JP2601581B2 - Manufacturing method of high strength composite structure cold rolled steel sheet with excellent workability - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a high-strength composite structure cold-rolled steel sheet having excellent workability.

[0002]

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

[0003] New low specific gravity materials (eg Al, Mg, etc.)
In the case of using steel, it is considered that it is premised that the steel is used in the coexistence state with the steel sheet which has been conventionally used as the main component of the vehicle body material from the viewpoint of price and stable supply. The most problematic in this case is the recycling of the scrap, and the steel sheet scrap mixed with other materials needs to be reused at the expense of a lot of energy and cost. Therefore, it is very important to reduce the weight of a single material (ie, steel) except for special parts in order to minimize energy and maintain the environment as a whole, and it is expected that steel will have even higher strength. Have been.

[0004] In addition to the above requirements, integral molding of vehicle body components is considered to be an important technical requirement for simplification and continuity of the manufacturing process. Among steel materials used in such a modern forming process, especially considering thin steel sheets,
Having good formability is a criterion for selecting the steel sheet. The formability of thin steel sheets is judged by elongation, Rankford's plastic strain ratio (r-value), work hardening index (n-value) and yield strength, and elongation and n-value for integral molding of complex parts. Is one requirement.

[0005] As an example of a steel sheet having a large elongation or n value, there is a dual ferrite and martensite dual-phase structure of Dual P
Hase (DP) steel is known. DP steel is No. 5
As disclosed in JP-A-6-18051 and JP-B-59-45735, a total elongation of up to about 30 to 35% can be obtained at 50 to 80 kgf / mm ² . However, it is hard to say that a sufficient strength-ductility balance is obtained when the steel sheet is applied to a part requiring complicated processing such as a thin steel sheet having a relatively low strength (35 to 45 kgf / mm ² ).

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

[0007] Steels utilizing transformation induced plasticity are known, for example, as TRIP steels, for example, Zackay et al.
F. Zackay et al .: Trans. ASM vol.60
As shown in (1967) 252), high ductility of up to about 90% is achieved at 70 kgf / mm ² or more. However, such a TRIP steel does not always meet the requirements here, for example, it is necessary to add a large amount of expensive alloying elements.

As a solution to such a problem, there is disclosed a method for producing a thin steel sheet suitable for an inexpensive use which is premised on mass production, such as a steel sheet for an automobile, as described in JP-A-61-157625. Have been. The technology described in this prior patent suppresses the precipitation of carbides by adding Si and promotes ferrite transformation (bainite transformation) at a low temperature, thereby effectively enriching carbon in untransformed austenite. , To stabilize austenite.

[0009]

In the prior art (for example, Japanese Patent Application Laid-Open No. 61-157625), when the microstructure of a hot-rolled steel sheet before cold rolling is different, a method for giving accurate annealing conditions is used. Are not obtained, and the best production conditions are not proposed for various component systems.

[0010]

SUMMARY OF THE INVENTION The present inventors have conducted experiments on a steel to which various alloying elements have been appropriately added, including a wide range of production conditions, and as a result, have finally obtained a material of a cold-rolled and annealed steel sheet. Can be optimized by appropriate combination of microstructure and annealing conditions of hot rolled sheet before cold rolling. That is, the gist of the present invention is as follows.

(1) By weight%, C: 0.05 to 0.40
%, Si: 0.5 to 3.00%, Mn: 0.5 to 2.5
0%, steel material consisting of balance Fe and unavoidable impurities
Rolled and cooled, bainite transformation temperature of steel(Bs) SuperWarmth
The structure of the hot-rolled steel sheet is ferrite + fine par
As a lightOr hot rolled steel at a temperature below Bs
Ferrite + bainite or ferrite
+ Perlite + bainite or bainite single phase
AfterCold rolled,To the winding temperature CT and hot rolled steel sheet structure
Accordingly, the annealing heating temperature Ts isS = (Ts−Ac₁) /
(Ac_Three-Ac₁)When CT> Bs,
0.2 ≦ S ≦ 0.75And when CT ≦ Bs
Is set in the range of 0.05 ≦ S ≦ 0.45 to annealAddition
Heating and then at a cooling rate of 1-10 ° C / sec.
Cooling to the range of 00 ° C followed by 10-200 ° C /
After cooling to 200-500 ° C at a cooling rate of
Hold for 15 seconds to 20 minutes in a temperature range of 00 to 500 ° C.
By cooling to temperature, ferrite and bainite
Ferrite as main phase + bainite + residual austenite
High strength with excellent workability characterized by a composite structure
Method of manufacturing cold rolled steel sheet with complex structure.

( 2 ) The method of (1) , wherein Ni, C
A method for producing a high-strength composite structure cold-rolled steel sheet excellent in workability, characterized in that one or more of r, Cu, Mo, Nb, and Ti are added in a total amount of 2% or less.

[0013]

The general high-strength cold-rolled steel sheet, except for the precipitation-strengthened type, is often wound at a relatively high temperature after hot rolling in order to maintain the strength of the steel sheet before cold rolling at a low level. Therefore, in this case, the initial structure before cold rolling usually has a two-phase structure of ferrite and pearlite, and is maintained at a high temperature for a long time, so that alloy elements such as Mn are sufficiently distributed to each phase. After the steel sheet is cold-rolled, it is heated to the ferrite + austenite two-phase region. At this time, the place where austenite is formed corresponds to the place where the pearlite structure (cementite phase in pearlite) was formed before cold rolling.

Therefore, allowing the alloying element (for example, Mn) to be concentrated in the cementite phase in pearlite leads to an increase in the hardenability of austenite generated by heating in the two-phase region. This means that austenite is hardly transformed into ferrite during the subsequent cooling, and the volume ratio of low-temperature transformation products (bainite) in the microstructure of the final steel sheet increases, and the strength of the steel sheet increases.

This is not always welcome in all cases. Bainite is hard but brittle compared to the ferrite phase formed at high temperatures. Therefore, in order to manufacture a steel sheet having a low strength level, it is necessary to limit the bainite phase to as small as possible. For this purpose, it is not desirable to increase the austenite hardenability more than necessary.

However, it is considered that increasing the alloy concentration (eg, Mn concentration) in austenite generated during heating in the two-phase region facilitates the retention of austenite by increasing the stability of austenite. Therefore, it is necessary to carefully select the hardenability of austenite and the subsequent heat treatment conditions.

The present inventors changed the structure before cold rolling to obtain 2
The most effective heat treatment conditions were investigated when the hardenability of austenite generated during heating in the phase region was changed, and the following facts were found.

When the coiling temperature after hot rolling is set to a temperature higher than the bainite transformation temperature of the steel sheet, the structure before cold rolling is ferrite and pearlite, and cementite in pearlite is rich in austenite stabilizing elements such as Mn. Become When the steel sheet is heated to the two-phase region after cold rolling, austenite is formed on Mn-enriched cementite, and as a result, the austenite has a very high hardenability. If a cooling history following the normal continuous annealing process is adopted thereafter, the strength of the finally obtained steel material increases more than necessary because the ferrite transformation from austenite is significantly suppressed, and as a result, the ductility Causes deterioration.

In order to cover such a defect, S = (Ts-A) determined by the temperature of Ac ₁ and Ac ₃ of the steel material.
c ₁ ) / (Ac ₃ −Ac ₁ ) is 0.2 ≦ S ≦ 0.
A relatively high annealing heating temperature Ts in the range of 75
Heating was found to be effective (see FIGS. 1 and 2).

FIG. 1 shows C / 0.12, Si / 1.2, Mn
/1.5% steel is rolled at 700 ° C and 450 ° C after hot rolling, cold rolled to a thickness of 0.8 mm, and then annealed at various annealing heating temperatures to change the S value over a wide range. Cool at 5 ° C / s to 680 ° C, then at 80 ° C / s,
It shows the S value dependence of the ductility of a steel sheet that is air-cooled to room temperature after isothermal holding at 00 ° C. for 5 minutes. FIG. 2 shows C / 0.2
The results of the same evaluation as in FIG. 1 are shown for steels of 0, Si / 1.2, Mn / 1.25% by weight.

On the other hand, when the coiling temperature after hot rolling is set to be lower than the bainite transformation temperature of the steel sheet, the microstructure before cold rolling may include ferrite + bainite and partly pearlite generated during cooling. Since the concentration of alloying elements such as Mn in the cementite in the phase is usually negligible, the hardenability of austenite generated on this cementite is almost the same as the hardenability of the steel sheet base material. Considered identical.

At this time, the two-phase zone heating after cold rolling is good when the above-mentioned S value is heated to a relatively low annealing heating temperature such that the value of S falls within the range of 0.05 ≦ S ≦ 0.45. It was found that excellent ductility was obtained (see FIGS. 1 and 2).

The details of the operation of the important elements of the present invention will be described below. C: As described above, an object of the present invention is to provide a method for producing a high-strength steel sheet exhibiting excellent workability by a work-induced transformation (TRIP effect) at room temperature. In order to allow austenite to remain at room temperature, it is necessary to contain a large amount of an austenite stabilizing element such as Mn or Ni, or to sufficiently concentrate C, which is a cheaper austenite stabilizing element. Therefore, C can be said to be the most important element in the present invention.

As described above, in order for austenite to exist stably even at room temperature, the carbonic acid concentration in austenite needs to be about 1% by weight. In order for the steel of the present invention containing retained austenite to exhibit a sufficient advantage over other high-strength steel sheets, the steel must contain C of 0.05% or more. I do. 0.4% on the other hand
An excessively high carbon concentration makes the steel sheet brittle and significantly deteriorates the weldability, so this is made the upper limit of the C content.

Si: Si is the most important additive element for concentrating carbon such that austenite is stable even at room temperature. Heating a steel sheet to the ferrite + austenite two-phase region and enriching carbon in austenite by allowing the ferrite transformation to proceed during cooling is central to the technology of the present invention. Increasing the formation of carbides (as the carbon concentration of the steel increases), producing pearlite at higher temperatures and upper bainite at lower temperatures, reducing the total carbon content in austenite and consequently reducing the amount of retained austenite Becomes

As is well known, Si does not form a solid solution in carbide (here, cementite), and therefore has a function of significantly delaying the formation of carbide. This enables efficient carbon enrichment to austenite without wasting carbon atoms in the form of carbides. 0.5 for this work
% Or more of Si is indispensable.

At this time, Si dissolves in the ferrite and strengthens the ferrite, so that an unnecessarily large amount of Si lowers the workability of the steel sheet. Therefore, the addition amount is 3%
Limited to the following. Since the addition of 2% or more causes a slight deterioration in ductility, the amount of Si added is preferably in the range of 0.5 to 2.0% by weight when the demand for strength is not particularly strong. .

Mn: Mn is an additive element that contributes to the retention of austenite because it has a function of delaying the formation of carbides like Si. In addition, the addition of Mn lowers the martensitic transformation start temperature of austenite. To stabilize austenite at room temperature, it is necessary to suppress the precipitation of carbides and increase the carbon concentration in austenite as described above. At the same time, it is also important to lower the martensitic transformation start temperature of the austenite.

If the martensite transformation temperature is higher than room temperature, a part of austenite is inevitably transformed into martensite, increasing the strength of the steel sheet and deteriorating the ductility. 0.5% for such purposes
Although the above Mn concentration is necessary, the addition of a large amount of Mn unnecessarily increases the hardenability of the steel sheet, and increases the strength and increases the possibility of deterioration in ductility. Therefore, the upper limit of Mn is set to 2.5%.

When the addition amount of Mn is large, the hardenability of the steel material is enhanced, and the dispersion of the material (strength, ductility, etc.) of the product is increased.
Set to the range of ２．０2.0% by weight.

Other additive elements: Ni and Cr can stabilize austenite by adding them alone or in combination, and are considered to be advantageous for remaining austenite. It is also effective to add an element such as Mo that increases the hardenability of steel. These elements are effective in retaining austenite because they slow the precipitation of cementite.

Nb and Ti are carbides (Nb (C, N), T
i (C, N)) is added for strength adjustment by precipitation strengthening. Further, since Cu is hardly dissolved in cementite like Si, it is used for delaying the start of precipitation of carbides, assisting the progress of carbon concentration of austenite, and adjusting the strength. However, the addition of these alloying elements more than necessary not only increases the production cost of the steel sheet, but also causes the ductility to deteriorate with the increase in strength. Therefore, the total addition is limited to 2% or less.

Winding conditions after hot rolling and annealing heating temperature: The winding temperature after hot rolling has a great effect on the concentration of alloy elements in cementite in the structure before cold rolling. Cementite obtained when the winding temperature is equal to or higher than the bainite transformation temperature of steel is markedly concentrated with austenite stabilizing elements such as Mn, and the hardenability of austenite generated on cementite during annealing in the two-phase region is markedly increased. Enhance. Therefore, when the value S: S = (Ts−Ac ₁ ) / (Ac ₃ −Ac ₁ ) defined by the Ac ₁ , Ac ₃ temperature and the annealing heating temperature Ts of the steel material is 0.2 or less, ferrite + austenite Since good ductility cannot be obtained even by heating in the two-phase region, this is set as the lower limit value of the annealing heating temperature in the case of performing winding at a high temperature.

As described above, when the hot-rolling coiling temperature is high, the higher the annealing temperature of the two-phase zone heating annealing, the better the material. However, when the S value exceeds 0.75, the ductility sharply increases. Since deterioration occurs, this is set as the upper limit of the annealing heating temperature when the winding is performed at a high temperature. When the coiling temperature after hot rolling is lower than the bainite transformation temperature of the steel material, the lower the annealing heating temperature in the two-phase region of ferrite and austenite, the better the ductility. However, when the S value is 0.05 or less, a sufficient amount of retained austenite cannot be obtained and good ductility cannot be achieved. Therefore, this was set as the lower limit of the annealing heating temperature when winding was performed at a low temperature. Since the ductility simply deteriorates when the annealing heating temperature is increased, the upper limit of the S value is limited to 0.45.

Heat treatment conditions after cold rolling: It has been found that good ductility depends not only on the presence of residual austenite but also on the presence of a soft ferrite phase. It is important that the ferrite transformation proceeds sufficiently. For this purpose,
In order to increase the amount of ferrite transformation after heating in the two-phase region as much as possible, a cooling rate of 1 to 10 ° C./sec.
Cool to 0 ° C. The lower limit of the cooling rate is 1 ° C./sec as the lowest cooling rate that can be achieved in actual operation. 10 ℃
Since the cooling of this temperature region at a cooling rate of / sec or more hinders the sufficient progress of ferrite transformation, the upper limit is set. When the cooling at 1 to 10 ° C./sec is performed to 550 ° C. or lower, not only ferrite but also pearlite is generated, and carbon in the steel is consumed in the form of carbide (cementite) and finally remains. It greatly inhibits carbon concentration to austenite, and when this cooling is completed at 700 ° C. or more, the obtained ferrite transformation amount is not sufficient.

After a sufficient ferrite transformation amount is obtained in this way, the steel sheet is cooled to 200 to 450 ° C. at a cooling rate of 10 to 200 ° C./sec. At this time, at a cooling rate of 10 ° C./second or less, the formation of pearlite was recognized, so this was set as the lower limit. The upper limit cooling rate was determined from the cooling rate that can be reached in the actual operation line.

It is necessary to avoid setting the cooling stop temperature to 200 ° C. or less because there is a possibility that martensitic transformation occurs. At 500 ° C. or more, carbide (cementite) precipitates at the same time as bainite transformation, and austenite is reduced to room temperature. Since it is disadvantageous to leave them, these are set as the upper and lower limits.

The temperature of the subsequent bainite transformation treatment (austempering treatment) is determined as follows.
Avoid at least 00 ° C, and at least 300 ° C at which the formation of lower bainite containing carbide in ferrite is not observed. From the production efficiency in the continuous annealing line which is the actual manufacturing process, the bainite transformation temperature is preferably set to 350 to 450 ° C.

When the cooling stop temperature is lower than the bainite transformation treatment temperature, it is necessary to introduce a rapid heating device before the bainite treatment. It has been confirmed that even if such reheating treatment is performed, a steel sheet having excellent workability can be obtained as long as the above-mentioned temperature range is secured.

It has been confirmed that at least 15 seconds or more are required to obtain a bainite transformation amount which is substantially meaningful for the carbon concentration of untransformed austenite at these appropriate bainite transformation treatment temperatures. However, holding for an unnecessarily long time causes the transformation of untransformed austenite to pearlite and the reduction of carbon concentration in austenite due to the precipitation of carbides from untransformed austenite, and as a result, the bainite transformation continues and the austenite remains. Not expected. To avoid this, the maximum holding time at the bainite transformation temperature is limited to 20 minutes or less.

[0041]

EXAMPLE Each steel type shown in Table 1 was hot-rolled to a thickness of 3.0 mm, then cooled and wound (100-780 ° C).
In Table 2, Bs is the bainite transformation temperature. In the case where the winding temperature CT is equal to or lower than Bs, the CT is marked with # . )
The hot-rolled steel sheet thus obtained was cold-rolled to a thickness of 1.0 mm and then annealed to investigate mechanical properties and quantify retained austenite.

The annealing conditions were the same as those shown in FIG.
The holding time at the annealing temperature (Ts / ° C.) is 90 seconds. After annealing, the steel sheet was cooled to T1 ° C. at 5, 50 ° C./sec, subsequently cooled to T2 ° C. at a cooling rate of 5 or 100 ° C./sec, kept isothermally at that temperature for a predetermined time, and then cooled to room temperature. .

Table 2 shows the mechanical properties and annealing conditions of the steel sheet obtained by annealing. In the table, Vg% represents the volume percentage of retained austenite in the steel sheet. As shown in the table, a steel sheet satisfying the conditions of the present invention has a large elongation at break, and TS (kgf / mm ² ) × El (%), which is one index of workability, is 2%.
It turns out that it has excellent workability of 000 or more.

[0044]

[Table 1]

[0045]

[Table 2]

[0046]

[Table 3]

[0047]

[Table 4]

[0048]

[Table 5]

[0049]

According to the present invention, 50 to 12
It is possible to manufacture a high-strength steel sheet having excellent ductility of 0 kgf / mm ² , and it can greatly contribute to reducing the weight of an automobile body when applied to automobile parts.

[Brief description of the drawings]

FIG. 1 is a chart showing the relationship between S value and elongation.

FIG. 2 is a chart showing the relationship between S value and elongation.

FIG. 3 is a chart of an annealing heat cycle of a steel sheet shown in an example.

Claims (2)

(57) [Claims] 1. C: 0.05 to 0.40% by weight, Si: 0.5 to 3.00%, Mn: 0.5 to 2.50% by weight, the balance being Fe and unavoidable impurities The steel material is hot rolled and then cooled, and is wound at a temperature higher than the bainite transformation temperature (hereinafter referred to as Bs) of the steel material, and the structure of the hot rolled steel sheet is made into ferrite + fine pearlite , or wound at a temperature equal to or lower than Bs. heat
Ferrite + bainite or ferrite
Light + perlite + bainite or bainite single phase
And then cold-rolled, and the winding temperature CT and hot-rolled steel sheet set
The annealing heating temperature Ts is set according to the weave as follows: S = (Ts−Ac ₁ )
/ (Ac ₃ −Ac ₁ ) , when CT> Bs, the range is 0.2 ≦ S ≦ 0.75, and when CT ≦ Bs
Is set in the range of 0.05 ≦ S ≦ 0.45, and is heated by annealing , and then at a cooling rate of 1 to 10 ° C./sec.
Cooling to the range of 00 ° C followed by 10-200 ° C /
After cooling to 200-500 ° C at a cooling rate of
Excellent workability characterized by having a ferrite and bainite main phase as a ferrite + bainite + retained austenite composite structure by holding for 15 seconds to 20 minutes in a temperature range of 00 to 500 ° C. and cooling to room temperature. Of manufacturing cold-rolled steel sheets with a high-strength composite structure. 2. The high workability according to claim 1, wherein one or more of Ni, Cr, Cu, Mo, Nb, and Ti are added in a total amount of 2% or less. A method for producing a cold-rolled steel sheet with a strong composite structure.