JPS5852438A - Production of zinc plated thin steel sheet having thermal hardenability - Google Patents

Abstract

PURPOSE:To produce a zinc-plated thin steel sheet having thermal hardenability by subjecting rimmed steel contg. limited compsns. of C, Mn, P, S, N, etc. to hot and cold rolling and to annealing for controlling the contents of C and N then to zinc plating. CONSTITUTION:The rimmed steel consisting of <=0.09% C, 0.15-0.45% Mn, <= 0.03% P, <= 0.03% S, <=0.006% N and the balance Fe and unavoidable impurities is subjected to ordinary hot rolling and is coiled at <=600 deg.C. After pickling, the hot-rolled steel plate is cold-rolled at >=40% draft and is then annealed by heating the same to a 580-740 deg.C temp. region in a gaseous atmosphere contg. H2 and N2 and holding the same at these temps. to control C to <=0.004% and N to 5-15ppm. In succession, the cold-rolled steel plate is heated to 450-800 deg.C and is applied with continuous zinc plating and an alloying treatment; thereafter the plated steel plate is subjected to skin pass rolling, whereby the zinc-plated thin steel sheet which has good deep drawability and formability and hardens during baking of painting is obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing a galvanized thin steel sheet which has good deep drawability and formability, and which is sometimes hardened by paint baking.

High-tensile cold-rolled thin steel sheets for processing are used, for example, as materials for automobile bodies, and in addition to fatigue properties and paint corrosion resistance, excellent press formability is required.In addition, in order to increase the plastic deformation resistance of the finished product, Thermosetting properties (baking is hardening properties) are often required.

Generally, aluminum killed steel is used as a cold rolled steel sheet for deep drawing. However, with conventional aluminum killed steel, it is difficult to achieve both good deep drawability and formability, and even if it were possible, defects such as orange peel and rough skin were likely to occur during processing.

Thermosetting steel sheets or paint-baking-hardenable steel sheets are slow-aging thin steel sheets whose yield point elongation recovers slowly at room temperature, but after pressing and baking at 170°C x 20 m4, the yield point increases significantly when treated. It is known to have thermosetting properties. This thermosetting property is determined by the amount of solid solute carbon remaining in the steel in the finished product or! ! I is influenced by the amount of dissolved nitrogen. Normal person
Since killed steel is used and nitrogen is fixed as a precipitate called AIN, thermosetting properties are imparted by controlling the amount of solid solute carbon.

In order to achieve both good deep drawability and formability, this invention first manufactures a cold-rolled steel sheet using rimmed steel, and then decarburizes the carbon content at the initial stage of annealing to reduce it and heat harden it. This paper proposes a method for producing galvanized thin steel sheets with improved properties by controlling the amount of solute nitrogen.

That is, this invention contains 0.095% or less of carbon, 0.15 to 0.45% of manganese, 0.035% or less of phosphorus, 0.03% or less of sulfur, 0.00696% or less of nitrogen, and the balance is iron and non-iron.

Rimmed steel consisting of avoidable impurities is subjected to normal hot rolling and rolled up at a temperature of 600'C or less, and after pickling, the pressing rate is 4.
After cold rolling at 0.0% or more, annealing is performed by heating and holding in an atmospheric gas containing hydrogen and nitrogen at a temperature range of 580 to 740°C for 3 hours or more to reduce carbon to 0.0045% or less and nitrogen. The gist is to control the mjc of 5 to 15 p, and then use this cold rolled product to heat it to 450 to soo'c, apply continuous zinc plating and alloying treatment, and then perform temper rolling.

In this invention, the components of the target steel are limited for the following reasons.

It is desirable to reduce the amount of carbon in the steel manufacturing stage to improve drawability, but on the other hand, to form a good rim layer, it is desirable to reduce the amount of carbon.
．． 09% or less. In addition, in the annealing stage, the carbon content of the final product is reduced to 0.004% in order to promote crystal grain growth and improve drawability and ductility, and to prevent aging due to carbon.
Decarburization treatment is performed in an oven coil annealing furnace until the following.

A small amount of manganese is desired to improve drawability, and 0.45% to prevent ductility deterioration due to solid solution hardening.
The following are desirable. However, if it is less than 0.15%, surface flaws due to red heat brittleness are prevented and the ability to form a good rim layer becomes insufficient, so the content is set at 0.15 to 0.45%.

Both phosphorus and sulfur are contained as impurities, but since they both have a negative effect on press formability, the content is set to 0.03% or less.

Nitrogen is a necessary component to obtain thermosetting properties, but in order to increase drawability after cold-pressure recrystallization, it is desired to be as small as possible even in the steel manufacturing stage, so nitrogen is set to 0.00696 or less.

The amount of nitrogen after annealing is the most essential part of this invention, and Figure 1 is a chart showing the relationship between the amount of nitrogen contained in the target steel, the amount of heat hardening, and the yield elongation. It exhibits 3 kf/- or more when the amount is more than 15 Dpm, and on the other hand, recovery of yield elongation becomes noticeable at 15 Dpm, making it easy to cause problems as an automobile exterior material, for example. Therefore 5~is
It was set as ppm.

The target steel in this invention is melted in a converter or the like, and hot-rolled into a slab by blooming from a steel ingot.

At this time, there is nothing wrong with using a direct feeding method (so-called direct roll) in which hot rolling is performed without water cooling after blooming. At this time, hot rolling is generally carried out by heating at 1100° C. or higher, finish rolling is completed at a temperature higher than the %ArI transformation point, and winding is performed at a low temperature of 600° C. or lower. In this winding, the lower the temperature, the better the r value can be obtained.

The hot rolled coil thus obtained is pickled to remove scale and then cold rolled, preferably at a rolling reduction of 40% or more from the viewpoint of drawing workability.

The annealing performed after the above cold rolling is necessary to control the carbon and nitrogen content, and for that purpose, 580 to 74
It is desirable to heat and maintain the material in an atmospheric gas containing hydrogen and nitrogen at a temperature of 0° C. for 3 hours or more. That is, under these conditions, sufficient decarburization and denitrification can be performed, and the carbon content is controlled to 0.004% or less and nitrogen to 5 to 15 pprn.

In order to make the amount of carbon content a predetermined amount, decarburization treatment is carried out by blowing steam into a surrounding gas containing 1 m of nitrogen such as AX gas in an oven coil annealing furnace.

This may be a commonly used method.

In order to keep the nitrogen content at a predetermined level, the Ha and N1 concentrations and soaking temperature should be adjusted to K in the atmosphere of the oven coil annealing furnace.
= Balance the N (nitrogen content) by setting the horse so that it corresponds to 9 (K at this time is given by, for example, 200 x 0.0981 x P (-1 plus chi)).

The soaking temperature at this time is 580 to 740°C, and the soaking is maintained for 3 hours or more. Generally, a stable amount can be obtained at a low temperature, but if it is too low, it will take a long time for N to equilibrate.
The temperature was set at 80°C, and since a high H-concentration is required on the high temperature side, the temperature was set at 740°C. Further, the heat cycle can be performed either by performing decarburization and nitrogen control at the same time or by performing nitrogen control after the decarburization is completed.

For zinc plating, the cold-rolled steel sheet produced as described above is heated to 450 to 800°C in a Sendzimir type continuous plating line, plated in a conventional manner, and then alloyed at 580°C.

In addition, temper rolling is carried out in a conventional manner within a range of 2% or less.

Next, embodiments of the invention will be described.

Example 1 A slab of rimmed steel whose chemical composition is shown in Table 1 was
The samples were heated to .degree. C., hot-rolled, finished at 900.degree. C., and wound into coils at the winding temperatures shown in Table 1, respectively. After winding, pickling and cold rolling at a reduction rate of 73% to produce a cold rolled steel plate with a thickness of 0.8 fi, steels 41 to 3 shown in Table 1 were prepared according to the present invention. For example, steel weight 4 was subjected to decarburization and nitrogen control, and for comparison, steel weight 4 was subjected to high coiling temperature (and nitrogen control was not performed), and steel A5 was annealed under different annealing conditions. After annealing under the conditions shown in , it was heated to 650'C in a Sendzimir type continuous plating line and plated, followed by alloying treatment and 196 temper 5 rolling.Table 1 shows the chemical Ingredients and processing conditions are shown.

Table 1 Chemical composition and processing conditions Samples were taken from the above six steels and tested for mechanical properties. The results are shown in Table 2.

Table 2 Test results From the above results, it can be seen that all the products according to the present invention have a high r value and excellent press formability.

Example 2 Carbon 0.05%, manganese 0.28%, phosphorus 0.012
%, 0.011% sulfur, 0.0024% nitrogen, and the remainder substantially iron, and controlled the nitrogen content to various degrees during annealing to improve thermosetting and yield elongation. I investigated the shadow it gives. The results are shown in FIG.

From this result, the effective amount of heat curing is required to be 3 kg/- or more, but for that purpose the nitrogen content must be 5 ppm.
It turns out that more than that is necessary. In addition, in order to keep the yield elongation to 0.5% or less, which does not cause any practical problems, the nitrogen content should be 151%.
) pm or less. Therefore, the nitrogen content is controlled to 5 to 15 ppm during annealing.

[Brief explanation of the drawing]

FIG. 1 is a chart showing the relationship between the amount of nitrogen contained, the amount of heat hardening, and the yield elongation of the target steel of the present invention. Applicant: Sumitomo Metal Industries, Ltd. r, 1

Claims (1)

[Claims] Carbon 0.09% or less, manganese 0.15-0.45%,
Phosphorus 0.03% or less, sulfur 0.03% or less, nitrogen 0.
Rimmed steel consisting of 0.006% or less, the balance iron and unavoidable impurities is subjected to normal hot rolling and coiled at a temperature of 600°C or less, pickled and then cold rolled at a rolling rate of 40% or more. A cold-rolled steel sheet is annealed by heating and holding in an atmosphere containing hydrogen and nitrogen for 3 hours or more in a temperature range of 580 to 740°C, and the carbon content is controlled to 0.004 S or less and nitrogen to 5 to 15 ppm. 1. A method for producing a thermosetting galvanized thin steel sheet, which comprises successively heating the steel sheet to 450 to 800 DEG C. to perform continuous galvanizing and alloying treatment, followed by temper rolling.