JPH08325644A - Production of high strength hot rolled steel sheet - Google Patents

Abstract

PURPOSE: To produce a high strength hot rolled steel sheet having excellent stretch-flanging properties and small in the variation in characteristics in a coil at a high yield. CONSTITUTION: A slab having a compsn. contg. 0.04 to 0.l5% C, 0.4 to 2.0% Si, 0.7 to 2.0% Mn, 0.005 to 0.10% P, 0.004 to 0.10% sol.Al and <=0.015% S or furthermore contg. one or more kinds among 0.0002 to 0.01% Ca, 0.005 to 0.15% Ti and 0.1 to 1.2% Cr, and the balance Fe with inevitable impurities is heated to >1200 deg.C and is subjected to hot rolling, and the hot rolling is finished at 830 to 940 deg.C final pass outlet side temp. Next, it is subjected to primary cooling of executing cooling to 540 to 640 deg.C at the cooling rate of >=50 deg.C/s, is successively subjected to secondary cooling of executing air cooling for 1 to 5sec, is continuously subjected to third cooling of executing cooling to 450 to 550 deg.C at a cooling rate of 5 to 30 deg.C/s and is coiled.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention is suitable for a machine structure such as an automobile underbody, which has excellent stretch-flangeability and has little variation in characteristics in the coil, and which requires both high strength and workability. The present invention relates to a method for manufacturing a hot rolled steel sheet with high productivity.

[0002]

2. Description of the Related Art So-called "hot rolled steel sheet" produced by continuous hot rolling is widely used as a relatively inexpensive structural material for various industrial equipment including automobiles.
Many of its applications are members formed by press working, so it is necessary to have both high strength and workability. Especially regarding workability, "stretch-flangeability" which is usually evaluated by a hole expansion test or the like is noted as one of the important indexes.

By the way, it is known that the addition of Si is effective for producing a hot rolled steel sheet having a good stretch flangeability. For example, JP-A-3-180426 and JP-A-3180426
No. 219049 and Japanese Patent Laid-Open No. 4-88125 disclose a method of improving the stretch flangeability of a hot rolled steel sheet by utilizing the cementite refining effect of Si.

However, when Si is added as described above, the descalability of the hot-rolled steel sheet deteriorates, and a scale having a non-uniform thickness remains on the steel sheet to deteriorate the surface quality, as well as the scale. -The cooling of the steel sheet was also non-uniform due to the non-uniformity of the thickness of the steel sheet, resulting in a large variation in the characteristics inside the coil, which was a cause of poor yield.

However, in Japanese Patent Laid-Open No. 3-219049 and Japanese Patent Laid-Open No. 4-88125, the heating temperature of the steel slab during hot rolling is suppressed to a low temperature of 1000 to 1200 ° C. and the descaling property deteriorates. Although it has been disclosed to suppress the generation of fairlight that causes the above-mentioned problem, this method has a problem that the rolling load (rolling load) becomes large because rolling is performed at a low temperature.

In view of the above, an object of the present invention is to provide a method capable of producing a high-strength hot-rolled steel sheet which has excellent stretch-flangeability and has little variation in characteristics in the coil with good yield. To establish.

[0007]

[Means for Solving the Problems] As described above, when Si is added to the material steel, the descaling property is deteriorated and a scale having a non-uniform thickness remains on the steel sheet. As a result of non-uniform cooling due to the difference in scale thickness in the subsequent cooling process, the characteristic variation in the coil becomes large.
The cause of this non-uniform cooling is that the heat retained by the steel sheet is not rapidly transferred to the scale surface in the portion where the scale remains thick due to the poor thermal conductivity of the scale, so only the surface of that portion Will be cooled quickly, and as a result, the scale
This is because only the portion where the thick film remains is rapidly transitioned from the film boiling cooling to the nucleate boiling region and is rapidly cooled.

[0008] Therefore, as a result of intensive studies on the method of suppressing these non-uniform cooling, the present inventors have found that most of the cooling of the steel sheet after hot rolling is performed before the cooling process in order to suppress the non-uniform cooling. It was found that it is important to carry out "cooling mainly in the first stage", which is carried out promptly in step 1, and to set the winding temperature to 450 ° C or higher. That is, the following findings have been made clear by the research conducted by the present inventors. If the cooling after hot rolling is set to "mainly cooling in the first stage", the amount of water and the 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 the low temperature side. Therefore, the scale
Even if the unevenness exists, uniform cooling can be performed. However, cooling after hot rolling is performed in the "temperature range below 450 ° C"
However, even if the cooling in the latter stage of the cooling process is reduced, uneven cooling cannot be avoided. Moreover, when the above-mentioned "mainly cooling in the first stage" is carried out, precipitation of cementite is suppressed, so that the stretch-flangeability of the hot-rolled steel sheet obtained also becomes good.

The present invention has been completed on the basis of the above-mentioned findings and the like. "C: 0.04 to 0.15% (hereinafter,% representing a component ratio shall be% by weight), Si: 0.4 to 2.0% , Mn:
0.7 ~ 2.0%, P: 0.005 ~ 0.10%, sol.Al: 0.004
~ 0.10%, S: 0.015% or less, or more
Ca: 0.0002 to 0.01%, Ti: 0.005 to 0.15%, C
r: A steel slab containing at least one of 0.1 to 1.2% and the balance consisting of Fe and unavoidable impurities is heated to a temperature over 1200 ° C. and hot-rolled, and the final pass outlet temperature 830
After finishing the hot rolling at ~ 940 ° C, the first stage cooling for cooling to 540 ~ 640 ° C at a cooling rate of 50 ° C / s or more, and the second stage cooling for subsequent air cooling for 1 ~ 5 seconds. And then 450 at a cooling rate of 5 to 30 ° C./s.
By applying the third stage cooling, which cools down to ~ 550 ° C, and then winding it up, we have made it possible to stably produce high-strength hot-rolled steel sheets with excellent stretch-flange formability and less variation in coil internal characteristics with good productivity. It has a great feature.

[0010]

The reason why the chemical composition of the steel slab and the treatment conditions are limited as described above in the present invention will be explained below. (A) Chemical composition of billet a) C C is a component necessary to secure the strength required for a high-strength steel sheet, but if the content is less than 0.04%, it becomes difficult to secure the required strength. . On the other hand, C increases the precipitation amount of carbide, which is the starting point of cracking during stretch flange forming, so the C content must be suppressed to 0.15% or less in order to ensure excellent stretch flangeability. Therefore, the C content is set to 0.04 to 0.15%.

B) Si Si is an important element for making the steel sheet have both high strength and stretch flangeability through the action of delaying the precipitation of cementite and promoting transformation strengthening. Further, Si is also a solid solution strengthening element, and has the effect of strengthening ferrite to improve the stretch flange. Then, in order to obtain sufficient stretch flangeability, it is necessary to secure a Si content of 0.4% or more, but if the content exceeds 2.0%, weldability and toughness will be deteriorated, so Si content 0.4 ~
It was set at 2.0%.

C) Mn Mn is an essential component for securing the strength required for a high-strength steel sheet and suppressing the precipitation of cementite,
For that purpose, it is necessary to contain 0.7% or more,
If it exceeds 2.0%, the weldability is deteriorated, which is not preferable. Therefore, the Mn content is set to 0.7 to 2.0%, but it is preferable to adjust it to 1.0 to 2.0%.

D) Pp is a component that contributes to the strengthening of the steel sheet through solid solution strengthening, and it is effective to secure a content of 0.005 or more for that, but if it exceeds 0.10%, workability is improved. Therefore, the P content is set to 0.005 to 0.10% because it causes deterioration of toughness.

E) sol.Al Al is an element added as a deoxidizer for steel, and in order to obtain a sufficient deoxidizing effect, it is necessary to secure a sol.Al content of 0.004% or more. However, excessive addition exceeding 0.10% leads to the formation of non-metallic inclusions. Therefore, change the Al content to
l.Al amount was set to 0.004 to 0.10%.

F) S S is an impurity element that combines with Mn in steel to form a non-metallic inclusion, so its content should be as low as possible. However, if the S content is 0.015% or less, the desired characteristics aimed at by the present invention can be secured, so the upper limit of the S content was set at 0.015%, but a more preferable range is 0.005%.
% Or less.

G) Ca, Ti and Cr Ca have a function of adjusting the shape of the inclusions and improving the cold workability, so they are components added as necessary.
If the content is less than 0.0002%, the desired effect due to the above-mentioned action cannot be obtained, while if it exceeds 0.01%, the inclusions in the steel become too large and the cold workability deteriorates. Therefore, the Ca content was determined to be 0.0002 to 0.01%. In addition, since Ti has the action of improving the strength of the steel sheet by precipitation strengthening and Cr by transformation strengthening, both are components that are also added if necessary, but in order to obtain the desired effect due to this action. Is 0.005% for Ti
It is necessary to secure the above content, and in the case of Cr, a content of 0.1% or more. However, 0.15% for Ti
If it is contained in excess of 1.2%, the effect is saturated, while in the case of Cr, if it exceeds 1.2%, the weldability deteriorates. Therefore, its content was set to 0.005 to 0.15% for Ti and 0.1 to 1.2% for Cr.

(B) Treatment condition In the present invention, the heating temperature of the steel slab during hot rolling is 12
It is considered to be over 00 ° C. This is a measure necessary to prevent the impurities in the steel from completely forming a solid solution and segregate, and to reduce the deformation resistance of the steel slab to reduce the rolling load (especially the load during rough rolling). Is.

The finishing temperature of hot rolling is 830 to 94.
Adjusted to 0 ° C. This is because when the finishing temperature was lower than 830 ° C and the amount of reduction in the unrecrystallized region was large, a remarkable band-like structure was formed and the hole expandability was deteriorated, while the finishing temperature exceeded 940 ° C. In this case, the austenite grain size becomes too large, the amount of ferrite produced decreases, and the ductility decreases.

After the hot rolling, the first stage of rapid cooling to a region of 540 to 640 ° C. at a cooling rate of 50 ° C./s or more (preferably 70 ° C./s or more) is continued in the winding. This is to reduce the load of cooling in the latter stage performed up to the take-up temperature to lower the film boiling-nucleate boiling transition temperature as described above, and to improve the hit rate of the take-up temperature, in addition to the grain boundary of cementite. It is also for suppressing the precipitation of.

In this case, the cooling rate of the first stage cooling is 50 ° C.
If it is less than / c or the end point temperature of the first stage cooling is higher than 640 ° C, precipitation of cementite at grain boundaries cannot be suppressed and the hole expandability deteriorates. Further, when the end point temperature of the first stage cooling is higher than 640 ° C., cooling after the air cooling (second stage cooling) that is subsequently performed is performed to cool to the winding temperature at which good hole expandability can be secured. Since the cooling rate of (third stage cooling) becomes faster and the film boiling-nucleus boiling transition temperature shifts to a relatively high temperature side, non-uniform cooling due to the island-shaped scale occurs and fluctuations in coil characteristics occur. Can't escape. On the other hand, when the end point temperature of the first stage cooling is less than 540 ° C, the ferrite is not sufficiently formed and the ductility decreases.

In the present invention, the second stage cooling (air cooling) performed after the first stage cooling is a very important step. That is, in the present invention, as a result of increasing the finishing temperature of hot rolling in order to suppress the formation of band structure, austenite grains become large and it becomes difficult to generate ferrite. Need to be done sufficiently. An air cooling time of 1 to 5 seconds is suitable, and if the time is less than 1 second, the above effect cannot be obtained. On the other hand, if air cooling is performed for a time of more than 5 seconds, cementite will be precipitated at the grain boundaries and the hole expandability will deteriorate. The air cooling time is preferably 1 to 3 seconds.

After the second stage cooling (air cooling), 5 to 3 are continued.
450 at a cooling rate of 0 ° C / s (preferably 5 to 20 ° C / s)
A "third stage cooling" is performed in which the coiling temperature is cooled to ~ 550 ° C. In this third stage cooling, the cooling rate is 30 ° C.
If it exceeds / s or the cooling end temperature (winding temperature) falls below 450 ° C, uneven cooling inside the coil due to island scale will occur and it will not be possible to avoid fluctuations in the coil characteristics. , Yield deteriorates. On the other hand, in this case, if the cooling rate is less than 5 ° C./s or the cooling end point temperature (winding temperature) exceeds 550 ° C., precipitation of cementite at grain boundaries occurs and the hole expandability deteriorates. . It should be noted that it is necessary to wind the film promptly after the completion of the third stage cooling.

Next, the present invention will be described with reference to examples.

EXAMPLE First, steel having the chemical composition shown in Table 1 was melted in a converter and continuously cast into a slab.

[0024]

[Table 1]

Then, slab reheating, hot rolling and cooling were sequentially carried out under the conditions shown in Tables 2 and 3, and then rapidly rolled to obtain a hot rolled steel sheet having a thickness of 2.6 mm. Further, in accordance with a conventional method, the hot rolled steel sheet was subsequently subjected to skin pass for shape correction and pickling.

[0026]

[Table 2]

[0027]

[Table 3]

Then, JIS No. 5 tensile test pieces were taken in the rolling direction from each of the hot-rolled steel sheets produced as described above, and their mechanical properties were investigated. Separately from this, a "hole expansion test" was also conducted in which a 12φ hole punched 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 shown in Tables 2 and 3 above.
Are also shown.

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 8 and Test Nos. 16 to 21 produced according to the conditions specified in the present invention are "excellent strength and ductility" and "100".
Has a hole expansion rate of more than%, and has a "winding temperature of ± 25
High cooling stability up to ℃ (which means that there is little variation in characteristics inside the coil) ”has been obtained.

On the other hand, in the hot-rolled steel sheets of Test No. 10 having a high air-cooling start temperature, Test No. 12 having a long air-cooling time, and Test No. 15 having a high coiling temperature, precipitation of carbides at grain boundaries was observed. Many holes have low expandability. Further, the hot-rolled steel sheets according to Test No. 9 which was not air-cooled and Test No. 11 having a low air-cooling start temperature both have low elongation. Further, in the hot-rolled steel sheets according to Test No. 13 having a high third-stage cooling rate and Test No. 14 having a low coiling temperature, uneven cooling due to the difference in scale thickness occurs, which causes variations in the coil characteristics. The variation in the winding temperature, which is the cause, is large. The hot-rolled steel sheet according to Test No. 22, which has a low Si content, has a good winding temperature hit rate but low hole expandability.

[0031]

[Summary of Effects] As described above, according to the present invention, there is provided a hot-rolled steel sheet which has both high strength and high stretch flangeability and is suitable for automobile underbody parts such as arms and members. In addition to being provided, the production yield is very good because the temperature control in the cooling process after hot rolling is simple in the production thereof, resulting in extremely useful effects in industry.

Claims (2)

[Claims] 1. A weight ratio of C: 0.04 to 0.15%, Si:
0.4 to 2.0%, Mn: 0.7 to 2.0%, P: 0.005 to 0.10.
%, Sol.Al: 0.004 to 0.10%, S: 0.015% or less, with the balance being Fe and inevitable impurities,
After performing hot rolling by heating to a temperature exceeding 1200 ° C. and finishing hot rolling at a final pass exit side temperature of 830 to 940 ° C., cooling to 540 to 640 ° C. at a cooling rate of 50 ° C./s or more. Third stage cooling is performed, followed by second stage cooling that performs air cooling for 1 to 5 seconds, and then cooling to 450 to 550 ° C at a cooling rate of 5 to 30 ° C / s. A method for producing a high-strength hot-rolled steel sheet, which is excellent in stretch-flange formability and has little variation in characteristics in a coil, which is characterized by performing step cooling and then winding. 2. A weight ratio of C: 0.04 to 0.15%, Si:
0.4 to 2.0%, Mn: 0.7 to 2.0%, P: 0.005 to 0.10.
%, Sol.Al: 0.004 to 0.10%, S: 0.015% or less, Ca: 0.0002 to 0.01%, Ti: 0.005 to 0.15
%, Cr: 0.1 to 1.2% of one or more and the balance consisting of Fe and unavoidable impurities are heated to a temperature over 1200 ° C. and hot-rolled, and the final pass outlet temperature 830 to 940. After finishing the hot rolling at 0 ° C, 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 subsequent air cooling for 1 to 5 seconds are performed. And then 4 at a cooling rate of 5 to 30 ° C / s.
A method for producing a high-strength hot-rolled steel sheet, which is excellent in stretch-flange formability and has little variation in characteristics in a coil, characterized by being subjected to a third-stage cooling for cooling to 50 to 550 ° C and then wound.