JPS61170518A - Production of high-strength hot rolled steel sheet having excellent formability - Google Patents

Abstract

PURPOSE:To obtain a high-strength hot rolled steel sheet having excellent formability by limiting the cooling conditions after finish rolling of a steel having a prescribed component compsn. to manufacture ferrite-martensite-bainite 3-phase mixed structure having a specific volumetric ratio. CONSTITUTION:The steel contg., by weight %, 0.03-0.25 C, <=1.3 Si, 0.6-1.6 Mn and 0.1-1 Al and contg. 1<=2C+Mn<=3 (symbol of element denote the weight % of the corresponding elements) is treated in the following manner: The steel is subjected to finish rolling at the temp. above the Ar3 point and is then cooled to have the two-phase co-existence state of ferrite and austenite and thereafter the steel is quickly cooled. The quickly cooled steel is coiled at <=230 deg.C and the steel sheet consisting of the ferrite-martensite-bainite 3-phase mixed structure having the prescribed volumetric ratio, i.e., the intended and titled high-strength hot rolled steel sheet is obtd.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a method for manufacturing a high-strength hot-rolled steel sheet with excellent formability.

(Prior art) High-strength steel plates have been widely used in the past, for example, to reduce the weight and improve safety of automobiles, but in recent years, high-strength steel plates have been widely used, especially for various parts that require complex forming processes. High-strength steel sheets have come to be used in many industries, and as a result, requirements for formability are becoming even more severe.

Hot-rolled steel sheets with excellent formability include ferritic/martensitic steel, which has a lower yield ratio than before, and
Ferritic bainite martensitic steel is particularly known for its excellent stretch flangeability. However, when manufacturing a hot rolled steel sheet having such a mixed structure, for example, as described in Japanese Patent Publication No. 58-24489, it has been conventionally known that solid solution carbon is present when coiling at a low temperature. Therefore, the strength-ductility balance of the obtained steel sheet deteriorates, so the coiling temperature should be set at 250°C or higher, preferably 30°C.
It is said that the temperature needs to be 0°C or higher.

(Object of the Invention) However, the present inventors discovered that when producing hot rolled steel sheets by low-temperature coiling, the amount of bake hardening is large and the content of dustite (ferrous oxide) in the scale is large.
This method has various advantages such as being able to shorten the pickling time, being able to immediately send the steel material to subsequent processes such as a skin bath or pickling after hot rolling, and being able to shorten the cooling yard. I found it. Therefore, the present inventors conducted intensive research on a method of imparting the same strength-ductility balance as conventional ferritic/martensitic steel to hot-rolled steel sheets by low-temperature coiling while taking advantage of these advantages. By controlling the cooling conditions after finishing rolling of steel with chemical composition of The present invention was achieved by discovering that a steel plate can be obtained.

(Structure of the Invention) The method for producing a hot rolled steel sheet with excellent formability according to the present invention includes:
CO, 03-0.25% by weight, Si 1.3% or less, Mn 0.6-1.6%, and /l! Contains 0.01-0.1%, and 1%≦2C
+Mn+Si≦3% (however, the element symbol indicates the weight% of the element), and after finish rolling a steel consisting of iron and unavoidable impurities at a temperature above the Ar+ point, ferrite and austenite are formed in the cooling process. The steel sheet is characterized in that it is brought into a two-phase coexistence state with ferrite, bainite, and martensite, and then, after being rapidly cooled, it is rolled up at a temperature of 230° C. or lower to obtain a steel sheet having a three-phase mixed structure of ferrite, bainite, and martensite.

First, the chemical composition of the steel used in the present invention will be explained.

C needs to be added in an amount of at least 0.03% to strengthen the steel and improve its hardenability, in particular to give the steel the required strength. However, when adding too much, it deteriorates the ductility of the steel and also the weldability.
The upper limit is set to 0.25%.

Si is an element that is necessary to improve the strength and ductility of steel, and also promotes the formation of low-temperature transformation phases of martensite and bainite to obtain the desired steel structure according to the present invention. . However, when adding too much, harmful effects such as embrittlement of the welded part may occur, so the upper limit of the amount added is set at 1.3%.

Like Si, Mn is also an element necessary to compensate for the decrease in strength due to lower C, and, like Si, to promote the formation of a low-temperature transformation phase to form the desired steel structure.
Furthermore, if it is too small, pearlite tends to be mixed in the steel sheet structure, leading to deterioration of properties, so in the present invention, it is necessary to add at least 0.6%. However, adding too much will deteriorate the ductility of the steel, so the upper limit is set at 1.6%.

Al is usually 0.601% to 0.1% for deoxidizing steel.
It is added within the range of

Further, in the present invention, in addition to the above-mentioned elements, at least one element selected from the group consisting of Ca and REM is added to the steel in order to control the shape of sulfide inclusions to make them harmless and to further improve formability. elements can be added. In order to effectively express the above effects, Ca should be set at o, o o o s% or more, and REM should be set at 0.0 s% or more.
It is necessary to add 0.5% or more. In addition, the upper limit of the amount added is 0.01% for Ca and 0.01% for REM.
．． It is 1%.

In the present invention, steel having the above-mentioned chemical composition is heated by Ar
After finish rolling at a temperature above the dot plate, a two-phase coexistence state of ferrite and austenite is created in the subsequent cooling process, and then, after rapid cooling, the product is wound at a temperature of 230°C or less to achieve a predetermined volume fraction. ferrite, bainite,
A martensitic three-phase mixed structure steel sheet is obtained.

When the finish rolling temperature is lower than the Ar3 point, ferrite is included in the steel structure and this ferrite is rolled. The rolled ferrite is subjected to strain and the dislocation density increases, resulting in significant deterioration of ductility. That is, since the processed ferrite is not recovered and is carried over until after coiling, the obtained hot rolled steel sheet becomes a processed ferrite-transformed polygonal ferrite-bainite-martensite structure, which reduces the ductility of the product.

Next, according to the method of the present invention, in the cooling process after the completion of hot rolling as described above, in order to obtain a three-phase mixed structure of ferrite, bainite, and martensite each having a predetermined volume fraction, Of the chemical components, C% S
and for Mn, 1%≦2C+Mn+Si≦3
% (however, the element symbol indicates the weight % of the element).

When the amount of the element represented by the above formula is less than 1%, the steel having the chemical composition described above is hot rolled, cooled, and
Even if the steel sheet is rolled at a temperature below 0°C, it is not possible to secure the required amount of martensite and the desired three-phase mixed structure of ferrite, bainite, and martensite cannot be obtained. is small, the yield ratio is 0 or more, and the elongation at yield is large. On the other hand, when the amount of the element shown by the above formula is more than 3%, a steel plate with a low yield ratio can be obtained by coiling at a temperature of 300°C or higher, but martensite accounts for the structure of the steel plate. If the amount is too large, stretch flangeability deteriorates, and manufacturing costs increase as the amount of added elements increases.

Furthermore, in the method of the present invention, in order to obtain the desired steel structure, the cooling conditions after hot rolling as well as the coiling temperature are important.

FIG. 1 shows a specific example of a cooling method after hot rolling. Cooling method a is that after the end of hot rolling, quenching is performed at an average cooling rate (C+) of 5 to b seconds to the quenching start temperature T, and then 2
The average cooling rate (Cs
) Rapid cooling at 20°C/sec or more. The cooling method is as follows: After hot rolling, Ar point and Al point, which is the temperature range for ferrite formation, are used.
The ferrite is rapidly cooled to a temperature between the points of In order to promote the formation of austenite and stabilize the austenite, the coiling temperature (T
After being rapidly cooled to 2), it is rolled up to obtain a hot-rolled steel sheet having a three-phase mixed structure of ferrite, bainite, and martensite.

In this way, in the present invention, after finish rolling, a two-phase coexistence state of ferrite and austenite is created in the cooling process, and then rapidly cooled and coiled at the above temperature to form a three-phase part having a predetermined volumetric composition. In the ferrite-bainite-martensite three-phase mixed structure steel sheet obtained in this way, which is a hot-rolled steel sheet consisting of a composite structure, ferrite is the basic structure for imparting ductility to the hot-rolled steel sheet. Therefore, it is necessary to contain 50% or more of equiaxed ferrite grains. In order to obtain such ferrite grains, as mentioned above, after hot rolling and before winding, a non-pouring zone is formed for about 3 to 15 seconds in a temperature range of 650 to 800°C, where ferrite transformation occurs. Setting and slow cooling is recommended.

Martensite is an essential structure for imparting strength to a steel sheet, eliminating elongation at yield point, and lowering yield ratio. On the other hand, when martensite is included in excess, it deteriorates stretch flangeability. In the present invention, as shown in Fig. 3, the relationship between the amount of martensite, elongation at yield point, yield ratio, and stretch flangeability (λ) is as follows: requires an amount of martensite of 2% or more. Although Fig. 3 shows the characteristics of a steel plate that has not been skin-passed, a light skin-pass for modifying the shape usually has a martensite content of 2.
% is sufficient, so in the present invention, the lower limit of the amount of martensite in the steel structure is set to 2%.

On the other hand, stretch flangeability was observed to decrease as the amount of martensite increased;
% is not preferable from the point of view of stretch flangeability, so the upper limit of the amount of martensite is set to 15%.

The bainite phase does not make a large contribution to martensite edge strength, but it is more ductile than martensite, but when it exceeds 50%, it reduces the total elongation. Therefore, in the present invention, the content of the bainite phase is set to 48% or less, which causes less decrease in elongation.

(Effects of the Invention) As described above, according to the present invention, 50% or more of the ferrite phase imparts ductility, and 2 to 15% of the martensite phase eliminates the elongation at yield, lowers the yield ratio, and increases strength. give,
Additionally, the 5-48% bainite phase provides strength;
A hot rolled steel sheet with excellent formability can be obtained.

(Example) The present invention will be explained below based on examples.

Examples Steels A-D of the present invention and comparative steels E and F having the chemical compositions shown in Table 1 were finish rolled at a temperature of 800 to 880°C, and then subjected to various cooling and winding conditions shown in Table 2. Thickness 3
．． Two cranes were made into hot-rolled steel sheets. In Table 1, cooling method a
and b, average cooling rate C, , C, and C3, winding temperature T
I and T2 are as previously explained based on FIG.

Table 3 shows the mechanical properties and steel structure of the obtained product steel plate. Stretch flangeability λ is the diameter of the hole drilled in the steel plate.
(10 m), when the hole diameter when the steel plate breaks due to hole expansion is d, no ((d-do)/do
)xi 00 (%).

All of the steels of the present invention have a yield point elongation of 0 and a yield ratio of 0.7.
It has excellent stretch flangeability and excellent strength-ductility balance.

In addition, if the amounts of Si and Mn added are increased, the yield point ratio can be reduced to 0.7 or less even by coiling at a temperature of 300°C, as seen in Comparative Steel F, but the element content As the amount of martensite increases, the manufacturing cost increases (in addition, the amount of martensite increases, which has a detrimental effect on stretch flangeability.

[Brief explanation of drawings]

Fig. 1 is a graph showing a preferred specific example of the cooling method after finish rolling in the method of the present invention, and Fig. 2 is a graph showing a preferred example of the cooling method after finish rolling in the method of the present invention.
FIG. 3 is a graph showing the relationship between the yield point ratio of a hot rolled steel sheet and the amount of n and the coiling temperature. FIG. 3 is a graph showing the relationship between the martensite volume fraction and mechanical properties of the hot rolled steel sheet. Figure 1 Figure 2 2C+ Sj + Mn ('10) Figure 3

Claims (2)

[Claims] (1) Contains C 0.03 to 0.25%, Si 1.3% or less, Mn 0.6 to 1.6%, and Al 0.01 to 0.1% by weight, and 1 %≦2C
+Mn+Si≦3% (however, the element symbol indicates the weight% of the element), and the balance consists of iron and unavoidable impurities. After finishing rolling at a temperature of Ar_3 or higher, ferrite and austenite are formed in the cooling process. A two-phase coexistence state with
A method for producing a high-strength hot-rolled steel sheet with excellent formability, characterized in that the steel sheet is rapidly cooled and then rolled at a temperature of 230° C. or lower to obtain a steel sheet having a three-phase mixed structure of ferrite, bainite, and martensite. (2) Contains (a) C 0.03 to 0.25%, Si 1.3% or less, Mn 0.6 to 1.6%, and Al 0.01 to 0.1% by weight; And 1%≦2C
+Mn+Si≦3% (however, the element symbol indicates the weight% of the element), and (b) Ca 0.0005 to 0.01%, and REM
After finish rolling a steel containing at least one element selected from the group consisting of 0.005 to 0.1%, with the balance consisting of iron and unavoidable impurities at a temperature of Ar_3 or higher, ferrite and austenite are formed in the cooling process. 2-phase coexistence state with ferrite and
A method for producing a high-strength hot-rolled steel sheet with excellent formability, characterized by obtaining a steel sheet having a bainite-martensite three-phase mixed structure.