JP3114107B2 - Method for producing alloyed hot-dip galvanized high-tensile cold-rolled steel sheet with excellent corrosion resistance and formability - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing an alloyed hot-dip galvanized high-tensile cold-rolled steel sheet having excellent corrosion resistance and press formability useful as a steel sheet for automobiles and the like.

[0002]

2. Description of the Related Art In recent years, the development of high-strength cold-rolled steel sheets for automobiles has been earnestly pursued for the purpose of weight reduction and safety improvement of automobile bodies. As a countermeasure, alloyed hot-dip galvanized cold-rolled steel sheets have attracted attention in terms of cost, rust prevention performance, paintability, and the like. Si, Mn, P
Solid solution strengthening method in which the crystal lattice is distorted by the addition of substitutional solid solution elements such as C and N and interstitial solid solution elements such as C and N;
V and other carbon-nitride forming elements are added, and the precipitation strengthening method of dispersing and precipitating the fine carbonitrides, the crystal structure of the steel sheet is changed to a structure in which a hard martensite phase is finely and uniformly dispersed in a soft ferrite phase. There is known a composite structure strengthening method (Japanese Patent Application Laid-Open Nos. 2-196442 and 2-290955).
JP-A-3-28325, JP-A-4-26744, etc.). In addition, in the galvannealing of cold-rolled steel sheets, the cold-rolled steel sheets are introduced into a continuous hot-dip galvanizing line, subjected to continuous annealing treatment for removing distortion, softening, non-aging, etc. It is fed into a zinc bath to form a galvanized layer with a predetermined adhesion amount, and then is sent to an alloying furnace where Fe and molten zinc are reacted by diffusion of Fe from the base steel sheet (Fe-Zn alloy). (Forming a plating layer).

[0003]

As described above, there are several methods for increasing the strength of a cold-rolled steel sheet. However, the tensile strength obtained by the solution strengthening method is about 30 to 45 kgf / mm ² . In order to obtain a high strength exceeding 50 kgf / mm ² by the same method, if a large amount of reinforcing elements such as Si, Mn, and P are added, the operation of each stage of the steel making process, the pickling process, and the hot-dip galvanizing process is performed. Remarkably deteriorates. In particular, in the hot-dip galvanizing process, Si,
An oxide film such as Mn deteriorates plating wettability and causes non-plating, and the reaction rate in the alloying treatment is extremely slow, so that not only takes a long time for the treatment, but also Zn
-The Fe alloy plating layer becomes inhomogeneous, and the desired corrosion resistance cannot be obtained. When applying the strengthening method that combines the precipitation strengthening method with the solid solution strengthening method, while avoiding the large addition of the solid solution strengthening element as described above and the disadvantages thereby,
As a synergistic effect between the ^two , a high strength exceeding about 60 kgf / mm ² can be obtained, but on the other hand, the yield ratio is increased, so that the shape freezing property in press molding is inferior and the balance between strength and elongation is good. Absent.

[0004] On the other hand, in the case of a strengthening method in which the crystal structure of the steel sheet is a composite structure composed of a ferrite phase and a martensite phase, while achieving a high strength exceeding 50 kgf / mm ² , it is possible to achieve a high strength. A high elongation and a low yield ratio can be obtained, and as compared with the above-mentioned strengthening method using both solid solution strengthening and precipitation strengthening, difficulties in press molding are alleviated, and the press shape is also advantageous in terms of freezeability. However, in this method, in order to enhance the hardenability necessary for forming a composite structure including a martensite phase, the plating property (plating wettability, plating wettability,
Alloying processability), which makes it difficult to ensure sufficient corrosion resistance of the alloyed hot-dip galvanizing.

[0005] As described above, it has been difficult for the conventional manufacturing method to simultaneously increase the strength of the cold-rolled steel sheet and improve the corrosion resistance. Therefore, the present invention is to maximize the strength of the composite structure strengthening method, a low yield ratio, good strength-improvement of the effect of improving the balance of elongation, while improving the plating property,
An object of the present invention is to provide a method for producing an alloyed hot-dip galvanized steel sheet that can ensure good corrosion resistance.

[0006]

The method for producing a galvannealed high-strength cold-rolled steel sheet according to the present invention comprises the steps of:
0.3%, Si: 2.0% or less, Mn: 2.0 to 3.5
%, P: 0.1% or less, S: 0.1% or less, steel containing the balance of Fe and unavoidable impurities is hot-rolled and wound at an Ar ₃ transformation point or higher, pickled, and then cold-rolled. After performing pre-plating of Fe-B alloy and heating and holding for 10 to 300 seconds in a temperature range of Ac _{1 to} Ac ₃ transformation point in a continuous hot-dip galvanizing line, an average cooling rate of 2
By cooling to a temperature below the Ms point at
The martensite phase in the ferrite matrix
Form dispersed ferrite-martensite composite structure
Then, it is introduced into a hot-dip galvanizing bath, and after hot-dip galvanizing, is alloyed in a temperature range of 450 to 600 ° C.

[0007]

[Action] In a continuous hot-dip galvanizing line, Ac _{1 to} Ac
After being heated and held at the annealing temperature at the _three transformation points, the cold-rolled steel sheet cooled from the same temperature range to a temperature of not more than the Ms point at an average cooling rate of 2 ° C./sec or more has a ferrite phase (quenched) due to its rapid cooling (quenching). A composite structure in which the martensite phase (α ′ phase) is finely and uniformly dispersed is formed in the matrix composed of (α phase). The surface of the cold-rolled steel sheet has good plating properties (plating wettability, alloying treatment properties) by Fe-B alloy plating applied as iron-based pre-plating, so hot-dip galvanizing without non-plating A layer is formed, and in the alloying treatment, the Zn-Fe alloying reaction can be achieved without a shortage of excess or deficiency by a short time treatment. The alloyed hot-dip galvanized steel sheet manufactured according to the present invention has a tensile strength of 60 kgf / mm ² or more, a low yield ratio that does not exceed 0.65 and sufficiently satisfies 0.60 or less, and 15% or more. High elongation and excellent and stable corrosion resistance due to a uniform Zn-Fe alloy plating layer without non-plating.

Hereinafter, the present invention will be described in detail. The reasons for limiting the chemical composition of steel in the present invention are as follows. All percentages indicating the content of elements are weight%. C: 0.05-0.3% C forms a solid solution in the steel to increase the strength of the steel. The reason for setting the lower limit of the content to 0.05% is that if the content is less than this, the effect of improving the strength is insufficient, and it becomes impossible to secure a strength level of 60 kgf / mm ² or more in tensile strength. Further, the upper limit is set to 0.3%, because if it exceeds this, ductility becomes insufficient and press molding becomes difficult.

[0009] Si: 2.0% or less The upper limit of the Si content is limited to 2.0%. Exceeding this limit lowers the plating wettability due to the formation of an oxide film on the surface of the steel sheet and the hot-dip galvanizing. This is because the deterioration of the alloying processability is remarkable, and the deterioration of the plating property cannot be sufficiently prevented even by performing the pre-plating of the Fe-B alloy .
Preferably, it is 1.5% or less. Note that Si is an element having the effect of strengthening the solid solution of steel and improving the hardenability, and its presence does not hinder the securing of a low yield ratio. It is effective to adjust the content thereof to increase the strength, and is preferably set to 0.02 to 1.5%.

Mn: 2.0 to 3.5% Mn is an element having an effect of improving the hardenability of steel. The reason why the lower limit of the content is set to 2.0% is that if the content is less than that, the amount of martensite phase occupying in the composite structure is insufficient due to insufficient hardenability of steel, and the high strength due to the composite structure is obtained. This is because it is not possible to sufficiently achieve the formation of a steel sheet, and this also causes an increase in the yield ratio of the steel sheet. On the other hand, the upper limit of 3.5% is that when the amount of Mn is increased beyond this, the amount of martensite formed becomes excessive, the ductility becomes insufficient, and the press formability deteriorates.

P: 0.1% or less The reason why the content of P is 0.1% or less is to prevent a reduction in workability due to embrittlement of steel. Preferably, 0.
Not more than 05%. However, since P is an element having a solid solution strengthening action, it is effective to increase the strength of steel by containing an appropriate amount of P, for example, 0.01% or more, within a range that does not cause deterioration in workability. It is. The presence of P suppresses the diffusion of Fe atoms in the galvannealing alloying treatment and causes a reduction in alloying treatment properties.
In the present invention in which the pre-plating of the Fe-B alloy is performed, even if the above-mentioned small amount of P is present, there is no actual harm to the alloying treatment.

S: 0.1% or less S causes embrittlement of steel and deteriorates workability.
Should not exceed 1%. Preferably, it is 0.05% or less.

Next, the hot rolling of the steel having the above chemical composition and the subsequent steps will be described. The finishing temperature in the hot rolling is set to the Ar ₃ transformation point or higher, and the hot rolling in the γ phase temperature range is performed in order to sufficiently exert the effect of improving the material properties of steel. This is because if hot rolling is performed in a two-phase temperature range (Ar _{1 to} Ar ₃ ) in which phases are mixed, the strength and workability of the final product steel sheet are reduced.
The hot-rolled steel sheet is rolled into a cold-rolled steel sheet having a predetermined thickness by cold rolling after the pickling treatment.

Prior to introducing the cold rolled steel sheet into the continuous hot-dip galvanizing line, the surface of the cold-rolled steel sheet is pre-plated with an iron-based Fe-B alloy in order to enhance the plating property of the steel sheet. That is, since the steel in the present invention has a composition containing a relatively large amount of Mn as a hardenability improving element, the surface of the cold-rolled steel sheet is liable to form an oxide film of Mn, and therefore, the plating property as it is. This is because (plating wettability, alloying treatment property) is not sufficient . Flops of plating can be carried out by electroplating. The plating composition may be pure iron, but an appropriate amount of B (about 10 to 30 p
pm) is particularly preferred because of its good compatibility with molten zinc. The amount of the pre-plated coating should be about 0.5 g /
m ² (per side) or more, but about 5 g / m ²
(Per side) is sufficient.

[0015] The annealing of the cold-rolled steel sheet that has been pre-plated with the Fe-B alloy and introduced into the continuous hot-dip galvanizing line is performed using Ac ₁
To be performed in the temperature range from the Ac ₃ transformation point to α + γ
The heating holding time was set to 10 seconds or more in order to sufficiently solidify and concentrate hardenability improving elements such as Mn, Si, and C in the γ phase.
The reason for setting the upper limit of the holding time to 300 seconds is that the solid solution concentration of the above element in the γ phase is almost completed within that time,
Further, the heating and holding for a long time exceeding the above range not only lowers the line efficiency, but also causes the growth of crystal grains to be coarse and the material properties to be lowered. By this heat holding, annealing (removal of strain, softening, non-aging, etc.) of the cold-rolled steel sheet is simultaneously achieved.

After heating and holding in the two-phase temperature range of the Ac _{1 to} Ac ₃ transformation points, the cooling from the same temperature range is rapidly cooled by γ
The phase is transformed into a martensite phase (α 'phase)
The reason why the average cooling rate was set to 2 ° C./sec or more in order to form a composite structure of α ′ phase is that at a lower cooling rate, bainite transformation occurs and the composite structure composed of α phase and α ′ phase. It is because it becomes impossible to secure.

The cold-rolled steel sheet into which the composite structure has been introduced is sent to a hot-dip galvanizing bath to be subjected to predetermined plating.
It is fed to an alloying furnace maintained at 50 to 600 ° C. to alloy the galvanized layer. No special conditions are added to the hot-dip galvanizing process, and it may be performed according to a conventional method.
On the surface thereof, a uniform galvanized layer without non-plating is formed due to good wettability by Fe-B alloy pre-plating. In addition, since the alloying property is good as an effect of pre-plating, a predetermined alloying reaction can be completed in a short time. The reason for setting the treatment temperature to 450 ° C. or higher is to promote the mutual diffusion of Fe and Zn atoms and efficiently form a uniform Fe—Zn alloy with no excess or deficiency in the entire plating layer. The reason for this is that, if it exceeds this, the martensite phase in the composite structure of the steel sheet is tempered, resulting in deterioration of the material properties, particularly the tensile strength and ductility.

[0018]

【Example】

[I] Production of test material A steel having the chemical composition shown in Table 1 was hot-rolled into a hot-rolled steel sheet having a thickness of 2.0 mm, and after pickling, subjected to cold rolling to obtain a steel sheet having a thickness of 0.8 mm. Was obtained. After performing Fe-B plating (B content: 15 ppm) by electroplating as a pre-plating on a cold-rolled steel sheet, the steel sheet is introduced into a continuous hot-dip galvanizing line, and is heated and held in an Ac ₁ to Ac ₃ transformation point temperature range. And a quenching (quenching) treatment from the same temperature, and a predetermined galvanizing by passing through a hot-dip galvanizing bath (bath temperature: 460 ° C.), followed by an alloying treatment. Finishing temperature in hot rolling, pre-coating weight of cold-rolled steel sheet, annealing conditions (temperature, time) in continuous plating line, cooling rate from annealing temperature, hot-dip galvanizing coating weight, and alloying treatment conditions ( Temperature and time) are also shown in Table 1.
The Ar ₃ transformation point of the steel sheet of the test material is about 700 ° C., and the temperature range of Ac _{1 to} Ac ₃ transformation point is about 700 to 800.
° C.

[II] Properties Various tensile properties of each test material were measured, and the quality of the plating was evaluated as to the presence or absence of non-plating and the alloying state of the plated layer (excess or insufficient Zn-Fe alloying reaction, surface Was evaluated, and the results shown in Table 2 were obtained. In Table 2, "O" in the column of "non-plating" indicates that the plating wettability was good and no non-plating was observed, and "x" indicates that non-plating occurred due to insufficient plating wettability. "O" in the column of "Potification state" indicates that the Zn-Fe is homogeneous over the entire surface of the plating layer.
The fact that alloying has been achieved, "x" indicates that the alloying reaction is insufficient or the reaction has progressed too much, and that the surface appearance is abnormal.

In the table, Nos. 1 to 5 are invention examples, and No. 101
-106 are comparative examples, and comparative examples No. 101 and N
o.102: Iron-based pre-plating of cold-rolled steel sheet (Fe-B alloy )
Example omitted gold), No.103, an example in which the Mn content in the chemical composition of the steel is insufficient, No.104, an example annealing temperature of the steel sheet in a continuous hot-dip plating line is too high, the No.105 No. 106 is an example where the cooling rate from the annealing temperature is too low, and No. 106 is an example where the alloying treatment temperature is too low.

The steel sheets of Nos. 1 to 5, which are examples of the invention, all have a tensile strength greatly exceeding 60 kgf / mm ² and a tensile strength of 75 kF / mm ^2.
It has a strength level of gf / mm ² or more and high strength, yet has a sufficiently low yield ratio of 0.45 or less and high elongation of 15% or more. Regarding the plating quality, the effect of the pre-plating is that there is no excess or deficiency in the non-plating or alloying reaction, and a sound Zn-Fe alloy plating layer is formed.

On the other hand, looking at Comparative Examples 101 to 106, No. 101 and No. 102 are Invention Examples No. 1
Although it has the same level of tensile properties as 5, but the pre-plating is omitted, the plating property of the steel sheet is poor,
It causes non-plating and poor alloying. The poor alloying (insufficient alloying reaction) cannot be avoided even if the alloying treatment temperature is set to the upper limit as in No. 102.

No. 10 with insufficient Mn content
3 has a tensile strength of at least 60 kgf / mm ² , but has undergone predetermined annealing and quenching treatment as a result of insufficient hardenability due to insufficient Mn content. The yield ratio is 0.76, which is significantly higher than that of the inventive example, and is inferior in press shape freezeability. In No. 104, unlike in No. 103, there was no shortage of Mn content, and although an appropriate chemical composition was given, the steel sheet was annealed at a temperature (γ phase temperature range) exceeding the Ac ₃ transformation point. Therefore, as a result of not forming the predetermined composite structure, the yield ratio was remarkably high at 0.79, and the tensile strength was at a lower level than that of the invention.

In No. 105, the chemical composition of the steel is appropriate, but the cooling rate from the annealing temperature is insufficient, so that the yield ratio is remarkably high at 0.85, and the tensile strength is lower than the strength level of the invention. It has fallen significantly below, and the growth is also insufficient. The cause of poor plating quality in this example is that the alloying treatment time was prolonged due to the necessity of adjusting the cooling rate after annealing the steel sheet and the line speed was reduced, and the alloying reaction proceeded excessively. This is not due to defects in the pre-plating effect of the steel sheet. No. 106 has improved tensile properties equivalent to those of the invention example, and non-plating is prevented as an effect of pre-plating. However, since the alloying treatment temperature is low, plating quality due to insufficient alloying reaction is high. It is bad.

[0025]

[Table 1]

[0026]

[Table 2]

[0027]

The alloyed hot-dip galvanized steel sheet produced by the method of the present invention has a tensile strength level exceeding 60 kgf / mm ² and also has a high level of rust prevention due to sound plating quality. In addition, while having high strength, it has a low yield ratio and high elongation, and is easy to press-form,
Excellent shape freezing. Therefore, it is suitable, for example, as a high-tensile rust-preventive steel plate or the like for meeting demands for reducing the weight of a vehicle body, improving safety, and extending the life.

────────────────────────────────────────────────── ─── Continuing on the front page (72) Fumihiro Ida 5th Ishizu Nishimachi, Sakai City, Osaka Prefecture Nisshin Steel Corporation Sakai Works (56) References JP-A-4-26744 (JP, A) 2-290955 (JP, A) JP-A-63-312960 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) C21D 8/02, 9/46 C22C 38/00-38 / 60 C23C 2 / 06,2 / 28

Claims (1)

(57) [Claims] 1. C: 0.05-0.3%, Si: 2.0
%, Mn: 2.0 to 3.5%, P: 0.1% or less,
S: steel containing 0.1% or less, the balance being Fe and unavoidable impurities is hot-rolled and rolled at an Ar ₃ transformation point or higher, pickled, cold-rolled into a thin steel sheet, and a Fe-B alloy after the pre-plating, in a continuous galvanizing line, it was heated and maintained for 10 to 300 seconds at a temperature range of Ac ₁ to Ac ₃ transformation point to a temperature below Ms point at an average cooling rate 2 ° C. / sec or higher By cooling , the ferrite phase
Ferrite matrix in which martensite phase is dispersed in a matrix
Forming a composite structure of rutensite , then introducing into a hot-dip galvanizing bath, and after hot-dip galvanizing, performing alloying treatment in a temperature range of 450 to 600 ° C., characterized by excellent corrosion resistance and formability; Manufacturing method of high tension cold rolled steel sheet.