JP3455567B2 - Method for producing high-strength hot-dip galvanized steel sheet with excellent workability - Google Patents

Description

Detailed Description of the Invention

[0001]

The present invention relates to the production of a high-strength hot-dip Zn-plated steel sheet having a tensile strength of more than 440 N / mm ^2, which is suitable for parts such as automobile underbody parts which require ductility and stretch flangeability. Regarding the method.

[0002]

2. Description of the Related Art In recent years, research and development have been carried out to reduce the weight of a vehicle body in order to improve fuel efficiency. With regard to steel materials as well, they have the same press formability as conventional products and exhibit excellent strength. Material is required. As materials that meet this requirement, dual-face steel sheets having a ferrite + martensite structure and high-strength steel sheets using retained austenite have been proposed. Dual face steel sheets have been introduced in, for example, Japanese Patent Publication No. 57-42127, Japanese Patent Publication No. 61-10009, Japanese Patent Publication No. 61-11291, and Japanese Patent Publication No. 57-143433. Many studies have been reported on high-strength steel sheets using retained austenite, and various methods for obtaining steel materials having an excellent strength-ductility balance have been proposed (Japanese Patent Laid-Open Nos. 62-196336 and 62-196336). 63-4017, JP-A-1-79345, etc.).

[0003]

In the conventional dual face steel sheet, since the low temperature transformation phase martensite is used, softening of the heat affected zone is inevitable when welding is performed. In addition, since the structure has a mixture of soft ferrite and hard martensite, when stretch-flange processing, stress concentrates at the phase interface and becomes the starting point of cracking, resulting in defects such as cracks and fractures in the processed product. It is easy to occur. That is, the dual face steel sheet is a material having poor stretch-flangeability, and it is in the current situation that a product having a target shape cannot be produced with a high yield. In order to obtain a desired retained austenite in a high-strength steel sheet using the retained austenite, it is necessary to strictly control cooling conditions after rolling, winding temperature, and the like. Therefore, the steel plate manufacturing process becomes complicated, and many problems remain in the manufacturing stability and material stability in the hot rolling line.

By the way, as materials are made thinner in order to reduce the weight of the vehicle body, requirements for rust prevention and corrosion prevention represented by perforation resistance and the like have also become strict. In this connection,
A method for producing a high-strength hot-dip galvanized steel sheet with improved corrosion resistance is disclosed in JP-A-63-72860 and JP-A-63-1.
It is introduced in Japanese Patent Publication No. 49321. However, none of them has been improved in stretch flangeability. Japanese Patent Laid-Open No. Hei 4
-280925 discloses a high-strength hot-dip galvanized steel sheet with improved stretch flangeability. But,
Since it is a steel sheet containing a large amount of Si, pretreatment such as flash plating is required, which limits the plating method. Therefore, it cannot be said that the high-strength hot-dip galvanized steel sheet has good manufacturability. The present invention has been devised to solve such a problem, and pearlite or cementite is finely dispersed in a fine ferrite matrix before hot-dip Zn plating by controlling the alloy design and hot rolling conditions. It is an object of the present invention to produce a high-strength hot-dip Zn-plated steel sheet having a fine structure, excellent workability such as ductility and stretch-flangeability and weldability, and high material stability.

[0005]

In order to achieve the object, the production method of the present invention is C: 0.02 to 0.10 wt%, Si: 0.2 wt% or less, Mn: 1.0 to. 2.0% by weight, P: 0.02 to 0.10% by weight, Nb: 0.01
A steel slab having a composition of 0.03 wt% to S: 0.005 wt% or less, and the balance being substantially Fe is used as a raw material. This steel slab further has Ti: 0.01 to 0.0
It can also include 5% by weight. 1000-8 steel slabs
After hot rolling at a finish rolling temperature of 50 ° C., the temperature region up to 600 ° C. is cooled at an average cooling rate of 40 ° C./sec or more, and the temperature region at 600 ° C. or less at an average cooling rate of 30 ° C./sec or less. It is cooled and wound in a temperature range of 550 to 400 ° C., and then hot-dip Zn plating is applied.

[0006]

The hot-rolled steel sheet used in the present invention has a structure in which pearlite or cementite is finely dispersed in a fine ferrite matrix before hot-dip galvanizing. In this structure, pearlite, which is relatively soft as compared to bainite and martensite, is finely dispersed to improve stretch flangeability. The term "ferrite" as used in the present specification is used to include polygonal ferrite and acicular ferrite. The structure in which pearlite or cementite is finely dispersed in a fine ferrite matrix is a structure in which ferrite having a small particle size is dispersed in the matrix, and as a result, pearlite or cementite generated after ferrite transformation is fine and dispersed. Among them, since pearlite is in a dispersed state without being continuously formed in a band shape, a steel material exhibiting good stretch flangeability can be obtained.

Such a structure defines the composition of the alloy, particularly the content of Nb effective for forming fine ferrite,
In addition, it is formed by performing cooling and winding after hot rolling under a predetermined temperature condition. Further, by defining the content of components such as Si, it becomes possible to perform hot dip Zn plating according to a conventional method without requiring pretreatment such as flash plating. Hereinafter, various conditions specified in the present invention will be described. C: 0.02 to 0.10% by weight It is an alloying element effective for securing strength and has a tensile strength of 4
To obtain the target strength of 40 N / mm ² or more, 0.02% by weight or more is necessary. However, if a large amount of C exceeding 0.10% by weight is contained, the formability of the steel material is significantly deteriorated.

Si: 0.2% by weight or less Concentrates easily on the surface layer of the steel sheet and significantly deteriorates the wettability to molten Zn. In order to form a hot-dip Zn plated layer having good adhesion, it is necessary to suppress the Si content to 0.2% by weight or less. Mn: 1.0 to 2.0 wt% To secure the strength, 1.0 wt% or more is required to be added. However, when a large amount of Mn is contained in excess of 2.0% by weight, center segregation is promoted in the slab and a band structure is formed. As a result, the stretch flangeability of the plate material deteriorates. Also, the weldability is reduced. P: 0.02 to 0.10% by weight It is a solid solution strengthening element and effectively contributes to the improvement of strength. To ensure the required strength, 0.02% by weight or more of P
Is included. However, if the P content exceeds 0.10% by weight, center segregation in the slab is promoted, and the toughness tends to deteriorate.

Nb: 0.01 to 0.03% by weight It is an alloying element which works as a precipitation strengthening element and is effective in improving strength. Further, it suppresses recrystallization of austenite during rolling, refines ferrite grains, and facilitates the production of polygonal ferrite effective for improving ductility and stretch flangeability. To secure these effects, 0.01
It is necessary to contain Nb in an amount of at least wt%. But 0.03
When a large amount of Nb is contained in excess of weight%, the strength is excessively increased due to precipitation strengthening, and the ductility is remarkably reduced. S: 0.005% by weight or less It is a harmful element that forms MnS and significantly deteriorates stretch flangeability. Therefore, in the present invention, the S content is restricted to 0.005% by weight or less, preferably 0.003% by weight or less.

Ti: 0.01 to 0.05% by weight It is an alloying element added as necessary in the steel material of the present invention. Ti combines with S to form TiS and suppresses the generation of MnS which deteriorates stretch flangeability. In this respect, addition of 0.01% by weight or more of Ti is extremely effective in improving stretch flangeability. However, the property improvement due to the addition of Ti is saturated at 0.05% by weight, and even if it is contained more than that, the economical efficiency at the time of steel production is adversely affected. Finishing temperature: 1000 to 850 ° C. Hot rolling is performed so that the finishing temperature is in the temperature range of 1000 to 850 ° C. At a finishing temperature of more than 1000 ° C., recrystallization of austenite proceeds during hot rolling, and it becomes difficult to obtain ferrite stably after cooling. As a result, the ductility of the obtained hot rolled sheet deteriorates. On the other hand, at a finishing temperature lower than 850 ° C., in the case of the steel containing Nb as in the present invention, the rolling is performed in the non-recrystallized state, so that the deformation resistance increases and the stripability deteriorates significantly. In addition, it causes deterioration of plate thickness accuracy and increase of electric power consumption.

Cooling rate: 40 ° C./sec or more at 600 ° C. or more, 30 ° C./sec or less at 600 ° C. or less, cooling after finish rolling is a target fine ferrite +
This is an important manufacturing condition for incorporating a pearlite or cementite structure into a hot rolled sheet. 600 ℃ after finish rolling
In the temperature range up to, while suppressing transformation of ferrite and pearlite, pearlite harmful to stretch-flange formability is finely dispersed. Therefore, this temperature range is 40
Cool at a cooling rate of not less than ° C / sec. On the other hand, in the temperature range of 600 ° C. or lower, the cooling rate is 30 ° C. to generate fine ferrite while suppressing the bainite transformation.
/ Sec or less.

Winding temperature: 550 to 400 ° C. From a number of experiments, the inventors of the present invention showed that when the winding temperature of the hot-rolled sheet was set to 550 ° C. or less, the formation of coarse pearlite was suppressed, and the ferrite was sufficiently formed. It was found that it would be miniaturized. However, at a winding temperature lower than 400 ° C., bainite is likely to be formed, which causes deterioration of ductility. When the hot-rolled sheet thus obtained is introduced into a hot-dip Zn plating tank, a Zn-plated layer is formed on the surface of the steel sheet with good adhesion, and high-strength hot-dip Zn excellent in corrosion resistance, workability, weldability, etc. A plated steel sheet is obtained.

[0013]

[Examples] Steels having the compositions shown in Table 1 were hot-rolled and then hot-dip Zn-plated to obtain hot-dip Zn-plated steel sheets with a plate thickness of 3 mm. The steel materials of Group A in Table 1 are materials that satisfy the requirements for the components specified in the present invention.
Further, the steel materials of Group B are materials whose components are out of the scope of the present invention. For the hot rolling, the conditions shown in Table 2 were adopted.

[0014]

[Table 1]

[0015]

[Table 2]

A test piece was cut out from the obtained hot-dip galvanized steel sheet, and a strength test was carried out with a JIS No. 5 tensile tester. Further, a punched hole was formed in the test piece with an initial hole diameter d ₀ of 10 mm, and the punched hole was expanded to the hole diameter d ₁ by a conical punch having an apex angle of 60 degrees, and the limit hole expansion that did not cause cracks or fractures in the test piece The stretch-flangeability was investigated by a hole expansion test measuring the ratio λ% [= (d ₁ −d ₀ ) / d ₀ × 100].

[0017]

[Table 3]

As is clear from Table 3 showing the test results,
The comparative examples of test numbers 9 to 12 were inferior in any of the material strength, ductility, and hole expandability. In addition, steel B having a high Si content
In Nos. 1 and B2, non-plating occurred. On the other hand, the test pieces of Group A satisfying the requirements defined in the present invention for both the components and the manufacturing conditions show no ductility and excellent ductility and stretch flangeability.

[0019]

As described above, in the present invention, the strength is high and the workability, ductility, stretch flangeability, weldability, etc. are excellent by specifying the combination of the steel material components and the hot rolling conditions. We manufacture high-strength hot-dip galvanized steel sheets. In particular, by reducing the Si content, the molten Z
Molten Zn with improved wettability to n and good adhesion
A plating layer is formed. As a result, the corrosion resistance also becomes good. The hot-dip Zn-plated steel sheet thus obtained is used in a wide range of fields including underbody parts for automobiles at 440N.
Used as a lightweight component with a tensile strength in excess of / mm ² .

─────────────────────────────────────────────────── ─── Continuation of front page (51) Int.Cl. ⁷ Identification symbol FI C23C 2/06 C23C 2/06 (72) Inventor Yuichi Higo 3-4-1, Marunouchi, Chiyoda-ku, Tokyo Within Nisshin Steel Co., Ltd. (56) References JP-A-4-88125 (JP, A) JP-A-4-2717 (JP, A) JP-A-63-286523 (JP, A) JP-A-6-264139 (JP, A) Kaihei 6-279853 (JP, A) JP-A-6-299236 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB name) C21D 9/46-9/48 C21D 8/00- 8/04 C22C 38/00-38/60

Claims (2)

(57) [Claims] 1. C: 0.02 to 0.10% by weight, Si:
0.2% by weight or less, Mn: 1.0 to 2.0% by weight, P:
0.02 to 0.10% by weight, Nb: 0.01 to 0.03
Wt% and S: includes 0.005 wt% or less, the balance being the actual
Using a steel slab with a composition that is qualitatively composed of Fe , 100
After hot rolling at a finishing rolling temperature of 0 to 850 ° C., 60
Cool the temperature range up to 0 ° C at an average cooling rate of 40 ° C / sec or more, cool the temperature range at 600 ° C or less at an average cooling rate of 30 ° C / sec or less, and wind it in the temperature range of 550 to 400 ° C. Then, a method for producing a high-strength hot-dip Zn-plated steel sheet having excellent workability, which is characterized by performing hot-dip Zn plating. 2. The manufacturing method according to claim 1, wherein a steel slab further containing Ti: 0.01 to 0.05% by weight is used.