JP2008274416A - Hot-rolled steel sheet excellent in fatigue characteristics and stretch-flanging, and producing method therefor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a hot-rolled steel sheet provided with both fatigue characteristics of 520-670 MPa class tensile strength and stretch-flanging workability, and a producing method thereof. <P>SOLUTION: The hot-rolled steel sheet excellent in the fatigue characteristics and the stretch-flanging workability contains 0.015 to <0.040% C, <0.05% Si, 0.9-1.8% Mn, <0.02% P, <0.01% S, <0.1% Al, <0.006% N, 0.06 to <0.11% Ti, 2.5 to <3.5% Ti/C and the balance Fe with inevitable impurities, wherein the maximum tensile strength is 520 to 720 MPa, the aging index (AI) is >15 MPa, product of bore-expanding ratio (λ)% and total elongation (EL)% is ≥2350, and the fatigue limit is ≥200 MPa. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a hot-rolled steel sheet excellent in fatigue characteristics and stretch flangeability, and a method for producing the same, and has a uniform microstructure that expresses particularly excellent stretch flangeability and a component that requires severe stretch flange processing. But it can be easily molded.

  In recent years, application of light metals such as Al alloys and high-strength steel sheets to automobile members has been promoted for the purpose of reducing the weight in order to improve the fuel efficiency of automobiles. However, although light metals such as Al alloys have the advantage of high specific strength, their application is limited to special applications because they are significantly more expensive than steel. Therefore, it is necessary to increase the strength of the steel sheet in order to promote the weight reduction of automobiles at a lower cost and in a wider range.

  Higher strength of materials generally deteriorates material properties such as formability (workability), so how to increase strength without deteriorating material properties is important in developing high strength steel sheets. Become. Stretch flangeability, ductility, fatigue durability, corrosion resistance, etc. are particularly important properties required for inner plate members, structural members, and steel plates for suspension members, and how to balance these properties at a high level with high strength. is important.

  In this way, in order to achieve both high strength and various properties, especially formability, the steel microstructure contains residual austenite, and thus the TRIP (Transformation Induced Plasticity) phenomenon is manifested during forming to dramatically improve formability. A TRIP steel with improved (ductility and deep drawability) is disclosed (see, for example, Patent Documents 1 and 2). However, the stretch flangeability is generally inferior. Therefore, a steel sheet having high strength and extremely excellent stretch flangeability is desired.

There are several disclosures regarding hot-rolled steel sheets with excellent stretch flangeability. Patent Document 3 discloses a hot rolled steel sheet having an acicular ferrite single phase structure. However, the structure of such a low temperature transformation product alone has low ductility and is difficult to use for applications other than stretch flange molding.
Patent Document 4 discloses a steel sheet having a structure composed of ferrite and bainite. With such a composite structure steel, although relatively good ductility can be obtained, hole expansion which is an index representing stretch flangeability is disclosed. The rate tends to be low.
Further, Patent Document 5 discloses a steel plate having a high ferrite volume fraction. However, since this contains a large amount of Si, there may be a problem in fatigue characteristics. In order to avoid such harmful effects due to Si, it is necessary to modify the surface during and / or after hot rolling, and there is a problem that special equipment must be introduced or productivity is deteriorated. Many.

Patent Documents 6 and 7 disclose hot-rolled steel sheets with good hole expansibility to which Ti is added. However, Ti / C is not properly controlled, and the hole expansion rate is not so high.
JP 2000-169935 A JP 2000-169936 A JP 2000-144259 A JP 61-130454 A JP-A-8-269617 JP-A-2005-248240 JP 2004-131802 A

  The present invention provides a hot-rolled steel sheet having excellent tensile flange formability and excellent ductility at a maximum tensile strength of 520 to 720 MPa and excellent fatigue characteristics, and a method for producing the same.

  The present inventors have intensively studied to overcome the above problems. As a result, first of all, it should be noted that Si is suppressed to the lowest level, that the structure is mainly composed of ferrite, and that even a small amount of solute C remains, and the ratio of Ti amount to C amount is noted. Newly found to be important.

That is, the gist of the present invention is as follows.
(1) In mass%, C: 0.015 or more and less than 0.040%, Si: less than 0.05%, Mn: 0.9 or more and 1.8% or less, P: less than 0.02%, S: Less than 0.01%, Al: less than 0.1%, N: less than 0.006%, Ti: 0.06 or more and less than 0.11%, Ti / C = 2.5 or more and less than 3.5, the balance Is a hot-rolled steel sheet having a component consisting of Fe and inevitable impurities, the maximum tensile strength is 520 MPa or more and less than 720 MPa,
Hot rolling with excellent fatigue characteristics and stretch flangeability characterized by an aging index AI of more than 15 MPa, a product of hole expansion ratio (λ)% and total elongation (El)% of 2350 or more, fatigue limit of 200 MPa or more steel sheet.
(2) The hot-rolled steel sheet according to the above (1) is further mass%, Nb: 0.001 to 0.04%, B: 0.0001 to 0.004%, Cu: 0.01 1.5% or less, Ni: 0.01 or more and 0.8% or less, Mo: 0.02 or more and 1.0% or less, V: 0.001 or more and 0.2% or less, Cr: 0.01 or more and 1 A hot-rolled steel sheet excellent in fatigue characteristics and stretch flangeability, characterized by containing one or more of 5% or less and W: 0.01 or more and 1.0% or less.
(3) The hot-rolled steel sheet according to any one of (1) or (2) above is further, in mass%, Ca: 0.0005 or more and 0.005% or less, REM: 0.0005 or more. A hot rolled steel sheet excellent in fatigue characteristics and stretch flangeability, characterized by containing one or two of 0.05% or less.
(4) A hot-rolled steel sheet excellent in fatigue characteristics and stretch flangeability, wherein the hot-rolled steel sheet according to any one of (1) to (3) is plated.

(5) When hot-rolling to obtain the hot-rolled steel sheet according to any one of (1) to (4) above, the steel slab having the above components is heated to 1100 ° C. or higher to perform rough rolling. Finish at a temperature of 1000 ° C. or higher, finish cooling in the temperature range of 830 to 980 ° C., air cool for 0.5 seconds or more, and average cooling rate in the temperature range of 750 to 600 ° C. in the range of 10 to 40 ° C./sec. A method for producing a hot-rolled steel sheet excellent in fatigue properties and stretch flangeability, characterized by being cooled at 440 to 560 ° C.
(6) In the hot rolling described in (5) above, the rough bar after completion of rough rolling of the steel slab is heated until the start of finish rolling and / or during the finish rolling of the coarse bar, A method for producing a hot-rolled steel sheet having excellent fatigue characteristics and stretch flangeability.
(7) In the hot rolling described in any one of the above (5) and (6), descaling is performed between the end of rough rolling and the start of finish rolling, and fatigue characteristics and stretch flanges A method for producing hot-rolled steel sheets with excellent properties.
(8) Production of a hot-rolled steel sheet having excellent fatigue characteristics and stretch flangeability, characterized by annealing at 780 ° C. or lower after hot rolling according to any one of (5) to (7) above. Method.
(9) After hot rolling according to any one of (5) to (7) above, the obtained hot-rolled steel sheet is heated at 780 ° C. or lower and then immersed in a plating bath to plate the steel sheet surface. A method for producing a hot-rolled steel sheet having excellent fatigue characteristics and stretch flangeability.
(10) A method for producing a hot-rolled steel sheet having excellent fatigue characteristics and stretch flangeability, characterized by performing a plating alloying treatment after plating in the production method according to (9) above.

    The present invention relates to a hot-rolled steel sheet having excellent stretch flangeability and a method for producing the same, and by using these steel sheets, severe stretch-flange processing typified by a decorative hole portion of a high-design wheel is required. Even parts can be easily molded. Also, the end face properties after the stretch flange processing are good with no secondary shear surface and similar defects. Excellent corrosion resistance after painting. In addition, the steel sheet strength has good fatigue characteristics, and the maximum tensile strength is as high as 520 to 670 MPa, so that the plate thickness can be reduced.

The present invention is described in detail below.
First, (1) will be described.
C is one of the most important elements in the present invention. If it contains 0.04% or more, the carbide that becomes the starting point of stretch flange crack increases, not only the hole expansion value deteriorates but also the strength increases and the workability deteriorates, so that it is less than 0.04%. To do. From the viewpoint of stretch flangeability, it is preferably less than 0.035%. Further, if less than 0.015%, the strength is insufficient, so 0.015% or more.

    Si forms a surface pattern called Si scale on the surface of the hot-rolled sheet, and not only deteriorates the surface properties of the molded product, but also roughens the surface roughness, so that the fatigue characteristics may be deteriorated. Moreover, chemical conversion processability deteriorates and, as a result, corrosion resistance also becomes inferior. Therefore, it is necessary to suppress the Si content as low as possible. Therefore, the upper limit is made less than 0.05%. This makes it possible to ensure good chemical conversion properties and post-coating corrosion resistance without high pressure descaling after rough rolling. The lower limit is not particularly defined, but it is a substantial lower limit because it causes a large cost increase to be less than 0.001%.

    Mn is an important element in the present invention. Since Mn lowers the ferrite transformation temperature, it has an effect of refining the structure and is preferable for fatigue characteristics. Further, since it is possible to increase the strength relatively inexpensively, 0.9% or more is added. Excessive addition deteriorates stretch flangeability and fatigue characteristics, so 1.8% or less is the upper limit. The upper limit is preferably less than 1.5%.

Since P deteriorates stretch flangeability, weldability, and fatigue strength of the welded portion, the upper limit is made less than 0.02%. Less than 0.01% is a more preferable upper limit. The lower limit is not particularly specified, but it is a practical lower limit because it is difficult to make it 0.001% or less in terms of steelmaking technology.
S not only causes cracking during hot rolling, but if it is too much, it generates A-based inclusions that deteriorate hole expandability, so it should be reduced as much as possible. It is. However, when high hole expandability is required, it is preferably less than 0.0040%, and when higher hole expandability is required, 0.0025% or less is desirable.

Al may be added for deoxidation of molten steel, but the upper limit is set to less than 0.1% because of an increase in cost. Further, if added too much, non-metallic inclusions are increased and elongation and hole expansibility are deteriorated, so the content is desirably less than 0.06%. Al may not be added.
N combines with Ti to form TiN and adversely affects hole expansibility and fatigue characteristics, so the upper limit is made less than 0.006%. Preferably it is less than 0.004%. Although there is no particular lower limit, it is difficult to stably obtain less than 0.0005%, which is a practical lower limit.

Ti is an extremely important element in the present invention. Ti is indispensable for increasing the strength and has the effect of improving the hole expandability. Therefore, addition of 0.06% or more is essential. However, if added too much, the strength may become too high, or the hole expansibility and fatigue characteristics may deteriorate, so the lower limit is made less than 0.11%. A range of 0.075% or more and less than 0.10% is a more preferable range.
Ti / C is 2.5 to less than 3.5 by mass ratio. If this is less than 2.5, high strength cannot be obtained stably. On the other hand, if it is 3.5 or more, it becomes difficult to secure solid solution C which is very important in the present invention described later, and as a result, hole expansibility and fatigue characteristics deteriorate.

    The maximum tensile strength of the hot-rolled steel sheet obtained in the present invention is less than 520 to 720 MPa. If it is less than 520 MPa, the merit of increasing the strength is small, and if it is 720 MPa or more, the formability deteriorates. On the other hand, when strict moldability and shape freezing property such as a high-design wheel are required, the pressure is more preferably less than 670 MPa. The tensile test is performed according to the method of JIS Z 2241.

    O is not particularly limited, but if it is too much, coarse oxides increase and the hole expandability is impaired, so 0.012% is a practical upper limit. More preferably, it is 0.006% or less, More preferably, it is 0.003% or less.

The aging index AI (Aging Index) is very important in the present invention. AI is an indicator of the amount of solid solution C and / or the amount of solid solution N, but in the present invention, since N binds to Ti and Al, the amount of solid solution N can be ignored. Therefore, AI corresponds to the amount of dissolved C. AI is greater than 15 MPa. If it is 15 MPa or less, good hole expansibility and fatigue characteristics cannot be ensured. The upper limit of AI is not particularly provided, but if it exceeds 80 MPa, the amount of solid solution C is too much and the moldability may be lowered, so this is the upper limit.
In the case of the steel sheet of the present invention, AI is measured as follows. First, a tensile strain of 6.5 to 8.5% is applied. The flow stress at this time is σ1. Once unloaded, the test piece is removed from the tensile tester and subjected to heat treatment that is held at 100 ° C. for 1 hour. Thereafter, the tensile test is performed again. The upper yield stress obtained there is assumed to be σ2. It is defined by AI (MPa) = σ2−σ1. The tensile test is performed according to the method of JIS Z 2241.

Stretch flangeability is more excellent as the balance between hole expansion value and total elongation is better. If the product of the hole expansion rate (%) and the total elongation (%) is less than 2350, the frequency of stretch flange cracking during molding increases, so the optimal range was limited to 2350 or more. 3400 or more is more preferable as a condition for preventing cracking even in a stricter molded product shape. In addition, when applying this invention steel plate to a wheel member with high designability, when a hole expansion rate is less than 140%, a crack may generate | occur | produce in a flange end surface, and a hole expansion rate may be 140% or more. desirable. More preferably, it is 160% or more. The hole expansion rate is determined according to the hole expansion test method described in Japan Iron and Steel Federation Standard JFS T 1001-1996.
The fatigue characteristics are evaluated by a complete swing bending fatigue test (stress ratio R = −1) with a constant stress amplitude, and the upper limit of the fatigue strength at the number of repetitions of 1 × 10 ⁷ times is defined as the fatigue limit. If the fatigue limit is less than 200 MPa, the molded product may undergo fatigue failure during use. Therefore, the appropriate fatigue limit range was limited to 200 MPa or more. 220 MPa or more is a more preferable range.

Next, (2) and (3) will be described.
Since Nb has an effect of improving fatigue characteristics, 0.001 to 0.04% may be added. If the addition is less than 0.001%, no particular effect is observed, so this is the lower limit. On the other hand, if over 0.04% is added, the hole expandability may be significantly reduced, so this is the upper limit. A range of more than 0.01% to less than 0.03% is a preferable addition amount.

    B may be added because it helps to increase the strength of the steel sheet at low cost by improving the hardenability. However, if it is less than 0.0001%, it is insufficient for obtaining the effect, and if it exceeds 0.004%, slab cracking may occur. Preferably, it is 0.0004% or more and 0.0025% or less.

    Furthermore, in order to impart strength, one or more of precipitation strengthening or solid solution strengthening elements of Cu, Ni, Mo, V, Cr, W may be added. However, if less than 0.01%, 0.01%, 0.02%, 0.001%, 0.01%, and 0.01%, respectively, the effect cannot be obtained. Moreover, even if added over 1.5%, 0.8%, 1.0%, 0.2%, 1.5%, and 1.0%, the effect is not only saturated but also the moldability is deteriorated. Incurs a cost increase.

    Ca and REM are elements that can be harmless by changing the form of non-metallic inclusions that can be a starting point of destruction or deteriorate workability. However, even if less than 0.0005% is added, there is no effect, and if Ca exceeds 0.005%, if REM exceeds 0.05%, the effect is saturated, so Ca = 0.005 to 0 It is desirable to add 0.005% and REM = 0.005 to 0.05%. Note that REM is a rare earth element such as La or Ce.

    Note that Zr, Sn, Co, Zn, and Mg may be contained in a total amount of 1% or less in steel containing these as main components. However, Sn is preferably 0.05% or less because wrinkles may occur during hot rolling.

Next, the above (4) will be described.
The steel plates described in the above (1) to (3) may be plated. The main component of the plating may be zinc, aluminum, tin, or any other plating. The plating may be electroplating in addition to hot dipping and alloying hot dipping. The chemical component of plating may contain one or more elements such as Fe, Mg, Al, Cr, Mn, Sn, Sb, and Zn in addition to the main component.

Next, the manufacturing method of the steel plate described in the above (5) to (10) will be described.
In the hot rolling, the steel slab needs to be heated to 1100 ° C. or higher. When this temperature (slab extraction temperature) is less than 1100 ° C., it is difficult to obtain sufficient strength. This is presumably because Ti-based carbides are not sufficiently dissolved when the temperature is lower than 1100 ° C., and as a result, the precipitates become coarse. 1140 degreeC or more is more preferable. Although there is no particular upper limit, even if the temperature exceeds 1300 ° C., there is no particular effect and the cost is increased, which is a practical upper limit.
The end temperature of rough rolling is extremely important in the present invention. That is, rough rolling needs to be completed at 1000 degreeC or more. This is because if it is less than 1000 ° C., the hole expandability deteriorates.
Therefore, this is the lower limit. More preferably, it is 1060 degreeC or more.

    The finishing temperature of hot rolling is 830 to 980 ° C. If this temperature is less than 830 ° C., the strength of the hot-rolled sheet varies greatly depending on the cooling and winding conditions after hot rolling, or the in-plane anisotropy of the tensile properties increases. Moreover, since the hole expandability also deteriorates, this is set as the lower limit. On the other hand, when the finishing temperature exceeds 980 ° C., the hot-rolled sheet becomes hard and the ductility may deteriorate. Moreover, since a hot-rolling roll tends to wear out, it is not preferable. Accordingly, 980 ° C. is the upper limit of the finishing temperature. 850-960 degreeC is preferable and 870-930 degreeC is a more preferable range.

    After the hot rolling finish rolling, air cooling is performed for 0.5 seconds or longer. If this is less than 0.5 seconds, good hole expansion characteristics cannot be obtained. The reason for this is not necessarily clear, but in less than 0.5 seconds, recrystallization of austenite does not proceed, and as a result, the anisotropy of mechanical properties increases and the hole expandability tends to decrease. More preferably, an air cooling time of more than 1.0 seconds is provided.

    In the subsequent cooling process, the average cooling rate in the temperature range of 750 to 600 ° C. is in the range of 10 to 40 ° C./s. If the cooling rate in this temperature range is low, coarse precipitates may be deposited and the hole expansibility may be reduced. On the other hand, if it exceeds 40 ° C./s, the structure becomes non-uniform and the hole expandability may be lowered, or the material may vary in the width direction or longitudinal direction of the coil, so this is the upper limit. 15-40 degreeC / s is preferable and more than 20-35 degreeC / s is a more preferable range.

    The winding temperature is 440 to 560 ° C. When the coiling temperature is less than 440 ° C., a hard structure such as bainite or martensite appears and the hole expandability deteriorates. If it exceeds 560 ° C., it is difficult to ensure the solid solution C, which is one of the most important requirements in the present invention, and as a result, the hole expandability may be deteriorated. A more preferable range of the coiling temperature is 460 to 540 ° C.

The rough bar after rough rolling may be subjected to heat treatment until finish rolling is completed (during finish rolling). Further, the heat treatment can be performed before the finish rolling is started on the rough bar after the rough rolling is completed. As a result, the temperature in the width direction and the longitudinal direction of the plate becomes uniform, and the material variation in the coil of the product is also reduced. The heating method is not particularly specified. What is necessary is just to perform by methods, such as a furnace heating, induction heating, electrical heating, and high frequency heating.
Similarly, descaling may be performed from the end of rough rolling to the start of finish rolling. This may reduce the surface roughness and improve fatigue characteristics and hole expansibility. The descaling method is not particularly specified, but the most common method is a high-pressure water stream.
The hot-rolled steel sheet thus obtained may be reheated (annealed). In this case, when the reheating temperature exceeds 780 ° C., the tensile strength and fatigue limit of the steel sheet decrease, so the appropriate range is limited to 780 ° C. or less. From the viewpoint of stretch flangeability, 680 ° C. or lower is a more preferable range. The heating method is not particularly specified, and may be performed by methods such as furnace heating, induction heating, energization heating, and high frequency heating. The heating time is not particularly defined, but when the heating and holding time of 550 ° C. or higher exceeds 30 minutes, the maximum heating temperature is desirably 720 ° C. or lower in order to obtain a strength of 520 MPa or higher.

  Skin pass rolling is effective in improving shape correction, aging, and fatigue properties, and therefore may be performed after pickling or before pickling. In the case of carrying out, it is desirable that the rolling reduction is 3% as an upper limit. This is because if it exceeds 3%, the formability of the steel sheet is impaired. Moreover, you may perform pickling according to the objective.

  After pickling the hot-rolled steel sheet thus obtained, the steel sheet may be heated and hot-dip plated using a continuous galvanizing facility or a continuous annealing galvanizing facility. When the heating temperature of the steel plate exceeds 780 ° C., the tensile strength and fatigue limit of the steel plate decrease, so the appropriate range of heating temperature is limited to 780 ° C. or less. Further, after hot-dip plating, alloyed hot-dip galvanization may be used. The heating temperature is more preferably 680 ° C. or less from the viewpoint of stretch flangeability.

The microstructure of the steel sheet in the present invention preferably has ferrite as the main phase for ensuring ductility. That is, the volume fraction of ferrite is preferably over 96%. More preferably, it is over 97%. The ferrite is one or more of polygonal ferrite (PF) and pseudo-polygonal ferrite (Quasi-Polygonal Ferrite, hereinafter referred to as αq). In addition, pearlite may be contained in a volume ratio of less than 4%. The microstructure does not include precipitated particles such as carbides such as cementite and TiC, sulfides such as MnS, nitrides such as TiN, and carbon sulfides such as Ti ₄ C ₂ S _2, and crystallized particles such as oxides.

  In the present invention, the production method preceding hot rolling is not particularly limited. In other words, following smelting with a blast furnace, converter, electric furnace, etc., the components are adjusted so that the desired component content is obtained by various secondary scouring, and then, in addition to normal continuous casting, casting by ingot method, thin slab What is necessary is just to cast by methods, such as casting. Scrap may be used as a raw material. In the case of a slab obtained by continuous casting, it may be directly sent to a hot rolling mill as it is a high-temperature slab, or may be hot-rolled after being reheated in a heating furnace after being cooled to room temperature.

Furthermore, it is desirable that the maximum height Ry of the steel sheet surface after finish rolling is 15 μm (15 μm Ry, l2.5 mm, ln12.5 mm) or less. This is clear from the fact that the fatigue strength of a hot-rolled or pickled steel sheet correlates with the maximum height Ry of the steel sheet surface, as described in, for example, Metal Material Fatigue Design Handbook, edited by the Japan Society of Materials Science, page 84. is there. Further, the subsequent finish rolling is desirably performed within 5 seconds in order to prevent the scale from being generated again after descaling. Ra is preferably less than 1.40 μm, more preferably less than 1.20 μm.
Moreover, a sheet bar may be joined between rough rolling and finish rolling, and finish rolling may be performed continuously. At that time, the coarse bar may be wound once in a coil shape, stored in a cover having a heat retaining function as necessary, and rewound again before joining.

The following examples further illustrate the present invention.
The steels A to P having chemical components shown in Table 1 are melted in a converter, and after continuous casting, are reheated under the conditions shown in Table 2, and the thickness is 4.5 mm by finish rolling following rough rolling. And then wound up. However, the display about the chemical composition in a table | surface is the mass%. Steel D, steel O, and steel P were subjected to descaling under the conditions of a collision pressure of 2.7 MP and a flow rate of 0.001 liter / cm ² after rough rolling. Furthermore, the steel I shown in Table 1 was galvanized at 450 ° C.

  Details of the manufacturing conditions are shown in Table 2. Here, “SRT” is the slab extraction temperature, “rough bar heating” is the time from the end of rough rolling to the start of finish rolling or / and whether or not the rough bar or rolled material is heated during finish rolling, and “RT” is the rough temperature. Rolling end temperature, “FT” is finish rolling end temperature, “time to start cooling” is time from finish rolling to start cooling, “cooling rate at 750 to 600 ° C.” is 750 during cooling The average cooling rate when passing through a temperature range of ˜600 ° C., “CT” indicates the coiling temperature.

  Table 3 shows that the slab reheated to 1200 ° C. is finished rolling temperature: 900 ° C., time to start cooling: 2 s, average cooling rate at 750 to 600 ° C .: 35 ° C./s, and heated at a winding temperature of 530 ° C. The example which annealed or galvanized, after giving the pickled material the pickling is shown. Steel A-3 and Steel A-4 are examples in which annealing was performed only in a box-type annealing furnace, and Steel B-3 and Steel B-4 were annealed in a continuous annealing plating facility and subsequently galvanized. For example, Steel C-3, Steel C-4, Steel D-3, Steel E-3, Steel F-3, Steel L-2, and Steel L-3 were annealed in a continuous annealing plating facility and subsequently zinc This is an example in which plating and plating alloying treatment were performed, and Steel M-2 and Steel N-2 were examples in which the pickled plate was heated to the galvanizing temperature and then subjected to galvanizing and plating alloying treatment. The galvanizing immersion temperature was 450 ° C. and the plating alloying temperature was 500 ° C.

Thus, the tensile test of the obtained thin steel plate performed the test material first to the 5th test piece of JISZ2201, and performed it according to the test method of JISZ2241.
The AI test is processed into a test piece No. 5 described in JIS Z 2201 in the same way as the tensile test, and after applying a 7% tensile pre-strain to the test piece, it is subjected to a heat treatment at 100 ° C. for 60 minutes and then a tensile test again. Carried out. Here, AI is defined as subtracting 10% tensile prestrain flow stress from the upper yield point in re-tensioning.
The stretch flangeability was evaluated by the hole expansion value according to the hole expansion test method described in Japan Iron and Steel Federation Standard JFS T 1001-1996.
In Table 2, “TS” is the maximum tensile strength, “YS” is the yield strength, “EI” is the elongation, “AI” is the aging index, and “λ” is the hole expansion ratio. .

    On the other hand, the microstructure is examined by grinding a sample cut from a 1/4 W or 3/4 W position of the steel plate width to a cross section in the rolling direction, etching using a Nital reagent, and 200-500 times magnification using an optical microscope. This was carried out with a photograph of the field of view at 1/4 t of the observed plate thickness. The volume fraction of the microstructure is defined by the area fraction in the metal structure photograph. As described above, the steel sheet of the present invention is mainly composed of PF and αq. αq is described in the Japan Iron and Steel Institute Basic Research Group Bainite Research Group / Edition; Recent Research on Bainite Structure and Transformation Behavior of Low Carbon Steels-Final Report of Bainite Research Group (1994 Japan Iron and Steel Institute) Thus, it is one of the microstructures defined as the transformation structure in the intermediate stage between the polygonal ferrite formed by the diffusive mechanism and the non-diffusible martensite. The internal structure of αq does not appear by etching like PF, but the shape is ash and is clearly distinguished from PF. Here, αq is a grain whose ratio (lq / dq) satisfies lq / dq ≧ 3.5 when the perimeter length lq of the target crystal grain is dq and the equivalent circle diameter is dq.

In the present invention example, a hot-rolled steel sheet containing a predetermined amount of a steel component, whose microstructure is mainly composed of uniform ferrite, and has both fatigue characteristics and stretch flangeability is obtained. That is, the hole expansion value evaluated by the method described in the present invention exceeds 140%.
Further, the fatigue characteristics were evaluated by a complete double-bending bending test and defined as the upper limit of the fatigue strength at the number of repetitions of 1 × 10 ⁷ times. As shown in Tables 2 and 3, the results of the present invention are excellent in fatigue strength.
On the other hand, it can be seen that those outside the present invention have chemical components or / and production methods outside the scope of the invention, resulting in inferior strength, hole expansibility, fatigue characteristics, and the like.
Moreover, in Table 2, since the fatigue limit is 200 or less in steel K-1, K-2 whose component is outside the present invention, it is out of the present invention.

  The steel sheet obtained by the present invention is particularly suitable for automobile chassis and suspension parts, and is particularly suitable for wheel disks. Since it has excellent formability such as stretch flangeability, the design freedom is increased and so-called high-design wheels are realized. It is excellent in corrosion resistance after painting, and because it has high strength, it is possible to reduce the plate thickness, contributing to the conservation of the global environment through weight reduction of the car body.

Claims (10)

In mass%
C: 0.015 or more and less than 0.040%,
Si: less than 0.05%,
Mn: 0.9 to 1.8%,
P: less than 0.02%,
S: less than 0.01%,
Al: less than 0.1%,
N: less than 0.006%,
Ti: 0.06 or more and less than 0.11%,
Including Ti / C = 2.5 or more and less than 3.5,
The balance is a hot-rolled steel sheet having a component composed of Fe and inevitable impurities,
The maximum tensile strength is 520 MPa or more and less than 720 MPa,
Aging index AI is over 15 MPa,
The product of hole expansion rate (λ)% and total elongation (El)% is 2350 or more,
A hot-rolled steel sheet excellent in fatigue characteristics and stretch flangeability, characterized by a fatigue limit of 200 MPa or more. The hot-rolled steel sheet according to claim 1, further in mass%,
Nb: 0.001 or more and 0.04% or less,
B: 0.0001 to 0.004%,
Cu: 0.01 to 1.5%,
Ni: 0.01 to 0.8%,
Mo: 0.02 to 1.0%,
V: 0.001 to 0.2%,
Cr: 0.01 to 1.5%,
W: A hot-rolled steel sheet excellent in fatigue characteristics and stretch flangeability, characterized by containing one or more of 0.01 to 1.0%. The hot-rolled steel sheet according to any one of claims 1 and 2, further in mass%,
Ca: 0.0005 or more and 0.005% or less,
REM: 0.0005 or more and 0.05% or less,
A hot-rolled steel sheet excellent in fatigue characteristics and stretch flangeability, characterized by containing one or two of the above.   A hot-rolled steel sheet excellent in fatigue characteristics and stretch flangeability, wherein the hot-rolled steel sheet according to any one of claims 1 to 3 is plated.   When hot-rolling to obtain the hot-rolled steel sheet according to any one of claims 1 to 4, the steel slab having the components is heated to 1100 ° C or higher, and rough rolling is performed at a temperature of 1000 ° C or higher. After finishing finish rolling in the temperature range of 830 to 980 ° C., air cooling is performed for 0.5 seconds or more, and the temperature range of 750 to 600 ° C. is cooled at an average cooling rate in the range of 10 to 40 ° C./sec. A method for producing a hot-rolled steel sheet having excellent fatigue characteristics and stretch flangeability, characterized by winding at 560 ° C.   Fatigue property, characterized in that, during the hot rolling according to claim 5, the rough bar after rough rolling of the steel slab is heated until the start of finish rolling and / or during the finish rolling of the rough bar. And a method for producing hot-rolled steel sheets with excellent stretch flangeability.   In the hot rolling according to any one of claims 5 and 6, descaling is performed from the end of rough rolling to the start of finish rolling, and is excellent in fatigue characteristics and stretch flangeability A method for producing a hot-rolled steel sheet.   A method for producing a hot-rolled steel sheet having excellent fatigue characteristics and stretch flangeability, characterized by performing annealing at 780 ° C or lower after hot rolling according to any one of claims 5 to 7.   The hot-rolled steel sheet obtained after hot rolling according to any one of claims 5 to 7 is heated at 780 ° C or lower, and then immersed in a plating bath to plate the steel sheet surface. A method for producing hot-rolled steel sheets with excellent fatigue characteristics and stretch flangeability.   A method for producing a hot-rolled steel sheet having excellent fatigue characteristics and stretch flangeability, characterized in that after the plating, the alloying treatment is performed after the plating.