JPH0670246B2 - Method for manufacturing high strength steel plate with good workability - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for producing a high-strength steel sheet having good workability.

(Prior Art) The running performance, comfort, and integrity of passenger cars have made remarkable progress as never before. This is largely due to an increase in electronic control parts and safety and security parts to be mounted and an increase in the projected area of the car body, but on the other hand, there is almost no change in the total weight of the car body. This is because the increase in weight is offset by the higher strength of the thin steel sheet for automobiles, which is the main constituent material. It is expected that consumers will continue to become more sophisticated and have higher performance due to the improvement of living standards, but at the same time, there is a keen awareness of costs due to the experience of the oil shock, and this trend continues. Be admitted. Therefore press workability in steel sheets for automobiles with excellent compares favorably with steel of the prior 30~40kgf / mm ² class strength while having a tensile strength greater than 60~100kgf / mm ² had a It would be required to have them at the same time.

Ferrite / martensite dual-phase steel, so-called Dual phase steel, proposed in Japanese Patent Publication No. 56-11741 and the like, is prominent as a thin steel sheet that achieves both high strength and workability. This is a steel in which the elongation is balanced by soft ferrite and the strength by hard martensite, and the balance between strength and ductility is improved to achieve both, but the product of tensile strength and total elongation, which is one index value, is still used. Is only about 2000 kgf / mm ² ·%, which makes it extremely difficult to press-form into a shape similar to that of conventional mild steel sheets. A further improved steel type is the transformation-induced plasticity (Transformation Induced P
A steel mixed with retained austenite for the purpose of utilizing lasticity is disclosed in JP-A-60-43430 and JP-A-61-15762.
It is proposed in Japanese Patent No. 5, etc. These steels, which can be manufactured using continuous annealing equipment for mild steel sheets, and which have a relatively simple chemical composition and contain as much as 10 to 15% retained austenite, have a total elongation of 30% inside and outside even with a tensile strength of 100 kgf / mm ² , for example. In addition, the bendability and hole expandability satisfy the level required for thin steel sheets for automobiles and are expected to be widely used in the future.

However, in order to obtain the best combination of high strength and workability by the method found so far, it is attempted to make residual austenite of about 10 to 15% or more coexist with ferrite and bainite. Although various measures have been taken as in Japanese Patent No. 188729, it is necessary to contain C of 0.20% or more, which is not generally considered in the category of conventional thin steel sheets. As described above, when the content of C is large, the ratio of pearlite is large and it is difficult to obtain a desired rolling rate in cold rolling, which causes various difficulties in the operation in the continuous heat treatment line. Not only that, because the required strength cannot be added to the joint by spot welding, which is often used when assembling automobile bodies, this type of steel plate is used despite its unprecedented high strength and extremely good workability. It prevented us from using it in large quantities.

(Problems to be Solved by the Invention) The present invention solves the problems of the prior art as described above, and a high-strength steel sheet having good workability equivalent to or better than that of the conventional invention brought about by retained austenite with a lower C content. The present invention provides a method for manufacturing the same.

(Means for Solving the Problems) The high-strength steel sheet according to the present invention is characterized by comprising a mixed structure of ferrite, bainite, and retained austenite,
It is necessary to coexist a required amount of retained austenite and bainite in order to secure the desired strength, and a required amount of retained austenite and ferrite in order to obtain good workability. Expand the austenite range on the equilibrium diagram A ₁
Generally, if the amount of the element that lowers the transformation point is increased, retained austenite is likely to be generated, and its stability can be adjusted to improve the elongation due to transformation-induced plasticity. Ni is a typical element of this kind and is added to metastable austenitic stainless steels such as SUS304.
Further, as in Japanese Patent Publication No. 44-738 and Japanese Patent Publication No. 46-13498, 4-10% of Ni was added inside and outside to improve the toughness at low temperature by mixing extremely fine retained austenite grains in the matrix. There is steel. As described above, Ni, which is a very useful element and has a small Clark number of 0.008, is expensive and cannot be added in such a large amount to thin steel sheets for automobiles, which is essential to be mass-produced and inexpensive. An example of a steel containing 2 to 4% of Ni that does not significantly impair economic efficiency is ASTM standard A203 steel, but it has no structural characteristics and has excellent toughness at low temperatures, but near room temperature. The strength-ductility balance at is obsolete. However, the present inventors have found that a structure in which a considerable amount of retained austenite is mixed can be obtained by optimizing the addition amounts of Si and Mn and performing a series of heat treatments including heat cycles similar to continuous annealing of mild steel sheets. Moreover, unlike the one which aimed at the stabilization of austenite only by Ni, C and Mn
The present invention has been made paying attention to the fact that the Ms point and the Md point have a large difference and the transformation-induced plasticity can be easily utilized because they are also stabilized by such elements as.

That is, in the present invention, C: 0.07 to 0.30% and Si: 0.30% by weight.
~ 1.50%, Mn: 0.20 ~ 2.00%, Ni: 1.00 ~ 6.00%, sol.A
l: 0.005-0.100% and 1 of Cu, Co, Cr as required
Steel containing at least 1% of one or two or more and the balance Fe and unavoidable impurities is hot-rolled and pickled and rolled at a rolling ratio of 35
~ 80% cold rolling, then heating in the two-phase coexistence temperature range of 600 ~ 800 ℃, hold for 15 seconds ~ 5 minutes, then at the rate of 1 ~ 200 ℃ / sec.
The gist is a method for producing a high-strength steel sheet with good workability, which is characterized by cooling to 50 to 500 ° C, holding for 5 seconds to 10 minutes within this temperature range, and then cooling to 150 ° C or less within 30 seconds. It is a thing.

(Operation) First, the reason for limiting the composition range of the steel targeted by the present invention will be described.

First, C is the lowest-cost austenite stabilizing element, and it moves from ferrite to austenite in a short time in the two-phase coexistence temperature range and bainite transformation temperature range to increase its stability. As a result, even if the Ms point determined by the chemical composition when the elements are uniformly distributed in the steel is much higher than room temperature, it synergizes with the effect of the distribution of Mn and Ni, and even if cooled to below room temperature The amount of austenite remains. As a result, unprecedented high strength and good workability can be obtained. The addition amount is preferably low in order to improve the weldability and impact properties, but if it is less than 0.07%, it is not possible to secure the amount of retained austenite enough to make the improvement of elongation apparent. On the other hand, when the content exceeds 0.30%, a large amount of retained austenite is obtained, but the amount of martensite that exists after press forming becomes considerable due to work-induced transformation, and the deterioration of various properties is significant, which is necessary for spot welding. It cannot be used for practical purposes because it cannot obtain the strength.

Since Si does not form a solid solution in cementite, it has the effect of suppressing its precipitation. During the temporary holding at 250 to 500 ° C, Si is concentrated in the untransformed austenite far exceeding the solid solubility limit, and its stability is improved. Increase. However, in the C content range of the present invention, such an effect is not clear when Si is less than 0.30%, and 0.70% or more is preferable for achieving the purpose. On the other hand, if it is added in excess, a scale to the extent that the pickling property is significantly deteriorated is generated during hot rolling, and C may be precipitated as graphite. Therefore, excessive addition of more than 1.50% should be avoided.

Mn suppresses the decomposition of austenite into pearlite during the cooling from the two-phase region to the bainite transformation region, and brings the austenite existing at the time of starting the quenching to 500 ° C or less as it is. It is an extremely effective element to add. Further, it is classified as an austenite forming element together with Ni, and widens the temperature range in which it is stable and facilitates the formation of retained austenite. If the amount is less than 0.20%, hot brittleness may occur during hot rolling, and a large amount of pearlite is generated during cooling to 500 ° C, which is not suitable. However, if the content exceeds 2.00%, the above-mentioned object can be achieved, but the stabilization reaction of austenite due to the concentration of C requires an extremely long time, and it is practically impossible to produce a large amount in a continuous line. It should be possible. Further, the characteristics may be deteriorated due to the formation of the band structure, so that it must be avoided.

Further, Ni is an austenite-forming element, which forms a solid solution in iron as a substitutional element, and lowers its Ms point. For this reason
Even if the C concentration is lower than that of the conventional invention example as disclosed in Japanese Patent Laid-Open No. 62-188729, austenite can remain without being transformed into martensite when cooled to room temperature or lower. Since it does not form a solid solution with cementite, it is 250 as with Si.
It promotes the enrichment into austenite while suppressing the precipitation of C as carbides during retention at ~ 500 ° C. However, if the amount is less than 1.00%, the effect of addition is not recognized. Further, if it exceeds 6.00%, according to the heat cycle of the present invention, the amount of retained austenite exceeds 30% after the heat treatment is completed, and austenite is likely to be present in a coupled state, so that plastic-induced transformation occurs at one time, and it is aimed. I can't get such a big growth.

Further, sol.Al is a deoxidizing element, and the material is improved by suppressing the grain refinement of the hot rolled material by AlN and the coarsening of the crystal grains in the series of heat treatment steps.
Add ~ 0.100%. If the amount is less than 0.005%, the desired effect is insufficient, and if it exceeds 0.100%, the toughness may deteriorate due to inclusions, so it must be avoided.

The steel of the present invention has the above-mentioned basic components, but in addition to these elements and Fe, it also contains P, S, N and other elements that are generally unavoidably mixed with steel. Further, it is preferable to increase the amount of retained austenite by adding Cu or Co, which is an austenite-forming element, or Cr, which is an element that increases hardenability, in order to achieve the object of the present invention. However, if added excessively, a compound having a complicated composition may be finely precipitated and workability may be significantly deteriorated. Therefore, the total amount is limited to 1% or less.

Next, the reasons for limitation in the process will be described in detail. In the present invention, the hot rolled steel sheet is pickled and cold rolled at a rolling ratio of 35 to 80%. This aims at obtaining a structure in which fine retained austenite is dispersed in ferrite and bainite after heat treatment consisting of a series of subsequent cycles. This rolling rate
If it is less than 35%, the micronization of the structure is insufficient, so even if the heat treatment specified in the present invention is applied, a sufficient amount of retained austenite cannot be obtained, and only those having poor properties such as elongation are obtained. Absent. The effect tends to be saturated as the rolling rate increases, and cold rolling exceeding 80% is not suitable because the power required for rolling becomes enormous and the effect is small.

First of all, in the heat treatment consisting of a series of cycles of the present invention,
Heat to the two-phase coexistence temperature range of 600 to 800 ° C and hold for 15 seconds to 5 minutes. When this heating is performed on the steel sheet having the component system of the present invention, the carbides above the solid solution limit are almost extinguished, and austenite becomes
A structure state where 40-80% is present and ferrite remains is revealed. C, which has a large diffusion constant, is concentrated in austenite and becomes rare in ferrite. Therefore, after a series of subsequent cycles, a structure in which retained austenite of 8 to 30% is finely dispersed in a mixture of ferrite and bainite can be obtained, and a steel sheet having high strength and good workability can be obtained.

When the heating temperature is less than 600 ° C, the carbide does not dissolve within the time that can be realized in the continuous line, the austenite content is very small, and the recrystallization is insufficient, so that the subsequent treatment is not possible. Even if it is defined in the present invention, it is not possible to obtain a steel sheet having high strength and good workability. On the other hand, heating to a temperature range exceeding 800 ° C. requires a large amount of energy, is uneconomical, and causes various undesirable phenomena such as deterioration of surface properties. If the holding time in this temperature range is less than 15 seconds, there is a high possibility that undissolved carbides will be present, and the desired amount of austenite will not be formed, making it impossible to achieve both strength and workability. on the other hand,
Holding for more than 5 minutes is not suitable for economical mass production in a continuous line, and even if it is carried out, the material may be deteriorated due to coarsening of crystal grains.

In the present invention, this is followed by cooling to 250 to 500 ° C at a rate of 1 to 200 ° C / sec. The purpose of this is to bring austenite produced by heating in the two-phase region to the bainite transformation region without transforming it into pearlite, and to obtain predetermined properties as residual austenite and bainite at room temperature by subsequent treatment. This cooling rate of less than 1 ° C / sec means that it is below the critical cooling rate, and most of the austenite undergoes pearlite transformation, so that after the heat treatment, the amount of bainite and retained austenite is extremely small and the strength is low and the workability is also low. Not good. On the other hand, if it exceeds 200 ° C / sec, acicular ferrite is formed, which causes deterioration of the strength-ductility balance. Further, it is difficult to finish cooling at the target temperature over the entire steel sheet, and the shape may not be able to withstand industrial use. When this cooling is completed at a temperature higher than 500 ° C, even if Si and Ni are contained, carbide is rapidly generated during the subsequent holding and the C concentration in austenite sharply decreases, so that it cannot be left at room temperature. Becomes On the other hand, although the austenite stabilizing elements such as Ni and Mn are added, C is not concentrated in austenite to such an extent that the Ms point is lowered to less than 250 ° C. in the state where it is formed in the two-phase region, so it is as it is. 250 ° C
If cooled to less than a large amount, a large amount of martensite is produced, and even if it is tempered after that, the workability cannot be said to be in a range in which it can be said to be good even if the strength is sufficient. After soaking in the two-phase region, 1 to 20 ℃ / sec up to 550 to 700 ℃, 25 to 200 ℃ below
Cooling at ℃ / sec and growing clean ferrite while concentrating Ni, Mn, etc. in the remaining austenite during slow cooling in the upper stage leads to an increase in the amount of retained austenite and further improves workability. Is the preferred method.

After completion of this cooling, in the present invention, the temperature is held at 250 to 500 ° C. for 5 seconds to 10 minutes, and then cooled to 150 or less within 30 seconds. This is because in the steel of the composition of the present invention, the transformation from austenite to bainite is separated into two stages, and bainite containing almost no carbide and C swept out from that portion are concentrated and stabilized by Ni, Mn, etc. In addition, the residual austenite whose Ms point has dropped below room temperature, the residual ferrite that has been purified during heating in the two-phase region, and the structure in which clean ferrite that has grown during the previous cooling are mixed are revealed, and high strength is achieved. It is to make good workability compatible. This holding temperature is 50
When the temperature is higher than 0 ° C, carbides are rapidly formed during that period, and the C concentration in austenite is rapidly reduced, so that it is impossible to remain. On the other hand, if the holding temperature is less than 250 ° C, it is substantially difficult to diffuse C, so that C is not concentrated in the untransformed austenite and the Ms point cannot be lowered to room temperature or lower, which is effective in obtaining retained austenite. It doesn't matter. If the holding time is less than 5 seconds, the progress of bainite transformation is insufficient, so austenite in which C is not sufficiently enriched undergoes martensite transformation during cooling to room temperature, and the resulting steel sheet has high strength but is difficult to work. It will be scarce. If the retention time exceeds 10 minutes, the bainite transformation proceeds further, and the austenite enriched with C in the previous reaction also precipitates carbides and decomposes into bainite, so the workability is improved by transformation-induced plasticity. Will bring. This is also the case when it takes more than 30 seconds to cool to 150 ° C. or lower, and the object of the invention cannot be achieved.

It should be noted that the heating temperature in the two-phase region and the holding temperature after the cooling from the two-phase region in the process described above, and the cooling rate during that period do not have to be constant as long as they are within the specified range. Even if it fluctuates within that range, the characteristics of the final product may not be deteriorated or improved in some cases.

(Example) Steels having the components shown in Table 1 were hot-rolled and pickled, and then cold-rolled under the conditions as shown in Table 2 and heat treatment consisting of a series of cycles was performed. Then, after subjecting to 0.8% temper rolling, a JIS No. 5 tensile test piece was prepared and subjected to a room temperature tensile test at a gauge length of 50 mm and a tensile speed of 10 mm / min. Got

Sample Nos. 2, 4, 5, 7, 8, 11, 14, which are samples of the present invention,
17, 18, 20, 22, 23, 26, 27, 30, 34 are all 2500 kg
As it can be judged from the product of tensile strength and total elongation exceeding f / mm ² ·%, it has high strength as well as good workability. On the other hand, steel a outside the composition range of the present invention,
c, g, h, i, and k are the same as in Sample Nos. 1, 3, 31 to 33, and 35 even if the treatment considered to be optimum is performed, and even in the case of the component steel of the present invention However, if there are inappropriate places, sample Nos. 6, 9, 10, 12, 13, 15, 16, 19, 21, 2
As shown in 4, 25, 28, 29, either tensile strength or total elongation is poor, or both are inferior, so only a product of tensile strength and total elongation of less than 2500 kgf / mm ² It cannot be a strong steel plate.

(Effects of the Invention) As is clear from the above examples, according to the present invention,
% Of residual austenite coexists with ferrite and bainite, it is possible to obtain a high-strength steel sheet with good workability over a wide range of tensile strength of 50 to 140 kgf / mm ² . Moreover, since such characteristics can be obtained with a much lower C content than in the conventional invention, the effect of the present invention is extremely great industrially.

[Brief description of drawings]

FIG. 1 is a diagram showing a heat treatment cycle applied to a steel sheet after cold rolling.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁵ Identification code Office reference number FI technical display location C22C 38/00 302 Z 38/52

Claims (3)

[Claims] 1. C: 0.07 to 0.30% by weight, Si: 0.30 to 1.50
%, Mn: 0.20 to 2.00%, Ni: 1.00 to 6.00%, sol.Al: 0.005
~ 0.100% and the balance Fe and unavoidable impurities are hot-rolled, pickled and cold-rolled at a rolling ratio of 35-80%, and then heated in the two-phase coexistence temperature range of 600-800 ° C. After holding for 15 seconds to 5 minutes, cool to 250 to 500 ° C at a rate of 1 to 200 ° C / sec,
Hold for 5 seconds to 10 minutes within this temperature range, then within 15 seconds for 15 seconds
A method for producing a high-strength steel sheet with good workability, which comprises cooling to 0 ° C or less. 2. The method for producing a high-strength steel sheet with good workability according to claim 1, wherein one or more of Cu, Co and Cr are added in a total amount of 1% or less by weight. 3. Cooling after heating in the two-phase coexistence temperature range and holding for 15 seconds to 5 minutes, 1 to 20 ° C./sec up to 550 to 700 ° C., and below
25-200 degreeC / sec is set, The manufacturing method of the high strength steel plate with favorable workability of Claim 1 or 2 characterized by the above-mentioned.