JPS59136418A - Manufacturing method of high toughness and high strength steel - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing high-toughness, high-strength steel, particularly steel suitable as a material for welded structures such as tanks, pressure vessels, and general bridges that require high strength and good weldability. Pertains to.

The present invention provides a method for inexpensively manufacturing steel that has a high degree of toughness and good weldability necessary for such uses, and also has a high tensile strength of 80 kg/rrrm2 or more.

80 kg class steel produced by the conventional off-line quench-temper heat treatment process is used in a variety of applications. However, many types of alloying elements are added to these steels in order to obtain high strength. Therefore, manufacturing costs are high, and
Strict considerations are normally taken during welding. Therefore, it has been strongly desired to reduce the manufacturing cost and improve weldability of this type of steel.

The present inventors lowered the carbon equivalent and 80 kii'/
As a result of various studies on the manufacturing method of steel to obtain high strength and toughness of mm2 or more, we found that even with a low carbon equivalent, Nb
It has been found that a high-toughness, high-strength steel with extremely excellent properties can be obtained by carrying out a composite addition of -Ti-Mo-H, on-line quenching, and then tempering. That is, the gist of the present invention is C: Q, 04
~0.10%, SS: 1.0% or less, B: O, 0
The basic component is 1% or less, and if necessary, v: o,
IS or less, Cu: 1% or less, Cr: 1% or less,
A steel containing one or more of Ca:O, 0.01% or less, the balance being Fe and unavoidable impurities, was heated at a heating temperature of 1i.
Ac1
The present invention provides a method for producing high-toughness, high-strength steel characterized by subjecting it to heat treatment for tempering at a temperature below 100%.

Conventionally, tempered steel containing Nb has been produced by off-line quenching and tempering. In this case, the quenching temperature is 900
Usually, the temperature is around 0.degree. C., and the solid solution of Nb cannot be expected to be strong.Therefore, precipitation hardening during tempering is not large, and the reduction of total elements by that amount is also limited. In addition, according to the results of conventional studies on direct quenching and tempering of Nb-containing steel, it is common knowledge that it is difficult to ensure hardenability unless the steel is heated at a low temperature.

As a result of various experiments and studies, the present inventors have found that by adding Nb-Ti-Mo-B in combination, it is possible to dramatically improve the hardenability due to B even at high temperature heating, which is contrary to common knowledge. It was confirmed that the strength and low-temperature toughness after tempering can be significantly improved.

In addition, the present invention makes it possible to increase the strength of the alloy by simply heating the Nb-containing steel at high temperature to sufficiently dissolve Nb in the solid solution, which exceeds that obtained by the method of causing precipitation hardening during tempering after rapid cooling. The amount of elements can be further reduced.

This method makes it possible to produce steel of 80 kf/1 ran or higher class that has good low-temperature toughness and weldability with relatively few chemical components.

Next, the reason for limiting the chemical composition of the target steel in the present invention will be described.

C is necessary in an amount of 0.04% or more to obtain high-strength copper, and as the amount increases, the strength increases, but low-temperature toughness decreases and
Since the weld cracking resistance deteriorates, the content should be reduced to 0.04 to 0.
, 10%.

Si usually exists to some extent as a deoxidizing element and is also useful for improving strength. However, if it exceeds 1.0%, the low-temperature toughness deteriorates significantly, so it is set at 1.0% or less.

Mn is required at 0.50% or more to obtain high strength, but if it exceeds 2.0%, low temperature toughness and weldability will be impaired, so the content was set at fi-0.50 to 2.0. .

Mo is useful for improving strength and toughness, but if it is less than 0.1%, it is ineffective, while if it exceeds 1%, the strength is too high, leading to a decrease in low-temperature toughness. Let's do it.

It is preferably 0.25 to 0.6%.

Nb is useful for grain refining and precipitation hardening, but 0.005
Less than % has no effect. Moreover, if it exceeds 0.05%, the cost will be high and weldability will be impaired, so the ratio was set at 0.005 to 0.05. The ratio is preferably 0.01 to 0.025.

T1 is fine grained and N is fixed and Nb -TI -M
.

The combined effect with carbon is useful for dramatically increasing the hardenability improvement effect of fiB, but if it is less than 0.005%, it is ineffective. However, if added in a large amount, the low temperature toughness will be impaired, so the upper limit -io is set at 05%.

A7 is added to deoxidize copper, but if it exceeds 0.1%, it impedes the cleanliness of the steel, so the upper limit is 0.1%.

B is useful for improving hardenability, but if added in a large amount, it inhibits low temperature toughness, so the upper limit is set at 0.01%.

Cr is useful for improving hardenability, but if added in large amounts it impairs weldability, so the upper limit fJ is set at 1.0%.

(2) is useful for increasing the strength of steel through precipitation hardening, but if too large it impairs low-temperature toughness, so it should be 0.1 inch or less.

Cu is useful for improving the hardenability of steel and increasing its strength, but the more it becomes the more expensive it becomes, so it should be kept at 1% or less.

Ca is useful when added during steelmaking to improve the deoxidation of steel, reduce inclusions, and completely improve low-temperature toughness, but when present in large amounts in steel, it can generate harmful nonmetallic inclusions. On the other hand, since it inhibits low-temperature toughness, it is set to 0.1 or less.

Next, P and S, which are unavoidably contained as impurities, are not particularly limited, but in order to stabilize the material through the cleanliness of the steel, the less the better, and from this point of view, P should be 0.020% or less It is desirable that S be 0.010% or less.

Next, the reasons for limiting the heating-rolling-heat treatment conditions for steel having the above composition will be described.

The heating temperature was set at 115 to enable solid solution of Nb.
The temperature shall be 0°C or higher. The rolling end temperature is set to 850° C. or higher, since the lower the temperature, the lower the hardenability and the lower the low-temperature toughness after tempering.

Next, immediately after rolling, quenching is performed, but since hardenability decreases when the cooling start temperature is low, the steel sheet is cooled at 850° C. by supplying a cooling medium such as man-over mist to the front and back surfaces of the steel sheet. If the temperature after quenching is high, it is difficult to obtain a completely hardened structure, so the upper limit is set at 200°C.

After the above-mentioned treatment, a tempering heat treatment is performed, and since tempering in the ferrite region is essential to obtain good low-temperature toughness, the upper limit temperature is set to A. 1 temperature.

Next, examples will be listed together with comparative examples.

Steel having the chemical composition shown in Table 1 was subjected to heating-rolling-heat treatment shown in Table 2. The mechanical properties and weld cracking resistance of the obtained steel plate are shown in Table 2.However, Examples A and B are examples of the first invention, and Examples C and D are shown in Table 2.
E shows an embodiment of the second invention.

As is clear from Table 2, in the case of the examples of the present invention, in addition to high strength of 80 kg/ran2 or more, -60
It was possible to produce a thick steel plate with extremely good weld cracking resistance, which exhibits high toughness of at least .degree. Among others, Examples C, D,
E corresponds to the second invention and is Cr, V, Cu, Ca
Containing one or more alloying elements, Example A
, H exhibits higher strength and toughness, and also has better weld cracking resistance.

Examples F and G show comparative examples, and Example F does not contain Ti or B and is DQ.
Although it is T-treated, it has low strength and poor toughness for 80kg steel.

Example G is a steel that has a high C content (no Ti) and has been subjected to the conventional offline quenching and tempering heat treatment, and has satisfactory strength as an 80kg steel, but has poor low-temperature toughness. The crack stop temperature is 100°C, making it a material with poor weld cracking resistance.

In summary, the present invention provides low C-Nb-Mo-Ti-B
Using steel, the process of high-temperature heating, high-temperature rolling, online quenching, and tempering was used, and in particular, the heating-rolling-online heat treatment from high-temperature heating to tempering maximized the precipitation hardening of Nb. Conventional 8
Ni, which was essential for 0 kg class steel, is completely omitted and the N
In addition to b, only a small amount of alloying is required, and as for heat treatment, offline quenching heat treatment can be omitted, resulting in the effect that high-toughness, high-strength steel can be produced at an extremely low cost.

Claims (2)

[Claims] (1) C: 0.04~0.10chi + Si: 1
．． Less than 0%. Mn: 0.50-2. O0%, Mo: O,1~
1.0%. T I: O-005~0.05%, Nb: 0.
005-0.05%. B: 0.01% or less, AL: 0.1% or less. Steel with balance Fe and unavoidable impurities and heating temperature 1
AC
A method for producing high-toughness, high-strength steel, characterized by subjecting it to heat treatment for tempering at one point or less. (2) C: 0.04-0.10%, Si: 1.0%
below. Mn: 0.50-2.00%, Mo: 0.1
~1.0%. Ti: 0.005 to 0.05 Ti, Nb: 0.005
~0.05%. B: 0.01% or less, Az: o, i% or less. and Cr: i% or less, V: O, 1% or less. Cu: 1% or less, Ca: 0.01% or less. A steel containing one or more of the following, with the remainder being Fe and unavoidable impurities, is heated and rolled at a heating temperature of 1150°C or higher and a rolling end temperature of 850°C or higher, and immediately after rolling is rapidly cooled to 200°C or lower. A method for producing high-toughness, high-strength steel, characterized by performing heat treatment for tempering at an Ae of 1 point or less.