JPH0559492A - High strength hot rolled steel sheet of composite structure excellent in ductility and its manufacture - Google Patents

Abstract

(57) [Summary] [Object] The present invention provides a method for producing a high-strength hot-rolled steel sheet having good ductility by adjusting the composition of the steel and the hot rolling / cooling method to control the amount of retained austenite in the steel sheet. provide. [Structure] C = 0.05 to 0.3, Si = 0.5 to 3.
Steel containing 0, Mn = 0.1-2.5, Mo = 0.05-2.0 wt% was hot-rolled near the Ar ₃ temperature, and 5 ° C.-1
By cooling at 20 ° C / sec and winding in the range of 300 to 475 ° C, the final steel sheet has a microstructure of 3 phases of ferrite, bainite, retained austenite, or 4 phases including some martensite, and high strength. It is possible to manufacture a steel sheet that achieves both high ductility. [Effect] The thickness of automobile steel plate for processing is reduced, which contributes to the weight reduction of automobile bodies.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high strength hot rolled steel sheet having excellent formability and a method for producing the same.

[0002]

2. Description of the Related Art In recent years, there has been an increasing demand for weight reduction of a vehicle body in addition to comfort and safety of an automobile. This is a demand for energy saving and environmental problems considered on a global scale, and its purpose is to improve vehicle fuel efficiency by reducing weight and reduce harmful exhaust gas such as CO ₂ . In order to achieve such an object, it is necessary to improve the strength of the material used for the vehicle body structure and reduce the material thickness, or
It is necessary to use new low specific gravity materials.

New low specific gravity materials (eg Al, Mg, etc.)
From the viewpoints of price and stable supply, it is premised that the steel is used in the state of coexistence with the steel sheet that has been conventionally used as the center of the body material. In this case, the most problematic problem is the recycling of scrap, and the steel plate scrap mixed with other materials needs to be reused with much energy and cost for subsequent use.

Therefore, in order to maintain the minimum energy and environment of the earth as a whole, it is very important to reduce the weight of a single material (that is, steel material) except for special parts, so that the strength of steel material is much higher. Is expected.

In addition to the above requirements, the integral molding of vehicle body constituent parts is considered to be an important technical requirement for simplification and continuation of the manufacturing process. Among the steel materials used in such modernizing forming process, especially considering thin steel plate,
Having good formability is the selection criterion for the steel sheet. The formability of thin steel sheet is judged by elongation, Rankford's plastic strain ratio (r value), work hardening index (n value) and yield strength. For integral molding of complex parts, elongation and n value Is a high requirement.

As an example of a steel sheet having a large elongation and a large n value, a conventional ferrite and martensite dual phase dual P
Hase (DP) steel is known. DP steel is Japanese Patent Sho5
As shown in Japanese Patent Publication No. 6-18051 and Japanese Patent Publication No. 59-45735, a maximum total elongation of about 30 to 35% can be obtained at 50 to 80 kgf / mm ² . However, it is difficult to say that the strength-ductility balance is sufficient when applied to a site where complicated processing is required.

As a method for further improving this material, a high-strength composite structure steel sheet having a microstructure of a mixed structure of ferrite, bainite and austenite (or partly containing martensite) has been proposed. This steel sheet utilizes “transformation-induced plasticity” which shows high ductility by transforming austenite remaining at room temperature into martensite during forming.

Steels utilizing transformation-induced plasticity are known as TRIP steels, for example, Zackay et al.
F. Zackay et al .: Trans. ASM vol.60
As shown in (1967) 252), high ductility of about 90% at maximum is achieved at 70 kgf / mm ² or more. However, such TRIP steel does not always meet the requirements here, such as the need to add a large amount of expensive alloying elements.

As a solution to such a problem, it can be applied to low-priced applications such as those disclosed in Japanese Patent Laid-Open Nos. 63-4017 and 64-79345, which are premised on mass production of automobile steel sheets. A method of manufacturing a conforming steel sheet is shown.

[0010]

The amount of retained austenite in the hot-rolled steel sheet that has been cooled and rolled after hot rolling has a great influence on the material of the final steel sheet, especially on the ductility. Regarding the effect of hot rolling cooling conditions on the amount of retained austenite, the case of containing no special alloying element is described in, for example, Kono et al.
oc. of Int. Conf. on “Physic
al Metallurgy and Thermom
mechanical Processing of S
tees and other material
s ", Tokyo Japan, ISIJ, 1988).
There is a report.

In order to increase the amount of retained austenite, rolling at low temperature (near Ar ₃ ), optimization of cooling conditions, winding near 400 ° C., and accelerated cooling after winding are reported to be effective. Has been done. However, these conditions are largely dependent on the capability of the manufacturing process and are not easily achieved in the normal hot rolling process, and particularly, in the normal manufacturing process, it is difficult to achieve them.

[0012]

Means for Solving the Problems As a result of an experiment based on such conventional knowledge, the present inventors have found that a strict restriction on manufacturing conditions is required by adding a small amount of a specific alloying element. It has been found that it is possible to easily increase the retained amount of austenite without. That is, according to the present invention, (1) wt% C: 0.05 to 0.30%, Si: 0.
5 to 3.0%, Mn: 0.1 to 2.5%, Mo: 0.0
5 to 2.0%, and 0.6 to 3.0% in total of the three elements including Mn and Mo with Ni added as necessary, and the balance F
A steel material comprising e and unavoidable impurities is hot-rolled and then cooled so that the final microstructure is three phases of ferrite, bainite, retained austenite, or four phases containing some martensite. High strength composite hot rolled steel sheet with ductility.

(2) C: 0.05 to 0.30 in% by weight
%, Si: 0.5 to 3.0%, Mn: 0.1 to 2.5
%, Mo: 0.05-2.0%, and a total of 0.6-3.
Hot rolling of a steel material containing 0% and the balance Fe and unavoidable impurities is completed within a range of the Ar ₃ transformation temperature of the steel −30 ° C. to the Ar ₃ transformation temperature + 100 ° C., and then 5 ° C./sec or more 120 ° C./sec or more. By cooling at a cooling rate of sec or less and winding in the range of 300 ° C to 475 ° C, the final microstructure is made to be three phases of ferrite, bainite, retained austenite or four phases containing some martensite. A method for producing a high-strength composite hot-rolled steel sheet having excellent ductility.

(3) Manufacture of a high strength composite structure hot rolled steel sheet having excellent ductility, characterized in that one or more elements of Cu, Cr, Nb and Ti are added in a total amount of 2% or less. Is the way.

[0015]

The high strength steel sheet containing retained austenite has been reported for both hot rolled steel sheet and cold rolled-annealed steel sheet. The major differences between hot rolling and cold rolling-continuous annealing are the presence or absence of hot working and the processing time at around 400 ° C. In the hot rolling step, it is unavoidable to keep the temperature around the coil for a long time after winding, so it is not easy to select an appropriate treatment time near 400 ° C.

Therefore, the amount of retained austenite increases if special cooling (for example, mist cooling or soaking in water) after winding is carried out by Kono et al. (Proc. Of I).
nt. Conf. on “Physical Meta
lurgy and Thermomechanic
al Processing of Steelsan
d Other Materials ”, Tokyo
It is also clear in the report of Japan, ISIJ, 1988).

However, the addition of such a step leads to an increase in manufacturing cost and hinders productivity. On the other hand, by adding an appropriate amount of a specific alloying element, even if it is exposed to the vicinity of the coiling temperature for a long time like hot-rolled steel sheet, it easily remains without introducing special cooling after coiling. It has been found that the amount of austenite can be increased.

The details of the operation of the important elements of the present invention will be described below. C: Carbon (C) is an essential element for obtaining retained austenite at room temperature in low alloy steel. That is, in order for austenite to exist stably at room temperature, either a sufficient amount of austenite-forming elements such as Mn and Ni is contained in austenite, or about 1% or more of C, which is the cheapest element, is contained. is necessary.

Since the present invention is characterized in that the use of expensive elements is minimized and austenite remains, the stabilization of austenite by C is the most important technique. Therefore, it can be said that the amount of residual austenite finally obtained is almost determined by the amount of C contained in the steel sheet. However, if the amount of C is increased, the hardenability of steel generally increases, and the strength of the finally obtained steel plate increases. Therefore, the appropriate amount of C changes according to the required intensity level.

However, a carbon content of at least 0.05% is required in order to have a clear advantage over steel sheets (conventional steels) utilizing other strengthening mechanisms (transformation structure strengthening, precipitation strengthening, etc.). Since this is the case, this is the lower limit of carbon. On the other hand, addition of more than 0.3% of C deteriorates weldability and embrittles the steel, so this was made the upper limit of C.

Si: Si is the most important additional element for carbon concentration so that austenite is stable even at room temperature. Although the core of the technique of the present invention is to concentrate carbon in the austenite by heating the steel sheet to the ferrite / austenite two-phase region and allowing the ferrite transformation to proceed during cooling, the ferrite transformation progresses (and thus the austenite). Carbides are more likely to occur (with increasing carbon content in the interior), pearlite is formed at higher temperatures and upper bainite is formed at lower temperatures, which reduces the total carbon content in austenite and consequently the retained austenite content. It will be.

As is well known, Si does not form a solid solution in a carbide (here, cementite), and therefore has a function of significantly delaying the formation of the carbide. This enables efficient carbon enrichment to austenite without wasting carbon atoms in the form of carbides.

For this purpose, addition of 0.5% or more of Si is indispensable. At this time, Si forms a solid solution in the ferrite and strengthens the ferrite. Therefore, addition of an unnecessarily large amount causes a decrease in the workability of the steel sheet. Therefore, the amount added is limited to 3% or less. However, if the addition of Si of 2% or more causes some deterioration in ductility, the addition amount of Si is preferably in the range of 0.5 to 2.0% when the demand for strength is not particularly strong.

Mn, Mo, Ni: Mn, Mo, Ni are also S
Like i, it is an additive element that contributes to the retention of austenite because it has a function of delaying the formation of carbides. In addition to this, the addition of these elements lowers the martensitic transformation start temperature of austenite. In order to stabilize austenite at room temperature, it is necessary to suppress the precipitation of carbides and increase the carbon concentration in the austenite as described above, but it is also important to lower the martensitic transformation start temperature of the austenite at the same time.

If the martensite transformation temperature is higher than room temperature, a part of austenite inevitably transforms to martensite, which increases the strength of the steel sheet and deteriorates ductility. For such purposes, Mn, M
Since the total addition amount of o and Ni needs to be 0.6% or more, this is the lower limit.

However, if the amount of Mn added is set to 0.1% or less, the cost in the steelmaking process increases, so this was made the lower limit of the amount of Mn added. Mo is one of the most important elements in the present invention.
That is, the proper winding temperature region after hot rolling is expanded by adding Mo. However, in order to obtain such an effect, it is necessary to add Mo in an amount of at least 0.05%, so this was made the lower limit of the amount of Mo added.

On the other hand, if a large amount of these alloy elements is added, the hardenability of the steel sheet is unnecessarily increased and the possibility of deterioration of ductility along with an increase in strength increases and at the same time the cost of the steel sheet increases. For this purpose, the upper limit of Mn is 2.5%, preferably 2.0% or less.

Similar to Mn, addition of a large amount of Mo causes unnecessary increase in strength and deterioration of ductility of the steel sheet, and also causes increase in cost of the steel sheet. Therefore, the upper limit of the amount of Mo added is set to 2.0%, and from the balance of strength and ductility, it is preferably 0.05 to
Set to 1.0%.

If the total amount of Mn and Mo added plus the amount of Ni added (which may not be added) exceeds 3.0%, the strength of the steel sheet becomes unnecessarily high and workability is required. Since it can not be applied to applications such as
The upper limit of the total addition amount of these three types of alloy elements was set to 3.0%. In order to reduce variations in the material (strength, ductility, etc.) of the steel sheet, this amount is preferably 2.0% or less.
In the component system of the present invention, the amount of Ni added has a great effect on the cost of the steel sheet, so it is preferably 2.0% or less.

Other additive element: Cr can stabilize austenite by its addition, and is considered to be advantageous for the retention of austenite. Also Nb, T
i may be added for strength adjustment by precipitation strengthening of carbides (Nb (C, N), Ti (C, N), etc.).
Further, since Cu is less likely to form a solid solution with cementite like Si, it delays the start of precipitation of carbides, assists the progress of carbon concentration in austenite, and is also used for strength adjustment. However, excessive addition of these alloying elements not only raises the manufacturing cost of the steel sheet but also causes deterioration of ductility due to an increase in strength. Therefore, the total addition is limited to 2% or less.

Hot rolling, cooling, and winding conditions: Hot rolling is important for making the final microstructure of a steel sheet finer, and lowering the rolling end temperature has a great effect. Steel Ar ₃
This effect cannot be expected at the end of rolling at the transformation point + 100 ° C or higher, and at Ar ₃ -30 ° C or lower, the workability of the steel sheet is remarkably deteriorated due to the mixed grains in the surface layer of the steel sheet and the appearance of worked ferrite. Set to an appropriate range for the rolling end temperature. Further, since the large reduction rolling in the post-rolling stage is particularly effective for grain refinement of the structure, it is desirable to set the total thickness reduction rate of the final two stages to 40% or more.

When the cooling rate after hot rolling is 5 ° C./sec or less, pearlite is generated during cooling and C to be used for remaining austenite is wasted, so this is the lower limit of the cooling rate. Increasing the cooling rate increases transformation products at low temperature, strengthens the steel sheet, and promotes grain refinement of the structure. However, at a cooling rate of 120 ° C./sec or more, this effect saturates, and since it is not realistic due to equipment restrictions, this is set as the upper limit of the cooling rate.

The finally obtained steel sheet has a microstructure of 80.
Steel sheets with a kgf / mm ² or less preferably have a structure in which ferrite is the main phase and include bainite, retained austenite, and some martensite. In steel sheets of higher strength, ferrite and bainite are the main phases and retained austenite and some A microstructure containing martensite is preferable.

At this time, generally, increasing the volume fraction of ferrite leads to an improvement in the strength-ductility balance of the steel sheet. Therefore, the ferritic transformation region is slowly cooled at a low temperature within the range where the pearlite transformation does not start. Alternatively, it is desirable to carry out isothermal holding.

The amount of retained austenite of the final product steel sheet strongly depends on the winding temperature, and at high temperature winding, the retained austenite cannot be expected due to the formation of pearlite or the formation of bainite containing carbide. On the other hand, when coiled at low temperature, martensite and lower bainite containing carbides in ferrite grains are generated, and a good material cannot always be obtained.

This proper winding condition changes depending on the addition of an alloy, and is different from the case where no alloy other than Si and Mn is contained.
When Mo was added, a wide appropriate winding temperature range was recognized (see FIG. 1). FIG. 1 shows C / 0.20, Si / 1.
850 for steel with 5, Mn / 1.7% (steel without Mo) and steel with 0.25% Mo added (steel with Mo) 850
50 ~ 60 ℃ after finishing hot rolling at ℃ to 3.0mm thickness
It is a chart of the residual austenite volume ratio contained in the steel plate cooled at various cooling temperatures and cooled at a cooling rate of / sec.

In order to improve the ductility due to the work-induced transformation, it is necessary to retain at least about 5% by volume of austenite. Therefore, as shown in FIG. Since it is easy to secure the yield, the yield of steel sheet products is significantly improved, which is a great economic advantage.

A similar phenomenon is recognized even when Mn + Ni + Mo composite is added, and the optimum winding temperature range is 300 ° C. to 475.
It was defined as the range of ° C. Good mechanical properties can also be obtained within the above-mentioned winding conditions even if the cooling after winding is allowed to cool, but rapid cooling after winding (mist, dobuzuke, rewinding, etc.)
It is desirable to ensure good mechanical properties.

[0039]

[Examples] For each of the steel types shown in Table 1, after hot rolling to a thickness of 3.0 mm, the mechanical properties of the cold rolled and rolled hot rolled steel sheet were investigated and the retained austenite was quantified. Table 2 shows the mechanical properties of the obtained steel sheet and the residual austenite volume percentage Vg% in the steel sheet.

From the table, a steel sheet satisfying the conditions of the present invention (indicated by the present invention steel in the table) shows better ductility than conventional steel (indicated by comparative steel), which is one of the indicators of workability. Is TS
A steel sheet having good workability such that (kgf / mm ² ) × El (%) is 2000 or more can be obtained.

[0041]

[Table 1]

[0042]

[Table 2]

[0043]

INDUSTRIAL APPLICABILITY The present invention makes it possible to produce a high strength hot-rolled steel sheet having excellent ductility of 50 to 120 kgf / mm ² , and when it is applied to, for example, automobile parts, it can greatly contribute to weight reduction of automobile bodies. it can.

[Brief description of drawings]

FIG. 1 is a table showing the volume ratio of retained austenite contained in steel sheets of Mo-free steel and Mo-containing steel and the winding temperature.

Claims (3)

[Claims] 1. C .: 0.05 to 0.30%, Si: 0.5 to 3.0%, Mn: 0.1 to 2.5%, Mo: 0.05 to 2.% by weight. A steel material consisting of 0% Mn and Mo, if necessary, with Ni added in total of 0.6 to 3.0% and the balance Fe and unavoidable impurities, is hot-rolled and then cooled to obtain a final microstructure. A high-strength composite hot-rolled steel sheet having excellent ductility, characterized in that the structure is a three-phase structure of ferrite, bainite, and retained austenite or a four-phase structure containing some martensite. 2. C: 0.05 to 0.30%, Si: 0.5 to 3.0%, Mn: 0.1 to 2.5%, Mo: 0.05 to 2.% by weight. A steel material consisting of 0% Mn and Mo, if necessary, with Ni added in total of 0.6 to 3.0%, and the balance Fe and unavoidable impurities, was added from the Ar ₃ transformation temperature of −30 ° C. of the steel material. Ar
₃ Finish the hot rolling within the range of transformation temperature + 100 ° C, then 5
Cool at a cooling rate of ℃ / sec or more and 120 ℃ / sec or less,
By winding in the range of 300 ° C to 475 ° C, the final microstructure was made to have three phases of ferrite, bainite, retained austenite or four phases containing a part of martensite, and had excellent ductility. Manufacturing method of high-strength composite microstructure hot-rolled steel sheet 3. A high-strength composite microstructure heat with excellent ductility according to claim 2, wherein one or more elements of Cu, Cr, Nb and Ti are added in a total amount of 2% or less. Manufacturing method of rolled steel sheet.