JPH0733551B2 - Method for producing high strength steel sheet having excellent formability - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for producing a high strength plate having excellent formability.

(Prior art) Simplification of the process is a general technical trend in recent years along with its continuity. This tendency is also recognized in the field of press forming of thin steel sheets, and one-step integral forming is being advanced, and steel sheets capable of coping with this are required. The formability of thin steel sheet is determined by elongation, Rankford's plastic strain ratio r value,
Judgment is made based on the work hardening index n value, yield strength, etc., but it is necessary to have a large elongation and a large n value in order to integrally mold a complicated part. As a high-strength steel sheet with a large elongation and a large n value, a dual structure consisting of a mixture of ferrite and martensite
Phase steel plate is generally used, and as disclosed in Japanese Patent Publication No. Sho 56-18051 and Japanese Patent Publication No. 59-45735, a maximum elongation of about 30 to 35% can be obtained with a tensile strength of 50 to 80 kgf / mm ² . This Du
Typical methods that have been proposed so far to improve the elongation and n value of al phase steel sheets are to increase the cleanliness of ferrite and increase its ratio, and to secure the strength without significantly degrading the n value. It is to add an element. In order to achieve the former purpose, the annealing temperature in the two-phase region is set to A
Set directly above _c1 or slow cooling to an appropriate temperature as in JP-B-57-45454, and then quenching it, or by performing batch annealing under appropriate conditions in advance before continuous annealing. There is a method of placing it, and elements such as P and Si can be cited as elements that achieve the latter purpose. However, there has not been reported an industrial scale production example of a steel sheet which has a tensile strength of 50 to 80 kgf / mm ² grade and an n value exceeding 0.25 at the same time by using such a method. That is, as the strength of the steel sheet increases, the elongation and the n value considerably decrease, so that it is impossible to form a complicated shape with the high strength steel sheet that has been developed so far. Thus, the so-called Dual phase
Even steel is unsatisfactory in formability, and the cost performance is not excellent, so even at present, it is applied to limited applications,
Taking into consideration the fact that it is difficult to obtain ideal characteristics in the existing general continuous annealing furnace having an overaging zone, it is difficult to expect further development in the state as it is.

(Problems to be Solved by the Invention) The present invention solves the problems of the prior art as described above, and has a tensile strength of about 50 to 80 kgf / mm ² and an elongation of more than 35 to 45%, and 0.25 to 0.30 or more. The present invention provides a method for producing a high-strength steel sheet having excellent formability that is recognized by having the same n value. In other words, while maximally utilizing the conventional knowledge, the transformation-induced plasticity of retained austenite is synergized to achieve both high strength and excellent formability that have not been considered in the past.

It is widely known that transformation-induced plasticity is a mechanism that brings high strength and high ductility out of the common range of strength-ductility balance. This is a phenomenon in which, in metastable stainless steel such as SUS301, austenite transforms to martensite as it is deformed, but at the same time, it shows a large elongation. If this phenomenon is effectively utilized, it will become "plasticity and processing".
As shown in Vol. 16, pp. 993-1000, with a tensile strength of 80 kgf / mm ² , a large elongation of 60% and an extremely large n of 0.70.
The value is obtained. However, such astonishing mechanical properties are exhibited only in a very limited temperature range, and are difficult to be applied to actual industrial production. In addition, as a steel that positively utilizes this transformation-induced plasticity, “Trans.ASM” No. 6
The so-called TRIP steel presented by Zackay et al. In Vol. 0, pp. 252-259, but its use because it contains a large amount of expensive alloying elements and complicated processes such as large reduction at low temperature must be used. Are very limited. However, in recent years,
It has been attempted to apply this phenomenon to applications such as steel sheets for automobiles, which require low cost and large-scale production, such as the invention described in JP-A-61-157625. The important point is that the addition of Si suppresses carbides and stabilizes the untransformed austenite in which C is concentrated. According to the invention of this prior application, stable mass production is possible using a general continuous annealing furnace without adding an expensive alloy element, and its significance is extremely large. However, in the invention of this prior application, the tensile strength is excessive due to the large amount of C and the large amount of retained austenite, and the adverse effects associated therewith are also found, and it has not reached widespread practical use.

(Means for Solving the Problems) Even if the present inventors carry out a process design that increases the cleanliness and the amount of ferrite at the same time as adding Si even if the C content is relatively low, the so-called Dual phase steel is obtained. Then, it was found that retained austenite, which largely contributes to transformation-induced plasticity, which was never observed, is obtained. As a result, it was revealed that a large amount of clean ferrite can be secured mainly in a large amount in the low strain region, and that a large n value can be secured in the high strain region due to the transformation-induced plasticity of retained austenite.

That is, in the present invention, C: 0.07 to 0.12% and Si: 0.50 to 2.
00%, Mn: 1.00 to 2.50%, sol.Al: 0.005 to 0.100% and, if necessary, one or more of Ni, Cr, Co and Cu, totaling 1% or less, balance Fe and unavoidable Of steel with mechanical impurities is pickled and cold-rolled at 30-85%, then A _c1
After heating at 700-800 ℃ above the transformation point and holding for 15 seconds-5 minutes, 1
Cool to 200-450 ° C at a rate of ~ 200 ° C / s, then 3
Within the range of 00 to 450 ℃, X = 5500 / (T + 273) -logt + 0.27 (Si (%) + Mn (%)) + 10 (C (%)) ² 7, the average temperature T ° C. is maintained for a time t seconds defined by the composition of the steel, and then the temperature is cooled to 150 ° C. or less within 60 seconds. The gist of the invention is a method of manufacturing a strength steel sheet.

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

First, C is an element that expands the austenite region of steel, and at the same time diffuses rapidly in the steel, so when ferrite and austenite coexist, they are concentrated in austenite to increase their stability. As a result, if heat treatment of the cycle defined in the present invention is performed, austenite can remain even after cooling to room temperature, and transformation-induced plasticity can be utilized. The amount of C is preferably small in order to secure good practical properties such as weldability, but 0.07%
If the amount is less than the above, it is difficult to obtain the retained austenite having appropriate stability that clearly shows the transformation-induced plasticity, so that the object of the present invention cannot be achieved. On the other hand, if it exceeds 0.12%, retained austenite is easily obtained, but it is difficult and possible to industrially secure the desired excellent formability at a tensile strength level of 50 to 80 kgf / mm ^2. However, there is no merit when considering the weldability.

Since Si does not form a solid solution in cementite, it has the effect of suppressing its precipitation and, at the same time, delays the transformation together with Mn and the like. Therefore, by holding the material at 300 to 450 ° C. for an appropriate time, C far exceeding the solid solubility limit is concentrated in the untransformed austenite, the stability of retained austenite is moderated, and the effect of transformation-induced plasticity is increased. To do. Such an effect is obtained with the C content and process conditions of the present invention.
Not observed when the Si content is less than 0.50%. However, if added excessively, a scale that deteriorates the pickling property remarkably occurs during hot rolling, and it causes graphitization of C, so the amount is 2.00
% Or less. The addition amount is preferably 1.40% or less so as not to impair the chemical conversion treatment property.

Further, Mn is not only an austenite forming element, but also suppresses the decomposition of austenite into pearlite in the process of cooling from the two-phase region to the bainite transformation region and at the same time delays the bainite transformation itself. Therefore, in order to complement the effect of Si as described above, it is possible to bring about transformation-induced plasticity even when the cooling method commonly used today in continuous annealing and the holding condition at around 400 ° C for the purpose of overaging are taken. Brings a metallographic structure with mixed retained austenite. Further, the addition thereof tends to make the structure fine, and not only strengthens the fine grains but also increases the amount and stability of retained austenite, which is effective for achieving the purpose. The above effects are not observed when the amount of Mn added is less than 1.00%.
However, if it exceeds 2.50%, even if the hot rolling conditions are controlled, the retention in the bainite transformation region is considerably long in order to maintain the stability of retained austenite to the extent that it exhibits the effect of transformation plasticity near room temperature. It must be time and should be avoided.

Further, sol.Al improves the material quality by suppressing the grain refining of the hot rolled material by AlN as a deoxidizing element 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 and its stability by adding austenite forming elements Ni, Cu and Co 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 precipitated and workability may be deteriorated, or extremely stable carbide may be formed to impair the role of C, so the total amount is 1%. Limited to:

Next, the reasons for limitation in the process will be described in detail.

The steel sheet according to the present invention is pickled and cold-rolled at 30 to 85%. The purpose is to form fine austenite grains centering on the triple boundary points of ferrite and pearlite when heated above the A _c1 transformation point, and after the subsequent series of cycles, fine retained austenite and bainite are formed. To obtain a metallic structure dispersed in a clean ferrite matrix. With this metal structure, it is possible to achieve both high strength and excellent formability, but if the cold rolling rate is less than 30%, the ferrite grains after recrystallization become coarse when heated at a normal rate, so the heat treatment specified in the present invention. However, even if it is carried out, it is difficult to retain austenite up to room temperature or below, and even if it is retained, the effect of transformation-induced plasticity is difficult to obtain, so the object cannot be achieved. On the other hand, even if cold rolling is performed at more than 80%, inclusions are extremely stretched, so that local elongation is deteriorated and no merit is brought about.

In the heat treatment consisting of a series of cycles of the present invention, first, it is heated to 700 to 800 ° C. which is higher than the A _c1 transformation point and kept for 15 seconds to 5 minutes. When this heating is applied to a steel sheet having the component system of the present invention, most of the carbides above the solid solution limit disappear, and 10-40% by volume of austenite in which C is concentrated near the eutectoid composition is formed. This austenite becomes residual austenite and bainite after a series of subsequent cycles,
Coexists with clean ferrite to bring a large n value from a low strain region to a high strain region. If the heating temperature is lower than 700 ° C., the carbide may not be completely dissolved within the time that can be realized in a continuous line, and further recrystallization may be in an insufficient state, so that the subsequent treatment is performed according to the present invention. However, even if it is specified by the above, it cannot be a steel sheet having high strength and excellent formability. Also, if the heating temperature range exceeds 800 ° C, it requires a large amount of energy and cannot be expected to have an effect commensurate with it, so it must be avoided.
If the time of heating and holding in this temperature range is less than 15 seconds, there is a possibility that undissolved carbides are present and austenite is hardly generated in some cases. On the other hand, even if it is kept for a long time such as more than 5 minutes, the characteristics cannot be improved corresponding to the increase in the energy released, and it does not meet the precondition of economical mass production in a continuous line.

In the present invention, this is followed by cooling to 200 to 450 ° C at a rate of 1 to 200 ° C / sec. The purpose of this is to bring austenite generated by heating above the A _c1 transformation point to the bainite transformation region as it is, and to obtain residual austenite in a structure coexisting with clean ferrite by subsequent treatment. If this cooling rate is less than 1 ° C./sec, transformation into pearlite starts and C forms carbides therein, so that retained austenite effective for transformation-induced plasticity cannot be secured. On the contrary, at a speed exceeding 200 ° C / sec, it is difficult to cool the entire steel plate under uniform conditions, and even if this difficulty is eliminated and such cooling is performed, ferrite will suddenly grow and acicular Since the structure is likely to be formed, the moldability becomes poor in spite of the strength. This cooling remains
When the temperature goes below 200 ° C, most of the austenite formed during heating transforms to martensite, so the strength increases, but its properties are not much different from conventional Dual phase steel, and elongation and n value are required. Does not reach the standard. On the other hand, ending this cooling at a temperature exceeding 450 ° C. is difficult because it is difficult to end the treatment in the desired microstructure state because the subsequent transformation rapidly progresses. In addition, changing the speed in the former and latter stages of this cooling to cool it to 1 to 20 ° C / sec up to 550 to 680 ° C, and to 15 to 200 ° C / sec below it is possible to use untransformed austenite during slow cooling of the former stage. The alloy element is concentrated to increase the amount of cleaner ferrite, which is more effective in achieving the object of the present invention.

Following this cooling, in the present invention, X = 5500 / (T + 273) -logt + 0.27 (Si (%) + Mn (%)) + 10 (C (%)) ^{2 in} the range of 300 to 450 ° C. After maintaining at the average temperature T ° C. for a time t seconds defined by the composition of the steel so that X given by the formula becomes 6 to 7, it is cooled to 150 ° C. or lower within 60 seconds. Precipitation of carbides does not occur during this retention because Si is added, and a portion of austenite transforms into a bainite-like structure and the untransformed austenite is further enriched with C and its stability is improved. Increase. As a result, the Ms point of untransformed austenite becomes a temperature significantly lower than room temperature, and a large elongation and n value are realized by coexisting with clean ferrite. If this holding temperature exceeds 450 ° C, the progress of transformation is rapid, so it will be difficult to finish the treatment in the desired microstructure unless a considerable amount of alloying elements are added, but this will cause a significant increase in cost, which is a reality. Not at all. On the other hand, holding at a temperature of less than 300 ° C. cannot be carried out because it requires a holding time that makes production in a continuous line practically impossible because the diffusion of C is extremely slow. When the holding time is short and the value of X is larger than 7, even if the retained austenite is obtained, the concentration of C is insufficient, so that the contribution as the transformation-induced plasticity is small. Therefore, the desired large elongation and n value are not shown,
Only properties that are not very different from conventional Dual phase steel can be obtained. On the contrary, when the value of X is smaller than 6, and most of the austenite is transformed into bainite when it is held for a long time. The resulting metallographic structure is a mixture of bainite centered around the ferrite grain boundaries, so its mechanical properties have no advantage over those of Dual phase steel.
This is also the case when it takes more than 60 seconds to cool to 150 ° C. or lower after the holding, which is far from reaching the object of the invention.

In the process described above, the temperature for heating and holding, the holding temperature after completion of cooling, or the cooling rate during that period does not need to be constant within a specified range, and does not fluctuate within that range and the final product. The effect on the properties of is negligible.

Further, the material of the present invention is, in principle, manufactured through normal steelmaking, casting, hot rolling process, but is manufactured by omitting a part or all of the hot rolling process by performing thin casting. There is no problem. The hot rolling may be carried out under conditions similar to those usually performed, including a winding temperature of about 500 to 750 ° C.

(Example) After hot-rolled steel sheet having the components shown in Table 1 was pickled and cold-rolled at the rolling rate shown in Table 2, heat treatment constituted by a series of conditions shown in the table and a 0.6% adjustment Quality rolling was performed. The average temperature held in the table, the value X determined by the time and the composition of the steel is the average temperature T ° C., the time t seconds and the composition of the steel X = 5500 / (T + 273) -log + 0. 27 (Si (%) + Mn (%)) ＋ 10 (C (%))
It is the requirement of the present invention that X is given by the formula ² and this value is 6 to 7. After that, JIS No. 5 tensile test pieces were sampled and subjected to a room temperature tensile test with a gauge length of 50 mm and a tensile speed of 10 mm / min, and the tensile strength, total elongation and n value as shown in the same table were obtained. Here, n value is tensile strain 10% and 20%
It is calculated on the assumption that the n-th power hardening rule holds between the two.

Sample No. 2,4,5,7,8,10,13,16,19 to 21,2 of the present invention sample
3,26,27,30,31,34,35 have a metallic structure in which a small amount of retained austenite existed with bainite and some martensite in a clean ferrite matrix, all of which are 50-80kgf / mm ² grade It has an excellent formability as well as high strength, as is clear from the fact that it has elongation exceeding 35 to 45% and n value of 0.25 to 0.30 or more. On the other hand, steels a, e, f, and g outside the composition range of the present invention were treated as sample Nos. 1, 37 to 39 even if heat treatment considered to be optimal, and even the composition steel of the present invention was treated. If any of the conditions is inappropriate, sample No. 3,6,9,11,12,14,15,17,18,22,24,25,28,2
Conventional technology has problems such as 9,32,33,36 having low strength, too high strength, or even at an appropriate strength level, there is a problem in formability due to insufficient elongation or n value. Cannot solve the problem.

(Effects of the Invention) As can be seen from the above embodiments, the present invention has a lower C than the conventional one.
Since it succeeded in producing retained austenite under a certain amount, a high-strength steel sheet having an unprecedentedly large elongation and excellent formability with an n value can be obtained. Moreover, the cost required to manufacture this steel sheet is less than or equal to that of conventional steel sheets having the same strength level, and the properties required for thin steel sheets in addition to formability such as weldability are also in a range that can be expected from the industrial viewpoint. Is extremely large.

[Brief description of drawings]

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

Claims (3)

[Claims] 1. C: 0.07 to 0.12% by weight%, Si: 0.50 to 2.00
%, Mn: 1.00 to 2.50%, sol.Al: 0.005 to 0.100%, balance Fe and inevitable impurities in steel, pickling and 30 to 85
% After cold rolling 700% to 8% above A _c1 transformation point
After heating to 00 ℃ and holding for 15 seconds to 5 minutes, cool to 200-450 ℃ at a speed of 1-200 ℃ / second, and then in the range of 300-450 ℃ X = 5500 / (T + 273)- logt0.27 (Si (%) + Mn (%)) + 10 (C (%)) ^{2 The} time given by the composition of the steel for t seconds such that X given by the formula becomes 6-7. A method for producing a high-strength steel sheet having excellent formability, which is characterized by cooling to an average temperature T ° C and then cooling to 150 ° C or less within 60 seconds. 2. A high-strength steel sheet having excellent formability according to claim 1, characterized in that one or more of Ni, Cr, Co, and Cu are added in a total amount of 1% or less by weight. Manufacturing method. 3. A method of heating at 700 to 800 ° C. above the A _c1 transformation point and holding for 15 seconds to 5 minutes, cooling at 550 to 680 ° C. at 1 to 20 ° C. /
The method for producing a high-strength steel sheet having excellent formability according to claim 1 or 2, characterized in that the second is set to 15 to 200 ° C / second.