JPH02236224A - Production of high tensile steel plate excellent in toughness - Google Patents

Abstract

PURPOSE:To produce a steel plate well balanced between strength and toughness by continuously casting a molten steel having a composition in which respective contents of C, Mn, and S are specified under specific conditions, applying slight-degree working in which total amount of strain is specified to the resulting cast slab, and then controlling cooling. CONSTITUTION:A steel having a composition consisting of, by weight, <=0.20% C, 0.6-3.0% Mn, 0.002-0.1% S, and the balance Fe with inevitable impurities is refined. The resulting molten steel is continuously cast while controlling average cooling rate in the range from solidification to the Ar3 transformation point to 1-30 deg.C/sec. The resulting cast slab is subjected, in the as-rolled state or in a temp. region of the Ar3 transformation point or above, to slight- degree working, such as light draft rolling of >=one pass in which total amount of strain is regulated to <=0.1 by logarithmic strain. Then, cooling is carried out while controlling average cooling rate from 900 down to 600 deg.C to >=10 deg.C/sec. By this method, the formation of ferrite can be inhibited and a fine ferritic bainite structure can be formed by means of slight-degree working alone. Moreover, if Ti is added to the above steel composition so that the relationship between Ti content and the content of inevitably contained N satisfies Ti>(48/14).N and further 0.0003-0.01% B is added, the intergranular nucleation of ferrite can be inhibited.

Description

Detailed Description of the Invention [Field of Industrial Application] The present invention relates to a method for manufacturing a steel plate having an excellent balance of strength and toughness, either as cast or by light processing after casting, such as light reduction rolling. ．． [Background Art] The basic idea of a method for manufacturing a steel plate with high toughness is to make ferrite grains fine and suppress the formation of barite. Following this idea, low-carbon systems ((≦
0.05wt%r) NiNb, Ti. High tensile strength Mn steel (lJn≧1.5 wtU) containing B, etc. has been developed.
Ductility and Toughness,Cl
imax Molibdanum Company
, Kyoto, 1971, p. 101-117). This type of steel suppresses the formation of pearlite due to its low carbon content,
Nb, Ti. This is an attempt to turn ferrite into cotton grains by utilizing the effect of controlled rolling by adding Mn.

On the other hand, recently, in the field of manufacturing steel sheets, there has been a strong trend toward simplifying and directing processes in order to reduce manufacturing costs. The technology of obtaining a thin strip with a thickness close to that of the product by continuous casting can be said to be one manifestation of this trend. However, the austenite grain size of as-cast steel is usually several mm, which is extremely coarse compared to the austenite grain size of steel that is achieved through the conventional hot rolling process.
The ferrite structure after metamorphosis also tends to become coarser. This is because the preferential production site of ferrite grains is usually an austenite grain boundary. Such a coarse structure deteriorates the balance of strength and toughness of the steel material.

As a method to suppress the formation of such coarse ferrite and refine the structure, it is possible to effectively utilize oxides etc. finely dispersed in steel to induce ferrite transformation not only at the austenite grain boundaries but also from within the grains. A technique has been developed to obtain an extremely fine structure by

An example of applying such technology to the production of steel sheets is the one disclosed in JP-A No. 61-213322, in which Ti-based oxides are used as the core of transformation, and the particle size of the oxides in steel and The amount of dispersion is being considered.

[Problems to be Solved by the Invention] In order to obtain a fine ferrite structure by light processing of as-cast or austenite region, such as light reduction rolling, as disclosed in the above-mentioned Japanese Patent Application Laid-Open No. 61-213322, Therefore, it is necessary to accurately control oxide inclusions during the melting process. However, such control has large variations due to subtle differences in operations such as the timing of addition of oxide-forming elements.
It is difficult to stably refine the ferrite structure.

The present invention provides a method for manufacturing a high-strength steel sheet with excellent toughness, which suppresses the formation of barite by only light processing such as light reduction rolling in the as-cast state or in the austenite region, and has a fine ferrite-bainite structure. It was done verbally.

[Means for Solving the Problems] The features of the present invention are as follows: 1% by weight, C≦0.21, Mn: 0.6 to 3.0
%, S:G. 002 ~ (1) Molten steel consisting of 1. and the rest: Fe and unavoidable impurities is continuously cast at an average cooling rate of 1 to 3 ot/s from solidification to the Ar3 transformation point, and is cast as-cast or at the Ar3 transformation point or higher. It is characterized by applying one or more light processing steps such that the total amount of strain becomes 1.0 or less in logarithmic strain in the temperature range of A method for manufacturing high-strength steel sheets with excellent toughness.

2 wt%te, C≦0.20%i, Mn+0.6 ~
Contains 3.0% F, S: 0.002 to 0.1k;
The relationship with N that is inevitably included is Ti> (48/14
)・N is added to satisfy Ti and B is added to 0.
Molten steel containing zero addition of 0003 to 0.01 and the remainder Fe and unavoidable impurities is continuously cast at an average cooling rate of 1 to 30°C/s from solidification to the Ar3 transformation point, and cast as-cast or ^r, After applying light processing for one or more passes in which the total strain becomes 1.0 or less in logarithmic strain in the temperature range above the transformation point, 9
00t: Average cooling rate from 600'C to 10°C
A method for producing a high tensile strength steel plate with poor toughness, characterized by cooling at a temperature of 1/s or more.

It is in.

The present invention will be explained in detail below.

The basic principle of the present invention is the above-mentioned intragranular transformation, and the precipitates present in the steel before the transformation are effectively used as the nucleus. Therefore, in the technology that utilizes intragranular transformation, it is important to control these precipitation behaviors as follows. Focusing on oxides, he mainly discusses the importance of temperature control during solidification.

However, as a result of a detailed study of the mechanism of intragranular nucleation in ferrite, the present inventors found that in order for intragranular nucleation to occur preferentially over grain boundary nucleation, it is necessary to add a certain amount of Mn and S. It was revealed that this is the case. This is γ/ after casting.
In the vicinity of the folded product of MnS generated during the α transformation, M
A depletion zone of n occurs, and along with this, γ in this small region
It has become clear that this is because the /α transformation temperature (Ar3 transformation point) becomes high and transformation occurs quickly.

The Mn concentration in the Mn depletion zone near the MnS precipitate is closely related to the Mns precipitation behavior, and in order to clarify this precipitation behavior, the present inventors created a Mns precipitation model and Using this method, we succeeded in finding the conditions that enable intragranular nucleation of ferrite using parameters such as the amounts of Mn, S, and C and the cooling rate. The present inventors conducted experiments based on these calculation results, and found that by controlling the above parameters after the normal melting process without performing any special oxide control, stable and fine transformation could be achieved. Established technology for forming organizations.

The present invention will be explained in more detail below.

First, the reasons for limiting the components of steel in the present invention will be explained.

C is an essential element for increasing the strength of steel materials, but when added in an excessive amount exceeding 0.20, it causes problems such as the formation of barite and deterioration of the weldability of the product.

On the other hand, the lower limit of C content is not particularly limited, but
When the C content becomes extremely low, the growth rate of the interface during ferrite transformation becomes extremely high.

Therefore, it is desirable that C be 0.005!6 or more.

Mn is one of the most important elements in the component system of the present invention, and if the Mn content is less than 0.0%, the amount of Mn depleted near MnS described above will not become sufficiently small, resulting in intragranular nucleation of ferrite. does not occur enough. On the other hand, 3.0!
Addition of Mn exceeding 6 deteriorates the strength/toughness balance of the product.

The reason for limiting the S content to within the range of 0.002 to 0.196 is that if S is not added at 0.002%i or more, MnS
This is because the precipitates are not sufficiently generated and the number of intragranular nucleation sites decreases, making it impossible to achieve sufficient refinement of ferrite.

On the other hand, in order to ensure product processability, the S content must be 0.1 zero or less.

The reason why Ti is added to satisfy the relationship of D > (4B/+4)・N is that N in the steel is precipitated in the form of TiN, eliminating solid IN, and B is precipitated in the form of BN. This is to enable ferrite to segregate at grain boundaries in a solid solution state, thereby suppressing grain boundary nucleation of ferrite.

The lower limit of the amount of B added is the minimum amount of O.B. required to suppress grain boundary nucleation. OQO2'4, and the upper limit is OQO2'4 at which deterioration of product processability becomes apparent. It's Olk.

In addition, in order to ensure the strength of the product, Ni. Si,C
Addition of r, Mo, Cu, Nb, P, etc. does not impair the spirit of the present invention.

Next, the reason for limiting the manufacturing conditions will be explained.

In the casting process? The lower limit of the average cooling rate from solidification to the ^r3 transformation point for Steel 8 was set at 1°C/s because if the average cooling rate is lower than this, the concentration distribution of Mn near MnS will change due to insufficient diffusion time. This is because the Mn concentration in the Mn depletion zone is not much different from the parent phase concentration because it is already smoothed during the γ/α transformation, and intragranular nucleation of ferrite does not occur significantly. On the other hand, from the solidification of molten steel, A
The reason why the upper limit of the average cooling rate to the r3 transformation point was set at 30°C/s is that if the average cooling rate exceeds this, sufficient diffusion time for Mn to form a depletion zone will not be obtained, and cooling will be slow. This is because, as in the case of too much ferrite, intragranular nucleation of ferrite does not occur sufficiently. That is, γ/ in the vicinity of MnS
An appropriate cooling rate range of 1 to 30° C./s exists to form the Mn depletion zone necessary to significantly raise the α transformation point.

On the other hand, when processing, such as rolling, is carried out to improve the surface quality and plate thickness accuracy of the product, it must be carried out at a temperature above the Ar transformation point, which does not cause deterioration of the material.

The processing amount, for example, the total reduction amount is 1 in logarithmic strain.
．． It needs to be less than 0.

This is because, as in the present invention, the intragranular transformation phenomenon leads to the coarsening and non-uniformity of the shape of ferrite generated from the grain boundaries, and the coarsening and non-uniformity of the structure, so the grain boundary area serving as the site for this phenomenon is as small as possible. This is because it is desirable. In this situation, it is disadvantageous that recrystallization progresses and the austenite grain size decreases due to processing, and processing in non-recrystallized areas accumulates strain at grain boundaries, which increases the activation of grain boundaries for transformation. This is still undesirable because it encourages

As mentioned above, the critical austenite grain size to avoid ferrite formation from grain boundaries is approximately 200 μm, and considering the initial grain size formed during solidification, the total strain should be 1.0 or less in terms of logarithmic strain. This condition can be satisfied if the processing is as follows.

In addition, regarding the cooling rate during transformation, if the cooling rate is too low, ferrite generated at grain boundaries will grow and the structure will become coarse.
Rapidly cool from 900°C to 6On°C. It is necessary to control the transformation of the grain boundary-generated ferrite generated in this temperature range, and to cause the intragranular transformation to occur from around 600° C., when the degree of supercooling of the transformation has become sufficiently high. For these reasons, in the present invention, the cooling rate in the temperature range of 900°C to 600°C is limited to 10°C/s or more.

[Example Table 1 shows the chemical composition of the test steel manufactured by vacuum melting. Table 2 shows the manufacturing conditions, product strength, and breakdown temperature (
vTrs).

No. produced by the method of the present invention. In samples 2.3, 7, 9.10, and 12, fine acicular ferrite with MnS as the nucleus was formed and exhibited excellent toughness. Sample No. 9 is
Since Ti and B are not added, ferrite is also generated from the grain boundaries, and sample N with the same composition contains Ti and B.
o. The value of fracture surface transition temperature (vTrs) is slightly higher than that of No. 2. However, it shows superior toughness compared to samples outside the scope of the present invention.

Average cooling rate from solidification to Ar=transformation point is 1-30℃
Sample No. that is not within the range of /s. 1.4, 8.11
In No. 1 and No. 13, the generation of intragranular ferrite did not occur sufficiently, resulting in coarsening of the structure.

In addition, sample No. with a total rolling reduction of 7096. In No. 5, coarse ferrite was formed from the grain boundaries and the toughness was poor. Sample No. had a low average cooling rate of 5°C in the temperature range from 900°C to 600°C. 6, coarse ferrite is similarly generated from the grain boundaries, and the fracture surface transition temperature (vTrs
) resulted in an increase in the value of

For comparison, steel E was heated to 1250°C and the total reduction rate was 9.
3. The tensile strength and fracture surface transition temperature (vTrs) of a control-rolled material finished at 850°C were determined.

Tensile strength is 78 kg/mm', fracture surface transition temperature (vTrs)
The value was -93°C. This value and sample No. As is clear from the comparison of No. 12, the steel according to the present invention exhibits almost the same toughness as the controlled rolled material despite the low cost and simple process of casting and simple processing. Accordingly, even in a thin-walled continuous casting process, according to the present invention, it is possible to produce a W4 material having properties equivalent to those of steel sheets produced by the prior art.

[Effects of the invention] As described above, the present invention has the advantage that even when using a thin-wall continuous casting process, which is the ultimate process aiming at cost reduction,
It makes it possible to manufacture steel plates with a balance of strength and toughness that is comparable to steel plates obtained by conventional techniques, and has high industrial value.

4 others

Claims (1)

[Claims] 1% by weight, C≦0.20%, Mn: 0.6-3.0
%, S: 0.002 to 0.1%, and the remainder: Fe and unavoidable impurities. Molten steel is continuously cast at an average cooling rate of 1 to 30°C/s from solidification to Ar_3 transformation point. After applying light processing for one or more baths in which the total strain becomes 1.0 or less in terms of logarithmic strain in the temperature range above or above the Ar_3 transformation point, the average cooling rate from 900℃ to 600℃ is reduced to 1.
A method for producing a high tensile strength steel plate with excellent toughness, characterized by cooling at 0° C./s or more. 2% by weight, C≦0.20%, Mn: 0.6-3.0
%, S: 0.002 to 0.1%, and furthermore, Ti is added so that the relationship with N that is unavoidably included satisfies Ti>(48/14)・N, and B is added from 0.0003 to 0.1%.
Molten steel with 0.01% addition and the balance consisting of Fe and unavoidable impurities is continuously cast at an average cooling rate of 1 to 30°C/s from solidification to the Ar_3 transformation point, and is cast as-cast or at a temperature above the Ar_3 transformation point. The total strain in the temperature range is 1.0 in terms of logarithmic strain.
A method for manufacturing a high-strength steel sheet with excellent toughness, which comprises performing one or more light workings as described below, and then cooling at an average cooling rate of 10°C/s or more from 900°C to 600°C.