JP4205853B2 - Hot-rolled steel sheet with excellent burring workability and fatigue characteristics and method for producing the same - Google Patents

Description

【００５６】
【表２】

【００５７】
【発明の効果】
以上詳述したように、本発明は、バーリング加工性と疲労特性に優れた引張強度６４０ＭＰａ以上の熱延鋼板、およびその鋼板を安定して製造できる製造方法を提供するものであり、これらの熱延鋼板を用いることにより、バーリング加工性（伸びフランジ性）を十分に確保しつつ疲労特性の大幅な改善が期待できるため、工業的価値が高い発明である。
【図面の簡単な説明】
【図１】本発明に至る予備実験の結果を、Ｔｉ* と穴拡げ値（穴拡げ率）の関係で示す図である。
【図２】本発明に至る予備実験の結果を、穴拡げ値（穴拡げ率）の範囲をＴｉ析出物サイズの範囲とＴｉ析出物の最小間隔の関係で示す図である。
【図３】本発明に至る予備実験の結果を、せん断や打ち抜き端面が存在する場合の疲労特性の範囲をＴｉ析出物サイズの範囲とＴｉ析出物の最小間隔の関係で示す図である。
【図４】疲労試験片の形状を説明する図である。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a hot-rolled steel sheet having a tensile strength of 640 MPa or more excellent in burring workability and fatigue characteristics, and a method for producing the same, and particularly suitable for automobile suspension parts such as suspension arm materials ( The present invention relates to a hot-rolled steel sheet having excellent hole expandability and fatigue characteristics and a method for producing the same.
[0002]
[Prior art]
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 has been limited to special applications because they are significantly more expensive than steel. In order to promote weight reduction of automobiles in a wider range, the application of inexpensive high-strength steel sheets is strongly demanded.
[0003]
In general, the higher the strength of a material, the lower the ductility and the worse the workability (formability). Steel materials are no exception, and attempts have been made so far to achieve both high strength and high ductility. On the other hand, in addition to these characteristics, materials used for undercarriage parts such as automobile suspension arms are required to have burring workability (hole expandability) and fatigue durability.
However, not only does the hole expandability tend to decrease with increasing strength, but increasing strength also increases notch sensitivity, so fatigue durability at stress-concentrated sites is improved as expected. do not do. Therefore, in the application of high-strength steel sheets to undercarriage parts and the like of automobiles having complicated shapes, not only the hole expandability but also fatigue durability becomes an important examination subject.
[0004]
As a high-strength hot-rolled steel sheet excellent in hole expansibility (stretch flangeability or burring workability), for example, in JP-A-6-200351, a high-strength hot-rolled steel sheet excellent in stretch flangeability is added with Ti and Nb. Thus, there is disclosed an invention obtained by reducing the second phase and precipitation strengthening TiC and NbC in polygonal ferrite which is the main phase.
Japanese Patent Laid-Open No. 7-11382 discloses that the second phase is reduced by adding Ti and Nb, and the microstructure is made of acicular ferrite and strengthened by precipitation strengthening with TiC and NbC. An invention for obtaining a high-strength hot-rolled steel sheet is disclosed.
[0005]
In JP-A-7-70696, Ti and Nb are added at C equivalents or more to make the microstructure a ferrite single phase and Cu is added, and ε-Cu is precipitated together with TiC and NbC to increase the strength. An invention for obtaining a high-strength hot-rolled steel sheet excellent in stretch flangeability is disclosed.
Further, JP-A-8-157957 discloses that Ti and Nb are added in a C equivalent or more to make the microstructure a ferrite single phase, and the value of Ni / Cu is defined to change the ferrite from polygonal to bainitic. An invention for obtaining a high-strength hot-rolled steel sheet having improved stretch flangeability and improved stretch flangeability is disclosed.
[0006]
  On the other hand, high strength with excellent fatigue propertiesFeverAs a rolled steel sheet, for example, in Japanese Patent Laid-Open No. 3-82708, a high-strength hot-rolled steel sheet having high fatigue properties with excellent fatigue characteristics is reduced as much as possible by reducing the second-phase structure such as cementite by reducing C to a minimum. An invention that obtains high strength and excellent fatigue properties by the combined addition of these is disclosed.
  In addition, in JP-A-6-287865, in addition to reducing the second phase by adding Ti and reducing the solid solution C in the ferrite, in addition to obtaining a strength of 690 MPa or more by precipitation strengthening of TiC or the like, An invention for obtaining a high-strength hot-rolled steel sheet that has improved fatigue characteristics by adding Cu and has excellent stretch flangeability and fatigue characteristics is disclosed.
[0007]
[Problems to be solved by the invention]
However, in some steel plates for parts such as suspension arms, fatigue durability at the end face of shearing and punching as well as workability such as burring workability is very important. I can't. Moreover, even if both characteristics are satisfied, it is important to provide a manufacturing method that can be stably manufactured at low cost, and the above-described conventional technology is insufficient.
[0008]
That is, in the invention described in JP-A-6-200351, polygonal ferrite having an area ratio of 85% or more is essential in order to obtain high stretch flangeability, but in order to obtain 85% or more polygonal ferrite. In order to promote the growth of ferrite grains after hot rolling, it is necessary to hold for a long time, which is not preferable in terms of operation cost.
Further, in the invention described in JP-A-7-11382, due to the microstructure having a high dislocation density and the precipitation of fine TiC and / or NbC, the ductility is only about 17% at 780 MPa and the moldability is insufficient.
[0009]
Further, in the invention described in JP-A-7-70696, since ε-Cu is precipitated in the ferrite phase, ductility may be lowered and workability may be deteriorated.
Further, in the invention described in JP-A-8-157957, due to precipitation of a microstructure having a high dislocation density and fine TiC and / or NbC, there is only about 20% ductility at 780 MPa and moldability is insufficient. .
[0010]
Furthermore, these inventions make no mention of fatigue properties. On the other hand, in the invention described in Japanese Patent Laid-Open No. 3-82708 as an invention that also refers to fatigue characteristics, 0.04 to 0.10% of P that segregates at a grain boundary and causes embrittlement of the grain boundary is added. Therefore, when grain boundary fracture, which is the starting point of fatigue fracture, occurs, fatigue characteristics may be significantly degraded. Furthermore, the publication does not describe anything about the addition of B that suppresses grain boundary embrittlement due to P or the like.
Furthermore, in the invention described in the above-mentioned Japanese Patent Application Laid-Open No. 6-287865, Cu precipitation strengthening is mainly used to improve fatigue characteristics, but Cu precipitation strengthening does not improve fatigue strength as much as static strength. There is a problem that the fatigue limit ratio is lowered.
[0011]
Therefore, the present invention provides a hot-rolled steel sheet having a tensile strength of 640 MPa or more excellent in burring workability and fatigue characteristics, and a manufacturing method capable of stably and inexpensively manufacturing the steel sheet, which can advantageously solve the above-described problems of the prior art. It is for the purpose.
[0012]
[Means for Solving the Problems]
The present inventors have made both the burring workability and fatigue characteristics of hot-rolled steel sheet compatible with the manufacturing process of hot-rolled steel sheet produced on an industrial scale by the continuous hot rolling equipment that is currently normally employed. We worked hard to achieve it.
As a result, the average size of the precipitates containing Ti of 5 nm or more among particles in steel is 10¹-10^ThreeMinimum spacing of 10 at nm¹More than 10 nm^FourIt has been found that a thickness of not more than nm is very effective for improving the burring workability and does not impair the ductility.
At the same time, the present inventors have newly found out that fatigue characteristics are improved when a fatigue crack propagates from a sheared and punched end face within the range of the average size and the minimum interval of the above-mentioned precipitates.
[0013]
  That is, the gist of the present invention is as follows.
(1) In mass%,
    C: 0.01 to 0.1%, S ≦0.0013%,
    N ≦ 0.005%,Si: 0.01-2%
    Mn: 0.05-2%      P ≦ 0.1%,
    Al: 0.005 to 1.0%,Ti: 0.05-0.240%
  Including
    Ti-48 / 12C-48 / 14N-48 / 32S ≧ 0%
Is a steel containing Ti and the inevitable impurities, and the average size of precipitates containing Ti of 5 nm or more in particles in the steel is 10¹-10³Minimum spacing of 10 at nm¹More than 10 nm⁴A hot-rolled steel sheet excellent in burring workability and fatigue characteristics, characterized by being less than or equal to nm.
(2) In mass%,
    C: 0.01 to 0.1%, S ≦0.0013%,
    N ≦ 0.005%,Si: 0.01-2%
    Mn: 0.05-2%      P ≦ 0.1%,
    Al: 0.005 to 1.0%,Ti: 0.05-0.240%,
    Nb: 0.01 to 0.5%
  Including
  Ti + 48 / 93Nb-48 / 12C-48 / 14N-48 / 32S ≧ 0%
Is a steel containing Ti and Nb in a range satisfying the following, the balance being Fe and inevitable impurities, and the average size of precipitates containing either one or both of Ti and Nb of 5 nm or more in particles in the steel is 10¹-10³Minimum spacing of 10 at nm¹More than 10 nm⁴A hot-rolled steel sheet excellent in burring workability and fatigue characteristics, characterized by being less than or equal to nm.
[0014]
(3) The steel is further mass%,
    B: 0.0002 to 0.002%
(1), characterized by comprisingOr (2)Hot-rolled steel sheet with excellent burring workability and fatigue properties as described in 1.
(4) The steel is further mass%,
    Ni: 0.1 to 1%
(1) to (1) characterized by containing3The hot-rolled steel sheet excellent in burring workability and fatigue properties according to any one of the above.
[0015]
(5) The steel is further mass%,
    Ca: 0.0005 to 0.02%,
    REM: 0.0005 to 0.2%
(1) thru | or (1) characterized by containing 1 type or 2 types of4The hot-rolled steel sheet excellent in burring workability and fatigue properties according to any one of the above.
(6) The steel is further mass%,
    Mo: 0.05 to 1%, V: 0.02 to 0.2%,
    Cr: 0.01 to 1%, Zr: 0.02 to 0.2%
1 type or 2 types or more, (1) thru | or (1) characterized by including5The hot-rolled steel sheet excellent in burring workability and fatigue properties according to any one of the above.
[0016]
(7(1) to (1)6) When hot-rolling a steel slab having the component according to any one of₃After finishing the hot finish rolling at the transformation point or higher, the steel is cooled to a temperature range of 350 ° C. to 750 ° C. and wound up, and the precipitates containing one or both of Ti and Nb having a particle size of 5 nm or more are contained in the steel. Average size is 10¹-10³Minimum spacing of 10 at nm¹More than 10 nm⁴A method for producing a hot-rolled steel sheet excellent in burring workability and fatigue characteristics, characterized by obtaining a steel sheet having a thickness of nm or less.
(8) During the hot rolling, after the rough rolling, high pressure descaling is performed, Ar₃The hot finish rolling is finished at the transformation point or higher,7) A method for producing a hot-rolled steel sheet having excellent burring workability and fatigue characteristics.
[0017]
DETAILED DESCRIPTION OF THE INVENTION
The basic research results that led to the present invention will be described below.
First, the effect of Ti * (Ti * = Ti-48 / 12C-48 / 14N-48 / 32S) on the hole expandability was investigated. The test material for that purpose was prepared as follows.
That is, the amount of Ti added based on 0.05% C-1.0% Si-1.4% Mn-0.01% P-0.001% S-0.03% Al-0.001% N After changing the components and adjusting the components, hot-rolling the molten slab and winding it at room temperature, isothermally holding it at 550 ° C for 1 hour, and then heat-treating it in a furnace, and various Ti * steel plates Obtained.
The hole expansion test result about these steel plates is shown in FIG. From this result, it has been newly found that the hole expansion value (hole expansion ratio) is remarkably improved when Ti * ≧ 0%. However, Ti * ≧ 0.05% is desirable in order to stably obtain an excellent hole expansion value in both the longitudinal direction and the width direction of the plate.
[0018]
Furthermore, the steel plate having the above components is rolled and heat-treated under various production conditions to change the precipitate size and precipitation interval containing Ti, and the relationship between the average size of the precipitate and the minimum interval between the precipitates and the hole expansion value. As a result, there is a strong correlation between the average size of the precipitates and the minimum interval between the precipitates and the hole expansion value.¹-10^Threein the range of nm and the minimum spacing of precipitates is 10¹More than 10 nm^FourIt has been newly found that the hole expansion value is remarkably improved at nm or less.
FIG. 2 shows the relationship between the hole expansion value (hole expansion ratio), the average value of the precipitate size including Ti, and the minimum interval between the precipitates. Here, the precipitate containing Ti is particles containing Ti, such as carbide, nitride, sulfide, and may contain oxide.
[0019]
This mechanism is not always clear, but if the precipitate is too large, voids are likely to form at the interface between the precipitate and the matrix, and cracks occur when expanding the hole, and if it is too small, the local ductility correlates with the hole expansion value. Therefore, it is estimated that the hole expansion value is improved at the optimum size and precipitate spacing.
However, the average size of precipitates containing Ti among particles in steel is 10²If it exceeds nm, there is a risk of starting a corrosion when precipitates appear on the surface of the steel sheet or on the fracture surface as punched or sheared, so the average size of precipitates containing Ti in the particles in steel is 10¹-10²A range of nm is desirable.
[0020]
Next, we investigated the fatigue characteristics in the presence of shear and punched end faces, and found that the average size of precipitates containing Ti and the minimum spacing between the precipitates, and the fatigue characteristics in the presence of shear and punched end faces. Has a strong correlation, and the average size of precipitates containing Ti of 5 nm or more among particles in steel is 10¹-10^Threein the range of nm and the minimum distance between the precipitates is 10¹More than 10 nm^FourIt has been newly found that the fatigue characteristics are significantly improved in the presence of shear and punched end faces at nm or less.
FIG. 3 shows the relationship between the fatigue characteristics in the presence of shear and punched end faces, the average value of the precipitate size including Ti, and the minimum interval between the precipitates.
[0021]
This mechanism is not always clear, but if the precipitate is too large, voids are likely to form at the interface between the precipitate and the parent phase, and the void becomes the starting point for fatigue cracks. Since the ratio of the fractured surface in the punched section that tends to be the starting point increases and the fatigue limit decreases, it is estimated that the fatigue characteristics are improved when shearing or a punched end face exists at the optimal size and precipitate spacing.
[0022]
In addition, about the mechanical property by a tensile test, it measured with the test method of JISZ2241 with the No. 5 test piece of JISZ2201. The fatigue characteristics in the presence of shear or punched end faces are as shown in FIG. 4 in the center of a fatigue test piece having a length of 98 mm, a width of 38 mm, a radius of curvature of the notch of 30 mm, and a minimum cross-sectional width of 30 mm. 1.0 × 10 obtained by a complete double swing plane bending fatigue test using punched holes with a clearance of about 11%.⁷The fatigue limit σWK at the time was divided by the tensile strength σB of the steel sheet (fatigue limit ratio σWK / σB). However, the surface of the fatigue test piece was left as pickled without any grinding.
[0023]
In addition, a precipitate containing Ti in the steel was obtained by collecting a transmission electron microscope sample from the test steel having a width of 1/4 W or 3/4 W and a thickness of 1/4, and energy dispersive X-ray spectroscopy ( Equipped with a field emission gun (FEG) with an accelerating voltage of 200 kV with the composition analysis function of Energy Dispersive X-ray Spectroscope (EDS) and Electron Energy Loss Spectroscope (EELS) Was observed with a transmission electron microscope. The observed particle composition was confirmed to be a precipitate containing Ti by EDS and EELS.
[0024]
In the present invention, the size of the precipitate to be defined is defined as the longest piece if it is rectangular, and the maximum length if it is stretched. Moreover, the average precipitate size prescribed | regulated by this invention is a simple average of the size in the one visual field about the thing of 5 nm or more among what measured the size of the precipitate with the magnification of 5000-500000 times. Furthermore, the minimum distance between the precipitates defined in the present invention is the minimum distance among the distances measured between the centers of the target precipitates of 5 nm or more. Here, the center of the precipitate is defined as the center of gravity of the area in the observation cross section of the precipitate.
[0025]
Next, the microstructure of the steel sheet in the present invention will be described.
The microstructure of the steel sheet is preferably a single ferrite phase in order to ensure excellent burring workability (stretch flangeability). However, some bainite is allowed as needed. In order to secure good stretch flangeability, the volume fraction of bainite is desirably 10% or less. Here, the volume fraction of ferrite and bainite is a sample cut from a 1/4 W or 3/4 W position of the steel plate width in a rolling direction cross section, etched using a Nital reagent, and 200 using an optical microscope. It is the area fraction of the microstructure at ¼t of the plate thickness, observed at a magnification of ˜500 times.
[0026]
Next, the reasons for limiting the chemical components of the present invention will be described. The amount of chemical component is mass%.
If the C content exceeds 0.1%, workability and weldability deteriorate, so the content is made 0.1% or less. Moreover, since intensity | strength will fall if it is less than 0.01%, it shall be 0.01% or more.
[0027]
  If S is too large, cracks during hot rolling will be caused, so it should be reduced as much as possible. However, it is acceptable if it is 0.03% or less.Note that the upper limit of S is set to 0.0013% or less based on the fact that S of K steel in Table 1 of the examples is 0.0013%.
[0028]
N forms precipitates with Ti and Nb at a higher temperature than C, and reduces Ti and Nb effective for fixing C. Therefore, it should be reduced as much as possible, but 0.005% or less is an acceptable range.
[0029]
  Ti is one of the most important elements in the present invention. That is, Ti contributes to an increase in strength of the steel sheet by precipitation strengthening. However, if it is less than 0.05%, this effect is insufficient, and if it exceeds 0.5%, the effect is not only saturated but also the alloy cost is increased. Therefore, the Ti content is 0.05% or more and 0.5% or less.The upper limit of the Ti content is set to 0.240% or less based on the fact that the Ti content of the F and L steels in Table 1 of the examples is 0.240%.
  Furthermore, Ti-48 / 12C-48 / 14N-48 / 32S ≧ 0% in order to precipitate and fix C which causes carbides such as cementite which deteriorates burring workability and contribute to improvement of burring workability. It is necessary to satisfy the following conditions.
[0030]
Nb contributes to an increase in the strength of the steel sheet by precipitation strengthening, like Ti, and is added as necessary. However, if the content is less than 0.01%, this effect is insufficient. Even if the content exceeds 0.5%, the effect is not only saturated but also the alloy cost is increased. Therefore, the Nb content is 0.01% or more and 0.5% or less.
Furthermore, in order to precipitate and fix C which causes carbides such as cementite which deteriorates burring workability, and contributes to improvement of burring workability, Ti + 48 / 93Nb-48 / 12C-48 / 14N-48 / 32S ≧ It is necessary to satisfy the condition of 0%.
[0031]
Since Si is effective as a solid solution strengthening element for increasing the strength, it is added as necessary. In order to obtain a desired strength, it is necessary to contain 0.01% or more. However, if it exceeds 2%, workability deteriorates. Therefore, the Si content is set to 0.01% or more and 2% or less.
[0032]
Since Mn is effective as a solid solution strengthening element for increasing the strength, it is added as necessary. In order to obtain a desired strength, 0.05% or more is necessary. Further, if added over 2%, slab cracking occurs, so it is made 2% or less.
[0033]
If P is contained in an amount exceeding 0.1%, workability and weldability are adversely affected.
[0034]
Al is added as necessary for molten steel deoxidation. Although it is necessary to add 0.005% or more, since it raises cost, the upper limit shall be 1.0%. Moreover, when adding too much, a nonmetallic inclusion will be increased and elongation will be degraded, Therefore Preferably it is 0.5% or less.
[0035]
Since B has an effect of increasing the fatigue limit by suppressing grain boundary embrittlement due to P, B is added as necessary. However, if it is less than 0.0002%, it is insufficient for obtaining the effect, and if added over 0.002%, slab cracking occurs. Therefore, the addition of B is set to 0.0002% or more and 0.002% or less.
[0036]
Ni is added as necessary to prevent hot brittleness due to Cu inclusion. However, if the content is less than 0.1%, the effect is small, and even if added over 1%, the effect is saturated.
[0037]
Ca and REM are elements that are detoxified by changing the form of non-metallic inclusions that become the starting point of destruction or deteriorate the workability. However, even if less than 0.0005% is added, there is no effect, and if Ca is more than 0.02%, and if REM is added more than 0.2%, the effect is saturated. It is desirable to add 0.02% and REM: 0.0005 to 0.2%.
[0038]
Further, in order to impart strength, one or more of precipitation strengthening or solid solution strengthening elements of Mo, V, Cr, and Zr may be added. However, the effect cannot be obtained if the content is less than 0.05%, 0.02%, 0.01%, and 0.02%, respectively. Moreover, the effect will be saturated even if it adds exceeding 1.0%, 0.2%, 1.0%, and 0.2%, respectively.
[0039]
Even if Sn is added, the effects of the present invention can be obtained, and the content of the Sn is not particularly required. However, since there is a risk of wrinkling during hot rolling, 0.05% or less is desirable.
[0040]
Next, the reasons for limiting the production method of the present invention will be described in detail below.
In the present invention, a slab obtained by casting a molten steel whose components are adjusted so as to have a desired component content may be directly sent to a hot rolling mill as a high-temperature slab, or after being cooled to room temperature, a heating furnace It may be hot-rolled after reheating at.
The reheating temperature is not particularly limited, but if it is 1400 ° C. or higher, the amount of scale-off increases and the yield decreases, so the reheating temperature is preferably less than 1400 ° C. Heating below 1100 ° C. not only re-dissolves precipitates containing Ti and / or Nb in the slab, but coarsens and loses the precipitation strengthening ability, but also has the desired size and distribution of Ti and / or burring processability. Alternatively, since the precipitate containing Nb does not precipitate, the reheating temperature is desirably 1100 ° C. or higher.
[0041]
In the hot rolling process, after the rough rolling is finished, finish rolling is performed, but the final pass temperature (FT) needs to be finished in a temperature range equal to or higher than the Ar3 transformation point. This is because if the rolling temperature falls below the Ar3 transformation point during hot rolling, strain remains and ductility decreases. The upper limit of the finishing temperature is not particularly required to obtain the effect of the present invention, but it is desirable that the upper limit of the finishing temperature is 1000 ° C. or less because scale soot may be generated in operation.
Here, when high-pressure descaling is performed after completion of rough rolling, the collision pressure P (MPa) of high-pressure water on the steel plate surface × flow rate L (liter / cm).²It is desirable to satisfy the condition of ≧ 0.0025.
[0042]
The collision pressure P of high-pressure water on the steel sheet surface is described as follows (see “Iron and Steel” 1991, vol. 77, No. 9, p1450).
P (MPa) = 5.64 × P_O× V / H²
However,
P_O(MPa): Fluid pressure
V (liter / min): Nozzle flow rate
H (cm): distance between the steel plate surface and the nozzle
[0043]
The flow rate L is described as follows.
L (liters / cm²) = V / (W × v)
However,
V (liter / min): Nozzle flow rate
W (cm): Width of spray liquid per nozzle hitting steel plate surface
v (cm / min): Feeding speed
The upper limit of the collision pressure P × flow rate L is not particularly required to obtain the effect of the present invention. However, increasing the nozzle flow rate causes inconveniences such as severe wear of the nozzle. The following is desirable.
[0044]
Furthermore, it is desirable that the maximum height Ry of the steel sheet after finish rolling is 15 μm (15 μm Ry, l2.5 mm, ln12.5 mm) or less. For example, as described in “Handbook of Fatigue Design for Metallic Materials”, edited by the Japan Society of Materials Science, page 84, the fatigue strength of a hot-rolled or pickled steel sheet correlates with the maximum height Ry of the steel sheet surface. It is clear from this. Further, the subsequent finish rolling is desirably performed within 5 seconds in order to prevent the scale from being generated again after descaling.
[0045]
After finishing rolling, the steel sheet is cooled to a specified winding temperature (CT), but the cooling rate is not particularly required to obtain the effect of the present invention. However, if the cooling rate is too slow, the size of the precipitate containing Ti and / or Nb becomes coarse and may not contribute to the increase in strength due to precipitation strengthening. Therefore, the lower limit of the cooling rate is preferably 20 ° C./s or more. In addition, the upper limit of the cooling rate is 100 ° C. or less in consideration of actual factory equipment capacity and the like.
[0046]
Next, if the coiling temperature is less than 350 ° C., sufficient Ti and / or Nb-containing precipitates are not generated, and solid solution C may remain in the steel, thereby reducing workability. In addition to the fact that the size of the precipitate containing Nb and / or Nb becomes coarse and does not contribute to the increase in strength due to precipitation strengthening, if the precipitate is too large, voids are likely to occur at the interface between the precipitate and the matrix, and the hole expandability is increased. May decrease. Accordingly, the coiling temperature is 350 to 750 ° C.
[0047]
【Example】
The following examples further illustrate the present invention.
The steels A to N having chemical components shown in Table 1 are melted in a converter, re-heated at the heating temperature (SRT) shown in Table 2 after continuous casting, and also shown in Table 2 after rough rolling. After rolling to a plate thickness of 1.2 to 5.4 mm at the finish rolling temperature (FT), each was wound at the winding temperature (CT) shown in Table 2. In some cases, after rough rolling, the collision pressure is 2.7 MPa, and the flow rate is 0.001 liter / cm.²High pressure descaling was performed under the following conditions. However, the display about the chemical composition in a table | surface is the mass%.
[0048]
The tensile test of the hot-rolled sheet thus obtained was carried out according to the test method described in JIS Z 2241 by first processing the specimen into a No. 5 test piece described in JIS Z 2201. Table 2 shows the test results. In addition, the thickness of the specimen cut from the 1/4 W or 3/4 W position of the steel sheet width was polished into a cross section in the rolling direction, etched using a Nital reagent, and the thickness measured at a magnification of 200 to 500 times using an optical microscope. Table 2 also shows the area fraction of the microstructure at 1/4 t.
[0049]
Furthermore, the fatigue characteristics in the presence of shear and punched end faces are as shown in FIG. 4 for the center of a fatigue test piece having a length of 98 mm, a width of 38 mm, a notch curvature radius of 30 mm, and a minimum cross-sectional width of 30 mm. 1.0 × 10 obtained by a complete double swing plane bending fatigue test using punched holes with a clearance of about 11%.⁷The value obtained by dividing the fatigue limit σWK at the time by the tensile strength σB of the steel sheet (fatigue limit ratio σWK / σB) was evaluated. However, the surface of the fatigue test piece was left as pickled without any grinding.
On the other hand, the burring workability (stretch flangeability) was evaluated according to the hole expansion test method described in Japan Iron and Steel Federation Standard JFS T 1001-1996.
[0050]
In addition, a precipitate containing Ti in the steel was obtained by collecting a transmission electron microscope sample from the test steel having a width of 1/4 W or 3/4 W and a thickness of 1/4, and energy dispersive X-ray spectroscopy ( Equipped with a field emission gun (FEG) with an accelerating voltage of 200 kV with the composition analysis function of Energy Dispersive X-ray Spectroscope (EDS) and Electron Energy Loss Spectroscope (EELS) Was observed with a transmission electron microscope.
[0051]
The observed particle composition was confirmed to be a precipitate containing Ti by EDS and EELS. Moreover, the size of the precipitate containing Ti and / or Nb is defined as the longest piece if it is rectangular, and the maximum length if it is stretched. The average precipitate size is a simple average of the sizes in one field of view of the precipitates measured at a magnification of 5000 to 500,000 times and having a size of 5 nm or more.
Furthermore, the minimum distance between precipitates is the minimum distance among the distances between the centers of the target precipitates of 5 nm or more. Here, the center of the precipitate is defined as the center of gravity of the area in the observation cross section of the precipitate.
[0052]
Consistent with the present invention are nine steels, steels A-1, B, F, I, J, K, L, M, and N, containing a predetermined amount of Ti and having a particle size in the steel of 5 nm or more. The average size of the precipitate containing Ti or Ti and Nb is 10¹-10^Threenm with a minimum spacing of 10¹More than 10 nm^FourA hot-rolled steel sheet excellent in burring workability and fatigue characteristics, characterized by being not more than nm, has been obtained.
[0053]
Steels other than the above are outside the scope of the present invention for the following reasons.
That is, steel A-2 has a finish rolling finish temperature (FT) outside the range of the present invention, so that not only does strain remain and ductility (El) decreases, but a sufficient hole expansion value (λ) is obtained. It is not done. Steel A-3 has a coiling temperature (CT) after hot rolling lower than the range of the present invention, so that sufficient Ti or Ti and Nb precipitation does not occur, and solid solution C remains in the steel. Sufficient ductility (El) and hole expansion value (λ) are not obtained.
Steel A-4 has a coiling temperature (CT) after hot rolling higher than the range of the present invention, so the size of precipitates containing Ti or Ti and Nb is coarsened and contributes to an increase in strength due to precipitation strengthening. The desired strength (TS) is not obtained, and the hole expansion value (λ) is low.
[0054]
In steel C, the C and Ti contents are outside the scope of the present invention, so that solid solution C remains in the steel, and sufficient ductility (El) and hole expansion value (λ) are not obtained. In Steel D, the content of S is outside the range of the present invention, so that sufficient ductility and hole expansion value (λ) are not obtained.
In Steel E, the content of N is outside the range of the present invention, so that sufficient ductility (El) and hole expansion value (λ) are not obtained. Steel G has a value of Ti * (Ti * = Ti + 48 / 93Nb-48 / 12C-48 / 14N-48 / 32S) smaller than the range of the present invention, so that solid solution C remains in the steel and sufficient ductility is achieved. (El) and the hole expansion value (λ) are not obtained. Steel H does not have sufficient strength (TS) because the C content is less than the range of the present invention.
[0055]
[Table 1]

[0056]
[Table 2]

[0057]
【The invention's effect】
As described above in detail, the present invention provides a hot-rolled steel sheet having a tensile strength of 640 MPa or more excellent in burring workability and fatigue characteristics, and a production method capable of stably producing the steel sheet. By using a rolled steel sheet, a significant improvement in fatigue properties can be expected while sufficiently securing burring workability (stretch flangeability), and therefore the invention has high industrial value.
[Brief description of the drawings]
FIG. 1 is a diagram showing the results of a preliminary experiment leading to the present invention in relation to Ti * and a hole expansion value (hole expansion ratio).
FIG. 2 is a diagram showing the results of a preliminary experiment leading to the present invention, showing the range of hole expansion values (hole expansion ratio) in relation to the range of Ti precipitate size and the minimum interval between Ti precipitates.
FIG. 3 is a diagram showing the results of a preliminary experiment leading to the present invention, showing the range of fatigue characteristics in the presence of shearing and punched end faces in relation to the range of Ti precipitate size and the minimum interval between Ti precipitates.
FIG. 4 is a diagram illustrating the shape of a fatigue test piece.

Claims (8)

In mass%
C: 0.01 to 0.1%,
S ≦ 0.0013 %,
N ≦ 0.005%,
Si: 0.01-2%
Mn: 0.05-2%
P ≦ 0.1%,
Al: 0.005 to 1.0%,
Ti: 0.05 to 0.240 %
Ti-48 / 12C-48 / 14N-48 / 32S ≧ 0%
Is a steel that contains Ti and the balance is Fe and inevitable impurities, and the average size of precipitates containing Ti of 5 nm or more in particles in the steel is 10 ¹ to 10 ³ nm and the minimum interval is A hot-rolled steel sheet excellent in burring workability and fatigue characteristics, characterized by being more than 10 ¹ nm and not more than 10 ⁴ nm. In mass%
C: 0.01 to 0.1%,
S ≦ 0.0013 %,
N ≦ 0.005%,
Si: 0.01-2%
Mn: 0.05-2%
P ≦ 0.1%,
Al: 0.005 to 1.0%,
Ti: 0.05 to 0.240 %,
Nb: 0.01 to 0.5%
Ti + 48 / 93Nb-48 / 12C-48 / 14N-48 / 32S ≧ 0%
Is a steel that contains Ti and Nb in the range satisfying the above, the balance is Fe and inevitable impurities, and the average size of precipitates containing either one or both of Ti and Nb of 5 nm or more in the particles in the steel is A hot-rolled steel sheet excellent in burring workability and fatigue properties, characterized in that the minimum interval is 10 ¹ to 10 ³ nm and the minimum interval is more than 10 ¹ nm and 10 ⁴ nm or less. The steel is further in mass%,
B: 0.0002 to 0.002%
The hot-rolled steel sheet excellent in burring workability and fatigue characteristics according to claim 1 or 2 , characterized by comprising: The steel is further in mass%,
Ni: 0.1 to 1%
The hot-rolled steel sheet having excellent burring workability and fatigue characteristics according to any one of claims 1 to 3 , wherein The steel is further in mass%,
Ca: 0.0005 to 0.02%,
REM: 0.0005 to 0.2%
The hot-rolled steel sheet excellent in burring workability and fatigue characteristics according to any one of claims 1 to 4 , characterized by containing one or two of the following. The steel is further in mass%,
Mo: 0.05 to 1%
V: 0.02-0.2%,
Cr: 0.01-1%,
Zr: 0.02 to 0.2%
The hot-rolled steel sheet excellent in burring workability and fatigue properties according to any one of claims 1 to 5 , characterized by containing one or more of the following. In the hot rolling of the steel slab having the component according to any one of claims 1 to 6 , after finishing the hot finish rolling at an Ar ₃ transformation point or higher, the steel slab is cooled to a temperature range of 350 ° C to 750 ° C. A steel plate having an average size of 10 ¹ to 10 ³ nm and a minimum interval of more than 10 ¹ nm and not more than 10 ⁴ nm with precipitates containing either one or both of Ti and Nb of 5 nm or more as particles in the steel A method for producing a hot-rolled steel sheet excellent in burring workability and fatigue characteristics. In the hot rolling, high pressure descaling is performed after the end of the rough rolling, and the hot finish rolling is finished at the Ar ₃ transformation point or more, and excellent in burring workability and fatigue characteristics according to claim 7 A method for producing a hot-rolled steel sheet.

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