JPS5921926B2 - High-toughness carbon steel or low alloy hot-rolled steel and its manufacturing method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing high-strength, high-toughness hot-rolled steel, which has a mixed structure of ferrite and pearlite during hot rolling that is equivalent to or higher than normalized steel. It is an object of the present invention to provide a method for manufacturing steel materials such as carbon steel or low alloy steel plates, round steel, shaped steel, and steel pipes, which have high strength and toughness.

The controlled rolling method is a well-known method for obtaining high strength and toughness in as-rolled steel. The conventional controlled rolling method focuses on the fact that the primary factor controlling the toughness of steel is the grain size, and (I) attempts to refine the austenite structure by rolling to obtain a fine-grained ferrite structure. Method.

I) A method of producing a fine ferrite structure by creating a large number of ferrite structure transformation nuclei within the austenite structure grains by low-temperature rolling that applies strong pressure regardless of the size of the austenite structure grains.

All of these methods are aimed at refining the ferrite structure. It is already common knowledge in the technical field that strength and toughness improve by refining the ferrite structure, but the strength and toughness of steel obtained by the controlled rolling method based on this common sense It was difficult to achieve the same level of shelf energy (especially in the Charpy test) as that of normalized steel. The present inventors investigated through many experiments the controlled rolling method that yields steel materials with high strength, especially high toughness. As a result, carbon steel or low alloy steel mainly composed of ferrite and pearlite structures has (I) grain size. In addition, the amount, form, and distribution of pearlite are major factors.

(I) Furthermore, dividing the pearlite band is more effective in improving the toughness than improving the toughness by finely and densely dispersing the pearlite band.

By focusing on this and creating a steel structure with improved pearlite fraction and structure parameters, which will be described later, toughness that could not previously be achieved without heat treatment or the addition of alloying elements can be obtained until rolling. I found out that. In other words, the present inventors have found that the desired steel material can be obtained by making carbon steel or low alloy steel consisting of a ferrite structure and a pearlite structure into a steel structure with a pearlite fragmentation ratio of 0.55 or more and a structure parameter of 1 or more. I found out.

The present invention was constructed based on this knowledge, and its gist is to heat a slab of carbon steel or low alloy steel to
After heating from a temperature exceeding 000'C to a temperature of 1100°C or less, rolling is performed at a cumulative reduction rate of 50 or more at a temperature of 850°C or less, and forced cooling is performed between 850 and 500°C during the rolling process. A method for producing hot-rolled high-toughness carbon steel or low-alloy steel.

Here, the definitions of the above-mentioned pearlite fragmentation rate and structure parameter 1 in the present invention will be explained.

First, the pearlite splitting rate will be explained. Using a grid of 18 lines arranged vertically (Lltie: l) at 2.5 mm intervals as shown in Figure 1, the number of pearlite phases each line segmented was counted on a microstructure photograph at 100x magnification. , calculate the average of this repetition three times, and divide this by (average number of L axis) ÷
The value obtained by applying the formula (average number on the Z axis) was defined as the pearlite fragmentation rate. That is, if there is a band of pearlite structure, the number of divisions along the Z axis increases, and the division ratio L/Z decreases. On the other hand, if the belt of pearlite structure is divided, the rate of division becomes large. Also, if the grain size of the ferrite structure is smaller (the grain size number becomes larger), the toughness will improve, so
It is thought that the larger the (particle size number) x (fractionation rate), the better the toughness. Therefore, the value obtained by the formula (particle size number) x (fractionation rate) was used as the structure parameter. The present invention will be explained in detail below.

Carbon steel manufactured by ingot making, blooming, or continuous casting of molten steel that has been melted in a melting furnace such as a converter, electric furnace, etc., or further through vacuum degassing treatment. It is heated to a temperature above 1100°C or less and rolled into a desired shape such as a steel plate, round steel, or shaped steel.

This rolling heating temperature is regulated based on the pearlite fractionation rate, rolling plastic workability, etc. At temperatures below 1000°C, the pearlite fractionation increases, but it is not stable and reduces the rolling plasticity of the steel material, resulting in poor toughness. Exacerbating the deterioration 1
At temperatures exceeding 100°C, the toughness decreases and the obtained toughness also varies. Moreover, since the pearlite fractionation ratio is less than 0.55 and the present invention employs low temperature strong reduction, there is a drawback that the waiting time for rolling temperature increases and the production efficiency per unit time is greatly reduced. Furthermore, in this rolling process, rolling is carried out at a temperature of 850°C or less at a sum of repeated rolling reductions, that is, a cumulative rolling reduction of 50cm or more, and at a temperature of 850 to 500°C.
℃ (including the temperature within this range after the end of rolling) is forcedly cooled (2 to 10℃/second). Such a limitation is intended to cause the pearlite structure to be separated and precipitated in the fine-grained ferrite structure to increase the pearlite breakage ratio to 0.55 or more and the structure parameter 1 to 4 or more, thereby improving strength and toughness. In other words, since the temperature range of 850 to 500°C is the range in which it is easy to form a ferrite structure with little strain and a large pearlite structure, the cumulative reduction rate is 5 when the temperature is 850°C or less.
A steel material having a fine-grained ferrite structure and a finely divided and dispersed pearlite structure can be obtained by strong reduction at a coefficient of 0 or higher and forced cooling in a temperature range of 850 to 500°C. The strength and toughness (particularly the steel energy in the Charpy test) of the steel produced by the method of the present invention are significantly superior to the normalized steel as rolled. Examples of the present invention are shown below.

Steel having the chemical composition shown in Table 1 was treated under the conditions shown in Table 2, and the properties obtained in this case are also listed in the same table.

Figure 2 shows the structural parameters of each sample.
It showed an impact value of 50°C.

In Table 2, A1 is a normalized normalized material and has an impact value of 21.1 kg・m at -50°C, but as-rolled A2 has a low impact value of 3.7 kg・m. I can't do it.
B1 to B3 are also as-rolled, but because they are subjected to controlled rolling, the ferrite grains become finer and the toughness is improved. In particular, B3 was subjected to a combination treatment of low temperature heating and controlled rolling, and the pearlite fragmentation rate was high and the toughness was considerably improved. In other words, when strong rolling is performed at a cumulative reduction rate of 70 at 850°C or lower, the ferrite grain size becomes finer than that of normalized material A1, and the impact value at -5.0°C is compared to that of normally rolled material A2. However, it has not reached the level of normalized material A1. Looking at the organization at this stage, Figure 3A
As shown in the photographs of ,b, the pearlite bands are fine and have a structure that is closely arranged in parallel. The pearlite fragmentation rate of this structure is 0.247 to 0.375, and the structure parameter is 2.37 to 3.60, both of which are lower than the AO material, and the impact value is low corresponding to these values. C1 to C6 are treated materials of the present invention, which involve heating the slab to over 1000°C to 1100°C, a large cumulative reduction rate of 50 factors or more at 850°C or less, and forced cooling between 850 and 500°C. By employing the rolling process, even though the ferrite particle size is at the same level as the normalized material A1, superior impact properties are obtained. As shown in the photograph of FIG. 4 Cpd and Table 2, the structure has a high pearlite fragmentation ratio of 0.55 or more, and the structure parameters are both 4 or more and thick. C7,C
8 is a comparative material, which was rolled under the rolling conditions of the present invention except for the slab heating temperature.

The pearlite fragmentation rate and structure parameters are both equivalent to the steel material according to the present invention, and the tensile test value is also excellent, but there is significant variation in the impact value. D
1 to D3 are comparative materials, which were rolled under the rolling conditions of the present invention except that the slab heating temperature was 1150°C.

The pearlite fragmentation rate is low, both below 0.55, and the ferrite particle size number is large, but the structure parameter is small at 4 or below, and the impact value at -50°C is also low. D4
is a comparative material, which was rolled under the rolling conditions of the present invention except that the slab heating temperature was 1000°C.

Although the structure parameter 1 is 4 or more, the pearlite fragmentation rate is lower than 0.55 and the impact value at -50°C is low. D5 is a plate processed according to the present invention with a thickness of 30 mm0
Although it is a hot-rolled steel material, its strength is comparable to normalized material A1, but its impact toughness at -50°C is significantly superior.

D6 to D9 are comparative materials that were conducted under the same conditions as the present invention except that the slab heating temperature was 930°C.

The pearlite fragmentation rate is low and there is significant variation depending on the plate thickness. Therefore, the structure parameters also vary, and the impact value at -50°C is inferior. ζ)ζ)

[Brief explanation of drawings]

Fig. 1 is an explanatory diagram of the method for calculating the pearlite fragmentation rate, Fig. 2 is a chart showing the relationship between structure parameters and impact values in the examples of the present invention, and Fig. 3a is the structure of comparative material B2 (XlO'O ), b is the structure of comparative material B1 (XiOO
), FIG. 4C shows the structure (X.iOO) of the processed material C5 of the present invention, and d shows the structure (X.iOO) of the processed material C6 of the invention
It is a micrograph figure showing XIOO).

Claims (1)

[Claims] 1 After heating a piece of carbon steel or low alloy steel to a temperature in the range of over 1000°C to 1100°C or less, 8
1. A method for producing high-toughness carbon steel or low-alloy hot-rolled steel material, which comprises rolling at 50°C or lower at a grade reduction rate of 50% or higher, and forced cooling to between 850 and 500°C during this rolling process.