JP2009084643A - Hot rolled steel sheet having excellent fatigue property and stretch flange formability balance - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a hot rolled steel sheet having excellent fatigue properties and an excellent balance in strength-stretch flange formability. <P>SOLUTION: The hot rolled steel sheet having excellent fatigue properties and stretch flange formability has a composition comprising, by mass, 0.01 to 0.10% C, ≤2.0% Si and 0.5 to 2.5% Mn, and further comprising one or more kinds of elements selected from V, Nb, Ti, Mo, Zr and W by 0.01 to 0.30%, respectively, and also, satisfying equations (1) and (2), and the balance Fe with inevitable impurities; wherein a ferrite fraction is >90%, a ferrite grain size is ≤10 μm, a precipitate average grain size r(nm) satisfies equation (3) (X(V-W) is expressed by equation (5)), and the average grain size r and a precipitation fraction f satisfy equation (4). <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a hot-rolled steel sheet exhibiting an excellent strength-stretch flangeability balance and high fatigue characteristics used for parts that require strength, workability, and fatigue characteristics such as automobile undercarriages and frame parts.

In recent years, the strength of automobile parts has been increased, and the strength of automobile undercarriage parts and frame parts has also been increased. However, in order to reduce the weight of parts, it is necessary to improve the fatigue strength as well as the static strength. Has been. Moreover, since it is processed into a complicated shape, compatibility with workability (ductility) is required.
As conventional knowledge about the strength, stretch flangeability and fatigue characteristics of hot-rolled steel sheets, Patent Document 1 describes the strength and elongation of a ferrite structure in which composite precipitates of Ti-Mo or Ti-Nb-Mo are finely dispersed. Patent Document 2 describes that a ferrite main structure in which fine Ti-Mo composite precipitates are dispersed is used to improve strength, fatigue characteristics and processing characteristics. Has been.

JP 2002-322540 A JP 2003-321748 A

In the hot-rolled steel sheets described in Patent Documents 1 and 2, carbides such as Ti, Mo, and Nb are finely dispersed and precipitated in the ferrite structure, and the main phase ferrite is strengthened by precipitation. These finely dispersed precipitates are considered to be obstacles to dislocation repetitive motion and improve fatigue characteristics. However, these hot-rolled steel sheets cannot be said to have a sufficient fatigue property improving effect.
Accordingly, an object of the present invention is to obtain a hot-rolled steel sheet having both excellent stretch flangeability and high fatigue characteristics.

  According to the research of the present inventors, the hot rolled steel sheet has a ferrite main structure, and the ferrite is precipitated and strengthened with carbides such as V, Ti, and Nb, so that the stretch flangeability can be improved while improving the strength. It was found that a high fatigue property improvement effect can be obtained by appropriately controlling the precipitate size (appropriately coarsening). This is because, by appropriately coarsening the precipitate size, the mechanism of dislocation passing through the precipitate changes from the cutting mechanism to the Orowan mechanism, and the precipitate becomes an effective obstacle to the repetitive movement of dislocations during the fatigue test. It is thought to be. Since the size of the precipitate that transitions from the cutting mechanism to the Orowan mechanism varies depending on the type of precipitate (component constituting the precipitate), it is necessary to consider the influence of the steel composition. In the hot-rolled steel sheets of Patent Documents 1 and 2, the precipitates were too fine (a suitable precipitate size according to the steel composition was not obtained), and a sufficient fatigue property improvement effect was not obtained. it is conceivable that.

The hot-rolled steel sheet having excellent fatigue characteristics and stretch flangeability according to the present invention is mass%, C: 0.01% or more, 0.10% or less, Si: 2.0% or less, Mn: 0.5% Including: 2.5% or less, V: 0.01% or more, 0.30% or less, Nb: 0.01% or more, 0.30% or less, Ti: 0.01% or more, 0.30 % Or less, Mo: 0.01% or more, 0.30% or less, Zr: 0.01% or more, 0.30% or less, W: 0.01% or more, 0.30% or less The above is included so as to satisfy the following conditional expressions (1) and (2), the balance is Fe and inevitable impurities, the ferrite fraction is over 90%, the ferrite grain size is 10 μm or less, and the average grain size r of the precipitates (Nm) satisfies the following conditional expression (3), and the average particle diameter r and the precipitate fraction f satisfy the following conditional expression (4): That.
Pc = C-12 × (V / 51 + Nb / 93 + Ti / 48 + Mo / 96 + Zr / 91 + W / 184) (1)
−0.03 ≦ Pc ≦ 0.01 (2)
r ≧ 207 ÷ (27.4X (V) + 23.5X (Nb) + 31.4X (Ti) + 17.6X (Mo) + 25.5X (Zr) + 23.5X (W)) (3)
r / f ≦ 13000 (4)
Here, X (M) (M: V, Nb, Ti, Mo, Zr, W) in the formula (3) is an average atomic weight ratio of elements constituting the precipitate, and is represented by the following general formula (5). The
X (M) = (mass% of M / atomic weight of M) / (V / 51 + Nb / 93 + Ti / 48 + Mo / 96 + Zr / 91 + W / 184) (5)
However, the element symbol in Formula (1), (5) means the mass% of the said element.

  The high-strength hot-rolled steel sheet is, as necessary, further, Cu: 1% or less, Ni: 1% or less, Cr: 1% or less, B: 20 ppm or less, one or more, or / and Al : 0.1% or less and P: 0.1% or less can be included.

According to the present invention, a hot-rolled steel sheet having excellent workability and fatigue characteristics can be obtained. The hot-rolled steel sheet according to the present invention is suitable for manufacturing automobile undercarriages and frame parts that require workability and fatigue characteristics.
In the present invention, workability is evaluated by strength-stretch flange balance (TS × λ), and fatigue characteristics are evaluated by fatigue limit ratio (FL / TS). TS means tensile strength, El means elongation, λ means hole expansion rate (stretch flangeability), and FL means fatigue strength.

First, the composition of the high-strength hot-rolled steel sheet according to the present invention and the reason for limiting the structure will be described.
C: 0.01% or more and 0.10% or less C forms carbide-based precipitates in the ferrite and contributes to precipitation strengthening of the ferrite. However, if it is less than 0.01%, the precipitation strengthening amount is insufficient and the fatigue characteristics are insufficient. If it exceeds 0.10%, the effect of improving the strength and fatigue characteristics by precipitation strengthening is saturated. Accordingly, the C content is as described above. Preferably, it is 0.02% or more and 0.08% or less.

・ Si: 2.0% or less (including 0%)
Si solidifies and strengthens ferrite and contributes to improvement of stretch flangeability and fatigue characteristics. However, Si is an element that promotes ferrite formation. If it exceeds 2.0%, the ferrite transformation temperature becomes high, the ferrite grain size becomes too large, and the stretch flangeability and fatigue characteristics deteriorate. Therefore, the Si content is as described above. Preferably they are 0.2% or more and 1.8% or less.
Mn: 0.5% or more and 2.5% or less Mn increases hardenability, lowers the ferrite transformation temperature, and refines the ferrite grain size. However, if it is less than 0.5%, the ferrite transformation temperature is high, the ferrite grain size becomes coarse, and the stretch flangeability and fatigue characteristics deteriorate. If it exceeds 2.5%, the hardenability becomes high, and the ferrite fraction is lowered by excessively suppressing the ferrite transformation, and the stretch flangeability is lowered. Therefore, the Mn content is as described above. Preferably they are 0.8% or more and 2.0% or less.

・ V: 0.01% or more, 0.30% or less ・ Nb: 0.01% or more, 0.30% or less ・ Ti: 0.01% or more, 0.30% or less ・ Mo: 0.01% or more 0.30% or less ・ Zr: 0.01% or more, 0.30% or less ・ W: 0.01% or more, 0.30% or less Addition of these elements which are carbide forming elements into ferrite Carbide-based precipitates can be dispersed and strengthened by precipitation to increase strength and fatigue characteristics. However, if it is less than the lower limit, the precipitation strengthening amount is insufficient and the fatigue characteristics are insufficient. When the upper limit value is exceeded, precipitation occurs during rolling in the austenite region, the precipitate size becomes coarse, r / f becomes excessive, and fatigue characteristics are insufficient. Therefore, the V, Nb, Ti, Mo, Zr, and W contents are as described above. Preferably, both are 0.03% or more and 0.2% or less. If the total content of these elements is too large, the precipitates are not completely dissolved by heating before hot rolling, and the amount of undissolved coarse precipitates increases and r / f becomes excessive. The preferable total content is 0.55% or less.

Pc = C-12 × (V / 51 + Nb / 93 + Ti / 48 + Mo / 96 + Zr / 91 + W / 184) (Expression (1))
−0.03 ≦ Pc ≦ 0.01 (Formula (2))
Pc (component parameter) means the amount of free carbon or carbide-forming element that is not fixed as carbide. When Pc is positive, free C exists, and when Pc is negative, it means that a free carbide-forming element exists. When Pc exceeds 0.01, the ferrite fraction becomes 90% or less, and the stretch flangeability deteriorates. If Pc is less than −0.03, the presence of free carbide-forming elements causes the precipitates to become coarse, resulting in excessive r / f, resulting in insufficient precipitation strengthening and reduced fatigue characteristics. Therefore, Pc is as described above. Preferably it is the range of -0.02-0.

Cu: 1% or less Ni: 1% or less Cr: 1% or less B: 20 ppm or less These elements increase the hardenability of the steel, lower the ferrite transformation temperature, and refine the ferrite grain size. As a result, fatigue characteristics and stretch flangeability are improved. However, if the content of these elements exceeds the upper limit, ferrite transformation is suppressed and the ferrite fraction is reduced. Accordingly, the Cu, Ni, Cr, and B contents are as described above. Preferably, Cu, Ni, and Cr are all 0.8% or less and B: 15 ppm or less.

・ P: 0.1% or less ・ Al: 0.1% or less P has the effect of solid solution strengthening, but if added too much, it segregates at the grain boundary and lowers the grain boundary strength, thereby reducing stretch flangeability. To do. It can be added up to 0.1%. Preferably, it is 0.03% or less.
Al is a deoxidizing element and also has an effect of fixing N, suppressing the aging effect by N, and suppressing embrittlement. You may add to about 0.1%. Preferably, it is 0.08% or less.

・ Others S, N, and O form nitrides and oxides, which become the starting point of fracture and deteriorate fatigue characteristics and stretch flangeability, so it is better to be lower. S: 0.010% or less, N: 0.00. It is desirable to regulate to 0060% or less and O: 0.00010% or less.
Ca, Mg, and REM contribute to improvement of stretch flangeability and fatigue characteristics by making inclusions finer, and may be added. In the case of addition, 0.01% or less is desirable in all cases.

-Ferrite fraction (area fraction): more than 90% Stretch flangeability can be improved by forming a uniform structure mainly composed of ferrite. When the ferrite fraction decreases and the second phase (bainite, martensite, retained austenite, pearlite) is mixed, stress concentration occurs at the interface between the second phase and the ferrite during deformation, and even small strains cause destruction. Stretch flangeability deteriorates. For this reason, the ferrite fraction is over 90%. The ferrite fraction is preferably 95% or more, more preferably 98% or more.

Average ferrite grain size: 10 μm or less Ferrite grain size has a Hall-Petch relationship (σ∝d ^−1/2 ) for fatigue properties (σ: yield stress, d: crystal grain size). Further, by reducing the ferrite grain size, the stress concentration at the ferrite grain boundary when deformation is applied is reduced and the fracture is suppressed, so that stretch flangeability is improved. For this reason, the average particle diameter of a ferrite shall be 10 micrometers or less. Preferably it is 8 micrometers or less, More preferably, it is 6 micrometers or less.

・ R ≧ 207 ÷ (27.4X (V) + 23.5X (Nb) + 31.4X (Ti) + 17.6X (Mo) + 25.5X (Zr) + 23.5X (W)) (Formula (3))
・ R / f ≦ 13000 (Formula (4))
These two rules mean that the average particle size r (nm) of the precipitates in the ferrite is controlled to a size that is not cut by dislocation, and at the same time, the interparticle distance (r / f) of the precipitates is limited to a small value. To do. f is a precipitate fraction (area fraction) in ferrite. As a result, the mechanism through which dislocations pass through precipitates changes from the cutting mechanism to the Orowan mechanism, and at the same time, the resistance to movement of dislocations during repeated stress application can be increased, and fatigue characteristics can be improved. In conditional expression (4), r / f is preferably 10,000 or less, more preferably 8000 or less.

The right side of the conditional expression (3) represents the minimum particle diameter (critical particle diameter) rc of the precipitate that is not cut by dislocation. This critical particle size is shown in “The Forefront of Precipitation Metallurgy of Steel”, p. 69-80 (Japan Steel Association Material Structure and Property Subcommittee, edited by Precipitation Control Metallurgy Study Group, published by Japan Iron and Steel Institute (2001)), there is an inverse relationship between hardness of precipitates (the above-mentioned literature) (See the graph of FIG. 10). The inventors presume that when the carbide forming element is added alone or in combination, the contribution of each element to the hardness of the precipitate is proportional to the average atomic weight ratio of the element (formula (5)), From the relationship between the critical particle size and the hardness of the precipitate (FIG. 10), the conditional expression (3) was derived approximately. In the conditional expression (3), the coefficient of the average atomic weight ratio X (M) is the Vickers hardness of the carbide of the element M (M: V, Nb, Ti, Mo, Zr, W) (FIG. 9 of the above-mentioned document). Reference).
Conditional expression (3) indicates that there is an appropriate precipitate size (size larger than the critical particle size) according to the steel composition in order for the Orowan mechanism to be expressed as a mechanism for dislocation passing through the precipitate. Show. The fact that this conditional expression (3) has technical significance in improving the fatigue properties of the hot-rolled steel sheet of the present invention has been proved by the examples described later.

Then, the manufacturing method of the high intensity | strength hot-rolled steel plate which concerns on this invention is demonstrated.
A typical manufacturing method includes heating a steel material, hot rolling including finish rolling, rapid cooling after hot rolling, winding after stopping the rapid cooling, and subsequently holding after winding. Hereinafter, each step will be described.
-Heating Heating before hot rolling is performed at 1100 ° C or higher and 1300 ° C or lower. By this heating, an austenite single phase is formed, and V, Nb, Ti, Mo, Zr, and W are dissolved in austenite. When the heating temperature is less than 1100 ° C., V, Nb, Ti, Mo, Zr, and W cannot be dissolved in austenite, and coarse carbides are formed, so that the effect of improving fatigue characteristics cannot be obtained. On the other hand, temperatures exceeding 1300 ° C. are difficult to operate. Desirable heating time is 10 minutes or more and 12 hours or less. If the heating time is short, the precipitate cannot be dissolved, and if the heating time is too long, productivity is hindered.

-Hot rolling Hot rolling is performed so that the finish rolling temperature is in the range of 850 ° C or higher and 1050 ° C or lower. If the finish rolling temperature is too low, carbides precipitate in the austenite during hot rolling and become too coarse, so that the desired r / f cannot be obtained. On the other hand, if it is too high, austenite is coarsened, the ferrite fraction is lowered, and the bainite and martensite fractions are increased, so that the stretch flangeability is lowered.
-Rapid cooling and winding after hot rolling After hot rolling, it is quenched at 20 ° C / s or higher to a temperature range of 650 ° C or higher and 800 ° C or lower, and then wound at 580 ° C or higher and 650 ° C or lower. This is because the ferrite precipitation nose is rapidly cooled to form ferrite.
If the quenching stop temperature is too high, ferrite is not formed, and if it is too low, bainite and martensite are formed, and fatigue characteristics and stretch flangeability are deteriorated.
If the cooling rate is slow, the austenite becomes coarse during cooling, so it is desirable that the cooling rate be fast. However, if it is too fast, it becomes difficult to control, so it is preferably less than 150 ° C / s, more preferably less than 120 ° C / s. And
If the winding temperature is too high, surface oxidation is promoted and the surface properties are deteriorated, which is not preferable. When the coiling temperature is too low, bainite is formed and stretch flangeability is deteriorated.

-Holding after winding After winding, annealing which hold | maintains at 500 degreeC or more and 700 degrees C or less for 1 hour or more is performed. Thereby, the average particle diameter of the precipitate in ferrite is adjusted to an appropriate size. If the temperature is too low or the holding time is short, a suitably coarse precipitate size cannot be obtained. If the temperature is too high or the holding time is long, the precipitate size becomes too coarse, r / f increases, and fatigue characteristics Decreases.

  A 50 kg ingot having the components shown in Table 1 was melted and hot rolled to form a 25 mm thick plate, which was used as a test material. All the test materials had an N content of 40 ppm.

This test material was hot-rolled under the conditions shown in Table 2 to produce a hot-rolled steel sheet. More specifically, after holding at the heating temperature shown in Table 2 for 1 hour, finish rolling was performed at the temperature shown in Table 2, and the finished plate thickness was 3 mm. Immediately after the finish rolling, the steel is cooled to the quenching stop temperature shown in Table 2 at a cooling rate of 30 ° C./s, wound at the temperature shown in Table 2, and subsequently annealed by holding at the temperature shown in Table 2 for 2 hr. It was cooled (part (b) was not annealed and was kept wound).
Samples were collected from the obtained hot-rolled steel sheets and subjected to structure observation, tensile test, fatigue test, and stretch flange characteristic test in the following manner.

-Microstructure observation (ferrite fraction, ferrite particle size)
The structure of the TD surface at the center of the steel plate was observed. The sample was polished to a mirror surface and then corroded with a 3% nital etchant, and observed and photographed at 5 fields with × 400. The white area of equiaxed crystal grains was used as ferrite, and the remaining area as other structures. The ferrite fraction is measured using image analysis software (Image Pro Plus manufactured by Micromedia), and the ferrite particle size is measured using the ferrite particle size measuring method (comparison method) described in JIS G0552, and the following formula is used. The ferrite particle size was determined from (7).
Ferrite grain size = 1 / (8 × 2 ^G ) ^1/2 (mm) (7)
G: Ferrite grain size number

・ Precipitation observation (r, r / f)
The average particle diameter r of the precipitates in the ferrite is determined by extracting the precipitates by the extraction replica method, and observing and photographing the 1 μm × 1 μm region at a magnification of 150,000 with a transmission electron microscope. The observed precipitate (equivalent circle diameter of 2 nm or more) was image-analyzed to determine the area of each particle, and the equivalent circle diameter was determined from the area to calculate the average value, which was defined as the average particle diameter r.
Further, the areas of the precipitates were added, the precipitate fraction (area ratio) f in the ferrite was determined from the precipitate area occupying the observation area, and r / f was calculated from the average particle diameter r and the precipitate fraction f. .

-Tensile test The tensile test processed the sample into the No. 5 test piece of JISZ2201, implemented according to JISZ2241, and measured the tensile strength (TS).
・ Fatigue test
In the fatigue test, the front and back surfaces of the sample were ground by 0.5 mm, and then the fatigue strength (FL) was measured by a plane bending test described in JISZ2275. Further, the fatigue limit ratio (FL / TS) was calculated from the fatigue strength (FL) and the tensile strength (TS).
As the evaluation of fatigue characteristics, the fatigue limit ratio (FL / TS) was evaluated as 0.60 or more as good and 0.65 or more as particularly good.

-Stretch flange characteristic test A hole expansion test was performed as an stretch flange characteristic test, and the hole expansion ratio (λ) was measured. The hole expansion test was performed in accordance with Japan Iron and Steel Federation Standard JFST1001. Further, the strength-elongation flange balance (λ × TS) was calculated from the hole expansion rate (γ) and the tensile strength (TS).
As the evaluation of stretch flange characteristics, the strength-stretch flange balance (λ × TS) was evaluated to be good when it was 50000 or more, and particularly good when it was 60000 or more.

From the measurement results in Tables 3 and 4, test example No. 1, 2, 5-9, 12-15, 17, 18, 20-26, 28, 30 are the composition, ferrite fraction, ferrite grain size, average grain size r of the precipitate, and r as defined in the claims. Each requirement of / f is satisfied, and FL / TS and TS × γ are excellent.
On the other hand, other test examples do not satisfy at least one requirement of the composition, ferrite fraction, ferrite particle size, average particle size r of precipitates, and r / f specified in the claims, and FL / TS and TS × One or both of γ is inferior.

Claims (4)

In mass%, C: 0.01% or more, 0.10% or less, Si: 2.0% or less, Mn: 0.5% or more, 2.5% or less, and V: 0.01% or more 0.30% or less, Nb: 0.01% or more, 0.30% or less, Ti: 0.01% or more, 0.30% or less, Mo: 0.01% or more, 0.30% or less, Zr : 0.01% or more and 0.30% or less, W: 0.01% or more and 0.30% or less including one or two or more so as to satisfy the following conditional expressions (1) and (2), The balance is Fe and inevitable impurities, the ferrite fraction is over 90%, the ferrite particle size is 10 μm or less, the average particle size r (nm) of the precipitate satisfies the following conditional expression (3), A hot-rolled steel excellent in fatigue characteristics and strength-stretch flangeability balance, wherein the precipitate fraction f satisfies the following conditional expression (4) .
Pc = C-12 × (V / 51 + Nb / 93 + Ti / 48 + Mo / 96 + Zr / 91 + W / 184) (1)
−0.03 ≦ Pc ≦ 0.01 (2)
r ≧ 207 ÷ (27.4X (V) + 23.5X (Nb) + 31.4X (Ti) + 17.6X (Mo) + 25.5X (Zr) + 23.5X (W)) (3)
r / f ≦ 13000 (4)
Here, X (M) (M: V, Nb, Ti, Mo, Zr, W) in the formula (3) is an average atomic weight ratio of elements constituting the precipitate, and is represented by the following general formula (5). The
X (M) = (mass% of M / atomic weight of M) / (V / 51 + Nb / 93 + Ti / 48 + Mo / 96 + Zr / 91 + W / 184) (5)
However, the element symbol in Formula (1), (5) means the mass% of the said element. The fatigue characteristics and strength according to claim 1, further comprising one or more of Cu: 1% or less, Ni: 1% or less, Cr: 1% or less, and B: 20ppm or less. -Hot rolled steel sheet with excellent stretch flangeability balance. Furthermore, Al: 0.1% or less is included, The hot-rolled steel sheet excellent in the fatigue characteristics and strength-stretch flangeability balance described in claim 1 or 2. Furthermore, P: 0.1% or less is contained, The hot-rolled steel plate excellent in the fatigue characteristics and strength-stretch flangeability balance as described in any one of Claims 1-3 characterized by the above-mentioned.