JP2734842B2 - High workability hot-rolled high-strength steel sheet and its manufacturing method - Google Patents

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 for processing excellent in ductility and hole expandability suitable as a steel sheet for high-strength members in automobiles, industrial machines, and the like, and a method for producing the same.

[0002]

2. Description of the Related Art Today, a so-called "hot rolled steel sheet" manufactured by continuous hot rolling has been widely applied in various fields as a relatively inexpensive structural material. There are many members that are molded by the above method, and therefore, it is a material that has high requirements for both strength and workability.

[0003] By the way, in the case of steel materials and the like, workability generally decreases as the strength is increased. However, DP steels such as those described in Japanese Patent Application Laid-Open No. 55-44551 are examples of steel materials that achieve both high strength and high ductility. (Dual Phase steel) is known. This DP steel is a two-phase structure steel of [ferrite + martensite], and is characterized by a low yield ratio and high ductility.
At 60 kgf / mm ² ), the elongation (El) is about 30% and the value of “TS × El” is 2
Less than 000, 80kg class high tensile strength (TS: 80kgf / mm ² ), elongation is about 20%, and the value of “TS × El” is less than 2000. It was impossible at present.

On the other hand, as a means for significantly improving the ductility of a conventional high-strength steel sheet, a "method utilizing TRIP (transformation-induced plasticity) of retained austenite" has been developed (for example, see Japanese Patent Application Laid-Open No. H11-157572). See Japanese Patent Application Publication No. 55-145121). According to this method, the tensile strength (TS): 110
kgf / mm ² or more, elongation: 22% or more, and it is considered possible to manufacture a high-ductility and high-strength steel sheet having a value of “TS × El” of more than 2400. However, this method reduces the C content.
Since the steel sheet needs to be adjusted to a relatively high value of 0.40 to 0.85% (hereafter,% representing the component ratio is referred to as weight%), the steel sheet obtained is inferior in terms of weldability, etc., and its application range as a steel sheet for automobiles is narrow. Met.

[0005] As a means for obtaining high a ductility in steel by securing retained austenite under a low C content, C: 0.2
%, Si: 1.5%, Mn: 1.5%
r A method has been proposed in which finish rolling is performed at about _three points, accelerated cooling at a cooling rate of 40 / s or more, and then wound at about 400 ° C. (JP-A-63-4017). However, in this method, although the elongation is greatly improved as compared with the DP steel, the “hole expanding property” which is one of the important indexes of the workability is not improved, and the hole expanding property is 30 times as large as the DP steel sheet. %. Therefore, it could not be used for parts requiring stretch flangeability.

Accordingly, an object of the present invention is to provide a means for stably providing a hot-rolled high-tensile steel sheet for processing having sufficiently excellent strength, ductility, weldability and hole expandability. It was to establish.

[0007]

In order to achieve the above-mentioned object, the present inventors have developed a steel sheet having a sufficiently high strength for automobiles, etc., particularly in the C content range where satisfactory weldability can be obtained. Shown and "TR that leads to excellent ductility and hole expandability
As a result of repeated investigations for the feasibility of a high-ductility hot-rolled high-strength steel sheet containing a sufficient amount of austenite to utilize the IP effect, the following findings were obtained. .

A) Si is an effective element for securing retained austenite in the low C range, but considering the hole expandability,
Even in steels with low Si content, Al
, It is possible to leave austenite, and thereby excellent tensile strength (TS) and elongation (El) balance can be obtained.

B) With the conventional Si-added residual austenitic high-strength steel, a high elongation can be obtained in a tensile test, but in a hole-expansion test, only a hole-expansion rate comparable to that of DP steel can be obtained. In the case of a retained austenitic high-strength steel, unexpectedly high elongation and a high hole expansion ratio can be simultaneously satisfied.

C) In addition, since Al is a ferrite stabilizing element, it was considered that Al addition is disadvantageous in retaining austenite. However, when an appropriate amount of Al is added, polygonal ferrite becomes finer. It is promoted to improve the elongation of the product, and the second phase is uniformly dispersed to be effective in improving the hole expandability.

D) In the Al-added residual austenitic high-strength steel, when an appropriate amount of Si is contained, the strength can be increased without causing significant deterioration of hole expandability. That is, Table 1 shows the results of an investigation of tensile properties and punching hole expandability of a Si + Al-added residual austenitic high-strength steel sheet (2.6 mm thick). The results are shown in comparison with the results for the retained austenitic high strength steel sheet. Here, the tensile property is J
Using IS5 test piece, the hole expansion rate is 2.6mm ×
A hole of 14 mmφ was punched out from a 120 mm × 120 mm steel slab with a clearance of 15%, and then examined using a conical punch. From Table 1, it can be seen that by adding an appropriate amount of Si to the Al-added residual austenitic high-strength steel, the strength can be increased without significantly deteriorating the hole expandability, and the tensile strength (TS ) hole expandability excellent tensile strength balance: 80 kgf / mm ² or more residual O - suggests that it is possible to realize a austenite-type high strength steel.

E) Therefore, C in which weldability is not significantly deteriorated
When the appropriate amount of Al and Si is added to the low Si steel in the content range to achieve a structure mainly composed of polygonal ferrite containing residual austenite with a volume ratio of 5% or more, the strength required for automotive steel sheets, etc. A hot-rolled high-strength steel sheet that fully satisfies various properties such as ductility, hole expandability, etc.

F) Further, the high-ductility hot-rolled high-strength steel sheet having excellent hole expandability can be stably manufactured with good reproducibility by hot rolling a steel slab having a predetermined composition in accordance with specific conditions. That.

The present invention has been completed by conducting further research based on the above findings and the like. "The hot-rolled high-strength steel sheet is made of C: 0.05 to 0.25%, Si: more than 0.05% to 1.0%. Less than , Mn: 0.8-2.5%, sol.Al: 0.8-2.5%, or Ca: 0.0002-0.01%, Zr: 0.01-0.10%, Rare earth element (REM): 0.01-0.10%, Nb: 0.005 to 0.10%, Ti: 0.005 to 0.10%, V: 0.005 to 0.20%, and the remaining component is Fe and unavoidable impurities, and the residual austenite is at least 5% by volume. By having a structure mainly composed of polygonal ferrite containing, excellent workability (ductility,
And hole expansion properties), and “C: 0.05 to 0.25%, Si: more than 0.05% to less than 1.0%, Mn: 0.8 to 2.5%, sol.Al: 0.8 Or Ca: 0.0002-0.01%, Zr: 0.01-0.10%, Rare earth element (REM): 0.01-0.10%, Nb: 0.005-0.10%, Ti: 0.005-0.10%, V: A steel containing at least one of 0.005 to 0.20% and the balance of which is composed of Fe and unavoidable impurities is heated and held at _three or more points of Ac, then hot-rolled, and finished at 780 to 840 ° C. After rolling is completed, 300 ~ at a cooling rate of 10 ~ 50 ° C / s
Accelerated cooling to 450 ° C and winding, or finish rolling at 780 to 940 ° C and accelerated cooling to a temperature range of 600 to 700 ° C at a cooling rate of 10 ° C / s or more, After air cooling for 10 seconds, further accelerated cooling to 300 to 450 ° C. at a cooling rate of 20 ° C./s or more and winding up, the “polygonal ferrite mainly containing residual austenite of 5% or more by volume ratio” Another important feature is that it enables stable production of hot-rolled, high-strength steel sheets having a microstructure with high ductility and excellent hole expandability.

Here, the "polygonal ferrite-based structure" is defined as "substantially polygonal structure" in which the structure other than residual austenite (eg, bainite) does not have a special effect on the properties of polygonal ferrite. Needless to say, it means “structure composed of null ferrite”.

[0016]

Next, the reason why the composition of the steel sheet (slab) and the production conditions thereof are limited as described above in the present invention will be described in detail together with the operation. A) Chemical composition C of steel sheet (slab) In the cooling process after hot rolling, C is concentrated in untransformed austenite with the progress of ferrite transformation to stabilize austenite and strengthen the steel sheet. If the content is less than 0.05%, the effect of securing strength and stabilizing austenite is not sufficient.
When the content of C exceeds 0.25%, the weldability is remarkably deteriorated, and the amount of polygonal ferrite is reduced and the amount of bainite is excessively increased, so that the hole expandability is also deteriorated. Therefore, C
The content was determined to be 0.05 to 0.25%.

Si In general, Si promotes the formation of polygonal ferrite to reduce C
Is an extremely effective element for retaining austenite because it has an effect of concentrating the untransformed austenite and delaying the precipitation of cementite. In the present invention, however, it is mainly polygonal. It is added to strengthen solid solution of ferrite and increase the strength of steel sheet. If the Si content is 0.05% or less, the above-mentioned strengthening effect is insufficient, and the desired strength cannot be secured. On the other hand, even if the Si content is 1.0% or more, the effect of the above-mentioned effect is saturated and hard martensite is hardened. Are generated, and the hole expandability is deteriorated. Therefore,
The Si content was determined to be in the range of more than 0.05% and less than 1.0% .

Mn Mn is an austenite stabilizing element, which reduces the Ms point of untransformed austenite, improves the hardenability, and suppresses the transformation of untransformed austenite into pearlite. Demonstrate. However, if the Mn content is less than 0.8%, the desired effect cannot be obtained by the above-mentioned action, while
%, It becomes difficult to generate sufficient polygonal ferrite in the cooling process after hot rolling, and therefore the concentration of C in the untransformed austenite becomes insufficient and the austenite is stabilized. Can not be converted. Therefore, the content of Mn was determined to be 0.8 to 2.5%.

Sol.Al Al is a particularly important component in the present invention. That is, Al
Like Si, it is a ferrite stabilizing element. It promotes the formation of polygonal ferrite, promotes the concentration of C into untransformed austenite, and promotes the retention of austenite through the action of delaying the precipitation of cementite. Moreover, the effect is more remarkable than when Si is added at the same weight ratio. In addition, it has the effect of promoting the uniform and fine generation of polygonal ferrite and suppressing the generation of coarse bainite, which deteriorates hole expandability. However, the Al content is so
l. If the content of Al is less than 0.8%, the desired effect due to the above-mentioned effect cannot be obtained. On the other hand, even if the content exceeds 2.5%, the effect is saturated and the generation of inclusions is promoted to increase ductility and hole expansion. In the present invention, the sol.Al content is 0.8
It is determined to be 2.5%. In order to stably generate residual austenite, the total amount of “sol.Al + Si” is 1.0%.
It is preferable to contain the above.

Ca, Zr, and rare earth elements (REM) Since each of these components has an effect of adjusting the shape of inclusions to improve cold workability, one or more of them may be used as necessary. However, in the case of Ca, the content is less than 0.0002%, in the case of Zr, the content is less than 0.01%, and in the case of a rare earth element, the content is less than 0.01%. Effect is not obtained, while Ca
If the content is more than 0.01%, the content of Zr is more than 0.10%, and the content of the rare earth element is more than 0.10%, the inclusions in the steel become too large and the workability is deteriorated. Therefore, the Ca content is 0.00
02-0.01%, Zr content was defined as 0.01-0.10%, and rare earth element content was defined as 0.01-0.10%, respectively.

Nb, Ti, and V Nb, Ti, and V are all elements that are precipitated as carbonitrides on ferrite ground and are effective elements for further increasing the strength of the steel sheet. Addition is made, but in any case, if the content is less than 0.005%, the desired effect cannot be obtained. On the other hand, if Nb or Ti exceeds 0.10% each, and if V exceeds 0.20%, the effect is saturated, so that it is not economical. Therefore, the Nb content is
0.005-0.10%, Ti content 0.005-0.10%, and V
The content was determined to be 0.005 to 0.20%.

The "unavoidable impurities" which are inevitably mixed into steel include P, S, Cu, Ni, Cr, Mo, and the like. For example, the contents of P and S are preferably as follows. It is desirable to regulate as follows. Since PP is an impurity element that adversely affects weldability, its content is desirably suppressed to 0.05% or less. However, from the viewpoint of further uniformly dispersing the polygonal ferrite,
More preferably, it is 0.010% or less. Since S is an impurity element that forms sulfide-based inclusions and lowers workability, its content is desirably suppressed to 0.05% or less, but from the viewpoint of securing more excellent workability. More preferably, it is 0.003% or less.

Incidentally, the steel having the above-described composition is melted by, for example, a converter, an electric furnace, or an open hearth furnace. The steel type may be any of rimed steel, capped steel, semi-killed steel and killed steel. Also, the production of steel slabs may be performed by any means of "ingot-bulk rolling" or "continuous casting".

B) Manufacturing conditions of hot-rolled steel sheet The hot-rolled steel sheet of the present invention having a composition mainly composed of polygonal ferrite and having a composition as described above and containing residual austenite in a volume ratio of 5% or more. The steel sheet was heated and maintained at a temperature of at least _three points of the steel slab having the above-mentioned composition, and then hot-rolled to obtain 78%.
After finishing rolling at 0 to 840 ° C, 10 to 50 ° C / s
It can be manufactured by accelerating cooling to 300 to 450 ° C. at a cooling rate of and winding up.

That is, by heating the steel slab to _three or more Ac points, the alloy element can be completely dissolved in austenite. The steel slab to be inserted into the heating furnace may be a slab kept at a high temperature after casting or a slab left at room temperature.

By finishing the finish rolling at 780 to 840 ° C., the austenite can be refined and the work hardening of the austenite can be caused to promote the formation of polygonal ferrite. A sufficient amount of polygonal ferrite can be produced during accelerated cooling at a cooling rate of 10 to 50 ° C./s after rolling. If the finishing temperature is lower than 780 ° C., ferrite is generated during hot rolling and becomes a processed ferrite, and thus the workability of the hot-rolled steel sheet is deteriorated. On the other hand, when the finishing temperature is 840
If the temperature exceeds 100 ° C., the work hardening of austenite becomes insufficient, so that polygonal ferrite is not sufficiently formed, and
Stenite is reduced.

The cooling rate after finish rolling is 10 ° C./s
If it is less than 1, pearlite is generated during cooling, and no austenite remains. On the other hand, if the cooling rate exceeds 50 ° C./s, a sufficient amount of polygonal ferrite is not formed, and no austenite remains.

Further, when the winding temperature exceeds 450 ° C.,
Light is generated and austenite does not remain sufficiently.
On the other hand, when winding is performed in a temperature range lower than 300 ° C., the formation of martensite is promoted, and the ductility and the hole-expandability deteriorate.

Incidentally, the high ductility and high tensile strength hot-rolled steel sheet according to the present invention may be obtained by hot rolling a steel slab having the predetermined component composition at _three or more points of Ac, and then performing hot rolling.
After finish rolling at 0 to 940 ° C., accelerated cooling to a temperature range of 600 to 700 ° C. at a cooling rate of 10 ° C./s or more, and air cooling at that temperature range for 2 to 10 seconds, and then 20 ° C.
It can also be produced by accelerating cooling to 300 to 450 ° C. at a cooling rate of / s or more and winding.

That is, austenite can be refined by finishing the finish rolling at 780 to 940 ° C., and then accelerated cooling to a temperature range of 600 to 700 ° C. and performed at that temperature range for 2 to 10 seconds. By air cooling, the formation of polygonal ferrite is promoted, and C is converted to untransformed austenite.
And a sufficient amount of austenite can be left. In this case, if the air cooling temperature range is less than 600 ° C. or the air cooling time is less than 2 seconds, the formation of polygonal ferrite is insufficient, while the air cooling temperature range is 700 ° C.
℃ and air cooling time exceeds 10 seconds.
Light is generated and no austenite remains.

The purpose of accelerated cooling at a cooling rate of 10 ° C./s or more after finish rolling is to secure an air cooling time of 2 to 10 seconds on a limited hot runtable. Also,
The reason why the accelerated cooling at 20 ° C./s or more after the air cooling is necessary is to suppress generation of pearlite.

Regarding the winding temperature, as in the case of the above-mentioned alternative method, pearlite is generated when the temperature exceeds 450 ° C., and austenite does not sufficiently remain. On the other hand, in the temperature range below 300 ° C. When winding is performed, the formation of martensite is promoted, and ductility and hole-expandability deteriorate.

[0033]

The hot-rolled steel sheet according to the present invention has a weldability of a level required for a steel sheet for high-strength members because the C content is 0.05 to 0.25%, and further uses TRIP. Contains a sufficient amount of austenite. In addition, by adjusting the contents of Al and Si, the hole expandability, which was a weak point of the Si-added residual austenitic steel sheet, is sufficiently improved.
Therefore, according to the present invention, despite the good weldability, the residual austenite is present and the tensile strength (T
A hot-rolled steel sheet having a good balance of S) and elongation (El) and excellent hole expandability can be realized. By subjecting the steel sheet of the present invention to a surface treatment such as hot-dip galvanizing, galvannealing, and electroplating, a surface-treated steel sheet having excellent ductility and hole expanding properties can be obtained.

Next, the effects of the present invention will be described more specifically with reference to examples.

EXAMPLE A slab made of steels A to S having the chemical composition shown in Table 2 was heated, hot rolled, and heated under the conditions shown in Tables 3 and 4.
Controlled cooling and winding were performed to obtain a hot-rolled steel sheet having a thickness of 2.3 mm.
Next, a JIS No. 5 tensile test piece was sampled from the obtained steel sheet, and its mechanical properties were examined. The dimensions are 2.3mm thick x 1
Holes of 14 mmφ were punched out from a steel plate test piece having a width of 20 mm and a length of 120 mm with a clearance of 15%, and the hole expandability of these holes was examined using a conical punch. Further, a test piece for an X-ray test was taken from the center of the steel sheet, and the amount of retained austenite was investigated. The results are shown in Tables 3 and 4.

[0035]

[Table 2]

[0036]

[Table 3]

[0037]

[Table 4]

As is clear from the results shown in Tables 3 and 4, the hot-rolled steel sheet produced according to the present invention has a high tensile strength of 70 kgf / mm ² or more and excellent ductility of 30% or more in elongation. At the same time, excellent punching workability with a hole expansion ratio of 90% or more was exhibited, and it was found that both high strength and high workability were satisfied.

On the other hand, in Test Nos. 20 and 45 using steel type O having a C content exceeding the specified range of the present invention, the elongation is relatively high but the hole expandability is inferior. In Test No. 21 using steel type P having a sol.Al content below the specified range of the present invention,
Both elongation and hole expansion rate are inferior. In Test No. 46 using steel type Q having a C content below the specified range of the present invention, the strength was significantly poor. In Test Nos. 22 and 47 using steel type R whose Mn content is below the specified range of the present invention, or Test Nos. 23 and 48 using steel type S whose sol.Al content exceeds the specified range of the present invention, the elongation and Both holes are inferior in spreadability.

Test numbers 16, 18 and 41 under conditions where the winding temperature is outside the specified range of the present invention, and test numbers whose finishing temperature, cooling rate, air cooling temperature, air cooling time, etc. are out of the range specified by the present invention.
In all of the steel sheets obtained in 17 to 19 and 41 to 44, residual austenite is not sufficiently generated, elongation is reduced, and hole expandability is not good.

[0041]

As described above, according to the present invention, it is possible to stably provide a high workability, high tensile strength hot rolled steel sheet having excellent strength, ductility, hole expandability and weldability.
The present invention can be applied to industrial equipment members such as automobile undercarriage parts and the like to further improve the performance and the life of those products, and provide industrially useful effects.

[Table 1]

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Claims (6)

(57) [Claims] 1. The composition contains, by weight, C: 0.05 to 0.25%, Si: more than 0.05% to less than 1.0%, Mn: 0.8 to 2.5%, and sol.Al: 0.8 to 2.5%, with the balance being Fe and High workability heat excellent in ductility and hole expansion, characterized by having a structure mainly composed of polygonal ferrite containing inevitable impurities and containing 5% or more by volume of retained austenite. Rolled high strength steel sheet. 2. The composition contains, by weight, C: 0.05 to 0.25%, Si: more than 0.05% to less than 1.0%, Mn: 0.8 to 2.5%, sol.Al: 0.8 to 2.5%, and Ca: 0.0002 to 0.01. %, Zr: 0.01 to 0.10%, Rare earth element: 0.01 to 0.10%, and the balance is Fe and unavoidable impurities, and contains residual austenite of 5% or more by volume. A hot-rolled, high-tensile steel sheet with excellent ductility and hole expandability, characterized by having a structure mainly composed of polygonal ferrite. 3. The composition contains, by weight, C: 0.05 to 0.25%, Si: more than 0.05% to less than 1.0%, Mn: 0.8 to 2.5%, sol.Al: 0.8 to 2.5%, and Nb: 0.005 to 0.10. %, Ti: 0.005 to 0.10%, V: 0.005 to 0.20%, and the balance is Fe and unavoidable impurities, and contains residual austenite of 5% or more by volume. High workability hot-rolled high-tensile steel sheet excellent in ductility and hole expandability, characterized by having a structure mainly composed of polygonal ferrite. 4. The composition contains, by weight, C: 0.05 to 0.25%, Si: more than 0.05% to less than 1.0%, Mn: 0.8 to 2.5%, sol.Al: 0.8 to 2.5%, and Ca: 0.0002 to 0.01. %, Zr: 0.01 to 0.10%, Rare earth element: One or more of 0.01 to 0.10%, Nb: 0.005 to 0.10%, Ti: 0.005 to 0.10%, V: 0.005 to 0.20% In addition, the balance is Fe and unavoidable impurities, and has a structure mainly composed of polygonal ferrite containing residual austenite in a volume ratio of 5% or more. Excellent high workability hot-rolled high-strength steel sheet. 5. The steel constituted according to any one of claims 1 to 4 is heated and held at _three or more points of Ac, then hot-rolled, and finish-rolled at 780 to 840 ° C. 5. The method according to claim 1, further comprising: cooling at a cooling rate of 10 to 50 [deg.] C./s to 300 to 450 [deg.] C. and winding. -A method for producing a hot-rolled high-strength steel sheet having a structure mainly composed of polygonal ferrite containing stainless steel and having excellent ductility and hole expandability. 6. Hot rolling is performed after heating and holding the steel having the component composition according to any one of claims 1 to 4 at _three or more points of Ac, and finishing rolling is completed at 780 to 940 ° C. Thereafter, accelerated cooling is performed at a cooling rate of 10 ° C./s or more to a temperature range of 600 to 700 ° C., air-cooled at that temperature range for 2 to 10 seconds, and then 300 to 450 at a cooling rate of 20 ° C./s or more.
2. Winding after accelerated cooling to a temperature of ℃.
5 to 5% or more of residual ozone according to any one of
A method for producing a hot-rolled high-strength steel sheet having high ductility and hole expandability having a structure mainly composed of polygonal ferrite containing stainless steel.