JP2002161340A - Hot rolled steel sheet superior in burring workability and fatigue characteristics, and manufacturing method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a hot rolled steel sheet superior in burring workability and fatigue characteristics, which has a tensile strength of 640 MPa or more, and a manufacturing method therefor. SOLUTION: The hot rolled steel sheet superior in burring workability and fatigue characteristics includes 0.01-0.1% C, 0.5-0.5% Ti by mass%, satisfies a relationship of Ti-48/12C-48/14N-48/32S>=0%, and is characterized by that an average size of precipitates including Ti with particle size of 5 nm or more in the steel is 101-103 nm, and that a minimum space between the precipitates is more than 101 nm and not more than 104 nm. The method for manufacturing the steel sheet is characterized by final hot rolling the steel with the above composition at a finishing temperature range of Ar3 transformation temperature or more, cooling it down to a temperature range of 350-750 deg.C, and winding it up.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hot-rolled steel sheet excellent in burring workability and fatigue properties and having a tensile strength of 640 MPa or more, and a method for producing the same. The present invention relates to a hot-rolled steel sheet which is excellent in burring workability (hole expanding property) and fatigue characteristics, and a method for producing the same.

[0002]

2. Description of the Related Art In recent years, the application of light metals such as Al alloys and high-strength steel sheets to automobile members has been promoted for the purpose of weight reduction in order to improve 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 reduce the weight of automobiles in a wider range, there is a strong demand for the use of inexpensive high-strength steel sheets.

In general, the higher the strength of a material, the lower the ductility and the worse the workability (moldability). Steel materials are no exception, and attempts have been made to achieve both high strength and high ductility. On the other hand, materials used for suspension parts such as suspension arms of automobiles are required to have burring workability (hole expanding property) and fatigue durability in addition to these properties. However, as the strength increases, not only the hole expandability tends to decrease, but also because the strength enhancement causes an increase in notch sensitivity, the fatigue durability at the stress concentration site is improved as expected. do not do. Therefore, when applying a high-strength steel plate to a vehicle-under-body part having a complicated shape, not only the hole expandability but also the fatigue durability are important considerations.

As a high-strength hot-rolled steel sheet having excellent hole expanding properties (stretch flangeability or burring workability), for example, Japanese Patent Application Laid-Open No. 6-200351 discloses a high-strength hot-rolled steel sheet having excellent stretch flangeability. By adding Nb, the second phase is reduced, and T in the main phase polygonal ferrite.
An invention obtained by strengthening the precipitation of iC and NbC is disclosed. Japanese Patent Application Laid-Open No. 7-11382 discloses that the second phase is reduced by adding Ti and Nb,
TiC, Nb with microstructure as acicular ferrite
The invention of obtaining a high-strength hot-rolled steel sheet having excellent stretch flangeability by precipitation strengthening with C is disclosed.

Japanese Patent Application Laid-Open No. 7-70696 discloses T
i and Nb are added in a C equivalent or more to make the microstructure into a ferrite single phase, Cu is added, and ε is added together with TiC and NbC.
There is disclosed an invention for obtaining a high-strength hot-rolled steel sheet having high stretch flangeability and enhanced strength by precipitating -Cu. Further, JP-A-8-157957 discloses that
Addition of Ti and Nb in an amount of C equivalent or more to make the microstructure a ferrite single phase, and also define the value of Ni / Cu to change the ferrite from polygonal to bainitic, thereby improving the stretch flangeability. To obtain a high-strength hot-rolled steel sheet excellent in the above.

On the other hand, as a high-strength and high-strength hot-rolled steel sheet having excellent fatigue properties, for example, Japanese Patent Application Laid-Open No. 3-82708 discloses a high-strength hot-rolled steel sheet for strong working having excellent fatigue properties.
To reduce the secondary phase structure such as cementite as much as possible.
The invention discloses that high strength and excellent fatigue characteristics are obtained by the combined addition of Cu and Cu. Also, Japanese Patent Application Laid-Open No. Hei 6-2876
No. 85 discloses that, in addition to obtaining a strength of 690 MPa or more by precipitation strengthening of TiC and the like, Cu is added in addition to reducing the second phase by adding Ti and reducing solid solution C in ferrite. There is disclosed an invention for obtaining a high-strength hot-rolled steel sheet which is improved in fatigue properties and excellent in stretch flangeability and fatigue properties.

[0007]

However, in some steel plates for parts such as suspension arms, not only the workability such as burring workability but also the fatigue durability at the end face as it is subjected to shearing and punching is very important. Satisfactory characteristics cannot be obtained with the above prior art. Even if both characteristics are satisfied, it is important to provide a manufacturing method that can be manufactured stably at a low cost, and the above-mentioned conventional technology has to be said to be insufficient.

That is, in the invention described in Japanese Patent Application Laid-Open No. Hei 6-200351, polygonal ferrite having an area ratio of 85% or more is essential to obtain high stretch flangeability, but polygonal ferrite having an area ratio of 85% or more is required. In order to obtain, it is necessary to maintain the ferrite grains for a long time after hot rolling to promote the growth of ferrite grains, which is not preferable in terms of operating costs. In the invention described in JP-A-7-11382, a microstructure having a high dislocation density and fine TiC and / or
Alternatively, due to precipitation of NbC, the ductility is only about 17% at 780 MPa, and the formability is insufficient.

In the invention described in Japanese Patent Application Laid-Open No. 7-70696, since ε-Cu is precipitated in the ferrite phase, ductility may be reduced and workability may be deteriorated. In the invention described in JP-A-8-157957, 20% at 780 MPa is obtained due to a microstructure having a high dislocation density and precipitation of fine TiC and / or NbC.
It has only a degree of ductility and the moldability is insufficient.

[0010] Further, these inventions do not mention any fatigue properties. On the other hand, in the invention described in Japanese Unexamined Patent Publication No. 3-82708 as an invention which also mentions fatigue properties, 0.04 to 0.10% of P which segregates at crystal grain boundaries and causes grain boundary embrittlement is added. Therefore, when grain boundary fracture, which is a starting point of fatigue fracture, occurs, fatigue properties may be significantly deteriorated. Further, this publication does not disclose the addition of B that suppresses grain boundary embrittlement due to P. Further, Japanese Patent Application Laid-Open No. 6-287
In the invention described in Japanese Patent No. 685, the precipitation strengthening of Cu is mainly used to improve the fatigue properties. However, since the precipitation strengthening of Cu does not improve the fatigue strength as much as the static strength, the fatigue limit ratio is lowered. There is a problem that.

Accordingly, the present invention is to provide a hot-rolled steel sheet having a tensile strength of 640 MPa or more, excellent in burring workability and fatigue properties, which can advantageously solve the above-mentioned problems of the prior art, and a method for manufacturing the steel sheet stably at low cost. The purpose is to provide.

[0012]

Means for Solving the Problems The present inventors considered the production process of a hot-rolled steel sheet produced on an industrial scale by a continuous hot-rolling equipment which is currently usually used, and considered the production process of the hot-rolled steel sheet. Intensive research was conducted to achieve both burring workability and fatigue characteristics. As a result, 5
The average size of the precipitate containing Ti of not less than nm is 10 ¹ to 1
It is very effective to improve the burring workability when the minimum interval at 0 ³ nm is more than 10 ¹ nm and 10 ⁴ nm or less.
It was also found that ductility was not impaired. At the same time, the present inventors newly found that the fatigue properties in the case where the fatigue crack propagates from the sheared and punched end faces within the range of the average size of the precipitates and the minimum spacing are improved, and have made the present invention.

That is, the gist of the present invention is as follows. (1) In mass%, C: 0.01 to 0.1%, S ≦ 0.03%, N ≦ 0.005%, Ti: 0.05 to 0.5%, and further Ti-48 / 12C-48 / 14N-48 / 32S ≧ 0% is a steel containing Ti, the balance being Fe and unavoidable impurities, and particles of the precipitate containing 5 nm or more of Ti in the steel. A hot-rolled steel sheet having excellent burring workability and fatigue properties, having an average size of 10 ¹ to 10 ³ nm and a minimum interval of more than 10 ¹ nm to 10 ⁴ nm or less. (2) In mass%, C: 0.01 to 0.1%, S ≦ 0.03%, N ≦ 0.005%, Ti: 0.05 to 0.5%, Nb: 0.01 to 0.5%, Ti + 48 / 93Nb-48 / 12C-48 / 14N-48 / 32S ≧ 0%, containing Ti and Nb, with the balance being Fe and inevitable impurities. 5nm for particles in steel
The average size of the precipitate containing one or both of Ti and Nb is 10 ¹ to 10 ³ nm and the minimum interval is 10
A hot-rolled steel sheet excellent in burring workability and fatigue properties, having a thickness of more than ¹ nm and not more than 10 ⁴ nm.

(3) The steel further contains, by mass%: Si: 0.01 to 2%, Mn: 0.05 to 2%, P ≦ 0.1%, Al: 0.005 to 1.0 %, Wherein the hot-rolled steel sheet is excellent in burring workability and fatigue properties according to the above (1) or (2). (4) The steel according to (1) to (3), wherein the steel further contains B: 0.0002 to 0.002% by mass%.
A hot-rolled steel sheet excellent in burring workability and fatigue properties according to any one of the above. (5) The steel according to (1) to (4), wherein the steel further contains Ni: 0.1 to 1% by mass%.
A hot-rolled steel sheet excellent in burring workability and fatigue properties according to any one of the above.

(6) The steel further contains one or two types of Ca: 0.0005 to 0.02% and REM: 0.0005 to 0.2% by mass%. A hot-rolled steel sheet excellent in burring workability and fatigue properties according to any one of the above (1) to (5). (7) The steel further contains, by mass%, Mo: 0.05 to 1%, V: 0.02 to 0.2%, Cr: 0.01 to 1%, Zr: 0.02 to 0. The hot-rolled steel sheet excellent in burring workability and fatigue characteristics according to any one of the above (1) to (6), characterized by containing one or more kinds of 2%.

(8) When hot rolling a steel slab having the components described in any one of the above (1) to (7),
3 After finishing hot finish rolling at the transformation point or higher, it is cooled to a temperature range of 350 ° C. to 750 ° C. and rolled up, and precipitates containing 5 nm or more of Ti and / or Nb in steel particles are contained. A method for producing a hot-rolled steel sheet having excellent burring workability and fatigue properties, characterized by obtaining a steel sheet having an average size of 10 ¹ to 10 ³ nm and a minimum interval of more than 10 ¹ nm to 10 ⁴ nm or less. (9) In the hot rolling, after rough rolling is completed, high-pressure descaling is performed, and hot finish rolling is completed at an Ar3 transformation point or higher, and the burring workability and fatigue characteristics according to the above (8), Method of manufacturing hot rolled steel sheet with excellent quality.

[0017]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The results of basic research that led to the present invention will be described below. First, Ti * (Ti * = Ti−
48 / 12C-48 / 14N-48 / 32S) on the hole-expandability. The test material for that was prepared as follows. That is, 0.05
% C-1.0% Si-1.4% Mn-0.01% P-
A steel sheet obtained by adjusting the composition by changing the amount of Ti added based on 0.001% S-0.03% Al-0.001% N, hot-rolling a smelted slab and winding it at room temperature, After maintaining the temperature isothermally at 550 ° C. for 1 hour, a heat treatment for furnace cooling was performed to obtain various Ti * steel sheets. FIG. 1 shows the results of the hole expansion test for these steel sheets. From these results, it was newly found that the hole expansion value (hole expansion ratio) was significantly improved when Ti * ≧ 0%. However, in order to stably obtain an excellent hole expansion value in both the longitudinal direction and the width direction of the plate, Ti * ≧ 0.
05% is desirable.

Further, the steel sheet having the above components is rolled and heat-treated under various manufacturing conditions to change the size of the precipitate containing Ti and the precipitation interval, the average size of the precipitate, the minimum spacing of the precipitate, and the hole expansion value. As a result, the average size of the precipitates, the minimum spacing of the precipitates, and the hole expansion value showed a strong correlation, and the average size of the precipitates containing 5 nm or more of Ti in the steel particles was 10 nm or more. ^{1-10 3} minimum spacing nm range a and precipitates of the hole expansion value below 10 ¹ nm ultra 10 ⁴ nm is finding new that significantly improved. 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. Where Ti
The precipitates containing Ti are particles containing Ti, such as carbides, nitrides, and sulfides, and may contain oxides and the like.

Although this mechanism is not always clear, if the precipitate is too large, voids are likely to be formed at the interface between the precipitate and the matrix, and it becomes a starting point of cracking when the hole is expanded, and if too small, the correlation with the hole expansion value is obtained. It is presumed that the hole expansion value is improved at an optimum size and precipitate interval because a certain local ductility is reduced. However, the average size of 10 ² nm than the precipitates containing Ti at grain in the steel, the risk of a starting point of corrosion when precipitates in the fracture surface of the steel sheet surface or punching and shearing while appeared on the surface Therefore, the average size of the precipitates containing Ti in the particles in the steel is 10 ¹ to 1
The range of 0 ² nm is desirable.

Next, when the fatigue characteristics in the case where the sheared or punched end face was present were examined, the average size of the precipitates including Ti and the minimum spacing of the precipitates and the case where the sheared or punched end face was present were examined. There is a strong correlation between the fatigue properties and the average size of precipitates containing 5 nm or more of Ti in the steel particles is in the range of 10 ¹ to 10 ³ nm, and the minimum interval between the precipitates is more than 10 ¹ nm and 10 ⁴ nm. In the following, it has been newly found that the fatigue properties in the case where shearing or punched end faces are present are significantly improved. FIG. 3 shows the relationship between the fatigue characteristics in the presence of shearing or punched end faces, the average value of the size of the precipitate containing Ti, and the minimum distance between the precipitates.

Although this mechanism is not necessarily clear, if the precipitate is too large, voids are likely to be formed at the interface between the precipitate and the matrix, and the voids serve as starting points for fatigue cracks. Since the percentage of the fractured surface in the punched cross section, which tends to be the crack initiation point, increases, and the fatigue limit decreases, it is estimated that the fatigue characteristics in the case where the shearing or punched end face exists at the optimal size and precipitate spacing are improved. .

The mechanical properties obtained by the tensile test were determined using a No. 5 test piece described in JIS Z 2201 according to JI.
It was measured by the test method described in SZ2241. FIG. 4 shows the fatigue characteristics in the presence of shearing or punched end faces.
As shown in the figure, a punched hole with a clearance of about 11% was provided at the center of a fatigue test piece having a length of 98 mm, a width of 38 mm, a radius of curvature of a notch of 30 mm, and a minimum cross section of 30 mm. Fatigue limit σW at 1.0 × 10 ⁷ times obtained by full swing plane bending fatigue test
K divided by the tensile strength σB of the steel sheet (fatigue limit ratio σW
K / σB). However, the surface of the fatigue test piece was a pickled surface without any grinding or the like.

As for the precipitates containing Ti in the steel, a transmission electron microscope sample was taken from a 1/4 W or 3/4 W position and 1/4 thickness of the test steel, and an energy dispersive X Energy Dispersive X-ray Spectrosco
pe: EDS) and electron energy loss spectroscopy (Electron E
Observation was made with a transmission electron microscope equipped with a 200 kV accelerating voltage field emission electron gun (Field Emission Gun: FEG) equipped with a composition analysis function of an energy loss spectroscope (EELS). It was confirmed by EDS and EELS that the observed particle composition was a precipitate containing Ti.

In the present invention, the size of the precipitate specified is
It is defined as the longest piece in the case of a rectangle and the maximum length in the case of a stretched shape. In addition, the average precipitate size defined in the present invention is a simple average of the size of a precipitate in a field of view of 5 nm or more among the precipitates measured at a magnification of 5,000 to 500,000 times. Furthermore, the minimum spacing of the precipitates defined in the present invention 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 observed cross section of the precipitate.

Next, the microstructure of the steel sheet according to the present invention will be described. The microstructure of the steel sheet is preferably a ferrite single phase in order to ensure excellent burring workability (stretch flangeability). However, it is allowed to partially contain bainite as necessary. 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 1/4 W or 3 times the width of the steel sheet.
The sample cut from the / 4W position is polished to a cross section in the rolling direction,
Etching using a nital reagent, 1/4 of the plate thickness observed at a magnification of 200 to 500 times using an optical microscope.
It is the area fraction of the microstructure at t.

Next, the reasons for limiting the chemical components of the present invention will be described. The amounts of the chemical components are% by mass. C is 0.
If the content exceeds 1%, workability and weldability deteriorate, so the content is set to 0.1% or less. If it is less than 0.01%, the strength is reduced.

If the content of S is too large, it causes cracking during hot rolling, so it should be reduced as much as possible, but if it is 0.03% or less, it is in an acceptable range.

N forms a precipitate with Ti and Nb at a higher temperature than C, and Ti and N are effective for fixing C.
b is reduced. Therefore, it should be reduced as much as possible,
If it is 0.005% or less, it is within an acceptable range.

[0029] Ti is one of the most important elements in the present invention. That is, Ti contributes to an increase in the strength of the steel sheet by precipitation strengthening. However, if the content is less than 0.05%, this effect is insufficient, and if the content exceeds 0.5%, the effect is not only saturated but also causes an increase in alloy cost. Therefore Ti
Is 0.05% or more and 0.5% or less. Further, in order to precipitate and fix C which causes carbide such as cementite which deteriorates burring workability and to contribute to improvement of burring workability, Ti-48 / 12C-48 /
It is necessary to satisfy the condition of 14N-48 / 32S ≧ 0%.

Since Nb contributes to an increase in the strength of the steel sheet by precipitation strengthening like Ti, it is added as necessary. However, if less than 0.01%, this effect is insufficient,
If the content exceeds 0.5%, not only the effect is saturated, but also the alloy cost is increased. Therefore, the content of Nb is 0.01
% Or more and 0.5% or less. Further, C, which causes carbides such as cementite, which deteriorates burring workability,
In order to contribute to the improvement of burring workability by precipitating and fixing Ti + 48 / 93Nb-48 / 12C-48 / 1.
It is necessary to satisfy the condition of 4N-48 / 32S ≧ 0%.

Since Si is effective as a solid solution strengthening element for increasing strength, it is added as necessary. In order to obtain a desired strength, it is necessary to contain 0.01% or more. However, if the content exceeds 2%, the workability deteriorates. Then, Si
Is 0.01% or more and 2% or less.

Since Mn is effective as a solid solution strengthening element for increasing the strength, Mn is added as necessary. To obtain the desired strength, 0.05% or more is required. Further, if added over 2%, slab cracks occur, so the content is set to 2% or less.

If P exceeds 0.1%, workability and weldability are adversely affected, so that P is set to 0.1% or less.

Al is added as necessary for deoxidizing molten steel. It is necessary to add 0.005% or more, but the cost is increased, so the upper limit is made 1.0%. Further, if added in an excessively large amount, nonmetallic inclusions are increased and elongation is deteriorated. Therefore, the content is preferably 0.5% or less.

B has the effect of increasing the fatigue limit by suppressing grain boundary embrittlement due to P, so B is added as necessary. However, if it is less than 0.0002%, it is insufficient to obtain the effect, and if it exceeds 0.002%, slab cracking occurs. Therefore, the addition of B is set to 0.0002% or more and 0.002% or less.

Ni is added as necessary to prevent hot brittleness due to the inclusion of Cu. However, if the content is less than 0.1%, the effect is small, and if the content exceeds 1%, the effect is saturated. Therefore, the content is set to 0.1 to 1%.

Ca and REM are elements that become the starting point of destruction or change the form of nonmetallic inclusions that degrade workability and render them harmless. However, even if added less than 0.0005%, the effect is not obtained.
If EM is added in excess of 0.2%, the effect is saturated, so Ca: 0.0005 to 0.02%, REM: 0.0
It is desirable to add 005 to 0.2%.

Further, in order to impart strength, Mo,
One, two or more of precipitation strengthening or solid solution strengthening elements of V, Cr and Zr may be added. However, the effect cannot be obtained if it is less than 0.05%, 0.02%, 0.01%, and 0.02%, respectively. The effect is saturated even if it exceeds 1.0%, 0.2%, 1.0% and 0.2%, respectively.

The effect of the present invention can be obtained even when Sn is added, and the content thereof need not be particularly determined. However, since flaws may be generated during hot rolling, the content is 0.05% or less. desirable.

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 molten steel whose components have been adjusted so as to have a target component content may be directly sent to a hot rolling mill as a high-temperature slab, or a heating furnace after cooling to room temperature. And then hot-rolled. The reheating temperature is not particularly limited, but if it is 1400 ° C. or more, the scale-off amount becomes large and the yield decreases, so the reheating temperature is 1400 ° C.
Less than is desirable. Heating at less than 1100 ° C.
And / or Nb-containing precipitates do not re-dissolve in the slab and become coarse, not only losing precipitation strengthening ability, but also having a desired size and distribution of Ti and / or for burring workability.
Alternatively, since the precipitate containing Nb does not precipitate, the reheating temperature is desirably 1100 ° C. or higher.

In the hot rolling step, finish rolling is performed after rough rolling is completed, but it is necessary to finish the final pass temperature (FT) in a temperature range not lower 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 does not need to be particularly determined in order to obtain the effects of the present invention, but is desirably set to 1000 ° C. or lower because scale flaws may occur during operation. Here, when high-pressure descaling is performed after the completion of rough rolling, the collision pressure P (MPa) of high-pressure water on the steel sheet surface × the flow rate L (liter / cm ² ) ≧ 0.
It is desirable to satisfy the condition of 0025.

The collision pressure P of the high-pressure water on the steel sheet surface is described as follows ("Iron and Steel", 1991, vol.
7, No. 9, p. 1450). P (MPa) = 5.64 × P O × V / H 2 However, P _O (MPa): liquid pressure V (liters / min): nozzle flow liquid quantity H (cm): the steel sheet surface and the distance between the nozzle

The flow rate L is described as follows. L (liter / cm ² ) = V / (W × v), where V (liter / min): Nozzle flow amount W (cm): Width of jet liquid per nozzle hitting steel sheet surface v (cm / min): Passing speed The upper limit of the collision pressure P × the flow rate L does not need to be particularly determined in order to obtain the effect of the present invention. However, increasing the flow rate of the nozzle causes inconvenience such as intensified wear of the nozzle.
It is desirable to set it to 0.02 or less.

Further, the maximum height R of the steel sheet after the finish rolling is performed.
y is 15 μm (15 μm Ry, 12.5 mm, ln1
2.5 mm) or less. This is because, as described in, for example, “Handbook for Fatigue Design of Metallic Materials”, edited by The Society of Materials Science, Japan, 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 that. Further, the subsequent finish rolling is desirably performed within 5 seconds in order to prevent scale from being formed again after descaling.

After finishing rolling, the roll is cooled to a specified winding temperature (CT), but the cooling rate does not need to be particularly determined 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 may become coarse and may not contribute to the increase in strength due to precipitation strengthening.
s or more is desirable. The upper limit of the cooling rate is 100 ° C. or less in consideration of the actual factory equipment capacity and the like.

Next, when the winding temperature is lower than 350 ° C., a precipitate containing sufficient Ti and / or Nb is not generated,
Solid solution C may remain in the steel to reduce workability. If the temperature exceeds 750 ° C., the size of the precipitate containing Ti and / or Nb becomes coarse, and not only does not contribute to the increase in strength due to precipitation strengthening. On the other hand, if the precipitate is too large, voids tend to be formed at the interface between the precipitate and the parent phase, and the hole expandability may be reduced. Therefore, the winding temperature is set to 350 to 750 ° C.

[0047]

The present invention will be further described below with reference to examples. The steels A to N having the chemical components shown in Table 1 were melted in a converter and continuously cast, and then heated at a temperature shown in Table 2 (SR
T), and after rough rolling, after rolling at a finish rolling temperature (FT) shown in Table 2 to a sheet thickness of 1.2 to 5.4 mm, winding at a winding temperature (CT) shown in Table 2 respectively. I took it. For some parts, after rough rolling, the collision pressure was 2.7MP.
a, High-pressure descaling was performed under the conditions of a flow rate of 0.001 liter / cm ² . However, the indication of the chemical composition in the table is% by mass.

The tensile test of the hot-rolled sheet obtained as described above was performed by first processing the test material into a No. 5 test piece described in JIS Z 2201 and following the test method described in JIS Z 2241. Table 2 shows the test results. Further, a sample cut from a 1 / 4W or 3 / 4W position of the steel plate width is polished to a cross section in the rolling direction, etched using a nital reagent, and observed with an optical microscope at a magnification of 200 to 500 times. Table 2 also shows the area fraction of the microstructure at 1 / 4t of the above.

Further, the fatigue characteristics in the presence of a sheared or punched end face are as shown in FIG. 4 in a fatigue test in which the length is 98 mm, the width is 38 mm, the radius of curvature of the notch is 30 mm, and the width of the minimum cross section is 30 mm. A specimen having a punched hole with a clearance of about 11% at the center of the piece was used.
The fatigue limit ShigumaWK at 10 ⁷ times was evaluated by the value obtained by dividing the tensile strength .sigma.B of the steel sheet (fatigue ratio σWK / σB). However, the surface of the fatigue test piece was a pickled surface without any grinding or the like. On the other hand, regarding the burring workability (stretch flangeability), Japan Iron and Steel Federation Standard JFS T 100
It was evaluated according to the hole expansion test method described in 1-196.

For the precipitates containing Ti in the steel, a transmission electron microscope sample was taken from a 1/4 W or 3/4 W position and a 1/4 thickness of the test steel, and the energy dispersive X Energy Dispersive X-ray Spectrosco
pe: EDS) and electron energy loss spectroscopy (Electron E
Observation was made with a transmission electron microscope equipped with a 200 kV accelerating voltage field emission electron gun (Field Emission Gun: FEG) equipped with a composition analysis function of an energy loss spectroscope (EELS).

The observed composition of the particles was confirmed to be a precipitate containing Ti by EDS and EELS. The size of the precipitate containing Ti and / or Nb is defined as the longest piece in the case of a rectangle and the maximum length in the case of a stretched state. In addition, the average precipitate size is a simple average of the size in one visual field of those having a size of 5 nm or more among those obtained by measuring the size of the precipitate at a magnification of 5,000 to 500,000. Further, the minimum distance between the precipitates is the minimum distance among the measured distances between 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 observed cross section of the precipitate.

According to the present invention, steels A-1, B, F,
Nine steels of I, J, K, L, M, N, and a predetermined amount of Ti
And the average size of a precipitate containing 5 nm or more of Ti or Ti and Nb in the steel particles is 10 ¹ to 10 ³ n.
m, the minimum interval is more than 10 ¹ nm and 10 ⁴ nm or less, and a hot-rolled steel sheet excellent in burring workability and fatigue properties is obtained.

Other steels are outside the scope of the present invention for the following reasons. That is, since the finish rolling temperature (FT) of the steel A-2 is out of the range of the present invention, not only the strain remains and the ductility (El) decreases, but also a sufficient hole expansion value (λ) is obtained. Not been. Since the winding temperature (CT) of the steel A-3 after hot rolling is lower than the range of the present invention, sufficient precipitation of Ti, or Ti and Nb 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 higher winding temperature (CT) after hot rolling than the scope of the present invention, so that Ti,
Or the size of the precipitate containing Ti and Nb is coarsened,
It does not contribute to the increase in strength due to precipitation strengthening, so that the desired strength (TS) cannot be obtained and the hole expansion value (λ) is low.

In steel C, since the contents of C and Ti are out of the range of the present invention, solid solution C remains in the steel and sufficient ductility (El) and hole expansion value (λ) are obtained. Absent. Steel D
Since the content of S is out of the range of the present invention, sufficient ductility and hole expansion value (λ) are not obtained. Steel E is N
Is out of the range of the present invention, so that sufficient ductility (E
1) and hole expansion value (λ) are not obtained. Steel G is
Ti * (Ti * = Ti + 48 / 93Nb-48 / 12C
−48 / 14N−48 / 32S) is smaller than the range of the present invention, so that solid solution C remains in the steel and sufficient ductility (E) is obtained.
1) and hole expansion value (λ) are not obtained. Steel H is
Since the content of C is less than the range of the present invention, sufficient strength (TS) has not been obtained.

[0055]

[Table 1]

[0056]

[Table 2]

[0057]

As described in detail above, the present invention provides a hot-rolled steel sheet having excellent burring workability and fatigue properties and a tensile strength of 640 MPa or more, and a manufacturing method capable of stably manufacturing the steel sheet. In addition, the use of these hot-rolled steel sheets can be expected to greatly improve fatigue characteristics while sufficiently securing burring workability (stretch flangeability), and is therefore an invention having high industrial value.

[Brief description of the drawings]

FIG. 1 is a diagram showing the results of preliminary experiments leading to the present invention in the relationship between Ti * and hole expansion value (hole expansion ratio).

FIG. 2 is a diagram showing the results of preliminary experiments leading to the present invention, showing the range of hole expansion value (hole expansion ratio) in the relationship between the range of Ti precipitate size and the minimum spacing of Ti precipitates.

FIG. 3 is a view showing the results of preliminary experiments leading to the present invention, showing the range of fatigue characteristics in the presence of shearing or punched end faces in the relationship between the range of Ti precipitate size and the minimum spacing of Ti precipitates.

FIG. 4 is a diagram illustrating the shape of a fatigue test piece.

 ──────────────────────────────────────────────────の Continuing on the front page (72) Inventor Riki Okamoto 5-3 Tokaicho, Tokai City Nippon Steel Corporation Nagoya Works (72) Inventor Hiroyuki Okada 5-3 Tokaicho, Tokai City Nippon Steel Corporation Company Nagoya Works F-term (reference) 4K037 EA01 EA02 EA04 EA05 EA09 EA11 EA15 EA17 EA18 EA19 EA20 EA23 EA25 EA31 EA32 EA35 EA36 EB03 EB06 FB10 FC07 FE01 FE02 FE03

Claims (9)

[Claims] 1. The composition according to claim 1, further comprising: C: 0.01 to 0.1%, S ≦ 0.03%, N ≦ 0.005%, and Ti: 0.05 to 0.5% by mass%. -48 / 12C-48 / 14N-48 / 32S ≧ 0
%, And the balance is Fe and unavoidable impurities, and the average size of the precipitates containing 5 nm or more of Ti 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 properties, having a thickness of more than 10 ¹ nm and 10 ⁴ nm or less. 2. In mass%, C: 0.01 to 0.1%, S ≦ 0.03%, N ≦ 0.005%, Ti: 0.05 to 0.5%, Nb: 0. 01-0.5%, and further Ti + 48 / 93Nb-48 / 12C-48 / 14N-
A steel containing Ti and Nb in a range satisfying 48 / 32S ≧ 0%, the balance being Fe and unavoidable impurities.
The average size of the precipitate containing one or both of Ti and Nb is 10 ¹ to 10 ³ nm and the minimum interval is 10
A hot-rolled steel sheet excellent in burring workability and fatigue properties, having a thickness of more than ¹ nm and not more than 10 ⁴ nm. 3. The steel further contains, in mass%, Si: 0.01 to 2%, Mn: 0.05 to 2%, P ≦ 0.1%, Al: 0.005 to 1.0%. The hot-rolled steel sheet having excellent burring workability and fatigue properties according to claim 1 or 2, characterized by containing: 4. The burring workability according to claim 1, wherein the steel further contains B: 0.0002 to 0.002% by mass%. Hot rolled steel sheet with excellent fatigue properties. 5. The burring workability and fatigue according to claim 1, wherein the steel further contains, in mass%, Ni: 0.1 to 1%. Hot rolled steel sheet with excellent properties. 6. The steel further comprises one or two of Ca: 0.0005 to 0.02% and REM: 0.0005 to 0.2% by mass%. A hot-rolled steel sheet excellent in burring workability and fatigue properties according to any one of claims 1 to 5. 7. The steel further comprises, by mass%, Mo: 0.05 to 1%, V: 0.02 to 0.2%, Cr: 0.01 to 1%, Zr: 0.02 to The hot-rolled steel sheet excellent in burring workability and fatigue properties according to any one of claims 1 to 6, wherein the hot-rolled steel sheet contains one or more kinds of 0.2%. 8. The hot rolling of a steel slab having the composition according to claim 1 is finished at 350 ° C. after hot finish rolling at an Ar3 transformation point or higher.
Cooled to a temperature range of 50 ° C and rolled up.
The average size of the precipitates containing either or both of nm or more Ti and Nb, characterized in that to obtain a 10 ¹ to 10 ³ minimum interval nm is not more than 10 ¹ nm ultra 10 ⁴ nm steel sheet, burring A method for manufacturing hot-rolled steel sheets with excellent workability and fatigue properties. 9. The burring workability and fatigue according to claim 8, wherein in the hot rolling, after rough rolling is completed, high-pressure descaling is performed, and hot finish rolling is completed at an Ar3 transformation point or higher. Manufacturing method of hot rolled steel sheet with excellent properties.