JPS5858264A - Production of low yield ratio and high tensile hot dipped steel plate with zinc - Google Patents

Abstract

PURPOSE:To produce a low yield ratio and high tensile hot dipped steel plate with zinc having good workability by subjecting a steel plate of the composite structure contg. a ferrite phase and a martensite phase to dip in molten zinc by way of an intermediate stage which suppresses the quantity of strains low. CONSTITUTION:The steel plate contg. 0.01-0.15% C, 1.9-3.0% Mn, 0.005-0.10% solAl, and <=1.0% Si, further contg. >=1 kind among <=1.0% Cr, 0.01-0.10% Nb and 0.01-0.2% Ti and consisting of the balance Fe and impurities unavoidable in production is annealed by box annealing at temps. above the A1 transformation point and below the A3 transformation point to have the composite structure contg. a ferrite phase and a martensite phase. Further in an intermediate stage up to a zinc plating treatment, the quantity of the strains applied upon the steel plate is suppressed down to <=1.3%. It is possible to apply an alloying treatment of iron zinc to said hot dipped steel plate in molten zinc.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing a hot-dip galvanized steel sheet using a steel sheet with a composite structure containing a ferrite phase and a martensitic phase as a base plate, and particularly relates to a method for manufacturing a hot-dip galvanized steel sheet using a steel sheet having a composite structure containing a ferrite phase and a martensitic phase. The present invention relates to a method for producing a low yield ratio, high tensile strength hot dip galvanized steel sheet whose specific properties are not impaired by hot dip galvanizing.

In recent years, the application of high-strength galvanized steel sheets has been considered as a material for automobiles, for example, due to the need for lightweight, strong materials that do not rust. A high-strength galvanized steel sheet using a reinforced steel sheet as the base sheet was proposed.

However, such steel sheets generally have a high yield ratio and low ductility, so they cannot be subjected to severe processing, and there are still problems with their actual application, such as large springback after processing.

The present invention has a yield ratio of 70 cm/- without such problems.
This was made for the purpose of providing the above-mentioned low yield ratio, high tensile strength hot-dip galvanized steel sheet, and solved the problem of workability when using composite structure high tensile strength steel as the plated original sheet. In general, even if a composite structure steel is adjusted to appropriately contain a ferrite phase and a martensitic phase by annealing, when it is subjected to hot-dip galvanizing, the yield ratio increases due to the heat applied during the plating. The present invention has been made specifically for the purpose of suppressing the increase in yield ratio during hot-dip galvanizing to obtain a hot-dip galvanized steel sheet that maintains the low yield ratio characteristic of composite structure steel. After conducting various experimental studies for this purpose, we found that the degree of increase in the yield ratio of composite structure steel during hot-dip galvanizing is greatly influenced by the strain applied to this steel before hot-dip galvanizing. It has been found that when the amount of strain applied is suppressed to 1.5% or less, the yield ratio can be reduced to 70% or less, and a high-strength hot-dip galvanized steel sheet with good workability can be obtained. That is, the present invention provides a method for hot dip galvanizing a composite structure steel sheet whose composition has been adjusted to exhibit a composite structure including a ferrite phase and a martensitic phase by annealing at a temperature above A1 transformation point and below A3 transformation point. A low yield ratio, high tensile strength galvanized steel sheet characterized by suppressing the amount of strain imparted to the steel sheet to 1.5% or less in an intermediate step between box-shaped annealing and hot-dip galvanizing so that the steel sheet exhibits The present invention provides a method for manufacturing.

In the present invention, the annealing process employs a box annealing (batch annealing) method instead of a continuous annealing method. In the case of continuous annealing method,
Although the reason for this is not clear, the yield ratio becomes extremely high due to the heating applied to the steel sheet during hot-dip galvanizing, and the low yield ratio characteristics are impaired. However, in the case of box-shaped annealing, the strain imparted by the annealing process itself is limited to the extent of unwinding the coil, and does not help increase the yield ratio due to the process operation. The annealing conditions for this box annealing are the conditions necessary for the applied steel to have a composite structure containing a ferrite phase and a martensitic phase, and are a temperature of A1 above the A1 transformation point and A below the transformation point.

The hot-dip galvanizing process can be carried out using a normal continuous hot-dip galvanizing line. For example, galvanizing can be carried out using a continuous hot-dip galvanizing line in which a steel sheet coil is processed through a coil roux, a pickling process, a reduction treatment process, and a zinc bath immersion process.

At that time, an alloying treatment step for forming an iron-zinc alloy layer may be added after immersion in the zinc bath.

Between the box annealing process and this hot dip galvanizing process,
In normal production of hot-dip galvanized steel sheets, the steel sheets are subjected to processes such as leveling and skin scraping to correct the shape of the coil, and root rolls that undergo repeated bending, which add distortion to the steel sheets. When strain is imparted to the composite structure steel, the yield ratio increases due to the heat applied during hot-dip galvanizing, as shown in Examples below.

Therefore, in the method of the present invention, such treatments that give strain to steel are avoided as much as possible, and the rolls of the coil looper installed at the entrance of the hot-dip galvanizing line are also made to have as large a diameter as possible. Reduce the amount of distortion as much as possible. As shown in the examples below, when the strain ε expressed as ε-t/r x 100, where t is the thickness of the steel plate and r is the roll radius, is 1.5 or less, By applying this level of strain, the increase in yield ratio can be suppressed to a practically sufficient level even by heating during hot-dip galvanizing.

Next, the steel to which the method of the present invention is applied is c; o, oi~O
j596, Mn; 1.9-5.0%, 5olJJ
0,005 to 0.10%, Sf; 1.0%, and further contains 1.0% or less of Or, 0.01 to 0.
A steel containing 10% of Nb-j and 0.01 to 0.20% of Tj, with the remainder being Fe and impurities unavoidable during manufacturing, is used.

C is preferably as low as possible in order to improve the workability and weldability of the steel, but it is extremely essential to increase the strength, and for the purpose of the present invention to obtain high tensile strength steel, it is at least 0.01. % is necessary, and in order to reduce it, the upper limit is set to 0.15%.

Mn is an element that effectively acts to obtain low yield ratio characteristics. If the Mn amount is less than L9 qA, a structure with a low yield ratio of 75f cannot be obtained, whereas if it exceeds 5 (J%), weldability and workability The range is 1.9 to 3.
Let it be 0chi.

Cr has the effect of lowering the yield ratio, but even if it is contained in an amount exceeding 1%, the effect is saturated.

Si is the most suitable element for improving tensile strength without impairing workability, but if a large amount of Si is contained, the surface appearance after plating will deteriorate and the incidence of unplated parts will be significant. Therefore, it is better to set the upper limit to 1.0ch.

Bol, A1. is necessary as a deoxidizer, and 0.0[15
If the deoxidizing effect is less than 0.0%, the deoxidizing effect will be small, while if it exceeds o or io%, alumina-based inclusions will increase, degrading the material properties and surface appearance of the steel.
Let it be 0chi.

Nb and T1 may be contained as necessary, and are particularly effective in increasing the tensile strength. If Nb is less than 0.01%, the effect of increasing tensile strength is small;
Since the effect is saturated even if the content exceeds 0.01% to 0.10%. T1 has the effect of controlling the form of inclusions and is also effective in improving the workability of steel, but if it is less than 0.01%, the effect of increasing tensile strength is small, and even if it is contained in excess of 0.20%, the effect of increasing the tensile strength is small. Since the effect is saturated, the range is preferably 0.01 to 0.20%.

The present invention will be specifically explained below with reference to Examples.

Example 1 A steel plate having the chemical composition shown in Table 1 and having a thickness t=2.0 was annealed at 750°C for 5 hours to obtain a composite structure steel plate having a low yield ratio shown in Table 1. This steel plate was bent along the outer periphery of a cylinder with radius r = 75 to 500 cm, and then similarly bent in the opposite direction to form a flat plate. The amount of strain ε on the steel plate at this time is ε−−
×100(9)). This sample was subjected to heat treatment equivalent to continuous hot-dip galvanizing at 450℃ and 600'C for 2 hours.
As is clear from the results in Table 2, the yield ratio increases as the amount of strain increases, and when the amount of strain exceeds 1.3 inches, the yield ratio increases to 70% or more. Yield characteristics are impaired.

Example 2 Steel plate f with chemical composition shown in Table 3, plate thickness t = 2.0 mm
Box annealing was performed at 30 C for 5 hours to obtain a composite structure steel sheet with a low yield ratio shown in the same table. This steel plate has a radius r = 100mm
(ε=t/r=20%), 600 ho(ε=-=Q,
After introducing the same bending strain as in Example 1 along the outer periphery of the cylinder of No. 07 H), the heat treatment shown in FIG. 1, which corresponds to pickling-reduction continuous hot-dip galvanizing, was applied. Table 4 shows the tensile test values of the steel plates thus obtained.

From the results in Table 4, it can be seen that the steel plates with a strain amount of ε-1r −〇 and 67% within the range of the present invention all exhibit a yield ratio of 70 inches or less, whereas ε-t/r = outside the range of the present invention. 2.0
It is clear that all of the steel plates shown in Table 1 have yield ratios exceeding 70%.

Example 3 Steel with the chemical composition shown in Table 5 was melted in a converter degassing facility,
Continuous casting, hot rolled gauze plate thickness t=2.0m7H and 4.5mm0
It was made into a hot rolled coil. After pickling this coil, it was heated to 725C for 1
Tight coil box type annealing was performed for 0 hours to obtain a composite structure steel strip with a low yield ratio shown in Table 6. Next, the radius r: 3 is applied to this coil without leveling or skin pass rolling.
After pickling through a coil looper with 6 rolls of 0.00 m, reduction treatment is performed at a temperature of 500 to 620 C, and 460 C.
After being immersed in a hot-dip galvanizing bath, the alloyed syllables were subjected to continuous hot-dip galvanizing. The tensile test values of the hot-dip galvanized steel sheets thus obtained are also shown in Table 6.

The material with a plate thickness of 2.1 ml in Table 6 is given a strain of ε = t/r X
100 = 1.5chi strain is applied,
The latter material with ε exceeding 1.3% showed a high yield ratio of 70% or more.

[Brief explanation of drawings]

FIG. 1 is a diagram showing an example of heat treatment in hot-dip galvanizing treatment in an embodiment of the present invention. Applicant: Nisshin Steel Co., Ltd. 1st Wj Time (seconds)

Claims (1)

[Claims] (In hot dip galvanizing a composite structure steel sheet whose composition has been adjusted to exhibit a composite structure including a ferrite phase and a martensitic phase by annealing at a temperature above 11A+transformation point and below A3 transformation point) A low-yield ratio, high-tensile welding process characterized by suppressing the amount of strain imparted to the steel sheet to 1.6% or less in an intermediate process from box-shaped annealing to exhibit a conjugated structure to hot-dip galvanizing. Method for producing galvanized steel sheet. (2) Steel sheet contains C; 0.01 to 0.15%, Mn;
1.9~3.0chi, sol, At; 0.005~0
．． 10%, Si; Contains 1.0% or less, and further contains 1
．． Contains one or more types of 0% or less Or, 0.01 to 0.10% Nb, or 0.01 to 0.20% T1, and the remainder consists of Fe and unavoidable impurities during manufacturing. A method for producing a low yield ratio, high tensile strength hot-dip galvanized steel sheet according to claim 1. (3) The hot-dip galvanizing process is performed in a continuous process line consisting of a pickling process, a reduction process, and a zinc bath immersion process. Manufacturing method of galvanized steel sheet. f41 A, a composite structure steel sheet whose composition has been adjusted to exhibit a complex structure including a ferrite phase and a martensitic phase by annealing at a temperature above the transformation point and below the A3 transformation point, is hot-dip galvanized, and further alloyed with iron-zinc. When performing the treatment, the strain applied to the steel sheet is suppressed to 1.3 inches or less in the intermediate process between box-shaped annealing to create a composite structure and hot-dip galvanizing. Manufacturing method of specific high tensile strength hot-dip galvanized steel sheet. (5) Steel plate contains C; 0.01 to 0.15%, Mn;
1.9~3.0chi, sol, A7; 0.005~
0.10%, Sj; Contains 1.0ch or less, and further,
Contains one or more of 1.0% or less of Or, 0.01 to 0.10% of NT) or 0.01 to 0.20% of T1, with the remainder being Fe and unavoidable in production. A method for producing a low yield ratio, high tensile strength hot-dip galvanized steel sheet according to claim 4, which comprises impurities.