JPH02185949A - High tensile plated steel sheet and its production - Google Patents

Abstract

PURPOSE:To manufacture a high tensile plated steel sheet excellent in workability and age hardenability and having moderate baking hardenability by subjecting a cold rolled sheet of dead-soft carbon steel containing specific small amounts of Ti to continuous annealing and then to galvanizing. CONSTITUTION:A steel slab having a composition which contains, by weight, 0.0015-0.0029% C, <0.04% Si, 0.12-0.20% Mn, 0.060-0.070% P, <0.008% S, 0.030-0.050% Al, <0.0030% N, And 0.010-0.030% Ti and in which available Ti quantity represented by an equation I is regulated to >=0 and a quantitative relationship satisfying an inequality II exists between C content and the above available Ti quantity is hot-rolled and then cold-rolled so as to be worked into a sheet metal. This sheet metal is continuously annealed at 730-770 deg.C, cooled, and subjected to galvanizing. By this method, the high tensile plated steel sheet excellent in workability and age hardenability, having moderate baking hardenability of 2.5-3.5kgf/mm<2>, and suitable for bicycle material can be obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a high-tensile plated steel sheet and a method for producing the same.
．． The present invention relates to a high tensile strength alloy plated steel sheet having a moderate bake hardenability of 5 kg4/ms'' and a method for manufacturing the same.

(Prior Art) In recent years, demand for plated tgJFi, such as alloyed hot-dip galvanized steel sheets, as materials for automobiles is rapidly increasing because of their good corrosion resistance and paintability. Automotive materials are required to have excellent workability so that they can be press-formed into complex shapes, and to have high strength in order to reduce the weight of vehicles to save energy. In order to have excellent workability, it is necessary to have a low yield point, high value, and good age hardenability.

By the way, in order to have high strength, it is necessary to have a high yield point, and materials with high strength generally have a low t value and tend to have poor workability. Therefore, in addition to the above properties, there is a property called bake hardenability that is required of these materials. This bake hardenability is a property in which the steel plate ages and hardens due to the heat generated during baking of the paint and its yield point increases, which has the advantage of being easier to process during molding and having higher strength after molding. However, if the bake hardenability is increased, the age hardenability deteriorates and aging occurs at room temperature, and the processability deteriorates due to age hardening between manufacturing and molding. Care must be taken when assigning. Generally, it is said that clear age hardening occurs when the bake hardenability, that is, the increase in hardness after baking exceeds 5 kgf/m1m'', but according to the inventor's investigation, when the bake hardenability exceeds 4 kgf/+wm'' It is considered that age hardenability cannot be ignored.

As mentioned above, various properties are required of materials for automobiles, and many new technologies have been published to meet these properties, and among these is the technology of using Ti-added steel.

Ti-added steel sheets have extremely excellent press formability, especially deep drawability, and this is because Ti can be replaced by TiN, TiS, and TiC.
This method takes advantage of the fact that solid J9T+ remains in the steel sheet by adding it in excess rather than forming Ti, and excellent deep drawability is obtained due to the action of this solid solution Ti. It also has the advantage that stretcher strain does not occur because no yield point elongation occurs.

(Problem to be solved by the invention) However, although such a steel plate has excellent deep drawability as described above, the C in the steel plate is completely fixed by the excess Ti, and therefore The strengthening effect of grain boundaries is lost, and as in the case of excessive decarburization of rimmed steel, a deep-drawn molded product will suffer from brittle cracking due to subsequent forming or small external forces, resulting in worsening of its resistance to secondary work brittleness. Also, since there is no solid solution CSN, the above-mentioned bake hardenability can hardly be expected.

Furthermore, since Ti-added steel sheets contain a relatively large amount of Ti, they tend to cause nozzle clogging or melting during continuous casting during injection of molten steel, and non-metallic inclusions such as TiO□
There were also manufacturing problems, such as the formation of large surface scratches.

In view of the various properties that will be required of galvanized steel sheets in the future, it is an object of the present invention to have good workability and appropriate bake hardenability, and to prevent the increase in yield point and decrease in elongation due to aging. It is an object of the present invention to provide a plated steel sheet that is less susceptible to deterioration and a method for manufacturing the same.

(Means for Solving the Problems) In order to achieve the above object, the present invention, as a result of many trial productions, has been developed by adding a small amount of Ti to improve deep drawability, and by adding a small amount of Ti to the steel sheet as a solid solution. A plated steel sheet with the desired properties can be obtained by retaining C to provide appropriate bake hardenability and secondary work brittleness, and by suppressing the addition of Si and Mn to improve wettability during plating. The present invention was completed based on the knowledge that

In addition, as prior art similar to the present invention, JP-A No. 53-114717: Manufacturing method of cold-rolled steel sheet for deep drawing hardened by post-processing heat treatment, ■ JP-A No. 57-63660: High-tensile cold rolling for deep drawing Steel plate, ■ JP-A-57-143464: High-strength cold-rolled steel sheet with excellent bake-hardenability and its manufacturing method, ■ JP-A-61-26757: Cold-rolled steel plate for deep drawing with bake-hardenability, ■ Special JP-A No. 61-113724: A method for producing a cold-rolled steel sheet with extremely excellent nibless formability, ■ JP-A-61-157639: A method for producing a cold-rolled steel sheet with excellent aging resistance and high bake hardenability, 1986
No.-276927: Method for producing cold-rolled steel sheet with good deep drawability, ■ JP-A-61. No.-276931: There is a method for manufacturing cold-rolled steel for deep drawing with bake hardenability, etc., but in ■, S combined with T1 is not taken into consideration at all in the formula for calculating effective Ti or solid solution C. Furthermore, there is no regulation regarding Si, which is extremely important for alloy-plated steel sheets. ■,■ and■
either do not mention anything about bake hardenability, or, if they do mention similar properties, they say rather that they do not have that property. In the case of ■, the specified range of Si, which is very important for alloy-plated steel sheets, is significantly different, and there is no specification regarding TI.
It is not a Tj-added steel, and is characterized by containing S and N at a certain number of times, and containing effective Ti, that is, the amount of Ti, in a range of 4 to 20 times that of C;
1, C-(12/4B) x 7i≦0, so the range of addition is completely different from that of the present invention.

■ is characterized by subjecting solid solution C to over-aging treatment for 5 minutes or more at a temperature of 120°C or less, and also mentions the addition of Ti, Nb, etc., but the reason is to increase the strength of the steel plate. Therefore, control of solid solution C is mainly carried out by over-aging treatment. (2) requires the simultaneous addition of Ti and Nb to fix solid solution C, and also states that the range of solid solution (Jl) is not specified, but according to the description of the examples, almost all of them are in solid solution. When the value of C is negative and the value of solid solution C is not negative, the composition range such as the amount of P is different from that of the present invention.

Here, the present invention, in weight%, C: 0.00]5 to 0.0029%, Si: 0.04%
Below, Mn: 0.12-0.20%, P: 0
．． 060-0.070%, S: 0.008% or less,
Eight Q: 0.030-0.050%, N: 0.0030
% or less, Ti: 0.010 to 0.030%, and when defined as TI umbrella>0 and 0.0015%≦c --Tit≦0.0025%
This is a high-strength plated steel sheet having a composition in which the balance consists of iron and unavoidable impurities, and is hot-dip galvanized, and has good workability and age hardenability, and has appropriate bake hardenability.

In addition, from another aspect of the present invention, a steel piece having the above composition,
Hot rolling, cold rolling, annealing temperature 730~770℃
After continuous annealing at
A method for producing a high-strength plated steel sheet having good workability and age hardenability and moderate bake hardenability, the method comprising cooling at 4 to 60 degrees Celsius and then applying hot-dip galvanizing. It is.

According to one embodiment of the invention, an alloying treatment may be performed after hot-dip galvanizing.

Note that the appropriate bake hardenability achieved by the present invention is 2
．． Approximately 5 to 3.5 kgf/m2 of garden is sufficient.

(Function) Next, the reason why the composition of the high tensile strength plated steel sheet of the present invention is limited as described above will be explained.

C exists as TiC or solid solution C in ultra-low C-Ti additive steel, and among these, solid J6C has the effect of making the steel have bake hardenability and improving secondary work brittleness.
For this purpose, the presence of 0.0015% or more is required.

On the other hand, if C is present in an excessive amount, age hardenability deteriorates and workability deteriorates, and the required amount of Ti to be added increases, increasing the alloy cost, and also tends to cause nozzle clogging and melting during continuous casting. .

Therefore, in the present invention, the upper limit is set to 0.0029%.

In addition, if an attempt is made to suppress C to less than 0.0015%, the cost due to vacuum degassing treatment will increase, and stable production will become difficult.

Si is an element added to high-strength steel sheets in order to obtain strength, but in the case of manufacturing alloy-plated steel sheets as in the present invention, Si becomes concentrated on the surface during continuous annealing, and adhesion with molten zinc occurs. Tension decreases significantly, plating adhesion deteriorates, and non-plating is more likely to occur.

In one study, when Si was 0.1θ%, the adhesion tension with zinc was 0.
Therefore, if it exceeds 0.10%, the adhesion tension becomes negative and zinc is repelled.

In the present invention, in order to obtain a good plated steel sheet, 0.
04% or less. Preferably it is 0.02% or less.

Mn is an element that is effective in fixing S, which is generally mixed as an impurity element in molten steel, and reducing its harmful effects, and, like Si, in increasing strength.

In the case of T1-added steel, most of the S can be considered to combine with Ti, so in the case of the present invention, there is no need to think too much about fixing S. However, when considering strength, the amount of Si added 0.1 in order to obtain the desired strength.
It is necessary to add 2% or more. - On the other hand, as the amount of addition increases, the F value decreases, and the amount of bake hardening decreases slightly.Furthermore, like St, concentration on the steel plate surface affects the plating properties, so the upper limit is set at 0.20%. .

Like Si and Mn, P also increases the strength and increases the c
When the amount of Ti within the range specified by the present invention is added to 1, a high f value can be obtained by adding 0.03% or more of Ti. However, when more than 0.10% of P is added, the elongation deteriorates significantly and the resistance to secondary work brittleness also deteriorates. Therefore, in the case of the present invention, since the Si content is small, it is necessary to compensate for the strength similarly to Mn, and in order to obtain the desired strength, it is necessary to add 0.060% or more.

However, in order to ensure elongation that provides good workability, the content is desirably 0.070% or less.

For this reason, P is 0.060-0.070%
shall be.

S is contained as an unavoidable impurity in steel, and reduces the ductility and f' value of the steel sheet. Also, as in the case of the present invention, T
In i-added steel, it combines with Ti during heating before hot rolling, for example, in the form of a slab, increasing the required amount of Ti addition, increasing alloy cost and causing hege defects due to coarse precipitates due to TiS. . In the present invention, Mn, Ti
Considering the balance with the amount added, S is set to 0.008% or less, preferably 0.006% or less.

M is added to fix oxygen in the steel,
This prevents Ti from being lost due to oxidation or becoming non-metallic inclusions (oxides) in the steel, suppresses the movement of solid i8C in the steel, and prevents age hardening until press working. It is a component that facilitates processing, assists precipitation of C in post-processing heat treatment, and enhances bake hardenability. Furthermore, 8Q has the effect of fixing N, which increases the required amount of Ti by combining with Ti, as AlN, thereby preventing room-temperature aging due to N. However, as the amount added increases, the alloy cost increases and Afb
(Inclusions such as h increase and the workability deteriorates.In the present invention, the amount of N is lowered and TI is added to fix N, so there is no need to add such a large amount. For this reason, Al is set to 0.030 to 0.050%.

Like S, N is contained in steel as an unavoidable impurity, and reduces the ductility and f-value of the steel sheet, as well as causing room temperature aging and deteriorating workability. In the case of Ti-added steel as in the present invention, like S, it combines with Ti before hot rolling to form a TiN
Since TiN is fixed as
This may cause hege defects due to t■ Ookawa extrusion, or may cause a decrease in total elongation and t value. For these reasons, N is set to 0.0030% or less.

Ti has a negative effect on the ductility and f-value of C in steel.
It is removed by fixing as C and is added for the purpose of improving deep drawability, but solid solution C
If all of these are fixed, the bake hardenability will be lost. Furthermore, before combining with C, N,
Since it combines with S and is consumed as TiN and TiS, it is necessary to add it in consideration of the amount consumed.

Therefore, in the present invention, the Ti-added steel must first have a Ti addition of 0.010% or more and 0.030% or less. However, if all C is fixed, bake hardenability will not be exhibited, so the effective amount of Ti is defined as follows, and the amount is limited as follows.

It is defined as Ti City>0 and 0°0015%≦C--Ti*≦0.0025%.

The Ti rate is the effective Ti amount, and the amount of N and S is determined from the added Ti amount.
This value is calculated by subtracting the amount of Ti equivalent to the amount of Ti that combines with N. If this value is positive, it means that Ti remains even after combining with N and S, and combines with a part of C to generate TiC. means. A portion of this TiC dissolves during the subsequent continuous annealing process and plays the role of finely adjusting the solid solution C, and also works to raise the recrystallization temperature and prevent the steel sheet strength from rapidly decreasing due to annealing. The characteristics of the product are less susceptible to fluctuations in manufacturing conditions, which has the effect of making the characteristics more stable.

From this point of view, it is desirable that the Ti content be 0.0010% or more, but such control is difficult with current steelmaking technology, so the Ti content is set to >0. In the present invention, the amount of solid solution C is 0.0 as understood from the above formula.
The alloy composition is adjusted to fall within the range of 0.015% and 0.0025%.

The high-strength plated steel sheet of the present invention, which has excellent workability and aging resistance and has appropriate bake hardenability, can be produced by ingot-forming, blooming, or continuous casting of molten steel having the above-mentioned chemical composition using a conventional method. After that, it is manufactured by hot rolling, cold rolling, annealing, and hot-dip galvanizing according to conventional methods.

According to a preferred embodiment of the present invention, A.

Finish hot rolling is carried out at a temperature above the transformation point, for example 600
After winding at a temperature of .degree. C. or higher and then cold-rolling at a rolling reduction of 50% or higher to obtain a cold-rolled coil, the coil is subjected to continuous annealing at a temperature of 730 to 770.degree. C. and then hot-dip galvanizing.

In the present invention, the molten galvanizing method itself is not particularly limited. It may be a common usage.

Thereafter, if necessary, an alloying treatment of about 500 to 600'Cxl□seconds is performed to produce a high tensile strength plated steel sheet. This alloying treatment may be carried out particularly when the steel sheet is used as an automobile steel sheet requiring improved paintability, and the treatment conditions may be conventional ones.

In the method according to the present invention, the reason why the annealing temperature is set to 730 to 770°C as described above is as follows. In other words, it is thought that a portion of TiC combined with a portion of C is redissolved during annealing and plays a role in finely adjusting the solid solution C.
730 to obtain the target bake hardenability and age hardenability.
Since the temperature range around 770° C. is most suitable than in many examples, it is considered that redissolution of TiC occurs in the best balance with the amount of solid solution C in this temperature range.

In addition, a certain degree of annealing temperature is necessary because the annealing process causes appropriate recrystallization and grain growth, elongation, and improves the f value, but if it is too high, the strength will decrease.

For the above reasons, if the annealing temperature is lower than 730°C, it is difficult to stably obtain good workability and bake hardenability, while if it is higher than 770°C, the strength may decrease and age hardenability may deteriorate. This is because the tendency for this to occur increases.

The heating time during the continuous annealing treatment may be, for example, about 60 seconds.

Furthermore, in the present invention, cooling is preferably performed at a cooling rate of 4 to 10°C for 7 seconds following the above-mentioned continuous annealing treatment, but if the cooling rate is slower than this, stable production is difficult, and This is because productivity is hindered, and on the other hand, if the temperature is cooled faster than this, it becomes difficult to obtain a sufficient soaking temperature and soaking time.

In this cooling stage, the solid 1c may not precipitate but remain in the structure and become a product.

In addition, "good workability" here means that the characteristics before aging are YP≦22kgf/mm, [L≧42%, P value≧1
．． 80, and good age hardenability means that the characteristics after aging due to accelerated aging treatment at 100°C for 60 minutes are YP deterioration ΔYP≦+〇, 5kg
f/■1, EL171 deterioration margin ΔEL≦0.5%, or-'
)YPF! ≦0.2%, and the strength as a high-tensile steel plate is TS≧35kg4/am2
It is assumed that

In addition, the elongation (EL) of alloyed plated steel sheets is about 2 to 3% compared to cold-rolled steel sheets with the same level of strength due to the hardness of the alloyed plating film. For the same reason, the Y value also tends to be 0.2 to 0.
It will be about 3% lower.

Next, the present invention will be explained in more detail with reference to examples thereof.

Example: Molten steel produced in a converter and subjected to vacuum degassing treatment was continuously cast according to a conventional method, heated to a temperature of 1200°C, hot rolled, coiled at about 690°C, and after pickling. , reduction rate 75%
A 1.01-inch cold-rolled steel plate was produced by cold rolling.

The obtained cold-rolled steel sheet was subjected to continuous annealing at 730 to 770°C for 60 seconds, then subjected to about 4-b to about 460°C, and then alloyed at 500 to 600°C for 10 seconds in an alloying furnace. An alloy-plated steel sheet was manufactured by applying the same process.

Table 1 summarizes the chemical composition and mechanical properties of these steel sheets. Note that materials whose components or manufacturing conditions differ from those of the present invention and other types of materials are shown as comparative materials.

As is clear from the results shown in Table 1, the plated steel sheet according to the present invention has better workability than the comparative plated steel sheet.
It is hard to cause age hardening and has a weight of 2.5 to 3.5 kgr.
/mm! Because of its bake hardenability, it is very suitable for automotive materials, especially exterior materials.

Claims (1)

[Claims] (1) In weight%, C: 0.0015 to 0.0029%, Si: 0.04%
Below, Mn: 0.12~0.20%, P: 0.060~
0.070%, S: 0.008% or less, Al: 0.03
0 to 0.050%, N: 0.0030% or less, Ti: 0.010 to 0.030%, and the effective Ti amount is Ti*=Ti-(48/14)N-(
48/32) When defined by S, Ti*>0 and 0.0015%≦C-(12/48)Ti*≦0.00
A high-strength plated steel sheet having a composition consisting of 25% balance iron and unavoidable impurities, and having good workability and age hardenability, as well as moderate bake hardenability, characterized by being hot-dip galvanized. (2) In weight%, C: 0.0015-0.0029%, Si: 0.04%
Below, Mn: 0.12~0.20%, P: 0.060~
0.070%, S: 0.008% or less, Al: 0.03
0 to 0.050%, N: 0.0030% or less, Ti: 0.010 to 0.030%, and the effective Ti amount is Ti*=Ti-(48/14)N-(
48/32) When defined by S, Ti*>0 and 0.0015%≦C-(12/48)Ti*≦0.00
hot rolling and cold rolling a steel billet having a composition consisting of 25% balance iron and unavoidable impurities;
A high-strength plated steel sheet having good workability and aging resistance and moderate bake hardenability, characterized by being continuously annealed at a temperature of 730 to 770°C, cooled, and then further hot-dip galvanized. manufacturing method.