JP4737761B2 - High strength hot-rolled steel sheet with excellent strength-elongation balance and fatigue properties - Google Patents

Description

  The present invention relates to a high-strength hot-rolled steel sheet exhibiting excellent strength-elongation balance and high fatigue properties used for parts that require strength, workability, and fatigue characteristics such as undercarriages and frame parts of automobiles.

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 improvement of strength, fatigue characteristics and workability, Patent Document 1 discloses that the average crystal grain size of polygonal ferrite, which is the main phase, gradually decreases from the center of the plate thickness to the surface layer, and fatigue fracture occurs. It is described that the fatigue characteristics of a hot-rolled steel sheet are improved by refining the structure of the surface layer portion to be made. In the production of this hot-rolled steel sheet, the above-mentioned grain size gradient structure is formed, so that repeated bending and unbending by a leveler is indispensable, and there is a problem that large-scale equipment introduction is necessary. In Patent Document 2, the C, Ti, and Mo ratios are controlled, and carbides containing Ti and Mo are dispersed and precipitated in the ferrite single phase structure to improve the strength, fatigue characteristics, and workability of the hot-rolled steel sheet. Are listed. However, since it is a ferrite single phase structure, the strength-elongation balance is insufficient.

  On the other hand, in order to improve the strength-elongation balance, it is effective to use steel (Dual steel, DP steel) having two structures with a large strength ratio. Further, as a method for improving the fatigue characteristics of DP steel, the strength is It is known that it is effective to reinforce a ferrite portion that is low and easily causes stress concentration. For example, Patent Document 3 describes that in a DP steel composed of a main phase ferrite precipitated and strengthened with a carbide of Ti or Nb and a hard second phase, the average ferrite grain size of the surface layer portion up to 20 μm is set to 5 μm or less. Patent Document 4 describes that in DP steel in which the second phase is martensite, acicular ferrite, and retained austenite, precipitation-strengthening of pro-eutectoid ferrite improves strength-workability-fatigue properties. ing.

JP 2004-2111199 A JP 2003-321726 A JP-A-9-137249 JP-A-11-189842

The hot-rolled steel sheets described in Patent Documents 3 and 4 have a retention and residence time of around 700 to 800 ° C., a dispersion of Ti and Nb carbides in ferrite, and precipitation strengthening of main phase ferrite. ing. In this hot-rolled steel sheet, it is considered that precipitates that are finely dispersed and precipitated by holding and staying in the above-mentioned temperature range for a short period of time become obstacles to repeated movement of dislocations and improve fatigue properties. . However, in the prior art, it could not be said that a sufficient fatigue property improving effect was obtained.
Accordingly, the first object of the present invention is to further improve the fatigue characteristics of DP steel having an excellent strength-elongation balance. A second object is to obtain DP steel having excellent stretch flange characteristics.

  According to the researches of the present inventors, when strengthening ferrite with precipitates such as Ti, Nb, and V in DP steel, the retention and residence time in the above temperature range is lengthened, and the precipitates are appropriately coarsened. Thus, it was found that a high effect of improving fatigue characteristics can be obtained. This is because, by appropriately coarsening the precipitates, the mechanism by which dislocations pass through the precipitates is changed from the cutting mechanism to the Orowan mechanism, and the precipitates become an effective obstacle to the repetitive movement of dislocations during fatigue tests. It is considered that the fatigue characteristics are improved.

The high-strength hot-rolled steel sheet excellent in strength-elongation balance and fatigue properties according to the present invention is mass%, C: more than 0.01%, 0.30% or less, Si: 0.1% or more, 2.0 %: Mn: 0.1% or more, 2.5% or less, V: 0.01% or more, 0.15% or less, Nb: 0.02% or more, 0.30% or less, Ti: 0 0.01% or more and 0.15% or less is included so as to satisfy the following conditional expression (1), the balance is made of Fe and inevitable impurities, and the ferrite fraction is 50% or more and 95% or less And a hard second phase fraction composed of martensite + residual austenite having a structure of 5% or more and 50% or less, and the average particle size r of the precipitate formed in the ferrite is 6 nm or more. The precipitate fraction f represented by r and the following formula (2) satisfies the following conditional formula (3). .
C-12 × (V / 51 + Nb / 93 + Ti / 48) ≧ 0.01 (1)
f = (2.08Ti + 1.69V + 1.14Nb) / 100 (2)
r / f ≦ 13000 (3)
Here, the element symbol in the above formulas (1) and (2) means mass% of the element.

In the high-strength hot-rolled steel sheet, it is desirable that the average particle diameter of the hard second phase is 5 μm or less. Thereby, stretch flangeability is improved.
If necessary, the high-strength hot-rolled steel sheet is further Cu: 0.02% or more, 2.0% or less, Ni: 0.02% or more, 2.0% or less, Cr: 0.02% or more, 2.0% or less, Mo: 0.01% or more, 0.5% or less, one or more, or / and P: 0.02% or more, 0.3% or less, Al: 0.02% As mentioned above, 0.3% or less of 1 type or 2 types can be included.

According to the present invention, a high-strength hot-rolled steel sheet having excellent workability and fatigue characteristics can be obtained. The high-strength hot-rolled steel sheet according to the present invention is suitable for manufacturing automobile undercarriages and frame parts that require strength, workability, and fatigue characteristics.
In the present invention, workability is evaluated by strength-elongation balance (TS × E1) and strength-elongation flange balance (TS × λ), and fatigue properties are evaluated by fatigue strength ratio (FL / TS). TS means tensile strength, El means elongation, λ means stretch flangeability (hole spreading ratio), 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: more than 0.01% and 0.30% or less C is a strengthening element, and the ferrite fraction decreases as the C content increases. If it is less than 0.01%, the required strength cannot be obtained, and if it exceeds 0.30%, the fraction of the second phase (martensite + retained austenite) becomes too large, and TS × El balance and TS × λ balance cannot be secured. . Preferably, it is more than 0.040% and 0.20% or less.

Si: 0.1% or more and 2.0% or less Si, as a solid solution strengthening element of ferrite, contributes to improving the TS × El balance and contributes to improving fatigue characteristics. However, if it is less than 0.15%, deoxidation is insufficient and the TS × E1 balance deteriorates, and if it exceeds 2.0%, the ferrite is excessively strengthened, and the TS × El balance and TS × λ balance deteriorate. Preferably they are 0.5% or more and 1.7% or less.
Mn: 0.1% or more, 2.5% or less Mn is added as a deoxidizing element, and contributes to the improvement of the TS × El balance by solid solution strengthening. However, if it is less than 0.1%, deoxidation is insufficient and the TS × E1 balance is deteriorated, and if it exceeds 2.5%, the hardenability becomes too high and the ferrite fraction decreases. Preferably they are 1.0% or more and 2.0% or less.

・ V: 0.01% or more, 0.15% or less ・ Nb: 0.02% or more, 0.30% or less ・ Ti: 0.01% or more, 0.15% or less These elements contain ferrite as carbides Contributing to the improvement of fatigue properties by strengthening precipitation. However, if it is less than the lower limit, the effect of precipitation strengthening is insufficient, and even if the content exceeds the upper limit, the effect of improving the characteristics cannot be obtained. Preferably, V is 0.03% or more and 0.12% or less, Nb is 0.05% or more and 0.25% or less, and Ti is 0.03% or more and 0.12% or less.
・ C-12 × (V / 51 + Nb / 93 + Ti / 48) ≧ 0.01 (Equation (1))
This equation means that the amount of free C not fixed by V, Nb, and Ti remains 0.01% or more. Free C contributes to securing the necessary hard second phase fraction. The calculated value (referred to as component parameter) on the left side is preferably 0.04% or more. In addition, the element symbol in a formula means the mass% of the said element.

Cu: 0.02% or more, 2.0% or less Ni: 0.02% or more, 2.0% or less Cr: 0.02% or more, 2.0% or less Mo: 0.01% or more 0.5% or less These elements have the effect of suppressing the formation of structures other than martensite and retained austenite by enhancing the hardenability of steel, and are added as necessary. However, if it is less than the lower limit, the effect cannot be obtained, and if it exceeds the upper limit, the ferrite becomes brittle and the strength-elongation balance is lowered.

・ P: 0.02% or more, 0.3% or less ・ Al: 0.03% or more, 0.3% or less These elements have the effect of improving the TS-El balance by solid solution strengthening. Added. However, if it is less than the lower limit, the effect cannot be obtained, and if it exceeds the upper limit, it segregates at the grain boundary, promotes grain boundary fracture, and lowers the TS-El balance.

-Ferrite fraction (area fraction): 50% or more, 95% or less-Hard second phase fraction (area fraction): 5% or more, 50% or less Ferrite fraction is less than 50% or hard second phase ( When the fraction of martensite + retained austenite) exceeds 50%, the hard second phase is connected to lower the TS x El balance, while the ferrite fraction exceeds 95% or the hard second phase fraction is If it is less than 5%, the TS × El balance improvement effect due to the multiphase organization cannot be obtained. Preferably, the ferrite fraction is 70% to 93%, and the martensite + retained austenite fraction is 7% to 30%.
The fraction of the structure (bainite, pearlite) other than the main phase ferrite and the hard second phase is preferably 5% or less. This is because the TS × El balance decreases due to the presence of a halfway hard phase.

・ Average particle size r of precipitates formed in ferrite: 6 nm or more ・ r / f ≦ 13000 (Equation (3))
These two rules mean that the average particle size r of the precipitate is controlled to a size that is not cut by dislocation, and at the same time, the interparticle distance (r / f) of the precipitate is limited to a small value. Thereby, the resistance force to the movement of dislocation during repeated stress application can be increased, and the fatigue characteristics can be improved. In addition, f is a precipitate fraction (area fraction), and is represented by the formula (2). Preferably r / f is 11000 or less.

-Average particle size of hard second phase: 5 μm or less By reducing the average particle size of hard second phase composed of martensite and / or retained austenite, local deformation is suppressed and stretch flangeability is improved. . If the average particle diameter of the hard second phase exceeds 5 μm, sufficient stretch flangeability cannot be obtained. When the amount of C and Mn is high and ferrite formation is suppressed, or when the austenite grain is coarsened due to high hot rolling finishing temperature, the average particle size of the hard second layer becomes large, so the average particle size of the hard second phase is 5 μm. In order to make it below, it is desirable that the amounts of C and Mn are not more than the upper limits of the above preferable ranges, and the hot rolling finishing temperature is not more than the upper limits of the preferable ranges 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 is hot rolling including finish rolling after heating a steel material, quenching after hot rolling, holding or staying after quenching stop, cooling after holding or staying, and 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 obtained, and V, Nb, and Ti are dissolved in austenite. When the heating temperature is less than 1100 ° C., V, Nb, and Ti 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.

-Hot rolling Hot rolling is performed so that the finish rolling temperature is in the range of 700 ° C or higher and 1050 ° C or lower. When the finish rolling temperature is less than 700 ° C., the hardenability is lowered, the ferrite transformation and the pearlite transformation are promoted, and the TS × El balance is lowered. On the other hand, when the temperature exceeds 1050 ° C., austenite is coarsened and hardenability is increased, so that a sufficient amount of ferrite cannot be secured. Moreover, the final structure is not refined. Preferably, they are 700 degreeC or more and 850 degrees C or less.
-Rapid cooling after hot rolling Rapid cooling after hot rolling is performed at a temperature range of 650 ° C or higher and 800 ° C or lower at a rate of 20 ° C / s or higher. This is because the ferrite precipitation nose is rapidly cooled to form ferrite. When the quenching stop temperature is less than 650 ° C., pearlite transformation or bainite transformation is promoted, and when it exceeds 800 ° C., ferrite transformation does not occur, and it is difficult to obtain DP steel having a predetermined phase fraction.

-Holding or retention after rapid cooling stop It is desirable that the retention in the temperature range after the rapid cooling stop or the retention in the temperature range (cooling by, for example, air cooling in the temperature range) be performed for a time exceeding 20 s. As a result, the ferrite transformation proceeds and the precipitates in the ferrite are appropriately coarsened. When the holding or staying time in the temperature range is short, the precipitates are not coarsened to a sufficient size and the fatigue characteristics are not sufficiently improved. On the other hand, if the holding or staying time is too long, the precipitates become too coarse and fatigue characteristics are not improved. Although the upper limit varies depending on the precipitate fraction f, it is generally preferable to select it within a range of 300 s or less.

-Cooling and winding after holding or dwelling After holding or dwelling in the above temperature range, in order to make the second phase martensite or retained austenite, it is cooled to 300 ° C or less at a cooling rate of 5 ° C / s or more, Wind up. When the temperature exceeds 300 ° C. or the cooling rate is less than 5 ° C./s, a structure other than martensite or retained austenite is formed, and the TS-El balance is not improved.

  A 50 kg ingot having the components shown in Tables 1 and 2 was melted and hot rolled into a 25 mm thick plate, which was used as a test material. In Tables 1 and 2, the component parameter means the calculated value on the left side of equation (1).

This test material was hot-rolled under the process shown in FIG. 1 and the conditions shown in Table 3 to produce a hot-rolled steel sheet. More specifically, after holding at the heating temperature shown in Table 3 for 30 minutes, finish rolling was performed at the temperature shown in Table 3, and the finished plate thickness was 3 mm. After finish rolling, the steel sheet was cooled to the quenching stop temperature shown in Table 3 at the cooling rate after hot rolling shown in Table 3, and held for the holding time shown in Table 3. Then, it cooled to the coiling | winding simulation temperature shown in Table 3 with the cooling rate after a holding shown in Table 3, and after having held for 30 minutes, it cooled in the furnace.
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.

-Structure observation The structure of the TD surface of the steel plate center part was observed. The sample is polished to a mirror surface and then corroded by a repeller reagent, and 5 fields of view are observed and photographed at × 400. The white area is martensite + retained austenite (hereinafter referred to as residual γ), the black area is other texture, intermediate color Using the image analysis software (Image Pro Plus, manufactured by Micromedia) as the ferrite region, the respective tissue fractions were obtained. Regarding the average particle size of martensite + residual γ, the area of each martensite + residual γ was measured using the above image analysis software, the equivalent circle diameter was calculated from the area, and the average was calculated as martensite + residual γ. Particle size.

-Average particle size r, r / f of precipitates
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 ferrite region with a transmission electron microscope at a magnification of 150,000 × 1 μm × 1 μm. The observed precipitates (equivalent circle diameter of 2 nm or more) were image-analyzed to determine the area of each particle, and the equivalent circle diameter was determined from the area to calculate the average value to obtain the average particle diameter.
Moreover, r / f was calculated from the obtained average particle diameter r and the precipitate fraction f (formula (3)).

-Tensile test The tensile test was carried out according to JISZ2241, after processing the sample into No. 5 test piece described in JISZ2201. Further, the strength-elongation balance (TS × El) was calculated from the tensile strength (TS) and the elongation (El).
・ Fatigue test
In the fatigue test, the front and back surfaces of the sample were ground 0.2 mm each, and then the fatigue strength 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).
-Stretch flange characteristic test A hole widening test was performed as the stretch flange characteristic test, and the hole spread ratio (λ) was measured. The hole expansion test was performed in accordance with Japan Iron and Steel Federation standard JFST1001, and the hole expansion ratio (λ) was measured.
Further, the strength-elongation flange balance (λ × TS) was calculated from the hole expansion rate (γ) and the tensile strength (TS).

  The measurement results are shown in Tables 4-7. In Tables 4 to 7, the tensile strength is evaluated as good at 590 MPa or more, the strength-elongation balance (TS × El) is evaluated as good at 17000 MPa% or more, particularly as good at 18000 MPa% or more, and the fatigue limit ratio (FL / TS ) Evaluated 0.62 or more as good, 0.65 or more as particularly good, and strength-elongation flange balance (λ × TS) as 64,000 MPa% or more as good.

The measurement results in Tables 4 to 7 will be briefly described below.
No. 3,4,17,19,21,25-34,38,43,44, each requirement of composition, phase fraction, precipitate particle size, r / f, martensite + residual γ particle size described in claims Satisfies and satisfies TS × El, FL / TS, TS × γ.
No. 2,10,11,14,23 satisfy each requirement of composition, phase fraction, precipitate particle size, r / f, martensite + residual γ particle size described in the claims, TS × El, FL / TS, Satisfies TS × γ. However, no. No. 2 has a slightly larger r / f and a slightly lower FL / TS. No. 10 has slightly less Si and TS × El slightly lower. No. 11 is slightly more Si, TS × γ is lower, No. 11 No. 14 has a slightly lower Mn and a slightly lower TS × E1. 23 has a slightly lower component parameter, a slightly lower martensite + residual γ fraction, and a slightly lower TS × El.
No. Nos. 7, 15, and 47 satisfy the requirements of the composition, phase fraction, precipitate particle size, and r / f described in the claims, and satisfy TS × El and FL / TS. However, no. No. 7 has a slightly larger martensite + residual γ fraction, the martensite + residual γ particle size exceeds the upper limit, TS × El is slightly lower, TS × λ is lower, No. 7 In Nos. 15 and 47, the martensite + residual γ particle size exceeds the upper limit, and TS × λ is low.

On the other hand, no. 1 does not contain V, Nb, and Ti, and has a low FL / TS.
No. No. 5, C is less than the lower limit value, the component parameter is less than the lower limit value, the martensite + residual γ fraction is less than the lower limit value, and TS × El is low.
No. In No. 8, C exceeds the upper limit, the martensite + residual γ fraction becomes excessive, and TS × El is low.
No. In No. 9, Si is less than the lower limit, and TS × El is low.
No. In No. 12, Si exceeds the upper limit, and TS × El and γ × TS are low.
No. In No. 13, Mn is less than the lower limit, and TS × El is low.
No. In No. 16, Mn exceeds the upper limit value, the martensite + residual γ fraction becomes excessive, and TS × El is low.
No. In 18, 20, and 22, the contents of V, Nb, and Ti are less than the lower limit, r / f exceeds the upper limit, and FL / TS is low.
No. In No. 24, the component parameter is less than the lower limit, the martensite + residual γ fraction is less than the lower limit, and TS × El is low.

No. 6,39,45 has r / f exceeding the upper limit and FL / TS is low.
No. 35-37 and 40-42 have a precipitate particle size of less than the lower limit and a low FL / TS.
No. No. 46 has a ferrite fraction lower than the lower limit, and TS × El is low.
No. In No. 48, the ferrite fraction is less than the lower limit, the martensite + residual γ fraction exceeds the upper limit, and TS × El is low.
No. No. 49 has a ferrite fraction lower than the lower limit, and TS × El is low.
No. No. 50 has a martensite + residual γ fraction less than the lower limit and a low TS × El.

It is a figure explaining the process of an Example.

Claims (3)

In mass%, C: more than 0.01%, 0.30% or less, Si: 0.1% or more, 2.0% or less, Mn: 0.1% or more, 2.5% or less, V: 0.01% or more, 0.15% or less, Nb: 0.02% or more, 0.30% or less, Ti: 0.01% or more, 0.15% or less, or one or more of the following conditional expressions (1) is included so that the balance is Fe and inevitable impurities, the ferrite fraction is 50% or more and 95% or less, and the hard second phase fraction consisting of martensite + retained austenite is 5% or more and 50%. Hereinafter , the structure fraction other than ferrite and hard second phase has a structure of 2.5% or less, the average particle size of hard second phase is 5 μm or less, and the average of precipitates formed in ferrite The particle size r is 6 nm or more, and the average particle size r and the precipitate fraction f represented by the following formula (2) are as follows: Strength and satisfies the equation (3) - a high-strength hot-rolled steel sheet with excellent elongation balance and fatigue characteristics.
C-12 × (V / 51 + Nb / 93 + Ti / 48) ≧ 0.01 (1)
f = (2.08Ti + 1.69V + 1.14Nb) / 100 (2)
r / f ≦ 13000 (3)
However, the element symbols in the above formulas (1) and (2) mean mass% of the elements. Furthermore, Cu: 0.02% or more, 2.0% or less, Ni: 0.02% or more, 2.0% or less, Cr: 0.02% or more, 2.0% or less, Mo: 0.01% or more The high-strength hot-rolled steel sheet having excellent strength-elongation balance and fatigue properties according to claim 1 , comprising one or more of 0.5% or less. Furthermore, P: 0.02% or more, 0.3% or less, Al: 0.02% or more, 0.3% or less 1 type or 2 types are included, It was described in Claim 1 or 2 characterized by the above-mentioned A high-strength hot-rolled steel sheet with excellent strength-elongation balance and fatigue properties.

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