JPH02236228A - Production of high strength steel plate - Google Patents

Abstract

PURPOSE:To produce a steel plate excellent in a balance between strength and toughness by casting and solidifying a steel with a specific composition, controlling cooling down to a specific temp. region, carrying out light draft rolling in which total amount of strain is specified, and then controlling cooling. CONSTITUTION:A steel having a composition consisting of, by weight, <=0.20% C, <=2.0% Si, <=2.0% Mn, 0.002-0.05% Ti, 0.0003-0.010% B, 0.002-0.05% S, <=0.01% N, and the balance Fe with inevitable impurities is refined. This steel is cast and solidified to 1-10mm casting thickness, and is cooled from solidification down to the Ar3 temp. at 5-30 deg.C/sec average cooling rate. Subsequently, in the as-cast state or at a temp. of the Ar3 point or above, the above steel is subjected to light draft rolling in which total amount of strain is regulated to <=1.0 by logarithmic strain by one pass or multiple passes. Then, cooling is applied to the resulting steel plate while regulating average cooling rate from 900 down to 600 deg.C to 10 deg.C/sec, by which a fine Widmanstaetten ferrite structure is formed.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a method for producing a steel plate having an excellent balance of strength and ductility, either as cast or by light reduction rolling after casting.

[Prior Art] In the manufacturing technology of steel plates, there has been a strong trend toward simplifying and directing processes in order to reduce manufacturing costs. One manifestation of this trend is the technology of casting slabs that are close to the thickness of the product.

However, the austenite grain size of as-cast steel is usually several IllITl, which is much coarser than the grain size that can be achieved by conventional hot rolling processes, so the ferrite structure after transformation also tends to be coarse. ．． This is because the preferential production site of ferrite grains is usually the austenite grain boundary. Such a coarse structure generally deteriorates mechanical properties such as strength-ductility balance and fatigue strength, and does not satisfy the strength and other properties required of high-strength steel sheets.

In order to solve these metallurgical structural problems, it has recently been possible to effectively utilize oxides that are finely dispersed in steel to induce ferrite transformation not only at the austenite grain boundaries but also within the grains. Techniques have been developed to obtain tissue. An example of applying this phenomenon to a steel plate manufacturing method is a method using a Ti-based oxide as a core disclosed in Japanese Patent Laid-Open No. 61-213322. The amount of dispersion is being considered.

However, in the above-mentioned publication, the manufacturing conditions and required mechanical properties were investigated mainly with the manufacture of thick plates in mind, and it was found that such an intragranular transformation structure could be formed when the casting thickness was several mm. There is no mention of whether or not it satisfies other properties such as ductility and fatigue strength. [Problems to be Solved by the Invention] The present invention provides a method for producing a high-strength steel plate with an excellent balance of strength and ductility by using the as-cast steel plate or only by light reduction rolling in the austenitic temperature range.

[A Thousand Steps to Solve the Problem] The basic principle of the present invention is the intragranular transformation described above, and the effective use of the precipitates that exist before the transformation as the nucleus. Therefore, how to control these precipitation behaviors is important in manufacturing technology, and the above-mentioned Japanese Patent Application Laid-Open No. 61-213322 focuses on oxides in steel and mainly describes in detail the temperature control during solidification. It had been disclosed. However, recent research has shown that these oxides do not directly act as transformation nuclei of ferrite; rather, the core of this phenomenon is precipitates such as MnS and TiN that precipitate in austenite after solidification. The results are becoming clear. Therefore, in this type of phenomenon, in austenite, that is, from 900°C to 140°C
Controlling the temperature range of 0°C is more important, and it is also important for manufacturing to clarify the composition of the elements that precipitate in these low temperature ranges.

Based on this phenomenon, the wooden invention has a cast plate thickness of 1 to 101 nI.
It forms a finer Widmanstätten ferrite structure through intragranular transformation than I1 steel, making it possible to manufacture high-strength steel sheets with an excellent balance of strength and ductility by as-cast or by only light reduction rolling. The method is characterized in that the critical rolling conditions are controlled in consideration of the cooling rate of the austenite region during production, the conditions for the formation of precipitates other than oxides, and the austenite grain size during solidification as described above.

The present invention will be explained in detail below. First, we will discuss the reasons for limiting the composition of the steel of the present invention. C is an essential element to increase the strength of steel materials, but excessive addition deteriorates weldability, so it was limited to 0.2% or less. Although not particularly limited, the lower limit of C is desirably 0.02% or more, at which a Widmanstätten ferrite structure can be obtained in a conventional process.

Sl is still an essential element for increasing the strength of steel materials, but excessive addition deteriorates the ductility of steel materials, so it was set at 2.0% or less.

Mn also increases the strength of steel materials, but in the present invention it is limited to 2.0% or less within the range where the cost of composition control in the steel manufacturing process can be kept low. Further, there is no particular restriction on the lower limit, but Mn is MnS, which is important in the present invention.
The content is preferably 0.1% or more so that local differences in Mnil4 degrees in the steel, which will be described later, will clearly appear. Ti combines with O, which is included as an inevitable element in steel,
It contributes to the nucleus of ferrite transformation as TiO or Ti,O, and on the other hand, it combines with N and fixes N in the steel as TiN, which has the effect of causing B to be segregated in a solid solution state at the austenite grain boundaries, which will be described later. It is the most important element in the steel of the present invention. Therefore, the lower limit is set at 0.002% to achieve the above effects, and the upper limit is set at 0.05% so that excessive addition will not promote the precipitation of Tic and cause deterioration of ductility due to precipitation hardening.

When added in a small amount, B segregates at austenite grain boundaries and has the effect of relatively promoting intragranular transformation by controlling ferrite generated from grain boundaries.
1) It is thought that the effect of directly contributing as a precipitate such as a is an element essential to the present invention. Therefore, the lower limit of this is 0.0003%, where the segregation effect occurs in conventional materials.
Suppose that The upper limit is set to 0.01% or more, since if it is too large, hot cracking may occur during transformation and the deterioration of ductility will become significant.

S is a constituent element of MnS that plays an important role in the present invention. However, excessive addition causes hot cracking, and the lower limit is limited to 0.002% to 0.05% due to problems such as increased desulfurization cost.

N is one of the inevitable components in steel, and in the steel of the present invention, it is important as a constituent element for forming TiN. However, addition of 0.01% or more causes deterioration of ductility, so this is limited.

Next, the manufacturing method will be described.

In the present invention, after casting the steel with the above-mentioned components to a casting thickness of 1 to 10 IIlffi, it is cooled at an average cooling rate of 5°C/s or more and 30°C or less from solidification to the 3-point temperature. Must be cooled. This causes precipitates such as MnS that precipitate in austenite to be in a non-equilibrium state, that is, in a state where the precipitates are in the process of growing.
This is to form a concentration gradient of the precipitate constituent elements near the interface between these precipitates and austenite. As a result, the tendency to form ferrite becomes noticeable around the precipitates, and intragranular transformation can be promoted. Such a concentration gradient cannot be formed by rapid cooling that does not cause precipitation, and if slow cooling or long-term retention in the γ range is performed, precipitation will progress to an equilibrium amount and the formed gradient will disappear. , an optimum cooling rate range as described above is required.

Furthermore, when rolling is performed in this temperature range while taking into account factors such as the surface shape of the steel sheet, the total amount of strain must be 1.0 or less in terms of logarithmic strain.

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

The critical austenite grain size to avoid ferrite formation from grain boundaries as described above is approximately 200μI, and considering the initial grain size formed during solidification, the total strain is 1
．． If it is less than 0, this condition can almost be satisfied.

Regarding the cooling rate during transformation, if the cooling rate is too slow, ferrite generated at grain boundaries will grow and the structure will become coarse.
It is necessary to rapidly cool from 900°C to 600°C to suppress the transformation of grain boundary-generated ferrite that conventionally occurs in this temperature range, and to allow intragranular transformation to occur from around 600°C, when the degree of supercooling of transformation has become sufficiently high. It is. For these reasons, in the present invention, the cooling rate is limited to 10°C/s or more in the temperature range of 900°C to 600°C. In addition, there is no particular restriction on temperature control below IiQO℃, but the temperature should be controlled at 100℃/s so that the final structure becomes fine Widmanstätten ferrite and to prevent the formation of coarse payinite or martensite structure. The following is desirable. [Example] Table 1 shows the chemical composition of the test steel produced by vacuum melting.

Table 2 shows the manufacturing conditions, the obtained structure, and its strength and ductility values. The underlined values shown in the comparative materials indicate values outside the conditions of the present invention.

Steels 1 to 5 manufactured by the method of the present invention all consist of a fine Widmanstättenferrite structure, and the strength level varies greatly depending on the composition, but the strength and
Looking at the ductility balance, it can be seen that all of them show values of 1700 or more. This is almost the same as the value of hot rolled steel sheets obtained by multi-stage rolling from thick slabs, which is the conventional process, and this shows that the method of the present invention can produce steel sheets equivalent to conventional materials even in the thin-walled CC process. Recognize. On the other hand, in Steel 6, which is a comparison material, the cooling rate in the transformation region is slow, so transformed ferrite grows from coarse austenite grain boundaries, resulting in a coarse ferrite structure throughout, resulting in a decrease in strength.

The austenite structure becomes non-uniform, and the resulting structure also exhibits mixed grains. Therefore, the balance of strength and ductility is extremely low.

Furthermore, &vI8 is an example in which the casting thickness was large and the rate of conversion of the austenite region was slow. In this steel, the degree of supersaturation near the precipitates in the steel, which should become the nuclei of ferrite, was reduced, and the ability to generate these nucleates was reduced, resulting in a coarser structure. In Steels 9 and 10, Tt and B, which are essential elements in the present invention, are not added, respectively, so the transformation within the liaustenite grains does not proceed, the structure becomes non-uniform, and the steels are particularly inferior in terms of ductility. There is. In this way, under conditions that deviate from the method of the present invention, the balance of strength and ductility is 1600 or less, which is considerably inferior to that of conventional materials.

In addition, since U47 had a large amount of distortion during processing, [Effects of the Invention] As described above, according to the method of the present invention, the casting thickness, which was conventionally thought to be mainly composed of coarse ferrite structure, was reduced.
Even in the thin-walled CC process where only light reduction is possible, a microstructure can be obtained, making it possible to produce high-strength steel sheets with the same balance of strength and ductility as conventional process materials.

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Claims (1)

[Claims] 1% by weight: C: 0.20% or less Si: 2.0% or less Mn: 2.0% or less Ti: 0.002 to 0.05% B: 0.0003 to 0.010 % S: 0.002 to 0.05% N: 0.01% or less as a basic component, and the balance consists of iron and unavoidable components. Molten steel is cast to a thickness of 1 to 10 mm and solidified from solidification to Ar_3 point temperature. With an average cooling rate of 5 to 30℃/s, as cast or after light reduction rolling with a total strain of 1.0 or less in logarithmic strain in one pass or multiple passes at Ar_3 point temperature or higher, 900℃ The average cooling rate from to 600℃ is 1
A method for producing a high-strength steel sheet having a fine Widmanstätten ferrite structure, characterized by cooling at a rate of 0° C./s or higher.