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EFFECTS OF ALLOYING ELEMENTS ON

MATERIAL PROPERTIES

CARBON

C is an element which adversely affects press-workability. The C content, therefore,should be not greater than 0.005 wt%, in order to obtain a steel sheet havingexcellent press-workability under a condition where heating and soaking have to bedone in short time as in the case of annealing in a continuous hot-dip galvanizingprocess.

TITANIUM

Ti is an element which reacts with inevitably existing elements such as N and C so as

to form TiN and TiC, thereby fixing such elements, thus eliminating any undesirableeffect of such elements on press-workability, and thus enhancing the effect of Bwhich will be mentioned later. In order to attain an appreciable effect of addition ofTi, the Ti content should be at least 0.005 wt%. On the other hand, however, additionof Ti in excess of 0.05 wt% causes burning defects in gal annealing process. The Ticontent therefore should not exceed 0.05 wt%.

ALUMINUM

Al is an element which is added to prevent oxidation of elements such as Ti, Nb and B

which are added to the molten steel. In order to sufficiently deoxidize the moltensteel, it is necessary that the Al is added in an amount which is not smaller than 0.01wt%. On the other hand, addition of Al in excess of 0.1 wt% causes a rise in the cost.The Al content, therefore, should be not smaller than 0.01 wt% and not greater than0.1 wt%.

NIOBIUM & BORON

Nb and B are elements which are effective in preventing softening of steel sheet athigh temperature. This advantageous effect is obtained only when both Nb and Bcoexist. In order to attain appreciable effect in preventing softening at high

temperature, the Nb content should not be smaller than 0.005 wt% and the B contentshould not be smaller than 0.0002 wt%. However, addition of Nb in excess of 0.015wt% undesirably reduces the ductility of the steel sheet at normal temperature, thusimpairing press-workability. On the other hand, any B content exceeding 0.002 wt%causes a reduction in the Lankford value r which is an index of deep-drawability inpress work, thus impairing press-workability. The Nb and B contents, therefore, arelimited to be from 0.005 to 0.015 wt% and from 0.0002 to 0.002 wt%, respectively.

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SILICON

Si is an element which is effective in strengthening the steel and is added inaccordance with the demand for strengthening. Addition of Si in excess of 0.1 wt%,however, adversely affects the deep-drawability and elongation so that Si content is

determined to be not greater than 0.1 wt%.

MANAGANESE

Mn also is an element which strengthen the steel. The Mn content, however, islimited to be not greater than 1.0 wt%, because Mn content exceeding 1.0 wt%undesirably reduces deep-drawability.

CONCLUDING REMARKS

A cold-rolled steel with the contents of components controlled as described aboveexhibits superior press-workability when annealed by being reheated to atemperature ranging between 770 and 900 C. When the annealing temperature isbelow 770 C., it is impossible to obtain sufficient recrystallization effect. On theother hand, when the annealing temperature exceeds 900 C., a transformationtakes place to reduce the Lankford value r, thus causing reduction in ductility. Theannealing temperature, therefore, should be determined to be from 770 C. to 900C.

The rate of cooling of the annealed cold-rolled steel sheet before entering a moltenzinc bath should be 10C./sec. This cooling rate causes a moderate level of internalstress to be generated in the steel sheet, thus imparting greater resistance tosoftening of the portions of the steel sheet thermally affected during spot welding.

In order to enhance this advantageous effect, it is preferred that the cooling beconducted at a rate which is 20C./sec or greater.

The cooling at such a fast rate, i.e., quenching, is ceased when the steel sheet isdipped in the molten zinc bath. It is necessary that the steel sheet is cooled to 530C. at the highest before entering the molten zinc bath. On the other hand, coolingdown below a lower limit temperature of 380 C. causes plating failure.

The Al content in the bath is not a factor which directly affects the spot-weldability,but produces an effect to effectively suppress exfoliation or peeling of the platinglayer during the press work particularly when the Fe content of the plating layer iscomparatively large. More specifically, it is possible to obtain a resistance toexfoliation or peeling of the plating layer during press work, high enough to enablethe plated steel sheet to be used as an automotive body part when the Fe content of

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the plating layer ranges between 9 and 12 wt%, provided that the Al content in theplating bath is 0.13 wt% or more, and preferably is equal to or higher than 0.15 wt%.

It is a critical feature of the present invention that the plating layer has an Fe contentnot smaller than 9 wt%. When the Fe content is below 9 wt%, it is impossible to

obtain the required spot-weldability even when the contents of the components ofthe base steel sheet are controlled as specified above. This is attributed to the factthat Fe content below 9 wt% undesirably allows presence of phase of low melting point in the plating layer so as to seriously promote the consumption of the spotwelding electrode. On the other hand, any Fe content in the plating layer exceeding12 wt% reduces the resistance to exfoliational peeling of the plating layer duringpress work, tending to cause a phenomenon known as "powdering". For thesereasons, the Fe content in the plating layer is limited to be from 9 wt% to 12 wt%.