JPH0774381B2 - Manufacturing method of medium strength steel - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Object of the Invention) The present invention has a wide range of uses as a structural steel such as construction materials, bridge materials, and pipe materials, and provides high yield point and high toughness after aging treatment. The present invention relates to a manufacturing method of a high-strength steel which is also excellent in economic efficiency.

(Prior art) Up to now, the yield strength of 40 to 50 kgf / mm ² class medium strength steel is obtained by hot rolling a slab manufactured by slab or continuous casting and releasing it to room temperature as a steel plate with a predetermined thickness. It has been manufactured by cooling, then reheating, and normalizing. As a problem of this method, in order to reheat the normalization to a temperature of Ac ₃ point or higher, compared with the as-rolled steel material, the toughness is improved but the yield strength and the tensile strength are lowered, Further, there is a problem that much energy is required for heating to a high temperature of Ac ₃ point or higher.

Further, Japanese Patent Application Laid-Open No. 56-25924 discloses a method for producing low temperature aluminum killed steel. This is done using aluminum-killed steel with a composition that does not contain Nb, after hot working including processing in the unrecrystallized temperature region of Ac ₃ points or higher, then rapidly cooled to room temperature and tempered at a temperature of Ac ₁ point or lower. Examples of the method include a method of cooling to room temperature by air cooling after the hot working, and a method of tempering at a temperature of Ac ₁ point or lower after the air cooling. However, this method has a problem that the strength is insufficient as a medium-strength steel because it suppresses the addition of hardening elements such as Nb in the steel composition.

(Problems to be Solved by the Invention) The present invention eliminates the problems of the above-mentioned conventional techniques, that is, the aging treatment for eliminating the decrease in yield strength and tensile strength due to tempering and reducing the thermal energy cost during tempering. To produce a medium strength steel that has a high yield point and high toughness.

(Means for Solving the Problems) The gist of the present invention is as follows.

1.C: 0.10wt% or more, 0.17wt% or less, Si: 0.10wt% or more, 0.60wt% or less, Mn: 0.80wt% or more, 1.60wt% or less, V: 0.005wt% or more, 0.050wt% or less, Austenite is formed by heating a steel slab containing Nb: 0.010 wt% or more and 0.050 wt% or less and Al: 0.05 wt% or less, and the balance being essentially Fe, in the temperature range of 1000 ° C to 1200 ° C. The aging treatment consists of performing controlled rolling with a cumulative rolling reduction of 30% or more at a temperature above the unrecrystallized region, and then air-cooling to room temperature, and then heating in the temperature range of 400 ° C to 600 ° C after the subsequent forming process. And a high yield point and a high toughness.

2.C: 0.10wt% or more, 0.17wt% or less, Si: 0.10wt% or more, 0.60wt% or less, Mn: 0.80wt% or more, 1.60wt% or less, V: 0.005wt% or more, 0.050wt% or less, Nb: 0.010 wt% or more, 0.050 wt% or less and Al: 0.05 wt% or less, and Cr: 0.5 wt% or less, Ni: less than 0.5 wt%, Ti: 0.03 wt% or less and Cu: 0.3 wt% or less A steel slab containing one or more selected from the above, the balance being essentially Fe
After heating in a temperature range of ℃ or less, controlled rolling with a cumulative reduction of 30% or more at a temperature of austenite unrecrystallized region or more, it is air-cooled to room temperature. A method for producing a medium-strength steel material, characterized by being subjected to an aging treatment in which it is heated in a temperature range of 600 ° C so as to have a high yield point and high toughness.

Here, the impurity components P and S are preferably 0.020 wt% or less and 0.015 wt% or less, respectively.

(Operation) In this invention, the crystal grain size is made fine by controlled rolling, the precipitate is dispersed and solid-dissolved and air-cooled to room temperature, and then heated in the temperature range of 400 ° C to 600 ° C below the Ac ₁ point. The inventors have found that by performing aging treatment, fine precipitates of carbonitrides are precipitated and ferrite is strengthened to achieve a high yield point and high toughness, and a medium-strength steel material can be obtained.

The ranges of chemical components in the steel composition of the present invention and the reasons for limiting the manufacturing conditions will be described below in order.

C: The purpose is to impart strength and toughness, but if the amount is less than 0.10 wt%, these effects cannot be obtained, so the lower limit is made 0.10 wt%.
On the other hand, if it exceeds 0.17 wt%, there is a problem that weldability and toughness deteriorate. Therefore, the upper limit is made 0.17 wt%.

Si: In addition to its role as a deoxidizer, it is a component that acts on strength and toughness, but if it is less than 0.10 wt%, those effects cannot be obtained, and if it exceeds 0.60 wt%, toughness deteriorates. Therefore, the lower limit is 0.10 wt% and the upper limit is 0.60 wt%.

Mn: An element essential for imparting strength and toughness, 0.80 wt% is required for high toughness, but if it exceeds 1.60 wt%, toughness deteriorates and weldability also deteriorates. . Therefore, the lower limit is 0.80 wt% and the upper limit is 1.60 wt%.

V: It is an important element for strengthening steel and reducing deterioration of weldability, and requires 0.005 wt%, but if it exceeds 0.05 wt%, toughness deteriorates, so the lower limit is 0.005 wt% and the upper limit is 0.05w
t%

Nb: an element useful for refining crystal grains, and for this reason 0.0
10 wt% is required, but if it exceeds 0.050 wt%, the weldability deteriorates. Therefore, the lower limit is 0.010 wt% and the upper limit is 0.050
wt%

Al: an element effective as a deoxidizer, and is also effective in refining austenite by combining with N in steel to form AlN and improving toughness, but if added excessively, it is contrary The toughness deteriorates. Therefore, the upper limit is 0.05%, which does not cause disappointment.

Cr, Ni, Ti, Cu: Equivalent components that are effective for improving strength, but as a level that does not have a negative effect on toughness, weldability, etc., Cr is 0.5 wt% or less and Ni is 0.5 wt% respectively. Less than Ti
Is 0.03 wt% or less and Cu is 0.3 wt% or less. Note that these Cr, Ni, Ti, and Cu may be added in combination within the above range.

Although P and S are not particularly limited, from the viewpoint of preventing deterioration of toughness and weldability, P is 0.020 wt% or less and S is 0.015 wt%.
% Or less is preferable.

Next, the reasons for limiting the manufacturing conditions will be described.

The heating temperature of the slab is set to 1000 ° C or more and 1200 ° C or less,
This is to re-dissolve the carbonitrides of the alloy components that were precipitated when the agglomerate or continuous cast slab was manufactured. It does not form a solid solution at 1000 ° C or less and remains until later, impairing the toughness of the steel material. This is because the austenite grains become coarse and the toughness is deteriorated at 1200 ° C. or higher.

Next, the reason why rolling at a temperature in the austenite unrecrystallized region or higher is set to a cumulative rolling reduction of 30% or more is to improve the toughness by refining the austenite.

That is, when the rolling reduction is less than 30%, when rolling in the austenite recrystallization region, the austenite is not sufficiently refined, and the rolling from the austenite recrystallization region to the austenite non-recrystallization region and the austenite non-recrystallization region are performed. In the case of rolling in the crystalline region, it is not possible to introduce a deformation zone into the austenite grains, so that the austenite grains are not refined by division, and therefore the toughness cannot be improved by the austenite refinement.

This is because the air-cooling to normal temperature after rolling has a low yield point in the as-rolled state.

Furthermore, by performing the aging treatment in the temperature range of 400 to 600 ° C, carbonitrides of about several Å will be precipitated in the ferrite, which will strengthen the ferrite, which will increase the yield point. It is because it is obtained.

A description will be added below based on experimental examples.

Steel slabs with the three chemical compositions shown in Table 1 that were melted in a converter and cast by continuous casting were rolled under the rolling conditions shown in Table 2 and then air-cooled to room temperature and the temperature from 400 ° C to 930 ° C. Tensile properties, impact properties, microstructures, etc. were investigated for the test pieces heated in.

In Table 2, the slab heated to 1200 ° C. was extracted from the soaking furnace, the rolling was immediately started, and then the rolling was stopped midway.
This indicates that the temperature was adjusted to cool it from 980 ° C to 830 ° C, then the rolling was restarted from 830 ° C and the controlled rolling was completed at the finishing temperature of 795 ° C.

The heat treatment was carried out by holding the heating furnace at a target heating temperature for 10 minutes in advance, then operating the test piece, holding it for 5 minutes after the test temperature reached the heating temperature, and then air-cooling the test piece. As shown in FIG. 1, the test piece was placed on a floor plate and heated, and the heating temperature was the temperature of the test piece.

The survey results are shown in FIGS. 2 and 3.

The tensile properties of Fig. 2 show that the conforming steel of the present invention has a higher yield point as the heating temperature rises compared to the as-rolled state. Although it is shown, it shows a higher value as-rolled up to 700 ° C. Also, looking at the toughness in Fig. 3, up to a heating temperature of 600 ° C, Charpy absorbed energy vE-60 at -60 ° C,
Fracture transition temperature vTrs shows almost no change and shows high toughness, but it drops sharply at 700 ° C.

Therefore, the heating temperature to obtain high yield point and high toughness is
It is better to have Ac less than ₁ point between 400 ℃ and 600 ℃.

In addition, according to the observation result of the microstructure, it has a fine structure composed of ferrite and pearlite, and precipitates of about several Å are seen in the ferrite. That is, the heat treatment causes the precipitates to be precipitated in the ferrite, strengthening the ferrite, and increasing the yield point.

The as-rolled product has a low yield point, and the phenomenon of increasing the yield point after aging by heating is effectively used to perform forming at a low yield point after rolling, which is easy to process, and then heat the product after aging. It can be advantageously applied to the application of treating to obtain a high yield point.

(Example) 13 types of steel slabs having the chemical composition shown in Table 3 which were melted in a converter and cast by continuous casting, and 3 types of steel slabs shown in Table 1 were used for rolling and tempering. The tensile properties and the impact properties were investigated.

Rolling was performed on all the steels shown in Tables 1 and 2 from the austenite recrystallized region to the non-recrystallized region, and for steel symbol B steel, rolling only in the austenite recrystallized region,
And rolling was performed only in the non-recrystallized region. Table 4 shows these rolling conditions.

Table 4 shows a temperature adjustment in which the slabs heated to the respective temperatures are extracted from the soaking furnace, the rolling is immediately started, the rolling is stopped midway, and the temperature is adjusted from the temperature control start temperature to the temperature control end temperature. After that, the rolling is restarted and the rolling is finished at each finishing temperature.

The aging treatment was the same as that described above, under the same conditions of heating temperature of 500 ° C. and holding time of 35 minutes. Table 5 shows the results of the examination of tensile properties and impact properties.

As is clear from Table 5, the conformity example of the present invention is applicable when the rolling is in the austenite recrystallization region (Sample No. 17), and when the rolling is from the austenite recrystallization region to the non-recrystallization region (Sample Nos.
7 to 16) in the unrecrystallized austenite region (Sample No. 1)
In any case of 8), yield point 46.8 kgf / mm ² or more, vE-60
It shows that the yield point is high and the toughness is high with 14.2 kgf · m or more and vTrs-59 ° C or less.

(Effect of the Invention) According to the present invention, the rolling yield is low and
By performing the aging treatment at a low temperature of ₁ point or less of Ac, a high yield point with excellent economic efficiency and high toughness medium strength steel can be obtained, which can be widely used as structural steel, and has a low yield point in the as-rolled state. Utilizing this fact, it can be advantageously applied to applications such as forming after rolling, which is easy to form, and then obtaining a high yield point by aging treatment.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a perspective view showing the adjustment state of a test piece during heat treatment, FIG. 2 is a diagram showing the relationship between heating temperature and tensile properties, and FIG. It is a diagram which shows the relationship between heating temperature and impact characteristics. 1 ... Floorboard 2 ... Tensile test piece 3 ... Charpy test piece 4 ... Thermocouple

 ─────────────────────────────────────────────────── ─── Continuation of the front page (56) Reference JP-A-55-104427 (JP, A) JP-A-51-20719 (JP, A) JP-A-3-61321 (JP, A)

Claims (2)

[Claims] 1. C: 0.10 wt% or more, 0.17 wt% or less, Si: 0.10 wt% or more, 0.60 wt% or less, Mn: 0.80 wt% or more, 1.60 wt% or less, V: 0.005 wt% or more, 0.050 wt %, Nb: 0.010 wt% or more, 0.050 wt% or less, and Al: 0.05 wt% or less, the balance being a steel slab with a composition essentially consisting of Fe in the temperature range of 1000 ° C to 1200 ° C. After performing controlled rolling with a cumulative rolling reduction of 30% or more at a temperature above the austenite unrecrystallized region and then cooling to room temperature, heating is performed in the temperature range of 400 ° C to 600 ° C after the subsequent forming process. A method for producing a medium-strength steel material, which comprises subjecting the steel to an aging treatment to bring about a high yield point and high toughness. 2. C: 0.10 wt% or more, 0.17 wt% or less, Si: 0.10 wt% or more, 0.60 wt% or less, Mn: 0.80 wt% or more, 1.60 wt% or less, V: 0.005 wt% or more, 0.050 wt % Or less, Nb: 0.010 wt% or more, 0.050 wt% or less and Al: 0.05 wt% or less, and Cr: 0.5 wt% or less, Ni: less than 0.5 wt%, Ti: 0.03 wt% or less and Cu: 03 wt% % Steel or steel slab containing one or more selected from the following, with the balance consisting essentially of Fe.
After heating in a temperature range of ℃ or less, controlled rolling with a cumulative reduction of 30% or more at a temperature of austenite unrecrystallized region or more, it is air-cooled to room temperature. A method for producing a medium-strength steel material, characterized by being subjected to an aging treatment in which it is heated in a temperature range of 600 ° C so as to have a high yield point and high toughness.