JP3781344B2 - Manufacturing method of hot-rolled steel sheet with excellent burring workability and fatigue characteristics - Google Patents

Description

【００５４】
【表２】

【００５５】
【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、本発明は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、Ｃｕ含有量と静的強度と疲労限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、Ｃｕ含有量と疲労限度比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、Ｔｉ* と穴拡げ率の関係で示す図である。
【図４】本発明に至る予備実験の結果を、穴拡げ率の範囲をＴｉを含む析出物の平均サイズの範囲とＴｉを含む析出物の最小析出間隔の関係で示す図である。
【図５】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.ofThis is related to the manufacturing method, and in particular, it is suitable for hole expandability (burring workability) and fatigue characteristics suitable for materials that require both hole expandability and durability for automobile undercarriage parts and load wheels. Excellent hot rolled steel sheetofIt relates to a manufacturing method.
[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, they are significantly more expensive than steel, so their application has been limited to special applications. 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, materials used for undercarriage parts such as road wheels of automobiles are required to have hole expandability (burring workability) and fatigue durability in addition to these characteristics. 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, when applying high-strength steel sheets to undercarriage parts and the like of automobiles having complicated shapes, not only the hole expandability but also fatigue durability is 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 Japanese Patent Application Laid-Open No. 6-200351, a high-strength hot-rolled steel sheet excellent in stretch flangeability is used as Ti and Nb. An invention obtained by reducing the second phase by addition and precipitation strengthening TiC and NbC in the polygonal ferrite as the main phase is disclosed.
JP-A-7-011382 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]
Japanese Patent Laid-Open No. 7-70696 discloses that high strength is obtained by adding more than C equivalent of Ti and Nb to make the microstructure a ferrite single phase and adding Cu to precipitate ε-Cu together with TiC and NbC. An invention for obtaining a high-strength hot-rolled steel sheet having excellent stretch flange workability 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, as a high-strength high-strength hot-rolled steel sheet excellent in fatigue characteristics, for example, in Japanese Patent Laid-Open No. 3-82708, a high-strength hot-rolled steel sheet for high-working excellent in fatigue characteristics is made of cementite or the like by extremely low C. An invention has been disclosed in which the second phase structure is reduced as much as possible and high strength and excellent fatigue properties are obtained by the combined addition of P and Cu.
Japanese Patent Laid-Open No. 6-268785 discloses that the addition of Ti reduces the second phase to reduce the solid solution C in the ferrite, and the precipitation strengthening of TiC or the like causes 70 kgf / mm.²In addition to obtaining a strength of (686.47 MPa) or more, an invention for obtaining a high-strength hot-rolled steel sheet that has improved fatigue properties by adding Cu and has excellent stretch flangeability and fatigue properties is disclosed. .
[0007]
[Problems to be solved by the invention]
However, in some steel plates for parts such as a disk of a road wheel, fatigue durability as well as workability such as burring workability is very important, and satisfactory characteristics cannot be obtained by the above-described conventional technology. 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 technique is insufficient.
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.
[0008]
In the invention described in JP-A-7-011382, 80 kgf / mm is obtained due to a microstructure having a high dislocation density and precipitation of fine TiC and / or NbC.²(784.53 MPa) has only a ductility of about 17%, and the moldability is insufficient.
Further, in the invention described in JP-A-7-070696, since ε-Cu is precipitated in the ferrite phase, ductility may be lowered and workability may be deteriorated.
Further, in the invention described in the above-mentioned Japanese Patent Application Laid-Open No. 8-157957, 80 kgf / mm is obtained due to a microstructure having a high dislocation density and precipitation of fine TiC and / or NbC.²(784.53 MPa) has only a ductility of about 20%, and the moldability is insufficient.
[0009]
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.
[0010]
  Accordingly, the present invention provides a hot-rolled steel sheet having a tensile strength of 640 MPa or more that is excellent in burring workability and fatigue characteristics, and that can advantageously solve the above-described problems of the prior art.TheAn object of the present invention is to provide a production method that can be stably produced at low cost.
[0011]
[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^Threenm and the minimum precipitation interval is 10¹More than 10 nm^FourIn order to improve fatigue properties in the manufacturing conditions applied to obtain the precipitates as described above, it is found that being less than nm is very effective for improving burring workability and that ductility is not impaired. The present inventors have newly found out that the effective Cu content range is Cu: 0.2 to 1.2%.
[0012]
  That is, the gist of the present invention is as follows.
(1) In mass%,
    C: 0.01 to 0.1%, Si: 0.01 to 2%,
    Mn: 0.05-2%, P ≦ 0.1%,
    S ≦ 0.03%, Al: 0.005 to 1.0%,
    N ≦ 0.005%, Ti: 0.05 to 0.5%,
    Cu: 0.2 to 1.2% is included, and
    Ti-48 / 12C-48 / 14N-48 / 32S ≧ 0%
Ti within a range that satisfiesIn hot rolling of a steel slab containing either or both of Nb and the balance consisting of Fe and inevitable impurities, after the rough rolling, collision pressure P (MPa) of high-pressure water on the steel sheet surface × flow rate L (liter) / Cm ² ) Perform high-pressure descaling to satisfy the condition of ≧ 0.0025, Ar _Three After finishing the hot finish rolling at the transformation point or higher, it is cooled to 350 ° C. to 750 ° C. and wound up,The average size of precipitates containing Ti of 5 nm or more among particles in steel is 10¹-10^Threenm and the minimum precipitation interval is 10¹More than 10 nm^Fournm or lessGet a steel plate that isHot rolled steel sheet with excellent burring workability and fatigue characteristicsManufacturing method.
  Here, P (MPa) = 5.64 × P _O × V / H ²
  However,
    P _O (MPa): Fluid pressure
    V (liter / min): Nozzle flow rate
    H (cm): distance between the steel plate surface and the nozzle
    L (liters / cm ² ) = 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
(2) In mass%,
    C: 0.01 to 0.1%, Si: 0.01 to 2%,
    Mn: 0.05-2%, P ≦ 0.1%,
    S ≦ 0.03%, Al: 0.005 to 1.0%,
    N ≦ 0.005%, Ti: 0.05 to 0.5%,
    Nb: 0.01-0.5% Cu: 0.2-1.2% is included, and
    Ti + 48 / 93Nb-48 / 12C-48 / 14N-48 / 32S ≧ 0%
Ti within a range that satisfiesIn hot rolling of a steel slab containing either or both of Nb and the balance consisting of Fe and inevitable impurities, after the rough rolling, collision pressure P (MPa) of high-pressure water on the steel sheet surface × flow rate L (liter) / Cm ² ) Perform high-pressure descaling to satisfy the condition of ≧ 0.0025, Ar _Three After finishing the hot finish rolling at the transformation point or higher, it is cooled to 350 ° C. to 750 ° C. and wound up,The average size of precipitates containing particles of steel and containing either or both of Ti and Nb of 5 nm or more is 10¹-10^Threenm and the minimum precipitation interval is 10¹More than 10 nm^Fournm or lessGet a steel plate that isHot rolled steel sheet with excellent burring workability and fatigue characteristicsManufacturing method.
  Here, P (MPa) = 5.64 × P _O × V / H ²
  However,
    P _O (MPa): Fluid pressure
    V (liter / min): Nozzle flow rate
    H (cm): distance between the steel plate surface and the nozzle
    L (liters / cm ² ) = 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
[0013]
(3) The steel slab further contains B: 0.0002 to 0.002% by mass%, and is excellent in burring workability and fatigue characteristics according to the above (1) or (2) Hot rolled steel sheetManufacturing method.
(4) The burring workability described in any one of (1) to (3) above, wherein the steel slab further contains Ni: 0.1 to 1% by mass%. Hot-rolled steel sheet with excellent fatigue propertiesManufacturing method.
(5) The steel slab further comprising one or two of Ca: 0.005 to 0.02% and REM: 0.005 to 0.2% in terms of% by mass ( The hot-rolled steel sheet excellent in burring workability and fatigue properties according to any one of 1) to (4)Manufacturing method.
(6) The steel slab is further in mass%, Mo: 0.05 to 1%, V: 0.02 to 0.2%, Cr: 0.01 to 1%, Zr: 0.02 to 0 .2% of one kind or two or more kinds are characterized by excellent burring workability and fatigue properties according to any one of (1) to (5)
Hot rolled steel sheetManufacturing method.
[0015]
DETAILED DESCRIPTION OF THE INVENTION
The basic research results that led to the present invention will be described below.
First, an investigation was made on the effect of Cu content on fatigue properties. The test material for that purpose was prepared as follows. That is, based on 0.05% C-1.0% Si-1.4% Mn-0.01% P-0.001% S-0.03% Al, the component content was adjusted by changing the Cu content. The melted slab was hot-rolled, wound up at room temperature, kept isothermal at 550 ° C. for 1 hour, and then subjected to a furnace-cooling heat treatment.
The results of mechanical tests and fatigue tests on these steel sheets are shown in FIGS. From this result, it was newly found that there is a strong correlation between the Cu content and the fatigue limit ratio, and that the fatigue limit ratio is significantly improved when the Cu content is 0.2 to 1.2%.
[0016]
Although this mechanism is not necessarily clear, solid solution Cu suppresses cross-slip under repeated load, changes the form of the surface slip step due to repeated load from a rough and deep state to a dense and shallow state, It is presumed that the fatigue crack initiation resistance is improved because the stress concentration is relaxed. However, if it is less than 0.2%, this effect is lost.
On the other hand, if it contains more than 1.2% Cu, it is considered that Cu is in a precipitated state under the above heat treatment conditions. Precipitated Cu significantly increases the static strength, but does not increase the fatigue limit so much, so it is assumed that the precipitate of Cu is re-dissolved under repeated loading and does not exhibit the effect of improving fatigue properties. The
[0017]
Next, an investigation was made on the effect of Ti * (Ti * = Ti-48 / 12C-48 / 14N-48 / 32S) on hole expandability. The test material for that purpose was prepared as follows. That is, 0.05% C-1.0% Si-1.4% Mn-0.01% P-0.001% S-0.03% Al-0.001% N-0.8% Cu To the base, the components were adjusted by changing the amount of Ti added, and the steel plate obtained by hot rolling the molten slab and winding it at room temperature was held isothermally at 550 ° C. for 1 hour, and then subjected to a furnace cooling heat treatment, Various Ti * steel plates were obtained.
The hole expansion test results for these steel plates are shown in FIG. From this result, it has been newly found that the hole expansion rate is improved when Ti * ≧ 0% or more, and the hole expansion rate is significantly improved at 0.05%.
[0018]
Furthermore, the steel plate of 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 rate As a result, there is a strong correlation between the average size of the precipitates, the minimum interval between the precipitates, and the hole expansion rate, and the average size of the precipitates containing Ti of 5 nm or more among particles in steel is 10.¹-10^Threenm and the minimum distance between precipitates is 10¹More than 10 nm^FourIt has been newly found that the hole expansion rate is remarkably improved at nm or less.
FIG. 4 shows the relationship between the results of the hole expansion test, the average size of precipitates containing Ti, and the minimum interval between the precipitates. Here, the precipitates containing Ti are particles containing Ti such as carbides, nitrides, sulfides, and may contain oxides and the like.
[0019]
This mechanism is not necessarily clear, but if the precipitate is too large, voids are likely to form at the interface between the precipitate and the matrix, and if it is too small, it becomes the starting point of cracks. Therefore, it is estimated that the hole expansion rate is improved at the optimum size and precipitate spacing.
However, the average size of precipitates containing Ti in the steel is 10²If it exceeds nm, there is a possibility that the precipitate appears on the surface of the fracture surface in the fractured surface as punched or sheared, so that it may become a starting point of fatigue fracture.¹-10²A range of nm is preferred.
[0020]
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. In addition, the fatigue properties of the steel sheet were as follows: a fatigue test piece having a plate thickness of 3.0 mm, a length of 98 mm, a width of 38 mm, a minimum cross-sectional width of 20 mm, and a notch curvature radius of 30 mm as shown in FIG. 2 × 10 obtained by double-bending plane bending fatigue test⁶The value obtained by dividing the fatigue strength σW at the time by the tensile strength σB of the steel sheet (fatigue limit ratio σW / σB) was evaluated.
[0021]
Moreover, as for the precipitate containing Ti in the steel, a transmission electron microscope sample is taken from a thickness of 1/4 of the test steel, and energy dispersive X-ray spectroscopy (EDS), Observation was made with a transmission electron microscope equipped with a field emission gun (FEG) having an acceleration voltage of 200 kV, to which a composition analysis function of electron energy loss spectroscopy (EELS) was added. The observed particle composition was confirmed to be a precipitate containing Ti by EDS and EELS.
[0022]
The size of the precipitate defined in the present invention is defined as the longest piece in the case of a rectangle and the maximum length in the case of an elongated shape. 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 (minimum precipitation interval) is the minimum distance among the measured 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.
[0023]
Next, the microstructure of the steel sheet and the presence state of Cu 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 preferably 10% or less. Here, the volume fraction of ferrite and bainite is defined by the area fraction of the microstructure in the microstructure observed 200 to 500 times with an optical microscope at 1/4 thickness of the cross-sectional thickness in the rolling direction of the steel sheet. The
[0024]
Further, the presence state of Cu in the steel is preferably a solid solution state. Thereby, fatigue characteristics can be improved while suppressing an increase in static strength that leads to deterioration of workability. On the other hand, when Cu is in a precipitated state, the static strength of the steel sheet is remarkably increased due to the precipitation strengthening of Cu, so that the workability is remarkably deteriorated. Further, with such Cu precipitation strengthening, the fatigue limit ratio does not improve as much as the increase in static strength, so the fatigue limit ratio decreases. Therefore, the presence state of Cu needs to be solid solution.
[0025]
Next, the reasons for limiting the chemical components of the present invention will be described.
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 that it is less than 0.01%, it is set as 0.01% or more.
[0026]
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.
[0027]
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.
[0028]
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.
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 condition of preferably ≧ 0.05%.
[0029]
Cu is one of the most important elements of the present invention, and has an effect of improving fatigue characteristics in a solid solution state. However, if the content is less than 0.2%, the effect is small. If the content exceeds 1.2%, precipitation occurs during winding and the static strength of the steel sheet is remarkably increased by precipitation strengthening, so that the workability is remarkably deteriorated. It will be. Further, with such Cu precipitation strengthening, the fatigue limit ratio does not improve as much as the increase in static strength, so the fatigue limit ratio decreases. Therefore, the Cu content is limited to a range of 0.2 to 1.2%.
[0030]
Nb, like Ti, contributes to an increase in the strength of the steel sheet by precipitation strengthening. 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%, preferably ≧ 0.05%.
[0031]
Si is effective for increasing the strength as a solid solution strengthening element. 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 to 2%.
[0032]
Mn is effective for increasing the strength as a solid solution strengthening element. 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 needs to be added in an amount of 0.005% or more for deoxidation of molten steel, but the cost is increased, so the upper limit is made 1.0%. On the other hand, if added too much, nonmetallic inclusions are increased and elongation is deteriorated, so the content is preferably made 0.5% or less.
[0035]
B suppresses grain boundary embrittlement due to P and has the effect of increasing the fatigue limit by being added in combination with Cu, so 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 to 0.002%.
[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 workability. However, even if less than 0.005% is added, the effect is not obtained. If Ca is more than 0.02% and if REM is added more than 0.2%, the effect is saturated. It is preferable to add 0.02% and REM: 0.005 to 0.2%.
[0038]
Furthermore, 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 as necessary. 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]
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 scale-off amount becomes large and the yield decreases, so the reheating temperature is preferably less than 1400 ° C. Further, when heating at less than 1100 ° C., the precipitate containing Ti or Nb or both does not re-dissolve in the slab and coarsens to lose the precipitation strengthening ability, but also has a favorable size and distribution for burring workability. , Nb, or a precipitate containing both of them does not precipitate, so the reheating temperature is preferably 1100 ° C. or higher.
[0040]
  Implement in the present inventionIn the hot rolling process, after finishing the rough rolling,After applying high-pressure descalingFinish rolling, but the final pass temperature (FT) is Ar_ThreeIt is necessary to finish in the temperature range above the transformation point. This is because the rolling temperature is Ar during hot rolling._ThreeThis is because when the transformation point is cut, 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 is preferably 1000 ° C. or less because scale soot may be generated in operation. Here, after the end of rough rollingHigh pressure descaling, High-pressure water collision pressure P (MPa) × flow rate L (liter / cm²) ≧ 0.0025Do as.
[0041]
The collision pressure P of high-pressure water on the steel plate 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
[0042]
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
[0043]
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 preferable.
Furthermore, the maximum height Ry of the steel sheet after finish rolling is preferably 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 has a correlation with the maximum height Ry of the steel sheet surface. It is clear from Further, the subsequent finish rolling is desirably performed within 5 seconds in order to prevent the scale from being generated again after descaling.
[0044]
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 either Ti or Nb or both may become coarse and may not contribute to the increase in strength due to precipitation strengthening, so the lower limit of the cooling rate is 20 ° C./s or more. Is desirable. In addition, the upper limit of the cooling rate is 100 ° C. or less in consideration of actual factory equipment capacity and the like.
[0045]
Next, when the coiling temperature is less than 350 ° C., there is no possibility that precipitates containing sufficient Ti or Nb or both are generated, and solid solution C remains in the steel and the workability may be lowered. If it exceeds ℃, the size of the precipitate containing Ti and / or Nb becomes coarse and does not contribute to the increase in strength due to precipitation strengthening, and if the precipitate is too large, voids are likely to occur at the interface between the precipitate and the matrix, There is a risk that hole expansibility will decrease. Accordingly, the winding temperature is set to 350 ° C. to 750 ° C.
[0046]
【Example】
  The following examples further illustrate the present invention.
  It has the chemical components shown in Table 1.B, G, I ~ KThe steel is melted in a converter, continuously cast, reheated at the heating temperature (SRT) shown in Table 2, and after rough rolling, the final rolling temperature (FT) shown in Table 2 is 1.2 to After rolling to a thickness of 5.4 mm, each was wound at the winding temperature (CT) shown in Table 2. part(Example of the present invention)For rough rolling, impact pressure is 2.7 MPa after rough rolling, flow rate is 0.001 liter / cm²High pressure descaling was performed under the following conditions.In additionThe indication for the chemical composition in the table is% by mass.
[0047]
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. Table 2 shows the volume ratio of the structure obtained by observing a thickness of ¼ of the cross-sectional thickness in the rolling direction of the steel sheet with an optical microscope at 200 to 500 times.
Furthermore, a complete double-bending plane bending fatigue test was performed on a plane bending fatigue test piece having a length of 98 mm, a width of 38 mm, a minimum cross-sectional width of 20 mm, and a notch curvature radius of 30 mm as shown in FIG. It was. The fatigue properties of the steel sheet are 2 × 10⁶The fatigue strength σW at the time was divided by the tensile strength σB of the steel sheet (fatigue limit ratio σW / σB). 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.
[0048]
Further, as for the precipitate containing Ti and / or Nb in the steel, a transmission electron microscope sample was taken from a thickness of 1/4 of the test steel, and energy dispersive X-ray spectroscopy (Energy Dispersive X-ray Spectroscope: Observation with a transmission electron microscope equipped with a field emission gun (FEG) with an accelerating voltage of 200 kV with the addition of the composition analysis function of EDS and electron energy loss spectroscopy (EELS) did.
[0049]
The observed particle composition was confirmed to be a precipitate containing Ti by EDS and EELS. Moreover, the size of the precipitate containing either or both of Ti and Nb is defined as the longest piece if it is rectangular, and the maximum length if it is stretched. In addition, the average precipitate size is a simple average of the sizes in one field of view of those having a size of 5000 to 500,000 times each of the precipitate sizes observed to be uniformly dispersed at a magnification of 5,000 to 500,000 times. It is. 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.
[0050]
  In accordance with the present invention is steel2 of J and KThe average size of precipitates containing steel, containing a predetermined amount of Cu and Ti, and containing either or both of Ti and Nb of 5 nm or more with particles in the steel is 10¹-10^Threenm and the minimum precipitation interval is 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.
[0051]
  Steels other than the above are outside the scope of the present invention for the following reasons..
[0052]
In Steel B, the Ti content is outside the range of the present invention, so that solid solution C remains in the steel and sufficient ductility and hole expansion rate (λ) are not obtained. Since the steel G has a Cu content larger than the range of the present invention, the steel G is precipitated during winding and the static strength of the steel sheet is remarkably increased by precipitation strengthening, so that the workability is remarkably deteriorated. Further, with such Cu precipitation strengthening, the fatigue limit ratio does not improve as much as the increase in static strength, so the fatigue limit ratio decreases. Accordingly, the effect of improving fatigue characteristics is small, and a sufficient fatigue limit ratio is not obtained. Steel I has less Cu content than the range of the present invention, so that the effect of improving fatigue characteristics is small, and a sufficient fatigue limit ratio is not obtained.
[0053]
[Table 1]

[0054]
[Table 2]

[0055]
【The invention's effect】
  As described above in detail, the present invention is a hot-rolled steel sheet having a tensile strength of 640 MPa or more and excellent in burring workability and fatigue characteristics.TheThis is a manufacturing method that can be manufactured stably. By using these hot-rolled steel sheets, it is possible to expect significant improvements in fatigue properties while ensuring sufficient burring workability (stretch flangeability). The invention is an invention with 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 Cu content, static strength, and fatigue limit.
FIG. 2 is a diagram showing the results of a preliminary experiment leading to the present invention in relation to the Cu content and the fatigue limit ratio.
FIG. 3 is a diagram showing a result of a preliminary experiment leading to the present invention in relation to Ti * and a hole expansion rate.
FIG. 4 is a diagram showing the results of a preliminary experiment leading to the present invention, in which the range of hole expansion rate is related to the range of the average size of precipitates containing Ti and the minimum precipitation interval of precipitates containing Ti.
FIG. 5 is a diagram illustrating the shape of a fatigue test piece.

Claims (6)

In mass%
C: 0.01 to 0.1%,
Si: 0.01-2%
Mn: 0.05-2%
P ≦ 0.1%,
S ≦ 0.03%,
Al: 0.005 to 1.0%,
N ≦ 0.005%,
Ti: 0.05 to 0.5%,
Cu: 0.2 to 1.2%,
Ti-48 / 12C-48 / 14N-48 / 32S ≧ 0%
In the hot rolling of the steel slab containing Ti and the balance of Fe and inevitable impurities in the range satisfying the above, after the rough rolling, the collision pressure P (MPa) of high-pressure water on the steel plate surface × flow rate L (liter / liter) cm ² ) ≧ 0.0025, high pressure descaling is performed, and after finishing the hot finish rolling above the Ar ₃ transformation point, the steel is cooled to a temperature range of 350 ° C. to 750 ° C. with an average size of 10 ¹ to 10 ³ nm particle precipitates containing 5nm or more Ti in in the burring workability characterized in that the minimum deposit spacing obtain steel sheet is 10 ¹ nm ultra 10 ⁴ nm or less A method for producing a hot-rolled steel sheet having excellent fatigue characteristics .
Here, P (MPa) = 5.64 × P _O × V / H ²
However,
P _O (MPa): Liquid pressure
V (liter / min): Nozzle flow rate
H (cm): distance between the steel plate surface and the nozzle
L (liter / cm ² ) = 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 In mass%
C: 0.01 to 0.1%,
Si: 0.01-2%
Mn: 0.05-2%
P ≦ 0.1%,
S ≦ 0.03%,
Al: 0.005 to 1.0%,
N ≦ 0.005%,
Ti: 0.05 to 0.5%,
Nb: 0.01-0.5%
Cu: 0.2 to 1.2%,
Ti + 48 / 93Nb-48 / 12C-48 / 14N-48 / 32S ≧ 0%
When hot-rolling a steel slab containing Ti and Nb with the balance being Fe and inevitable impurities, and after the rough rolling, the collision pressure P (MPa) of high-pressure water on the surface of the steel plate × flow rate L ( Liter / cm ² ) High pressure descaling is performed so as to satisfy the condition of ≧ 0.0025 , the hot finish rolling is finished at the Ar ₃ transformation point or higher, and then cooled to a temperature range of 350 ° C. to 750 ° C. And a steel plate having an average size of 10 ¹ to 10 ³ nm and a minimum precipitation interval of more than 10 ¹ nm and not more than 10 ⁴ nm with precipitates containing either or both of Ti and Nb of 5 nm or more. A method for producing a hot-rolled steel sheet having excellent burring workability and fatigue characteristics .
Here, P (MPa) = 5.64 × P _O × V / H ²
However,
P _O (MPa): Liquid pressure
V (liter / min): Nozzle flow rate
H (cm): distance between the steel plate surface and the nozzle
L (liter / cm ² ) = 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 steel slab is further in mass%,
B: 0.0002 to 0.002%
The method for producing a hot-rolled steel sheet having excellent burring workability and fatigue characteristics according to claim 1 or 2, characterized by comprising : The steel slab is further in mass%,
Ni: 0.1 to 1%
The method for producing a hot-rolled steel sheet excellent in burring workability and fatigue characteristics according to any one of claims 1 to 3, wherein The steel slab is further in mass%,
Ca: 0.005 to 0.02%,
REM: 0.005 to 0.2%
One type or two types of these are contained , The manufacturing method of the hot-rolled steel plate excellent in burring workability and fatigue characteristics of any one of Claims 1-4 characterized by the above-mentioned. The steel slab is further in mass%,
Mo: 0.05 to 1%
V: 0.02-0.2%,
Cr: 0.01-1%,
Zr: 0.02 to 0.2%
One or two or more of these are contained , The manufacturing method of the hot-rolled steel plate excellent in burring workability and fatigue characteristics of any one of Claims 1-5 characterized by the above-mentioned.

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