JP4112993B2 - High-strength hot-rolled steel sheet excellent in stretch flangeability and manufacturing method thereof - Google Patents

Description

【００２９】
【表１】

【００３０】
【表２】

【００３１】
その結果を表３にまとめて示す。
また断面組織を観察して構成する組織を調べた。組織は鏡面研磨後、ナイタール液で現出させ、表面から板厚の１／４相当内部位置を４００倍で観察し、下記の技術文献１を参考に存在する組織を同定した。その結果、認められた相は、いずれの鋼板、箇所においてもベイニティック・フェライトおよびポリゴナル・フェライトの２相のみであり、その他の相は認められなかった。
［「鋼のベーナイト写真集−１」（平成４年６月２９日、（社）日本鉄鋼協会発行、例えば２１頁・Ｆｉｇ２．９）］（技術文献１）
表３から明らかなように、本発明を用いれば、引張強さ、延性、および穴広げ性に優れ、穴広げ値のバラツキも１０％以下の鋼板を得ることができた。
【００３２】
【表３】

【００３３】
（実施例２）
表４に化学成分を示す鋼のスラブを表５に記載の条件にて熱間圧延し、厚さ２．６ｍｍの熱延板を得た。このようにして得られたコイルの引張強さσ_B、延性δ、および穴広げ値λを上記実施例１と同様に調べた。その結果の内、Δλｍａｘと圧延終了後最初に行う緩冷却の条件との関係を、Δλｍａｘが１０％以下のものとそれ以外に分けて図１に示す。横軸は冷却速度Ｒｃ（℃／秒）、縦軸は冷却時間ｔｃ（秒）である。図を用いてΔλｍａｘ≦１０％（白丸（〇））を囲む境界線Ｉ、ＩＩ、およびＩＩＩを求めた。なお、境界線上もΔλｍａｘ≦１０％である。
【００３４】
３本の境界線は下記のように求められた。
すなわち、（Ｉ）：ｔｃ＝３３．５−１．７×Ｒｃ
（ＩＩ）：Ｒｃ＝１４
（ＩＩＩ）：ｔｃ＝１１−０．４×Ｒｃ
である。
以上より、冷却速度をＲｃ（℃／秒）、冷却時間をｔｃ（秒）として、圧延完了後、最初に、Ｒｃ≦１４（℃／秒）、かつ１１−０．４×Ｒｃ≦ｔｃ≦３３．５−１．７Ｒｃを満たすような緩冷却を行えば穴広げ性のバラツキが１０％以下と小さい鋼板を得ることができることが示された。
【００３５】
【表４】

【００３６】
【表５】

【００３７】
【発明の効果】
以上述べたように、本発明によれば、伸びフランジ性に優れた高強度熱延鋼板を得ることが出来る優れた効果を奏するものである。
【図面の簡単な説明】
【図１】 鋼板の伸びフランジ性のバラツキを冷却速度を横軸に、冷却時間を縦軸にして表したグラフである。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a high-strength hot-rolled steel sheet excellent in stretch flangeability and a method for producing the same.
[0002]
[Prior art]
High-strength steel plates are widely used for automobile and construction machine parts. Among them, hot-rolled steel sheets are used for applications in which the plate thickness is relatively thick and design properties are not particularly required. Steel sheets used in such applications are required to have high workability in a narrow sense, that is, high stretch flangeability represented by a hole expansion test value as well as ductility. Among these properties, the one that pursues the improvement of the former is known as a dual phase steel such as DP steel or TRIP steel, but the mechanism that makes these steels highly ductile is stretch flangeability. Since steel is a cause of deterioration, steel sheet development for enhancing stretch flangeability without greatly impairing ductility has been underway.
[0003]
Examples of such steel sheets include JP-A-6-172924 (Patent Document 1) and JP-A-2001-20030 (Patent Document 2). Patent Document 1 has a predetermined chemical component, and by using bainitic ferrite as the main microstructure, it is possible to obtain a hot-rolled steel sheet having a tensile strength of 500 N / mm ² class or more and excellent stretch flangeability. Technology is disclosed. In Patent Document 2, a hot-rolled steel sheet having a predetermined chemical component and having extremely fine stretch flangeability is obtained by using granular bainitic ferrite and / or bainitic ferrite as a main microstructure. Techniques for disclosing are disclosed.
[0004]
[Cited document]
(1) Patent Document 1 (Japanese Patent Laid-Open No. 6-172924)
(2) Patent Document 2 (Japanese Patent Laid-Open No. 2001-20030)
[0005]
[Problems to be solved by the invention]
The inventors first examined the above prior art in detail. According to it, the most essential thing that makes these steel sheets have good stretch flangeability is the bainitic ferrite structure and granular bainitic ferrite structure that compose them. In order to achieve this, we came to the conclusion that the cooling method that is performed after rolling is the key technology. Specifically, Patent Document 1 describes a method for cooling to a predetermined temperature at a cooling rate of 50 ° C./second after the end of hot rolling as an example of conditions for obtaining the structure. On the other hand, Patent Document 2 discloses a method for cooling to a predetermined temperature at a cooling rate of 15 to 70 ° C./second after hot rolling as a condition for obtaining the structure.
[0006]
Thus, according to the prior art, after hot rolling, it is necessary to cool at a relatively fast cooling rate. However, the hot rolling equipment of a type where the distance from the final rolling stand to the cooling bed is long, In a hot rolling facility where the foremost part is set to a low cooling capacity in order to avoid adverse effects on the temperature measurement system, it is extremely difficult to cool at this cooling rate, that is, to create a structure, so that this point can be solved. It has become clear that there is a need to modify or newly install equipment, resulting in an increase in manufacturing costs.
[0007]
In addition, according to the experiments by the present inventors, when the tissue is formed by cooling at a relatively fast cooling rate, it is considered that the characteristic is caused by an increase in cooling unevenness (temperature variation due to the part). In particular, there has been a concern about unevenness in stretch flangeability. Of course, such a problem can be said to be anxious if it is a state-of-the-art facility designed to minimize cooling unevenness, but it cannot be easily implemented without modification of the facility for a manufacturer having only a low-capacity facility. It is also obvious that it is a thing.
[0008]
Therefore, there is a strong demand for a method for obtaining a steel plate that has excellent stretch flangeability and small variations even when the cooling rate immediately after rolling and the ability to uniformly cool the steel plate are limited as described above. However, there are no examples focusing on these points. An object of the present invention is to provide a high-strength hot-rolled steel sheet having excellent stretch flangeability and small variations, and a method for producing the same.
[0009]
[Means for Solving the Problems]
In view of such a situation, the present inventors have made extensive studies. As a result, the present inventors have found that combining a predetermined chemical component and hot rolling conditions characterized by cooling conditions is an effective means, and thus completed the present invention. In particular, after hot rolling, firstly, cooling conditions of slow cooling and then cooling at a predetermined cooling rate are the most important, and the gist thereof is as follows. That is, (1) in mass%, C: 0.02 to 0.06%, Si: 0.05 to 1.2%, Mn: 0.5 to 2.0%, P: 0.05% or less, S: 0.01% or less, N: 0.01% or less, Al: 0.005-1.0%, Ti: 4 × [C] + 0.01-4 × [C] +0.07 ([C] Is a method for producing a steel sheet having a microstructure composed of bainitic ferrite and polygonal ferrite, the balance being Fe and inevitable impurities, and having the chemical components After heating the steel material to 1200 ° C or higher and completing rolling at a temperature of Ar ₃ or higher, assuming that the cooling rate is Rc (° C / second) and the cooling time is tc (second), Rc≤14 ° C / second, and 11- Cooling satisfying 0.4 × Rc ≦ tc ≦ 33.5-1.7Rc is performed, and subsequently, Rc ≧ 3 ° C. / sec performs cooling, the method of producing a high strength hot-rolled steel sheet excellent in stretch flange formability, characterized in that winding at 450 to 600 ° C..
[0010]
(2) In addition to the chemical component described in (1 ) above, it is characterized by containing Cu: 0.8 to 2.0% and Ni: 0.4 to 1.0% in mass%. The manufacturing method of the high intensity | strength hot-rolled steel plate excellent in the stretch flangeability as described in said (1).
[0011]
( 3 ) In addition to the chemical component described in (1) or (2 ) above , in mass%, Nb: 0.01 to 0.3%, Mo: 0.05 to 0.3%, V: 0 The method for producing a high-strength hot-rolled steel sheet having excellent stretch flangeability according to the above (1) or (2), comprising at least one of 0.025 to 0.20% .
[0012]
DETAILED DESCRIPTION OF THE INVENTION
First, the experiment that led to the completion of the present invention will be described.
The present inventors, in mass%, C: 0.035%, Si: 1.0%, Mn: 1.5%, P: 0.01%, S: 0.001%, N: 0.00. Steel strips containing 0035%, Al: 0.033%, Ti: 0.12 to 0.22% and the balance being Fe were melted, and these were hot rolled to produce hot rolled coils. The heating temperature of the slab is 1250 ° C., the rolling end temperature is 900 ° C., and the cooling after the end of rolling is first cooled to 804 ° C. with Rc: 8 ° C./second and tc: 12 seconds, followed by Rc: 35 ° C./second. Was cooled to 450 ° C and wound up at 450 ° C. While pickling the obtained hot-rolled coil, the steel plate was extract | collected from the front-end | tip part, the center part, and the tail part. And about each of the steel plate in these three site | parts, the test piece for characteristic evaluation and the micro structure | tissue investigation sample were extract | collected from the both ends and center part of the width direction. The test pieces are tensile test pieces for evaluating strength ductility (JIS No. 5, sampled so that the tensile direction is perpendicular to the rolling direction) and test pieces for measuring the hole expansion value (150 mm × 150 mm). Produced.
[0013]
Using these test pieces, the strength (tensile strength) σ _B (MPa), the ductility (elongation) δ (%), and the hole expansion value λ (%) were investigated. The tensile test was performed by a method based on JIS Z 2241 using an Instron type testing machine. The hole expansion value λ was measured in accordance with Japan Iron and Steel Federation Standard JFS T1001, and the clearance was 12.5%. The obtained characteristic values are arranged at five points collected from the same part, three parts in the width direction, and three parts on the coil, and the relationship between the distribution of characteristic values (variation) and the Ti concentration of the steel sheet is examined. did. As a result, the tensile strength σ _B and ductility δ do not vary greatly between five points in the same part and between three parts in the width direction, and the difference between the maximum value and the minimum value is 2% of the simple average value. Was within.
[0014]
On the other hand, the degree of variation of the hole expansion value λ may be large even between the same parts. As a result of various analysis, it was found that the degree of variation does not depend on the Ti concentration itself, but increases when the relationship between the Ti concentration and the C concentration deviates from a specific condition. Then, next, experiment which examines the cooling conditions after completion | finish of rolling using the steel slab which contains 0.17% of Ti and the other component is the same as the above. As a result, it has been found that the degree of variation becomes small when the combination of the cooling rate and the cooling time is within a specific range.
[0015]
Although the details of the examination described above will be described in the examples, the present invention is based on such basic experiments and is a result of further earnest research, and shows a predetermined stretch flangeability with less variation. A hot-rolled steel sheet and a manufacturing method thereof are provided. In view of the fact that “less variation” is a feature demanded by steel sheet users today, a high-strength hot-rolled steel plate with little variation and excellent stretch flangeability is simply obtained by stretching flanges. It was described as a high-strength hot-rolled steel sheet with excellent properties.
[0016]
The reasons for limiting the present invention will be described below.
First, the reasons for limiting chemical components will be described. All the component description is mass%.
C is an essential element for ensuring the strength of the steel sheet, and at least 0.02% is necessary to obtain a high-strength steel sheet. However, if it is contained excessively, the formation of a phase unfavorable for stretch flangeability such as cementite and martensite cannot be avoided, so the content is made 0.06% or less.
[0017]
Si is an element effective for ensuring strength without deteriorating stretch flangeability, and is defined as containing 0.05% or more. However, if it is excessively contained, the surface properties are impaired, so the upper limit is 1. .2%.
Mn is an element effective for increasing the strength of a steel sheet and should be contained in an amount of 0.5% or more. However, if the content exceeds 2.0%, the ductility deteriorates, so the upper limit is 2.0%. To do.
P is effective as a solid solution strengthening element. However, since there is a concern about deterioration of workability due to segregation, it is necessary to make it 0.05% or less.
[0018]
S should be suppressed as much as possible because it forms inclusions such as MnS and deteriorates the stretch flangeability. However, it should be 0.01% or less.
N forms nitrides and reduces ductility and stretch flangeability. Therefore, it should be suppressed as much as possible, but 0.01% or less is acceptable.
Al is used as a deoxidizer, and 0.005% is necessary to obtain an appropriate cleanliness. However, if it is excessively contained, ductility and stretch flangeability deteriorate. The upper limit is 0%.
[0019]
Ti forms a carbide with C and functions to suppress the formation of cementite, which is undesirable for stretch flangeability. In particular, when receiving gentle cooling after rolling, it is necessary to make the most of its function to suppress the formation of cementite. On the other hand, when it is added more than necessary, that is, when it is added in an amount exceeding a sufficient amount to form carbides and fix C, they are present in the steel as solute Ti, which causes damage to ductility. Therefore, it is necessary to pay close attention to the amount added. The range is 4 × [C] +0.01 to 4 × [C] +0.07 ([C] is C concentration) as shown in the examples. In addition, said formula is a formula which the present inventors newly found out by calculating | requiring the range from which the variation of the hole expansion value (lambda) becomes less than 10% by the Example mentioned later.
[0020]
Cu can be used as a solid solution strengthening element or a precipitation strengthening element to increase the strength of the steel sheet. Further, the fatigue strength can be further improved by the addition thereof. The effect is not exhibited unless 0.8% or more is added. On the other hand, if the content exceeds 2.0%, the surface properties of the steel sheet after hot rolling deteriorate, so 2.0% is made the upper limit.
Ni has an effect of relieving the deterioration of hot rolled surface properties caused by Cu, and it is desirable to add about half of Cu as a guide. Therefore, the lower limit is 0.4%. On the other hand, even if added over 1.0%, the effect is saturated and only leads to an increase in cost, so 1.0% is made the upper limit.
[0021]
Nb, Mo, and V all work together to form carbides and increase the strength, and the generated carbonitride can also be expected to suppress the softening of the steel sheet due to the heat effect of welding. These effects are manifested when Nb: 0.01%, Mo: 0.05% or V: 0.025% or more, while Nb: 0.3%, Mo: 0.3% or V: 0.20% If the content exceeds V, the hard phase is formed and the hole expansion value λ is deteriorated.
In the present invention, components other than those described above are Fe, but inevitable impurities mixed from melting raw materials such as scrap are allowed.
[0022]
Next, the structure of the steel plate will be described.
In order to obtain excellent stretch flangeability, a microstructure composed of bainitic ferrite and polygonal ferrite is required. As the remaining structure other than these, one or more of pearlite, retained austenite, and martensite should be eliminated as much as possible, and it is desirable that the total area ratio thereof is 3% or less. On the other hand, any composition ratio (area ratio) can be used as long as it is a combination of two phases of bainitic ferrite and polygonal ferrite, and can be designed based on required strength, ductility and stretch flangeability. . For example, in applications where high ductility is the main focus and high stretch flangeability is required, polygonal ferrite is the main phase, and a method of ensuring the required strength using precipitates can be selected. On the other hand, a method using bainitic ferrite as the main phase can be selected for applications in which it is most important to reduce the number of constituent elements as much as possible and to provide a low-cost steel sheet.
[0023]
Finally, each condition of heating, rolling, cooling and winding will be described.
The heating temperature is required to be 1200 ° C. or higher in order to temporarily dissolve carbonitride in the steel. By dissolving these in a solid solution, the strength of the steel sheet can be increased by finely dispersing carbonitride in the cooling process after rolling. On the other hand, there is no upper limit to the heating temperature for the purpose of dissolving carbonitride, but when the temperature exceeds 1300 ° C., oxidation of the slab surface becomes significant, and in particular, wedge-shaped that seems to be caused by selective oxidation of grain boundaries. Since surface defects remain after descaling and impair the surface quality after rolling, it is desirable that the upper limit is 1300 ° C.
[0024]
The rolling end temperature is important for controlling the structure of the steel sheet. If it is less than Ar ₃ point, the structure of the surface layer is not uniform, and the material is not stable, which is not preferable in terms of characteristics. On the other hand, when the Ar ₃ point is higher than + 100 ° C., the austenite grain size becomes coarse during the slow cooling after the end of rolling, and the composition and the fraction of the phase generated during the subsequent cooling become unstable, making it difficult to operate. It is desirable to set the temperature as the upper limit. The cooling rate Rc (° C./second) and the cooling time tc (second) after rolling are the most important conditions in the present invention. In particular, a steel plate with small variations in the hole expansion value λ cannot be obtained unless the first cooling condition after the rolling is within a specific combination range.
[0025]
The range is Rc ≦ 14 (° C./second) and 11−0.4 × Rc ≦ tc ≦ 33.5-1.7Rc as described in the examples. At present, it is not clear why the variation is small within this range, but it is probably that the transformation from austenite to ferrite is made uniform in a wide field of view by slow cooling under specific conditions. Is believed to contribute to the stability of the hole expansibility λ, that is, the reduction of the distribution range. The above relational expression was newly found by the present inventors by obtaining a range in which the variation of the hole expansion value λ is within 10% according to an example described later. The lower limit of the cooling rate Rc is the air cooling rate when the sheet is passed at the minimum speed of the manufacturing equipment, and varies depending on the rolling end temperature, the steel plate shape (plate thickness, width, etc.), the transport roll, the ambient temperature, etc. However, it may be 2.5 ° C./second practically.
[0026]
The cooling performed subsequent to the slow cooling needs to satisfy Rc ≧ 30 ° C./second. Below this speed, it is difficult to produce bainitic ferrite. In this case, it is necessary to design a higher strength that depends on precipitation strengthening and solid solution strengthening, but this gives an excellent hole expansion value λ. Because it is not possible. On the one hand 100 ° C. / sec greater, particular upper limit 100 ° C. cooling rate C _R after slow cooling since the material dispersion caused by uneven cooling in the width direction of the coil is concerned, and preferably in a 70 ° C..
The winding temperature is in the range of 450 ° C to 600 ° C. If it is less than 450 degreeC, there exists a possibility that a martensite may produce | generate or it may become a structure | tissue where a dislocation density is too high. On the other hand, if the temperature exceeds 600 ° C., the hot-rolled scale is remarkably generated and the pickling property is impaired.
[0027]
【Example】
Examples of the present invention will be described below together with comparative examples.
(Example 1)
A steel slab having chemical components shown in Table 1 was hot-rolled under the conditions shown in Table 2 to obtain a coil having a thickness of 3.2 mm. The tensile strength σ _B , ductility δ, and hole expansion value λ at the nine positions (tip, center, tail, each widthwise both ends and center) of the coils were examined. Each of the nine locations was examined with 5 specimens. As a result, the variation in tensile strength σ _B and ductility δ was small, and the value obtained by dividing the difference between the maximum and minimum measured values of 45 pieces (9 locations x 5 specimens) by their simple average value was 2% Since it was below, it was set as the characteristic value by the simple average value. On the other hand, since the degree of variation of the hole expansion value λ was various, the difference between the maximum value and the minimum value (for 5 specimens) at each location
[Formula 1]

[0029]
[Table 1]

[0030]
[Table 2]

[0031]
The results are summarized in Table 3.
Further, the structure formed by observing the cross-sectional structure was examined. The structure was mirror-polished and then revealed with a nital solution, and the internal position corresponding to 1/4 of the plate thickness was observed from the surface at 400 times, and the existing structure was identified with reference to the following technical document 1. As a result, the recognized phases were only two phases of bainitic ferrite and polygonal ferrite in any steel plate and location, and no other phases were observed.
["Steel Bainite Photobook-1" (June 29, 1992, published by Japan Iron and Steel Institute, for example, page 21, FIG. 2.9)] (Technical Reference 1)
As apparent from Table 3, by using the present invention, it was possible to obtain a steel sheet having excellent tensile strength, ductility, and hole expandability and having a hole spread value variation of 10% or less.
[0032]
[Table 3]

[0033]
(Example 2)
A steel slab having chemical components shown in Table 4 was hot-rolled under the conditions shown in Table 5 to obtain a hot-rolled sheet having a thickness of 2.6 mm. The tensile strength σ _B , ductility δ, and hole expansion value λ of the coil thus obtained were examined in the same manner as in Example 1. Among the results, the relationship between Δλmax and the first slow cooling condition after rolling is shown in FIG. 1 for Δλmax of 10% or less and the others. The horizontal axis represents the cooling rate Rc (° C./second), and the vertical axis represents the cooling time tc (second). Using the figure, boundary lines I, II, and III surrounding Δλmax ≦ 10% (white circle (◯)) were obtained. Note that Δλmax ≦ 10% also on the boundary line.
[0034]
Three boundary lines were obtained as follows.
That is, (I): tc = 33.5-1.7 × Rc
(II): Rc = 14
(III): tc = 11−0.4 × Rc
It is.
From the above, assuming that the cooling rate is Rc (° C./second) and the cooling time is tc (second), first after rolling, Rc ≦ 14 (° C./second) and 11−0.4 × Rc ≦ tc ≦ 33 It was shown that a steel sheet having a small variation in hole expansibility of 10% or less can be obtained by slow cooling that satisfies .5 to 1.7Rc.
[0035]
[Table 4]

[0036]
[Table 5]

[0037]
【The invention's effect】
As described above, according to the present invention, there is an excellent effect that a high-strength hot-rolled steel sheet excellent in stretch flangeability can be obtained.
[Brief description of the drawings]
FIG. 1 is a graph showing variation in stretch flangeability of steel sheets with the cooling rate on the horizontal axis and the cooling time on the vertical axis.

Claims (3)

In mass%
C: 0.02 to 0.06%,
Si: 0.05-1.2%
Mn: 0.5 to 2.0%
P: 0.05% or less,
S: 0.01% or less,
N: 0.01% or less,
Al: 0.005 to 1.0%,
Ti: 4 × [C] +0.01 to 4 × [C] +0.07 ([C] represents C concentration), with the balance being Fe and inevitable impurities, bainitic ferrite and polygonal A method of manufacturing a steel sheet having a microstructure composed of ferrite, in which a steel material having the chemical component is heated to 1200 ° C. or higher, and rolling is completed at a temperature of Ar ₃ or higher, and the cooling rate is set to Rc (° C. / Sec), and cooling time tc (sec), Rc ≦ 14 ° C./sec and cooling satisfying 11−0.4 × Rc ≦ tc ≦ 33.5-1.7Rc is performed, followed by Rc ≧ 30 A method for producing a high-strength hot-rolled steel sheet having excellent stretch flangeability, wherein the steel sheet is cooled at a temperature of ° C / second and wound at 450 to 600 ° C. In addition to the chemical component according to claim 1, in mass%, Cu: 0.8 to 2.0%,
Ni: 0.4-1.0%
The manufacturing method of the high strength hot-rolled steel plate excellent in stretch flangeability of Claim 1 characterized by the above-mentioned . In addition to the chemical component according to claim 1 or 2, in mass%, Nb: 0.01 to 0.3%,
Mo: 0.05-0.3%
V: 0.025 to 0.20%
The manufacturing method of the high strength hot-rolled steel plate excellent in stretch flangeability of Claim 1 or 2 characterized by including 1 or more types of these .

Images