JPH02217425A - Production of high strength steel sheet having superior formability - Google Patents

Abstract

PURPOSE:To obtain a high strength steel sheet reduced in C content and excellent in formability by applying pickling and cold rolling to a steel containing specific amounts of Si, Mn, and Al, subjecting the resulting steel sheet to heating and cooling under respectively specified conditions, and further subjecting the above steel sheet to holding for a time defined according to steel components and then to cooling. CONSTITUTION:A steel having a composition consisting of, by weight, 0.07-0.12% C, 0.50-2.00% Si, 1.00-2.50% Mn, 0.005-0.100% solAl, and the balance Fe with inevitable impurities is pickled and cold-rolled at 30-85% draft. Subsequently, the resulting steel sheet is heated up to a temp. of the Ac1 point or above, 700-800 deg.C, held at the above temp. for 15sec-5min, and cooled down to 200-450 deg.C at 1-200 deg.C/sec cooling rate. Successively, at 300-450 deg.C, the above steel sheet is held at an average temp. T deg.C for a time (t)sec defined according to steel composition so that the value of X given by an equation is regulated to 6-7, which is then cooled down to <=150 deg.C within 60sec. If neccessary, one or more elements among Ni, Cr, Co, and Cu are added by <=1% in total to the above steel composition. This steel sheet has satisfactory weldability, etc.

Description

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

(Prior Art) Process simplification, along with process continuity, is a general technological trend in recent years. This trend is also recognized in the field of press forming of thin steel sheets, where single-stage integral forming is progressing, and steel sheets that can handle this are being sought. The quality of formability of a thin steel plate is determined by its elongation, Lankford's plastic strain ratio r value,
This is determined by the work hardening index n value, yield strength, etc., and in order to mold complex parts in one step, it is necessary to have a large elongation and n value. Du, which has a mixed structure of ferrite and martensite, is a high-strength steel sheet with large elongation and n-value.
Al phase steel plates are common, and
As stated in Publication No. 8051 and Japanese Patent Publication No. 59-45735, the maximum tensile strength is 30 to 3 with a tensile strength of 50 to 80 kgf/-.
An elongation of about 5% can be obtained. This dual phase
The typical methods that have been proposed so far to improve the elongation and n-value of steel sheets are increasing the cleanliness of ferrite and its ratio, and adding elements that can ensure strength without significantly deteriorating the n-value. It is to add. To achieve the former purpose, the two-phase region annealing temperature should be A,
(It is possible to set the temperature directly above the surface, to cool slowly to an appropriate temperature and then rapidly cool it as in Japanese Patent Publication No. 57-45454, or to carry out batch annealing under appropriate conditions before continuous annealing. P and Si can be cited as elements that achieve the latter purpose. However, even if such a method is used, it will not be possible to achieve a tensile strength of 50 to 80 kgf/- class and at the same time an n value exceeding 0.25. There have been no reports of an example of manufacturing a steel plate with this type of bonding on an industrial scale.In other words, as the strength of the steel plate increases, the elongation and n-value decrease considerably. Therefore, it is impossible to mold a complex shape in one step.Thus, even the so-called Dua Iphaset!A has poor moldability and is not good in cost performance, so it is currently only applicable for limited applications. Considering the fact that it is difficult to obtain ideal characteristics in existing general continuous annealing furnaces that have an over-aged zone, it is difficult to expect further development in the current state.

(Problems to be Solved by the Invention) The present invention solves the problems of the prior art as described above, and achieves a tensile strength of about 50 to 80 kgf/- and a tensile strength of about 35 to 35 kgf/-.
The present invention provides a method for producing a high-strength steel plate having excellent formability as evidenced by an elongation exceeding 45% and an n value of 0.25 to 0.30 or more. In other words, by making full use of conventional knowledge and synergizing the transformation-induced plasticity of retained austenite, we aim to achieve both high strength and excellent formability that were previously unimaginable.

It is widely known that transformation-induced plasticity is a mechanism that brings about high strength and high ductility beyond the range of common-sense strength-ductility balance. This is a phenomenon in which, for example, in a metastable stainless steel such as 5US301, austenite transforms into martensite as it deforms, but at the same time it exhibits large elongation. If this phenomenon is effectively utilized, the 80k
Large elongation of 60% with tensile strength of gf/- and 0.
An extremely large n value of 70 is obtained. However, such amazing mechanical properties are exhibited only in an extremely limited temperature range, making it difficult to apply them to actual industrial production. In addition, rTrans, ^SM J Vol. 60, 252-2, is a steel that actively utilizes this transformation-induced plasticity.
On page 59, the so-called TRIP! presented by Zackay et al.
There is li1, but its use is extremely limited because it contains a large amount of expensive alloying elements and requires complicated processes such as pressure injection at low temperatures. However, in recent years, attempts have been made to apply this phenomenon to applications such as steel plates for automobiles, which require inexpensive and mass production, as in the invention described in JP-A-61-157625. The important point is that the addition of Si suppresses the precipitation of carbides and stabilizes the untransformed austenite enriched with C. According to this prior invention, stable mass production is possible using a general continuous annealing furnace and without adding expensive alloying elements, which is extremely significant.

However, in this prior invention, the tensile strength is excessive due to the high content of C and the large amount of retained austenite, and the accompanying disadvantages have been observed here and there, so that it has not been put into widespread practical use.

(Means for Solving the Problems) The present inventors have designed a process that increases the cleanliness and amount of ferrite at the same time as adding Si even when the C content is relatively low. It was discovered that retained austenite, which greatly contributes to transformation-induced plasticity, which had never been recognized before, could be obtained. As a result, it became clear that in the low strain range, a large n value can be secured mainly due to the abundant clean ferrite, and in the high strain range, due to the transformation-induced plasticity of retained austenite, which led us to carry out the present invention.

That is, in the present invention, C: 0.07 to 0.12% by weight
, Si: 0.50-2. O0%, Mn: 10
0-2.50%, sal, N: 0.005-0.
100% and as necessary Ni, Cr, Co, C
A steel containing one or more of u in a total of 1% or less, with the remainder being Fe and unavoidable impurities, is pickled and
~85% cold rolling, then heated to 700 ~ 800 ° C above the ACl transformation point and held for 15 seconds ~ 5 minutes, 1
Cooling to 200-450°C at a rate of ~200°C/sec, followed by X=5500/(T+273)-jogt+0.27(
Si (χ) + Mn (χ)) + 10 (C (X)) The gist of the present invention is to provide a method for manufacturing a high-strength steel sheet having excellent formability, which comprises cooling the steel sheet to 150° C. or less within 60 seconds after maintaining the temperature at a temperature of 150° C. or less.

(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 an element that expands the austenite region of steel and at the same time diffuses quickly in steel, so when ferrite and austenite coexist, it concentrates in austenite and increases its stability. As a result, if heat treatment is performed in a cycle as specified in the present invention, austenite can remain even after cooling to room temperature, and transformation-induced plasticity can be utilized. It is preferable that the amount of C is small in order to ensure good practical properties such as weldability, but 0.0
If it is less than 7%, it is difficult to obtain retained austenite with appropriate stability that clearly exhibits transformation-induced plasticity, making it impossible to achieve the object of the present invention. On the other hand, if the content exceeds 0.12%, it is easier to obtain retained austenite, but it is currently difficult or impossible to industrially secure the desired excellent formability at a tensile strength level of 50 to 80 kgf/-. Even if there were, there would be no benefit when considering weldability etc.

Since Si does not form a solid solution in cementite, it has the effect of suppressing its precipitation, and at the same time, it retards transformation together with Mn and the like. Therefore, by holding the temperature at 300 to 450°C for a certain period of time, it is possible to concentrate C far beyond the solid solubility limit in untransformed austenite, moderate the stability of retained austenite, and increase the effect of transformation-induced plasticity. do.

Such an effect is obtained under the C conditions and process conditions of the present invention.
It is not acceptable if the amount is less than 0.50%. However, if added in excess, scale will occur during hot rolling that will significantly worsen pickling properties, and will also lead to graphitization of C, so the amount should be 2.
．． 00% or less. In addition, in order not to impair chemical conversion treatment properties, the amount added is preferably 1.40% or less.

Moreover, Mn is not only an austenite-forming element, but also suppresses the decomposition of austenite into pearlite during the cooling process from the two-phase region to the bainite transformation region, and at the same time retards the bainite transformation itself. Therefore, by complementing the effect played by Si as mentioned above, continuous annealing is used for cooling methods used today in boat fishing and for over-aging purposes.
“Even when holding conditions around C are adopted, a metal structure containing residual austenite that brings about transformation-induced plasticity is brought about.Additionally, the addition of retained austenite tends to make the structure finer, and not only can fine-grain reinforcement be achieved. It can increase the amount and stability of retained austenite and is effective in achieving the objective.

The above effects are not observed when the amount of Mn added is less than 00%. However, when it exceeds 2.50%, even if the hot rolling conditions are controlled, it is necessary to maintain the retained austenite in the bainite transformation region in order to maintain the stability of the retained austenite to the extent that it exhibits the effect of transformation-induced plasticity near room temperature. It is necessary to take a considerable amount of time and should be avoided.

In addition, sol and N act as deoxidizing elements, and improve the material quality by reducing the grain size of the hot-rolled material with AZN and suppressing the coarsening of crystal grains during a series of heat treatment steps.
, oos~o, ioo%. The amount is 0.0
If it is less than 0.05%, the desired effect is insufficient, and 0.1
If it exceeds 0.00%, the toughness may deteriorate due to inclusions, so it must be avoided.

The steel of the present invention has the above basic components, but in addition to these elements and Fe, it also contains P, S, N, and other elements that are generally unavoidably mixed in steel. In addition, it is preferable to increase the amount of retained austenite and its stability by adding Nt, Cu, and CO, which are austenite-forming elements, or Cr, which is an element that increases hardenability. If this happens, compounds with complex compositions may precipitate, impairing workability, or extremely stable carbides may be formed, inhibiting the role of C, so the total amount thereof is limited to 1% or less.

Next, the reasons for the limitations on the process will be explained in detail.

The steel plate according to the present invention has been pickled and cold rolled by 30-85%. The purpose is A (to form fine austenite grains around the grain boundary triple points of ferrite and pearlite when heated above the 1 transformation point, and after completing a series of subsequent cycles, fine residual austenite and bainite are formed). The purpose is to obtain a metal structure that is dispersed between a clean ferrite matrix.This metal structure allows for both high strength and excellent formability, but if the cold rolling rate is less than 30%, the metal structure is heated at a normal speed. Sometimes, the ferrite grains after recrystallization become coarse, so even if the heat treatment specified in the present invention is performed, it is difficult to allow austenite to remain below room temperature, and even if it remains, it is difficult to obtain the effect of transformation-induced plasticity, so the purpose cannot be achieved. On the other hand, even if cold rolling exceeds 80%, the inclusions are extremely elongated, which only causes deterioration of local elongation and does not bring any benefit.

In the heat treatment consisting of a series of cycles of the present invention, first ACl
Heat to 700-800°C above the transformation point and hold for 15 seconds to 5 minutes. When this heating is applied to a steel sheet having the composition system of the present invention, most of the carbides exceeding the solid solubility limit disappear, and austenite with a concentration of C close to the eutectoid composition is formed at a volume fraction of 10.
~40% produced. After passing through a series of subsequent cycles, this austenite becomes retained austenite and bainite, which coexists with clean ferrite and provides a large n value from a low strain region to a high strain region. Heating temperature is 700℃
If it is less than that, the carbide may not be completely dissolved within the time that can be realized in a continuous line, and furthermore, recrystallization may be insufficient, so the subsequent treatment is as specified in the present invention. Even if it were, it would not be possible to produce a steel plate with high strength and excellent formability. Furthermore, heating in a temperature range exceeding 800° C. requires a large amount of energy, and no commensurate effect can be expected, so it must be avoided. The time to heat and maintain in this temperature range is 1
If the heating time is less than 5 seconds, there is a high possibility that undissolved carbides exist, and almost no austenite may be formed. On the other hand, even if it is held for a long time, such as over 5 minutes, it is not possible to improve the properties commensurate with the increase in input energy, and in the first place it does not meet the prerequisites for economical mass production on a continuous line.

In the present invention, it is then cooled to 1-b 200-450°C. The purpose of this is to bring the austenite generated by heating above the ACl transformation point to the bainite transformation region as it is, and to obtain retained austenite in a structure coexisting with clean ferrite through subsequent processing. This cooling rate is 1℃/
If it is less than seconds, transformation to pearlite will begin, and C will form carbides therein, so retained austenite, which is effective for transformation-induced plasticity, cannot be secured. On the other hand, at speeds exceeding 200°C/sec, it is difficult to cool the entire steel plate under uniform conditions, and even if such cooling is done without difficulty, ferrite will suddenly grow and become acicular. Since a structure is easily formed, the formability becomes inferior in comparison to the strength. If this cooling continues to a temperature lower than 200 °C, most of the austenite generated during heating will transform into martensite, so although the strength will increase, its properties will not be much different from conventional dual phase steel, and the elongation and n The value does not reach the required level. On the other hand, it is necessary to avoid ending the cooling at a temperature exceeding 450° C., since the subsequent transformation will proceed rapidly and it will be difficult to finish the process with the desired structure. The speed at the front and rear stages of this cooling is changed,
1-20℃/sec up to 550-680℃, 1-20℃/sec below
Cooling at a rate of 5 to 200°C/second allows the alloying elements to become concentrated in the untransformed austenite during the previous stage of slow cooling, increasing the amount of cleaner ferrite, which is more effective in achieving the purpose of the present invention. It is something that brings about an effect.

Following this cooling, in the present invention, X=5500/(T+273)-fogt+0.27(
Si (χ) + Mn (χ)) + 1.0 (Cα)) 2, so that X given by 2 becomes 6 to 7. After holding, the temperature is cooled to below 150°C within 60 seconds. Si
Because of the addition of carbon, carbide precipitation does not occur during this holding, and part of the austenite transforms into a bainite-like structure, and the untransformed austenite is further enriched with C, increasing its stability. . As a result, the Ms point of untransformed austenite becomes significantly lower than room temperature,
Large elongation and n due to coexistence with clean ferrite
Value is realized. If the holding temperature exceeds 450°C, the transformation progresses so rapidly that it becomes difficult to finish the process in the desired fiber state unless a considerable amount of alloying elements are added, but this results in a significant increase in cost. On the other hand, holding at a temperature below 300°C would result in extremely slow diffusion of C, requiring a long holding time that would make production in a continuous line virtually impossible. It cannot be done. If the holding time is short and the value of Therefore, the desired large elongation and n value are not exhibited, and only characteristics that are not significantly different from those of the conventional dual phaseli are obtained. On the other hand, when the value of The resulting metal structure is one in which bainite is mixed around the grain boundaries of ferrite, so its mechanical properties are dual phase.
It has no more advantages than steel. After this hold 150
The same is true if it takes more than 60 seconds to cool down to below .degree. C., which is far from achieving the purpose of the invention.

In addition, in the process explained above, the heating and holding temperature, the holding temperature after cooling, and the cooling rate during that period do not need to be constant as long as they are within a specified range, and if they fluctuate within that range, the final product will be affected. The effect on the properties of is negligible.

Additionally, although the material of the present invention is, in principle, manufactured through normal steelmaking, casting, and hot rolling processes, it is manufactured by performing thin casting and omitting part or all of the hot rolling process. There is no problem either. The coiling temperature for hot rolling is 500 to 750℃.
Conditions similar to those normally practiced may be used.

(Example) A hot-rolled steel sheet whose composition is shown in Table 1 is pickled and cold-rolled at the rolling rate shown in Table 2, followed by heat treatment consisting of a series of conditions shown in the table and 0.6% Temper rolling was carried out.・
Here, the value X determined by the average temperature, time and steel composition listed in the table is the average temperature T''C101 hours.
+0.27(Si(χ)+Mn(χ))+10(C(χ)
)) 2, and it is a requirement of the present invention that this value is 6 to 7. After that, a JIS No. 5 tensile test piece was taken, and the gauge length was 50 mm and the tensile speed was 1 f.
) + ma/min, the tensile strength, total elongation, and n value as shown in the same table were obtained. Here, the n value is calculated on the assumption that the n-th power hardening law holds between 10% and 20% of tensile strain.

Sample Nα2.4.5, 7.8°10.
13.16.19-21,23,26,27゜30.3
1.34.35 has a metal structure in which a small amount of residual austenite existed together with bainite and some martensite between the clean ferrite matrix, and both of them are 5
It has excellent formability and high strength, as is clear from its combination of tensile strength of 0 to 80 kgf/- class, elongation exceeding 35 to 45%, and n value of 0.25 to 0.30 or more. I am balancing both.

On the other hand, steels a, e, and f, which are outside the composition range of the present invention.

g is the sample Nal even after the heat treatment that can be considered optimal.

As shown in 37 to 39, even if the composition steel of the present invention is used, if one of the processing conditions is inappropriate, the sample blockage will be 3°6.9, 1.
1.12,14,15,17,18゜22.24,25
, 28. 29, 32, 33. 36, the strength is low or too high, or even if the strength is at an appropriate level, the elongation or n value is insufficient and there are problems with formability. Unable to solve problems faced by technology.

((1) In the table, - indicates that it is out of the scope of the present invention.

(Effect of the invention) As can be seen from the above examples, the present invention has a lower C than the conventional one.
Since we have succeeded in generating retained austenite in a controlled amount, a high-strength steel plate with unprecedented formability and greater elongation and n-value can be obtained. Moreover, the cost required to manufacture this steel plate is less than that of conventional steel plates with the same strength level, and the properties required for thin steel plates in addition to formability such as weldability are within the range that satisfies the expected industrial effects. is extremely large.

[Brief explanation of the drawing]

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

Claims (3)

[Claims] (1) C: 0.07-0.12%, Si: 0.
50-2.00%, Mn: 1.00-2.50%, so
l, Al: 0.005-0.100%, balance Fe
pickling and 30 to 85%
% cold rolling, then heated to 700-800°C above A_c_1 transformation point and held for 15 seconds to 5 minutes, then 1-2
Cooling to 200-450°C at a rate of 00°C/sec, followed by cooling in the range of 300-450°C with X=5500/(T+273)-logt+0.27(
Maintain the average temperature at T°C for a time t seconds defined by the steel composition so that X given by the formula Si (%) + Mn (%)) + 10 (C (%)) is 6 to 7. A method for producing a high-strength steel sheet having excellent formability, which comprises cooling the steel sheet to 150° C. or less within 60 seconds after heating. (2) Production of a high-strength steel sheet with excellent formability according to claim 1, characterized in that one or more of Ni, Cr, Co, and Cu is added in a total weight percentage of 1% or less. Method. (3) A_c_1 Heating to 700 to 800°C above the transformation point and holding for 15 seconds to 5 minutes, then cooling at 1 to 20°C/second from 550 to 680°C, and 15 to 200°C/second below that. The method for producing a high-strength steel plate having excellent formability according to claim 1 or 2.