JPS5839770A - Production of high-strength zinc hot dipped steel plate - Google Patents

Abstract

PURPOSE:To make the production of a high-strength zinc hot dipped steel plate having a low yield ratio, and excellent strength-elongation balance and stretch- flangability in the production of a zinc hot dipped steel plate by forming the structure of the steel plate into specific 3-phase composite structure. CONSTITUTION:An ingot contg. 0.005-0.15% C, <1% Si, 0.7-2.5% Mn, <0.1% P or further <0.5% Cr is hot and cold rolled to a cold-rolled steel plate. After the steel plate is annealed in a direct firing type heating furnace of a non- oxidative atmosphere at 900-1,100 deg.C and a radiation heating furnace of a reducing atmopshere, the steel plate is passed through a slow cooling zone, a quick cooling zone and a cooling control furnace then through the inside of a zinc hot dipping bath of about 500 deg.C, whereby zinc hot dipping is applied thereon. The ferrite-bainite-martensite 3-phase structure contg. 5-50% area rate of bainite and 1-20% area rate of martensite is formed, and the high strength zinc hot dipped steel plate of the composite structure having a low yield ratio and excellent strength-elongation balance and stretch-flangability is obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing a high-strength galvanized steel sheet with a composite structure having a low yield ratio and excellent strength-elongation balance and stretch flangeability.

In general, alloying elements are added in various countries to increase the strength of steel sheets, but hot-dip galvanized steel sheets are annealed above the recrystallization temperature, making it difficult to obtain strength. There is a problem that the galvanizing properties deteriorate.

Recently, a composite copper plate with martensite dispersed in a ferrite matrix has been attracting attention as a high-strength steel plate with good workability.This steel plate has a low yield ratio and excellent workability and time hardening after press forming. There is. However, when this composite structure steel sheet is used as a hot-dip galvanized steel sheet, martensite dispersed in the ferrite matrix is tempered during hot-dip galvanizing at approximately 500 tons, resulting in a decrease in strength and an increase in yield ratio. There is a problem with doing so.

In view of the above-mentioned circumstances, the present invention utilizes the thermal history during continuous hot-dip galvanizing line passing during the production of high-strength hot-dip galvanized steel sheets to achieve low drop-down ratio and good strength, which are characteristics of composite structure steel. The purpose of this invention is to provide a method for manufacturing a high-strength cold-rolled steel sheet that maintains elongation balance and has excellent stretch flangeability.

In other words, this formula is c O,005-0,15%,
st 1% or less.

A cold-rolled steel sheet obtained by hot rolling copper containing MmO, 7 to 2.5%, P 0.1% or less, and Or 0.5% or less as necessary, is coated with zinc on a continuous hot-dip galvanizing line. Through this plating process, the steel plate structure is made into an 8-phase composite structure with a bainite area ratio of 6 to 60% and a martensite area ratio of 1 to 2096 & ferrite + bainite + martensite, which is characterized by a low yield ratio and strength-elongation balance. and a method for producing a high-strength hot-dip galvanized steel sheet with excellent stretch flangeability.

The thermal history in the continuous hot-dip galvanizing process of the hot-dip galvanized steel sheet that is the object of the present invention is usually as follows.

That is, a cold rolled steel sheet manufactured through hot rolling and cold rolling was annealed in a direct fire heating furnace in a non-oxidizing atmosphere and a radiant heating furnace in a reducing atmosphere at a furnace temperature of 900' to 1100''C. After passing through the slow cooling zone, rapid cooling zone, and cooling adjustment furnace, it cools for about 5 minutes.
When manufacturing a hot-dip galvanized steel sheet having a mixed structure consisting of ferrite, bainite with an area ratio of 5 to 70%, and martensite of 1 to 2096, which passes through a galvanizing bath at 00°C, a non-oxidizing atmosphere or a reducing atmosphere is used. After heating the steel plate to a temperature in the (α + γ) range in a heating furnace and further enriching the γ phase with 0, only bainite transformation proceeds in the quenching zone, and while the γ phase remains. After completing galvanizing, it is necessary to rapidly cool the residual r to below the M1 point and turn it into martensite.

To achieve this, it is necessary to suppress phase decomposition as much as possible when cooling a steel sheet with a specific composition regulated by hot-dip galvanizing property, mixed structure, etc. after heating in the (α + γ) region, and to suppress the decomposition of the phase in the α phase that coexists with the γ phase. It is essential to cool the galvanized steel under conditions that reduce the temperature as much as possible, and to maintain the temperature of the galvanized steel for a sufficient period of time to obtain bainite transformation that is sufficient to improve stretch flangeability but not excessive. As mentioned above, in the present invention, a high-strength hot-dip galvanized steel sheet is manufactured under restricted conditions in a continuous hot-dip galvanizing line, but the structure of the copper sheet is modified by adding ferrite, bainite, and martensite to appropriate layers. It is important to form an 8-phase liquid structure with a certain proportion.

That is, Figures 1 to 8 show the tensile strength, total elongation, and
This is a diagram showing the relationship between yield stress and stretch flangeability. First, as shown in Figure 1, the relationship between strength and elongation is as follows. It has a better strength-elongation balance than steel.

Next, regarding the yield ratio, as is known from FIG. 2, the yield ratio is the lowest for ferrite + martensitic copper, and the yield ratio for bainitic steel on ferrite is around 70LN, which is about the same as that for ferrite + pearlite steel. When the amount of bainite is reduced and martensite is further introduced to create an eight-phase structure of polygonal ferrite + bainite + martensite, the yield ratio decreases to a value similar to that of ferrite + martensitic steel.

Furthermore, looking at the relationship between strength and stretch flangeability, as shown in Figure 8, the stretch flangeability of ferritic + martensitic steel deteriorates rapidly as the strength increases, whereas Good values can be obtained for steel despite the increased strength. If the steel has an eight-phase structure of ferrite + bainite / martensite, it will have extremely good stretch flangeability, although it is slightly inferior to bainite-on-ferrite steel.

As is known from these results, a steel sheet with an eight-phase structure of ferrite + bainite + martensite has a ferrite + bainite + martensite structure.
It can be said that this steel plate incorporates only the advantages of martensitic steel and bainite steel, and has a low yield ratio, excellent strength-elongation balance, and excellent stretch flangeability.

As is known from these examples, the area ratio of bainite in the eight-phase steel of the present invention should be 5 to 60%; if it exceeds 50%, the effect of reducing the yield ratio due to the introduction of martensite becomes small. If the content is less than 6%, the steel has a ferrite + martensitic structure. The area ratio of this bainite is preferably 10
~86g6.

Next, the area ratio of martensite should be 1 to 20%; if it exceeds 2096, the hole expansion rate will decrease and the yield ratio will increase, while if it is less than 1%, the effect of introducing martensite will increase. The area ratio of martensite is preferably 8 to 15%.

In the present invention, ferrite mainly means polygonal ferrite, and martensite includes a portion of retained austenite.

Next, the chemical composition of the target steel in the present invention will be described.

C is an essential element for maintaining the necessary strength and forming low-temperature transformation products such as bainiite and martensite. In particular, in the case of the present invention, the volume fraction of the γ phase when heated to (α+r) is determined by o1 in the steel and its heating temperature, and is important because it also affects martensite and bainite after transformation. Mechanical properties such as strength are greatly influenced by the fraction of these low-temperature transformation products and their hardness. If 0 is less than 0.005 m, not only will refining costs increase, but the effect of improving strengthening and hardenability will not be exhibited. As the martensite fraction increases, workability, especially stretch flangeability, decreases, and the yield ratio also increases to 0.7 or more, so 0.005AJ0.
It is necessary to keep it within the 15% range.

8i is an element that improves ductility such as elongation by reducing the amount of solid solution in the α phase, but if it exceeds 196, zinc cracking occurs, so it must be kept at 1% or less.

Earth is a solid solution strengthening element and is also important for suppressing ferrite transformation and stabilizing the γ phase in the mixed structure. In particular, when such an 8-phase mixed structure steel sheet is to be manufactured on a continuous hot-dip galvanizing line as in the present invention,
It is difficult to obtain an 8-phase mixed structure because thermal cycle constraints for galvanizing cannot be excluded. For example, after recrystallization annealing, immediately before galvanizing, the steel sheet temperature is lowered to 50°C.
It is necessary to maintain the temperature after 0 t' bending, and the martensitic transformation must be carried out by subsequent cooling. Therefore, the cooling conditions and temperature maintenance before galvanizing are set to suppress the ferrite transformation and promote the bainite transformation. I have to control it, M! If the amount of zinc is less than 0.7%, an eight-phase mixed structure cannot be obtained no matter how many combinations are made within the constraints of the zinc plated line structure.In other words, at 0.796Mn, no matter how rapidly the γ phase is stabilized, no matter how rapidly the γ phase is stabilized. Inevitably, the temperature holding time before galvanizing becomes longer, so all remaining austenite transforms into bainite, and martensite no longer suffocates. On the other hand, if Mn is more than 2.54, the deterioration of galvanizing properties exceeds the permissible limit, so Mn is more than 0.
It must be within the range of 7 to 2.596.

P is a solid solution strengthening element and is also an important element for suppressing the decomposition of the γ phase during cooling, but P is 0.1
If it exceeds 4, the ductility deteriorates, so P needs to be 0.1% or less.

In addition, in the present invention, Or% can be contained as necessary. Or is an element with strong quench hardenability,
The stability of the γ phase is increased in proportion to the amount of iron contained, and its decomposition is suppressed, but if the amount exceeds 0.54, the zinc plating properties and the phosphoric acid peel properties in the case of single-sided plating deteriorate, so the maximum
．． It is desirable to set it to 54.

Next, examples of the present invention will be shown together with comparative examples.

Steel having the chemical composition shown in Table 1 was melted in a converter. Then, it was formed into a slab by the blooming method and hot rolled under normal conditions to form a hot coil with a thickness of 2.8 mm. The hot rolling finishing temperature is 850~900″C9 winding temperature is approximately 600″
It was t. This hot coil has a plate thickness of Q1g after pickling.
The specimens were cold-rolled to a length of 100 m and subjected to continuous hot-dip galvanizing under the conditions shown in Table 2.

For the coils A1 to A6, the pre-plating temperature holding time inevitably changes when the cooling conditions after zinc plating for lead metal are constant and the cooling rate from the (a+,) range temperature to the zinc plating is sequentially changed. Note that the coil A6 was continuously cooled without maintaining its temperature.

Coils A7 to A9 are the case where the cooling conditions from Ca+1) region temperature to zinc plating are kept constant and the cooling rate is sequentially changed after that, and A9 is the case where, like coil A9, the temperature was not determined before plating. be.

Coils /1610 to 12 are coils with varying ground or Cv content.

The hot-dip galvanized steel sheet thus obtained was subjected to a tensile test and microstructural observation without being subjected to temper rolling. These results are shown in Tables 2 and 8.

As is clear from Tables 2 and 8, the hot-dip galvanized 11N4 root consisting of polygonal ferrite, bainite, and martensite structures with the area ratio specified by the present invention has a higher total elongation and stretch flangeability than those other than those specified by the present invention. is good,
It has a low yield ratio and no yield elongation, meaning it has excellent workability.

Table 2 Manufacturing conditions and structure 0 & cooling rate to galvanization (t'/-...)OR
1 “After” (t/s*s)

[Brief explanation of the drawing]

Figure 1 is a diagram showing the relationship between tensile strength and total elongation for steels with various structures, Figure 2 is a diagram showing the relationship between tensile strength and yield stress, and Figure 8 is also a diagram showing the relationship between tensile strength and total elongation. 7 is a diagram showing the relationship with lunge property (hole expansion rate). In the figure, they are F+ ferrite, B black bainite, MI fumarunsite, and PI pearlite.

Claims (2)

[Claims] (1) 00.005A (1,15*, former 14 or less 1
A cold-rolled steel plate obtained by hot rolling and cold rolling a steel containing 0.1% or less of M-0.7-L5 Kotobuki is galvanized on a continuous hot-dip galvanizing line, and the steel plate structure is changed through this process. Featuring an 8-phase composite structure of ferrite + bainite + martensite containing a bainite area ratio of 5 to 50* + martensite area ratio of 1 to 20%, it has a low yield ratio and excellent strength-elongation balance and stretch flangeability. A method for manufacturing high-strength hot-dip galvanized steel sheets. (2) CG, 005-0.15%, 81 1
% or less, Mu O, 7~muscle%, pO, 1% or less,
A cold-rolled steel sheet obtained by hot rolling and cold rolling a steel containing 0.5% or less of Or is galvanized on a continuous hot-dip galvanizing line, and the steel sheet structure is changed to a bainite area ratio of 5 to 50 through the galvanizing process. %, martensite area ratio 1-20
Ferrite including g6 + bainite + martensite 8
A method for producing a high-strength hot-dip galvanized steel sheet with a low yield ratio, excellent strength-elongation balance and stretch flangeability, characterized by having a phase composite structure.