延鋼板の全伸びの値を劣化させることなく行う必要がある。 On the other hand, in many cases, automobile parts are used after being processed into a part shape by press molding, and therefore, excellent press formability is required. A typical index of press formability is a value of total elongation, and many high-strength hot-rolled steel sheets for automobiles are manufactured so as to obtain a required total elongation value. In improving the above-described fatigue strength, it is necessary to carry out without deteriorating the total elongation value of the target high-strength hot-rolled steel sheet.
本発明は、コスト増加を招くことなく、また全伸びの劣化を招くことなく、疲労強度を改善した高強度熱延鋼板、及びその製造方法を提供することを目的とする。 An object of the present invention is to provide a high-strength hot-rolled steel sheet having improved fatigue strength without incurring an increase in cost and without causing deterioration in total elongation, and a method for producing the same.
本発明者らは、上記課題を解決すべく鋭意研究した結果、鋼板表裏層において所定の組織制御を行い、かつそのような表裏層が板厚中心より硬質である鋼板とすることで、疲労特性に優れた高強度熱延鋼板を得ることができることを見出して、本発明を完成した。
本発明の要旨は、以下の通りである。
As a result of diligent research to solve the above-mentioned problems, the inventors have performed a predetermined structure control in the front and back layers of the steel sheet, and the steel sheet having such a front and back layer that is harder than the center of the plate thickness, fatigue characteristics. The present invention was completed by finding that a high-strength hot-rolled steel sheet excellent in resistance could be obtained.
The gist of the present invention is as follows.
（１）発明１は、質量％で、
Ｃ：０．１０〜０．２０％、
Ｓｉ：０．０１〜２．００％、
Ｍｎ：０．１０〜２．００％、
Ｐ≦０．１００％、
Ｓ≦０．０１００％、
Ａｌ：０．００５〜０．０５０％、
Ｎ≦０．０１００％、
を含有し、残部がＦｅ及び不可避的不純物の組成からなり、引張強度が４６７ＭＰａ以上、５９４ＭＰａ以下であり、鋼板の表裏面から板厚の少なくとも１０％に相当する厚みの領域のミクロ組織がベイナイト単相であり、板厚中心部の板厚の少なくとも５０％に相当する厚みの領域のミクロ組織が５０％以上のフェライト面積分率を有し、前記表裏面から板厚の少なくとも１０％に相当する厚みの領域の硬さが前記板厚中心部の板厚の少なくとも５０％に相当する厚みの領域の平均硬さの１．１５倍以上であることを特徴とする疲労特性に優れた高強度熱延鋼板である。
(1) Invention 1 is mass%,
C: 0.10 to 0.20%,
Si: 0.01 to 2.00%
Mn: 0.10 to 2.00%,
P ≦ 0.100%,
S ≦ 0.0100%,
Al: 0.005 to 0.050%,
N ≦ 0.0100%,
The balance is composed of Fe and unavoidable impurities, the tensile strength is 467 MPa or more and 594 MPa or less, and the microstructure in the region corresponding to at least 10% of the plate thickness from the front and back surfaces of the steel plate has a bainite unit. The microstructure of the thickness region corresponding to at least 50% of the plate thickness at the center of the plate thickness has a ferrite area fraction of 50% or more, and corresponds to at least 10% of the plate thickness from the front and back surfaces. High strength heat excellent in fatigue characteristics, characterized in that the hardness of the thickness region is 1.15 times or more of the average hardness of the thickness region corresponding to at least 50% of the thickness of the central portion of the plate thickness It is a rolled steel sheet.
（２）発明２は、質量％でさらに、
Ｎｂ：０．０５０％以下、
Ｔｉ：０．３００％以下、
Ｖ：０．１０％以下、
Ｃｕ：１．００％以下、
Ｎｉ：１．００％以下、
Ｃｒ：１．００％以下、
Ｂ：０．００５０％以下、
Ｃａ：０．００３０％以下、
ＲＥＭ：０．０２００％以下、
のうちの１種または２種以上を含むことを特徴とする発明１に記載の疲労特性に優れた高強度熱延鋼板である。
(2) Invention 2 is further in mass%,
Nb: 0.050% or less,
Ti: 0.300% or less,
V: 0.10% or less,
Cu: 1.00% or less,
Ni: 1.00% or less,
Cr: 1.00% or less,
B: 0.0050% or less,
Ca: 0.0030% or less,
REM: 0.0200% or less,
The high strength hot-rolled steel sheet having excellent fatigue characteristics according to the first aspect of the present invention, comprising one or more of the above.
（３）発明３は、発明１または発明２に記載の疲労特性に優れた高強度熱延鋼板の製造方法であって、発明１または発明２の組成を有する鋳片を１１５０℃以上に加熱し、粗圧延した後、Ａｒ３温度＋５０（℃）以上の温度にて仕上げ圧延を行い、その後、仕上げ圧延終了温度から６５０℃までの間の平均の熱伝達係数αを下記式（１）で示される範囲内として鋼板の表面及び裏面から冷却を行い、その後鋼板全厚の平均の温度を６００℃以下として巻取りを行うことを特徴とする疲労特性に優れた高強度熱延鋼板の製造方法である。
９１×板厚（ｍｍ）＋７５６
≦α（Ｊ／ｍ２ｓｅｃＫ）≦９１×板厚（ｍｍ）＋１８００ ・・・（１）
(3) Invention 3 is a method for producing a high-strength hot-rolled steel sheet having excellent fatigue properties as described in Invention 1 or Invention 2, wherein a slab having the composition of Invention 1 or Invention 2 is heated to 1150 ° C. or higher. After rough rolling, finish rolling is performed at a temperature of Ar3 temperature +50 (° C.) or higher, and then an average heat transfer coefficient α between the finish rolling end temperature and 650 ° C. is expressed by the following formula (1). A method for producing a high-strength hot-rolled steel sheet with excellent fatigue characteristics, characterized in that cooling is performed from the front and back surfaces of the steel sheet within the range, and then winding is performed with an average temperature of the total thickness of the steel sheet set to 600 ° C. .
91 x plate thickness (mm) + 756
≦ α (J / m2secK) ≦ 91 × plate thickness (mm) +1800 (1)
（４）発明４は、発明１または発明２に記載の疲労特性に優れた高強度熱延鋼板の製造方法であって、発明１または発明２の組成を有する鋳片を１１５０℃以上に加熱し、粗圧延した後、Ａｒ３温度＋５０（℃）以上の温度にて仕上げ圧延を行い、その後、仕上げ圧延終了温度から６５０℃までの間の冷却での平均の水量密度Ｗを下記式（２）で示される範囲内として鋼板の表面及び裏面から冷却を行い、その後鋼板全厚の平均の温度を６００℃以下として巻取りを行うことを特徴とする疲労特性に優れた高強度熱延鋼板の製造方法である。
０．００４８×板厚（ｍｍ）＋０．００３５７
≦Ｗ（ｍ３／ｓｅｃ／ｍ２）≦０．００４８×板厚（ｍｍ）＋０．０５５ ・（２）
(4) Invention 4 is a method for producing a high-strength hot-rolled steel sheet having excellent fatigue properties as described in Invention 1 or Invention 2, wherein a slab having the composition of Invention 1 or Invention 2 is heated to 1150 ° C. or higher. After rough rolling, finish rolling is performed at a temperature of Ar3 temperature + 50 (° C.) or higher, and then the average water density W in cooling from the finish rolling end temperature to 650 ° C. is expressed by the following formula (2). be within the range indicated perform cooling from the front and rear surfaces of the steel plate, then the steel sheet of high-strength hot-rolled steel sheet excellent in fatigue properties, characterized in that the temperature of the average of the total thickness performing coiling as 600 ° C. or less It is a manufacturing method.
0.0048 × plate thickness (mm) +0.00357
≦ W (m 3 / sec / m 2 ) ≦ 0.0048 × plate thickness (mm) +0.055 (2)
本発明によれば、従来と比べ、コスト増加を招くことなく、加工性を劣化（全伸びを劣化）させることなく、疲労限度比、すなわち疲労強度と引張強度のバランスに優れた熱延鋼板を得ることができ、自動車軽量化に寄与する。 According to the present invention, a hot-rolled steel sheet having an excellent fatigue limit ratio, that is, a balance between fatigue strength and tensile strength, without causing an increase in cost and without degrading workability (deteriorating total elongation), compared with the conventional steel sheet. It can be obtained and contributes to the weight reduction of automobiles.
以下に本発明について、詳細に説明する。 The present invention is described in detail below.
本発明者らは、疲労特性に優れた高強度熱延鋼板の開発のため、鋼板表裏層において所定の組織制御を行い、かつそのような表裏層が板厚中心より硬質である鋼板、及びその容易な製造方法を開発した。 In order to develop a high-strength hot-rolled steel sheet having excellent fatigue properties, the present inventors perform a predetermined structure control in the front and back layers of the steel sheet, and such a steel sheet in which the front and back layers are harder than the center of the plate thickness, and its An easy manufacturing method was developed.
鋼板の表裏層をその板厚内部より硬質とすることにより、鋼材の疲労限度比が上昇するのは以下の理由による。鋼材の疲労強度は鋼材が高強度なほど一般的には高い。これは、鋼板の強度が高いほど、一定レベルの繰り返し応力下において、鋼材の表面からき裂が生じにくいためである。従って、表裏層を高強度、即ち硬質とすることにより繰り返し応力の負荷時に表面からの疲労き裂の発生を抑制することができる。その一方で、板厚内部を表裏層より軟質とすることにより鋼板の加工性の劣化は避けることができる。従って、鋼板の表裏層を板厚内部より硬質とした鋼板を得ることにより、鋼板内部の加工性を維持したまま鋼材の疲労限度比を改善することが可能となる。 The reason why the fatigue limit ratio of the steel material is increased by making the front and back layers of the steel plate harder than the inside of the plate thickness is as follows. The fatigue strength of steel is generally higher as the strength of the steel is higher. This is because the higher the strength of the steel plate, the less likely it is to crack from the surface of the steel material under a certain level of repeated stress. Therefore, by making the front and back layers high in strength, that is, hard, it is possible to suppress the occurrence of fatigue cracks from the surface when a repeated stress is applied. On the other hand, deterioration of the workability of the steel sheet can be avoided by making the inside of the plate thickness softer than the front and back layers. Therefore, by obtaining a steel plate in which the front and back layers of the steel plate are harder than the inside of the plate thickness, it becomes possible to improve the fatigue limit ratio of the steel material while maintaining the workability inside the steel plate.
また、本発明においては、上記の原理を応用することに加え、表裏層のミクロ組織を微細な炭化物を含むベイナイト単相の組織とすることにより疲労特性の改善を図った。ベイナイト組織とすることにより、疲労き裂の発生原因となりうる粗大な炭化物が生成しなくなり、疲労き裂が発生しにくくなる。 Further, in the present invention, in addition to applying the above principle, fatigue characteristics are improved by making the microstructures of the front and back layers into a bainite single-phase structure containing fine carbides. By employing a bainite structure, coarse carbides that can cause fatigue cracks are not generated, and fatigue cracks are less likely to occur.
本発明においては、鋼板の表裏層の強度を増加させる一方で、鋼板内部を軟質なフェライト主体の組織とすることで鋼板全体の成形性（全伸び）は良好に保つことができる。 In the present invention, while increasing the strength of the front and back layers of the steel sheet, the formability (total elongation) of the entire steel sheet can be kept good by making the inside of the steel sheet a soft ferrite-based structure.
本発明者らは、表裏層のみが硬質となっており、かつベイナイト単相組織となっている高強度熱延鋼板の開発に取り組み、熱間圧延後の所定の温度域における冷却において、冷却水量等の冷却条件により変動する鋼板と冷却水の間の熱伝達係数を、板厚に応じた所定以上の値以上に制御し、表裏層と板厚中心の冷却速度の差を所定量大きくし、表裏層のみを板厚中心より著しく硬質とし、かつベイナイト単相組織とする製造方法及びそれによる高強度熱延鋼板を開発した。 The present inventors worked on the development of a high-strength hot-rolled steel sheet in which only the front and back layers are hard and have a bainite single-phase structure, and in cooling in a predetermined temperature range after hot rolling, the amount of cooling water The heat transfer coefficient between the steel sheet and the cooling water, which varies depending on the cooling conditions such as, is controlled to a value greater than or equal to a predetermined value according to the plate thickness, and the difference in cooling rate between the front and back layers and the plate thickness center is increased by a predetermined amount, A manufacturing method in which only the front and back layers are remarkably harder than the center of the plate thickness and has a bainite single phase structure, and a high-strength hot-rolled steel plate using the same have been developed.
尚、熱伝達係数α（Ｊ／ｍ２ｓｅｃＫ）とは、２種類の物資間での熱エネルギーの伝え易さを表す値であり、単位面積、単位時間、単位温度差あたりの伝熱量（すなわち単位温度差あたりの熱流束密度）である。熱伝達係数は、冷却に用いる流体の速度等の条件によって大きく異なる。 The heat transfer coefficient α (J / m 2 secK) is a value representing the ease of transferring heat energy between two kinds of materials, and the amount of heat transfer per unit area, unit time, and unit temperature difference (that is, Heat flux density per unit temperature difference). The heat transfer coefficient varies greatly depending on conditions such as the speed of the fluid used for cooling.
熱伝達係数を、板厚に応じた所定以上の値以上に制御することにより、表裏層のみが硬質となっている高強度熱延鋼板の開発に向け、本発明者らが行った実験について次に説明する。 By controlling the heat transfer coefficient to a value greater than or equal to a predetermined value according to the plate thickness, the following experiments were conducted by the present inventors toward the development of a high-strength hot-rolled steel sheet in which only the front and back layers are hard. Explained.
図１は連続熱間圧延工程における冷却パターンに示している。即ち、仕上げ圧延後の６５０℃までの急速冷却とその後に通常冷却（放冷）をして巻き取る工程までの表層冷却と板厚中央部（１／２ｔと表示）冷却の冷却パターンの概要を示す。なお、圧延後冷却開始までの時間（秒）を２．５秒以下、好ましくは１．６秒以下にすることが望ましい。圧延後の冷却開始までの時間とは、仕上げ圧延機とランアウトテーブルの冷却ゾーンの間を鋼板が走行する時間である。仕上げ圧延機とランアウトテーブルの冷却ゾーンとは、それらの間に通常温度計等の計測装置が設置されており積極的な冷却が行われないため、鋼板が空冷されるゾーンである。 FIG. 1 shows a cooling pattern in a continuous hot rolling process. That is, the outline of the cooling pattern of rapid cooling to 650 ° C. after finish rolling, and subsequent cooling to normal temperature (cooling) and winding up to the step of winding up and the central part thickness (indicated as 1 / 2t). Show. In addition, it is desirable that the time (second) from the rolling to the start of cooling is 2.5 seconds or less, preferably 1.6 seconds or less. The time until the start of cooling after rolling is the time for the steel plate to travel between the cooling zone of the finish rolling mill and the run-out table. The cooling zone of the finish rolling mill and the run-out table is a zone in which the steel sheet is air-cooled because a measuring device such as a thermometer is usually installed between them and active cooling is not performed.
表１に示す鋼Ａの組成からなる鋳片を用いて、図１及び表２−１及び表２−２に示す熱延条件にて、板厚３ｍｍ〜１２ｍｍの熱延鋼板の製造を行った。ここで、表２−１及び表２−２の仕上げ圧延温度、巻き取り温度は放射温度計により測定した値である。放射温度計による温度の測定値は、鋼材の表面（圧延時の上側の面）の最表層の温度の測定値である。 Using a slab comprising the composition of steel A shown in Table 1, a hot rolled steel sheet having a thickness of 3 mm to 12 mm was manufactured under the hot rolling conditions shown in FIG. 1, Table 2-1, and Table 2-2. . Here, the finish rolling temperature and the winding temperature in Table 2-1 and Table 2-2 are values measured by a radiation thermometer. The measured value of the temperature by the radiation thermometer is a measured value of the temperature of the outermost layer on the surface of the steel material (upper surface during rolling).
仕上げ圧延後の冷却の際、冷却水の量等で定まる鋼板と冷却水の間の熱伝達係数は、鋼板の表面（圧延時の上側の面）、裏面（圧延時の下側の面）ともに同程度となるようにした。 表２−２に示す熱伝達係数は、所定の冷却条件で鋼材が冷却されている場合に、鋼材表面のからの抜熱量と鋼材の温度低下量の関係示し、一定条件での冷却においては下記式（３）のαで示される値である。この熱伝達係数は、冷却水量、鋼材の表面の状態などに依存する。
熱伝達係数α＝Ｑ／（Ｔｗ−Ｔａ）・・・（３）
ここで、Ｑ：単位面積当たりの熱移動量（Ｗ）、Ｔｗ：鋼板の表面温度（Ｋ）、Ｔａ ：冷却水の温度（Ｋ）、ただしＴｗ＞Ｔａとする。
When cooling after finish rolling, the heat transfer coefficient between the steel sheet and the cooling water, which is determined by the amount of cooling water, etc., is the surface of the steel sheet (upper surface during rolling) and the rear surface (lower surface during rolling). It was set to be the same level. The heat transfer coefficient shown in Table 2-2 indicates the relationship between the amount of heat removed from the steel surface and the temperature drop of the steel material when the steel material is cooled under a predetermined cooling condition. This is the value indicated by α in equation (3). This heat transfer coefficient depends on the amount of cooling water, the surface state of the steel material, and the like.
Heat transfer coefficient α = Q / (Tw−Ta) (3)
Here, Q: heat transfer amount per unit area (W), Tw: steel plate surface temperature (K), Ta: cooling water temperature (K), where Tw> Ta.
表２−２に示す熱伝達係数は、例えば特公平６−８８０６０号公報に記載されるような、冷却帯の入側及び出側及び冷却ゾーン内の中間温度計にて測定された温度実績値に基づいて、逐次最小自乗法を用いることで、水冷時における上部各冷却バンクの熱伝達係数、下部各冷却バンクの熱伝達係数、及び空冷時における上部各冷却バンク、下部各冷却バンクの熱伝達係数を修正する技術を用いて求めた。 The heat transfer coefficients shown in Table 2-2 are actual temperature values measured by intermediate thermometers in the cooling zone on the entry and exit sides and in the cooling zone, as described in, for example, Japanese Patent Publication No. 6-88060. Based on the above, the heat transfer coefficient of each upper cooling bank during water cooling, the heat transfer coefficient of each lower cooling bank during water cooling, and the heat transfer between each upper cooling bank and each lower cooling bank during air cooling Obtained using a technique to correct the coefficients.
熱伝達係数は冷却の温度域の違いによる変動が見られたが、表２−２に示す値は、６５０℃以上の温度域での熱伝達係数の平均値である。また、連続熱間圧延工程では、通常、仕上げ圧延の後ランアウトテーブルでの冷却が始まるまでの間の数秒間、水冷が行われず空冷される領域が存在するが、表２における熱伝達係数の平均値はその温度域を通過した直後の水冷開始温度から６５０℃の間の平均値である。 Although the heat transfer coefficient fluctuated due to the difference in the cooling temperature range, the values shown in Table 2-2 are average values of the heat transfer coefficient in the temperature range of 650 ° C. or higher. In the continuous hot rolling process, there is usually a region where air cooling is not performed without water cooling for several seconds after finishing rolling until cooling on the run-out table is started. A value is an average value between 650 degreeC from the water-cooling start temperature immediately after passing through the temperature range.
なお、熱伝達係数は以下のようにして求めた。
まず、各々の水量密度による冷却を行った場合のランアウトテーブル中における鋼板の温度をランアウトテーブル中の数ケ所で測定し、それにより鋼板の温度履歴を求めた。次に、上記式（３）及び比熱の値を用いて、ランアウトテーブル内の各位置における温度降下量を求め、それより鋼板の温度履歴を求めた。そして、それが実測と一致するように熱伝達係数を求めた。
得られた熱延鋼板の幅方向中央部より２枚の幅方向のＪＩＳ５号引張試験片、幅方向の疲労試験片、ミクロ組織観察用試験片を採取した。ミクロ組織観察用試験片の圧延方向断面（幅方向と垂直な断面）を埋め込み、研磨を行い、ナイタール腐食の後、ミクロ組織の観察を行った。
The heat transfer coefficient was determined as follows.
First, the temperature of the steel plate in the run-out table when cooling with each water density was measured at several places in the run-out table, thereby obtaining the temperature history of the steel plate. Next, the amount of temperature drop at each position in the run-out table was determined using the above equation (3) and the specific heat value, and the temperature history of the steel sheet was determined therefrom. And the heat transfer coefficient was calculated | required so that it might correspond with measurement.
Two JIS No. 5 tensile test pieces in the width direction, fatigue test pieces in the width direction, and microstructural observation test pieces were collected from the center in the width direction of the obtained hot-rolled steel sheet. A cross section in the rolling direction (cross section perpendicular to the width direction) of the specimen for microstructural observation was embedded, polished, and microscopically observed after nital corrosion.
その後、同じ断面の鏡面研磨を行い、板厚中心部、及び表層及び裏層の３か所においてビッカース硬さ測定を行い、そこでの硬さの測定値の平均値を求めた。 After that, mirror polishing of the same cross section was performed, and Vickers hardness measurement was performed at the plate thickness center portion and the surface layer and the back layer at three locations, and the average value of the measured hardness values was obtained.
鋼板の板厚中心部の硬さ（ＨＶｃ）の測定においては、板厚中心部に位置する、全板厚の５０％に相当する厚さを有する層の中を、板厚方向（板面と垂直な方向）に０．１ｍｍ間隔で、硬さ（ＨＶ）を測定し、その平均値（算術平均以下同じ）を求めた。その硬さ測定の際の荷重は１ｋｇとした。鋼板の表裏層の硬さの測定は以下のように行った。 In the measurement of the hardness (HVc) at the center of the plate thickness of the steel plate, a layer having a thickness corresponding to 50% of the total plate thickness located at the center of the plate thickness is taken in the plate thickness direction (plate surface and The hardness (HV) was measured at intervals of 0.1 mm in the vertical direction), and the average value (the same as the arithmetic average) was obtained. The load for the hardness measurement was 1 kg. The hardness of the front and back layers of the steel sheet was measured as follows.
鋼板の表裏層の硬さ（ＨＶｓ）の測定においては、鋼板の表層、及び裏層から、鋼板全板厚の１０％に相当する距離だけ離れた板厚方向位置において、鋼板の圧延方向と平行方向な線上で０．１ｍｍ間隔の距離を置いて１０点の硬さ（ＨＶ）測定を行い、表裏層における測定値の平均値を求め、さらに表層と裏層の各々の平均値から表裏層の硬さの平均値を求めた。その際、硬さ測定の荷重は１ｋｇとした。尚、表裏層各々の硬さの各々の平均値の差は互いに±５％以内であり、小さかった。 In the measurement of the hardness (HVs) of the front and back layers of the steel plate, it is parallel to the rolling direction of the steel plate at a position in the plate thickness direction separated from the front and back layers of the steel plate by a distance corresponding to 10% of the total thickness of the steel plate. Measure the hardness (HV) at 10 points at a distance of 0.1 mm on a directional line, determine the average value of the measured values in the front and back layers, and further determine the average value of the front and back layers from the average values of the front and back layers. The average value of hardness was calculated. At that time, the hardness measurement load was 1 kg. In addition, the difference of the average value of each hardness of front and back layers was within ± 5% of each other and was small.
鋼板の疲労強度（ＦＬ）の評価は、表面が熱延ままの鋼板から図２に示す寸法の疲労試験片１を採取し、その中央部の表裏面に試験片の長手方向に所定の曲げの繰り返し応力を加え、試験片が疲労破壊するまでの繰り返し数である平面曲げ疲労寿命を求めた。ここで、Ｌは圧延方向、Ｗは板厚方向である。そして、応力レベルを変えて疲労寿命を求め、そして、１０７回の繰り返し数においても破壊しなかった最低の応力を疲労強度（ＦＬ）として求めた。疲労強度（ＦＬ）を求める際は、その疲労強度近傍の応力レベルにおいては付加する繰り返し応力を１０ＭＰａごとに変えて繰り返し応力を付加する試験を行った。この疲労強度を、引張強度（ＴＳ）で除した値を疲労限度比（ＦＬ／ＴＳ）とした。 The fatigue strength (FL) of the steel sheet is evaluated by taking a fatigue test piece 1 having the dimensions shown in FIG. 2 from the steel sheet whose surface is hot-rolled, and applying a predetermined bend in the longitudinal direction of the test piece on the front and back surfaces of the center part. Repeated stress was applied, and the plane bending fatigue life, which was the number of repetitions until the specimen was fatigued, was determined. Here, L is the rolling direction and W is the thickness direction. Then, the fatigue life was obtained by changing the stress level, and the lowest stress that did not break even after 10 7 repetitions was obtained as the fatigue strength (FL). When the fatigue strength (FL) was determined, a test was performed in which repeated stress was applied by changing the repeated stress to be applied every 10 MPa at a stress level near the fatigue strength. A value obtained by dividing the fatigue strength by the tensile strength (TS) was defined as a fatigue limit ratio (FL / TS).
このとき、試験片に加える繰り返し応力の条件は、完全両振り、即ち、応力振幅＝σ０とした場合に、応力の時間変化が、最大応力＝σ０、最小応力＝−σ０、応力の平均値＝０の正弦波となるような応力を加える条件とした。また、疲労寿命を評価するうえでは、同じ応力振幅σ０の値での試験を試験数Ｎ＝３として複数回行い、得られた各試験ごとの測定値を算術平均して平面曲げ疲労寿命の平均値を求め、その求めた平均値により評価することとした。その他の試験条件はＪＩＳ Ｚ ２２７５に準拠するものとした。 At this time, when the condition of the repeated stress applied to the test piece is complete swing, that is, when the stress amplitude = σ 0 , the time change of the stress is the maximum stress = σ 0 , the minimum stress = −σ 0 , The stress was applied so that the average value = 0 was a sine wave. Further, in evaluating the fatigue life, a test with the same stress amplitude σ 0 is performed a plurality of times with the number of tests N = 3, and the measured values obtained for each test are arithmetically averaged to obtain the plane bending fatigue life. An average value was obtained and evaluated based on the obtained average value. Other test conditions were based on JIS Z 2275.
図３に得られた熱延鋼板の、表層（表裏層）の硬さ（ＨＶｓ）と板厚中心部の硬さ（ＨＶｃ）の平均値の硬さ比（以降単に「硬さ比」と称することがある）と疲労限度比の関係を示す。硬さ比（ＨＶｓ／ＨＶｃ）を１．１５以上とすることにより疲労限度比を０．５０以上とすることができることが分かる。ここで、硬さは表層と裏層ではほぼ同じであった。尚、ここで、表裏層は硬いほど疲労特性は改善する。しかし、本発明では鉄鋼材料を急冷した場合に生じる硬質なミクロ組織を用いて表裏層を硬質としており、その観点から、板厚中心と表裏層の硬さ比の上限は特に限定するものではないが、大きくても３．０倍が限度であり、２．０倍が実用的である。 The hardness ratio of the average value of the hardness (HVs) of the surface layer (front and back layers) and the hardness (HVc) of the thickness center of the hot-rolled steel sheet obtained in FIG. 3 (hereinafter simply referred to as “hardness ratio”). ) And the fatigue limit ratio. It can be seen that the fatigue limit ratio can be 0.50 or more by setting the hardness ratio (HVs / HVc) to 1.15 or more. Here, the hardness was almost the same in the surface layer and the back layer. Here, the fatigue properties improve as the front and back layers become harder. However, in the present invention, the front and back layers are made hard using a hard microstructure generated when the steel material is rapidly cooled, and from this viewpoint, the upper limit of the hardness ratio between the center of the plate thickness and the front and back layers is not particularly limited. However, the maximum is 3.0 times, and 2.0 times is practical.
ここで、充分な疲労強度を得るために、板厚中心部に対して十分な硬さを有する表裏層の厚みを、表裏層から全板厚の１０％に相当する位置までの領域とすればよいのは以下の理由によるものと推定される。
疲労き裂は鋼材の表裏面において、繰り返し応力により転位が移動し、それが蓄積して表面に凹凸が生じることにより発生するとされている。表裏層から板厚の１０％に相当する位置までの領域を硬くすることにより、鋼板表層部での転位の移動を抑えることが可能となり、表裏面で生じる疲労き裂の発生を遅延させることが可能となることを本発明者は知見している。尚、本発明のようにランアウトテーブルにおいて、表裏層の表面からの抜熱により鋼板を冷却する場合、表裏層の表面に近い位置ほど冷却速度は大きいので、表裏層に近い位置ほどより多くの低温変態組織が現れるようにあり、硬さは増加する。従って、上述の場合、板厚の１０％の厚さより表層側及び裏層側にある組織は、表裏層から全板厚の１０％のそれぞれの位置の硬さより硬い組織となる。
Here, in order to obtain sufficient fatigue strength, if the thickness of the front and back layers having sufficient hardness with respect to the center portion of the plate thickness is a region from the front and back layers to a position corresponding to 10% of the total plate thickness, The good reason is presumed to be as follows.
Fatigue cracks are said to occur when dislocations move due to repeated stress on the front and back surfaces of steel materials and accumulate, resulting in irregularities on the surface. By stiffening the region from the front and back layers to the position corresponding to 10% of the plate thickness, it becomes possible to suppress the movement of dislocations at the surface layer portion of the steel sheet, and delay the occurrence of fatigue cracks occurring on the front and back surfaces. The present inventor has found that this is possible. In the run-out table as in the present invention, when cooling the steel sheet by removing heat from the surface of the front and back layers, the cooling rate is larger as the position is closer to the surface of the front and back layers. The metamorphosis appears to appear and the hardness increases. Therefore, in the above-mentioned case, the structure on the surface layer side and the back layer side from the thickness of 10% of the plate thickness is a structure harder than the hardness at each position of 10% of the total plate thickness from the front and back layers.
図４に６５０℃以上の熱伝達係数α（Ｊ／ｍ２ｓｅｃＫ）と硬さ比（ＨＶｓ／ＨＶｃ）の関係を示す。熱伝達係数α（Ｊ／ｍ２ｓｅｃＫ）が大きくなるほど、表裏層の硬さ（ＨＶｓ）と板厚中心部の硬さ（ＨＶｃ）との硬さ比（ＨＶｓ／ＨＶｃ）も増加する。硬さ比が増加するのは熱伝達係数が増加した場合、表裏層での抜熱量が増加して表裏層が急冷される一方で、鋼板の内部の冷却速度は表裏層ほど大きく増加しないため、板厚中心と表裏層の冷却速度の差が大きくなるためである。 FIG. 4 shows the relationship between the heat transfer coefficient α (J / m 2 secK) at 650 ° C. or higher and the hardness ratio (HVs / HVc). As the heat transfer coefficient α (J / m 2 secK) increases, the hardness ratio (HVs / HVc) between the hardness (HVs) of the front and back layers and the hardness (HVc) of the thickness center portion increases. The hardness ratio increases because when the heat transfer coefficient increases, the amount of heat removed from the front and back layers increases and the front and back layers are quenched, while the cooling rate inside the steel sheet does not increase as much as the front and back layers. This is because the difference in cooling rate between the center of the plate thickness and the front and back layers increases.
また、図４から、硬さ比（ＨＶｓ／ＨＶｃ）は同じ熱伝達係数αで冷却した場合、板厚３ｍｍ、６ｍｍ、１２ｍｍと板厚が異なる鋼板の硬さ比からみて、板厚が小さいほど硬さ比が大きいことが判明した。 Also, from FIG. 4, when the hardness ratio (HVs / HVc) is cooled with the same heat transfer coefficient α, the smaller the plate thickness is, the more the plate thickness is 3 mm, 6 mm, and 12 mm. It was found that the hardness ratio was large.
図５は、６５０℃以上の熱伝達係数αと板厚が異なる場合の硬さ比の変化を示す。同じ板厚で熱伝達係数が変化した場合の硬さ比の変化に着目すると、熱伝達係数が増加するにつれ硬さ比は増加するが、過度に熱伝達係数が大きくなると硬さ比は逆に低下することが分かる。熱伝達係数が増加するにつれ硬さ比は増加するのは、鋼板表裏層の組織がベイナイト単相組織となるためである。一方、過度に熱伝達係数が大きくなると硬さ比が小さくなるのは、その場合に鋼板の板厚中心部（１／２ｔと表示）でフェライト組織が減少し、表裏層との組織の差が小さくなるためである。図中の○印に付与した数字は、表裏層の硬さ（ＨＶｓ）と板厚中心部の硬さ（ＨＶｃ）との硬さ比（ＨＶｓ／ＨＶｃ）を示す数字である。
硬さ比を１．１５以上にするために必要な熱伝達係数の範囲は板厚が大きいほど大きく、図５から十分な硬さ比１．１５以上を得るためには式（１）で表わされる熱伝達係数α（Ｊ／ｍ２ｓｅｃＫ）の範囲内とする必要があることが分かる。
９１×板厚（ｍｍ）＋７５６
≦α（Ｊ／ｍ２ｓｅｃＫ）≦９１×板厚（ｍｍ）＋１８００ ・・・（１）
FIG. 5 shows changes in the hardness ratio when the heat transfer coefficient α of 650 ° C. or higher and the plate thickness are different. Focusing on the change in the hardness ratio when the heat transfer coefficient changes with the same thickness, the hardness ratio increases as the heat transfer coefficient increases, but the hardness ratio is reversed when the heat transfer coefficient becomes excessively large. It turns out that it falls. The reason why the hardness ratio increases as the heat transfer coefficient increases is that the structure of the steel sheet front and back layers becomes a bainite single-phase structure. On the other hand, if the heat transfer coefficient becomes excessively large, the hardness ratio decreases. In this case, the ferrite structure decreases at the center of the plate thickness (indicated as 1 / 2t), and the difference in structure between the front and back layers This is because it becomes smaller. The numbers given to the circles in the figure are the numbers indicating the hardness ratio (HVs / HVc) between the hardness (HVs) of the front and back layers and the hardness (HVc) of the center of the plate thickness.
The range of the heat transfer coefficient necessary for setting the hardness ratio to 1.15 or more is larger as the plate thickness is larger, and in order to obtain a sufficient hardness ratio of 1.15 or more from FIG. It can be seen that the heat transfer coefficient α (J / m 2 secK) must be within the range.
91 x plate thickness (mm) + 756
≦ α (J / m 2 secK) ≦ 91 × plate thickness (mm) +1800 (1)
図６は、６５０℃以上の水量密度Ｗ（ｍ３／ｓｅｃ／ｍ２）と板厚（ｍｍ）が異なる場合の硬さ比（ＨＶｓ／ＨＶｃ）の変化を示す。同じ板厚で水量密度が変化した場合の硬さ比の変化に着目すると、水量密度が増加するにつれ硬さ比は増加するが、過度に水量密度が大きくなると硬さ比は逆に低下することが分かる。水量が増加するにつれ硬さ比は増加するのは、熱伝達係数の増加により鋼板表裏層の組織がベイナイト単相組織となるためである。一方、過度に水量密度が大きくなると硬さ比が小さくなるのは、その場合に過度に熱伝達係数が増加し、鋼板の板厚中心部（１／２ｔと表示）でフェライト組織が減少し、表裏層との組織の差が小さくなるためである。図中の○印に付与した数字は、表裏層の硬さ（ＨＶｓ）と板厚中心部の硬さ（ＨＶｃ）との硬さ比（ＨＶｓ／ＨＶｃ）を示す数字である。
硬さ比を１．１５以上とするために必要な水量密度の範囲は板厚が大きいほど大きく、図６から十分な硬さ比１．１５以上を得るためには、水量密度Ｗ（ｍ３／ｓｅｃ／ｍ２）を下記式（２）で表わされる範囲内とする必要があることが分かる。
０．００４８×板厚（ｍｍ）＋０．００３５７
≦Ｗ（ｍ３／ｓｅｃ／ｍ２）≦０．００４８×板厚（ｍｍ）＋０．０５５ ・（２）
FIG. 6 shows the change in the hardness ratio (HVs / HVc) when the water density W (m 3 / sec / m 2 ) and the plate thickness (mm) at 650 ° C. or higher are different. Focusing on the change in the hardness ratio when the water density changes at the same thickness, the hardness ratio increases as the water density increases, but the hardness ratio decreases as the water density increases excessively. I understand. The reason why the hardness ratio increases as the amount of water increases is that the structure of the steel sheet front and back layers becomes a bainite single-phase structure due to an increase in the heat transfer coefficient. On the other hand, when the water density is excessively increased, the hardness ratio is decreased. In this case, the heat transfer coefficient is excessively increased, and the ferrite structure is decreased at the center of the plate thickness (indicated as 1 / 2t). This is because the difference in structure between the front and back layers is reduced. The numbers given to the circles in the figure are the numbers indicating the hardness ratio (HVs / HVc) between the hardness (HVs) of the front and back layers and the hardness (HVc) of the center of the plate thickness.
The range of the water density necessary for setting the hardness ratio to 1.15 or more increases as the plate thickness increases. From FIG. 6, in order to obtain a sufficient hardness ratio of 1.15 or more, the water density W (m 3 It can be seen that / sec / m 2 ) needs to be within the range represented by the following formula (2).
0.0048 × plate thickness (mm) +0.00357
≦ W (m 3 / sec / m 2 ) ≦ 0.0048 × plate thickness (mm) +0.055 (2)
また、図７（ａ）及び（ｃ）に示すように、表裏層部のミクロ組織をベイナイト単相組織とすることにより、良好な疲労限度比を得ることができることが判明した。図７（ｃ）は、図７（ａ）の表層部のミクロ組織の拡大写真である。良好な疲労限度比が得られるのは、ベイナイト組織には微細な炭化物が含まれるが、その微細な炭化物により表裏層の強度が増加する一方で、疲労き裂自体は炭化物が微細であるために抑制されるためである。本発明の鋼板における良好な疲労強度は、表裏層が硬質であることによる効果に加え、表裏層の鋼組織をベイナイト単相とすることの効果も合わせて得ることにより得られる。表裏層においてそのような組織を得る為には、上述のような熱伝達係数での急速冷却を鋼板表裏層温度が少なくとも６５０℃となるまで行う必要があることが判明した。 Further, as shown in FIGS. 7A and 7C, it was found that a favorable fatigue limit ratio can be obtained by making the microstructure of the front and back layer portions a bainite single phase structure. FIG. 7C is an enlarged photograph of the microstructure of the surface layer portion of FIG. A good fatigue limit ratio is obtained because the bainite structure contains fine carbides, but the fine carbides increase the strength of the front and back layers, while the fatigue crack itself is because the carbides are fine. This is because it is suppressed. Good fatigue strength in the steel sheet of the present invention can be obtained by obtaining the effect of making the steel structure of the front and back layers a bainite single phase in addition to the effect of the hard front and back layers. In order to obtain such a structure in the front and back layers, it has been found that it is necessary to perform rapid cooling with the heat transfer coefficient as described above until the steel plate front and back layer temperature is at least 650 ° C.
本発明において、図７（ｃ）で示すベイナイト組織は、表裏層部に存在し、表層及び裏層のそれぞれは板厚の１０％以上の厚みを有する必要がある。これは、以下の理由によるものと推定している。
本発明において、表裏層のベイナイト組織により、表裏層表面近傍のミクロ組織中における炭化物を起点とした疲労き裂の発生が遅延される。疲労き裂は、表裏層近傍の板厚のおおよそ１０％の厚みを有する層の中の転位の蓄積を通じて生じるので、本発明者の知見によれば表層及び裏層のベイナイト層の厚みは、それぞれ板厚の１０％以上とする必要がある。一方、ベイナイト単相組織の表裏層のそれぞれの厚さの最大値は、後述のように、軟質な組織の層を板厚中心部に全板厚の５０％以上の厚さに設ける必要性があることから、表裏層の厚さはそれぞれ全板厚の２５％となる。また、ここで鋼板の表層は圧延時に上側であった面であり、裏層とは圧延時に下側であった面である。但し、本発明では、表面・裏面の組織、特性は板厚中心に対して大凡対象であることを前提としており、表層と裏層を区別する必要はない。
In the present invention, the bainite structure shown in FIG. 7C exists in the front and back layer portions, and each of the front layer and the back layer needs to have a thickness of 10% or more of the plate thickness. This is presumed to be due to the following reason.
In the present invention, the bainite structure of the front and back layers delays the occurrence of fatigue cracks starting from carbides in the microstructure near the front and back layer surfaces. Since fatigue cracks occur through the accumulation of dislocations in a layer having a thickness of approximately 10% of the thickness in the vicinity of the front and back layers, according to the knowledge of the present inventors, the thicknesses of the bainite layers of the front and back layers are respectively It is necessary to be 10% or more of the plate thickness. On the other hand, the maximum value of the thicknesses of the front and back layers of the bainite single-phase structure requires that a soft structure layer be provided at a thickness of 50% or more of the total thickness at the center of the thickness as described later. Therefore, the thicknesses of the front and back layers are each 25% of the total plate thickness. Moreover, the surface layer of a steel plate is the surface which was the upper side at the time of rolling here, and a back layer is the surface which was the lower side at the time of rolling. However, in the present invention, it is premised that the structure and characteristics of the front and back surfaces are roughly targeted with respect to the center of the plate thickness, and it is not necessary to distinguish between the front layer and the back layer.
本発明において、鋼板の成形性（全伸び）を良好とするためには、図７（ｂ）に示すように、板厚中心部の組織は軟質なフェライト組織とする必要がある。軟質なフェライト組織による全伸びを良好とする効果を得るためには、その軟質層の厚みは全板厚の５０％以上とする必要がある。そして、その板厚中心部のフェライトを含む軟質な層におけるフェライト以外の組織は、ランアウトテーブルでの冷却中にフェライトの次に生じるベイナイトまたはパーライトまたはその複合組織となる。この軟質な層の厚さの上限は全板厚の８０％である。これは、前述のように、鋼板の表層および裏層にそれぞれ全板厚の１０％以上の厚さを有する硬質な層を設ける必要があるためである。 In the present invention, in order to improve the formability (total elongation) of the steel sheet, as shown in FIG. 7B, the structure at the center of the sheet thickness needs to be a soft ferrite structure. In order to obtain the effect of improving the total elongation due to the soft ferrite structure, the thickness of the soft layer needs to be 50% or more of the total plate thickness. The structure other than ferrite in the soft layer containing ferrite at the center of the plate thickness becomes bainite, pearlite, or a composite structure generated next to ferrite during cooling on the run-out table. The upper limit of the thickness of this soft layer is 80% of the total plate thickness. This is because, as described above, it is necessary to provide a hard layer having a thickness of 10% or more of the total thickness on the front and back layers of the steel plate.
本発明の鋼板のミクロ組織については、鋼板の表裏層組織は硬質なベイナイト組織とする必要がある。これは、表裏層を十分硬くして、かつ不可避的に生じる炭化物が粗大に析出することを防ぐことにより表裏層からの疲労き裂の発生を抑制することができるためである。一方、板厚中心部のミクロ組織はフェライト面積分率が５０％以上となるようにする必要がある。これは、良好な疲労特性を得ながらも、得られる鋼板の成形性を良好に保つためである。ミクロ組織はフェライト単独の組織であってもよいが、通常の製造工程からして、フェライト組織以外にベイナイトまたはパーライトまたはその複合組織を含んでいてもよい。 Regarding the microstructure of the steel sheet of the present invention, the front and back layer structure of the steel sheet needs to be a hard bainite structure. This is because it is possible to suppress the occurrence of fatigue cracks from the front and back layers by making the front and back layers sufficiently hard and preventing the unavoidable carbides from precipitating out. On the other hand, the microstructure in the central portion of the plate thickness needs to have a ferrite area fraction of 50% or more. This is for maintaining good formability of the obtained steel sheet while obtaining good fatigue characteristics. Although the microstructure may be a structure of ferrite alone, it may contain bainite, pearlite, or a composite structure thereof in addition to the ferrite structure from a normal manufacturing process.
次に、本発明の鋼板の化学成分の限定理由について説明する。ここで、成分についての「％」は質量％である。 Next, the reasons for limiting the chemical components of the steel sheet of the present invention will be described. Here, “%” for the component is mass%.
（Ｃ：０．１０〜０．２０％）
Ｃは、０．２０％超含有していると加工性及び溶接性が劣化するので、０．２０％以下とする。また、Ｃが高すぎると、フェライト変態が遅延しベイナイトが増加する。そのため、板厚中心部におけるフェライト面積分率が低下する。そのため、急冷却時にも表裏層と板厚中心の硬さ比を大きくすることができなくなる。この観点からもＣの上限は０．２０％とする。Ｃが低すぎるとフェライト変態が速くなり、急冷却を行っても鋼板表裏層をベイナイト単相組織とはできなくなる。そこで、Ｃの下限は０．１０％とする。上記の観点から、Ｃは０．１０〜０．２０％としたが０．１３〜０．１８％であることが好ましい。
(C: 0.10 to 0.20%)
If the C content exceeds 0.20%, workability and weldability deteriorate, so the content is made 0.20% or less. On the other hand, if C is too high, ferrite transformation is delayed and bainite increases. Therefore, the ferrite area fraction in the center portion of the plate thickness is lowered. For this reason, the hardness ratio between the front and back layers and the thickness center cannot be increased even during rapid cooling. Also from this viewpoint, the upper limit of C is 0.20%. If C is too low, the ferrite transformation becomes fast, and even if rapid cooling is performed, the steel sheet front and back layers cannot have a bainite single phase structure. Therefore, the lower limit of C is 0.10%. From the above viewpoint, C is 0.10 to 0.20%, but is preferably 0.13 to 0.18%.
（Ｓｉ：０．０１〜２．００％）
Ｓｉは、予備脱酸に必要な元素である。所定の効果を得るためには０．０１％以上含有する必要がある。しかし、２．００％超とした場合、変態点が過度に高温となるため、本発明に必要な圧延温度の確保が困難となるためその上限は２．００％、好ましくは１．４０％である。 上記の観点から、Ｓｉは０．０１〜２．００％としたが０．０１〜１．４０％であることが好ましい。
(Si: 0.01-2.00%)
Si is an element necessary for preliminary deoxidation. In order to obtain a predetermined effect, it is necessary to contain 0.01% or more. However, if it exceeds 2.00%, the transformation point becomes excessively high, and it becomes difficult to secure the rolling temperature necessary for the present invention, so the upper limit is 2.00%, preferably 1.40%. is there. From the above viewpoint, Si is set to 0.01 to 2.00%, but preferably 0.01 to 1.40%.
（Ｍｎ：０．１０〜２．００％）
Ｍｎは、固溶強化元素として強度上昇に有効である。所望の強度を得るためには０．１０％以上必要であるが、０．４０％以上とすることが望ましい。一方、２．００％超添加するとスラブ割れを生ずるため、２．００％以下とする。また、Ｍｎはオーステナイトフォーマーでありフェライト変態を遅延させる。従って、Ｍｎが過多にあると板厚中心部のフェライトが減少し、ベイナイトを増加させ、表裏層と板厚中心部の硬さ比を大きくすることができなくなる。この観点からもＭｎの上限は２．００％、好ましくは１．６０％である。上記の観点から、Ｍｎは０．１０〜２．００％としたが０．４０〜１．６０％であることが好ましい。
(Mn: 0.10 to 2.00%)
Mn is effective for increasing the strength as a solid solution strengthening element. In order to obtain a desired strength, 0.10% or more is necessary, but 0.40% or more is desirable. On the other hand, if added over 2.00%, slab cracking occurs, so the content is made 2.00% or less. Mn is an austenite former and delays ferrite transformation. Therefore, if Mn is excessive, ferrite in the center portion of the plate thickness decreases, bainite increases, and the hardness ratio between the front and back layers and the center portion of the plate thickness cannot be increased. Also from this viewpoint, the upper limit of Mn is 2.00%, preferably 1.60%. From the above viewpoint, Mn is 0.10 to 2.00%, but is preferably 0.40 to 1.60%.
（Ｐ≦０．１００％）
Ｐは、不可避的に含有される不純物元素であり低いほど望ましく、０．１００％超含有すると加工性や溶接性に悪影響を及ぼすと共に疲労特性も低下させるので、０．１００％以下とするが、好ましくは０．０２０％以下である。
(P ≦ 0.100%)
P is an impurity element that is inevitably contained, and is desirably as low as possible. If contained over 0.100%, workability and weldability are adversely affected and fatigue characteristics are also reduced. Preferably it is 0.020% or less.
（Ｓ≦０．０１００％）
Ｓは、Ｐと同様に不可避的に含有される不純物元素であり低いほど望ましく、多すぎるとＭｎＳ等の粗大な介在物となって成形性を劣化させるので、０．０１００％以下とする必要があるが、Ｓの上限は好ましくは０．００３％である。
(S ≦ 0.0100%)
S is an impurity element that is inevitably contained in the same manner as P, and is desirably as low as possible. If it is too large, it becomes coarse inclusions such as MnS and deteriorates moldability, so it is necessary to make it 0.0100% or less. However, the upper limit of S is preferably 0.003%.
（Ａｌ：０．００５〜０．０５０％）
Ａ１は、溶鋼の脱酸に必要な元素である。その効果を得るには０．００５％以上、好ましくは０．０１０％以上含有させることが望ましい。しかし、過多に添加すると、変態点を極度に上昇させ、本発明に必要な圧延温度の確保が困難となるためその上限は０．０５０％、好ましくは０．０３０％とする。以上の観点から、Ａｌは、０．００５〜０．０５０％としたが、０．０１０〜０．０３０％とすることが望ましい。
(Al: 0.005 to 0.050%)
A1 is an element necessary for deoxidation of molten steel. In order to acquire the effect, it is desirable to make it contain 0.005% or more, preferably 0.010% or more. However, if excessively added, the transformation point is extremely increased, and it becomes difficult to secure the rolling temperature necessary for the present invention, so the upper limit is made 0.050%, preferably 0.030%. From the above viewpoint, Al is 0.005 to 0.050%, but is preferably 0.010 to 0.030%.
（Ｎ≦０．０１００％）
Ｎは、成分調整段階で溶鋼に混入する不可避的不純物である。過多にあると、鋼材の時効を促進し加工性を劣化させる可能性があるので０．０１００％以下とする。好ましくは、０．００４０％以下である。
(N ≦ 0.0100%)
N is an unavoidable impurity mixed in the molten steel in the component adjustment stage. If it is excessive, the aging of the steel material is accelerated and the workability may be deteriorated, so the content is made 0.0100% or less. Preferably, it is 0.0040% or less.
以上を基本的な組成とする。 The above is the basic composition.
次いで、必要に応じて選択的に添加させることができる成分（元素）について説明する。これらの成分はいずれも鋼板の強度を増加するに寄与する成分である。上記基本成分に加えて、必要に応じて、強度を得る為に以下の元素の内一種類以上を添加してもよい。 Next, components (elements) that can be selectively added as necessary will be described. These components are all components that contribute to increasing the strength of the steel sheet. In addition to the above basic components, if necessary, one or more of the following elements may be added to obtain strength.
（Ｎｂ：０．０５０％以下）
Ｎｂは析出強化により強度を増加させる元素である。強度を得る為に、必要に応じてＮｂを添加してもよい。十分な強度増加の効果を得るためには、０．００５％以上の添加をすることが望ましい。しかし過多にあるとその効果は飽和する一方で、コストを増加させる要因となるので、上限を０．０５０％とする。
(Nb: 0.050% or less)
Nb is an element that increases the strength by precipitation strengthening. In order to obtain strength, Nb may be added as necessary. In order to obtain a sufficient strength increase effect, it is desirable to add 0.005% or more. However, if the amount is excessive, the effect is saturated, but the cost increases, so the upper limit is made 0.050%.
（Ｔｉ：０．３００％以下）
Ｔｉは析出強化により強度を増加させる元素である。強度を得る為に、必要に応じてＴｉを添加してもよい。十分な強度増加の効果を得るためには、０．００５％以上の添加をすることが望ましい。しかし過多にあるとその効果は飽和する一方で、コストを増加させる要因となるので、上限を０．３００％とする。
(Ti: 0.300% or less)
Ti is an element that increases the strength by precipitation strengthening. In order to obtain strength, Ti may be added as necessary. In order to obtain a sufficient strength increase effect, it is desirable to add 0.005% or more. However, if the amount is excessive, the effect is saturated, but the cost is increased. Therefore, the upper limit is set to 0.300%.
（Ｖ：０．１０％以下）
Ｖは析出強化により強度を増加させる元素である。強度を得る為に、必要に応じてＶを添加してもよい。十分な強度増加の効果を得るためには、０．０１％以上の添加をすることが望ましい。しかし過多にあるとその効果は飽和する一方で、コストを増加させる要因となるので、上限を０．１０％とする。
(V: 0.10% or less)
V is an element that increases the strength by precipitation strengthening. In order to obtain strength, V may be added as necessary. In order to obtain a sufficient strength increase effect, it is desirable to add 0.01% or more. However, if the amount is excessive, the effect is saturated, but the cost increases, so the upper limit is made 0.10%.
（Ｃｕ：１．０％以下）
Ｃｕは固溶強化により強度を増加させる元素である。強度を得る為に、必要に応じてＣｕを添加してもよい。十分な強度増加の効果を得るためには、０．１０％以上の添加をすることが望ましい。しかし過多にあるとその効果は飽和する一方で、コストを増加させる要因となるので、上限を１．０％とする。
(Cu: 1.0% or less)
Cu is an element that increases the strength by solid solution strengthening. In order to obtain strength, Cu may be added as necessary. In order to obtain a sufficient strength increase effect, it is desirable to add 0.10% or more. However, if the amount is excessive, the effect is saturated, but the cost is increased, so the upper limit is set to 1.0%.
（Ｎｉ：１．０％以下）
Ｎｉは固溶強化により強度を増加させる元素である。強度を得る為に、必要に応じてＮｉを添加してもよい。十分な強度増加の効果を得るためには、０．１０％以上の添加をすることが望ましい。しかし過多にあるとその効果は飽和する一方で、コストを増加させる要因となるので、上限を１．０％とする。
(Ni: 1.0% or less)
Ni is an element that increases the strength by solid solution strengthening. In order to obtain strength, Ni may be added as necessary. In order to obtain a sufficient strength increase effect, it is desirable to add 0.10% or more. However, if the amount is excessive, the effect is saturated, but the cost is increased, so the upper limit is set to 1.0%.
（Ｃｒ：１．０％以下）
Ｃｒは固溶強化により強度を増加させる元素である。強度を得る為に、必要に応じてＣｒを添加してもよい。十分な強度増加の効果を得るためには、０．１０％以上の添加をすることが望ましい。しかし過多にあるとその効果は飽和する一方で、コストを増加させる要因となるので、上限を１．０％とする。
(Cr: 1.0% or less)
Cr is an element that increases the strength by solid solution strengthening. In order to obtain strength, Cr may be added as necessary. In order to obtain a sufficient strength increase effect, it is desirable to add 0.10% or more. However, if the amount is excessive, the effect is saturated, but the cost is increased, so the upper limit is set to 1.0%.
（Ｂ：０．００５０％以下）
Ｂは焼き入れ強化により強度を増加させる元素である。強度を得る為に、必要に応じてＢを添加してもよい。十分な強度増加の効果を得るためには、０．０００１％以上の添加をすることが望ましい。しかし過多にあるとその効果は飽和する一方で、コストを増加させる要因となるので、上限を０．００５０％とする。
(B: 0.0050% or less)
B is an element that increases the strength by quenching strengthening. In order to obtain strength, B may be added as necessary. In order to obtain a sufficient strength increase effect, it is desirable to add 0.0001% or more. However, if the amount is excessive, the effect is saturated, but the cost increases, so the upper limit is set to 0.0050%.
（Ｃａ：０．００３０％以下）
硫化物の形態制御を行い、強度を増加し、加工性を改善するために、Ｃａを添加してもよい。形態制御のため必要な効果を得る為には０．０００５％以上を添加することが望ましい。一方、過多にあると効果が飽和し、かつコスト増加要因となるので、それを防ぐ観点から上限をＣａ：０．００３０％とする。
(Ca: 0.0030% or less)
Ca may be added to control the form of the sulfide, increase the strength, and improve workability. In order to obtain a necessary effect for form control, it is desirable to add 0.0005% or more. On the other hand, if the amount is excessive, the effect is saturated and the cost increases, so the upper limit is set to Ca: 0.0030% from the viewpoint of preventing it.
（ＲＥＭ：０．０２００％以下）
ＲＥＭもＣａと同様に硫化物の形態制御を行い、強度を増加し、加工性を改善するために、ＲＥＭ（希土類元素）を添加してもよい。形態制御のため必要な効果を得る為には０．０００５％以上を添加することが望ましい。一方、過多にあると効果が飽和し、かつコスト増加要因となるので、それを防ぐ観点から上限をＲＥＭ：０．０２００％とする。
(REM: 0.0200% or less)
Similarly to Ca, REM may be added with REM (rare earth element) in order to control the form of sulfides, increase strength, and improve workability. In order to obtain a necessary effect for form control, it is desirable to add 0.0005% or more. On the other hand, if the amount is excessive, the effect is saturated and the cost increases, so the upper limit is set to REM: 0.0200% from the viewpoint of preventing it.
以上必要に応じて選択的に含有させる成分について説明したが、これらの選択成分は上記に説明した下限値以下を含有しても本発明の疲労特性に優れた高強度熱延鋼板の効果を損なうものではないので、本発明はその下限値以下をも含有することを許容するものである。また、上記に述べた成分の残部はＦｅおよび不可避不純物である。 Although the components to be selectively contained as necessary have been described above, these selected components impair the effect of the high-strength hot-rolled steel sheet excellent in fatigue characteristics of the present invention even if they contain the lower limit value or less described above. Since it is not a thing, this invention accept | permits containing below the lower limit. The balance of the components described above is Fe and inevitable impurities.
次に、本発明の製造方法の限定理由について、詳細に述べる。 Next, the reason for limiting the production method of the present invention will be described in detail.
本発明の製造方法は、初めに、上記に述べた成分に調整された鋳片を精錬工程、連続鋳造工程を用いて製造する。次に、加熱、粗圧延、仕上げ圧延、冷却、巻き取り及び精整工程からなる連続熱延工程により熱延鋼板を得る。以下に具体的製造条件について述べる。 In the production method of the present invention, first, a slab adjusted to the above-described components is produced using a refining process and a continuous casting process. Next, a hot-rolled steel sheet is obtained by a continuous hot-rolling process including heating, rough rolling, finish rolling, cooling, winding and refining processes. Specific manufacturing conditions are described below.
（１１５０℃以上に加熱）
加熱温度は、粗圧延、仕上げ圧延からなる連続熱間圧延工程により熱延鋼板を得るために必要とされる温度が好ましい。この加熱温度は常法では、仕上げ圧延の温度を所定以上とする観点から１１５０℃以上である。加熱温度が高すぎると加熱中に生じる酸化層に起因した表面疵が生じる。また、過度に加熱温度を上げることは、生産コストの観点からも好ましくない。この観点から加熱温度の上限は１３００℃程度が望ましい。
(Heating above 1150 ° C)
The heating temperature is preferably a temperature required to obtain a hot-rolled steel sheet by a continuous hot rolling process including rough rolling and finish rolling. This heating temperature is usually 1150 ° C. or higher from the viewpoint of setting the finish rolling temperature to a predetermined level or higher. If the heating temperature is too high, surface flaws caused by an oxide layer generated during heating will occur. Moreover, raising the heating temperature excessively is not preferable from the viewpoint of production cost. From this viewpoint, the upper limit of the heating temperature is preferably about 1300 ° C.
粗圧延は、加熱炉から加熱した鋳片を抽出した後から仕上げ圧延の間の圧延工程であるが、その温度域も常法に従う。 Rough rolling is a rolling process between extracting a cast slab heated from a heating furnace and then finishing rolling, and its temperature range also follows a conventional method.
（Ａｒ３温度＋５０（℃）以上の温度にて仕上げ圧延）
仕上げ圧延温度は、Ａｒ３温度＋５０℃以上とする必要がある。これは圧延温度がそれより低い場合、フェライト変態が鋼板表裏層において促進され、ランアウトテーブルでの熱伝達係数の増加による表裏層硬さの増加効果が得られないためである。尚、Ａｒ３変態温度は以下の公知の式（４）で求めるものとする。仕上げ圧延温度が過度に高いと、圧延前の鋼板に付着する酸化層の厚みが増加し、それが圧延時に噛みこまれ、鋼板に疵を残す。この観点から、仕上げ圧延温度の上限は概ね１１００℃とする。
Ａｒ３＝８６８−３９６×Ｃ＋２５×Ｓｉ−６８×Ｍｎ−３６×Ｎｉ−２１×Ｃｕ−２５×Ｃｒ＋３０×Ｍｏ ・・・（４）
ここで、各元素は鋼板中に含有された元素の含有量（質量％）である。なお、含有されていない元素の含有量は０質量％とする。
(Finish rolling at Ar3 temperature + 50 (° C) or higher)
The finish rolling temperature needs to be Ar 3 temperature + 50 ° C. or higher. This is because when the rolling temperature is lower than that, ferrite transformation is promoted in the front and back layers of the steel sheet, and the effect of increasing the front and back layer hardness due to the increase in the heat transfer coefficient at the run-out table cannot be obtained. Incidentally, Ar 3 transformation temperature shall be determined by the following known formula (4). If the finish rolling temperature is excessively high, the thickness of the oxide layer adhering to the steel plate before rolling increases, and it is bitten during rolling, leaving wrinkles on the steel plate. From this viewpoint, the upper limit of the finish rolling temperature is approximately 1100 ° C.
Ar 3 = 868-396 × C + 25 × Si-68 × Mn-36 × Ni-21 × Cu-25 × Cr + 30 × Mo (4)
Here, each element is content (mass%) of the element contained in the steel plate. The content of elements not contained is 0% by mass.
（６５０℃までの間の平均の熱伝達係数α）
所定の硬さ比を得る為に必要な、仕上げ圧延終了温度から６５０℃までの間の平均の鋼板表裏層の熱伝達係数αの範囲は板厚に依存し、その許容範囲は下記式（１）で表わされる。
９１×板厚（ｍｍ）＋７５６
≦α（Ｊ／ｍ２ｓｅｃＫ）≦９１×板厚（ｍｍ）＋１８００ ・・・（１）
そして、硬さ比を最大とする観点から、熱伝達係数αは下記式（１−２）を満たすことが望ましい。
９１×板厚（ｍｍ）＋９００
≦α（Ｊ／ｍ２ｓｅｃＫ）≦９１×板厚（ｍｍ）＋１５００ ・・（１−２）
(Average heat transfer coefficient α up to 650 ° C)
The range of the average heat transfer coefficient α of the front and back layers of the steel sheet from the finish rolling end temperature to 650 ° C., which is necessary for obtaining a predetermined hardness ratio, depends on the plate thickness, and the allowable range is the following formula (1 ).
91 x plate thickness (mm) + 756
≦ α (J / m 2 secK) ≦ 91 × plate thickness (mm) +1800 (1)
And from the viewpoint of maximizing the hardness ratio, it is desirable that the heat transfer coefficient α satisfies the following formula (1-2).
91 x plate thickness (mm) + 900
≦ α (J / m 2 secK) ≦ 91 × plate thickness (mm) +1500 (1-2)
ここで、必要な熱伝達係数α（Ｊ／ｍ２ｓｅｃＫ）に下限（式１の左辺）が存在するのは、それが小さすぎると鋼板表裏層がベイナイト単相組織とならないためである。一方、それに上限（式１の右辺）が存在するのは、それが大きすぎると板厚中心部のフェライトが減少し、そこの硬さが増加するためである。 Here, the lower limit (the left side of Formula 1) exists in the necessary heat transfer coefficient α (J / m 2 secK) because the steel sheet front and back layers do not have a bainite single phase structure if it is too small. On the other hand, there is an upper limit (the right side of Formula 1) because if it is too large, ferrite in the central portion of the plate thickness decreases and the hardness thereof increases.
そのような仕上げ圧延終了温度からの冷却条件を満たす急冷却の温度域を６５０℃以上としたのは、急冷却に伴って生じる硬さ比を増加させるミクロ組織の変化は、６５０℃以上の温度域の冷却速度の変化により生じるためである。 The reason why the rapid cooling temperature range satisfying the cooling condition from the finish rolling finish temperature is set to 650 ° C. or more is that the change in the microstructure that increases the hardness ratio caused by rapid cooling is a temperature of 650 ° C. or more. This is due to the change in the cooling rate of the area.
（６５０℃までの間の冷却での平均の水量密度Ｗ）
所定の硬さ比を得る為に必要な、６５０℃以上の冷却における水量密度の範囲は板厚に依存し、その許容範囲は下記式（２）で表わされる。
０．００４８×板厚（ｍｍ）＋０．００３５７
≦Ｗ（ｍ３／ｓｅｃ／ｍ２）≦０．００４８×板厚（ｍｍ）＋０．０５５ ・（２）
そして、硬さ比を最大とする観点からは、６５０℃以上の冷却における水量密度Ｗ（ｍ３／ｓｅｃ／ｍ２）の範囲は下記式（２−２）を満たすことが好ましい。
０．００５×板厚（ｍｍ）＋０．０１０
≦Ｗ（ｍ３／ｓｅｃ／ｍ２）≦０．００５×板厚（ｍｍ）＋０．０４０・・（２−２）
(Average water density W in cooling to 650 ° C.)
The range of water density necessary for obtaining a predetermined hardness ratio in cooling at 650 ° C. or higher depends on the plate thickness, and the allowable range is expressed by the following formula (2).
0.0048 × plate thickness (mm) +0.00357
≦ W (m 3 / sec / m 2 ) ≦ 0.0048 × plate thickness (mm) +0.055 (2)
From the viewpoint of maximizing the hardness ratio, the range of the water density W (m 3 / sec / m 2 ) in cooling at 650 ° C. or higher preferably satisfies the following formula (2-2).
0.005 x plate thickness (mm) + 0.010
≦ W (m 3 / sec / m 2 ) ≦ 0.005 × plate thickness (mm) +0.040 (2−2)
ここで、必要な水量密度Ｗ（ｍ３／ｓｅｃ／ｍ２）に下限（式２の左辺）が存在するのは、それが小さすぎると熱伝達係数が低下し鋼板表裏層のベイナイト層の厚さが小さくなり、また、表裏層の硬さも小さくなるためである。一方、それに上限（式２の右辺）が存在するのは、それが大きすぎると熱伝達係数が過度に増加して板厚中心部のフェライトが減少し、そこの硬さが増加するためである。 Here, there is a lower limit (the left side of Formula 2 ) in the required water density W (m 3 / sec / m 2 ). If it is too small, the heat transfer coefficient decreases and the thickness of the bainite layer on the front and back layers of the steel sheet This is because the hardness of the front and back layers is reduced. On the other hand, there is an upper limit (the right side of Formula 2) because if it is too large, the heat transfer coefficient increases excessively, the ferrite at the center of the plate thickness decreases, and the hardness increases. .
そのような水量密度での冷却を必要とする急冷却の温度域を６５０℃以上としたのは、急冷却に伴って生じる硬さ比を増加させるミクロ組織の変化は、６５０℃以上の温度域の冷却速度の変化により生じるためである。 The reason why the temperature range of rapid cooling that requires cooling at such a water density is 650 ° C. or higher is that the change in the microstructure that increases the hardness ratio that occurs with rapid cooling is the temperature range of 650 ° C. or higher. This is because it occurs due to a change in the cooling rate.
（６００℃以下として巻取り）
巻取り温度は６００℃以下とする。これは、巻取り温度が６００℃を超える場合、十分な熱伝達係数αを得ても十分な表裏層硬度の増加効果が得られないためである。これは、巻き取り温度が６００℃を超える場合、巻き取り後に生成するベイナイトが軟質となり、所定の硬さ比が得られないためである。表裏層の硬度を増加させる観点からは、巻き取り温度は５６０℃以下とすることが好ましい。
(Winding at 600 ° C or less)
The winding temperature is 600 ° C. or lower. This is because, when the coiling temperature exceeds 600 ° C., even if a sufficient heat transfer coefficient α is obtained, a sufficient effect of increasing the front and back layer hardness cannot be obtained. This is because when the winding temperature exceeds 600 ° C., the bainite generated after winding becomes soft and a predetermined hardness ratio cannot be obtained. From the viewpoint of increasing the hardness of the front and back layers, the winding temperature is preferably 560 ° C. or lower.
尚、本発明において、６５０℃から巻き取り温度の間のランアウトテーブル上での冷却は、特段の冷却を行わない空冷、または水冷で行うものとする。ここでの冷却速度は速い方が、より大きな硬さ比を得る上で好ましい。しかし、過度に大きくすると冷却停止温度のばらつきも大きくなるので、特に規定はしない。
巻き取り温度の下限は特に規定しないが、３５０℃以下の場合、巻き取り温度の精度が劣化するので、巻き取り温度は３５０℃以上が好ましい。
尚、ここで、仕上げ圧延温度、圧延後の冷却の速度、巻き取り温度は、鋼板表面温度ではなく、全板厚の平均温度である。全板厚の平均温度は、表面温度の測定値に合うように鋼板の伝熱計算を行うなどして全板厚の温度を算出し、それらを平均（算術平均）して求める。
本発明において、鋳片が加熱炉を出た後の加熱温度、粗圧延の温度、時間、パススケジュール、仕上げ圧延のパススケジュール、仕上げ圧延終了から冷却を開始するまでの時間等の条件は、常法に従うものとする。
In the present invention, the cooling on the run-out table between 650 ° C. and the coiling temperature is performed by air cooling or water cooling without special cooling. A faster cooling rate is preferable for obtaining a larger hardness ratio. However, if it is excessively increased, the variation in the cooling stop temperature also increases, so there is no particular limitation.
The lower limit of the winding temperature is not particularly specified, but when it is 350 ° C. or lower, the winding temperature is deteriorated, and therefore the winding temperature is preferably 350 ° C. or higher.
Here, the finish rolling temperature, the cooling speed after rolling, and the winding temperature are not the steel sheet surface temperature but the average temperature of the entire sheet thickness. The average temperature of the total plate thickness is obtained by calculating the temperature of the total plate thickness by performing heat transfer calculation of the steel plate so as to match the measured value of the surface temperature, and averaging them (arithmetic average).
In the present invention, conditions such as the heating temperature after the slab exits the heating furnace, the temperature of rough rolling, the time, the pass schedule, the pass schedule of finish rolling, the time from the end of finish rolling to the start of cooling are usually Follow the law.
以下に、本発明の実施例を具体的に説明する。 Examples of the present invention will be specifically described below.
表１に示す成分の鋼を転炉にて溶製した後、連続鋳造により鋳片とした。その後、表２−１及び表２−２に示す条件にて、再加熱を行い、粗圧延、仕上げ圧延、冷却、巻取りを行う事により熱延鋼板とした。なお、表２−２中に記載の熱伝達係数の上下限の数値は式（１）に、そして、水量密度の上下限の数値は式（２）による数値である。
得られた鋼板の組織、機械的特性を表３−１及び表３−２に示した。
Steels having the components shown in Table 1 were melted in a converter and then cast into slabs by continuous casting. Then, it reheated on the conditions shown in Table 2-1 and Table 2-2, and it was set as the hot-rolled steel plate by performing rough rolling, finish rolling, cooling, and winding. In addition, the numerical value of the upper and lower limit of the heat transfer coefficient as described in Table 2-2 is a numerical value by Formula (1), and the numerical value of the upper and lower limit of water quantity density is a numerical value by Formula (2).
The structure and mechanical properties of the obtained steel sheet are shown in Tables 3-1 and 3-2.
鋼板の幅方向中心部より採取した試験片を用いて、鋼板の引張試験、圧延方向断面の組織観察を行い、それと同じ断面のビッカース硬さ（ＨＶ）測定を行った。その際、鋼板の表層、及び裏層から、鋼板全板厚の１０％に相当する距離だけ離れた板厚方向位置において、鋼板の圧延方向と平行方向な線上で０．１ｍｍ間隔の距離を置いて１０点の硬さ測定を行い、表裏層における測定値の平均値を求め、さらに表層と裏層の平均値の平均値を求めた。硬さ測定の荷重は１ｋｇとした。尚、表裏層各々の硬さの平均値の差は互いに±５％以内であり、小さかった。ここで、鋼板の表層、裏層とは、それぞれ圧延時にそれぞれ上側、下側であった面を指す。 Using a test piece collected from the central part in the width direction of the steel sheet, a tensile test of the steel sheet and a structure observation of the cross section in the rolling direction were performed, and the same Vickers hardness (HV) measurement was performed. At that time, a distance of 0.1 mm is placed on a line parallel to the rolling direction of the steel sheet at a position in the thickness direction that is separated from the surface layer and the back layer of the steel sheet by a distance corresponding to 10% of the total thickness of the steel sheet. Ten points of hardness were measured, the average value of the measured values in the front and back layers was determined, and the average value of the average values of the front and back layers was determined. The load for hardness measurement was 1 kg. The difference in the average hardness of the front and back layers was within ± 5% of each other and was small. Here, the surface layer and the back layer of the steel sheet refer to the surfaces that were the upper side and the lower side, respectively, during rolling.
条件１−２、２、３−２、４、４−２、６、７、８、１１〜１８、２２〜２６は本発明例であり、良好な加工性（全伸びの劣化がない）と良好な疲労限度比［Ｌ（疲労強度）／ＴＳ（引張強度）］０．５０以上が得られている。 Conditions 1-2, 2, 3-2, 4, 4-2, 6, 7, 8, 11-18, 22-26 are examples of the present invention, and good workability (no deterioration of total elongation) A good fatigue limit ratio [L (fatigue strength) / TS (tensile strength)] of 0.50 or more is obtained.
条件１（比較例）は急速冷却域の熱伝達係数が小さすぎるため、鋼板表裏層と板厚中心部の硬さの比が小さい。また、表裏層のベイナイト層の厚みが小さい。このため疲労限度比が小さい。 In condition 1 (comparative example), the heat transfer coefficient in the rapid cooling region is too small, so the ratio of the hardness between the front and back layers of the steel plate and the central portion of the plate thickness is small. Moreover, the thickness of the bainite layer of the front and back layers is small. For this reason, the fatigue limit ratio is small.
条件２−２（比較例）は、急速冷却域の熱伝達係数が大きすぎるため、鋼板表裏層と板厚中心部の硬さの比が小さい。このため疲労限度比が小さい。 In Condition 2-2 (Comparative Example), the heat transfer coefficient in the rapid cooling region is too large, and thus the hardness ratio between the steel sheet front and back layers and the plate thickness center portion is small. For this reason, the fatigue limit ratio is small.
条件３（比較例）は急速冷却域の熱伝達係数が小さすぎるため、また、表裏層のベイナイト層の厚みが小さい。鋼板表裏層と板厚中心部の硬さの比が小さい。このため疲労限度比が小さい。 In condition 3 (comparative example), the heat transfer coefficient in the rapid cooling region is too small, and the thickness of the bainite layer of the front and back layers is small. The ratio of the hardness of the steel sheet front and back layers and the center of the plate thickness is small. For this reason, the fatigue limit ratio is small.
条件４−３（比較例）は、急速冷却域の熱伝達係数が大きすぎるため、鋼板表裏層と板厚中心部の硬さの比が小さい。このため疲労限度比が小さい。 In Condition 4-3 (Comparative Example), the heat transfer coefficient in the rapid cooling region is too large, and thus the hardness ratio between the steel sheet front and back layers and the plate thickness center portion is small. For this reason, the fatigue limit ratio is small.
条件５及び条件５−２（比較例）は急速冷却域の熱伝達係数が小さすぎるため、鋼板表裏層と板厚中心部の硬さの比が小さい。また、表裏層のベイナイト層の厚みが小さい。このため疲労限度比が小さい。 In Condition 5 and Condition 5-2 (Comparative Example), the heat transfer coefficient in the rapid cooling region is too small, and thus the hardness ratio between the steel sheet front and back layers and the center of the plate thickness is small. Moreover, the thickness of the bainite layer of the front and back layers is small. For this reason, the fatigue limit ratio is small.
条件９（比較例）は、圧延温度が低すぎる為、鋼板表裏層と板厚中心部の硬さの比が小さい。また、表裏層のベイナイト層の厚みが小さい。このため、このため疲労限度比が小さい。 In condition 9 (comparative example), since the rolling temperature is too low, the hardness ratio between the steel sheet front and back layers and the thickness center portion is small. Moreover, the thickness of the bainite layer of the front and back layers is small. For this reason, the fatigue limit ratio is small for this reason.
条件１０（比較例）は、巻取り温度が高すぎる為、鋼板表裏層で軟質のベイナイトが生成し、そのため板厚中心部の硬さの比が小さい。このため、このため疲労限度比が小さい。 In condition 10 (comparative example), since the coiling temperature is too high, soft bainite is generated in the front and back layers of the steel sheet, and therefore the hardness ratio of the center portion of the sheet thickness is small. For this reason, the fatigue limit ratio is small for this reason.
条件１９（比較例）は、鋼成分中のＭｎ量が所定より高い。そのため、所定の硬さ比が得られておらず、疲労限度比が小さい。 Condition 19 (Comparative Example) is that the amount of Mn in the steel component is higher than a predetermined value. Therefore, a predetermined hardness ratio is not obtained and the fatigue limit ratio is small.
条件２０（比較例）は、鋼成分中のＣ量が所定より高い。そのため、所定の硬さ比が得られておらず、疲労限度比が小さい。 In condition 20 (comparative example), the amount of C in the steel component is higher than a predetermined value. Therefore, a predetermined hardness ratio is not obtained and the fatigue limit ratio is small.
１ 疲労試験片 1 Fatigue specimen
Claims (4)
Ｃ：０．１０〜０．２０％、
Ｓｉ：０．０１〜２．００％、
Ｍｎ：０．１０〜２．００％、
Ｐ≦０．１００％、
Ｓ≦０．０１００％、
Ａｌ：０．００５〜０．０５０％、
Ｎ≦０．０１００％、
を含有し、残部がＦｅ及び不可避的不純物の組成からなり、引張強度が４６７ＭＰａ以上、５９４ＭＰａ以下であり、鋼板の表裏面から板厚の少なくとも１０％に相当する厚みの領域のミクロ組織がベイナイト単相であり、板厚中心部の板厚の少なくとも５０％に相当する厚みの領域のミクロ組織が５０％以上のフェライト面積分率を有し、前記表裏面から板厚の少なくとも１０％に相当する厚みの領域の硬さが前記板厚中心部の板厚の少なくとも５０％に相当する厚みの領域の平均硬さの１．１５倍以上であることを特徴とする疲労特性に優れた高強度熱延鋼板。 C: 0.10 to 0.20% by mass%,
Si: 0.01 to 2.00%
Mn: 0.10 to 2.00%,
P ≦ 0.100%,
S ≦ 0.0100%,
Al: 0.005 to 0.050%,
N ≦ 0.0100%,
The balance is composed of Fe and unavoidable impurities, the tensile strength is 467 MPa or more and 594 MPa or less, and the microstructure in the region corresponding to at least 10% of the plate thickness from the front and back surfaces of the steel plate has a bainite unit. The microstructure of the thickness region corresponding to at least 50% of the plate thickness at the center of the plate thickness has a ferrite area fraction of 50% or more, and corresponds to at least 10% of the plate thickness from the front and back surfaces. High strength heat excellent in fatigue characteristics, characterized in that the hardness of the thickness region is 1.15 times or more of the average hardness of the thickness region corresponding to at least 50% of the thickness of the central portion of the plate thickness Rolled steel sheet.
Ｎｂ：０．０５０％以下、
Ｔｉ：０．３００％以下、
Ｖ：０．１０％以下、
Ｃｕ：１．００％以下、
Ｎｉ：１．００％以下、
Ｃｒ：１．００％以下、
Ｂ：０．００５０％以下、
Ｃａ：０．００３０％以下、
ＲＥＭ：０．０２００％以下、
のうちの１種または２種以上を含むことを特徴とする請求項１に記載の疲労特性に優れた高強度熱延鋼板。 In addition by mass%
Nb: 0.050% or less,
Ti: 0.300% or less,
V: 0.10% or less,
Cu: 1.00% or less,
Ni: 1.00% or less,
Cr: 1.00% or less,
B: 0.0050% or less,
Ca: 0.0030% or less,
REM : 0 . 0200% or less,
The high-strength hot-rolled steel sheet having excellent fatigue characteristics according to claim 1, comprising one or more of them.
請求項１または請求項２の組成を有する鋳片を１１５０℃以上に加熱し、粗圧延した後、Ａｒ３温度＋５０（℃）以上の温度にて仕上げ圧延を行い、その後、仕上げ圧延終了温度から６５０℃までの間の平均の熱伝達係数αを下記式（１）で示される範囲内として鋼板の表面及び裏面から冷却を行い、その後鋼板全厚の平均の温度を６００℃以下として巻取りを行うことを特徴とする疲労特性に優れた高強度熱延鋼板の製造方法。
９１×板厚（ｍｍ）＋７５６
≦α（J／ｍ２ｓｅｃＫ）≦９１×板厚（ｍｍ）＋１８００ ・・・（１） A method for producing a high-strength hot-rolled steel sheet having excellent fatigue properties according to claim 1 or 2,
The slab having the composition of claim 1 or claim 2 is heated to 1150 ° C. or higher and rough-rolled, and then finish-rolled at a temperature of Ar 3 temperature + 50 (° C.) or higher, and then 650 from the finish rolling finish temperature. The average heat transfer coefficient α up to ℃ is set within the range represented by the following formula (1), and cooling is performed from the front and back surfaces of the steel sheet, and then winding is performed with the average temperature of the total thickness of the steel sheet being 600 ℃ or less. A method for producing a high-strength hot-rolled steel sheet having excellent fatigue characteristics.
91 x plate thickness (mm) + 756
≦ α (J / m 2 secK) ≦ 91 × plate thickness (mm) +1800 (1)
請求項１または請求項２の組成を有する鋳片を１１５０℃以上に加熱し、粗圧延した後、Ａｒ３温度＋５０（℃）以上の温度にて仕上げ圧延を行い、その後、仕上げ圧延終了温度から６５０℃までの間の冷却での平均の水量密度Ｗを下記式（２）で示される範囲内として鋼板の表面及び裏面から冷却を行い、その後鋼板全厚の平均の温度を６００℃以下として巻取りを行うことを特徴とする疲労特性に優れた高強度熱延鋼板の製造方法。
０．００４８×板厚（ｍｍ）＋０．００３５７
≦Ｗ（ｍ３／ｓｅｃ／ｍ２）≦０．００４８×板厚（ｍｍ）＋０．０５５・・（２） A method for producing a high-strength hot-rolled steel sheet having excellent fatigue properties according to claim 1 or 2,
The slab having the composition of claim 1 or claim 2 is heated to 1150 ° C. or higher and rough-rolled, and then finish-rolled at a temperature of Ar 3 temperature + 50 (° C.) or higher, and then 650 from the finish rolling finish temperature. ° C. the average water flow rate W in cooling until be within the range represented by the following formula (2) performs cooling from front and rear surfaces of the steel plate, after which the average temperature of the steel sheet total thickness as 600 ° C. or less A method for producing a high-strength hot-rolled steel sheet having excellent fatigue characteristics, characterized by winding.
0.0048 × plate thickness (mm) +0.00357
≦ W (m 3 / sec / m 2 ) ≦ 0.0048 × plate thickness (mm) +0.055 (2)
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