JP2003003240A - High-strength hot-rolled steel sheet excellent in hole expandability and HAZ fatigue property and method for producing the same - Google Patents

Abstract

(57) [Summary] [Problem] Hole expansion and stretch flangeability to withstand severe workability required for underbody members, and HAZ after arc welding
Provided are a high-strength steel sheet excellent in part fatigue properties and a method for manufacturing the same. SOLUTION: The amount of inclusions contained in the steel and the structure formed by the hot-rolled sheet are composed of ferrite and a second phase mainly composed of bainite. C: 0.03 to 0.08% by weight,
Si: 0.1 to 2%, Mn: 0.5 to 3%, S: 0.0005 to 0.0030%, P: 0.02% or less, N: 0.0035% or less, O: 0.0035% or less, and Mg 0.0005 to 0.02% After adding the remaining Fe and unavoidable impurity elements to a slab by continuous casting, rough rolling is performed immediately after reheating or casting, and Ar
Finishing rolling is completed in a temperature range of ₃ transformation points or more, and the winding temperature is 400 to 550 ° C.
A high-strength hot-rolled steel sheet having excellent Z-part fatigue properties and a method for producing the same.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hot-rolled steel sheet and a method for manufacturing the hot-rolled steel sheet. More specifically, in a high-strength hot-rolled steel sheet used for an automobile underbody member, HAZ after welding.
TECHNICAL FIELD The present invention relates to a high-strength hot-rolled steel sheet excellent in fatigue property of a HAZ portion that imparts particularly high hole expandability without deterioration of fatigue strength in a portion, and a manufacturing method thereof.

[0002]

2. Description of the Related Art Arc welding is mainly used as a welding method for assembling parts for underbody parts. Therefore, the amount of heat input is much larger than that of spot welding, and there is concern that the structure of the HAZ will become coarse. This may cause deterioration of the fatigue characteristics of the welded portion, which is a fatal defect particularly for parts. Further, in order to improve the workability, especially the hole expandability, it is necessary to reduce the C content as much as possible because the second phase fraction is made as low as possible. Particularly, with respect to the former problem, as described in JP-A No. 11-124652, in order to prevent the coarsening of the structure in the HAZ part, Ti-
A method of controlling the dispersion and morphology of Mg-based oxides is disclosed. This is due to the formation of fine oxides in the steel
To prevent the coarsening of the structure due to heat input during welding, GBF (Gr
Ain Boundary Ferrite) and FSP (Ferrite Side Plate) are miniaturized and IGF (Intra Granuler Ferrite) is formed.
As a result, the HAZ toughness is greatly improved. On the other hand, regarding the latter problem, Japanese Patent Laid-Open No.
As disclosed in Japanese Patent No. 293910-90, a method of securing a ferrite content of 90% or more and reducing the fraction of the second phase is disclosed.

[0003]

Particularly, since the welding conditions for the parts for undercarriage of an automobile are about 5 kJ, the heat input amount during welding is not so large, and the effect on the heat input amount less than 30 kJ is disclosed in JP-A-11-. 124652
It is not disclosed in the publication. Furthermore, there is no description regarding fatigue characteristics. In addition, JP-A-6-29391
No. 0 discloses the improvement of workability, especially the burring property by the microstructure, but there is no description of the influence of inclusions, and it must be said that the consideration of the fatigue property of the weld is insufficient. I don't get it.

[0004]

[Means for Solving the Problems] In order to solve the above-mentioned problems, the present inventors have developed a low carbon steel (0.069C-0.2Si-1.1Mn-0.010P).
-0.0015N), the results of a laboratory investigation on the effects of additional elements and winding temperature after finish rolling on the hole expandability and fatigue properties of arc welds were found to be as follows.

(1) The results of investigating the effects of S, O and Mg on the hole expandability are shown in FIGS. 1, 2 and 3. The hot rolling conditions are as follows: heating temperature: 1200 ° C, finishing temperature:
The temperature was 870 ° C and the coiling temperature was 500 ° C. 10mm diameter for hole expansion test
The punching hole of was squeezed out with a 60 ° conical punch with the burr on the outside. At that time, it obtains the ratio of the hole diameter at the time the crack penetrated the plate thickness (d) and an initial hole diameter _{(d 0) (d / d} 0), and an indicator of hole expandability. First, the effect of S is shown in FIG. When the S content is 0.003% or less, d / d ₀ > 2.3 is obtained. This property is also affected by the amount of oxygen in the steel, and as shown in Fig. 2, O
When the amount is 0.0035% or less, d / d ₀ > 2.3. In addition, Mg
With respect to the effect of, as shown in FIG. 3, it was found that when S and O were within the range of the present invention, d / d ₀ was further improved. In order to elucidate the cause of these effects,
Table 1 shows the results of an investigation focusing on the inclusions in the steel sheet. S
As the morphology of inclusions is decreased by decreasing the amounts of O and O, and further by adding Mg, the amount of so-called A-type inclusions is decreased, and the total amount of inclusions is also decreased. As a result, it is considered that the factors that promote the initiation and propagation of cracks were reduced. The inclusions were investigated according to the method described in JIS G 0555. That is, the measurement was performed at a magnification of 400 times using a grid plate having a grid thickness of 5 μm, the number of grid lines being 20 in each of the vertical and horizontal directions, and a grid spacing of 0.4 mm. The number of fields of view is 60, and the cleanliness is S = {n / (f ×
p)} × 100. Here, f is the number of fields of view, p is the total number of lattice points, and n is the lattice point occupied by inclusions.

[0006]

[Table 1]

(2) Among the materials studied in the above (1), the material to which Mg was added was subjected to a heating experiment at 1400 ° C. to investigate the grain growth property. The results are shown in Fig. 4. Mg
It was found that the grain growth property was significantly suppressed by the addition.

Based on the above findings, a high-strength hot-rolled steel sheet having excellent hole expandability during burring and excellent HAZ fatigue characteristics and a method for producing the same were established.

The gist of the present invention is as follows: (1) It consists of ferrite and a second phase mainly composed of bainite, and the total amount of inclusions in the steel sheet is 0.05% or less and the amount of inclusions is A-type + B. A high-strength hot-rolled steel sheet with excellent hole expandability and HAZ fatigue characteristics, which is characterized by containing 0.01% or less of total inclusions. (2) By weight, C: 0.03 to 0.08%, Si: 0.1 to 2%, Mn:
0.5 to 3%, S: 0.0005 to 0.0030%, including Al: 0.005 to 0.1% or less, P: 0.02% or less, N: 0.0035% or less, O: 0.0035% or less, and 0.0005 to 0.02% Mg added However, a high-strength hot-rolled steel sheet which is excellent in hole expansibility and HAZ part fatigue characteristics according to (1), which consists of the balance Fe and unavoidable impurity elements. (3) The hot-rolled steel sheet according to (2) contains 0.2% or less of each of Ti, Nb, and V in an amount of 0.1% or less, and H expandability and H.
High-strength hot-rolled steel sheet with excellent AZ fatigue properties. (4) 0.5% Mo is added to the hot rolled steel sheet according to (2) or (3).
A high-strength hot-rolled steel sheet with excellent hole expandability and HAZ fatigue characteristics. (5) 0.005% of Ca in the hot rolled steel sheet according to (2) to (4)
A high-strength hot-rolled steel sheet with excellent hole expandability and HAZ fatigue characteristics. (6) 0.003% of B in the hot rolled steel sheet according to (2) to (5)
A high-strength hot-rolled steel sheet with excellent hole expandability and HAZ fatigue characteristics. (7) A high-strength hot-rolled steel sheet excellent in hole expandability and HAZ part fatigue characteristics, which has a plating layer on the surface of the hot-rolled steel sheet according to (1) to (6). (8) After slab is formed by continuous casting, rough rolling is performed immediately after reheating or casting, and finish rolling is completed in a temperature range of Ar ₃ transformation point or higher, and the winding temperature is 400 to 550 ° C. Hole expandability and HAZ according to (1) to (6)
A method for producing a high-strength hot-rolled steel sheet having excellent fatigue properties. (9) The hole as described in (8), characterized in that the rough rolling is finished, the sheet bar is once wound around a coil, and is subjected to finish rolling as it is, or is finished rolling after being connected to a preceding sheet bar. A method for manufacturing a high-strength hot-rolled steel sheet having excellent expandability and HAZ part toughness. (10) A high-strength hot-rolled steel sheet excellent in hole expandability and HAZ part fatigue characteristics according to (8) or (9), characterized by performing rough rolling immediately after casting into a slab of 100 mm or less Production method.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION First, regarding the reason for limiting the structure in the present invention, it is necessary to minimize the difference in hardness from ferrite in order to secure the hole expansibility. It is better to form. Furthermore, in order to reduce this variation in hole expandability and raise the minimum value, it is preferable that the number of inclusions formed in the steel is small, and it is particularly necessary to minimize the amount of A- and B-type inclusions. Therefore, the total amount of inclusions formed in the steel sheet should be 0.05% or less, and in particular, the total amount of A-type + B-type inclusions should be 0.01% or less.

Next, the reasons for limiting the component composition in the present invention will be described. If C is less than 0.03%, it becomes difficult to secure the strength, so this is the lower limit. On the other hand, if it is added in excess of 0.08%, not only the hole expandability deteriorates because the amount of carbides formed increases, but also the weldability deteriorates. That is,
There is concern about deterioration of threadability during steel plate manufacturing and weldability during parts assembly.

[0012] Si is one of the elements added to increase the strength of the steel sheet. In order to exert its effect, 0.1% or more must be added. On the other hand, excessive addition causes welding defects in the joints and parts assembly parts during steel sheet manufacturing, so 2%
Is the upper limit.

[0013] Mn is also added when the strength of steel is increased, but excessive addition significantly deteriorates the ductility and the weldability in various welding methods, so the upper limit is 3%.
On the other hand, it is necessary to add 0.5% or more to secure the strength.

S is a type A inclusion (JIS G055
5) is formed and not only the hole expandability but also the ductility is deteriorated, so 0.003% is made the upper limit. Further, if it is lower than 0.0005%, the cost in steelmaking is significantly increased, which is not preferable.

P is an element added mainly for the purpose of increasing the strength. However, if it is contained excessively, not only the ductility is lowered, but also the secondary workability is deteriorated.
02% or less.

Al is an element added for deoxidation.
If it is less than 0.005%, the intended effect is not exhibited, while if it exceeds 0.1%, it remains in the steel as an oxide, so there is concern that the hole expandability will deteriorate significantly or the ductility will decrease.

The N content is preferably as small as possible, but an excessive decrease would significantly increase the cost in steelmaking, so the upper limit is 0.0035%.

O is an element which plays an important role together with Mg in the present invention. If it exceeds 0.0035%, it is feared that a large amount of inclusions will remain as an oxide and the hole expandability will be deteriorated, so this is made the upper limit.

Mg is the most important element in the present invention. That is, forming fine oxides in steel,
In order to reduce the amount of precipitation of A-based inclusions and, as a result, to exert the effect of suppressing the grain growth of austenite, 0.00
05% or more is required. However, even if 0.02% or more is added, not only the effect is saturated, but also the cost is significantly increased, so this is the upper limit.

Ti, Nb and V are added in order to obtain higher strength, and the precipitation strengthening by carbonitride formed with these elements is utilized. The effect can be obtained by adding each element alone or in combination, but excessive addition deteriorates the workability, so 0.2% is the upper limit for one or more elements. 0.00 for each strength improvement effect
It is preferable to add 5% or more.

Mo is an element added to secure the strength, but is mainly added to improve the hardenability. In particular, the purpose is to ensure strength under the cooling conditions in the hot dip galvanizing process, but excessive addition causes deterioration of ductility, so the upper limit is 0.5%. In order to ensure hardenability, it is preferable to add 0.1% or more.

Ca is added for controlling the morphology of MnS formed in steel. In the present invention, since the amount of S is suppressed to a low level, excessive addition will rather cause inclusions to remain in the steel, so the content is made 0.005% or less. In order to keep the steelmaking cost low and to exert the effect, it is preferable to add 0.001% or more.

B is added together with Mn in order to stably form bainite necessary for ensuring strength at the stage of hot rolling. If added in excess of 0.003%, cracks will occur during the slab manufacturing stage, so this is the upper limit. Further, in order to effectively bring out the effect of B, 0.0001% or more is preferably added.

It should be noted that trace amounts of Cu, N due to the use of scrap
The mixing of i, Sn and Cr does not impair the effect of the present invention.

The steel-making process is carried out by, for example, blowing Mg alloy powder into the molten steel as a method for adding Mg, or continuously supplying a wire filled with the Mg-containing alloy powder into the molten steel. There are ways.

Among the hot rolling conditions in the present invention, the finishing temperature and the coiling temperature are important factors for obtaining the desired steel sheet properties. That is, finish rolling needs to be carried out in a temperature range above the Ar ₃ transformation point. If it is carried out at a temperature lower than the transformation point, the structure becomes non-uniform and there is a concern that ductility and hole expandability will deteriorate. On the other hand, the coiling temperature needs to be in the range of 400 to 550 ° C. in order to make the microstructure formed in the steel sheet the second phase mainly composed of bainite from the viewpoint of ensuring hole expandability. If the temperature is higher than 550 ° C, pearlite will be formed, and there is concern that the hole expandability may deteriorate. On the other hand, if it becomes lower, the hardness of bainite itself increases, so that there is a concern that the hole expandability will deteriorate.

In carrying out the above-mentioned hot rolling, even if the preceding sheet bar is joined after the rough rolling by using laser welding or the like and the rolling is carried out, the effect in the present invention is not changed at all.

Further, also in the case of producing a slab, it is not impairing the effect of the present invention to produce a thin slab having a so-called near net shape of 100 mm or less and to immediately produce it under the above-mentioned rolling conditions.

Regarding the plating step, for example, when performing hot dip galvanizing, the surface of the steel sheet is heated to the same temperature as the temperature of the zinc bath and immersed in the zinc bath at that temperature. At that time, as a preferable condition, if the temperature becomes too high, the strength of the hot-rolled sheet is changed due to the change of the structure, so that the upper limit is 550 ° C.
On the other hand, if it is too low, poor plating and non-uniformity of the plating layer thickness are caused, so the lower limit is 420 ° C.

[0030]

Examples Example 1 C: 0.063%, Si: 0.35%, Mn: 1.1%, P: 0.014%, S: 0.002%, Al: 0.0
Steel containing 27%, N: 0.0024%, O: 0.0017%, Mg: 0.0007% was tapped from the converter and continuously cast into a slab. After hot rolling at 1200 ° C., rough rolling was performed, and then hot rolling was completed under the conditions shown in Table 2 to obtain a 4.5 mm hot rolled sheet. Here, the Ar ₃ transformation point is approximately 85 when calculated as 916−50 [C (%)] + 27 [Si (%)] − 64 [Mn].
2 ° C. The size of the oxide containing Mg and the average interparticle distance were 0.07 and 3.5 μm, respectively.

The amount of inclusions formed in the steel was measured by the method described in JIS G 555 described above, and A system: 0%, B system:
It was 0.008% and D system: 0.0025%. Material evaluation is JIS Z
The No. 5 test piece described in 2201 was processed, and a tensile test was performed according to the test method described in JIS Z 2241. In addition, the hole expandability was investigated by punching out a hole with a diameter of 10 mm (d ₀ ), using a 60-degree conical punch to spread the hole so that the burr was on the outside, and when the crack penetrated the plate thickness. Measure the hole diameter (d) of
It was evaluated by / d ₀ . The width of the test piece and the longitudinal direction 3
The samples were collected in rows, and the average value and the minimum value of d / d ₀ were calculated. Further, the steel sheet according to No. 4 was subjected to a plane bending fatigue test by butt welding with a heat input of 3.5 kJ and butt welding.
At this time, as a comparison, welding was similarly performed and fatigue tests were carried out for those to which Mg was not added. The stress range at this time was set to 196 to -196 MPa, and the number of repetitions until fracture was obtained.

When Mg is not added, the HAZ part is fractured, so that the fracture life is 812566 times.
When added, the base material fractures and the fracture life is 305322.
It was extended four times by three times.

Table 2 shows the mechanical properties of the hot rolled sheet obtained. In Nos. 2, 3, 4, 5, 6 and 8 according to the present invention, high elongation and hole expandability are obtained. On the other hand, when the finishing temperature or the winding temperature is out of the range of the present invention, the ductility and the hole expandability are low. In particular, No. 1 and 7 have poor perforability due to the formation of pearlite and No. 9 due to the formation of a large amount of carbide in bainite. Furthermore N
In o.10, since the finishing temperature was low and it came off, a non-uniform structure was formed, resulting in poor ductility and hole expandability (minimum d / d ₀ value).

[0034]

[Table 2]

Example 2 Various steels shown in Table 3 were tapped from a converter and continuously cast into slabs. After hot rolling at 1150 to 1250 ° C., rough rolling and finish rolling were carried out to manufacture hot rolled sheets having plate thicknesses shown in Table 4. The finish rolling was completed in the temperature range above the Ar ₃ transformation point. Further, the amount of water in the cooling zone was adjusted so that the cooling rate after finish rolling was within the range of the present invention. After cooling, winding was performed at 400 to 500 ° C., and material evaluation and hole expandability evaluation by a tensile test were carried out in the same manner as in Example 1. Arc welding was also performed under the same conditions as in Example 1, and the fracture position was investigated as the weldability and the fatigue characteristics of the welded portion.

[0036]

[Table 3]

[0037]

[Table 4]

The results are shown in the same table. A according to the invention,
B, C, E, F, G, H and I steels have high ductility and hole expandability of more than 2.3, good weldability, and no fatigue fracture at HAZ. On the other hand, in the D and M steels in which Mg is out of the range of the present invention, fatigue fracture occurs in the HAZ part, and inclusions of A type and B type are formed beyond the range of the present invention, In particular, M steel has poor S expandability because it also has a high S. Steel J, which has a high C content and has a large amount of carbide, not only has poor hole expandability due to the formation of a large amount of carbide, but also has poor weldability. In addition, in the K and L steels with a high Si or Mn deviation, a large number of A-based and B-based oxides are formed, resulting in variations in hole expansibility and a low minimum value. Also,
Weldability is also poor. Furthermore, due to the large number of defects formed in the welds of J steel and the large number of A- and B-type oxides formed in K and L steels, the fracture at the bead is remarkable as a fatigue property. Becomes On the other hand, N steel has a large amount of O and is out of order, so many oxides are formed on the whole and the hole expandability is poor.

Example 3 With respect to the B and C steels according to the scope of the present invention of Example 2, a production process of immediately sending to a rough rolling process after casting by a thin slab continuous casting method, and a hot rolling process after completion of rough rolling were preceded. It was manufactured by a so-called continuous hot rolling process in which the material is connected and rolled. Table 5 shows the manufacturing process. The finishing temperature, cooling conditions and winding temperature were the same as in Example 2. The obtained materials are shown in the same table. The obtained materials of a, a, u, d, o and mosquito have almost the same characteristics as those of the second embodiment.

[0040]

[Table 5]

[0041]

EFFECTS OF THE INVENTION According to the present invention, it is possible to produce a high strength hot-rolled steel sheet used for automobile underbody members, which has particularly high hole expandability and does not deteriorate the fatigue strength of the HAZ portion after welding. it can.

[Brief description of drawings]

FIG. 1 is a diagram showing a relationship between an S amount and d / d ₀ .

FIG. 2 is a diagram showing the relationship between the O amount and d / d ₀ .

FIG. 3 is a diagram showing a relationship between Mg amount and d / d ₀ .

FIG. 4 is a diagram showing a difference in grain growth behavior of austenite with and without addition of Mg.

─────────────────────────────────────────────────── ─── Continuation of front page (51) Int.Cl. ⁷ Identification code FI theme code (reference) C22C 38/14 C22C 38/14 // B21B 1/46 B21B 1/46 BF term (reference) 4E002 AA04 AD04 BC06 BD02 BD03 CB01 4K037 EA01 EA02 EA05 EA09 EA14 EA15 EA16 EA17 EA18 EA19 EA22 EA23 EA25 EA27 EA28 EA31 EA32 EB05 EB11 EC01 FA02 FA03 FC07 FD04 FE01

Claims (10)

[Claims] 1. A ferrite comprising a ferrite and a second phase mainly composed of bainite, and having a total amount of inclusions of 0.05 in the steel sheet.
% And a total of A-type + B-type inclusions of 0.01% or less, a high-strength hot-rolled steel sheet excellent in hole expandability and HAZ part fatigue characteristics. 2. C: 0.03 to 0.08% and Si: 0.1 to 2 by weight.
%, Mn: 0.5 to 3%, S: 0.0005 to 0.0030%, Al: 0.005 to 0.1
% Or less, P: 0.02% or less, N: 0.0035% or less, O: 0.003
5% or less, and 0.0005 to 0.02% Mg is added, and the balance F
The high-strength hot-rolled steel sheet having excellent hole expandability and HAZ part fatigue characteristics according to claim 1, characterized in that the hot-rolled steel sheet comprises e and unavoidable impurity elements. 3. The hot-rolled steel sheet according to claim 2, which contains 0.2% or less of each of Ti, Nb, and V in an amount of 0.2% or less, and has a high strength hot rolling excellent in HAZ part fatigue property. steel sheet. 4. The hot rolled steel sheet according to claim 2 or 3,
A high-strength hot-rolled steel sheet that contains 0.5% or less of aluminum and has excellent hole expandability and HAZ fatigue characteristics. 5. A hot-rolled steel sheet according to any one of claims 2 to 4, which contains 0.005% or less of Ca and is excellent in hole expandability and HAZ portion fatigue characteristics. 6. A high-strength hot-rolled steel sheet according to any one of claims 2 to 5, which contains 0.003% or less of B and has excellent hole expansibility and HAZ part fatigue characteristics. 7. A high-strength hot-rolled steel sheet having excellent hole expandability and HAZ part fatigue characteristics, which has a plating layer on the surface of the hot-rolled steel sheet according to claim 1. 8. A slab is formed by continuous casting, and then rough rolling is performed immediately after reheating or casting, and finish rolling is completed in a temperature range of the Ar ₃ transformation point or higher, and a winding temperature is 400 to 550 ° C. The method for producing a high-strength hot-rolled steel sheet having excellent hole expansibility and HAZ part fatigue characteristics according to any one of claims 1 to 6. 9. The method according to claim 8, wherein the rough rolling is completed, the sheet bar is once wound around a coil, and the sheet is directly subjected to finish rolling, or the sheet bar is connected to a preceding sheet bar and then finish rolling is performed. Of high strength hot rolled steel sheet with excellent hole expandability and HAZ part toughness. 10. A high-strength hot-rolled steel sheet excellent in hole expandability and HAZ part fatigue characteristics according to claim 8 or 9, characterized by performing rough rolling immediately after casting into a slab of 100 mm or less. Production method.