JPS5929087B2 - Manufacturing method for low yield point hot-dipped galvanized steel sheets - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing hot-dip plated steel sheets with a low yield point of 23 kg/1 or less using a Sendzimer-type continuous hot-dip plating line.

The hot-dip coated steel sheets obtained by the Sendzimer-type continuous hot-dip plating line with built-in in-line annealing are superior to the steel sheets produced by the flux-type hot-dip plating line, which uses outline annealing before plating, in terms of production efficiency, economy, labor savings, etc. Although it has many advantages, the material is hard due to rapid heating and rapid cooling during continuous annealing, making it extremely difficult to produce a soft material. That is, due to the rapid heating and cooling of continuous annealing, (1) ferrite crystal grains become fine, (2) it is difficult to control the optimum texture, and (3) ferrite grains become fine.
) Hardening is unavoidable due to quenching aging due to supersaturated solid solute carbon, and it has good shape freezing properties during processing and prevents the occurrence of stretcher strain.
Moreover, it is extremely difficult to manufacture soft materials with a low yield point (for example, a yield point of 23 kg/1 or less) that can be processed manually for applications that require simple bending. It was usually considered difficult. For example, in recent years, with the aim of producing soft hot-dip plated steel sheets with a low yield point and good workability on a Sendzimer-type continuous hot-dip plating line, the values (Mn, S)
Adjustment of chemical components using the O content ratio (a value that expresses a specific relationship) as an index, promotion of grain growth by high-temperature winding during hot rolling, precipitation of supersaturated solid solution carbon by over-aging treatment, etc. Attempts have been made to implement this treatment, but due to the difficulties in steelmaking technology that can accommodate value adjustment and restrictions on capital investment for establishing an over-aging zone, it has not been possible to generalize the process, and
As with the case of flux-type hot-dip galvanized steel sheets that are pre-outline annealed, in the end, the use of special materials such as decarburized annealed pre-plating material to which special elements such as Al are added is used as the base material coil. It is normal that it is on.

However, in this case, not only does the manufacturing cost inevitably increase, but the reaction products generated during the pre-plating annealing remain without being removed in the reducing atmosphere of the plating pre-treatment, which may lead to non-plating. There were problems with deterioration of surface quality and yield loss. The present invention solves the above-mentioned various problems and develops a new method for efficiently manufacturing low-yield point hot-plated steel plates made of low carbon steel using a continuous hot-plating line of the Zenzima type. However, as described in the claims, when producing a hot-dip galvanized steel sheet with a low yield point using the Zenzima type 1 continuous hot-melt plating line, the C content is set to 0.02 to 0.1.
5% low carbon steel is hot rolled at a coiling temperature of 650°C or higher to produce a hot coil, followed by normal cold rolling and then softening annealing in a non-decarburizing atmosphere. After further cold rolling at a cold rolling rate of 10 to 20%, the sheet is passed through a Sendzima type 1 continuous melt plating line, and in-line annealed in this Sendzima type 1 continuous melt plating line with 650 molten C-ACl ( The present invention provides a method for producing a low-yield-point hot-dip galvanized steel sheet, characterized in that plating is carried out in a temperature range of 723° C.).

The principles of the present invention are as follows: (1) Reaction products generated during pre-plating annealing can be destroyed if the reaction products are subsequently cold rolled at a cold rolling rate above a certain level, (2) In addition, in combination with the subsequent in-line annealing of the Zenzima type continuous melting line, rolling can provide the effect of the so-called "strain annealing method", and the cold rolling rate in this cold rolling can be adjusted to an appropriate level. If it acts as strain energy (prestrain), it can contribute to reducing the yield point through the growth and coarsening of ferrite crystal grains.
When using the treatment method 2), the requirement that the yield point be 23 kg/ml or less means that the material of the base coil does not need to be a decarburized annealed material as is the case when special elements such as A1 are added. This is based on the knowledge that ordinary non-decarburized annealed materials can sufficiently satisfy the requirements, and in addition to this knowledge, the hot coil winding temperature in hot rolling is set at a predetermined temperature or higher to produce ferrite. After assisting the coarsening of grains, after cold rolling as usual, softening annealing is performed in a non-decarburizing atmosphere, and then secondary cold rolling is performed again at a light cold rolling rate. At the same time, strain energy is applied to destroy the reaction product, and the strain annealing method is performed by in-line annealing of the Zenzima type continuous melting line, thereby achieving low yield point melting while further growing and coarsening the ferrite crystal grains. The purpose is to produce plated steel plates stably and inexpensively.

In other words, the method of the present invention lowers the yield point by growing and coarsening ferrite crystal grains, and the methods include controlling the winding temperature of the hot coil, specific secondary cold rolling, and specific in-line annealing control. It is characterized by the application of Ferrite crystal grains are an important factor governing mechanical properties, and it is well known, for example, as in the Hall-Petch relationship, that the coarser the ferrite crystal grains, the lower the yield point.

The method of the present invention cleverly utilizes this relationship between ferrite crystal grains and yield point in the Zenzima type continuous melt plating line, but in order to obtain a low yield point melt plated steel sheet with a yield point of 23 kg/i or less, It was confirmed that it is necessary to make the particle size NO of the grains 6 or less. In other words, in the present invention, the ferrite crystal grains are larger than a predetermined value (for example, the particle size NO is 6 or less) in the Zenzima type continuous melting line.
It can be said that we have discovered a method for manufacturing hot-dip galvanized steel sheets. Each requirement of the present invention and the reason for numerical limitation will be individually explained in detail below. First, the great invention is to manufacture a hot coil by winding low carbon steel with a C content of 0.02 to 0.15% at a coiling temperature of 650°C or higher during hot rolling. Lower limit of amount 0.02%
The reason for this is that it is difficult to melt steel with a lower C content using normal steel manufacturing methods, and the present invention requires decarburization using a vacuum degassing device and an open coil annealing furnace. This is because one of the objectives of the system, which is low cost and economic efficiency, is impaired.

Further, the upper limit of the C content is limited to 0.15% because a C content exceeding 0.15% has a strong material hardening effect. In addition, the coil winding temperature during hot rolling was set to 65
The reason why the temperature is set at 0°C or higher is that by high-temperature winding at a temperature higher than this temperature, the carbides are aggregated and coarsened to reduce the grain boundary movement inhibiting effect of the carbonization proboscis, thereby causing the growth and coarsening of ferrite crystal grains. It's for a reason. At a winding temperature of less than 650°C, this effect is insufficient and the object of the present invention cannot be achieved even when combined with subsequent steps.

3 The hot coil thus obtained is then subjected to usual pickling and cold rolling, and then softening annealing in a non-decarburizing atmosphere.
Then, secondary cold rolling is performed again at a cold rolling rate of 10 to 20%. This secondary cold rolling is carried out, and the cold rolling rate is 10 to 201%.
A major feature of the present invention lies in the range of . This secondary cold rolling, which is carried out after softening annealing in a non-decarburizing atmosphere, is due to the destructive effect of the reaction products generated in this pre-plating annealing as described above, and the in-line of the Zenzima type continuous melt plating line that will be carried out subsequently. It provides the effect of imparting strain energy (prestrain) that assists in the growth and coarsening of ferrite crystal grains during annealing. In the former case, the destructive effect of reaction products increases as the cold rolling rate increases, but in the latter case, if the cold rolling rate is too high, the prestrain becomes excessive and the fine grains of the ferrite crystal grains This will cause the phenomenon of
Through a series of experiments, it was found that there is a preferable range for selecting this cold rolling rate. Figure 1 shows a representative example of the results, and is a plot of the relationship between the cold rolling rate and the Funierite grain size NO in the secondary cold rolling. This figure 1 shows the secondary cold rolling rate when applying prestrain in this secondary cold rolling when attempting to grow and coarsen ferrite crystal grains in combination with in-line annealing of the Sendzima type continuous melt plating line. It has been shown that NO affects the ferrite crystal grain size NO in a specific relationship, and that a cold rolling rate of about 15% has the greatest effect on the growth and coarsening of ferrite crystal grains. According to the experiments conducted by the present inventors, in order to make the yield point 23 kg/1ii or less, the grain size NO of the ferrite crystal grains must be 6 or less. As such, the secondary cold rolling ratio must be limited to a range of 10 to 20%. In other words, when the secondary cold rolling ratio is less than 10%, the ferrite crystal grains do not grow uniformly and become coarse due to the lack of strain energy (prestrain), and the coarse grains center around the center of the plate thickness. As a result, a mixed grain structure in which fine grains remain is exhibited, and as shown in FIG. 1, the grain size of the ferrite crystal grains becomes difficult.

Therefore, the in-line annealing temperature in this Zenzima type 1 continuous melt plating line was set at 650°C to ACl (723°C).
℃) is an extremely important requirement for the present invention. Examples will be described below.

Example Table 1 shows the chemical composition values (wt%) of the sample materials.

This low carbon steel was melted in a converter and seven slabs were manufactured. These seven slabs (A-G) were each rolled at the hot-rolling temperature shown in Table 2 to produce a 2.7 x 930 hot coil.Next, each hot coil was pickled and cold-rolled as usual. Rolling (first stage) is carried out, and in this first stage cold rolling, the five coils A to E are rolled from 2.7% to 0.7%.
♂, the F coil goes from 2,7 to 0.86', and the G
The coil was cold rolled from 2.7% to 0.63~. Next, each coil was subjected to softening annealing in a non-decarburizing atmosphere, and then subjected to secondary cold rolling at the cold rolling rate shown in Table 2.

That is, the five coils A-E are 0.7 to 0.6
(cold rolling rate 14%), F coil is 0.86% to 0
．． 6% (cold rolling ratio: 30%), and the G coil was subjected to secondary cold rolling from 0.63% to 0.6% (light rolling, tempering ratio: 5%) to impart strain energy. A cold-rolled coil was manufactured. Each of the obtained cold-rolled coils was passed through a Zenzima type continuous hot-dip galvanizing line, and the in-line annealing temperature was controlled under the conditions of 600, 700, or 750 degrees C as shown in Table 2. Zinc adhesion amount 609/
TrIO? A galvanized steel sheet was manufactured.

The mechanical properties (especially yield point), ferrite grain number (Figs.

Claims (1)

[Claims] 1. When manufacturing hot-dip plated steel sheets with a low yield point on a Sendzimer-type continuous hot-dip plating line, the C content is reduced to 0.02.
650% during hot rolling of low carbon steel containing ~0.15%
A hot coil is manufactured by coiling at a coiling temperature of ℃ or above, followed by normal cold rolling, followed by softening annealing in a non-decarburizing atmosphere, and further cold rolling at a cold rolling rate of 10 to 20%. After rolling, the plate is passed through a Sendzimer continuous hot-dip plating line, and in-line annealed at 650°C to Ac_1 (723°C) in this Sendzimer continuous hot-dip plating line.
A method for producing a low-yield-point hot-dip plated steel sheet, characterized in that plating is carried out in a temperature range of . 2. The manufacturing method according to claim 1, wherein the yield point of the hot-dip plated steel sheet is 23 kg/mm^2 or less.