JPS5952207B2 - Manufacturing method of low yield ratio, high toughness, high tensile strength steel plate - Google Patents

Description

[Detailed description of the invention] The present invention relates to a method for manufacturing high-strength steel for low temperature use.

1. In recent years, with the shift to clean energy, there has been an increasing need to develop thick-walled, low-temperature steel materials for materials such as line pipes for natural gas transportation, storage tanks, and tankers for storage and transportation.

By the way, it is of course necessary for these materials to have high strength and excellent low-temperature toughness, but on the other hand, when forming them into a structure, elastic deformation recovery occurs due to springback, resulting in shape fixability. If it is defective, its uses will be severely limited.

As is clear from conventional knowledge, this shape fixability is better as the ratio of yield strength to tensile strength (referred to as yield ratio of yield strength ÷ tensile strength 2 or less) is lower.

In general, high tensile strength steel has good shape fixability if the yield ratio is 0.78 or less.

High-strength steel materials are generally as-rolled and rolled (
In normalizing) material, the yield ratio is 0.7.
It is often less than 8.

However, these materials cannot be quenched and tempered.
High toughness and high strength characteristics cannot be obtained as in the case of the (and Temper) materials.

On the other hand, although high toughness can be obtained with quenched and tempered materials, it is almost impossible to obtain a yield ratio of about 0.85 or higher < 0.78.

There are various possible reasons for this, but it is also believed that in the conventional quenching and tempering process, the structure is mainly composed of tempered martensite, and the tempering temperature is naturally below the Ac1 transformation point.

As described above, it can be said that it is nearly impossible to obtain steel with a low yield ratio while maintaining the high toughness and high tensile strength properties that are the objectives of the present invention using conventional techniques.

Now, the present invention is considered to be able to solve these problems all at once, and its principle outline is as follows.

Normal construction for steel with quantitative restrictions on C, Si, Mn, Al, or other additive elements to be added.

The austenite grains are made finer by setting the cumulative reduction rate of 950° C. or less at 30% or more and 90% or less during hot rolling after forming a slab using the lump/blowing method or continuous casting method.

On the other hand, during hot rolling, at least one cross-direction rolling (rolled slab to steel ingot, etc.)

By performing rolling (meaning rolling in a direction perpendicular to the casting direction during production of steel billets), uniform austenite grains can be obtained by reducing the anisotropy of components, especially carbides, in the rolling direction.

, contributes to improving the toughness of the steel plate in the direction perpendicular to rolling.

It has been discovered that by combining both of these conditions, a uniform fine structure with high toughness and a low yield ratio can be obtained in the heat treatment in the next step.

Now, next is heat treatment, which is heated to above the Ae3 transformation point, then rapidly cooled and quenched in the same way as normal quenching, and then heated to a temperature range between the AC1 transformation point and AC3 transformation point. After heating, air cooling is performed.

As a result, the first step of quenching creates a structure with a large amount of martensite or bainite.
The AC1 transformation point of the next second step.

By air cooling after reheating to the AC3 transformation point, a part of the martensite or bainite becomes a ferrite structure.

In addition, from the viewpoint of toughness, the most preferable ratio of ferrite is 15 to 17%.

Typical examples of this structure are multi-site (65%) and ferrite (35%), as shown in Figure 1.
This results in a fine mixed structure, and it is possible to manufacture steel materials with a low yield ratio and high strength and toughness.

What should be noted here is the difference between the quenching process from the so-called two-phase region temperature between the Acm transformation point and the AC3 transformation point, which exists in the prior art.

Although this conventional method does produce a mixed structure of martensite and ferrite, relatively coarse ferrite grains remain as they are after rolling without going through the transformation process, so the low-temperature toughness remains at a sufficiently good value. does not indicate.

On the other hand, according to the method of the present invention, the coarse ferrite grains after rolling are transformed from fine austenite grains to fine martensite etc. by the quenching treatment in the first step, and then the reheating treatment in the second step is performed. and air cooling, some martensite etc. change from fine austenite to ASTM
The grain size changes to finer ferrite grains than grain size number 9, and as a result, as shown in Figure 1, a very fine mixed structure of mulch and ferrite with grain size number 10.5 is formed, which is different from the conventional method. It is thought that the toughness was significantly improved as a result.

In addition, in order to make the structure of the product as the hot-rolled steel material of the present invention as fine as possible, it is necessary to obtain a sufficient cumulative reduction in the non-austenite/recrystallized region during hot rolling, and the cumulative reduction rate at 950°C or less is required. 30% or more.

The upper limit of the rolling reduction is within an industrially applicable range, and is usually 90% or less.

In addition, the finishing temperature in the rolling method of the present invention that employs cross-direction rolling is usually in the range of AC1 transformation point or higher, and the finishing temperature in the method of the present invention is also in the AC1 transformation point or higher.
m transformation point or higher.

The reasons for limiting the scope of the claims of the present invention will be described below.

C is effective in increasing the strength of steel, with a minimum content of 0.01%
is necessary.

On the other hand, if it exceeds 0.30%, pearlite is often formed and toughness is deteriorated.

Note that if it is 0.15 or less, weldability is greatly improved.

In addition to the purpose of deoxidizing steel, Si is a stable strengthening element; at least 0.01% is required.

On the other hand, if the content exceeds 1.0%, the toughness will deteriorate.

Note that if it is 0.6% or less, weldability is also good.

Mn is an essential element for strengthening steel, and a minimum content of 0.5% is required.

On the other hand, if the content exceeds 3%, the toughness will deteriorate significantly.

AI is essential for deoxidizing steel, and a minimum of 0.005% is required.

On the other hand, if the content exceeds 0.2%, scratches due to Al 203 clusters will occur frequently.

. A consisting of Nb, V, Mo, Ti, W, Zr
In addition to strengthening steel, the seven elements in the group are effective in suppressing the growth of austite grains during heating, and one or more of them is added in a total of 0.005%.
The above is necessary.

On the other hand, if the content exceeds 0.80%, the toughness will deteriorate significantly.

Rare earth elements, Ca, Mg, and group B elements consisting of A total of 0.0004% or more of the above is necessary.

On the other hand, if it exceeds 0.10%, large oxides of those elements will be formed, leading to a decrease in toughness.

Group C elements consisting of Cu, Ni, and Cr are effective in improving the strength of steel without significantly reducing its toughness, and at least 0.05% of one or more of these elements is required in total.

On the other hand, if the content exceeds 10%, the surface quality of the product will deteriorate.

In addition to the above elements, S is an impurity element that is inevitably contained, but if it is preferably 0.010% or less, the toughness is significantly improved.

P is also an element that is inevitably included, but if it is 0.025% or less, the toughness is improved, and if it is 0.015% or less, the toughness is significantly improved.

Next, we will discuss the reasons for the limitations in the manufacturing process.

First, the steel is quenched.The heating temperature at this time needs to be completely austenitic, so it needs to be higher than the AC3 transformation point.On the other hand, if the temperature is too high than the AC3 transformation point, the austenite grains will become too coarse. Since no improvement in toughness can be expected, the upper limit is AC3 transformation point + 100°C.
shall be.

Next, regarding the heating temperature of the reheating treatment, one aim of the present invention is to leave a part of the hardened structure such as martensite as mentioned above and transform the rest into ferrite grains, so the reheating temperature for that purpose is AC1. It needs to be between the transformation point and the AC3 transformation point.

In this case, within this range, the higher the temperature, the easier the strength will be, while the lower the temperature, the easier the toughness will be.

However, from the viewpoint of strength, toughness, and yield ratio (Ac1
The optimum reheating temperature is from (transformation point +50°C) to (Ac3 transformation point -20°C).

Examples of the present invention are shown below.

Example 1 Steel with the chemical composition shown in Table 1 was tapped in a converter (Bl, B2, and B3 are electric furnaces), and some (E and F) were cast using the continuous casting method, while the others were cast using the ingot making and blooming methods. After making it into a slab, hot rolling was performed.

The hot rolling conditions are shown in Table 1. Table 1 shows the thickness of the hot rolled sheets.

This was heat treated at the heating temperature for quenching treatment and the heating temperature for reheating treatment shown in Table 1.

Here, A2. B2, C2, B2. F was tempered at 630° C. for 30 minutes after quenching.

Table 1 shows the mechanical properties and toughness of these steels.

Here, the tensile test is performed using the API test method, and the Charpy impact test is performed using a 2mm V notch test piece (plate thickness 10mm x 10mmf).
ll 5ize), also DWTT (DropWeig
ht Tear Te5t) test is API method,
The test temperature is 85% FATT (F
Rature Appearance Tr
AnsitionTemperature).

Note that both the mechanical test and the impact test are values in the direction perpendicular to the rolling direction.

Now, as shown in Table 1, the steel of the present invention not only has excellent strength and toughness, but is also characterized by a particularly low yield ratio.

On the other hand, the comparative steels are inferior in either strength, toughness, or yield ratio.

It should be noted that it is also effective to apply the heat treatment of the present invention to cold-rolled sheets after cold rolling for materials that require high strength and yield ratio, such as high-strength cold-rolled steel sheets. A rolled steel plate is obtained.

Example 2 Steel with the chemical composition and AC1 transformation point and AC3 transformation point shown in Table 2 was tapped in a converter, made into a slab by normal ingot making and blooming methods, and then subjected to hot rolling conditions, that is, rolling in the cross direction. Starting temperature and 9
Hot rolling was performed at a cumulative reduction rate of 50° C. or lower as shown in Table 3, and then various heat treatments shown in Table 3 were performed.

Table 3 shows the mechanical properties and toughness of the product, the proportion of ferrite in the product structure, and the particle size number.

゛As shown in Table 3, in the comparative steel, K2゜M
In the conventional method of quenching from a certain temperature in the temperature range between the Ac1 transformation point and AC3 transformation point as shown in 2, the yield ratio is certainly small, but the grain refinement is insufficient, so the strength and toughness are both poor. Inferior to invented steel.

In addition, cross direction rolling is performed during hot rolling, and M8 or 9
M9 with a cumulative reduction ratio of less than 30% at 50° C. or less has low toughness and a high yield ratio.

Furthermore, by applying the heat treatment method of the present invention after cold rolling, products with low yield ratio characteristics and excellent strength and toughness were obtained.

[Brief explanation of drawings]

The drawing is an optical micrograph (magnification: 500 times) of the invention steel A1 in the examples of the invention.

Claims (1)

[Claims] I Coo, 01-0.30% Si; 0.01-1.0% Mn; Q, 5-3% Al: 0.005-0.20%, balance iron and unavoidable Steel consisting of impurities is used as a steel billet, and when the steel billet is hot rolled, the cumulative reduction rate at 950°C or less is 30% or more and 90% or less, and at least once in the cross direction during the whole hot rolling process. is rolled to form a steel plate, then heated at a temperature between AC3 transformation point and AC3 transformation point +100°C, then quenched, then heated at a temperature between Ac1 transformation point and Ac3 transformation point, and then air cooled. A method for manufacturing a steel plate with a low yield ratio, high toughness, and high tensile strength. 2C; 0.01-0.30% Si: 0.01-1.0% Mn: Q, 5-3% Al: 0.005-0.20%, further Nb
, ■, MOlTi, W, Zr, one or more of the elements of group A, total 0.005-0.80%, rare earth elements, Ca, Mg, one or more of the elements of group B, consisting of 13. A total of 0.0004 to 0.10%, one or more of the C group elements consisting of Cu, Ni, and Cr, and a total of 0.05 to 10%, and the balance consists of iron and inevitable impurities. When the steel is made into a steel billet and the steel billet is hot rolled, 950
The cumulative reduction rate below ℃ is 30% or more and 90% or less, and cross direction rolling is performed at least once during the entire hot rolling process to form a steel plate, and then AC3 transformation point and AC3
After heating at a temperature between the transformation point +100°C, quenching is performed, and then A
A method for producing a low yield ratio, high toughness, and high tensile strength steel plate, characterized by heating and then air cooling at a temperature between the cm transformation point and the AC3 transformation point.