JPH042718A - Production of medium strength steel plate having high yield point and high toughness - Google Patents

Abstract

PURPOSE:To produce a medium strength steel plate having high yield point and high toughness by subjecting a slab of a steel containing specific amounts of C, Si, Mn, V, Nb, and Al to heating up to specific temp., to specific rolling, to cooling, and further to specific thermal refining. CONSTITUTION:A slab of a steel which has a composition containing, by weight, 0.10-0.17% C, 0.1-0.60% Si, 0.80-1.60% Mn, 0.005-0.050% V, 0.010-0.050% Nb, and <=0.05% Al and further containing, if necessary, one or more kinds among <=0.5% Cr, <=0.5% Ni, <=0.03% Ti, and <=0.3% Cu is heated at 1000-1200 deg.C. Subsequently, this steel slab is rolled at a temp. in the austenite unrecrystallized region or above at >=30% cumulative reduction of area and cooled down to ordinary temp. The resulting rolled plate is further subjected to thermal refining at a temp. not higher than the Ac1 point. By this method, the medium strength steel plate having high yield point and high toughness and suitable for economical structural material can be obtained.

Description

[Detailed Description of the Invention] (Objective of the Invention) This invention aims to improve the economic efficiency of medium-strength steel with a high yield point and high toughness, which has a wide range of uses as structural steel, such as building materials, bridge materials, and pipe materials. The present invention relates to a manufacturing method that is also excellent in terms of manufacturing methods.

(Prior art) Until now, the yield point was 40 to 50 kgf/n+m2
Grade medium-strength steel is manufactured by hot rolling a slab manufactured by blooming or continuous casting, cooling it to room temperature as a steel plate of a specified thickness, and then reheating and normalizing it. It has been. The problem with this method is that the normalizing
c In order to reheat to a temperature of 3 points or more, although the toughness is improved compared to the as-rolled steel material, the yield strength and tensile strength are reduced, and furthermore, it is heated to a high temperature of 3 points or more. Therefore, there were problems such as requiring a lot of energy.

Furthermore, Japanese Patent Application Laid-Open No. 56-25924 discloses a method for manufacturing aluminum killed steel for low temperature use. This is done by using aluminum killed steel with a composition that does not contain Nb, hot working including working in the non-recrystallization temperature range of at least one A c point, and then rapidly cooling it to room temperature. or, after the above-mentioned hot working, cooling to room temperature by air cooling; furthermore, after the above-mentioned air cooling, tempering is performed at a temperature below the self-temperature point. However, this method requires that fA′M! Since the addition of hardening elements such as Nb is suppressed during the i-forming process, there is a problem that the strength is insufficient as a medium-strength steel.

(Problems to be Solved by the Invention) The present invention eliminates the problems of the prior art described above, namely, the decrease in yield strength and tensile strength due to heat treatment, and reduces the thermal energy cost during heat treatment to increase the The purpose is to produce medium strength steel with high yield point and high toughness.

(Means for solving problems) The gist of this invention is as follows.

1. C: 0.10wt% or more, 0.17wt%
below.

Si: 0.10iyt% or more, 0.60wt
% or less Mn: 0.80wt% or more, 1.60w
t% or less.

V: 0.005 wt% or more, 0.050 w
t% or less Nb: 0.010 iwt% or more, 0.
050 wt% or less, and A 12 : 0.05 wt% or less.

A steel slab containing Ac A method for producing a medium strength steel plate having a high yield point and high toughness, which is characterized by heating and tempering at a desired temperature.

2. C: 0.10ivt% or more, 0.17wt
%below.

Si: 0.10wt% or more, 0.60ivt
%below.

Mn: 0.80wt% or more and 1.60wt% or less.

V: 0.005 wt% or more, 0.050 w
t% or less Nb: 0.010 wt% or more, 0.0
50 ivt% or less and A1: 0.05wt% or less.

and contains one or more selected from Cr: 0.5 wt% or less, Ni: 0.5 wt% or less, Ti: 0.03 wt% or less, and Cu: 0.3 wt% or less. A steel slab is heated in a temperature range of 1000°C or higher and 1200°C or lower, rolled at a cumulative reduction rate of 30% or higher at a temperature higher than the austenite non-recrystallized region, then cooled to room temperature, and further rolled at the A C1 point. A method for producing a medium-strength steel plate having a high yield point and high toughness, the method comprising heating and tempering at the following temperatures.

Here, the temperature below A c 1 point is from 400℃ to 60℃.
Means a temperature range of 0°C.

In addition, the impurity components P and S are each 0.020wt.
% or less, preferably 0.015 wt% or less.

(Function) This invention refines the grain size by controlled rolling, disperses the precipitates into a solid solution, and then heats and tempers the material in a temperature range of 400°C to 600°C below A c 1 point. It was discovered that a medium-strength steel plate having a high yield point and high toughness can be obtained by precipitating fine carbonitride precipitates to strengthen ferrite.

Below, the range of chemical components in the steel composition of the present invention and the reasons for limiting the manufacturing conditions will be described in order.

C: The purpose is to impart strength and toughness, but these effects cannot be obtained at less than 0.10% t%, so the lower limit is set to 0.1.
The content shall be 0wt% or less. On the other hand, if it exceeds 0.17 wt%, there is a problem that weldability and toughness deteriorate. Therefore, the upper limit is set to 0.17 wt%.

Si: In addition to acting as a deoxidizing agent, it is a component that affects strength and toughness, but if it is less than 0.10 wt%, these effects cannot be obtained, and on the other hand, if it exceeds 0.60 wt%, toughness deteriorates. . Therefore, the lower limit is set to 0.10 wt% and the upper limit is set to 0.60 wt%.

Mn: An essential element for imparting strength and toughness, 0.80 wt% is required for high toughness, but 1.
If it exceeds 60 wt%, the toughness will deteriorate, and weldability will also deteriorate. Therefore, the lower limit is set to 0.80 wt% and the upper limit is set to 1.60 wt%.

V: An important element for strengthening @ and reducing deterioration of weldability, and requires 0.005 wt%,
If it exceeds 0.05wt%, the toughness deteriorates, so the lower limit is set to 0.
．． 005 ivt%, and the upper limit is 0.05wt%.

Nb: An element useful for refining crystal grains. For this purpose, 0.010 wt% is required, but 0.050 wt% is required.
If it exceeds wt%, weldability deteriorates. Therefore, the lower limit is set to 0.010 wt% and the upper limit is set to 0.050 wt%.

Al = An effective element as a deoxidizer, and is effective in refining austenite and improving toughness by combining with N in steel to form AIN, but when added in excess, it has the opposite effect. Toughness deteriorates. Therefore, the upper limit is set at 0.05 wt%, which is not satisfactory.

Cr, Ni, Ti, Cu: These are effective components that are effective in improving strength, but the upper limit is set at 0.5 wt for each as an amount that does not have a negative effect on toughness, weldability, etc.
%, Ni 0. Si%, Ti 0.03wt%, Cu
is 0.3 wt%. In addition, these Cr, Ni
, Ti, and Cu may be added in combination within the above ranges.

Note that P and S are not particularly limited, but from the viewpoint of preventing deterioration of toughness and weldability, P is 0.020 wt% or less,
It is preferable that S be 0.015 i1t% or less.

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

The heating temperature of the slab was set to 1000°C or more and 1200"C or less in order to re-dissolve the carbonitrides, which are alloy components, that were precipitated when the blooming or continuous casting slab was produced.
This is because if the temperature is below 00°C, the austenite grains will not become a sufficient solid solution and will remain until later, impairing the toughness of the steel material, and if the temperature is above 1200°C, the austenite grains will become coarse and the toughness will deteriorate.

Next, the reason why rolling at a temperature above the austenite non-recrystallized region is performed at a cumulative reduction rate of 30% or more is to improve toughness by making the austenite finer.

In other words, if the cumulative reduction ratio is less than 30%, sufficient refinement of austenite will not occur when rolling is carried out in the austenite recrystallization region, and rolling from the austenite recrystallization region to the austenite non-recrystallization region and the austenite non-recrystallization In the case of rolling in the crystalline region, it is not possible to introduce deformation bands into the austenite grains, and therefore, the austenite grains are not refined by splitting, and therefore the toughness cannot be improved by refining the austenite.

Furthermore, by performing heat refining at a temperature below A c 1 point after rolling cooling, that is, in the temperature range from 400°C to 600°C, several carbonitrides are precipitated in the ferrite, and this This is because it strengthens the steel and provides a high yield point.

Explanation will be added below based on experimental examples.

Steel slabs having the three types of chemical compositions shown in Table 1 that were melted in a converter, continuously cast, and cast were rolled under the rolling conditions shown in Table 2, cooled, and then tempered at temperatures from 400°C to 930°C. The tensile properties, impact properties, microstructure, etc. of the tested specimens were investigated.

This shows that the rolling was restarted at a temperature of 83° C. and completed at a finishing temperature of 795° C.

In addition, in the heat refining treatment, the test piece was operated after holding the heating furnace at the target heat refining temperature for 10 minutes, and after the test piece reached the heat refining temperature, the test piece was held for 5 minutes and then air cooled. The test piece was placed on a bed plate and tempered as shown in FIG. 1, and the tempering temperature was determined by measuring the temperature of the test piece.

The survey results are shown in Figures 2 and 3.

Looking at the tensile properties in Figure 2, the yield point of the steel conforming to this invention increases as the tempering temperature increases compared to the as-rolled state, reaching a maximum of 500°C, and then decreasing after this point. However, up to 700°C, the value is higher than that of the as-rolled steel. Also, looking at the toughness in Figure 3, the tempering temperature is 600.
Charpy absorbed energy v at -60℃ up to ℃
Both E-60 and fracture surface transition temperature vTrs showed almost no change, indicating high toughness, but they rapidly decreased when the temperature reached 700°C.

Therefore, the annealing temperature for obtaining a high yield point and high toughness is preferably between 400°C and 600°C.

Furthermore, according to the microstructure observation results, the microstructure is composed of ferrite and pearlite, and several precipitates can be seen in the ferrite. That is, the above-mentioned precipitates precipitate in the ferrite by the heat treatment, strengthen the ferrite, and increase the yield point.

In addition, by effectively utilizing the phenomenon that rolling has a low yield point and becomes a high yield point after tempering, the forming process is performed at a time when the yield point is low after rolling when it is easy to process, and after working, A c , as desired It can also be applied to applications where a high yield point is obtained by performing heat refining at a temperature of .

(Example) Using 13 types of steel slabs having the chemical compositions shown in Table 3, which were melted in a converter and continuously cast, and the three types of steel slabs shown in Table 1 described above, rolling and tempering were carried out. The tensile properties and impact properties were investigated.

For all steels shown in Tables 1 and 2, rolling was carried out from the austenite recrystallized region to the non-recrystallized region, and for steel code B, rolling was performed only in the austenite recrystallized region.
Then, only the non-recrystallized area was rolled. Table 4 shows these rolling conditions.

In addition, the tempering was performed using the same method as described above, and the tempering temperature was set to 5.
The test was carried out under the same conditions of 00° C. 2 holding time and 3511 Iin. Table 5 shows the results of the investigation of tensile properties and impact properties.

As is clear from Table 5, the applicable examples of the present invention are when the rolling is in the austenite recrystallized region (sample number 17), and when the rolling is from the austenite recrystallized region to the non-recrystallized region (sample number 2).
, 3.7-16), in the case of austenite-end recrystallization region (
In both cases of sample number 18), the yield point was 46.8 k.
gf/mm2 or more, vE -6014, 2kgf-m
As described above, v T rs is −59° C. or less, which indicates a high yield point and high toughness value.

(Effects of the Invention) According to the present invention, by performing heat refining at a low temperature of A c 1 point or less, a medium-strength steel with a high yield point and high toughness that is excellent in economic efficiency can be obtained and can be widely used as a structural steel. In addition to the A
It can also be applied to applications where a high yield point is obtained by tempering at a temperature below the C1 point.

[Brief explanation of drawings]

FIG. 1 is a perspective view showing the adjustment status of a test piece during thermal refining treatment, FIG. 2 is a diagram, and FIG. 3 is a diagram. 1... Bottom plate 2... Tensile test piece 3... Charpy test piece 4... Line showing the relationship between thermocouple heat refining temperature and tensile properties Diagram 3 showing the relationship between heat refining temperature and impact properties Mashima Diagram 2 thermal quality i degree (τ) of and -

Claims (1)

[Claims] 1. C: 0.10 wt% or more, 0.17 wt% or less, S
i: 0.10 wt% or more, 0.60 wt% or less, Mn:
0.80wt% or more, 1.60wt% or less, V: 0.0
05wt% or more, 0.050wt% or less, Nb: 0.0
A steel slab containing 10wt% or more, 0.050wt% or less, and Al: 0.05wt% or less is heated in a temperature range of 1000°C or more and 1200°C or less, and the cumulative A method for manufacturing a medium-strength steel plate having a high yield point and high toughness, which comprises rolling with a reduction ratio of 30% or more, cooling to room temperature, and further heat-refining at a temperature of Ac_1 point or less. 2, C: 0.10wt% or more, 0.17wt% or less, S
i: 0.10 wt% or more, 0.60 wt% or less, Mn:
0.80wt% or more, 1.60wt% or less, V: 0.0
05wt% or more, 0.050wt% or less, Nb: 0.0
10 wt% or more, 0.050 wt% or less, and Al: 0.05 wt% or less, and Cr: 0.5 wt% or less, Ni: 0.5 wt% or less, Ti: 0.03 wt% or less, and Cu: 0. A steel slab containing one or more selected from 3wt% or less is heated in a temperature range of 1000°C or more and 1200°C or less, and the cumulative reduction rate is 30% or more at a temperature above the austenite non-recrystallized region. A method for producing a medium strength steel sheet having a high yield point and high toughness, which comprises rolling the steel sheet, cooling it to room temperature, and further heat-refining it at a temperature of Ac_1 point or lower.