JP2001316765A - Hot rolled steel sheet excellent in fatigue characteristic and ductility and its production method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a hot rolled steel sheet having excellent ductility and fatigue characteristics by controlling the composition, crystal grain size, hot rolling conditions or the like within specified ranges, and to provide its production method. SOLUTION: This hot rolled steel sheet excellent in fatigue characteristics and ductility has a composition containing, by mass, 0.0009 to 0.0045% C, 0.05 to 0.7% Mn, 0.04 to 0.09% P, 0.002 to 0.01% S, <=0.006% N and Ti: (4C+3.4N)×1.2 to 0.75%, and the balance Fe with inevitable impurities, and in which the average crystal grain size of ferrite in a position of the thickness of 1/4 to the inside of the sheet thickness from the steel sheet surface is <=10 μm.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

TECHNICAL FIELD The present invention relates to a 340-480M
It relates to a hot-rolled steel sheet having a tensile strength of Pa,
The present invention relates to a hot-rolled steel sheet having excellent fatigue properties in which the composition, crystal grain size, slab heating temperature, cooling conditions, and the like of a steel sheet are in specific ranges, and a method for producing the same.

[0002]

2. Description of the Related Art Various techniques for manufacturing a hot-rolled steel sheet having excellent fatigue characteristics have been devised.
And Japanese Patent Application Laid-Open No. 5-295485, there is a method of forming ferrite and fine benite. However, these techniques are directed to a steel sheet having a tensile strength of 490 MPa or more, and have a tensile strength of 480M.
Since the ductility is lower and the yield strength is higher than that of a steel sheet having a pressure of Pa or less, it is not suitable for parts requiring high working. Many parts used for automobiles and the like have a strength of 480 MPa or less, have high ductility, and require higher fatigue properties than ever before.

The production of a hot-rolled steel sheet having a tensile strength of 480 MPa or less and excellent in fatigue properties is disclosed in
As described in JP-A-345948, there is a method of adding Cu to extremely low carbon steel. However, in this method, expensive Cu is added for the purpose of strengthening at a high temperature, and further expensive N is added to prevent cracking in hot rolling.
The addition of i is also required, and the increase in production cost due to the addition of these elements is a problem.

[0004]

Accordingly, the present inventors have developed a steel sheet having a tensile strength of 340 MPa or more and 480 MPa or less in a composition range that does not require a special additive element, which significantly increases the cost. ) ×
An object of the present invention is to provide a steel sheet having ductility (%) ≧ 15000 and further having fatigue characteristics of (fatigue limit / tensile strength) ≧ 0.53 and a method for producing the same.

[0005]

Means for Solving the Problems The present inventors maintain good ductility by setting the composition of steel, the crystal grain size of a hot-rolled steel sheet, hot-rolling conditions, and the like in specific ranges, and have extremely good fatigue properties. I found it to be.

The gist of the invention is that C: 0.000 by mass ratio.
9 to 0.0045%, Mn: 0.05 to 0.7%, P:
0.04 to 0.09%, S: 0.002 to 0.01%,
N: 0.006% or less, Ti: (4C + 3.4N) ×
1.2 to 0.075%, the balance being Fe and unavoidable impurities, and 1 in the sheet thickness from the steel sheet surface.
The average grain size of ferrite at the position of / 4 thickness is 1
It is a hot-rolled steel sheet having excellent fatigue properties and ductility characterized by being 0 μm or less. In order to manufacture the hot-rolled steel sheet, hot rolling is performed at a temperature of 900 ° C. or less and 860 ° C. or more in hot rolling of a slab. It is characterized by cooling at an average cooling rate of 80 ° C./s or more in a temperature range of 800 ° C. or more after rolling, and then winding.

[0007]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The reasons for limiting the chemical components of steel in the present invention will be described.

[0008] The smaller the amount of C, the better the strength-ductility balance, and the effect becomes greater at 0.0045% or less. However, if it is less than 0.0009%, the cost for decarburization increases, which is not preferable. Therefore, the C content is limited to 0.0009% to 0.0045%.

Mn plays a role of improving tensile strength and fatigue properties by solid solution strengthening, and it is required to be 0.05% or more. However, when it exceeds 0.7%, the ductility decreases.
Therefore, Mn must be 0.05-0.7%.

P is an element whose tensile strength is greatly improved by increasing the amount of P added, and is an element suitable for improving strength at low cost. In order to have a tensile strength of 340 MPa or more, the addition of 0.04% or more is effective. However, when the content exceeds 0.09%, the reduction in fatigue strength with respect to ductility and tensile strength increases. Therefore, it was limited to 0.04 to 0.09%.

[0011] S forms sulfides and may be a starting point of cracking, and reduces ductility and fatigue properties. for that reason,
0.01% or less is required. However, when the content is 0.002% or less, the average ferrite grain size increases and the fatigue strength decreases. Therefore, the amount of S was limited to 0.002 to 0.01%.

N lowers the ductility with respect to strength, so it is preferable that N is low, and it must be 0.006% or less.

[0013] Ti is an element necessary for preventing the aging deterioration of the steel sheet and ensuring the ductility by reducing solid solution C and solid solution N by binding to C and N in the steel. Further, in the present invention, it is an element necessary for preventing the crystal grains of the hot-rolled sheet from becoming coarse and improving the fatigue characteristics.
To fully associate with C and N, (4C + 3.4
N) is required, but in the steel of the present invention, the amount is 1.2 times or more, that is, (4C + 3.4N) ×
The present inventors have found that 1.2 or more is necessary. If Ti is less than this amount, Ti and C, and T
The amount of precipitates in which i and N are combined is small, and coarse grains are likely to be generated, and fatigue is reduced due to coarsening of crystal grains.
Further, aging due to solid solution C and solid solution N occurs, so that ductility is significantly reduced. However, when the amount exceeds 0.075%, the effect is saturated. Therefore, the amount of Ti was limited to (4C + 3.4N) × 1.2 to 0.075%. Conventionally, Ti is a special element, and if it is used in large amounts, the cost will be greatly increased. It was done.

In addition to the above-mentioned elements, Si and AL used for deoxidation when producing steel may remain in the steel. However, in order to prevent a significant decrease in ductility, the content of Si is preferably 0.2% or less, and the content of Al is preferably 0.1% or less.

In the hot-rolled steel sheet having the composition described above, the ferrite must have an average crystal grain size of 10 μm or less in order to have good fatigue characteristics and good strength-EL balance.

Table 2 shows that a slab having a composition within the range of the present invention shown in Table 1 is heated to 1250 ° C., and hot rolled to a finish temperature of 860 to 900 ° C. within the range of the present invention to 3.0 mm. After finishing, the temperature range up to 800 ° C is 7 ~ 230 ° C /
s at various cooling rates and winding at 600 ° C. to form a ferrite average grain size in a range of 7.6 to 17 μm at a position of 1/4 thickness from the steel sheet surface to the inside of the sheet thickness. It shows the average ferrite grain size, tensile strength, and ductility of a steel sheet. In cooling, 7 to 6 outside the present invention
When cooled at 0 ° C./s, the average ferrite grain size is 1
0.5 μm or more, which is outside the scope of the present invention. The strength-ductility balance of these steel sheets is determined by the tensile strength (MP
a) Judgment was made based on the value of × EL (%), and a target of 15,000 or more was set as a measure of good workability in a steel sheet having a strength range intended in the present invention. In Table 2, the values are all 15000.
The above is good.

[0017]

[Table 1]

[0018]

[Table 2]

FIG. 1 shows the relationship between the average ferrite grain size and the fatigue limit of the steel sheet shown in Table 2 above.
It can be seen that the fatigue limit is significantly improved when the thickness is 10 μm or less. FIG. 2 shows the relationship between the average ferrite grain size and the fatigue limit ratio. It can be seen that the fatigue limit ratio is 0.53 or more when the average particle size is 10 μm or less, which is excellent. On the other hand, when the average ferrite grain size is 10.4 μm or more, the ratio of the fatigue limit to the tensile strength is 0.53.
Less than the conventional steel plate.

As described above, by setting the average ferrite grain size of the steel sheet of the present invention to 10 μm or less, a good strength-ductility balance can be maintained and the fatigue limit ratio can be increased. The reason is considered to be that the generation and propagation of fatigue cracks can be prevented by making the grains finer than 10 μm.

The tensile strength and ductility of the steel sheet are as described in J.
This is a value measured by an IS5 tensile test piece. Fatigue limit performs a fatigue test by repeated bending test pieces wearing both end surfaces 30R of the notch, there is shown a maximum stress cracking did not occur after 10 ⁷ times of repetition. The average ferrite grain size was determined by nital etching the cross section of the steel sheet, taking a photograph magnified 1000 times with an optical microscope, and photographing the photograph by a cutting method according to JIS G 0552.
The number of crystal grains in a 25 mm square at 0 is obtained, and the diameter of one crystal grain when all the crystal grains are circles of the same size is shown.

Further, in order to manufacture the steel sheet according to the first aspect of the present invention, the finishing temperature of hot rolling must be 860 ° C. or more and 900 ° C.
It must be below ° C. At a finishing temperature exceeding 900 ° C., it is difficult to secure a ferrite grain size of 10 μm or less at a position １ ／ of the inside of the sheet thickness from the steel sheet surface, and the fatigue limit ratio decreases. When the finishing temperature is lower than 860 ° C., the ductility is significantly reduced. After hot rolling, it is necessary to cool at least the temperature range of 800 ° C. or more at an average cooling rate of 80 ° C./s or more. If the cooling rate is slower than this, the average ferrite grain size exceeds 10 μm and the fatigue limit The ratio decreases.

[0023]

EXAMPLES Examples of the present invention will be described below together with comparative examples.

A steel ingot having the chemical composition shown in Table 3 was produced, and a hot-rolled steel sheet of 2.6 mm was formed under the same hot rolling conditions as shown in Table 3. From the surface of the obtained hot-rolled steel sheet, 1 /
Table 3 also shows the average ferrite grain size at the four thickness positions.

[0025]

[Table 3]

In Table 3, No. Nos. 1 to 11 are within the scope of the present invention, both in steel composition, hot-rolling finishing temperature, average cooling rate in a temperature range of 800 ° C or higher, and average ferrite grain size. No. 12
19 to 19, any one or more of steel composition, hot-rolling finishing temperature, average cooling rate in a temperature range of 800 ° C. or more, and ferrite average particle diameter, which are underlined in Table 3, are out of the range of the present invention. ing.

The tensile strength, ductility, and fatigue limit of these hot-rolled steel sheets were measured by the same methods as described above. In order to determine the strength / ductility balance, TS (MP
a) × EL (%) was used, and the value of fatigue limit / tensile strength (fatigue limit ratio) was used to judge the quality of fatigue strength.

Table 4 shows the results. From Table 4, the components and the hot rolling conditions that are all within the range of the present invention are those in which the average particle size of ferrite is also within the range of the present invention.
The value of TS (MPa) × EL (%) is 15,000 or more, which is good. Furthermore, the fatigue limit ratio is 0.5
3 or more, which is also good.

[0029]

[Table 4]

On the other hand, those whose composition, average ferrite grain size, etc. deviate from the present invention do not have a fatigue limit ratio of 0.53 or more, and have a lower fatigue limit ratio than the steel of the present invention. Even if the crystal grain size falls within the range of the present invention, one or more of the composition and the hot rolling conditions are out of the range of the present invention,
The value of TS (MPa) × EL (%) is less than 15,000, and the fatigue limit ratio is lower than that of the steel of the present invention.

As described above, only the steel sheet whose composition, hot-rolling conditions, and ferrite average grain size are within the range of the present invention have a TS (MPa) × EL (%) value of 15,000 or more and a fatigue limit ratio. It is understood that both are good when the ratio is 0.53 or more.

[0032]

As described above, the tensile strength (MPa) .times.ductility (%). Gtoreq.15000 is maintained by setting the composition of the steel sheet, the average ferrite grain size, and the like to specific ranges, and furthermore, (fatigue limit / tensile strength). ) A steel sheet having a fatigue property of ≧ 0.53 was obtained. Further, in order to manufacture the steel sheet in the hot rolling process, the finishing temperature in hot rolling is 800 ° C.
It could be manufactured by setting the average cooling rate in the above temperature range to a specific range.

[Brief description of the drawings]

FIG. 1 is a diagram showing the relationship between fatigue strength and the average ferrite grain size of a hot-rolled steel sheet.

FIG. 2 is a graph showing a relationship between a fatigue limit ratio and an average ferrite grain size of a hot-rolled steel sheet.

Claims (2)

[Claims] 1. A mass ratio of C: 0.0009 to 0.0
045%, Mn: 0.05 to 0.7%, P: 0.04 to
0.09%, S: 0.002 to 0.01%, N: 0.0
06% or less, Ti: (4C + 3.4N) × 1.2-0.
075%, the balance being Fe and unavoidable impurities, and further having an average crystal grain size of ferrite at a position of 1/4 thickness inside the plate thickness from the steel plate surface is 10 μm or less. Hot rolled steel sheet with excellent ductility. 2. C: 0.0009 to 0.0 by mass ratio
045%, Mn: 0.05 to 0.7%, P: 0.04 to
0.09%, S: 0.002 to 0.01%, N: 0.0
06% or less, Ti: (4C + 3.4N) × 1.2-0.
In making a cast slab containing 075% and a balance of Fe and unavoidable impurities into a hot-rolled steel sheet through a hot-rolling step,
Hot rolling of the slab is completed at a temperature of 900 ° C or lower and 860 ° C or higher, and 80 ° C /
A method for producing a hot-rolled steel sheet having excellent fatigue properties and ductility, wherein the hot-rolled steel sheet is cooled at an average cooling rate of s or more and then wound up.