JPS5821007B2 - Kokiyoujinseikouzainoseizouhou - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION This invention provides excellent toughness, ductility, and
The present invention relates to a method for producing a low alloy steel material having a tensile strength of 0.00 kq/-.

Conventional methods for obtaining high-strength steel materials include various tempering heat treatments such as quenching and tempering, and mechanical strength strengthening through cold working, but these methods are effective in increasing strength. However, it was not sufficient to combine high strength with high toughness and high ductility.

This invention proposes a method for manufacturing high-strength steel materials with high toughness and high ductility that solves the above-mentioned problems by conducting various studies on heat treatment and processing of low-alloy steel.

The feature of this invention is that by appropriately selecting and combining the three conditions of steel chemical composition, heat treatment, and processing, a fine grain structure that could not be obtained by conventional methods is achieved, resulting in high strength and excellent It is possible to impart additional toughness and ductility.

The low alloy steel targeted by this invention is carbon 0.1-10
%, silicon 0.01-2.0%, manganese 0.1-2.
0%, acid-soluble aluminum 0.01-0.2%, further contains vanadium 0.005-0.3%, niobium 0.
．． Steel containing one or more of the following: 0.002-0.2%, titanium 0.005-03%, zirconium 0.002-0.2%, and chromium 0.1-2.0% as necessary. ,
Nickel 0.1-2.0%, steel 0.1-1.0%, molybdenum 0.05-1.0%, boron 0.0005-0.
The target is low alloy steel containing one or more of 0.01% and 1050
Pretreatment by quenching after high-temperature heating at ~1,300°C completely and uniformly dissolves the alloying elements into a martensite structure or a mixed structure of martensite and bainite, which has an Ac3 transformation point ~ Ac3 transformation point +200°C. Heating in a temperature range at a heating rate of 5°C/second or more produces an austenitic structure consisting of extremely fine austenite grains and extremely finely dispersed precipitated compounds, and cools this to a temperature between the A3 transformation point before the Ms point. It is rapidly cooled at a rate of 4°C/second or more, and processed at a reduction rate of 15% or more (preferably, 50% or more of the area reduction is processed before the transformation is completed) in that temperature range to form a very fine material. This is a method for producing a high-strength steel material, which is characterized by forming a structure consisting of crystal grains and carbides, and holding the steel material for 30 seconds to 10 minutes.

In the above production method, Ac3 transformation point to Ac3 transformation.

Processing with an area reduction rate of 10% or more in the temperature range of point +200℃, or an area reduction rate of 10 to 95% at room temperature after production.
The quality of the steel can be further improved by adding processing.

In the production method of this invention, first, the Ac3 transformation point or higher.

-Heating and quenching the top to form martensite or a mixed structure of martensite and bainite is necessary as a pretreatment to obtain fine austenite grains,
At this time, it is desirable to heat to a temperature range of 1050 to 1300°C.

That is, in order to austenitize steel, Ac3
It must be heated above the transformation point, but in that case 1
If the temperature exceeds 050°C, carbide-forming elements such as vanadium, titanium, and zirconium can be uniformly dissolved in the austenite structure, but if the temperature exceeds 1300°C, cracks will occur in the steel due to burning of the steel during heating.

Next, quenching is performed to change the structure of the steel to a martensitic structure or a mixed structure of martensite and bainite, which is a necessary condition for carrying out the subsequent steps and achieving the object of the present invention. .

After quenching, Ac3
Heating to a temperature range from the transformation point to the Ac3 transformation point +200°C is necessary to obtain the desired fine austenite grain structure, and the structure must be austenitized at a temperature higher than the Ac3 transformation point and at a temperature lower than the Ac3 transformation point +200°C. This prevents crystal grains from becoming coarser.

In addition, after reheating, Ac3 transformation point ~ Ac3 transformation point +200
By performing processing with an area reduction rate of 10% or more in the temperature range of °C, the austenite grains before transformation can be made finer, and this can have a positive effect on improving the toughness, strength, etc. of the steel.

The steel obtained as described above is rapidly cooled to a temperature range between the Ac3 transformation point and the Ms transformation point at a cooling rate of 4° C./sec or more, and is transformed by processing at least 15% in the temperature range.

Rapid cooling to the above temperature range is effective in making the austenite grains finer before transformation, and this treatment provides properties that are significantly superior to those produced by conventional ausforming or isoforming.

In addition, performing hot working with an area reduction rate of 15% or more is because the precipitation of added compounds such as vanadium, niobium, titanium, and zirconium is further promoted, and fine compounds can be dispersed in the matrix. .

For this reason, the products generated in the subsequent transformation are made more uniform and finer, contributing to improvement of various properties of the steel material.

When cooling at a rate of 4°C/sec or more from a temperature above the Ac3 transformation point temperature, if it is less than 4°C/sec, the transformation will complete during cooling for low-alloy steel, resulting in a decrease in strength. This is because martensite occurs in the structure below the Mss transformation point, and no transformation of the steel structure occurs above the A3 transformation point. This is because the steel structure cannot be obtained.

In addition, processing is performed with an area reduction rate of 15% or more (however, it is desirable to add 50% or more of the total area reduction rate before the completion of transformation) by maintaining the temperature in the above Ms transformation point to A3 transformation point. This is necessary to increase strength and improve toughness by refining the structure.

In particular, if the processing is performed to a total area reduction rate of 50% or more before the transformation is completed, the toughness will be significantly improved.

Furthermore, in the temperature range of Ms transformation point to A3 transformation point, 30
The purpose of holding the cementite for 10 seconds to 10 minutes is to sufficiently complete the transformation, but if the holding time exceeds 10 minutes, the cementite becomes spheroidal and its strength decreases, which is not desirable.

Here, in order to further improve the mechanical properties, processing with an area reduction rate of 10 to 95% is performed at room temperature.

In this case, if the area reduction rate is less than 10%, the effect will not be fully exhibited, and if it exceeds 95%, the repeatability of bending and drawability will deteriorate, which is not desirable.

In addition, the transformation points of the target steel in this invention are Ac3 point and A3 point of 720 to 900°C, and Ms point of 220 to 400°C.
It is located in ℃.

The steel material obtained by processing according to the present invention has a uniform fine grain structure, and has superior mechanical properties compared to steel material obtained by conventional manufacturing methods.

Next, embodiments of the invention will be described.

Carbon 0.61%, silicon 0.33%, manganese 1.18
This invention method was carried out using hot-rolled steel plates with thicknesses of 10.5 mm, 15 mm, and 25 mm containing 0.06% Nagi vanadium, 0.06% niobium, and 0.055% solid solution aluminum, with the remainder made of iron. and produced by comparative methods;
Mechanical properties were tested.

The results are shown in Table 1.

In addition, the high-temperature Caloe shown in Table 1 refers to the temperature between Ac3 transformation point and Ac3 transformation point.
It refers to processing in the temperature range of 3 transformation point + 200°C, and processing in the transformation temperature range refers to processing in the temperature range of Ms transformation point to A3 transformation point.

The A3 point and AC3 point of the above low alloy steel are 745.
°C, Ms point is 283 °C.

Further, maintenance in the transformation temperature range was performed in a salt bath.

Further, test numbers 6°7.11 and 12 in the comparative method were not pretreated by quenching after heating to a temperature above the Ac3 transformation point, but all test materials were allowed to cool after hot rolling.

JI814 tensile test piece, JISl No.4 tensile test piece made from steel material made by the treatment shown in Table 1. A notched tensile test piece with a notch groove with a depth of 2 mm and a bottom curvature of 0.05 mm in the center of the JISI No. 4 tensile test piece, diameter 10 m1. Delayed fracture test pieces having a notch groove with a depth of 1 wrr L and a bottom curvature of 0.05 y++ m in the center of a rod-shaped test piece with a length of 340 m were prepared and tested.

Note that a JI814 tensile test piece was used for the relaxation test.

The delayed fracture test was carried out by four-point support bending in a 0.1N hydrochloric acid solution for up to 240 hours with a nominal stress of 150 kyf/vtyj applied to the notch bottom. %, and the relaxation value after 10 hours at °C was calculated from equation (1) and shown.

0.7XTs - σ Relaxation value - In the drawing test, the obtained steel material was polished to a thickness of 2 mm and a width of 20 mm, and the specimen was sandwiched between a pair of rolls with a radius of 10 mm and repeatedly bent back and forth at 90 degrees until cracking occurred. The number of bends was counted.

Note that when the plate was bent by 90°, the number of bends was counted as one.

From the results shown in Table 1, it can be seen that the mechanical properties of the products produced by this invention are significantly superior to those produced by a comparative method that does not depend on this invention.

Claims (1)

[Claims] 1. After heating the low alloy steel to the Ac3 transformation point or higher, it is quenched to form a martensitic structure or a mixed structure of martensite and bainite, and then heated to a temperature range of Ac3 transformation point to Ac3 transformation point + 200°C. Raise the temperature at a heating rate of 5°C/second or higher, and cool it at a cooling rate of 4°C/sec to the temperature range from the MS transformation point to the A3 transformation point.
1. A method for producing a high strength and toughness steel material, which comprises rapidly cooling the material for at least 1 second, subjecting it to processing with an area reduction rate of at least 15% in the temperature range, and holding the product for 30 seconds to 10 minutes. 2. After heating the low alloy steel to a temperature above the Ac3 transformation point, it is quenched to form a martensite structure or a mixed structure of martensite and bainite, and then heated to a temperature between the Ac3 transformation point and Ac3.
The temperature is raised to a temperature range of transformation point +200 °C at a heating rate of 5 °C/second or more, hot working is performed with an area reduction rate of 10% or more in this temperature range, and then cooled to a temperature range of Ms transformation point - A3 transformation point. Rapid cooling at a rate of 4°C/sec or higher, with an area reduction rate of 15 in the temperature range
A method for producing a high strength and toughness steel material, which is characterized by subjecting the material to a process of % or more and holding it for 30 seconds to 10 minutes. 3. After heating the low alloy steel to the Ac3 transformation point or above, it is quenched to form a martensitic structure or a mixed structure of martensite and bainite, and then the Ac3 transformation point ~A c3
Raise the temperature at a heating rate of 5°C/sec or more to the temperature range of transformation point +200°C, and cool at a cooling rate of 4°C to the temperature range of MS transformation point to A3 transformation point.
It is characterized by rapidly cooling at a temperature of 15% or more in the temperature range, holding for 30 seconds to 10 minutes, and then processing at room temperature with an area reduction of 10 to 95%. Manufacturing method for high strength steel materials. 4 After heating the low alloy steel to the Ac3 transformation point or higher, it is quenched to form a martensitic structure or a mixed structure of martensite and bainite, and then heated at a heating rate of 5°C/sec or more in the temperature range of Ac3 transformation point to Ac3 transformation point + 200°C. The temperature is increased at Area reduction rate 15%
A method for producing a high strength and toughness steel material, which comprises holding the above-described processing for 30 seconds to 10 minutes, and then processing the area reduction rate of 10 to 95% at room temperature.