JPS59182915A - Production of high tensile steel - Google Patents

Abstract

PURPOSE:To obtain a low-Mn high-tensile steel having no strain aging embrittlement by subjecting a steel having a prescribed component compsn. to hot rolling at a specific finishing temp. after heating then cooling the steel at the regulated average cooling rate in prescribed temp. range and specifying the temp. decreasing condition from said temp. down to room temp. CONSTITUTION:A steel consisting, by weight, of 0.1-0.2% C, <=0.5% Si, 0.6- 1.1% Mn, 0.01-0.08% sol.Al, and <=0.0030% N and the balance Fe and unavoidable impurities is prepd. The steel is heated to 900-1,200 deg.C to accomplish substantially formation of gamma iron and uniform solutionization of carbide and thereafter the steel is subjected to hot rolling at the finishing temp. of 900- 750 deg.C to improve toughness. The above-described steel after the hot-rolling is quickly cooled at 10-60 deg.C/sec average cooling rate in 800-500 deg.C temp. range to about <=300 deg.C. The steel after the quick cooling is cooled by taking >=5min in 300-100 deg.C temp. range to precipitate solid solution of C and solid solution of N and thereafter the temp. is decreased down to room temp., by which the intended low-Mn high-tensile steel is obtd.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for manufacturing low-grade high-strength steel that does not cause strain-age embrittlement.

Conventionally, it has been thought that in At-killed steel, the solid solute N, which is the main cause of aging, becomes AtN and precipitates sufficiently, so that aging does not occur. At-killed steel was widely used in various fields without doing so.

However, in recent years, At-killed high-strength steel has been developed that is used without heat treatment, with the resulting ferrite-bainite structure maintained by water cooling after hot rolling, and produced by controlled rolling. It has been put to practical use mainly in thick plate materials for shipbuilding, as it has a higher strength than other materials, and the demand for it is also increasing. It has become increasingly common to receive information that there may be a significant deterioration in toughness.

Therefore, the present inventors conducted research to investigate the cause of toughness deterioration that occurs in At-killed steel products produced by hot-rolling, drawing, and cooling, and found that even with At-killed steel, rapid cooling If this is done, solid solution C and solid solution N will not completely precipitate during cooling, and will remain in solid solution, especially in the ferrite region, causing strain aging. They came to the conclusion that strain concentrates and particularly deteriorates the toughness of that part. It was also found that this strain aging is particularly large in low Mn materials.

From the above-mentioned viewpoint, the present inventors continued research to produce low-grade high-strength steel that does not undergo strain aging embrittlement at a low cost and with high efficiency. As a result, the inventors conducted rapid cooling after hot rolling. When manufacturing high-strength steel using steel, the N content in the steel is kept low, and quenching is stopped at around 300°C to 250°C.
When cooled between 00 and 100 degrees Celsius, preferably between 300 and 220 degrees Celsius over 5 minutes or more, supersaturated N and C are sufficiently precipitated, resulting in non-aging properties similar to ordinary At-killed steel. We also found that a steel material with mechanical properties comparable to those of conventional quenched materials can be obtained, and furthermore, the bond toughness during high heat input welding of steel in the low Mn range can be improved by reducing the N content of the steel. It was also confirmed that there was a significant improvement.

This improvement in bond toughness during high heat input welding is particularly desirable for shipbuilding plates where high heat input welding is frequently performed.

This invention was made based on the above findings, and includes c: 0.1 to 0.2% (hereinafter, "ch" is weight percentage), St: 0.5% or less.

Mn: 0.6-1.1%.

sol, At: 0.01-0.08%.

N: 0.0030 or less.

and, if necessary, further include V: 0.0
8% or less.

Ti: 0.08 inch or less.

Nb: 0.08% or less.

B: 0.0 s% or less.

After heating a steel having a composition consisting of one or more of the following, Fe and unavoidable impurities: the remainder to 900 to 1200 °C,
Finishing temperature: Hot rolling at 900-750°C, followed by an average cooling rate of 10-60°C/800-500°C.
By carrying out rapid cooling for 1 lec to a temperature of 300°C, and then cooling between 300 and 100°C over 5 minutes to lower the temperature to room temperature, strain aging embrittlement does not occur and large This method is characterized by the ability to efficiently and reliably manufacture high-strength steel with excellent bond toughness during heat input welding.

Next, in the method for producing high-strength steel of the present invention, the reason why the composition of the target steel and the heating, rolling, and cooling conditions are limited as described above will be explained.

A) Composition (a) C The C component has the effect of ensuring the strength of steel, and is included in a predetermined amount in relation to the amount of Mn, which has the same effect. If it is less than 11, sufficient strength of the steel in the low Mn region cannot be ensured, and on the other hand, if it is 0.2
If the content exceeds 0%, the strength will increase excessively and the toughness of the base material will deteriorate.
．． It was set at 1% to 0.2%.

(b) 5i The Si component is added for various purposes such as deoxidizing steel and adjusting the strength of the base metal, but if it is contained in excess of 0.50%, it may cause deterioration of the toughness of the base metal. Therefore, its content was set at 0.5% or less.

(c) Mn The Mn component has the effect of improving the strength and toughness of the base material, but if its content is less than 0.6%, the desired effect cannot be obtained.

On the other hand, steel with a mineralization content of more than 1.1% has low aging, and even if the treatment of the present invention is applied, it is not possible to obtain an effect commensurate with the cost. ．．
It was set as 1ch.

Figure 1 shows C: 0.17%, Si: 0.25%
, Mn: 0.63%, sol, At: 0.043%
Fig. 1 is a diagram showing the relationship between the 3% strain aging effect and the Mn content when steel containing Mn is water-cooled to room temperature after rolling. It is clear that the amount of embrittlement after 3% strain aging is large. The strain aging effect is calculated by the difference between the fracture surface transition temperature in the Charpy impact test before strain aging and the fracture surface transition temperature after strain aging, that is, ΔvTs = vTs (before strain aging) - vTs (after strain aging). It was expressed as ΔvTs.

(d) sol, At 5o1. The At component is used as a deoxidizing agent and also has the effect of fixing N in steel as AtN, but if its content is less than 0.01%, the desired effect on the N fixation effect is not achieved. On the other hand, if the content exceeds O,OS, the cleanliness of the steel will decrease, so the content was set at 0.01 to 0.08.

(e) N If N is contained in excess of 0.03%, as is clear from Figure 1, the strain aging properties of the base metal will increase and at the same time the toughness of the weld bond will deteriorate. amount to 0
．． 0.003% or less.

Figure 2 is a diagram examining the effect of N content on high heat input welding bond toughness for the same steel as in Figure 1, but the N content should be 0.003% or less. It can be seen that the weld bond toughness is extremely good. Note that high Mn copper exhibits good weld bond toughness even if the N content is increased to some extent, but this results in a disadvantage of increased cost.

Therefore, it can be seen from FIGS. 1 and 2 that by using a low Mn-low N steel, it is possible to obtain a steel material that has good base material performance and bond toughness and is also inexpensive.

(f) V, Ti, Nb, and B These components each have a carbide- or nitride-forming action, and fix solid solution C and N in steel to improve the strength of the base metal, so they may be added as necessary. One or more types are added,
If each content exceeds 0.08%, it tends to deteriorate the toughness of the base metal and has a negative effect on the weld bond toughness, so the content of each is determined to be 0.08% or less. In addition, for ■, it is better to limit the content to 0.075% or less, and for Ti, Nb, and B, it is better to limit it to 0.07% or less, 0.04% or less, and 0.004% or less, respectively. preferable.

B) Heating temperature If the heating temperature is less than 900°C, the γ-hardening of the steel and the solid solution homogenization of carbides will be insufficient, and on the other hand, even if the steel is heated above 1200°C, there will be no change in the γ-hardening effect. However, since the R grains become too large and deteriorate the toughness of the base material, the temperature was set at 900 to 1200°C.

C) Rolling Finishing Temperature If the finishing temperature exceeds 900°C, the effect of processing is small and no improvement in toughness can be expected through processing.
On the other hand, if rolling is completed at a temperature below 750°C, the processing is too strong and the influence of α+r2 phase region rolling appears, which in turn deteriorates the toughness. Established.

D) Average cooling rate between 800 and 500°C: 800-500
The average cooling rate between ℃ is 10℃/l! If it is less than ec, the desired strength of the steel material cannot be secured;
If the cooling rate exceeds ℃/sec, the base metal will harden too much and the toughness will deteriorate. 8o of steel material water-cooled to 300°C and then cooled between 300 and 220°C for 8 minutes to promote precipitation.

Although it is a diagram showing the relationship between the average cooling rate and mechanical properties between ~500°C, it is also clear from FIG.
0-b It is obvious that

E) Quenching stop temperature If the quenching stop temperature such as water cooling is higher than 300℃, the strengthening effect of the steel will be small and it will be difficult to secure the desired strength. We decided to do this. Of course, the temperature of the rapid cooling may be up to 300° C., and care should be taken because if the temperature is much lower than that, it will be difficult to fix solid solution C and solid solution N in the next step.

F) Cooling time between 300 and 100°C After rapid cooling following hot rolling, the process of cooling between 300 and 100°C over 5 minutes or more is performed to precipitate solid solution C and solid solution N. However, if slow cooling is performed from a temperature exceeding 300°C, precipitation will occur easily, but the strength of the steel will be greatly reduced, while if the temperature is below 100°C, precipitation will not occur sufficiently. If the temperature is not kept within this temperature range for 5 minutes or more, sufficient precipitation will not occur and strain aging will remain, so it was decided that cooling should be carried out between 300 and 100°C over 5 minutes or more. In addition, the slow cooling temperature range is
Preferably, the temperature is 300 to 220°C. This is because the degree of precipitation decreases rapidly below 220°C, and around 200°C is the temperature range of so-called blue brittleness.
This is because precipitation aging embrittlement is large. Therefore, 20
A large amount of solid solution N is precipitated before the temperature drops to around 0℃,
It is recommended to prevent precipitation aging embrittlement caused by N.

Figure 4 shows the 3% strain aging effect and solid solution C,N when water-cooled to room temperature after hot rolling for the same steel as in Figure 1.
Relationship between precipitation treatment temperatures (after water cooling to room temperature, 10% at each temperature)
This is a diagram showing the relationship between temperature and ΔvTs when heated and held for minutes.
It can be seen that treatment between 100°C and 300°C and 220°C is effective in preventing aging. In addition, with high N steel, a sufficient effect cannot be obtained even after holding for about 10 minutes, and the
It is predicted that the process will require 0 minutes or more.

Figure 5 is a diagram showing the effect of slow cooling (aging treatment) between 300 and 100°C on 3% strain aging of a similar steel that was hot rolled and water cooled to 300°C. In the case of cooling, it is clear that it is necessary to cool down for 5 minutes or more.

Next, the present invention will be explained with reference to examples and in comparison with comparative steel.

Example First, steel A-T having the chemical composition shown in Table 1 was melted by a melting method that combines the converter steelmaking method and the vacuum degassing method (RH method), and N was absorbed from the air. Please do not
A slab with a thickness of 150 cm was obtained by continuous casting while always shielding with an inert gas such as.

Subsequently, each of these was heated to the temperatures shown in Table 2, and then hot rolled under the conditions shown in Table 2 to obtain a finished plate thickness of 30 to 20 m (however, test number 2 was
5 m), and immediately water-cooled to 300°C (excluding test numbers 21 and 22). 800 at this time
The average cooling rate between ~500°C and 30~b was as shown in Table 2, although there were some differences depending on the plate thickness.

Following this, for test numbers 1-20, temperature range = 3
After slowly cooling from 00 to 100°C for 20 minutes (however, about 7 minutes from 300 to 220°C), it was allowed to cool in the atmosphere. Test numbers 21 and 22 were at 450°C and 100°C, respectively.
After cooling with water until the temperature reached 100 degrees, it was left to cool in the air.

The mechanical properties of the copper material thus obtained were investigated, and the results are also shown in Table 2. Well, the welding is single-sided SA.
The W method was used at a heat input of 170,000 to 230,000 J/cmO.

As is clear from the results shown in Table 2, the high tensile strength steels obtained by methods 1 to 15 of the present invention have a Δ
vTs is also small at less than 20℃, and tensile strength is 50kg.
In addition to exhibiting excellent properties such as f/W" or higher and VT8 of less than -40°C, the toughness of the weld bond is high with an impact energy absorption value (v E-20) of more than 7 kr-m at -20°C. On the other hand, the steel obtained by the comparative method, Test No. 16, has a large ΔvTs due to high N, and the steel obtained by Test No. 17, which is a comparative method, shows good performance due to the high Mn steel, but it has poor performance compared to the low Mn steel. This results in an approximately 20% increase in cost compared to test number 18, and the test number 18 has a too low Mn content and is inferior in strength and toughness.
Because the C content is low, the tensile strength is 50 kgf.
/simple 2, and it can be seen that the one according to test number 20 has an excessively high C content and is therefore inferior in toughness.

The steel materials obtained by the methods of test numbers 8 to 15 were added with trace elements of V, Ti, Nb, and B, and although there was a slight decrease in the toughness of the weld bond, there was a significant increase in the toughness. It can be seen that the toughness of the base material is also improved.

As described above, according to the present invention, it is possible to easily obtain a low-cost, low-Mn high-strength steel that has high manufacturing efficiency and does not cause strain-age embrittlement, and the safety of welded structures is further improved. This brings about industrially useful effects such as:

[Brief explanation of drawings]

Figure 1 is a diagram showing the relationship between 3% strain aging effect and Mn content, Figure 2 is a diagram showing the influence of N content on high heat input welding bond toughness, and Figure @3 is a diagram showing the relationship between 3% strain aging effect and Mn content. A diagram showing the relationship between the average cooling rate and mechanical properties at 500℃, Figure 4 is 3%
FIG. 5 is a diagram showing the relationship between the strain aging effect and the precipitation treatment temperature of solid solution C1N, and FIG. 5 is a diagram showing the influence of the cooling time between 300 and 100° C. on 3% strain aging. Applicant Sumitomo Metal Industries Co., Ltd. Agent Tomi 1) Kazuo and 1 other person Academic 1 Figure 2 0 0.00IQ, 002Q, 0O30,0040QO
5QOO6Q, 007N01 (11%)

Claims (1)

[Claims] (1) c: 0.1-0.2%. St: o, 5% or less. Mn: 0.6-1.1%. sol, At: 0.01-0.08%. N: 0.0030% or less. Contains Fe and unavoidable impurities: remainder. Steel with a composition (more than 900 to 120% by weight) consisting of
After heating to 0°C, hot rolling is performed at a finishing temperature of 900 to 750°C, followed by an average cooling rate of 1 between 800 and 500°C.
A high-temperature method characterized by rapidly cooling from 0 to 609C/se until the temperature reaches 300C or less, then cooling from 300 to 100C over 5 minutes, and cooling at 1 - to room temperature. Method of manufacturing tension steel. (21C: 0.1-0.2%. sl: 0.5% or less. Mn: 0-6-1-1%. 5ol-At: 0.01-0-08%. N: 0.0030% Contains the following, and also contains one or more of the following: V: o, o s% or less; Ti: o, □ 3% or less; Nb: 0.08% or less; B: 0.08% or less. Contains Fe and unavoidable impurities: remaining, steel with a composition (more than Vi content%) of 900 to 12
After heating to 00°C, hot rolling is performed at a finishing temperature of 900 to 750°C, followed by rapid cooling at an average cooling rate of lθ to 60°C between 800 and 500°C until the temperature reaches 300°C or less. A method for manufacturing high-strength steel, which comprises cooling from 300 to 100°C over 5 minutes or more to lower the temperature to room temperature.