JP2001049391A - High strength hot dip galvanized steel sheet excellent in workability and platability, its manufacture, and automobile member using the same steel sheet - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a hot dip galvanized steel sheet having high strength and combining excellent workability with excellent platability, its manufacturing method, and an automobile member manufactured by using the steel sheet. SOLUTION: The high strength hot dip galvanized steel sheet is obtained by forming a hot dip galvanizing layer on a steel sheet as a base material. The steel sheet has a composition which contains, by mass, 0.02 to 0.20% C, 1.50 to 2.40% Mn, 0.03 to 1.50% Cr, 0.03 to 1.50% Mo, 0.010 to 0.150% Al, and Fe as essential components and satisfies 3Mn+6Cr+Mo<=8.1% and Mn+6Cr+10 Mo>=3.5% and in which the contents of Si, P, S, and Ti are limited to <=0.04%, <=0.06%, <=0.030%, and <=0.01%, respectively, and also has a dual-phase structure composed essentially of ferrite and martensite. Further, the hot dip galvanizing layer includes that which is subjected to alloying treatment after plating.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

TECHNICAL FIELD The present invention relates to a tensile strength of 440 to 440.
The present invention relates to a high-strength hot-dip galvanized steel sheet (including an alloyed hot-dip galvanized steel sheet) having a strength of about 780 N / mm ² and having excellent workability and plating property.

[0002]

2. Description of the Related Art In recent years, steel sheets for structural members such as members that play a role of absorbing energy at the time of a collision as a frame member of an automobile have been developed from the viewpoint of improving safety and improving fuel efficiency as a measure against environmental problems. From the viewpoint of weight reduction of
High strength steel sheets are rapidly being used. On the other hand, an increase in strength leads to deterioration of workability and makes it difficult to press the part itself, so that a steel sheet having both strength and workability is strongly desired.

In response to such a demand, Japanese Patent Application Laid-Open No. 4-26
744, JP-A-5-331537, JP-A-4-128320, JP-A-4-128321, JP-A-4-173945, JP-A-9-255
No. 37, Japanese Patent Application Laid-Open No. 9-263883, and the like, a structure utilizing a hard phase such as a high-strength steel sheet or martensite having excellent elongation by utilizing the work-induced transformation of retained austenite. Hot-dip galvanized steel sheets and alloyed hot-dip galvanized steel sheets using reinforced steel sheets as base steel sheets have been widely used as automotive steel sheets having excellent corrosion resistance.

[0004]

However, in the technique described in Japanese Patent Application Laid-Open No. Hei 4-26744, high strength steel sheets with excellent elongation are obtained by generating retained austenite in a hot dip galvanizing line. Since the amount of Si is high, the plating property is poor, and in some embodiments, additional steps such as surface grinding and pre-plating are required. Japanese Patent Application Laid-Open No. 5-31537 discloses a method for producing a steel sheet having a composite structure of ferrite and martensite. However, in this technique, aggressive measures are taken to enhance the hardenability of the steel sheet. Amount of Si added to M
The plating property decreases with the n content, and the reduction in the plating property is avoided by performing an additional step called pre-plating. As described above, it is extremely difficult to achieve both workability and plating property, and so far, it has been responded by adding manufacturing steps such as surface grinding and pre-plating. Also, Japanese Patent Application Laid-Open Nos. 4-128320 and 4-128321
The technology described in Japanese Patent Publication No.
Although a high-strength steel sheet with high elongation can be obtained, in order to cause martensitic transformation, C concentration in austenite by adding Si is employed as a means of improving hardenability. Incidentally, in Examples, 0.17 to 0.20% of Si
Is added. However, excessive addition of Si causes poor plating when alloying treatment is performed as described in the same gazette, but before that surface defects due to the wettability of hot-dip galvanizing called non-plating occur. Occurs. For this reason, it is necessary to control much more strictly than the amount of Si described in the example. The technique described in Japanese Patent Application Laid-Open No. 4-173945 is a method for producing a steel sheet having a composite structure of bainite, ferrite, and martensite, which is mainly composed of bainite. In order to improve the ductility of the ferrite due to discharge, it is necessary to add Si, and its adverse effect is as described above. Furthermore, the structure is mainly composed of bainite, which is a relatively soft low-temperature transformation phase, and is excellent in local deformability such as bending workability, but is not necessarily advantageous in elongation characteristics. Also, Japanese Patent Application Laid-Open No. 9-2553
The technology described in No. 7 bulletin and JP-A-9-263883 relates to a high-strength hot-rolled steel sheet and a cold-rolled steel sheet comprising ferrite and a low-temperature transformation product of martensite, tempered martensite, and bainite. However, it is necessary to add a certain amount or more of Ti in order to improve the pitting corrosion resistance. However, Ti not only forms coarse nitrides in steel and deteriorates local deformability, but also forms carbides and reduces the amount of C itself necessary for forming a low-temperature transformation phase. Furthermore, it greatly affects the alloying behavior of plating, and is likely to make stable production difficult.

[0005] The present invention has been made in view of the above-mentioned problems, and a galvanized steel sheet having high strength and excellent workability and plating properties without adding a surface grinding step or a pre-plating step. And a method for producing the same.

[0006]

The high-strength hot-dip galvanized steel sheet of the present invention has a mass% of C: 0.02 to 0.20%, Mn: 1.50 to 2.40%, and Cr: 0. 03 to 1.50%, Mo: 0.03 to 1.50%, 3Mn + 6Cr + Mo: 8.1% or less, Mn + 6Cr + 10Mo: 3.5% or more, Al: 0.010 to 0.150%, and Fe as main components , And Si: 0.04% or less,
P: 0.060% or less, S: 0.030% or less, Ti:
It is limited to 0.01% or less, and a hot-dip galvanized layer is formed on a base steel sheet having a composite structure mainly composed of ferrite and martensite. In the present invention, the hot-dip galvanized layer also includes a layer subjected to an alloying treatment after plating.

Hereinafter, the reasons for limiting the chemical composition (unit: mass%) of the base steel sheet of the hot-dip galvanized steel sheet of the present invention will be described. C: 0.02 to 0.20% C has a large effect on the strength of steel, and affects the elongation and stretch flangeability by changing the amount and form of low-temperature transformation products.
If it is less than 0.02%, it is difficult to obtain a high strength of 440 N / mm ² or more, while if it exceeds 0.20%, the weldability is reduced. Therefore, the lower limit of the amount of C is set to 0.02.
%, Preferably 0.04%, with the upper limit being 0.20%.
%, Preferably 0.15%.

Mn: 1.50 to 2.40% Mn is an element that stabilizes austenite, changes the amount of solid solution C in austenite, and greatly affects the characteristics of low-temperature transformation products generated in a cooling process. In the invention, it is added for the production of this low-temperature transformation product. In order to obtain the characteristics of a high-strength steel sheet having extremely excellent workability, it is necessary to add at least 1.50%. But 2.40%
If it exceeds, not only the melting becomes difficult but also the weldability is adversely affected, so this is made the upper limit.

Mo: 0.03 to 1.50%, Cr: 0.
03-1.50% In the present invention, Cr and Mo are added as a hardenability improving element. Mn is also basically a hardenability improving element. These elements not only enrich C in austenite, increase stability and facilitate formation of martensite, but also form oxides on the steel sheet surface. This affects the plating property. The effect is unknown, but C
By making a composite addition of r, Mo, and Mn, the respective elements complement each other, and both the plating property and the workability can be efficiently achieved. If the addition amounts of Cr and Mo are each less than 0.03%, the effect of improving hardenability cannot be expected. On the other hand, if each of them exceeds 1.50%, the effect is not only saturated, but also the cost is disadvantageous. Therefore, the lower limits of the amounts of Cr and Mo are set to 0.03%, and the upper limits thereof are set to 1.50%.

3Mn + 6Cr + Mo: 8.1% or less, M
n + 6Cr + 10Mo: 3.5% or more (the symbol of the element in the formula on the left indicates the amount of addition of the element.) As for Cr, Mo and Mn, as is clear from the examples described later, the amount of addition of these elements is appropriate. It needs to be balanced. First, in order to efficiently produce martensitic transformation and obtain a composite structure suitable for workability, Mn + 6
Cr + 10Mo needs to be 3.5% or more, preferably 5.0% or more. If it is less than 3.5%, soft low-temperature transformation products such as bainite tend to be excessively generated, and an excellent strength-elongation balance cannot be obtained. On the other hand, from the viewpoint of plating properties, 3Mn + 6Cr + M
It is necessary to make o less than 8.1%, preferably 7.0% or less. If the content exceeds 8.1%, it is considered that the composition of the generated oxide is affected. However, a portion (unplated portion) where no plating is formed in a pinhole shape during hot-dip galvanizing frequently occurs on the steel sheet surface, and the appearance quality is significantly impaired. Become like

Al: 0.010 to 0.150% Al is added at least 0.010% or more for deoxidation. However, an excessive amount of addition not only increases the cost but also deteriorates the surface properties.
0%.

The base steel sheet of the present invention contains the above basic component and Fe as main components.
In addition to the case of the basic component, Fe and unavoidable impurities, it does not hinder the action and effect of the basic component, or does not hinder the addition of an element that further improves the properties of the steel sheet,
For example, one or more of the following B and Ca can be added to the main component to obtain the following chemical components (1) and (2). (1) Main component + B (2) Main component or component (1) + Ca

B: 0.0050% or less B has the effect of improving weldability and enhancing hardenability. In order to effectively exhibit such an effect, it is preferable to add 0.0005% or more. However, an excessive addition not only saturates these effects, but also deteriorates ductility and reduces workability, so the upper limit is made 0.0050%, preferably 0.0030%.

Ca: 0.0050% or less Ca has the effect of controlling the form of inclusions, increasing ductility and improving workability. In order to effectively exhibit such an effect, it is preferable to add 0.0010% or more. However, if added excessively, the amount of inclusions in the steel increases, the ductility deteriorates, and the workability decreases,
The upper limit is 0.0050%, preferably 0.0040%
And

In the present invention, in order to achieve the intended purpose, Si, P, S,
It is necessary to limit the amount of Ti to a predetermined amount or less. Hereinafter, the reasons for limiting these impurity components will be described. Si is an element that forms an oxide film on the surface of a steel sheet and extremely deteriorates the wettability of plating. Therefore, Si is basically not added in the present invention. %, Preferably 0.02%
It is necessary to stop. Further, if P exceeds 0.060%, plating unevenness is likely to occur, and alloying treatment becomes difficult. Therefore, when P is mixed as an unavoidable impurity,
It is necessary to keep the upper limit at 0.060%, preferably 0.030%. In addition, S is fixed as a precipitate in the steel, but if its amount increases, it causes deterioration of elongation and stretch flangeability, so when it is mixed as an unavoidable impurity,
It is necessary to keep the upper limit at 0.030%, preferably at 0.015%. In addition, Ti forms carbonitrides, degrades local deformability, causes an adverse effect on securing a predetermined transformed structure, and is an element that remarkably promotes alloying of hot-dip galvanizing. Addition causes excessive alloying, which leads to powdering or the problem of peeling of the plating layer called flaking due to formation of a hard and brittle Γ phase at the interface between the plating layer and the ground iron during steel plate processing. Therefore, it is necessary to strictly control the addition amount, and the upper limit is set to 0.01%.

The structure of the base steel sheet of the present invention is ferrite +
This is a composite structure mainly composed of martensite. A steel sheet having the above-mentioned chemical composition is made into ferrite + austenite in a soaking step (annealing step) before plating, and after the plating or further alloying, is heated to 5 ° C./sec or more. At an average cooling rate of not more than the Ms point. The main component is an unavoidable level having an area ratio of 5% or less, preferably 3% or less in the structure other than ferrite and martensite, because the workability is deteriorated when different structures such as pearlite and bainite are mixed. Means that Under the above chemical composition, the area ratio of ferrite is 50 to 95%.
%, And 5 to 50% of martensite, and can have both excellent workability and plating property under high strength. By adjusting the structure of the ferrite to 50% or more, preferably more than 70%, more preferably 75% or more, an extremely excellent strength-elongation balance (T
S * El) can be achieved, and by setting the amount of ferrite to 75% or more, preferably 80% or more, the local deformability is improved, and the strength-hole expanding balance is also improved.

In a preferred production method of the present invention, a steel slab having the above-mentioned chemical composition is hot-rolled, wound up at 700 ° C. or lower, pickled if necessary, and then cold-rolled.
After soaking at a temperature of at least _one point Ac in a continuous hot-dip galvanizing line, it is cooled to a plating bath temperature at an average cooling rate of 1 ° C / sec or more to perform hot-dip galvanizing, or further alloying treatment. And cooling at an average cooling rate of 5 ° C./sec or more.

In the present invention, hot rolling may be performed according to a conventional method. Desirably, the heating temperature of the slab is 1000 ° C. to 1300 ° C. from the viewpoint of securing the finishing temperature and preventing the austenite grain size from becoming coarse. The finishing temperature of the hot rolling is set to 800 so that a texture that hinders workability is not formed.
C. to 950 ° C., and the cooling rate after finish rolling may be 30 to 120 ° C./sec in order to suppress generation of pearlite. The reason why the winding temperature is specified to be 700 ° C. or less is that if the film is wound at a temperature higher than this temperature, the scale of the steel sheet surface becomes thick and the pickling property is deteriorated. The lower limit of the winding temperature is not particularly defined, but if it is too low, it becomes hard and lowers the cold rollability, so the lower limit is 250 ° C, preferably 400 ° C.
It is good to do.

After hot rolling, pickling and cold rolling are performed according to a conventional method. It is desirable to carry out the cold rolling at a rate of 30% or more.
If it is less than 30%, in order to obtain a desired product, the hot rolled sheet becomes thin and long, and the productivity at the time of pickling decreases.

After cold rolling, the base material cold-rolled steel sheet may be heated and held at _one or more points of Ac before plating in a continuous hot-dip galvanizing line. In order to stabilize the temperature, the component steel of the present invention is preferably heated to 780 ° C. or higher, which is about 50 ° C. higher than the Ac ₁ point. There is no particular upper limit, but there is no problem if the temperature is 900 ° C. or lower. The holding time is 10 for processing at high temperature.
If it is longer than sec, the heat is sufficiently soaked and a ferrite + austenite structure is obtained.

After maintaining the temperature by heating and equalizing the temperature, it is cooled to a plating bath temperature (normally 440 to 470 ° C.) at an average rate of 1 ° C./sec or more, and subjected to a plating treatment. If it is less than 1 ° C./sec, pearlite is generated and remains as a final structure, so that workability is deteriorated. Therefore, the lower limit of the cooling rate after soaking is set to 1 ° C./sec, preferably 5 ° C./sec. Although the upper limit of the cooling rate is not particularly defined, it is preferably 50 ° C./sec in order to control the sheet temperature and suppress the increase in equipment cost. When performing the alloying treatment, after plating, heating is performed at a temperature of about 500 to 750 ° C. as usual. After plating without alloying treatment, and after alloying treatment with alloying treatment, cool austenite to martensite at 5 ° C / sec or higher to transform austenite into martensite, and convert ferrite + martensite. Get the organization that will be the subject. 5 ℃
If it is less than / sec, pearlite or bainite may be formed, so the lower limit of the cooling rate is set to 5 ° C / sec, preferably 10 ° C / sec.

[0022]

EXAMPLES Example 1 Molten steel containing various Mn contents and Cr contents was prepared by adding Mn and Cr to the following chemical components as base components. 1
Heat to 150 ° C, 2.6 at finish temperature 870-900 ° C
mm hot rolled, with an average cooling rate of 40 ° C / sec,
Each was wound at 480 ° C. After pickling, cold rolling was performed at a cold rolling reduction of 46% to a thickness of 1.4 mm, soaking was performed at 800 ° C. for 20 seconds, and the average cooling rate was 15 ° C./se.
The resultant was cooled in Step c, and subsequently plated in a hot-dip galvanizing bath at 460 ° C., cooled to room temperature at 25 ° C./sec or higher, and temper-rolled at a reduction of 0.8% to produce a hot-dip galvanized steel sheet. -Base component (mass%, balance substantially Fe) C: 0.06%, Si: 0.01%, P: 0.010
%, S: 0.001%, Al: 0.030%, Mo: 0.1
0%

Using the obtained hot-dip galvanized steel sheet, the type and area% of the structure at the center of the base steel sheet thickness were determined by SE.
Inspection by M observation (1000 times, area% is the average value of 20 visual fields), a JIS No. 5 test piece was sampled, and tensile strength (TS) and elongation (El) were measured by a tensile test.
The strength-ductility balance (TS × El) was determined. The occurrence of non-plating was visually observed. Figure 1 shows the survey results
Shown in From FIG. 1, it can be seen that the steel sheet having a component within the range of the present invention (hatched portion in the figure) has not only excellent mechanical properties (TS × El) but also no occurrence of non-plating.
On the other hand, a steel sheet having a component outside the range of the present invention is observed to have deteriorated mechanical properties and / or non-plated. The structure of the steel sheet satisfying the components of the present invention was all a two-phase structure of ferrite and martensite, and the martensite content was 13 to 24 area%.

Example 2 The following chemical components were used as base components, to which Mo and Cr were added to obtain various Mn contents and Cr contents.
A cold rolled steel sheet was produced under the same conditions as in Example 1 and subjected to a soaking heat treatment at 810 ° C. for 60 seconds, followed by cooling at an average cooling rate of 30 ° C./sec.
Subsequently, plating is performed in a hot-dip galvanizing bath at 460 ° C.
It was cooled to room temperature at a temperature of 15 ° C./sec or more, and temper-rolled at a draft of 0.5% to produce a hot-dip galvanized steel sheet. -Base component (mass%, balance substantially Fe) C: 0.04%, Si: 0.01%, Mn: 1.6%, P: 0.005%, S: 0.003%, Al: 0.02
0%

Using the obtained hot-dip galvanized steel sheet, in the same manner as in Example 1, the strength-ductility balance (TS × E
1) and the occurrence of non-plating were investigated. FIG. 2 shows the results of the investigation. From FIG. 2, it is within the scope of the present invention (shaded area in the figure)
The steel sheet having the component of not only has excellent mechanical properties (TS × El) but also has no non-plating,
Steel sheets having components outside the range of the present invention are observed to have deteriorated mechanical properties and / or non-plated. Also in Example 2, the structure of the steel sheet satisfying the components of the present invention was all a two-phase structure of ferrite + martensite, and the martensite content was 8 to 16 area%.

Example 3 A steel having the chemical composition shown in Table 1 was melted, and a cold-rolled steel sheet was manufactured under the same conditions as in Example 1.
After performing soaking at a temperature of 0 ° C. for 40 seconds, cooling was carried out at an average cooling rate of 25 ° C./sec, followed by plating in a hot-dip galvanizing bath at 460 ° C., and for samples Nos. 1 and 2 at 550 ° C. Alloying, then 30 ℃ /
Cool to room temperature at a cooling rate of more than sec.
To produce a hot-dip galvanized steel sheet (including an alloyed hot-dip galvanized steel sheet). Using the obtained hot-dip galvanized steel sheet, in the same manner as in Example 1, the type and amount of the structure, the mechanical properties, and the occurrence of non-plating were investigated.
Furthermore, regarding the hole expandability, the Japan Iron and Steel Federation Standar
d, Method of hole expanding Test): JFST10
A hole expansion test was performed based on 01-1996, and a hole expansion ratio λ was determined. In the hole expansion test, an initial hole (hole diameter d1 = φ10 mm) punched out from a steel plate was formed with an apex angle of 6 from the side opposite to the burr.
The hole diameter d2 at the time when a crack that penetrates the plate thickness is generated by pushing in a 0 degree conical punch is measured. The hole expansion ratio λ is calculated by the following equation. Table 2 shows the results of these investigations. λ (%) = (d2−d1) / d1 × 100

[0027]

[Table 1]

[0028]

[Table 2]

As is clear from Table 2, the invention examples (sample N
o. 1-4), the tensile strength is as high as 450 MPa or higher, the yield ratio is low, a good strength-ductility balance (TS × El ≧ 17000 MPa ·%) is exhibited, and the workability is excellent. Also, there is no occurrence of non-plating. Furthermore, those having a high ferrite area ratio (Sample Nos. 1, 2,
In 4), the strength-hole expanding balance (TS * λ) is 29.
000MPa% or more, local deformability (local ductility)
Also excellent.

On the other hand, in Comparative Examples (Sample Nos. 5 to 11) in which the range of the chemical components was different from the conditions of the present invention, No.
In Nos. 5, 7, 9, and 11, structures other than ferrite and martensite (bainite and pearlite) are generated in a large amount exceeding 5%, and the strength-ductility balance is remarkably inferior to that of the invention. On the other hand, No. 6 and 3Mn + 6Cr + Mo in which the amount of Si exceeds the limit range of the present invention.
It can be seen that non-plating occurred in Comparative Examples (Sample Nos. 8 and 10) whose amounts were outside the range of the present invention.

[0031]

According to the hot-dip galvanized steel sheet of the present invention,
It has a high tensile strength of about 440 to 780 N / mm ² , a low yield ratio, and a good strength-ductility balance.
Excellent workability. In addition, non-plating does not occur, and excellent plating properties are obtained. Further, according to the production method of the present invention, the high-strength hot-dip galvanized steel sheet can be easily produced, and the productivity is excellent. Further, the automotive member manufactured by using the hot-dip galvanized steel sheet of the present invention contributes to the weight reduction of the vehicle body, and is unlikely to cause problems such as peeling of the hot-dip galvanized layer during processing from the time of use. It has excellent rust prevention and durability.

[Brief description of the drawings]

FIG. 1 is a graph showing the influence of the amounts of Cr and Mn on strength-ductility balance and plating properties in Example 1.

FIG. 2 is a graph showing the effect of Cr content and Mo content on strength-ductility balance and plating properties in Example 2.

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Claims (5)

[Claims] 1. Mass%, C: 0.02 to 0.20%, Mn: 1.50 to 2.40%, Cr: 0.03 to 1.50%, Mo: 0.03 to 1. 50%, 3Mn + 6Cr + Mo: 8.1% or less, Mn + 6Cr + 10Mo: 3.5% or more, Al: 0.010 to 0.150%, Fe: 0.04% or less, P: 0.060 % Or less, S: 0.030% or less, Ti: 0.01% or less, and a hot-dip galvanized layer was formed on a base steel sheet having a composite structure mainly composed of ferrite and martensite. High strength hot-dip galvanized steel sheet with workability and plating properties. 2. The high-strength hot-dip galvanized steel sheet according to claim 1, wherein the composite structure mainly composed of ferrite and martensite has a structure other than ferrite and martensite in an area ratio of 5% or less. 3. The high-strength galvanized steel sheet according to claim 1, wherein the composite structure mainly composed of ferrite and martensite contains ferrite in an area ratio of 50% or more. 4. A steel slab having the chemical composition described in claim 1 is hot-rolled, wound up at 700 ° C. or lower, pickled as needed, cold-rolled, and continuously rolled. After soaking at a temperature of at least _one point Ac in a hot-dip galvanizing line, it is cooled to a plating bath temperature at an average cooling rate of 1 ° C./sec or more and subjected to hot-dip galvanizing, or further alloyed. 5. A method for producing a high-strength hot-dip galvanized steel sheet having excellent workability and plating properties that is cooled at an average cooling rate of 5 ° C./sec or more. 5. An automotive member manufactured using the high-strength hot-dip galvanized steel sheet according to claim 1. Description: