JPH0543779B2 - - Google Patents

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. Generally, various alloying elements are added 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, and the addition of alloying elements There is a problem that the galvanizing properties deteriorate. Recently, a composite steel sheet with martensite dispersed in a ferrite matrix has been attracting attention as a high-strength steel sheet with good workability.This steel sheet has a low yield ratio and has excellent workability and age hardening after press working. . However, when this composite structure steel sheet is used as a hot-dip galvanized steel sheet, the martensite dispersed in the ferrite matrix is tempered during hot-dip galvanizing at approximately 500℃, 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 yield ratio and good strength, which are the 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. That is, the present invention provides cold rolling steel obtained by hot rolling steel containing 0.005 to 0.15% C, 1% or less Si, 0.7 to 2.5% Mn, 0.1% or less P, and optionally less than 0.5% Cr. When galvanizing inter-rolled steel sheets on a continuous hot-dip galvanizing line, they are cooled at a rate of 10 to 45°C/sec in a quenching zone after the soaking furnace in the plating line.
Bainitic transformation is caused by holding at a temperature of 450 to 550°C for 5 to 60 seconds, then hot-dip galvanizing, followed by a quenching zone after alloy ratio treatment.
Rapid cooling at a rate of ℃/sec or higher causes martensitic transformation, changing the steel sheet structure to a bainite area ratio of 5 to 50% and a martensite area ratio of 3 to 15%.
% ferrite + bainite + martensite three-phase composite structure, which is characterized by a low yield ratio and excellent strength-elongation balance and 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. In other words, cold-rolled steel sheets manufactured through hot rolling and cold rolling are 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 an annealing zone, a rapid cooling zone, and a cooling adjustment furnace, it passes through a galvanizing bath at about 500℃, then is heated to around 600℃, alloyed, and then cooled. When manufacturing a hot-dip galvanized steel sheet with a mixed structure consisting of ferrite, bainite with an area ratio of 5 to 50%, and martensite of 3 to 15%, the steel sheet is heated in a heating furnace in a non-oxidizing or reducing atmosphere (α + γ ), and after soaking to sufficiently concentrate C in the γ phase,
By rapidly cooling at a rate of 10 to 45°C/sec and holding at 450 to 550°C, only bainite transformation proceeds,
The galvanizing process (including the heat treatment process for alloying treatment) is completed while the γ phase remains, and the residual γ is rapidly cooled to below the Ms point at a rate of 7°C/sec or higher to form martensite. It is necessary to do so. For this purpose, steel sheets with specific compositions (α+
During cooling after heating in the γ) region, the decomposition of the γ phase is suppressed as much as possible, and then the amount of C in the α phase coexisting with the γ phase is reduced as much as possible under conditions (i.e., 10 to 45°C/sec).
While cooling to 450 to 550℃, hold the temperature for a period of time (i.e., 5 to 60 seconds) necessary to obtain sufficient but not excessive bainite transformation to improve stretch flangeability by holding the temperature before galvanizing. After hot-dip galvanizing and then alloying heat treatment, Ms.
It is necessary to generate martensite by rapid cooling at a rate of 7° C./sec or higher to below the point. This alloying treatment is a process that is very commonly adopted when manufacturing high-strength hot-dip galvanized steel sheets, and when selecting the subsequent quenching conditions, it is important to take into account the type of steel used. Carbon equivalent (Ceq=C%+
[13-
It is best to set the quenching conditions so that the temperature is 20×Ceg (°C/sec) or higher. As described above, in the present invention, high-strength hot-dip galvanized steel sheets are manufactured under restricted conditions on a continuous hot-dip galvanizing line.
It is important that the structure of the steel sheet be a three-phase composite structure in which ferrite, bainite, and martensite are formed in appropriate proportions. That is, Figures 1 to 3 are diagrams showing the relationship between tensile strength, total elongation, yield stress, and stretch flangeability for hot-dip galvanized steel sheets having various structures shown in Examples. -The relationship between elongation is shown in Figure 1.
The bainite + martensitic structure provides a better strength-elongation balance than ferrite + martensitic steel. Next, regarding the yield ratio, as is known from Figure 2, ferrite + martensitic steel has the lowest yield ratio, while ferrite + bainite steel has the lowest yield ratio.
It is around 70%, which is about the same as ferrite + pearlite steel. When the amount of bainite is reduced and martensite is further introduced to create a three-phase structure of polygonal ferrite + bainite + martensite, the yield ratio decreases to a value similar to that of ferrite + martensitic steel. Also, looking at the relationship between strength and stretch flangeability,
As is known from Fig. 3, the stretch flangeability of ferrite + martensitic steel deteriorates rapidly as the strength increases, whereas the stretch flangeability of ferrite +
Good values can be obtained for bainitic steel despite the increased strength. If the steel has a three-layer structure of ferrite + bainite + martensite, it will have extremely good stretch flangeability, although it is slightly inferior to ferrite + bainite steel. As is known from these results, the three-phase structure steel sheet of ferrite + bainite + martensite incorporates only the advantages of ferrite + martensite steel and bainite combination, has a low yield ratio, and has - It can be said that this steel plate has excellent elongation balance and stretch flangeability. As is known from these examples, the area ratio of bainite in the three-layer steel of the present invention is 5.
It should be ~50%; if it exceeds 50%, the effect of lowering the yield ratio due to the introduction of martensite becomes small, and if it is less than 5%, it becomes no different from steel with a ferrite + martensitic structure. Note that the area ratio of this bainite is desirably 10 to 35%. Next, the area ratio of martensite should be 3 to 15%; if it exceeds 15%, the hole expandability will decrease and the yield ratio will increase, while if it is less than 3%, the area ratio of martensite will increase. The effect of introduction is small. 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 bainite and martensite. In particular, in the case of the present invention, the volume fraction of the γ phase when heated to the (α + γ) region is determined by the amount of C in the steel and the heating temperature, and is important because it also affects the amount of 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 C is less than 0.005%, not only will refining costs increase, but the effect of improving strengthening and hardenability will not be exhibited. On the other hand, if it exceeds 0.15%, the spot weldability of the steel sheet will be significantly deteriorated, and the martensite content in the steel sheet will be reduced. As the yield ratio increases, workability, particularly stretch flangeability, decreases, and the yield ratio also increases to 0.7 or more, so it is necessary to keep it within the range of 0.005 to 0.15%. Si is an element that improves ductility such as elongation by reducing the amount of solid solution C in the α phase, but 1%
If it exceeds 1%, galvanizing defects will occur, so it must be kept at 1% or less. Mn 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 attempting to manufacture such a three-phase mixed structure steel sheet on a continuous hot-dip galvanizing line as in the present invention, it is difficult to obtain a three-phase mixed structure because the thermal cycle constraints for galvanizing cannot be excluded, such as recrystallization. After annealing and immediately before galvanizing, the temperature of the steel sheet should be lowered.
It is necessary to maintain the temperature between 450 and 550°C, and the martensitic transformation must be carried out by subsequent cooling. Therefore, by setting the cooling conditions and temperature maintenance before galvanizing as described above, it is necessary to suppress ferrite transformation and control so that only bainite transformation proceeds, but if the Mn content is less than 0.7%, the galvanizing line No matter how they are combined within structural constraints, a three-phase mixed structure cannot be obtained. i.e. 0.7%
In the case of Mn, no matter how rapidly the γ phase is stabilized, the temperature holding time before galvanizing inevitably becomes long, and all remaining austenite transforms into bainite, making it impossible to obtain martensite. On the other hand, if Mn is more than 2.5%, the deterioration of galvanizing properties will exceed the allowable limit, so Mn should be 0.7 to 2.5%.
Must be within the range. P is a solid solution strengthening element and is also an important element for suppressing the decomposition of the γ phase during cooling, but if it exceeds 0.1%, the elongation will deteriorate.
P needs to be 0.1% or less. In addition to this, in the present invention, Cr can be contained as necessary. Cr is an element with strong quench hardening properties, and increases the stability of the γ phase in proportion to its content and suppresses its decomposition, but if the content exceeds 0.5%, it may deteriorate the galvanizing properties or phosphorus in the case of single-sided plating. Since it deteriorates the acid film properties, it is desirable that the maximum content be 0.5%. 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 then hot rolled under normal conditions to form a hot coil with a thickness of 2.8 mm. Note that the hot rolling finishing temperature was 850 to 900°C, and the winding temperature was about 600°C. After pickling this hot coil,
The sheets were cold rolled to a thickness of 0.8 mm, and continuous hot-dip galvanizing was performed under the conditions shown in Table 2 with the plating line speed kept approximately constant. Coils No. 1 to No. 6 were cooled to (α + γ) range temperature galvanizing for steel A, with the cooling conditions after galvanizing (cooling rate after alloying heat treatment) being approximately constant (9 to 11 °C/sec). This is an example where the speed is changed sequentially, and since the plating line speed is kept constant, the temperature holding time immediately before plating changes according to changes in the cooling rate. Coil No. 6 was continuously cooled without maintaining its temperature. Coils No. 7 to No. 8 are cases in which the cooling conditions from the (α + γ) range temperature to galvanization are constant, and the cooling rate after the subsequent alloying heat treatment is sequentially changed.
Similar to coil No. 6, No. 9 is a case in which the temperature was not maintained before plating. Coils Nos. 10 to 12 have different Mn or Cr contents. The hot-dip galvanized steel sheets thus obtained were subjected to tensile tests and microstructural observations without being subjected to temper rolling. These results are shown in Tables 2 and 3. As is clear from Tables 2 and 3, the hot-dip galvanized steel sheets (No. 2, 3, 4, 8, 11, 12) consisting of polygonal ferrite, bainite, and martensite structures having the area ratio specified in the present invention are as follows: It has good total stretch flangeability, low yield ratio, no yield elongation, and excellent workability. On the other hand, Nos. 1, 5, 6, 7, 9, and 10 are comparative steels that lack any of the requirements specified by the present invention as described below, and have problems in some of the physical properties. No. 1: Because the cooling rate until galvanizing is slow and the holding time before plating is short, martensitic structure is not formed and bainite structure is insufficient. No. 5: The cooling rate before galvanizing is too fast and the holding time before galvanizing is too long, so bainite transformation progresses too much and an appropriate three-phase composite structure cannot be obtained. No. 6, 9: Since the holding time before plating is zero,
Almost no bainite was produced, and martensite was also excessive, making it impossible to obtain an appropriate three-phase composite structure. No.7: The cooling rate after galvanizing is too slow.
The amount of martensite and bainite is insufficient. No. 10: Due to the insufficient amount of Mn in the steel, a martensitic structure is not generated even if the cooling rate and holding time are properly controlled, and a three-phase composite structure is not formed.

【table】

【table】

【table】 [Brief explanation of the drawing]

Figure 1 is a graph showing the relationship between tensile strength and total elongation for steels with various structures, Figure 2 is a graph showing the relationship between tensile strength and yield stress, and Figure 3 is a graph showing the relationship between tensile strength and total elongation. It is a figure showing the relationship with flangeability (hole expansion rate). F in the figure: ferrite,
B: bainite, M: martensite, P: pearlite.

Claims (1)

[Claims] 1 C: 0.005 to 0.15%, Si: 1% or less, Mn: 0.7
~2.5%, P: 0.1% or less, and Al: 0.1% or less when galvanizing a cold-rolled steel sheet obtained by hot rolling and cold rolling on a continuous hot-dip galvanizing line. It is cooled at a rate of 10 to 45℃/sec in the quenching zone after the soaking furnace in the Metsuki line.
Bainitic transformation is caused by holding at a temperature of 450 to 550°C for 5 to 60 seconds, followed by hot-dip galvanizing, followed by 750°C in a quenching zone after alloying.
By rapidly cooling at a rate of ℃/sec or more, martensitic transformation is caused, and the steel plate structure is changed to a bainite area ratio of 5 to 50% and a martensite area ratio of 3.
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 three-phase composite structure of ferrite + bainite + martensite containing ~15%. 2 C: 0.005-0.15%, Si: 1% or less, Mn: 0.7
~2.5%, P: 0.1% or less, Cr: less than 0.5%, and Al: 0.1% or less. A cold rolled steel plate obtained by hot rolling and cold rolling is processed on a continuous melt plating line. When galvanizing, the temperature is 10 to 45℃/sec in the quenching zone after the soaking furnace in the galvanizing line.
bainite transformation is caused by cooling at a rate of
Next, after hot-dip galvanizing, martensitic transformation is caused by rapid cooling at a rate of 7°C/sec or more in a quenching zone after alloying treatment, and the steel plate structure is changed to a bainite area ratio of 5 to 50% and a martensite area ratio. : 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 three-phase composite structure of ferrite + bainite + martensite containing 3 to 15%.