JPH0670247B2 - Method for producing high strength steel sheet with good formability - Google Patents

Description

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

(Prior Art) The runnability, comfort, and integrity of passenger vehicles have made remarkable progress in recent years. 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, strength exceeding 80 to 100 kgf / mm ² will be required for thin steel sheets for automobiles at the same time as severe press formability for obtaining the required shape.

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 formability. 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 ·%, and if the tensile strength exceeds 100 kgf / mm ² , it becomes extremely difficult to press-form into the required shape. A further improved steel grade is transformation-induced plasticity (Tr
Japanese Patent Laid-Open No. 60-43430 discloses a steel containing mixed austenite for the purpose of utilizing the ansformation induced plasticity.
It is proposed in Japanese Laid-Open Patent Publication No. 61-157625.
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 or outside even with a tensile strength of 100 kgf / mm ^2. Depending on the method, the bendability and hole expandability will be equal to the best class that has been obtained so far in terms of strength level, and it is expected to be widely used as a thin steel sheet for automobiles in the future.

However, in order to obtain the best combination of high strength and formability by the method found up to now, it is necessary to perform a fairly rapid cooling of about 100 ° C / s or more from around 700 ° C to around 400 ° C. Although it is generally possible to cool a steel sheet at this rate according to a method commonly called steam cooling, it is a target for steel having a chemical composition as defined in the invention described in each of the above patent publications. Careful handling is required because it becomes extremely hard and brittle with a slight deviation from the temperature. Therefore, it is not always easy to manufacture a large amount of this type of steel sheet in actual operation, and it has been difficult to meet the demand due to its excellent characteristics, although it is in great demand.

(Problems to be Solved by the Invention) The present invention solves the problems of the prior art as described above, and has a cooling rate that facilitates temperature control, and a steel sheet having the same characteristics as those of the best prior art. 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,
In order to secure the required strength, it is necessary to bring austenite formed in the two-phase coexistence temperature range to the bainite transformation temperature range without transforming it into pearlite or the like. This critical cooling rate becomes smaller as the amount of alloying elements such as Mn and Cr increases, but at the same time, the time required for the austenite to stabilize becomes significantly long, making it difficult to manufacture in a continuous line. In addition, the banding of the structure becomes remarkable, and the characteristics are deteriorated accordingly. On the other hand, when a very small amount of B is added, the critical cooling rate is greatly reduced, but the time required for stabilizing austenite is almost unchanged. Therefore, the present inventors have found that if this is effectively utilized, it is possible to stably manufacture in a continuous line even at a cooling rate that is relatively slow and easily controlled, and the present invention has been made.

That is, in the present invention, C: 0.12 to 0.40% by weight, Si: 0.30
~ 1.50%, Mn: 1.50 ~ 3.00%, B: 0.0003 ~ 0.0050%, so
l.Al: 0.005 to 0.100% and Ni, Cu, C depending on the case
Steel containing 1% or more of o and Cr in total of 1% or less, the balance Fe and unavoidable impurities, and placed in the temperature range of 600 to 700 ° C for 1 to 20 hours after completion of hot rolling. After pickling and cold rolling at a rolling rate of 35-80%, 700-850 ° C
After heating in the two-phase coexistence temperature range of 15 seconds to 5 minutes and then 1 to 30 ℃
Manufacture of high-strength steel sheets with good formability, characterized by cooling to 300 to 450 ° C at a speed of / sec, holding for 15 seconds to 10 minutes within this temperature range, and then cooling to 150 ° C or less within 30 seconds. The method is the gist.

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

First, C is an austenite stabilizing element, which migrates from ferrite to austenite in the two-phase coexistence temperature range and bainite transformation temperature range to increase its stability. As a result, 10 to 25% remains even when cooled to room temperature, resulting in a large elongation due to transformation-induced plasticity. The addition amount is preferably low in order to obtain good weldability and excellent impact properties, but if it is less than 0.12%, it is not possible to secure the amount of retained austenite that causes a clear improvement in elongation. On the other hand, if it exceeds 0.40%, a large amount of retained austenite is obtained, but the amount of martensite that will be present after press forming due to work-induced transformation is also considerable, so the impact characteristics deteriorate significantly and it is not practical .

Since Si does not form a solid solution in cementite, it has an action of suppressing its precipitation, and it is possible to achieve the concentration of C far exceeding the solid solubility limit in untransformed austenite while holding it at 300 to 450 ° C for a while. . However, in the C amount range of the present invention, Si is
If it is less than 0.30%, such an effect is not clear, and addition of 0.70% or more is preferable to achieve 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. For this reason, excessive addition exceeding 1.50% must be avoided.

Further, Mn is an austenite-forming element and, at the same time, suppresses the decomposition of austenite into pearlite during cooling from the two-phase region to the bainite transformation region. Therefore, even with relatively slow cooling that can be easily controlled, the addition of austenite that exists during heating in the two-phase region is extremely effective in bringing the austenite to 450 ° C or lower as it is. If the amount is less than 1.50%, the effect is not recognized. But 3.
This goal can be achieved if it exceeds 00%,
The stabilization reaction of austenite due to the concentration of C requires an extremely long time, which makes practically impossible a large-scale production in a continuous line. It must be avoided because the formation of a band structure deteriorates the properties and, at the same time, the spot welding easily causes fracture in the nugget.

B is generally known as an element that increases the hardenability, but has little effect on the retention conditions at 300 to 450 ° C. in the heat treatment cycle of the present invention. Therefore, it is a very useful additive element for industrial stable production. A minimum of 0.0003% addition is required to reach the goal. But 0.0050
If it exceeds%, ear cracking etc. occurs and hot rolling becomes difficult, so it must be avoided.

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 residual austenite by adding Ni, Cu or Co which is an austenite forming element or Cr which is an element which 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.

The steel according to the present invention is wound after completion of hot rolling, and is then left as it is or cooled to room temperature and then reheated and placed in a temperature range of 600 to 700 ° C. for 1 to 20 hours. The purpose of this is to relax the conditions required for the subsequent heat cycle by concentrating Mn in the carbide portion that forms the core of retained austenite after a series of heat treatments after cold rolling. If this temperature is higher than 700 ° C., the carbides become coarse, and the retained austenite grains that finally exist become large, which makes it difficult to make the work-induced transformation effective, and therefore it is necessary to avoid it. In addition, Mn existing as a substitutional solid solution element in steel
The diffusion is slow, and if the temperature is lower than 600 ° C, the purpose cannot be achieved in a time that can be industrially carried out, and it is not worth doing. If this time is less than 1 hour, the alloy element cannot be concentrated properly. On the other hand, if it exceeds 20 hours, the energy cost will be huge and it will not be economically attractive, and since the Mn concentration in the carbide will become excessive and its stability will increase, it will not dissolve even when heating in the two-phase region after cold rolling. You must avoid it because you cannot reach your final goal.

In the present invention, the hot-rolled steel sheet having such a history undergoes pickling and cold rolling 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. If the rolling ratio is less than 35%, the micronization of the structure is insufficient, so that a sufficient amount of retained austenite cannot be obtained even when the heat treatment specified in the present invention is performed,
Only those with poor properties such as elongation can be obtained.
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 is enormous and the effect is small.

First of all, in the heat treatment consisting of a series of cycles of the present invention,
Heat in the two-phase coexistence temperature range of 700 to 850 ° 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 to 80% exists and ferrite accounts for the rest appears. C, which has a large diffusion constant, becomes concentrated in austenite and becomes rare in ferrite, but a certain amount of Mn distribution is achieved by passing through the temperature history specified above between hot rolling and cold rolling. Therefore, after a series of subsequent cycles, a structure in which 10 to 25% of retained austenite is finely dispersed in a mixture of ferrite and bainite can be obtained, and a steel sheet having high strength and good formability can be obtained.

When the heating temperature is less than 700 ° C, the carbide does not dissolve within the time that can be realized in the continuous line, the austenite content is very small, and recrystallization is insufficient, so that the subsequent treatment is Even if it is defined in the present invention, it is not possible to obtain a steel sheet having high strength and good formability. On the other hand, heating to a temperature range exceeding 850 ° 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 formability. on the other hand,
If it is held for more than 5 minutes, not only the crystal grains may become coarse, but also the distribution of the preferable alloying elements obtained by the temperature history before cold rolling is eliminated, and it is compared with the case of holding for a shorter time. The strength-ductility balance becomes poor.

In the present invention, after this, it is cooled to 300 to 450 ° C at a rate of 1 to 30 ° 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 since most of the austenite undergoes pearlite transformation, bainite and retained austenite become very small amounts after heat treatment, resulting in low strength and low formability. Not good. On the other hand, if the temperature exceeds 30 ° C / sec, deviation from the set temperature is likely to occur, often resulting in properties greatly different from the intended mechanical properties, so it must be avoided. When this cooling is completed at a temperature higher than 450 ° C., carbides are rapidly formed during the subsequent holding and the C concentration in austenite is rapidly reduced, so that the residue is not recognized. On the other hand, in the state where it is generated in the two-phase region, C and Mn are not concentrated enough to lower the Ms point of austenite to less than 300 ° C.
If it is cooled to less than ℃, a large amount of martensite is produced, and even if it is tempered after that, the moldability cannot be said to be in a good range even if the strength is sufficient.

After completion of cooling in the two-phase region, in the present invention, at 300 to 450 ° C. for 15 seconds to
Hold for 10 minutes and then cool to below 150 ° C within 30 seconds. This utilizes the fact that the transformation from austenite to bainite is separated into two stages due to the inclusion of Si, and the bainite containing almost no carbide and the C swept out from that part are concentrated and the Mn point falls below room temperature. The purpose is to develop a structure in which the retained austenite and the residual ferrite that has been cleaned during heating in the two-phase region are mixed, and to achieve both high strength and good formability. If this holding temperature is higher than 450 ° C., carbides are rapidly formed during that period, and the C concentration in austenite decreases sharply, so that it becomes impossible to remain. On the other hand, if the holding temperature is less than 300 ° C., it becomes substantially difficult to diffuse C, so that C is not concentrated in the untransformed austenite and residual austenite cannot be obtained when cooled to room temperature. This retention time is feasible in a continuous line despite the amount of Mn addition that results in a relatively small critical cooling rate because it contains 0.0003 to 0.0050% of B, but the retention of bainite transformation is less than 15 seconds. However, austenite in which C is not sufficiently concentrated undergoes martensitic transformation during cooling to room temperature, and the resulting steel sheet has high strength but poor formability. If the retention time exceeds 10 minutes, the bainite transformation proceeds further, and the austenite enriched with C in the preceding reaction also precipitates carbides and decomposes into bainite, which improves the formability by transformation-induced plasticity. Will bring. After this, 30 to cool down below 150 ℃
The same is true when a time exceeding seconds is required, 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 pickled from 600 to 700 ° C for the time shown in Table 2 after completion of hot rolling, and then cold rolled and heat treated at the rates shown in the table and 0.8%. Was temper-rolled.
After that, the JIS No. 5 tensile test piece was sampled and the gauge length was 50 mm,
When a room temperature tensile test was conducted at a tensile speed of 10 mm / min, the tensile strength and total elongation as shown in the same table were obtained. Further, when the amount of retained austenite was determined by the X-ray diffraction method when the surface layer was located at the center of 1/4 of the plate thickness, the values in the table were obtained.

Sample Nos. 2, 4, 5, 8, 11, 14, 17, which are samples of the present invention,
20, 23, 26, 29, 30 all contain 10-25% residual austenite and the product of tensile strength and total elongation is 2500 kgf / mm ² %
As can be seen from the above, it has high strength and extremely good moldability, and it can be realized at a cooling rate with which temperature control is easy, which has not been possible in the prior art. On the other hand, steels a, f, g which are out of the composition range of the present invention
h and i are sample No. 1, 31-
34, and even in the case of the composition steel of the present invention, if any one of the processing conditions is unsuitable, sample Nos. 3 and 6,
7, 9, 10, 12, 13, 15, 16, 18, 19, 21, 22, 24, 2
As shown in 5, 27, 28, the strength or formability is poor as shown by the product of tensile strength of less than 2500 kgf / mm ² ·% and total elongation, and even if they are satisfactory, the process It will not have the merit of doing it through difficulty.

(Effects of the Invention) As can be seen from the above examples, according to the present invention, a high-strength steel sheet having a good formability equivalent to the best in the conventional invention at a cooling rate that facilitates temperature control is obtained. be able to. For this reason, it becomes possible to manufacture a large amount without much difficulty even in the actual operation, and a very remarkable industrial effect can be expected.

[Brief description of drawings]

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

Claims (2)

[Claims] 1. C: 0.12 to 0.40% by weight%, Si: 0.30 to 1.50
%, Mn: 1.50 to 3.00%, B: 0.0003 to 0.0050%, sol.Al: 0.
005 to 0.100%, balance Fe and unavoidable impurities, and 1 to 20 within the temperature range of 600 to 700 ℃ after hot rolling.
After the pickled steel is pickled and cold rolled at a rolling rate of 35 to 80%, it is heated to a two-phase coexistence temperature range of 700 to 850 ° C and held for 15 seconds to 5 minutes, then 1 to 30 ° C / sec. Cool to 300-450 ℃ at a rate of 15 seconds to 10 minutes within this temperature range, then 150 seconds within 30 seconds.
A method for producing a high-strength steel sheet having good formability, which comprises cooling to ℃ or less. 2. The production of a high-strength steel sheet with good formability according to claim 1, characterized in that one or more of Ni, Cu, Co and Cr are added in a total amount of 1% or less by weight. Method.