JPH0344423A - Manufacture of galvanized hot rolled steel sheet having excellent workability - Google Patents

Abstract

PURPOSE:To stably manufacture the galvanized hot rolled steel sheet having excellent strength, galvanizing adhesion and workability such as spreading properties by regulating a hot rolled steel sheet having controlled conditions in cooling and coiling after hot-rolled as stock, reheating the stock to a specified temp. and thereafter passing it into a hot dip galvanizing bath. CONSTITUTION:A slab contg., by weight 0.003 to 0.18% C, <1.2% Si, <2.0% Mn, <0.10% Sol.Al and <0.005% S or furthermore contg. one or more kinds among 0.01 to 0.10% Nb, V, Ti and Zr, 0.002 to 0.10% REM and 0.002 to 0.01% Ca is hot-rolled, and finish rolling is completed in the temp. area of the Ar3 point or above. The slab is rapidly cooled at >=10 deg.C/sec cooling rate and is coiled in the temp. area of <=500 deg.C. The hot rolled steel sheet is reheated to the (alpha+gamma) two phase area and is thereafter subjected to hot dip galvanizing through a hot dip galvanizing bath. The structure of the steel sheet is transferred into a two phase structure one of extremely fine-pulverized ferrite and pearlite or cementite, by which the hot dip galvanized hot rolled steel sheet having excellent workability such as spreading properties can stably be manufactured at a low cost.

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Application Field> The present invention relates to a method for producing a galvanized steel sheet that is particularly excellent in workability, particularly in hole expansion properties.

<Prior art and its problems> At present, so-called “
"Hot-rolled steel sheets" are widely used as a relatively inexpensive structural material in various industrial machinery such as automobiles, but recently, galvanized steel sheets have been used mainly for parts used in automobile undercarriages, for example. Demand for hot-rolled steel sheets with enhanced rust resistance has been on the rise.

Incidentally, such "galvanized hot-rolled steel sheets" are usually produced by directly utilizing a continuous annealing hot-dip galvanizing line, which is a galvanizing line for cold-rolled steel sheets.

In this case, heating for annealing is not required unlike when cold-rolled steel sheets are treated, but in order to ensure the adhesion of hot-dip plating, the hot-rolled steel sheets are zinc-plated before being passed through a molten zinc bath. It was necessary to preheat to above the melting temperature of (approximately 460° C.).

However, on the other hand, if a hot rolled steel plate is preheated as described above,
Cementite in the steel plate melts and solid solution C in ferrite
It has been pointed out that as a result, the processability of the resulting galvanized hot-rolled steel sheet, especially its hole expandability, becomes noticeably deteriorated.

Therefore, in order to avoid the above problems, a method was proposed in which galvanized hot-rolled steel sheets were manufactured using hot-rolled steel sheets with an extremely low carbon content (for example,
-179024). However, the C of the material steel plate
With this method of limiting the amount, it is difficult to ensure sufficient strength for galvanized hot-rolled steel sheet products, and it is also economically disadvantageous, so it could not be said to be a particularly preferable method.

Therefore, it is an object of the present invention to be able to stably mass-produce hot-dip galvanized steel sheets that have better workability (particularly hole expandability) than the hot-rolled steel sheets used as the base material, without any economic disadvantage. placed on providing the means.

(Means for Solving the Problems) In order to achieve the above object, the present inventors conducted research while repeating numerous zinc-coated hot-rolled steel sheet manufacturing experiments, particularly by treating the hot-dip galvanizing line as a type of heat treatment line. As a result of repeated efforts, we were able to use a hot-rolled steel sheet with an appropriately adjusted fractional composition and regulated cooling and coiling conditions after hot rolling, and also set the reheating temperature to a specific value before passing it through the zinc bath. By applying hot-dip galvanizing under controlled conditions, it is possible to stably produce galvanized hot-rolled steel sheets that not only ensure sufficient strength and plating adhesion, but also have significantly improved hole expansion characteristics of the base material. I came to discover that.

That is, on a galvanizing line, a low Mntl material with a controlled Mn content is hot rolled to prevent martensite from forming.
Next, a hot-rolled steel sheet is used that has been rapidly cooled to a temperature below a specific temperature range to refine the ferrite and cementite, and the hot-rolled steel sheet is heated to a (α+γ) two-phase region prior to hot-dip galvanizing on a galvanizing line. When the cementite that existed at the grain boundaries melts into the matrix and is finely dispersed within the grains, ferrite is generated from the austenite generated during reheating in this state, resulting in extremely fine grains. A structure of ferrite and pallite or cementite is formed. It has been found that this method can sufficiently improve workability (hole expandability) and at the same time ensure good plating adhesion and strength.

The present invention has been made based on the above-mentioned findings, etc. r C: 0.003 to 0°, 18% (hereinafter, % representing the component ratio is % by weight).

St: 1.2% or less, Mn: 2.0%
below.

sol, Pdl: 0.10% or less, S:
Contains 0.005% or less, or further Nb: 0.01 to 0.10%, v: o,
oi~o,io%.

Ti: 0.01-0.10%, REM: 0.
002-0.10%.

Ca: 0.002-0.01%, Zr: 0
．． A.
r, finish rolling in the intended temperature range, then rapidly cool at a cooling rate of 10°C/39C or higher, and coil the hot rolled steel sheet in a temperature range of 500°C or lower, before passing it through a molten zinc bath. (α+
γ) By reheating to a two-phase region and then hot-dip galvanizing, it is possible to stably produce galvanized steel sheets with excellent workability in which the base steel sheet is ferrite-pearlite or ferrite-cementite m-woven. It is characterized by the fact that

Note that Japanese Patent Publication No. 57-9831 describes a method of reheating a hot-rolled steel sheet to the (α+γ) two-phase region and then rapidly cooling it to form a ferrite-martensite structure in order to improve its strength properties. However, hot-rolled steel sheets manufactured by this method have significantly deteriorated hole expandability due to the inclusion of martensite, making it difficult to manufacture galvanized hot-rolled steel sheets with excellent hole expandability as the object of the present invention. It is not applicable to

In addition, Japanese Patent Publication No. 56-108830 states that high-Mn hot-rolled steel sheets are box-annealed in a two-phase temperature range to form a composite structure containing ferrite and martensite, and then hot-dip galvanized. A method for manufacturing galvanized steel sheets is disclosed, but in this case too, martensite is mixed into the base metal structure of the product galvanized steel sheet, and when martensite is mixed into the base metal structure in this way, a low yield ratio is achieved. However, since the hole expandability of the steel sheet is significantly deteriorated, the galvanized hot-rolled steel sheet with excellent hole expandability, which is the object of the present invention, cannot be realized.

Hereinafter, the reason why the chemical composition and processing conditions of the steel material are limited as described above in the present invention will be explained in detail along with their effects.

^) Chemical composition C has the effect of ensuring the strength of the steel plate, but if its content is less than 0.003%, the desired effect due to the above effect cannot be obtained; If the C content exceeds 0.003 to 0.0%, it will cause deterioration of weldability.
．． It was set at 18%.

St St is a preferred component that improves the strength and ductility of steel sheets through solid solution strengthening. However, if the Si content exceeds 1.2%, weldability deteriorates, so the Si content should be set at 1.2%.
It has been set at 2% or less.

Mn Mn is a preferred element that improves the strength and ductility of steel sheets through solid solution strengthening. However, if the content exceeds 2.0%, weldability deteriorates, and the austenite produced by heating in the two-phase region does not transform even after passing through the zinc bath, resulting in the presence of martensite in the final product. This causes deterioration in hole expandability. Therefore, the Mn content is 2.0%
It was determined as follows.

Sol and UIM are added as deoxidizing agents for steel, but if they are contained in excess of 0.10% as soj and Af, the amount of alumina-based inclusions will increase, degrading workability. Therefore, the amount of A1 added was determined to be 0.10% or less in terms of sol and AI content.

S is an impurity element that forms MnS-based inclusions and reduces workability. Therefore, in order to avoid the above-mentioned disadvantages as much as possible,
The S content was determined to be no more than o, oos.

Nb, V, Ti, REM (-element), Ca
These elements have the effect of improving the strength or workability of the steel sheet, so one or more of these elements can be included as necessary, but the reason for limiting the content of each element is as follows. It is.

That is, Nb, V, and Ti have the effect of strengthening the steel sheet by combining with C and N and precipitating as carbonitrides. However, the contents of Nb, V and Ti are each 0.01
If the content is less than 0.10%, the desired effect cannot be obtained. On the other hand, if the content exceeds 0.10%, the strength increasing effect will be saturated, which is economically disadvantageous.

Therefore, the content of each of these components is o, oi ~ 0.1, respectively.
It was set as 0%.

Furthermore, REM, Ca, Zr, and rare earth elements all have the effect of adjusting the shape of inclusions and improving the cold workability of the steel sheet. However, the content of C
If it is less than 0.002% for a, less than 0.01% for Zr, and less than 0.002% for REM, the desired effect cannot be obtained by the above action, while on the other hand, 0.01% for Ca,
If Zr is contained in excess of 0.10% and REM is contained in excess of 0.10%, on the contrary, inclusions in the steel will increase too much, leading to deterioration of cold workability. from,
Ca content is 0.002-0.01%, Zr content is 0.01-0.10%, and REM content is 0.00%.
It was set at 2 to 0.10%, respectively.

In addition, P is known as an element that inevitably mixes into steel.
P has the effect of improving the strength and ductility of steel sheets through solid solution strengthening, but on the other hand it has a negative effect on weldability, so in order to ensure the desired weldability, the content of P must be reduced to 0.
It is preferable to suppress it to 0.7% or less.

B) Manufacturing conditions for hot rolled steel sheets During hot rolling, there is no particular problem whether a direct slab is used or the slab is reheated, but if the finish rolling temperature is below Ar, the steel may be transformed and formed. The processed structure is mixed into the ferrite grains, and this cannot be recovered even by reheating before zinc coating, leading to deterioration of workability. Therefore, the finish rolling temperature was determined to be Ar and above the dot plate.

In addition, after hot rolling is completed, cooling rate: 10"C/s
It is rapidly cooled at a temperature of EC or higher and wound up at a temperature of 500°C or lower for the following reason. That is, the cooling rate after hot rolling is 10°C.
If it is less than /sec, the ferrite grains will not be refined sufficiently, and if the winding temperature exceeds 500°C, very coarse cementite will be produced, both of which will lead to deterioration of the hole expandability in the hot rolled sheet. This is because the bridge is continued even after plating.

C) Processing conditions for hot-rolled sheets during hot-dip galvanizing In the present invention, hot-rolled steel sheets manufactured under the above conditions are hot-dip galvanized without being cold rolled. Before passing through the zinc bath, it is reheated to the two-phase region (α0T).This reheating improves the strength-hole expansion balance by making the ferrite grains finer and the cementite finely dispersing, and also improves the plating adhesion. will also be improved.

Next, the effects of the present invention will be explained in more detail with reference to Examples.

(Example) 50 kg of steel having the chemical composition shown in Table 1 is melted in a vacuum melting furnace, and 60 kg is melted by casting into a mold and directly or by hot forging.
It was made into a slab with a thickness of m.

Next, each slab is hot-rolled under the conditions shown in Table 2 to form a 2 fl thick hot-rolled steel plate, and then reheated to the temperature shown in Table 2 before being passed through a molten zinc bath. A galvanized hot-rolled steel sheet was manufactured by plating treatment.

Table 2 also shows the mechanical properties of the hot rolled steel sheet before the plating treatment and the galvanized hot rolled steel sheet after the plating treatment.

As is clear from the results shown in Table 2, the galvanized hot-rolled steel sheet produced according to the conditions specified in the present invention has an excellent strength-hole expansion balance; Under test number 6, where the finishing temperature is outside the lower limit specified by the present invention, the strength of the steel plate increases, and the strength-hole expansion balance (tensile strength x hole expansion) of the hot-rolled and plated steel plates increases. It can be seen that the ratio) is getting worse.

In addition, in Test No. 7, in which the cooling rate after rolling was outside the lower limit specified by the present invention, and in Test No. 8, in which the coiling temperature was outside the upper limit specified in the present invention, coarse cementite was formed, and the heat Strength of steel plate as rolled and after galvanizing
The hole expansion balance is poor.

Furthermore, in Test No. 9, in which the reheating temperature before passing through the molten zinc bath was outside the upper limit specified by the present invention, the hole expandability of the steel sheet decreased due to coarsening of the ferrite; It can be seen that in test number 10, in which the reheating temperature was outside the lower limit defined by the present invention, the structural change in the steel plate was small, and only the hole expandability deteriorated due to the increase in solid solution C.

On the other hand, it was confirmed that the galvanized hot-rolled steel sheet obtained in Test No. 21.22, in which the C and S+ contents of the steel sheet exceeded the upper limit specified by the present invention, caused poor arc welding.

In addition, in test number 23 in which the Mn content of the steel sheet exceeds the specified value of the present invention, martensite is present in the final product (in the galvanized hot-rolled steel sheet), and the steel sheet has a two-phase structure of ferrite and martensite. Yield ratio (YR)
It can be seen that although the strength is low, the strength-hole expansion balance becomes poor.

Summary of Effects> As explained above, according to the present invention, sufficient strength and
It is possible to stably produce galvanized hot-rolled steel sheets that have plating adhesion and excellent workability including hole expandability, which brings about extremely useful effects industrially.

Claims (2)

[Claims] (1) Weight percentage: C: 0.003 to 0.18%, Si: 1.2% or less, M
n: 2.0% or less, sol. Al: 0.10% or less, S
: 0.005% or less, the balance substantially consisting of Fe and unavoidable impurities is hot-rolled, finish rolling is completed in a temperature range of Ar_3 points or higher, and then cooling rate: 10°C/sec or higher. 5.
A hot-rolled steel sheet rolled in a temperature range of 00℃ or less is reheated to a two-phase region (α + γ) before being passed through a molten zinc bath and then hot-dip galvanized.The base material steel sheet is ferrite.・A method for manufacturing galvanized steel sheets with pearlite or ferrite-cementite structure and excellent workability. (2) Weight percentage: C: 0.003 to 0.18%, Si: 1.2% or less, N
n: 2.0% or less, sol. Al: 0.10% or less, S
: 0.005% or less, and Nb: 0.01 to 0.10%, V: 0.01 to 0.10
%, Ti: 0.01-0.10%, REM: 0.002
~0.10%, Ca:0.002~0.01%, Zr:
A steel billet containing 0.01 to 0.10% of one or more elements with the remainder substantially consisting of Fe and unavoidable impurities is hot rolled.
Finish rolling is completed in a temperature range of r_3 points or higher, followed by rapid cooling at a cooling rate of 10°C/sec or higher and coiled in a temperature range of 500°C or lower, before passing it through a molten zinc bath ( α
+γ) A method for producing a galvanized steel sheet with excellent workability, in which the base steel sheet has a ferrite-pearlite or ferrite-cementite structure, characterized by hot-dip galvanizing after reheating to a two-phase region.