JPS63145711A - Production of high tension steel plate having excellent low temperature toughness - Google Patents

Abstract

PURPOSE:To obtain high tension steel plate for structure having excellent low temp. toughness, by adding a little qualities of Cr, Mo and Nb, and very little quality of Ti, hot-rolling after heating the steel slab specifying the welding crack sensibility index at low temp. and then quenching and tempering under the specific condition. CONSTITUTION:The steel slab containing by wt% of 0.02-0.08% C, 0.05-0.80% Si, 1.00-1.60% Mn, 0.01-0.08% Al, 0.30-1.50% Cr, 0.10-1.0% Mo, 0.010-0.080% Nb, 0.005-0.025% Ti and the remaining part of Fe and inevitable impurities, and <=0.21% the welding crack sensibility index PCM shown in the equation is heated at <=1,000 deg.C and hot-rolled. Next, after rapid cooling it from >=800 deg.C to <=200 deg.C, the tempering treatment is executed at the temp. of <=Ac1 point. In the steel having the above composition, in case of need, <=1% Cu, <=2% Ni, <=0.1% V, <=0.01% B, etc., is added. In this way, the steel plate for using suitably welding structure having excellent low temp. toughness and >=80 kgf/mm<2> tensile strength is obtd.

Description

[Detailed description of the invention] The present invention has a tensile strength of 80 kgf/mm or more, excellent low-temperature toughness, and is used for welded structures in pressure vessels, bridges, construction machinery, etc. The present invention relates to a method for manufacturing a high-strength steel plate that can be suitably used as a steel plate.

Conventionally, high-strength steel sheets with a tensile strength of 80 kgf/mn+" or more have been manufactured by a quenching and tempering method, but in order to ensure the required strength and toughness, Ni, C' % M
Therefore, such steel sheets have a high sensitivity to cold cracking, and when welding requires high-temperature preheating during welding, there is a significant decrease in welding efficiency. I'm forced to.

Therefore, as one method for improving the weldability of such steel sheets, Japanese Patent Application Laid-Open No. 61-3833 describes Nb-
By significantly increasing the hardenability of Mo and B by solid solution Nb through precipitation hardening by Mo-based precipitates and by reducing Nb, it is possible to reduce the amount of alloying elements added and, by extension, to improve weldability. is proposed. However, according to this method, in order to ensure 1l of solid solution Nb, 1000
Since it is necessary to heat the slab at a high temperature of ℃ or higher, it causes coarsening of the austenite and, together with precipitation hardening during tempering, causes the problem of deterioration of low-temperature toughness, especially for thick-walled materials. There is a strong demand for improvement.

The source ■ tries to do FX. As a result of intensive research to solve the above-mentioned problems in the production of high-strength steel sheets of 80 kgf/mm" or higher, the present inventors added appropriate amounts of Nb, Cr, and MO, and
By applying slab low-temperature heating and direct quenching to a steel made by adding a small amount of Ti, the
The present invention was achieved by discovering that it is possible to obtain a high-strength steel plate with excellent low-temperature toughness and having a strength of 100° C. or less and a fracture surface transition temperature of 160° C. or less.

Therefore, the present invention has a tensile strength of 80 with excellent low temperature toughness.
The object of the present invention is to provide a method for manufacturing a high tensile strength steel plate with a tensile strength of 1 kgf/mm or more.

Measures to solve the problem Tensile strength of 80 kgf/excellent low temperature toughness according to the present invention
The manufacturing method for high-strength steel sheets of mm'' or more is as follows: C0.02-0.08%, 3i 0.05-0.80%, Mn 1.OO-1.60%, A7!o, 01-0 in weight%. .08% Cr 0.30-1.50%, Mo 0.10-1°0%, Nb 0.010-0.080%, Ti 0.005-0.025%, balance iron and inevitable impurities, And, the steel slab whose weld cracking susceptibility index PCM expressed by is 0.21% or less is 100%.
Hot rolled by heating to a temperature of 0°C or less, then 800°C
After rapidly cooling from a temperature above ℃ to a temperature below 200℃,
Ac, characterized by tempering at a desired temperature.

According to the present invention, for steel whose weld cracking susceptibility index PCM is lowered by adding small amounts of Cr and MO, a trace amount of T
By adding i and a small amount of Nb, heating the steel slab at low temperature, hot rolling, and quenching and tempering it under specified conditions, the remarkable strength-improving effect of Nb results in low PCM.
In addition to ensuring a strength of 80 kgf/mn+" or more for steel, it also prevents the deterioration of toughness that decreases as Nbt increases, and thus has a tensile strength of 80 kgf/mm+" or more and quickly improves low-temperature toughness. It is possible to obtain a high tensile strength steel plate.

First, the effect of adding Nb and Tt and the effect of low-temperature heating of the steel slab will be explained.

Regarding steel whose basic components are C0.04%, Si0.25%, Mn 1.45%, Cr0.75%, Mo0.35%, and A10.03%, Ti was replaced with O-
0.015% and Nb added in the range of O to 0.06% are melted, heated to 980 to 1100°C, and then rolled to a thickness of 5011 at a temperature of 850°C. ,
Thereafter, steel plates were produced by direct quenching at a temperature of 800°C, and their strength and toughness were measured.

As shown in Figure 1, the results show that N
b-h<80 with remarkable effect, PcH value 0.18%
It is possible to secure a strength of 1 kgf/mm or more.

On the other hand, low-temperature toughness deteriorates as Nbl increases, but low-temperature toughness can be significantly improved by heating the steel slab to g-temperature and adding a small amount of Ti.

The method according to the present invention was completed based on this new effect, and this effect is due to the refinement of austenite grains during slab heating and subsequent rolling.
This appears to be due to the improvement in the toughness of the soil due to the fixation of solid solution N.

Next, the chemical composition of steel in the method of the present invention will be explained.

In order to ensure a tensile strength of 80 kgf/mm" or more, it is necessary to add C in an amount of 0.02% or more. However, when the amount added exceeds 0.08%,
It becomes difficult to secure a sufficient amount of solid solution Nb when heating the slab, and it also increases the hardenability of the weld heat-affected zone and deteriorates weldability.

St is an element necessary for deoxidation, and for this purpose it is necessary to add at least 0.05%. However, 0.8
If added in excess of 0%, not only will the above effects be saturated, but the weldability will deteriorate, so the amount added is 0.80%.
% or less.

In order to ensure the required strength, Mn should be contained in a small amount (both 1.
It is necessary to add 0.00% or more. However, addition of an excessive amount exceeding 1.60% impairs weldability and causes internal defects.

AI also needs to be added at least 0.01% for deoxidation, but even if it is added in excess, the effect will be saturated and
Since it deteriorates cleanliness, the upper limit of the amount added is set at 0.08%.

Cr needs to be added in an amount of 0.30% or more in order to obtain the required strength. However, when added in excess of 1.50%, weldability is inhibited.

MO also requires addition of 0.10% or more to obtain the required strength. However, when added in excess of 1.0%, weldability deteriorates.

Nb has the effects of improving hardenability and strengthening the precipitation described above, and in the present invention, in order to effectively obtain these effects, it is necessary to add 0.010% or more. However, when added in excess, low-temperature toughness is deteriorated, so the amount added should be within the range of o, oso% or less.

As mentioned above, Ti is an essential element in the present invention for improving low-temperature toughness, and in order to effectively obtain the above effect, it is necessary to add 0.005% or more. However, adding too much will not only saturate the effect, but will also impede low-temperature toughness, so the upper limit of the amount added can be set to 0.
．． 025%.

Furthermore, in the present invention, it is necessary to set the PCM value to 0.21% or less so that weldability is excellent.

In the present invention, the steel sheet contains, in addition to the above-mentioned elements, at least one element selected from the group consisting of Cu 1% or less, Ni 2% or less, V 0.1% or less, and B 0.01% or less. can contain.

Cu is added to improve hardening.

In order to effectively obtain this effect, it is preferable to add 0.10% or more. However, when the amount added exceeds 1%,
Makes the steel plate surface more likely to crack.

Ni is also added in an amount of 0.1% to improve hardening, but since Ni is an expensive element, the upper limit of the amount added is 2%, mainly from the economic point of view.

■ is also preferably 0°01% in order to improve hardenability.
It is added in the above range. However, addition of an excessive amount exceeding 0.1% deteriorates weldability.

B has the effect of improving hardenability when added in a small amount, and in the present invention, B has the effect of improving hardenability when added in a small amount. It is preferable to add OOO3% or more. However, even if added in excess, the effect will be saturated, so it is added within a range of 0.01% or less.

Furthermore, in the present invention, the steel plate can contain Ca. Ca is effective in improving low-temperature toughness by controlling the morphology of sulfide-based inclusions, and is preferably 0.0005%.
It is added in the above range. However, when added in excess, harmful nonmetallic inclusions are generated, so the amount added is 0.
．． The range shall be 0.01% or less.

Next, manufacturing conditions in the method of the present invention will be explained.

According to the method of the present invention, a steel slab having the above-mentioned composition is heated to a temperature of 1000°C or less and hot rolled,
By rapidly cooling from a temperature of 00°C or higher to a temperature of 200°C or lower, and then tempering at a temperature below Ac, it is possible to obtain a high-strength steel plate with a tensile strength of 80 kgf/mm” or higher with excellent low-temperature toughness. can.

In the present invention, as mentioned above, in order to ensure low-temperature toughness, it is necessary to set the slab heating temperature to 1000°C or less. It is possible to achieve refinement of the austenite grains at the time and, by extension, of the crystal grains after direct quenching.

Although the rolling temperature is not particularly limited, it is preferably a high temperature in order to ensure the cooling start temperature, and from this point of view, the rolling end temperature is usually preferably 850° C. or higher.

In this quenching after hot rolling, the cooling start temperature is set at a lower limit temperature of 800°C in order to achieve quenching in the method of the present invention. cannot be obtained, so the temperature is set at 200°C or lower.

In the present invention, after direct quenching, tempering is performed to adjust the strength to the required level depending on the chemical composition and plate thickness. However, when this tempering temperature exceeds the Ac point, the hardening effect disappears and the strength decreases significantly, so in the present invention, the tempering temperature is set to Ac.

As described above, according to the present invention, the weld cracking susceptibility index PCM value obtained by adding a small amount of C and Mo is 0.21%.
By adding a small amount of Ti and a small amount of Nb to the following steel, heating the steel slab at a low temperature, and directly quenching and tempering it under predetermined conditions after hot rolling, Nb has a remarkable strength-improving effect. The deterioration of toughness, which decreases as the temperature increases, can be prevented by low-temperature heating and f&ff1T+, and in this way, a high-strength steel plate with a tensile strength of 80 kgf/mmt or more and excellent low-temperature toughness can be obtained.

EXAMPLES The present invention will be described below with reference to Examples, but the present invention is not limited to these Examples in any way.

The steel whose chemical composition is shown in Table 1 was heated and hot rolled under the conditions shown in Table 2, and then directly quenched and tempered to obtain a steel plate of a predetermined thickness. Table 2 shows the mechanical properties and Y-slit cracking prevention temperatures of these steel plates.

The steel plates of the present invention all have a tensile strength of 80 kgf/mm" or more, and a transition temperature of -60°C or less,
Excellent low temperature toughness. Further, the PCM value is 0°21% or less, and in the Y-slit weld cracking test, the preheating temperature is 25°C or less in all cases, so welding can be performed without preheating.

On the other hand, Comparative Steel Sheet I does not contain Nb and therefore does not have a strength of 80 kg4/mm'', and Comparative Steel Sheet J does not contain Ti and therefore has poor low-temperature toughness.On the other hand, The steel plate of the comparative example had a high steel slab heating temperature.
It is inferior in low temperature toughness, and since the comparative example steel plate has a low cooling start temperature during quenching, it is inferior in both strength and toughness.

Furthermore, the comparative example steel sheet M produced by the conventional quenching and tempering method has p
Since the cs value is high, weldability is poor.

[Brief explanation of the drawing]

Figure 1 shows 0.04χC-1,45χMn-0.75X
It is a graph showing the effect of addition of Ti and Nb and the effect of steel slab heating temperature for steel whose basic components are Cr-0,35χMo-0,03χAl. Patent Applicant Kobe Steel Co., Ltd. Representative Patent Attorney Itsu Makino Department (Figure 1, 11)

Claims (4)

[Claims] (1) C 0.02~0.08%, Si 0.05~0.80%, Mn 1.00~1.60%, Al 0.01~0.08% Cr 0.30~ 1.50%, Mo 0.10~1.0%, Nb 0.010~0.080%, Ti 0.005~0.025%, the balance consisting of iron and inevitable impurities, and
/30)+(Mn/20)+(Cu/20)+(Ni/
60) + (Cr/20) + (Mo/15) + (V/10
) + 5B (%) Weld cracking susceptibility index P_C
A steel slab with _M of 0.21% or less is heated to a temperature of 1000°C or less, hot rolled, then rapidly cooled from a temperature of 800°C or more to a temperature of 200°C or less, and then Ac_1
A method for producing a high-strength steel plate with excellent low-temperature toughness and a tensile strength of 80 kgf/mm^2 or more, characterized by tempering at a temperature below 100 lbs. (2) In weight% (a) C 0.02-0.08%, Si 0.05-0.80%, Mn 1.00-1.60%, Al 0.01-0.08% Cr 0 .30 to 1.50%, Mo 0.10 to 1.0%, Nb 0.010 to 0.080%, Ti 0.005 to 0.025%, and further (b)
Contains at least one element selected from the group consisting of Cu 1% or less, Ni 2% or less, V 0.1% or less, and B 0.01% or less, the balance consisting of iron and inevitable impurities, and C+(Si
/30)+(Mn/20)+(Cu/20)+(Ni/
60) + (Cr/20) + (Mo/15) + (V/10
) + 5B (%) Weld cracking susceptibility index P_C
A steel slab with _M of 0.21% or less is heated to a temperature of 1000°C or less, hot rolled, then rapidly cooled from a temperature of 800°C or more to a temperature of 200°C or less, and then Ac_1
A method for producing a high-strength steel plate with excellent low-temperature toughness and a tensile strength of 80 kgf/mm^2 or more, characterized by tempering at a temperature below 100 lbs. (3) C 0.02~0.08%, Si 0.05~0.80%, Mn 1.00~1.60%, Al 0.01~0.08% Cr 0.30~ 1.50%, Mo 0.10~1.0%, Nb 0.010~0.080%, Ti 0.005~0.025%, Ca 0.01% or less, the balance consisting of iron and inevitable impurities. , and C+(Si
/30)+(Mn/20)+(Cu/20)+(Ni/
60) + (Cr/20) + (Mo/15) + (V/10
) + 5B (%) Weld cracking susceptibility index P_C
A steel slab with _M of 0.21% or less is heated to a temperature of 1000°C or less, hot rolled, then rapidly cooled from a temperature of 800°C or more to a temperature of 200°C or less, and then Ac_1
A method for producing a high-strength steel plate with excellent low-temperature toughness and a tensile strength of 80 kgf/mm^2 or more, characterized by tempering at a temperature below 100 lbs. (4) In weight% (a) C 0.02-0.08%, Si 0.05-0.80%, Mn 1.00-1.60%, Al 0.01-0.08% Cr 0 .30 to 1.50%, Mo 0.10 to 1.0%, Nb 0.010 to 0.080%, Ti 0.005 to 0.025%, and further (b)
Contains at least one element selected from the group consisting of Cu 1% or less, Ni 2% or less, V 0.1% or less, and B 0.01% or less, and (c) Ca 0.01% or less, The balance consists of iron and unavoidable impurities, and C+(Si
/30)+(Mn/20)+(Cu/20)+(Ni/
60) + (Cr/20) + (Mo/15) + (V/10
) + 5B (%) Weld cracking susceptibility index P_C
A steel slab with _M of 0.21% or less is heated to a temperature of 1000°C or less, hot rolled, then rapidly cooled from a temperature of 800°C or more to a temperature of 200°C or less, and then Ac_1
A method for producing a high-strength steel plate with excellent low-temperature toughness and a tensile strength of 80 kgf/mm^2 or more, characterized by tempering at a temperature below 100 lbs.