JP2002105594A - High burring property hot rolled steel sheet having excellent low cycle fatigue strength and its production method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high burring property hot rolled steel sheet having excellent low cycle fatigue strength and to provide a production method by which the steel sheet can inexpensively and stably be produced. SOLUTION: This high burring property hot rolled steel sheet having excellent low cycle fatigue strength is composed of steel having a composition containing 0.01 to 0.2% C, 0.01 to 2% Si, 0.05 to 3% Mn, <=0.1% P and <=0.01% S, containing Al and N by <=0.2% Al and 0.001 to 0.1% N also so as to satisfy 0.52Al/N<=10, further containing one or more kinds selected from Cr, Mo and V by <=2.5% Cr, <=1% Mo and <=0.1% V also so as to satisfy (Cr+3.5Mo+39V)>=0.1, and the balance Fe with inevitable impurities, whose microstructure with the maximum volume ratio is composed of bainite or of the composite one of ferrite and bainite, and the area ratio of a cell structure in the dislocated structure observed after a fatigue test is <=50%. In the method for producing the above steel sheet, the steel having the above components is subjected to hot finish rolling so as to be finished at the Ar3 transformation point or higher, is thereafter cooled at a cooling rate of >=20 deg.C/s and is coiled at a coiling temperature in the temperature range of 450 to 650 deg.C.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hot-rolled hot-rolled steel sheet having excellent low-cycle fatigue strength and a method for producing the same. More particularly, it relates to durability and burring of automobile undercarriage parts such as road wheels. The present invention relates to a high burring hot-rolled steel sheet excellent in low cycle fatigue strength, which is suitable as a material of a member requiring both workability 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 is limited to special applications because they are significantly more expensive than steel. Therefore, in order to promote the weight reduction of automobiles in a wider range, it is strongly required to use inexpensive high-strength steel sheets.

[0003] In response to such demands for high strength, steel sheets having both strength and deep drawability in the field of cold-rolled steel sheets used for white bodies and panels that occupy about 1/4 of the body weight. And bake-hardening steel plates have been developed, which has contributed to weight reduction of vehicle bodies. However, at present, the object of weight reduction is shifting to structural members and undercarriage members occupying about 20% of the vehicle body weight, and there is an urgent need to develop high-strength hot-rolled steel sheets used for these members.

However, since high strength generally deteriorates material properties such as formability (workability), how to achieve high strength without deteriorating material properties is the key to the development of high strength steel sheets. Become. In particular, burring workability, fatigue durability, corrosion resistance, and the like are important as characteristics required for a steel sheet for structural members and suspension members, and it is important how to balance these characteristics with high strength and high dimensions. For example, fatigue durability is particularly important as a characteristic required for a steel plate for a road wheel disc. This is because the fatigue durability is controlled by the strictest standards in the member properties of the wheel.

At present, steel sheets of 440 to 590 MPa class are used as the hot-rolled steel sheets for road wheel discs, but the required strength level of these steel sheets for members is from 590 MPa class to 780 MPa class, and further higher strength is required. It is heading. On the other hand, thinning, which is the purpose of strengthening, causes an increase in the strain level applied to the wheel, and some parts have been exposed to an amplitude at a strain level exceeding the yield point.

Hitherto, in applying a high-strength steel plate to a part around a foot such as a road wheel, in order to improve the fatigue durability, the fatigue limit under a repeated load below the yield point has been regarded as important. However, as described above, recently, improvement in low cycle fatigue characteristics (fatigue characteristics up to about 105 times) at a strain level exceeding the yield point has been desired. However, at present, almost no technology for improving low cycle fatigue characteristics is found.

[0007]

SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide a high burring hot-rolled steel sheet having excellent low cycle fatigue strength and a method of manufacturing the steel sheet stably at low cost. is there.

[0008]

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 an improvement in low cycle fatigue strength. As a result, the microstructure having the largest volume fraction was composed of bainite or a composite structure of ferrite and bainite, and the area ratio of the cell structure among the dislocation structures observed after the fatigue test was 50% or less. The present inventors have newly found that they are very effective in improving the strength, and have made the present invention.

That is, the gist of the present invention is as follows. (1) In mass%, C: 0.01 to 0.2%, Si:
0.01-2%, Mn: 0.05-3%, P ≦ 0.1
%, S ≦ 0.01%, Al ≦ 0.2%, N:
0.001 to 0.1%, 0.52 Al / N is contained so as to satisfy Al / N ≦ 10, and one or two or more of Cr, Mo, and V are Cr ≦ 2.5%, Mo ≦ 1%, V
≦ 0.1% and (Cr + 3.5Mo + 39V) ≧ 0.
1 and the balance consisting of Fe and unavoidable impurities, and the microstructure having the largest volume fraction consists of bainite or a composite structure of ferrite and bainite, which is observed after a fatigue test. A high burring hot-rolled steel sheet having excellent low cycle fatigue strength, wherein the area ratio of the cell structure in the dislocation structure is 50% or less.

(2) The steel further comprises, by mass%, C
u: The high burring hot-rolled steel sheet having excellent low cycle fatigue strength according to the above (1), characterized by containing 0.2 to 1.2%. (3) The steel further contains B: 0.000% by mass.
High-burring hot-rolled steel sheet having excellent low cycle fatigue strength according to the above (1) or (2), characterized by containing 2 to 0.002%.

(4) The steel further comprises N
i: The high-burring hot-rolled steel sheet having excellent low cycle fatigue strength according to any one of the above (1) to (3), characterized by containing 0.1 to 0.6%. (5) The steel further contains Ca: 0.00% by mass.
05-0.002%, REM: 0.0005-0.02
%, Characterized in that the hot-rolled steel sheet has excellent low cycle fatigue strength according to any one of the above (1) to (4).

(6) The steel further comprises N
b: 0.001 to 0.1% and N-0.15Nb ≧ 0.
0005%, Ti: 0.001-0.1% and N-0.
29Ti ≧ 0.0005%, Zr: 0.001 to 0.2
% Or more than one type,
The high burring hot-rolled steel sheet having excellent low cycle fatigue strength according to any one of (1) to (5).

(7) In the hot rolling of the steel slab having the component described in any one of the above (1) to (6), Ar
After finishing hot finish rolling at ₃ transformation point temperature or more, 20 ℃
/ S at a cooling rate of 450 ° C or more and 650 ° C or more.
The microstructure having the largest volume fraction is composed of bainite or a composite structure of ferrite and bainite. The area ratio of the cell structure among the dislocation structures observed after the fatigue test is 50%. %. A method for producing a high burring hot-rolled steel sheet having excellent low cycle fatigue strength, characterized by obtaining a steel sheet of not more than 0.1%. (8) In the hot rolling, after rough rolling is completed, high-pressure descaling is performed, and hot finish rolling is completed at an Ar ₃ transformation point temperature or higher. An excellent method for producing a hot rolled steel sheet having high burring properties.

[0014]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The results of basic research that led to the present invention will be described below. First, the effect of the addition amount of Al, N, Cr, Mo, and V on the dislocation structure after the fatigue test was investigated. The test material for that was prepared as follows.
That is, 0.06% C-0.9% Si-1.2% Mn
-0.01% P-0.001% S as a basic component of Al,
Hot finish rolling was performed on the ingot by adjusting the composition by changing the amounts of N, Cr, Mo, and V so that the plate thickness became 3.5 mm at any temperature equal to or higher than the Ar ₃ transformation point temperature. After finishing, cooling at a cooling rate of 20 ° C./s or more
It was wound up in a normal temperature range.

From the steel sheet thus obtained, a fatigue test piece having the shape shown in FIG.
Samples were taken from the 4W position so that the rolling direction was on the long side, and subjected to a fatigue test. However, the surface of the fatigue test piece was a ground surface with a three-sided finish. The fatigue test was performed using an electrohydraulic servo-type fatigue tester, and the test method was in accordance with ASTM E606-92. As shown in FIG. 2, the test conditions were as follows: a complete swing-pull compression load with a triangular wave in the axial direction, and a total strain amplitude of 0.2
0.6% and a strain rate of 4.0 × 10 ⁻³ / sec. The test was performed while recording changes in strain response and stress response. After the fatigue test, the condition of total strain amplitude is 2
As shown in FIG. 3, with respect to a test piece which was tested in the range of ≦ 2100 × ε _a / YP ≦ 4, a processing strain was not introduced into a transmission electron microscope sample (thin film) from a portion having a thickness of 1/4 near the fractured portion. And the dislocation structure was observed with a transmission electron microscope. However, observation with a transmission electron microscope was performed by changing the crystal grains at a magnification of 2000 to 10000 times.
Observation was made for zero or more visual fields. Where YP: yield stress or 0.
2% yield strength (MPa), ε _a : total strain amplitude (%).

FIGS. 4 and 5 show examples of observation. In each case, the total strain amplitude ε _a = 0.3%. FIG. 4 shows an example of the scope of the present invention, and FIG. 5 shows an example of the scope of the present invention. 4 outside the scope of the invention shows a typical cell structure, whereas FIG. 5 outside the scope of the invention does not show a cell structure. Here, the cell structure means a cell wall (wall, vei) having a high dislocation density specific to the fatigue phenomenon.
n, debris). In addition, a cell is defined as having a completely closed structure surrounded by cell walls on all sides.

On the other hand, the area ratio of the cell structure is a so-called value obtained by adding the value of the area ratio obtained by visual observation or image processing in each field observed in one sample for each observation field, and dividing the result by the number of observation fields. Average value. Further, in the present invention, the dislocation structure is observed at a resolution of about 500 times that of an optical microscope at a resolution of about 500 times that of a microstructured steel containing ferrite grains. In the structure steel, the ferrite phase is a phase other than the cementite phase in the bainite structure.

The present inventors have examined these experimental results in detail, and as a result, as shown in FIG. 6, there is a very strong correlation between the dislocation structure observed after the fatigue test and the low cycle fatigue strength. It was newly found that when the area ratio of the cell structure was 50% or less, the low cycle fatigue strength was improved. Further, the dislocation structure, the value of 0.52 Al / N and C
As shown in FIG. 7, a strong correlation was also observed in the relationship with the value of r + 3.5Mo + 39V, and 0.52 Al / N
It was newly found that the area ratio of the cell structure was 50% or less in a region of ≦ 10 and (Cr + 3.5Mo + 39V) ≧ 0.1. Although this mechanism is not always clear, it is presumed as follows. Usually, repeated strain concentrates on the ferrite phase, which is a soft phase, so that softening occurs repeatedly and low cycle fatigue strength is reduced. Therefore, in order to improve low cycle fatigue strength, it is necessary to suppress repetitive softening in the ferrite phase which is a soft phase.

N, C and C in a solid solution state as in the present invention
When r, Mo, and V are contained in a specific range, N and C, which are intrusion-type solid solution elements, and Cr, Mo, and V form pairs and clusters in the ferrite phase, and the cross-slip of dislocations under repeated load is prevented. This suppresses repeated softening due to rearrangement of dislocations (formation of a cell structure). Further, N and C, which are intrusion-type solid solution elements, escape from pairs or clusters of Cr, Mo, and V due to the action of atomic vacancies generated by the application of the cyclic load, and fix dislocations to cause repeated hardening. Cycle fatigue strength is improved. It has also been newly found that by restricting the hot rolling conditions and the like, it is possible to control the state of N and C, which are intrusion-type solid solution elements in the ferrite phase, to produce a steel sheet having excellent low cycle fatigue strength.

In the present invention, the low cycle fatigue strength is defined as a value obtained by dividing a cyclic yield stress by a tensile strength. Here, the repeated yield stress can be obtained as follows. Changes in strain response and stress response during the fatigue test at a constant total strain amplitude are schematically represented as a hysteresis loop as shown in FIG. The material softens or hardens due to repeated strain, and this change is obtained as a change in Δσ. The value of Δσ of the material is almost saturated and stable at a repetition number of の of the fracture life (Nf). Therefore, Δ at this repetition rate
σ / 2 is defined as the stress amplitude σ _a at that strain amplitude. FIG. 9 schematically shows σ _a for each strain amplitude. Here, an intersection obtained by extrapolating a straight line obtained by linearly approximating these σ _a to the strain to the stress-strain curve is defined as a yield point repeatedly. In addition, this intersection may be an intersection with an elastic straight line obtained when the material is assumed to be a linear elastic body (Hooke's body).

Next, the microstructure of the steel sheet according to the present invention will be described in detail. The microstructure of the steel sheet was ferrite, or a composite structure of ferrite and bainite, with the maximum volume fraction in order to achieve both fatigue characteristics and burring workability. However, the inclusion of unavoidable pearlite, retained austenite, and martensite is permitted. Here, hereinafter, bainite includes bainitic ferrite and ashular ferrite structures. In order to secure good fatigue characteristics, the volume fraction of pearlite is desirably 5% or less. In order to obtain good burring workability, the volume fraction of bainite is desirably 30% or more, and the volume fraction of martensite is desirably less than 5%. Further, in order to obtain good ductility, the volume fraction of bainite is desirably 70% or less. Here, the volume fraction of ferrite, bainite, retained austenite, pearlite, and martensite is 1% of the steel sheet width.
A sample cut from the / 4W or 3 / 4W position is polished and etched into a cross section in the rolling direction, and is polished and etched using an optical microscope.
It is defined as the area fraction of the microstructure at 1 / 4t of the plate thickness observed at a magnification of ~ 500 times.

Next, the reasons for limiting the chemical components of the present invention will be described. C is an element necessary for obtaining a desired microstructure. However, if the content exceeds 0.2%, workability and weldability deteriorate, so the content is set to 0.2% or less. On the other hand, if it is less than 0.01%, the strength is reduced.
1% or more. Further, C existing in a solid solution state forms pairs or clusters with Cr, Mo, and V, like N, and is thus effective in improving low cycle fatigue strength. In the present invention, N is sufficiently added and the range of the solute C amount is not particularly limited. However, a sufficient amount of solid solution C for obtaining the effect is secured in the above range of the total C content lower limit or more, and the range is 0.0005% or more and 0.00.0% or more.
Desirably, it is at most 04%.

Si is necessary for obtaining a desired microstructure and is effective for increasing the strength as a solid solution strengthening element. In order to obtain a desired strength, the content needs to be 0.01% or more. However, if the content exceeds 2%, the workability deteriorates. Therefore, the content of Si is set to 0.01% or more and 2% or less. Mn is effective for increasing strength as a solid solution strengthening element. To obtain the desired strength, 0.05% or more is required. On the other hand, if added over 3%, slab cracks occur, so the content is set to 3% or less.

P is an impurity and is preferably as low as possible.
If the content exceeds 0.1%, the workability and the weldability are adversely affected and the fatigue characteristics are also reduced. S is an impurity and is preferably as low as possible. If it is too high, A-based inclusions that deteriorate local ductility and burring workability are generated. Therefore, S should be reduced as much as possible.
Below is an acceptable range.

Al may be used as a deoxidizing agent.
However, since Al combines with N to form AlN, C
Since the effective amount of N that forms a pair or a cluster with r, Mo, and V is reduced, it is desirable that the addition thereof be kept to a minimum necessary within a range that is reasonable in production technology. That is, A
If the amount of l exceeds 0.2%, a large amount of N must be added in order to secure an effective amount of N for forming pairs and clusters with Cr, Mo, and V, and the production cost and processing by precipitation of AlN are required. It is disadvantageous in terms of property deterioration. Therefore, the upper limit of the addition amount of Al is set to 0.2% or less. Further, Al may form non-metallic inclusions such as Al ₂ O ₃ and may cause flaws and decrease in local ductility. Therefore, the amount of Al added is preferably 0.05% or less. Further, in order to easily contain N in steel within a range that does not deteriorate the production cost and the operation efficiency, the content is further desirably 0.02% or less. Although the lower limit of Al is not particularly defined,
If the content is less than 0.001%, the production cost and the operation efficiency are deteriorated. Therefore, the content is desirably 0.001% or more.

N is one of the important elements in the present invention. In the present invention, N and C, which are intrusion type solid solution elements in a solid solution state, form pairs or clusters in the ferrite phase with Cr, Mo, and V, and suppress cross slip of dislocations under repeated loading. Suppresses repetitive softening due to rearrangement of dislocations (formation of a cell structure), and furthermore, N and C, which are intrusion-type solid solution elements, are formed of Cr, Mo, and V by the action of atomic vacancies generated by the repetitive load. The low cycle fatigue strength is improved due to repeated hardening due to escape from the metal and clusters to fix dislocations. Therefore, addition of 0.001% or more is essential. On the other hand, in order to add a large amount of N to molten steel, special equipment and operation such as pressurization are required, so the upper limit is 0.1%. On the other hand, if N is too large, yield point elongation occurs and workability deteriorates, so that the N content is more preferably 0.01% or less.

Further, since N is an element which easily forms AlN by combining with Al, 0.52 Al / N ≦ 10 in order to secure solid solution N which contributes to improvement of low cycle fatigue strength.
Limited. When the value of 0.52 Al / N exceeds 10,
Since AlN is easily precipitated during the cooling process after hot rolling or during winding, this is set as the upper limit. If this value is 10 or less, excessive precipitation of AlN can be avoided by controlling the cooling rate after hot rolling and the winding temperature within the range of the present invention. Further, when the value of 0.52 Al / N is 5 or less, deterioration of workability due to precipitation of fine AlN is improved, so that 0.52 Al / N ≦ 5 is more preferable.

On the other hand, the amount of solute N may be adjusted within the above-mentioned total N content range.
0.004% is desirable. If the solute N is less than 0.0005%, excellent low cycle fatigue strength cannot be obtained,
If it exceeds 0.004%, yield point elongation occurs and workability deteriorates. Further, from the viewpoint of suppressing generation of waist break flaw, the amount of solute N is
0.0012 to 0.003% is desirable.

Here, the solute N includes not only N present alone in Fe, but also N forming a pair or a cluster with substitutional solute elements such as Cr, Mo, V, Mn, Si and P. . The amount of solute N is determined by a heating extraction method in a hydrogen stream. In this method, the sample is heated to a temperature range of about 200 to 500 ° C., and the dissolved N and hydrogen are reacted to form ammonia,
This is subjected to mass spectrometry, and the analysis value is converted to obtain the amount of solute N. The amount of solid solution N is calculated from the total amount of N to Al.
It can also be determined from a value obtained by subtracting the amount of N present as a compound such as N, NbN, VN, TiN, or BN (quantified from the chemical analysis of the extraction residue). Furthermore, the internal friction method and FIM (Field Ion Microscopy)
May be determined by:

Cr, Mo, and V are important elements in the present invention. The upper limits of the added amounts of Cr, Mo, and V are determined from the viewpoint of securing workability and cost, and are 2.5 and 2.5, respectively.
1, 0.1%. Particularly, if V is added in an excessive amount, nitrides are formed depending on hot rolling conditions, and it may be difficult to secure solid solution N effective for improving low cycle fatigue strength. It is desirable that on the other hand,
In order to obtain excellent low cycle fatigue strength, (Cr + 3.
5Mo + 39V) ≧ 0.1. Further, in order to avoid deterioration of workability due to the occurrence of elongation at the yield point, (Cr + 3.5Mo + 39V) ≧ 0.4 is a more desirable range. In order to avoid deterioration in workability due to the occurrence of yield point elongation, it is more effective to add two or more types of Cr, Mo, and V in combination than to add Cr, Mo, and V alone.

Since Cu has an effect of improving fatigue characteristics in a solid solution state, Cu is added as necessary. However, if the content is less than 0.2%, the effect is small, and if the content exceeds 1.2%, it may precipitate during winding and significantly deteriorate workability. Therefore, the content of Cu is set in the range of 0.2 to 1.2%. B is added as necessary since it is effective to increase the fatigue limit by being combined with Cu. 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 0.0002% or more and 0.1% or more.
002% or less. Further, when B is added in excess of 0.0004%, BN is formed, and there is a possibility that the effective amount of solute N that forms pairs or clusters with Cr, Mo, and V may be reduced. Therefore, the addition of B is 0.0002% or more and 0.002% or more.
04% or less is a more desirable range.

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 0.6%, the effect is saturated. Therefore, the content is set to 0.1 to 0.6%. 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 it is added less than 0.0005%, there is no effect, Ca exceeds 0.002%, and REM 0.0%.
Even if more than 2% is added, the effect is saturated.
005-0.002%, REM: 0.0005-0.0
It is desirable to add 2%.

Nb is added as necessary because it is effective for improving workability and increasing strength by making the structure finer and more uniform. However, if the amount of addition is less than 0.001%, no effect is exhibited, and even if added over 0.1%, the effect is saturated. Further, when the value of N-0.15Nb is more than 0.0005%, it becomes difficult to secure solid solution N effective for improving low cycle fatigue strength. Therefore, the added amount of Nb is set to 0.001 to 0.1% and N-0.15Nb ≧ 0.0005%. On the other hand, N
If b is added in excess of 0.012%, NbN is likely to be formed, and it may be difficult to secure solid solution N effective for improving low cycle fatigue strength. Therefore, 0.001 to 0.012% is more desirable.

Since Ti has the same effect as Nb, it is added as necessary. However, if the amount of addition is less than 0.001%, no effect is exhibited, and even if added over 0.1%, the effect is saturated. The value of N-0.29Ti is 0.000
It becomes difficult to secure the solute N effective for improving the low cycle fatigue strength of more than 5%. Therefore, the addition amount of Ti is 0.001.
% To 0.1% and N−0.29Ti ≧ 0.0005%. On the other hand, if more than 0.012% of Ti is added, it may precipitate or crystallize as TiN, and it may be difficult to secure solid solution N effective for improving low cycle fatigue strength. .012% is more desirable.

Further, in order to impart strength, Zr may be added as a precipitation strengthening or solid solution strengthening element. However, if it is less than 0.001%, the effect cannot be obtained. The effect is saturated even if it is added in excess of 0.2%. Therefore, Zr is added in the range of 0.001% to 0.2%. However, since Zr forms ZrN and may reduce the amount of solute N effective for improving the low cycle fatigue strength, it is preferably set to 0.01% or less. Steel containing these as main components may contain Sn, Co, Zn, W, and Mg in a total amount of 1% or less. However, since Sn may cause flaws during hot rolling, 0.05% or less is 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. However, if the temperature is 1400 ° C. or more, the scale-off amount becomes large and the yield decreases.
Desirably less than ° C. Further, since the heating at a temperature lower than 1000 ° C. significantly impairs the operation efficiency on a schedule, the reheating temperature is desirably 1000 ° C. or higher. Further, when it is necessary to dissolve AlN to secure solid solution N, 115
It is desirable that the temperature be 0 ° 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 Ar ₃ transformation point temperature. This is because if the rolling temperature falls below the Ar ₃ transformation point during hot rolling, strain remains and ductility is reduced, thereby deteriorating workability. On the other hand, the upper limit of the finishing temperature is not particularly limited, but if the temperature exceeds the Ar ₃ transformation point + 200 ° C., it is practically impossible to secure the temperature before finish rolling, so the upper limit of the finishing temperature is the Ar ₃ transformation point temperature + 200 ° C. or less. Is desirable. Here, when performing high-pressure descaling after rough rolling,
It is desirable that the condition of collision pressure P (MPa) of high-pressure water on the steel sheet surface × flow rate L (liter / cm ² ) ≧ 0.0025 is satisfied.

The collision pressure P of the high-pressure water on the steel plate surface is described as follows. ("Iron and Steel" 1991 vol. 77
No. 9 P1450) P (MPa) = 5.64 × P ₀ × V / H ² where P ₀ (MPa): liquid pressure V (liter / min): nozzle flow H (cm): steel sheet surface and nozzle Distance between

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 apparent from the fact that the fatigue strength of a hot-rolled or pickled steel sheet is correlated with the maximum height Ry of the steel sheet surface, as described in, for example, Handbook of Fatigue Design for Metallic Materials, edited by The Society of Materials Science, Japan, page 84. is there. Further, the subsequent finish rolling is performed in order to prevent scale from being formed again after descaling.
It is desirable to do this within seconds.

In the present invention, the process up to winding at a predetermined winding temperature after finishing rolling is not particularly limited except for the cooling rate during the process, but the ductility and the burring workability are not significantly deteriorated. If you aim to balance
It may be retained for 1 to 20 seconds in a temperature range from the r ₃ transformation point to the Ar ₁ transformation point (two-phase region of ferrite and austenite). The retention here is performed in order to promote ferrite transformation in the two-phase region, but if less than 1 second, the ferrite transformation in the two-phase region is insufficient, so that sufficient ductility cannot be obtained. Pearlite is generated, and bainite or a composite structure of ferrite and bainite cannot be obtained as a target microstructure having the maximum volume fraction. In order to facilitate the ferrite transformation, the temperature range in which the stagnation is maintained for 1 to 20 seconds is at least the Ar ₁ transformation point.
C or lower is desirable. Further, from the viewpoint of suppressing the precipitation of AlN, 700 ° C. or lower is more preferable. In addition, 1
The residence time for up to 20 seconds is desirably 1 to 10 seconds in order not to significantly reduce the productivity.

In order to satisfy these conditions, it is necessary to quickly reach the temperature range at a cooling rate of 20 ° C./s or more after finish rolling. The upper limit of the cooling rate is not particularly defined, but 300 ° C./s or less is a reasonable cooling rate in view of the capacity of the cooling equipment. Further, if the cooling rate is too high, the cooling end temperature cannot be controlled, and there is a possibility of overshoot and overcooling to the Ar ₁ transformation point or less, and the effect of improving ductility is lost. The speed is desirably 150 ° C./s or less.

After finishing rolling, the cooling rate until winding at a predetermined winding temperature (CT) is 20 ° C./s or more. If the cooling rate is less than 20 ° C./s, pearlite may be generated, and bainite or a composite structure of ferrite and bainite, which is a microstructure having the maximum desired volume fraction, cannot be obtained. On the other hand, the effect of the present invention can be obtained without particularly setting the upper limit of the cooling rate to the winding temperature.

If the winding temperature is higher than 650 ° C., pearlite is formed, and the desired microstructure of bainite or ferrite and bainite having the maximum volume fraction cannot be obtained. Limit to below ° C. On the other hand, when the winding temperature is lower than 450 ° C., a large amount of retained austenite or martensite harmful to burring workability may be generated, and bainite, which is the microstructure having the maximum volume fraction, or microstructure comprising ferrite and bainite Since a texture cannot be obtained, the winding temperature is limited to 450 ° C. or higher. After the completion of the hot rolling step, pickling may be performed as necessary, and then a skin pass with a rolling reduction of 10% or less or cold rolling to a rolling reduction of about 40% may be performed in-line or off-line.

[0045]

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.

[0046]

[Table 1]

In the tensile test of the hot-rolled sheet obtained in this manner, the test material was first processed into a No. 5 test piece described in JIS Z2201, and was subjected to a test method described in JIS Z2241. Furthermore, burring workability (hole expanding property)
About Japan Iron and Steel Federation Standard JFS T 1001-1
The evaluation was performed according to the hole expansion test method described in 996. Table 2 shows the test results. Here, the volume fraction of retained austenite, ferrite, bainite, pearlite and martensite refers to a sample cut out from a 1 / 4W or 3 / 4W position of a steel sheet width, polished and etched into a cross section in the rolling direction, and using an optical microscope. It is defined as the area fraction of the microstructure at 1 / 4t of the plate thickness observed at a magnification of 200 to 500 times.

[0048]

[Table 2]

Next, a fatigue test piece having the shape shown in FIG. 1 was sampled from the 1/4 W or 3/4 W position of the steel sheet width so that the rolling direction became the longer side, and subjected to a low cycle fatigue test. However, the surface of the fatigue test piece was a ground surface with a three-sided finish.
The fatigue test was performed using an electrohydraulic servo-type fatigue tester, and the test method was in accordance with ASTM E606-92. As shown in FIG. 2, the test conditions were as follows: a complete triangular wave in the axial direction, a complete swinging tensile compression load, a total strain amplitude of 0.3 to 0.6% and a strain rate of 4.0 × 10 ⁻³ / sec. And The test was performed while recording changes in strain response and stress response. After the fatigue test, the condition of the total strain amplitude is 2 ≦ 2100 × ε _a
As shown in FIG. 3, a transmission electron microscope sample (thin film) was sampled from a portion having a thickness of 1/4 near the fractured portion so that no processing strain was introduced, and the transmission was performed on the test piece which was tested in the range of / YP ≦ 4. The dislocation structure was observed with a scanning electron microscope.
Table 2 shows the area ratio of the cell structure as S _cell . However, observation with a transmission electron microscope is 2000 to 1000
The crystal grains were changed at a magnification of 0, and observation was made for 10 visual fields or more.
Where YP: yield stress or 0.2% proof stress (MPa),
ε _a : total strain amplitude (%).

The low cycle fatigue strength of the steel sheet was evaluated by dividing the yield stress by the tensile strength. Here, the cyclic yield stress is defined as the intersection of a stress-strain curve or an elastic straight line extrapolating a straight line obtained by linearly approximating the stress amplitude σ _a at a repetition number of １ ／ of the fracture life (Nf) to the strain. did.
According to the present invention, steel A, B, D, E, G, H, J-
1, J-6, N, O, 10 steels, containing a predetermined amount of steel components, the microstructure of the largest volume fraction consisting of bainite or a composite structure of ferrite and bainite, observed after a fatigue test A high burring hot-rolled steel sheet excellent in low cycle fatigue strength, characterized in that the area ratio of the cell structure among the dislocation structures to be formed is 50% or less.

Other steels are outside the scope of the present invention for the following reasons. That is, since the steel C has a C content outside the range of the present invention, a desired microstructure cannot be obtained and a sufficient fatigue strength (TS) cannot be obtained. Steel F is
Since the S content is out of the range of the present invention, a sufficient hole expansion value (λ) is not obtained. Steel I has a sufficient low cycle fatigue strength (CYS) because the content of P is out of the range of the present invention.
/ TS) is not obtained. Steel J-2 has a finish rolling end temperature (FT) lower than the range of the present invention, a strain remaining, and a low ductility (El). Steel J-3 has a winding temperature (CT)
However, the target microstructure was not obtained and a sufficient hole expansion value (λ) was not obtained.

The steel J-4 has a winding temperature (CT) lower than the range of the present invention, so that a desired microstructure cannot be obtained and a sufficient hole expansion value (λ) cannot be obtained. In steel J-5, the cooling rate (CR) after finish rolling was lower than the range of the present invention, and the desired microstructure was not obtained and a sufficient hole expansion value (λ) was not obtained. Steel K has a sufficiently low cycle fatigue strength (CYS) because the value of 0.52 Al / N is out of the range of the present invention.
/ TS) is not obtained. Steel L is Cr + 3.5Mo
Since the value of +39 V is out of the range of the present invention, sufficient low cycle fatigue strength (CYS / TS) has not been obtained. Steel M
However, since the content of C is out of the range of the present invention, a desired microstructure cannot be obtained and a sufficient hole expansion value (λ) cannot be obtained.

[0053]

As described above in detail, the present invention relates to a high burring hot-rolled steel sheet having excellent low cycle fatigue strength and a method for producing the hot-rolled hot-rolled steel sheet. Since the present invention can be expected to significantly improve low cycle fatigue characteristics, which is one of the important characteristics in a member that requires both durability and burring workability of parts and the like, the present invention is an invention having high industrial value. It can be said.

[Brief description of the drawings]

FIG. 1 is a diagram illustrating the shape of a fatigue test piece.

FIG. 2 is a diagram illustrating a method of loading a fatigue test.

FIG. 3 is a diagram illustrating a transmission electron microscope sample collection position.

FIG. 4 is an electron micrograph showing an example of a cell structure among dislocation structures observed after a fatigue test.

FIG. 5 is an electron micrograph showing an example of a dislocation structure observed after a fatigue test, other than a cell structure.

FIG. 6 is a diagram showing the results of a preliminary experiment leading to the present invention in a relationship between the cell structure area ratio after a fatigue test and low cycle fatigue strength (a value obtained by dividing a cyclic yield stress by a tensile strength).

FIG. 7 shows the results of preliminary experiments leading to the present invention,
It is a figure which shows in the relationship of the range of the value of / N, the range of the value of Cr + 3.5Mo + 39V, and the cell structure area ratio after a fatigue test.

FIG. 8 shows a stress amplitude σ _a at 1 / 2Nf in a fatigue test.
FIG.

FIG. 9 is a diagram illustrating a repeated yield stress CYS in a fatigue test.

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) C22C 38/58 C22C 38/58 (72) Inventor Manabu Takahashi 20-1 Shintomi, Futtsu-shi, Chiba Nippon Steel Corporation F-term (Reference) 4K037 EA01 EA02 EA05 EA06 EA09 EA11 EA13 EA15 EA16 EA17 EA18 EA19 EA20 EA23 EA25 EA27 EA28 EA31 EA32 EA35 EA36 EB03 EB06 EB07 EB08 EB09

Claims (8)

[Claims] C .: 0.01 to 0.2%, Si: 0.01 to 2%, Mn: 0.05 to 3%, P ≦ 0.1%, S ≦ 0. Al ≦ N%, N: 0.001 to 0.1%, 0.52 Al / N is contained so as to satisfy Al / N ≦ 10, and one of Cr, Mo, and V Or, two or more kinds are contained so as to satisfy Cr ≦ 2.5%, Mo ≦ 1%, V ≦ 0.1%, and (Cr + 3.5Mo + 39V) ≧ 0.1, and the balance consists of Fe and inevitable impurities. In steel, the microstructure having the largest volume fraction is bainite or a composite structure of ferrite and bainite, and the area ratio of the cell structure among the dislocation structures observed after the fatigue test is 50% or less. High burring hot rolled steel sheet with excellent low cycle fatigue strength. 2. The steel according to claim 1, further comprising:
The high burring hot-rolled steel sheet having excellent low cycle fatigue strength according to claim 1, characterized by containing 0.2 to 1.2%. 3. The steel according to claim 1, further comprising:
The high burring hot-rolled steel sheet having excellent low cycle fatigue strength according to claim 1 or 2, characterized by containing 0.0002 to 0.002%. 4. The steel according to claim 1, further comprising:
The high burring hot-rolled steel sheet having excellent low cycle fatigue strength according to any one of claims 1 to 3, characterized by containing 0.1 to 0.6%. 5. The steel further comprises, in mass%, one or two of Ca: 0.0005 to 0.002% and REM: 0.0005 to 0.02%. The high burring hot-rolled steel sheet according to any one of claims 1 to 4, which is excellent in low cycle fatigue strength. 6. The steel according to claim 1, wherein the steel further comprises Nb:
0.001-0.1% and N-0.15Nb ≧ 0.00
05%, Ti: 0.001-0.1% and N-0.29
The low cycle fatigue according to any one of claims 1 to 5, wherein one or more of Ti ≧ 0.0005% and Zr: 0.001 to 0.2% is contained. High burring hot rolled steel sheet with excellent strength. 7. The hot rolling of a slab having the composition described in any one of claims 1 to 6, after finishing the hot finish rolling at a temperature not lower than the Ar ₃ transformation point, at 20 ° C. / s
Cooling at the above cooling rate, winding at a winding temperature in the temperature range of 450 ° C. or more and 650 ° C. or less, the microstructure having the largest volume fraction is composed of bainite or a composite structure of ferrite and bainite, and is subjected to a fatigue test. A method for producing a high burring hot-rolled steel sheet having excellent low cycle fatigue strength, comprising obtaining a steel sheet having an area ratio of a cell structure of 50% or less among dislocation structures observed later. 8. The low cycle fatigue strength according to claim 7, wherein, during the hot rolling, after the rough rolling is completed, high-pressure descaling is performed, and the hot finish rolling is completed at the Ar ₃ transformation point temperature or higher. For producing high burring hot rolled steel sheet with excellent heat resistance.