JP2001294979A - High strength hot rolled steel sheet excellent in fatigue characteristic of weld zone and its producing method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high strength hot rolled steel sheet having a excellent weld fatigue resistance by controlling its composition and metallic structure to specified ranges, and to provide its producing method. SOLUTION: This high strength hot rolled steel sheet excellent in the weld fatigue resistance has a composition containing, by mass, 0.04 to 0.16% C, 0.9 to 3% Si, 0.5 to 3% Mn, <=0.07% P, <=0.01% S, 0.02 to 0.3% Al and 0.001 to 0.008% N, in which Cu of 1 to 2.5% is made into a state of solid solution, containing Ni by 0.4×Cu to 1.5×Cu%, and the balance Fe with inevitable impurities, further, in which the ferrite average crystal grain size in the cross-sectional crystal structure of the steel sheet at a position of 1/4 thickness inwardly to the sheet thickness from the surface of the steel sheet is <=10 μm, and its cross- sectional area ratio is >=70% and moreover having tensile strength of >=500 MPa.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-strength hot-rolled steel sheet having a tensile strength of 500 MPa or more, wherein the composition, structure, finishing temperature in hot rolling, and cooling conditions after finishing are specified. An object of the present invention is to provide a high-strength hot-rolled steel sheet excellent in fatigue properties of a welded portion by setting the content within the range, and a method for producing the same.

[0002]

2. Description of the Related Art In recent years, in order to reduce the weight of vehicles and improve the safety of collisions, high strength steel plates are increasingly used for vehicles. When a steel sheet is formed and then welded when manufacturing an automobile part, it is an important issue to increase not only the fatigue strength of the base material but also the fatigue strength of the welded portion. In particular, a hot-rolled steel sheet having a tensile strength of 500 MPa or more, after being processed into parts,
The problem is that as the heat input of welding increases, the strength of the heat affected zone tends to decrease, and the fatigue strength of that portion also tends to decrease. In flash butt welding, arc welding, and the like, which have a higher welding heat input than spot welding, improvement of the fatigue strength of the heat affected zone is an issue.

Various methods for improving the fatigue strength of a steel sheet base material have been studied so far.
In JP-A-74377 and Japanese Patent Application No. 8-78100, a steel sheet having a fatigue limit ratio of a base material of 0.52 or more by forming a structure mainly composed of ferrite and martensite has been found. However, in this steel sheet, the fatigue strength of the heat-affected zone in flash butt welding and arc welding is the same or lower than that of a steel sheet mainly composed of a ferrite-pearlite structure having the same tensile strength. Despite its high fatigue strength, there is a problem that it is difficult to use it for parts that require high fatigue properties in the weld.

[0004] In addition, as described in Japanese Patent Application No. 8-78102, there is known a method for improving fatigue characteristics by forming a structure containing residual γ in a hot-rolled sheet. However, even with this method, the fatigue strength of the flash butt weld and the arc weld has not been significantly improved.

[0005]

Therefore, the present invention relates to
An object of the present invention is to provide a hot-rolled steel sheet having a tensile strength of 00 MPa or more and to provide a steel sheet having excellent weld fatigue properties and to provide a method of manufacturing the steel sheet.

[0006]

Means for Solving the Problems The inventors of the present invention have found that by setting the composition and the metal structure of a hot-rolled steel sheet manufactured in a hot-rolling process to specific ranges, the fatigue resistance of a welded part can be extremely improved. It has been found that, in order to manufacture this, it is necessary to set the finishing temperature in hot rolling and the subsequent cooling conditions within a specific range.

The present invention has been completed based on these findings, and the gist thereof is as follows.

(1) By mass ratio, C: 0.04-0.
16%, Si: 0.9-3%, Mn: 0.5-3%,
P: 0.07% or less, S: 0.01% or less, AL: 0.
02-0.3%, N: 0.001-0.008%, C
u: 1 to 2.5%, Ni: 0.4 × Cu to 1.5 × Cu
%, The balance being Fe and inevitable impurities, and the Cu exists in a solid solution state in the steel sheet, and furthermore, the cross-sectional crystal structure of the steel sheet at a position of 1/4 thickness inside the steel sheet from the steel sheet surface Ferrite average crystal grain size is 10 μm
A high-strength hot-rolled steel sheet excellent in fatigue properties of a weld portion, wherein a ferrite cross-sectional area ratio is 70% or more and a tensile strength is 500 MPa or more.

(2) Ca: 0.0005 by mass ratio
The high-strength hot-rolled steel sheet having excellent weld fatigue properties according to the above (1), wherein the high-strength steel sheet contains 0.1 to 0.01%.

(3) In terms of mass ratio, C: 0.04-0.
16%, Si: 0.9-3%, Mn: 0.5-3%,
P: 0.07% or less, S: 0.01% or less, AL: 0.
02-0.3%, N: 0.001-0.008%, C
u: 1 to 2.5%, Ni: 0.4 × Cu to 1.5 × Cu
%, With the balance consisting of Fe and unavoidable impurities being hot-rolled and finish-rolled at a temperature of 800 to 880 ° C, and after the obtained steel strip is hot-rolled and finished, it is 30 ° C to a range of 780 to 720 ° C. / S at a cooling rate of not less than 1 / s, and after the first cooling, from 7 to 25 ° C./s to a range of 680 to 630 ° C.
A second cooling at a cooling rate of
A method for producing a high-strength hot-rolled steel sheet having excellent fatigue properties at a weld portion, wherein a third cooling is performed at a cooling rate of 50 ° C / s or more to a range of 0 to 300 ° C and a steel strip is wound.

(4) The steel slab is expressed by mass ratio of Ca: 0.0
(3) The method for producing a high-strength hot-rolled steel sheet having excellent weld fatigue properties according to the above (3), wherein the high-strength steel sheet contains 005 to 0.01%.

[0012]

First, the reasons for limiting the chemical components of steel in the present invention will be described.

C has the effect of increasing the fatigue strength by reducing the size of ferrite grains and strengthening the ferrite. In order to exert its effect sufficiently, it is necessary to add 0.04% or more. However, if it exceeds 0.16%, a hard phase is likely to appear in cooling after welding, and fatigue near the heat affected zone of the weld is increased. Strength decreases. Also, when the amount is less than 0.04%, 10 μm
m or less fine ferrite cannot be obtained, and the fatigue strength of the weld decreases. Therefore, it was set to 0.04 to 0.16%.

[0014] Si promotes the formation of ferrite by cooling after hot rolling, suppresses the formation of carbides, and has the effect of forming a solid solution in ferrite to increase the fatigue strength of the base material and the fatigue strength of the welded portion. In order to exhibit this effect, it is necessary to add 0.9% or more. However, if it exceeds 3%, a hard phase is likely to appear during cooling after welding, and the fatigue strength near the heat affected zone of the weld is reduced, so the upper limit is 3%.

Mn has a role of strengthening and miniaturizing ferrite, and has an effect of improving fatigue strength. 0.5
%, The effect is exhibited. If the content is less than this, fine ferrite of 10 μm or less cannot be obtained, and the fatigue strength of the welded portion decreases. However, if it exceeds 3%, a hard phase is likely to appear in cooling after welding, and the fatigue properties near the heat affected zone of the weld are reduced, so the upper limit is 3%.

P has the effect of increasing the fatigue strength by the solid solution strengthening action, but in the composition of the present invention, if it exceeds 0.07%, the weld becomes brittle and the fatigue properties of the weld deteriorate. Therefore, the upper limit must be 0.07%.

If the amount of S is too large, it forms sulfides and deteriorates the fatigue characteristics of the base metal and the welded portion. Therefore, the smaller the amount, the better.
007% or less is good.

[0018] AL has a role to prevent a decrease in ductility due to aging caused by solid solution N by binding to N in steel. In order to have this effect, 0.0
2% is needed. However, the effect is saturated even if it exceeds 0.3%. Therefore, 0.02-0.3
%.

It is known that N reduces the ductility of steel due to aging, and its amount must be 0.008% or less. However, when the content is less than 0.001%, coarse grains are easily generated in the welded portion, which causes a decrease in the strength of the welded portion. Therefore, 0.001 to 0.0
08%.

It has been found by the present inventors that Cu has a great effect on improving the fatigue strength of a welded portion when it is in a specific state, and is a major feature of the present invention. .

That is, it has been found that 1% or more of Cu is contained in the steel, and by making this into a solid solution state, it has an effect of greatly increasing the fatigue strength of the welded portion of the hot-rolled steel sheet.

Table 1 shows the composition of a steel ingot containing 0.1 to 2.8% of Cu and 0.5 × Cu% of Ni and other elements having chemical components within the range of the present invention. Things.
This was formed into a thin steel sheet of 3.0 mm by hot rolling, and mechanical properties and fatigue properties were investigated. In the hot rolling, the slab was heated at 1200 ° C. and the finishing temperature at 830 ° C., and cooled under the following cooling conditions.

Cooling condition A: 30 ° C. to 750 ° C. after hot rolling
/ S, then cooled to 680 ° C at 20 ° C / s, further cooled to 405 ° C at 80 ° C / s, and wound immediately.

The average ferrite grain size of the obtained hot-rolled steel sheet is 5.5 to 6.2 μm, and the ferrite cross-sectional area ratio is 8
1-87%. The measured values of the tensile strength of these steel sheets with a JIS No. 5 tensile test piece were 620 to 690 MPa.
a.

Cooling condition B: 30 ° C. to 750 ° C. after hot rolling
/ S, then cooled to 405 ° C at 10 ° C / s, and immediately wound up.

The obtained hot-rolled steel sheet had an average ferrite grain size of 11.5 to 12.4 μm and a cross-sectional area ratio of the ferrite of 85 to 90%. The measured values of the tensile strength of these steel sheets with a JIS No. 5 tensile test piece increased with the amount of Cu, and were 740 to 810 MPa. As described above, the reason why the tensile strength of the steel sheet is higher in the cooling condition B than in the cooling condition A is because fine Cu is precipitated in the steel.

[0027]

[Table 1]

These hot-rolled sheets of the same type are butt-welded to each other by flash butt welding, and the burrs at the welded portion are polished with paper, and the welded portion is set at the center of the notch.
A notch interval was set to 20 mm, and a 50R notched flat bending fatigue test piece was processed. Using a flat bending fatigue tester, a fatigue test for repeatedly applying bending stress to the welded portion was performed. In this test evaluation, the fatigue strength was defined as a maximum stress that would not break when subjected to 5 × 10 ⁶ repeated bending stresses. Since the tensile strength of the steel sheet is different, the effect was determined by comparing the value of fatigue strength / tensile strength of the steel sheet. (Hereinafter, the value of fatigue strength / tensile strength of steel sheet is referred to as “fatigue strength ratio”)

FIG. 1 shows the effect of solid solution Cu on the fatigue strength ratio. It can be seen that the alloys containing 1% or more of Cu and cooled under cooling condition A after hot rolling have a large improvement in the fatigue strength ratio. This is because the average particle size of the ferrite is 10 μm or less and Cu is in a solid solution state.

On the other hand, even if Cu is added in an amount of 1.0% or more, the steel cooled under the cooling condition B does not improve the fatigue strength. This is because the average grain size of the ferrite exceeds 10 μm and Cu is not in a solid solution state but in a precipitated state.

When the amount of solute Cu exceeds 2.5%, the fatigue strength can be ensured, but the effect is saturated. Therefore, the amount of solid solution Cu was limited to the range of 1 to 2.5%. In order to further improve the fatigue strength within the range of the present invention, 1.2
It is preferable to use Cu in the range of up to 2.5%.

In the prior art, as disclosed in Japanese Patent Application Laid-Open Nos. 4-110418 and 4-329848, an invention in which Cu is added to a steel sheet containing 0.04% or more of C in steel. Are disclosed, but all of them are intended to ensure strength, and are different from the present invention in which the fatigue strength of the welded portion is improved by making Cu into a solid solution state. 1 to
Even if 2.5% of Cu is contained, if it is not dissolved in steel but precipitates as fine Cu, the tensile strength is improved, but it contributes to the improvement of the fatigue strength of the welded portion. do not do.

The mechanism by which the effect of improving the fatigue strength of the welded portion by making 1 to 2.5% of Cu into a solid solution state in steel as described above is not clearly understood. However, according to the investigations of the present inventors, when Cu is brought into a solid solution state, the time until the initial fatigue crack initiation and the time of the subsequent fatigue crack propagation in the steel sheet subjected to repeated loading are both delayed. Has been confirmed, and this is considered to be the reason for improving the fatigue strength.

Since Ni has a role of preventing cracking in hot rolling by Cu, it is preferable to add Ni as necessary. In order to exert its effect, the total amount of Cu must be 0.4.
It is effective to add at least twice as much, and preferably at least 0.8 times. However, an increase in the amount of addition leads to an increase in cost, and the effect saturates at 1.5 times the total amount of Cu. Therefore, the upper limit is set.

Ca is added as necessary because Ca has the effect of increasing the workability of the steel by spheroidizing the precipitate in the steel. In that case, 0.0005% or more is effective. However, if the content exceeds 0.01%, the effect is saturated and the fatigue strength of the welded portion is also reduced.
It is limited to the range of 005 to 0.01%.

In order to obtain high fatigue strength at the welded portion in the hot-rolled steel sheet having the composition described above, it is necessary to contain a ferrite phase having an average crystal grain size of 10 μm or less in a cross-sectional area ratio of 70% or more. .

When the average crystal grain size of ferrite exceeds 10 μm, the fatigue strength of the base metal and the welded portion is greatly reduced.
The reason is that the generation and propagation of fatigue cracks can be prevented by fine ferrite grains of 10 μm or less.

In the composition of the present invention, when the ferrite phase is reduced, the hard phase such as the pearlite phase is apt to increase, and the crack is easily generated by the repeated load, so that the fatigue strength of the welded portion is reduced. To prevent this, the ferrite phase must have a cross-sectional area ratio of 70% or more. The phase other than the ferrite phase is preferably a residual γ phase or a benite phase, and the smaller the pearlite phase or martensite phase, the better the fatigue strength of the weld.

The average crystal grain size of ferrite as used herein means that after polishing a cross section of a steel sheet and etching it with nital, a plane corresponding to a position of 1/4 of the thickness in the thickness direction from the surface of the steel sheet is scanned. A photograph magnified 1000 times or more was taken with a scanning electron microscope, and the photograph was taken according to JIS G0
It is a value obtained by measuring by the cutting method of 552. The cross-sectional area ratio of ferrite is defined as 10 parts of distilled water.
A solution obtained by mixing a solution obtained by adding 7 g of sodium thiosulfate to 0 ml, a solution obtained by adding 5 g of picric acid to 100 ml of ethyl alcohol, and a solution obtained by adding 4 ml of nitric acid to 95 ml of ethyl alcohol were respectively mixed at a ratio of about 5: 5: 1. Using a polished steel plate, the cross section was corroded, and the surface corresponding to a 1 / 4th of the plate thickness in the thickness direction from the steel plate surface was photographed with an optical microscope at a magnification of 1000 times in color. This value is obtained by performing point counting using a mesh having a grid of 400 points or more at intervals.

Next, a method of manufacturing the steel sheet according to the present invention will be described.

First, the finishing temperature at the time of hot rolling the slab must be in the austenite range, but in order to make the ferrite grains fine, a low temperature is better in that temperature range.
In the composition of the present invention, the average particle size of ferrite is 10 μm.
m or less, the upper limit is 880 ° C. But,
If the temperature is lower than 800 ° C., on the contrary, some coarse ferrite grains are formed, which causes a reduction in fatigue strength. Therefore, it was limited to the range of 800 to 880 ° C.

After the hot rolling, (1) the first cooling was performed at a cooling rate of 30 ° C./s or more in the range of 780 to 720 ° C.
(2) Further, after the first cooling, a second cooling is performed at a cooling rate of 5 to 25 ° C./s in a range of 680 to 630 ° C., and (3) In a range of 450 to 300 ° C. after the second cooling, 50 ° C / s
It is necessary to perform third cooling at the above cooling rate and wind up the steel strip.

In the first cooling of (1), when the temperature exceeds 780 ° C. or the cooling rate is lower than 30 ° C./s,
In the second cooling of (2), when cooling to a temperature exceeding 680 ° C. or when the cooling rate is slower than 7 ° C./s, the ferrite phase having an average particle size of 10 μm or less can be obtained. Therefore, the fatigue characteristics of the weld cannot be secured.

In the first cooling of (1), 72
(3) when cooling to a temperature range below 0 ° C., when cooling to a temperature range below 630 ° C. in the second cooling of (2), and when the cooling rate is faster than 25 ° C./s,
In the third cooling, the temperature exceeding 450 ° C or 300
When cooled to a temperature below ℃, the cooling rate is 50
When the temperature is lower than ° C./s, the ferrite phase cannot have a cross-sectional area ratio of 70% or more in any case, and the fatigue characteristics of the welded portion cannot be secured.

Further, in the cooling of (2), 630 ° C.
When cooled to a temperature lower than, the cooling rate is 7 ° C /
When the cooling speed is lower than 450 ° C., when the cooling rate is lower than 50 ° C./s, the Cu cannot be kept in a solid solution state in the cooling of (3). The fatigue properties of the part cannot be secured.

As described above, it is possible to manufacture a hot-rolled steel sheet having a high fatigue property at the welded portion by limiting the composition of the steel sheet, cooling at a finishing temperature in hot rolling or a specific temperature range, and the like. This is a feature of the invention.

[0047]

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

[0048] Steel ingots having the chemical components shown in Table 2 were produced and thinned by hot rolling. Steel symbols A to H are compositions within the scope of the present invention. Any one or more of the compositions I to P deviate from the scope of the present invention.

The heating temperature of the slab is 1200.degree. C., the finishing temperature is 770 to 880.degree.
Cool at 60 ° C./s (first cooling: cooling speed a), and then
Cool to 650 ° C at 5 to 60 ° C / s (second cooling: cooling speed b), further cool to 405 ° C at 30 to 100 ° C / s (third cooling: cooling speed c), and immediately wind I took it.

The finishing temperature (FT) and cooling rate conditions of the hot rolling are as shown in Table 3. The ferrite grain size, the ferrite area ratio, and the tensile strength of the obtained hot-rolled sheet were measured, and a flash butt welding and a fatigue test of a welded portion were performed in the same manner as in Example 1 to determine the fatigue strength.
Table 3 shows the measured ferrite fraction, ferrite grain size, and fatigue strength ratio.

The contents underlined in Tables 2 and 3 are out of the scope of the present invention.

[0052]

[Table 2]

[0053]

[Table 3]

The TS is targeted at 500 MPa or more, and the fatigue strength ratio of the flash butt welded portion is targeted at 0.35 or more which is superior to the conventional steel sheet at this strength level. Tables 2 and 3 show that the components, the finishing temperature in hot rolling, the cooling rate after finishing, and the cooling temperature range that are all within the range of the present invention have an average ferrite grain size of 10 μm.
m and a metal structure occupying 70% or more in cross-sectional area ratio, and the fatigue strength ratio of these welds is 0.35.
That was all. On the other hand, the fatigue strength ratio of the welded portion of the steel sheet outside the range of the present invention was less than 0.35. From this,
It can be seen that the steel sheet within the range of the present invention has an excellent fatigue strength ratio at the welded portion.

The measurement of the tensile strength of the hot-rolled steel sheet was conducted according to J
A tensile test was performed using a IS-5 tensile test piece according to the method described in JIS Z 2241.

The ferrite grain size number is obtained by etching a steel plate with nital, taking a photograph magnified 1000 times with an optical microscope, and measuring the photograph by a cutting method.

[0057]

As described above, it has been found that by setting the composition and structure of the steel sheet to a specific range, it is possible to obtain a steel sheet having high fatigue properties of a welded portion. Further, it has been found that this steel sheet can be manufactured by setting the hot rolling finish temperature and the cooling conditions after the hot rolling finish to specific ranges.

[Brief description of the drawings]

FIG. 1 is a view showing the influence of Cu on the fatigue strength ratio of a hot-rolled steel sheet.

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 4K037 EA01 EA05 EA06 EA09 EA13 EA15 EA16 EA18 EA20 EA21 EA23 EA25 EA27 EA28 EB05 EB07 EB08 EB09 FC03 FC04 FD02 FD03 FD04 FE01 JA07

Claims (4)

[Claims] 1. A mass ratio of C: 0.04 to 0.16.
%, Si: 0.9-3%, Mn: 0.5-3%, P:
0.07% or less, S: 0.01% or less, AL: 0.02
-0.3%, N: 0.001-0.008%, Cu: 1
２．５2.5%, Ni: 0.4 × Cu to 1.5 × Cu%, the balance being Fe and unavoidable impurities, and the Cu is present in a solid solution state in the steel sheet; Furthermore, the ferrite average crystal grain size of the cross-sectional crystal structure of the steel sheet at a position 1/4 thicker inside the steel sheet than the steel sheet surface is 10 µm or less, the ferrite cross-sectional area ratio is 70% or more, and the tensile strength is 500 MPa or more. High-strength hot-rolled steel sheet with excellent weld fatigue characteristics. 2. A mass ratio of Ca: 0.0005 to 0.5.
The high-strength hot-rolled steel sheet according to claim 1, wherein the high-strength steel sheet contains 0.01%. 3. A mass ratio of C: 0.04 to 0.16.
%, Si: 0.9-3%, Mn: 0.5-3%, P:
0.07% or less, S: 0.01% or less, AL: 0.02
-0.3%, N: 0.001-0.008%, Cu: 1
２．５2.5%, Ni: 0.4 × Cu to 1.5 × Cu%, a steel slab composed of a balance of Fe and unavoidable impurities is finish-rolled by hot rolling at a temperature of 800 to 880 ° C., The obtained steel strip is first cooled at a cooling rate of 30 ° C./s or more to a range of 780 to 720 ° C. after hot-rolling, and after the first cooling, is cooled to a range of 7 to 25 ° C. to a range of 680 to 630 ° C. second cooling at a cooling rate of s, and 450 to 3 after the second cooling.
A method for producing a high-strength hot-rolled steel sheet excellent in fatigue properties of a weld portion, wherein a third cooling is performed at a cooling rate of 50 ° C / s or more to a temperature range of 00 ° C and a steel strip is wound. 4. A steel slab having a mass ratio of Ca: 0.0005.
The method for producing a high-strength hot-rolled steel sheet having excellent weld fatigue properties according to claim 3, characterized in that the content of the hot-rolled steel sheet is 0.01% to 0.01%.