JPH0297620A - Production of high-strength steel sheet having excellent workability - Google Patents

Abstract

PURPOSE:To obtain the title low-carbon high-strength steel sheet having excellent workability by hot-rolling a steel having specified contents of C, Si, Mn, Ni, Al, and Fe, then cold-rolling, heat-treating, and cooling the steel under specified conditions. CONSTITUTION:A steel contg. 0.07-0.30wt.% C, 0.30-1.50wt.% Si, 0.20-2.00wt.% Mn, 1.00-6.00wt.% Ni, 0.005-0.100wt.% SolAl, <=1wt.% of >=1 kind among Cu, Co, and Cr as required, the balance Fe, and inevitable impurities is hot-rolled. The hot-rolled steel sheet is pickled, and then cold-rolled at 35-80% draft. The cold-rolled steel sheet is then heated to a two-phase coexistent temp. region at 600-800 deg.C, and kept at that temp. for 15sec to 5 min. The steel sheet is then cooled to 250-500 deg.C at a rate of 1-200 deg.C/sec, and kept at that temp. for 5sec to 10min. The cooling is preferably carried out at a rate of 1-20 deg.C/sec to 550-700 deg.C, and then at a rate of 25-200 deg.C/sec. The steel sheet is subsequently cooled to <=150 deg.C for <=30sec. As a result, a high-strength steel sheet having excellent workability is obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a method for manufacturing a high-strength steel plate with good workability.

(Prior Art) In recent years, the driving performance, comfort, and safety of passenger cars have made remarkable advances to an extent never seen before. This is largely due to an increase in the number of electronic control parts and safety and security parts installed and an increase in the projected area of the car body, but there is almost no change in the total weight of the car body. This is because the weight increase is offset by the increased strength of automobile thin steel sheets, which are the main constituent material. It is thought that consumers' preference for luxury and high performance will continue due to the improvement in living standards, but at the same time, this trend will continue as there is a deep-rooted heightened cost consciousness resulting from the experience of the oil crisis. It will be recognized. Therefore, thin steel sheets for automobiles require 60 to 100 kg.
f/mm! While having a tensile strength exceeding 30 to 4
At the same time, it will be required to have excellent press workability comparable to that of steel with a strength of 0 kgf/1III1z class.

A thin steel sheet that achieves both high strength and workability is the so-called ferrite-martensitic dual phase steel proposed in Japanese Patent Publication No. 11741/1983.
hasefi is famous. This is a steel that has an improved balance of strength and ductility by having soft ferrite share the elongation and hard martensite share the strength.
Even so, the product of tensile strength and total elongation, which is an index value, is only about 2000 kgf/mm"%, making it extremely difficult to press-form it into the same shape as conventional mild steel plates. Steel types that have been improved include transformation-induced plasticity (Transformation Ind.
A rope mixed with retained austenite was developed in JP-A-60-43 with the intention of utilizing plasticity (uced plasticity).
This method has been proposed in Japanese Patent Application Laid-Open No. 430, Japanese Patent Application Laid-Open No. 157625/1984, and the like. It can be manufactured using continuous annealing equipment for mild steel sheets, and it has a relatively simple chemical composition, but it is 10 to 15%
These steels containing retained austenite are, for example, 1
Even with a tensile strength of 0.00 kgf/mm2, the total elongation reaches around 30%, and its bendability and hole expandability meet the levels required for thin steel sheets for automobiles, so it is expected to be widely used in the future.

However, in order to obtain the best combination of high strength and workability using the methods discovered so far, it has been attempted to coexist 10 to 15% retained austenite with ferrite and bainite. -188
Items with various improvements such as Publication No. 7290
It is necessary to contain an amount of C that has not been generally considered in the conventional thin steel sheet range of about 0.20% or less. When the content of C is high as described above, the ratio of pearlite is large, making it difficult to obtain a desirable rolling ratio in cold rolling, which causes considerable operational difficulties in a continuous heat treatment line. In addition, spot welding, which is often used when assembling car bodies, cannot provide the required strength to the joints, so this type of steel plate, despite having unprecedentedly high strength and extremely good workability, is This prevented it from being used in large quantities.

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

(Another Means to Solve the Problems) The high-strength steel sheet according to the present invention is characterized by having a mixed structure of ferrite, bainite, and retained austenite,
It is necessary to have a required amount of retained austenite and bainite coexist with ferrite to ensure the desired strength, and a required amount of retained austenite and bainite to obtain good workability.

By expanding the austenite region on the equilibrium phase diagram and increasing the amount of elements that lower the A+ transformation point, retained austenite generally becomes easier to generate, and its stability can be adjusted to improve elongation due to transformation-induced plasticity. Ni is a typical element of this type and is added to metastable austenitic stainless steels such as 5US304. Also, Special Publication No. 44-738 and Special Publication No. 46-13498
There is a steel in which 4 to 10% Ni is added to mix ultrafine retained austenite grains in the matrix class, as disclosed in Japanese Patent No. 3, to improve the toughness at low temperatures. Although Ni is an extremely useful element, it is expensive, with a small Clark number of o, o o s, and cannot be added in such a large amount to thin steel sheets for automobiles, which are mass-produced and must be inexpensive. An example of a steel with 2 to 4% Ni added that does not significantly impair economic efficiency is ASTM standard A203 steel, but it has no structural characteristics and although its toughness at low temperatures is excellent, its toughness remains close to room temperature. The strength-ductility balance in However, the present inventors optimized the amount of Si and Mn added,
It has been found that by applying a series of heat treatments consisting of heat cycles similar to continuous annealing of mild steel sheets, a structure containing a considerable amount of retained austenite can be obtained. Moreover, unlike the case where austenite is stabilized with only Ni, stabilization is also achieved with elements such as C and Mn, so the difference between the Ms point and the Md point is large, making it easy to utilize transformation-induced plasticity. This is what the present invention has focused on.

That is, the present invention has C: 0.07 to 0.30 in weight%.
%, Si: 0.30-1.50%, Mn: 0.2
0-2.00%, Ni: 1.00-6.00%, sol
, N: 0.005 to 0, 100%, and if necessary, one or more of Cu, Co, and Cr in a total of 1
% or less, with the remainder being Fe and unavoidable impurities, hot rolled, pickled and cold rolled at a rolling reduction of 35 to 80%, then heated to a two-phase coexistence temperature range of 600 to 800°C.
After holding for 5 seconds to 5 minutes, heat at a rate of 1 to 200°C/sec.
The gist of the present invention is a method for producing a high-strength steel sheet with good workability, which is characterized by cooling to 50 to 500°C, holding in this temperature range for 5 seconds to 10 minutes, and then cooling to 150°C or less within 30 seconds. It is something.

(Function) First, the reason for limiting the range of components of steel that is the object of the present invention will be described.

First, C is the lowest-cost austenite stabilizing element, and moves from ferrite to austenite in a short time in the two-phase coexistence temperature range and bainite transformation temperature range, increasing its stability. As a result, even if the Ms point determined by the chemical composition is considerably higher than room temperature when the elements are uniformly distributed in the steel, the effect combined with the distribution of Mn and Ni makes it possible to maintain the temperature even when cooled to below room temperature. A large amount of austenite remains. As a result, unprecedented high strength and good workability can be obtained. It is preferable that the amount added be low in order to achieve excellent weldability and impact properties, but if it is less than 0.07%, it will not be possible to secure enough retained austenite to clearly improve elongation. If it exceeds 30%, a large amount of retained austenite can be obtained, but due to processing-induced transformation, the amount of martensite that will be present after press forming will also be considerable, resulting in a significant deterioration of properties, and the required It cannot be put to practical use because it cannot obtain strength.

Since Si is not solid-solubilized in cementite, it has the effect of suppressing its precipitation, and during temporary holding at 250 to 500°C, C is concentrated in untransformed austenite far exceeding the solid-solubility limit, and its stability is reduced. increase.

However, in the range of C amount for this vocalization, such an effect is not obvious if Si is less than 0.30%, and to achieve the purpose, 0.70% or more is preferable. During hot rolling, a scale that significantly deteriorates the properties may be generated, and C may be precipitated as graphite. Therefore, excessive addition of more than 1.50% must be avoided.

Mn suppresses the decomposition of austenite into pearlite during cooling from the two-phase region to the bainite transformation region, and is effective in bringing the austenite that is present at the start of rapid cooling to below 500°C as it is. It is an extremely effective element to add. It is also classified as an austenite-forming element along with Ni, and it broadens the temperature range in which it remains stable and facilitates the formation of retained austenite. The amount is 0.20
If it is less than 2.00%, hot brittleness may occur during hot rolling, and a large amount of pearlite will be produced during cooling to 500°C, so it is not suitable. However, as mentioned earlier, if it exceeds 2.00%, Although this objective is achieved, the stabilization reaction of austenite due to concentration of C requires an extremely long time, making mass production in a continuous line virtually impossible.

Furthermore, the formation of a band structure may deteriorate the characteristics and must be avoided.

Further, Ni is an austenite-forming element and dissolves in solid solution as a substitutional element in iron, lowering its Ms point.

Therefore, even if the C concentration is lower than that of the conventional invention example shown in JP-A-62-188729, austenite can remain without being transformed into martensite when cooled to room temperature or below. In addition, because it does not dissolve in cementite, C
Precipitates as carbides, and promotes concentration into austenite while suppressing the formation of carbides. However, the amount is 1.00%
If it is less than 6.00%, the effect of addition is not recognized.
According to the heat cycle of the present invention, the amount of retained austenite exceeds 30% after the heat treatment is completed, and the austenite becomes connected and tends to exist, so plasticity-induced transformation occurs all at once, resulting in the desired large elongation. is not obtained.

Furthermore, sol and kl can be used as deoxidizing elements, and AZN
Since the quality of the material is improved by making the hot rolled material finer and suppressing the coarsening of crystal grains during a series of heat treatment steps, 0.005 to 0.000% is added. The amount is 0
．． If it is less than 0.05%, the desired effect is insufficient, and
．． If it exceeds 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 Re, it also contains P, S, N, and other elements that are generally unavoidably mixed in steel. In addition, it is preferable to increase the residual austenite 1-ffi by adding Cu or Co, which is an austenite-forming element, or Cr, which is an element that increases hardenability. However, if excessively added, compounds with complex compositions may be finely precipitated and workability may be significantly deteriorated, so the total amount is limited to 1% or less.

Next, the reason for the limitations on the process will be explained in detail. In the present invention, a hot rolled steel plate is pickled and cold rolled at a rolling reduction of 35 to 80%.

The purpose of this is to obtain a structure in which fine residual austenite is dispersed in ferrite and bainite after a heat treatment consisting of a series of successive cycles. If this rolling rate is less than 35%, the structure will not be refined enough, so
Even if the heat treatment specified in the present invention is performed, a sufficient amount of retained austenite cannot be obtained, and only those with poor properties such as elongation can be obtained. The effect tends to be saturated as the rolling ratio increases, and cold rolling exceeding 80% is not appropriate because the power required for rolling becomes enormous and the effect is small.

In the heat treatment consisting of a series of cycles of the present invention, first, heating is performed to a two-phase coexistence temperature range of 600 to 800 °C for 15 seconds to
Hold for 5 minutes. When a steel sheet having the composition system of the present invention is heated in this way, most of the carbides above the solid solubility limit disappear, and a structure is created in which 40 to 80% austenite exists and ferrite accounts for the remainder. C, which has a large diffusion constant, is concentrated in austenite and diluted in ferrite. Therefore, after a series of successive cycles, 8-30%
A structure in which residual austenite is finely dispersed in a mixture of ferrite and bainite is obtained, and a steel sheet with high strength and good workability can be obtained.

When the heating temperature is less than 600°C, the carbides do not dissolve and the amount of austenite is very small within the time that can be achieved in a continuous line, and recrystallization is insufficient, so subsequent processing is difficult. Even if it is as defined in the present invention, it is not possible to obtain a steel plate with high strength and good workability.

On the other hand, heating to a temperature range exceeding 800° C. not only requires a large amount of energy and is uneconomical, but also 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 exist, 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 on a continuous line, and even if held, the quality of the material may deteriorate due to coarsening of crystal grains, etc.

In the present invention, the temperature after this is 1 to 200°C/! Cool to 250-500°C at IEIC rate. The purpose of this is to bring the austenite generated by heating to the two-phase region to the bainite transformation region without transforming into pearlite, and to obtain predetermined characteristics as residual austenite and bainite at room temperature through subsequent treatment. This cooling rate is 1°C
Less than /sec means that the cooling rate is below the critical cooling rate, and since most of the austenite transforms into pearlite, the amount of bainite and residual austenite becomes very small after heat treatment, and the strength is low and the workability is not good. On the other hand, if it exceeds 200°C/sec, acicular ferrite will be produced, causing deterioration of the strength-ductility balance. Further, it is difficult to finish cooling the entire steel plate at a target temperature, and the shape may not be suitable for industrial use. If this cooling ends at a temperature higher than 500°C, carbides will rapidly form during the subsequent holding even if they contain Si and Ni, and the Cfi degree in austenite will rapidly decrease, so it is impossible to leave them until room temperature. It becomes possible. On the other hand, although austenite stabilizing elements such as Ni and Mn are added, C is not concentrated in austenite enough to lower the Ms point to below 250°C when it is still formed in the two-phase region. 25
If it is cooled to less than 0°C, a large amount of martensite will be produced, and even if it is subsequently tempered, the strength will be sufficient but the workability will not be within the range that can be said to be good. After soaking in the two-phase region, cooling is performed from 550 to 700°C at a rate of 1 to 200° (/sec).
Growing clean ferrite while concentrating , Mn, etc. in the remaining austenite is a preferred method for increasing the amount of retained austenite and further improving workability.

After this cooling is completed, in the present invention, the temperature is increased to 250-500"C
Hold for 10 seconds to 10 minutes, then cool to below 150 within 30 seconds. This takes advantage of the fact that the transformation from austenite to bainite is separated into two stages in the composition of the present invention, and the bainite, which contains almost no carbides, and the C swept out from that part are concentrated and stabilized by Ni, Mn, etc. In addition, a structure containing retained austenite whose Ms point has decreased to below room temperature, residual ferrite that has been purified during the two-phase region heating, and clean ferrite that has grown during the previous cooling is revealed, resulting in high strength and The aim is to achieve both good workability. If this holding temperature is higher than 500° C., carbides will rapidly form during this period and the CW11 degree in austenite will rapidly decrease, making it impossible to retain them. On the other hand, if the detection temperature is less than 250°C, it becomes difficult for C to diffuse effectively, so C does not concentrate in untransformed austenite and the Ms point cannot be lowered below room temperature, which is effective in obtaining retained austenite. It doesn't become something. This retention time is 5
In the second milli, the progress of bainite transformation is insufficient, so the austenite that is not sufficiently enriched with carbon undergoes martensitic transformation during cooling to room temperature, and the resulting steel sheet has high strength but poor workability. . Also, the retention time is 1
If the time exceeds 0 minutes, the bainite transformation will further progress, and the austenite enriched with C in the previous reaction will also precipitate carbides and decompose into bainite, resulting in an insufficient amount of retained austenite to improve workability due to transformation-induced plasticity. Become. The same applies if it takes more than 30 seconds to cool down to 150° C. or less, and the object of the invention cannot be achieved.

In addition, in the process explained above, the heating temperature in the two-phase region, the holding temperature after cooling from the two-phase region, and the cooling rate during that period do not need to be constant as long as they are within the specified range. , even if it fluctuates within that range, it does not degrade the properties of the final product in any way, and may even improve them.

(Example) After hot rolling and pickling the steel whose components are shown in Table 1, it was subjected to heat treatment consisting of cold rolling and a series of cycles under the conditions shown in Table 2. After that, a JIS No. 5 tensile test piece was prepared after 0.8% temper rolling, and the gauge length was 50.
(2) A room temperature tensile test was conducted at a tensile rate of 10 min/ml, and the tensile strength and total elongation as shown in the table were obtained.

Sample Nα2.4.5.7.8, l 1 which is a sample of the present invention
．． 14.17.18.20.22.23.26 27
No. 30.34 both have high strength and good workability, as can be judged from the product of tensile strength and total elongation exceeding 2500 kgf/--%. On the other hand, for steels a, cSg, h, and t, which are outside the composition range of the present invention, samples kl, 3.31~
As shown in 33.35, even if the composition steel of the present invention is used, if there is even one inappropriate point in the processing conditions, sample N
116.9.10, 12.13.15.16.19.2
1.24.25.28.29, the tensile strength, total elongation, or both are inferior, so the weight is 2500 kg.
Only the product of tensile strength and total elongation less than f/-% can be obtained,
It cannot be made into a high-strength steel plate with good workability.

(Effect of the invention) As is clear from the above examples, according to the present invention, 5 to 3
0% retained austenite coexists with ferrite and bainite, resulting in a tensile strength of 50 to 140 kgf/
- It is possible to obtain high-strength steel sheets with good workability over a wide range of conditions. Furthermore, since such characteristics can be achieved with a much lower C content than in the conventional invention, the effects of the present invention are extremely significant industrially.

[Brief explanation of drawings]

FIG. 1 is a diagram showing a heat treatment cycle applied to a steel plate after cold rolling. Required cooling interval Cd

Claims (1)

[Claims] 1. C: 0.07 to 0.30% by weight, Si: 0.3
0-1.50%, Mn: 0.20-2.00%, Ni:
1.00-6.00%, sol. Al: 0.005~0
．． A steel containing 100% Fe and unavoidable impurities is hot-rolled, pickled and cold-rolled at a rolling reduction of 35-80%, then heated to a two-phase coexistence temperature range of 600-800°C. After holding for 1 to 5 minutes, cool to 250 to 500℃ at a rate of 1 to 200℃/sec, and hold for 5 to 10 minutes within this temperature range.
A method for manufacturing a high-strength steel sheet with good workability, characterized by holding the plate for a minute and then cooling it to 150°C or less within 30 seconds. 2. Claim 1 characterized in that one or more of Cu, Co, and Cr are added in a total weight percent of 1% or less.
The method for producing a high-strength steel plate with good workability as described above. 3. Cooling after heating to a two-phase coexistence temperature range and holding for 15 seconds to 5 minutes is 1 to 20 °C/sec up to 550 to 700 °C, and 25 to 200 °C/sec below that. The method for producing a high-strength steel plate with good workability according to claim 1 or 2.