JP3823338B2 - Manufacturing method of high strength hot-rolled steel sheet - Google Patents

Description

【００２５】
次いで、表２及び表３に示す条件でスラブの再加熱，熱間圧延，冷却を順次実施した後、速やかに巻取って 2.6mm厚の熱延鋼板を得た。
更に、常法に従い引続いてこの熱延鋼板に形状修正のためのスキンパスと酸洗とを施した。
【００２６】
【表２】

【００２７】
【表３】

【００２８】
そして、上述のように製造された各熱延鋼板から圧延方向にＪＩＳ５号引張試験片を採取し、その機械的性質を調べた。
また、これとは別に、各熱延鋼板から採取した試験片に１０％クリアランスで打ち抜いた１２φ穴を６０°円錐ポンチで穴拡げするという“穴拡げ試験”も実施した。
これらの結果を前記表２及び表３に併せて示す。
【００２９】
表２及び表３に示される結果からは、次のことを確認できる。
即ち、本発明で規定する条件に従って製造された試験番号１〜７及び試験番号16〜21に係る熱延鋼板は、何れも“優れた強度及び延性”と“１００％を超える穴拡げ率”を有し、また“巻取温度で±２５℃に入る高い冷却安定性（これはコイル内特性変動の少ないことを意味する）”が得られている。
【００３０】
これに対して、空冷開始温度の高い試験番号10と、空冷時間の長い試験番号12と、巻取温度の高い試験番号15に係る熱延鋼板は、粒界での炭化物の析出が多く穴拡げ性が低い。
また、空冷を実施しなかった試験番号９と、空冷開始温度の低い試験番号11に係る熱延鋼板は、何れも伸びが低い。
更に、第３段冷却速度の速い試験番号13と、巻取温度の低い試験番号14に係る熱延鋼板は、スケ−ル厚みの差に起因した冷却の不均一が生じ、コイル内特性バラツキの原因となる巻取温度のバラツキが大きくなっている。
そして、Si含有量の低い試験番号22に係る熱延鋼板は、巻取温度の的中率は良いが穴拡げ性が低い。
【００３１】
【効果の総括】
以上に説明した如く、この発明によれば、高強度と高伸びフランジ性を両立していてア−ムやメンバ−等の自動車足廻り部品用等として好適な熱延鋼板を提供することができる上、その製造に際しては熱間圧延後の冷却過程での温度制御が簡単なために製造歩留が非常に良好であるなど、産業上極めて有用な効果がもたらされる。[0001]
[Industrial application fields]
The present invention provides a hot-rolled steel sheet suitable for use in machine structures such as automobile undercarriage materials that have both excellent stretch flangeability and small fluctuations in characteristics in the coil and require both high strength and workability. It relates to a method of manufacturing well.
[0002]
[Prior art]
The so-called “hot rolled steel sheet” manufactured by continuous hot rolling is widely used as a relatively inexpensive structural material in various industrial equipment including automobiles. There are many members, so it is necessary to achieve both high strength and workability.
In particular, with respect to workability, “stretch flangeability”, which is usually evaluated by a hole expansion test or the like, is attracting attention as one of the important indicators.
[0003]
By the way, it is known that addition of Si is effective for manufacturing a hot-rolled steel sheet having good stretch flangeability. For example, JP-A-3-180426, JP-A-3-219049, and JP-A-4-88125 disclose a method for improving stretch flangeability of a hot-rolled steel sheet by utilizing the cementite refinement effect of Si. It is disclosed.
[0004]
However, when Si is added as described above, the descaling property of the hot-rolled steel sheet deteriorates, the scale having a non-uniform thickness remains on the steel sheet, and the surface texture deteriorates. Due to the non-uniformity of the steel sheet, the cooling of the steel sheet also became non-uniform, and as a result, the fluctuation of the characteristics in the coil became large, causing the yield to deteriorate.
[0005]
However, the above-mentioned JP-A-3-219049 and JP-A-4-88125 disclose that the heating temperature of the steel slab at the time of hot rolling is as low as 1000 to 1200 ° C., which causes the deterioration of descaling properties. Although it is also disclosed to suppress the formation of fairylite, there is a problem that the rolling load (rolling load) becomes large because the rolling is performed at a low temperature in the case of this measure.
[0006]
For this reason, the object of the present invention is to establish a method capable of producing a high-strength hot-rolled steel sheet having excellent stretch flangeability and little fluctuation in characteristics in the coil with high yield. It is.
[0007]
[Means for Solving the Problems]
As described above, when Si is added to the material steel, the descalability deteriorates and a non-uniform thickness scale remains on the steel plate, and the thickness of the scale is reduced during the cooling process after hot rolling. As a result of non-uniform cooling due to the difference, the characteristic fluctuation in the coil increases.
The cause of this non-uniform cooling is that the thermal conductivity of the scale is poor, so that the heat retained in the steel sheet is not quickly transmitted to the scale surface in the part where the scale remains thick. As a result, only the portion where the scale remains thick is rapidly changed from the film boiling cooling to the nucleate boiling region and rapidly cooled.
[0008]
Therefore, as a result of intensive studies on the method for suppressing such non-uniform cooling, the present inventors have conducted most of the cooling of the steel sheet after hot rolling immediately before the cooling process in order to suppress non-uniform cooling. It was found that it is important to perform “cooling mainly in the previous stage” to be performed and to set the winding temperature to 450 ° C. or higher.
That is, the following has been clarified by the study of the present inventors.
If the cooling after hot rolling is “primary cooling”, the amount of water and water pressure in the latter stage of the cooling process can be reduced, so the transition temperature from film boiling cooling to nucleate boiling cooling shifts to lower temperatures. For this reason, even if the unevenness of the scale exists, uniform cooling can be performed. However, if the cooling after hot rolling is carried out to “a temperature range lower than 450 ° C.”, uneven cooling cannot be avoided even if the cooling after the cooling process is reduced.
In addition, when the “cooling of the main part of the previous stage” is performed, the stretch flangeability of the hot-rolled steel sheet obtained because cementite precipitation is suppressed is improved.
[0009]
The present invention has been completed on the basis of the above knowledge and the like, and “C: 0.04 to 0.15% (hereinafter,“% ”representing the component ratio is“% by weight ”),
Si: 0.4-2.0%, Mn: 0.7-2.0%, P: 0.005-0.10%,
sol.Al: 0.004 to 0.10%, S: 0.015% or less, or further
Ca: 0.0002 to 0.01%, Ti: 0.005 to 0.15%, Cr: 0.1 to 1.2%
The steel slab containing at least one of the above and the balance consisting of Fe and inevitable impurities is heated to a temperature exceeding 1200 ° C. and subjected to hot rolling, and the hot rolling is finished at the final pass outlet temperature of 830 to 940 ° C. After that, the first stage cooling to 540 to 640 ° C. at a cooling rate of 50 ° C./s or more and the second stage cooling to perform air cooling for 1 to 5 seconds following this are performed. High-strength hot-rolled steel sheet with excellent stretch flangeability and little fluctuation in coil characteristics is produced by winding after winding the third stage cooling to 450-550 ° C at a cooling rate of -30 ° C / s The point that it was possible to manufacture stably with good quality. ''
Has major features.
[0010]
[Action]
Hereinafter, the reason why the chemical composition of the steel slab and the processing conditions are limited as described above in the present invention will be described.
(A) Chemical composition of billet
a) C
C is a component necessary for ensuring the strength required as a high-tensile steel plate, but if the content is less than 0.04%, it is difficult to ensure the required strength. On the other hand, C increases the precipitation amount of carbide, which is the starting point of cracking during stretch flange molding, so the C content must be suppressed to 0.15% or less in order to ensure excellent stretch flangeability. Therefore, the C content is determined to be 0.04 to 0.15%.
[0011]
b) Si
Si is an important element for achieving both high strength and stretch flangeability in a steel sheet through the action of delaying precipitation of cementite and promoting transformation strengthening. Furthermore, Si is also a solid solution strengthening element and has the effect of strengthening ferrite and improving the stretch flange. And in order to obtain sufficient stretch flangeability, it is necessary to secure a Si content of 0.4% or more. However, if the content exceeds 2.0%, weldability and toughness will be deteriorated. The Si content was determined to be 0.4 to 2.0%.
[0012]
c) Mn
Mn is an essential component for securing the necessary strength as a high-tensile steel sheet and suppressing the precipitation of cementite. For that purpose, it is necessary to contain 0.7% or more, but if it is contained more than 2.0% This is not preferable because it causes deterioration of weldability. Therefore, the Mn content is set to 0.7 to 2.0%, but it is preferable to adjust to 1.0 to 2.0%.
[0013]
d) P
P is a component that contributes to strengthening of the steel sheet through solid solution strengthening. For that purpose, it is effective to secure a content of 0.005 % or more, but if it exceeds 0.10%, workability and toughness are deteriorated. Therefore, the P content is determined to be 0.005 to 0.10%.
[0014]
e) sol.Al
Al is an element added as a deoxidizing material for steel. In order to obtain a sufficient deoxidizing effect, it is necessary to secure a content of 0.004% or more in terms of sol.Al content. Addition leads to the formation of non-metallic inclusions. Therefore, the Al content is determined to be 0.004 to 0.10% in terms of sol.Al content.
[0015]
f) S
Since S is an impurity element that combines with Mn in steel to form non-metallic inclusions, the content is preferably as low as possible. However, if the S content is 0.015% or less, it is possible to ensure the desired characteristics of the present invention. Therefore, the upper limit of the S content is set to 0.015%, but a more preferable range is 0.005% or less. .
[0016]
g) Ca, Ti and Cr
Ca is a component added as necessary because it has the effect of improving the cold workability by adjusting the shape of inclusions, but if its content is less than 0.0002%, the desired effect due to the above effect is obtained. On the other hand, if the content exceeds 0.01%, the amount of inclusions in the steel becomes excessive and the cold workability deteriorates. Therefore, the Ca content is determined to be 0.0002 to 0.01%.
In addition, Ti has the effect of improving the strength of the steel sheet by precipitation strengthening, and Cr is strengthened by transformation strengthening, so all are components added as necessary, but in order to obtain the desired effect by this action Therefore, it is necessary to secure a content of 0.005% or more in the case of Ti and 0.1% or more in the case of Cr. However, in the case of Ti, if the content exceeds 0.15%, the effect is saturated, while in the case of Cr, if it exceeds 1.2%, the weldability deteriorates. Therefore, the content is determined to be 0.005 to 0.15% for Ti and 0.1 to 1.2% for Cr.
[0017]
(B) Treatment conditions In the present invention, the heating temperature of the steel slab at the time of hot rolling exceeds 1200 ° C. This is necessary to reduce the rolling load (especially the load during rough rolling) by reducing the deformation resistance of the steel slab and preventing the segregation by completely dissolving impurities in the steel. It is.
[0018]
Moreover, the finishing temperature of hot rolling is adjusted to 830-940 degreeC. This is because when the finishing temperature is less than 830 ° C. and the amount of reduction in the non-recrystallized region is increased, a remarkable band-like structure is generated and the hole expandability is deteriorated, while the finishing temperature exceeds 940 ° C. In this case, the austenite grain size becomes too large, the amount of ferrite produced decreases, and the ductility decreases.
[0019]
And after hot rolling, the first stage of rapid cooling to the region of 540 to 640 ° C. at a cooling rate of 50 ° C./s or more (preferably 70 ° C./s or more) is continued up to the coiling temperature. This is to reduce the cooling load of the subsequent stage to lower the film boiling-nuclear boiling transition temperature as described above, and to improve the accuracy of the coiling temperature, and in addition, precipitate cementite at the grain boundaries. It is also for suppressing.
[0020]
In this case, if the cooling rate of the first stage cooling is less than 50 ° C./c or the end point temperature of the first stage cooling exceeds 640 ° C., the precipitation of cementite at the grain boundary is not suppressed and the hole expandability is suppressed. Deteriorates. In addition, when the end point temperature of the first stage cooling is higher than 640 ° C., the cooling after the air cooling (second stage cooling) that is subsequently performed to cool down to the winding temperature at which good hole expansibility can be secured. The cooling rate of (third stage cooling) is increased, and the film boiling-nucleate boiling transition temperature shifts to a relatively high temperature side, resulting in uneven cooling due to island scale, resulting in fluctuations in the coil characteristics. Can not escape.
On the other hand, when the end point temperature of the first stage cooling is less than 540 ° C., ferrite is not sufficiently generated and ductility is lowered.
[0021]
In the present invention, the second stage cooling (air cooling) performed following the first stage cooling is a very important process. That is, in the present invention, as the result of increasing the hot rolling finishing temperature in order to suppress the formation of the band structure, the austenite grains become large and it is difficult to generate ferrite. Need to be fully implemented.
The air cooling time is suitably 1 to 5 seconds. If the air cooling time is less than 1 second, the above effect cannot be obtained. On the other hand, if air cooling is performed for more than 5 seconds, cementite precipitates at the crystal grain boundaries and the hole expandability deteriorates. The air cooling time is preferably 1 to 3 seconds.
[0022]
After the second stage cooling (air cooling), “third stage cooling” is performed to cool to a coiling temperature of 450 to 550 ° C. at a cooling rate of 5 to 30 ° C./s (preferably 5 to 20 ° C./s). To be implemented.
In this third stage cooling, if the cooling rate exceeds 30 ° C./s or the cooling end point temperature (winding temperature) falls below 450 ° C., uneven cooling in the coil due to the island-shaped scale will occur. As a result, fluctuations in the characteristics within the coil cannot be avoided and the yield deteriorates. On the other hand, in this case, when the cooling rate is less than 5 ° C./s or the cooling end point temperature (winding temperature) is higher than 550 ° C., precipitation of cementite at grain boundaries occurs and the hole expandability deteriorates. .
In addition, it is necessary to wind up promptly after the end of the third stage cooling.
[0023]
Next, an example explains the present invention.
【Example】
First, steel having the chemical composition shown in Table 1 was melted in a converter and made into a slab by continuous casting.
[0024]
[Table 1]

[0025]
Next, the slab was reheated, hot-rolled, and cooled sequentially under the conditions shown in Tables 2 and 3, and then quickly wound to obtain a 2.6 mm thick hot-rolled steel sheet.
Further, the hot-rolled steel sheet was subsequently subjected to skin pass and shape pickling for shape correction in accordance with a conventional method.
[0026]
[Table 2]

[0027]
[Table 3]

[0028]
And the JIS No. 5 tensile test piece was extract | collected in the rolling direction from each hot-rolled steel plate manufactured as mentioned above, and the mechanical property was investigated.
Separately from this, a “hole expansion test” was carried out in which a 12φ hole punched out with a 10% clearance was expanded on a test piece taken from each hot-rolled steel sheet with a 60 ° conical punch.
These results are also shown in Tables 2 and 3 above.
[0029]
From the results shown in Tables 2 and 3, the following can be confirmed.
That is, the hot-rolled steel sheets according to Test Nos. 1 to 7 and Test Nos. 16 to 21 manufactured according to the conditions specified in the present invention all have “excellent strength and ductility” and “a hole expansion ratio exceeding 100%”. In addition, “high cooling stability within ± 25 ° C. at the coiling temperature (this means that there is little fluctuation in the characteristics in the coil)” is obtained.
[0030]
On the other hand, the hot rolled steel sheets according to test number 10 with a high air cooling start temperature, test number 12 with a long air cooling time, and test number 15 with a high coiling temperature have a large amount of carbide precipitation at the grain boundaries and expand the hole. The nature is low.
In addition, the hot-rolled steel sheet according to test number 9 in which air cooling was not performed and test number 11 in which the air cooling start temperature is low has low elongation.
Furthermore, in the hot-rolled steel sheet according to test number 13 having a high third stage cooling rate and test number 14 having a low coiling temperature, cooling non-uniformity is caused by the difference in scale thickness, and variations in the characteristics in the coil occur. The variation in the coiling temperature that is the cause is large.
And the hot-rolled steel sheet according to test number 22 having a low Si content has a good midpoint ratio of the coiling temperature but has low hole expandability.
[0031]
[Summary of effects]
As described above, according to the present invention, it is possible to provide a hot-rolled steel sheet that has both high strength and high stretch flangeability and is suitable for automobile undercarriage parts such as arms and members. In addition, since the temperature control in the cooling process after the hot rolling is simple in the production, the production yield is very good, and thus an extremely useful effect in the industry is brought about.

Claims (2)

By weight ratio: C: 0.04-0.15%, Si: 0.4-2.0%, Mn: 0.7-2.0%,
P: 0.005 to 0.10%, sol.Al: 0.004 to 0.10%, S: A steel slab containing 0.015% or less and the balance consisting of Fe and inevitable impurities is heated to a temperature exceeding 1200 ° C. and subjected to hot rolling. Then, after finishing the hot rolling at the final pass outlet temperature of 830 to 940 ° C., the first stage cooling to be cooled to 540 to 640 ° C. at a cooling rate of 50 ° C./s or more, followed by 1 to 5 seconds. It is characterized in that it is subjected to second stage cooling that performs air cooling, and then wound after performing third stage cooling that performs cooling to 450 to 550 ° C. at a cooling rate of 5 to 30 ° C./s. A method for producing high-strength hot-rolled steel sheets with excellent stretch flangeability and less fluctuations in the properties of the coil. By weight ratio: C: 0.04-0.15%, Si: 0.4-2.0%, Mn: 0.7-2.0%,
P: 0.005 to 0.10%, sol.Al: 0.004 to 0.10%, S: 0.015% or less, and
Ca: 0.0002 to 0.01%, Ti: 0.005 to 0.15%, Cr: 0.1 to 1.2%
A steel slab comprising at least one of the above and the balance comprising Fe and inevitable impurities was heated to a temperature exceeding 1200 ° C. to perform hot rolling, and the hot rolling was completed at a final pass outlet temperature of 830 to 940 ° C. Thereafter, the first stage cooling for cooling to 540 to 640 ° C. at a cooling rate of 50 ° C./s or more, and the second stage cooling for air cooling for 1 to 5 seconds, followed by 5 to 5 ° C. A high-strength hot-rolled steel sheet with excellent stretch flangeability and little fluctuation in coil characteristics, which is wound after being subjected to third-stage cooling that cools to 450 to 550 ° C. at a cooling rate of 30 ° C./s Manufacturing method.