JPS6013029A - Production of high-tension steel bar material - Google Patents

Abstract

PURPOSE:To prevent deterioration in breaking strength occuring in warm working by hot rolling a specifically composed steel to a bar shape and subjecting the bar to hardening then tempering thereby producing a high-tension steel bar material to be used with application of warm working. CONSTITUTION:A steel contg. 0.20-0.50% C, 0.5-2.0% Si, 0.9-2.0% Mn, 0.10- 0.60% Cr, <0.005% B and 0.010-0.10% Al or Ti and having 0.85+ or -0.35 value of (Si)/(Mn) is used as a blank material for a steel bar such as PC steel bar which is subjected to a temper heat treatment such as hardening and tempering after hot rolling and is used after warm working. The billet of such steel is hot rolled to a bar shape and is tempered at >=280 deg.C after hardening and thereafter the steel bar is subjected to warm working such as heading. The high-tension steel bar is obtd. without decrease in the breaking strength in the warm worked part.

Description

Detailed Description of the Invention Technical Field High-strength steel bars, of which PC steel bars are a typical example, are generally hot-rolled and then subjected to tempering heat treatment, that is, quenching and tempering, and then used to construct concrete structures using the above-mentioned PC steel bars. Warm processing (including sub-hot processing), a typical example of which is heading, which facilitates the introduction of prestress into objects.
It is often put to use after doing so.

Rapid heating such as high-frequency induction heating is usually performed for the ninth heating in such warm processing.

Therefore, the following technical details related to the effective avoidance of such deterioration of bar performance in warm-worked parts are based on the composition adjustment of high-strength steel bars. Aiming at improving the tempered structure, it is generally located in the field of technology that includes methods for producing high-strength steel bars, of which PC steel bars are a typical example.

Problems: As mentioned above, this type of steel bar material is used, for example, as a PC steel bar in the production of PO piles, etc., and usually has a tensile strength of 14+5/+g/;,- or more.

However, in manufacturing PC piles, the PC steel bars are combined with auxiliary reinforcing bars and woven into reinforcing bars by spot welding, etc., but prior to that, heading processing is often applied to the ends of the steel bars, and a large number of PC steel bars are assembled. It is provided for the convenience of fixing the end layer to the end layer.

If the head section made in this way were to break under the load for introducing prestress into the steel bar, there is a risk that fragments of the steel bar head section would fly off and cause a serious accident. At 10,000 yen, it is required to have sufficient strength at the end of the steel bar, and the conditions of this heading processing, that is, sub-hot processing, are particularly permissible over a wide temperature range, and the strength is maintained stably. needs to be expected.

In general, this raw steel bar material (30J5% by weight (hereinafter referred to simply as ``chi''), containing about 5% Mno, s%, and subjected to quenching and tempering treatment has been widely used. In addition to elongation after spot welding of 75ζ, In particular, problems remain in terms of the narrow heating temperature range for ladder processing in order to achieve sufficient strength in the header section. As the heating means, short-time heating by high frequency is usually used, and the high frequency output and heating time are adjusted.The strength of the head formed here and its fracture form are greatly influenced by the heating conditions.

Therefore, conventionally, due to variations in the header itself depending on the lot of steel bars, or in some cases even within the same lot, there was a disadvantage in that it was necessary to change the high frequency output and heating time settings in order to provide sufficient head strength.

Therefore, the header processing conditions, that is, the heating conditions, can be selected from a wide range, and the advantages of achieving sufficient head strength are technically extremely large.

Now, according to the results of the fracture test (first drawing) on the head part, the fracture forms can be roughly divided into five types as shown in Figure 2.

In (a), the fracture occurred in a place not affected by the ladder force, and in this case no strength reduction occurred due to the ladder processing.

(b) , (Creation is caused by heating during header processing,
It is thought that the head portion or the lower part of the head was softened to a large extent, and the strength often decreased.

In cases (d) and (e), it is thought that the inside of the head or the lower part of the head has become brittle or cracked due to header processing, and the strength may be significantly reduced.

As a way to deal with this problem, we first proposed a means to improve warm workability and increase temper softening resistance. 55-11726 Publication
I made a suggestion.

However, according to that disclosure, the fracture mode (t)), (C+
) It is effective in preventing softening of the lower part of the head during header processing, but if the heating time during header processing deviates from short to long, it often causes damage to the inside of the head or the lower part of the head. Fracture mode due to embrittlement (C
I), (occurred at 133') and could not be completely suppressed.

The origin of the ideaThe inventors investigated various causes of such embrittlement and found that the cause was due to a striped structure based on the micro-segregation of Si and Mn among the material components.
) By controlling the ratio, the striped structure can be eliminated, and P
It was possible to completely prevent the embrittlement of the lower part of the head of the C steel bar.

Purpose of the Invention Based on the above findings, the performance of high-strength steel bars derived from the warm processing of high-strength steel bars that are subjected to hot rolling, temper heat treatment, and then warm working is improved. It is an object of this invention to avoid deterioration of.

Composition of the invention The above objectives will be effectively achieved through the following:

As a high-strength steel bar material that undergoes hot rolling, complete processing heat treatment, and then warm working, it is used as a 0:0
．． 20-0.50%, si: 0.5-2.0%.

In: 0.9-2.0% and Or: 0.10
~0.60% together with 0.005% or less of B or further 0.010 to 0.10 of AA and/or T1, and its 81 and Mn content'c (Sill, (In) Ratio of both T hail (
”)/(Mn) = (1.85 ± 0.85 By hot rolling, quenching, and then tempering at a temperature of 280° or higher, Production of a high-strength steel bar that does not contain a striped structure, has excellent warm workability, and can satisfactorily improve the spot weldability relaxation value based on microsegregation of Si and Mn during subsequent warm working. Law.

The molten steel with the above-mentioned composition adjustment is made into a bar by hot rolling after ingot formation or continuous casting according to the conventional method, followed by quenching to rapidly cool from the austenitic region to 280°C.
By applying tempering heat treatment to temper at the above temperature, the head part can be used as a prestressing steel bar, for example, regardless of the heating conditions of sub-hot processing or temperature fluctuations. It has been confirmed that the strength of the bar is not impaired, and embrittlement of the lower part of the head is completely prevented.In addition, it has been confirmed that the bar has excellent spot weldability and relaxation values, which are required for bar materials. .

The reasons for limiting the components in the steel are as follows.

0: U, 2 to 0.5% It is related to the tensile strength after Ofd tempering. If it is less than 0.2 inch, it will be difficult to achieve the required tensile strength by tempering at 280"C or more, and If it exceeds 0.5 inches, the elongation under welding conditions that provide sufficient welding strength will deteriorate significantly, so a range of 0.2 to 0.5% is specified.

Here, the above [8i]/[
By effectively controlling the Mn ratio and preventing embrittlement, the upper limit of C was regulated at 0.3% in Toyo Tokuko Publication No. 11726/1980, but the upper limit of C was regulated at 0.5%. It can be expanded up to.

Si: 0.5 to 2.0% As for Sl, (Sl)l(In)= which is essential to suppress the embrittlement of the lower part of the head during the herating process.
In order to satisfy the condition of 0.85±0.35, the minimum value is 0.
．． 45% is the lower limit of 0.5, allowing for a slight margin.
Let's do it. Further, since Si has characteristics as a metalloid, increasing the amount added tends to impair the toughness of the material, so the upper limit was set at 2.0 inches.

Mn: 0.9-2.0% In addition to its normal deoxidizing and hardening effects, Mn is useful for improving spot weldability, and has higher properties in the range of 0.9-2.0% than conventional regular steel materials. Set on the eyes.

Or: 0.1-0.6% Or is an effective component for improving workability in sub-hot ladder processing and maintaining stable head strength under a wide range of heating conditions. ,! If the content is less than 0.1%, it will not be effective in expanding the range of heating conditions for header processing, and if the content exceeds 0.6%, no enhancement of the effect will be expected and the economy will be poor. It was limited to a range of 0.1 to 0.6%.

For the purpose of improving hardenability, B is added at 0.0050% or less, and if necessary, 0.01% is added to ensure effective B.
Add 0 to 0.100 Ti and/or A71.

Note that when the tempering temperature for tempering heat treatment is lower than 280°C, it is difficult to obtain a good relaxation y value.
Therefore, a temperature of 280°C or higher is required.

Examples Conventional products include low Si and low In materials (A), CB)
, high Si low Mn materials (0), CD) and (E) and low Si.

High Mn materials (F, G, H)'e as comparative examples, whereas Examples (1), (J) and (K) according to the present invention
The results of the head fracture test conducted on the head are shown below.

Table 1 lists the components of the test materials, and FIG. 3 shows the results of a fracture test on the head of a steel bar, which was subjected to header processing while keeping the high-frequency heating power constant and varying the heating time.

In the comparative example, the breaking strength decreased when the heating time exceeded 6 seconds, whereas in the example, there was almost no decrease in strength.

Next, we separated the symbols for fractures with only mode (a) and those that included other fracture modes, and calculated the relationship between the [81)/(Kn) ratio and the fracture strength at the heating time with the greatest heat load during header processing. The time period of 7.5 seconds is shown in Figure 4.

As is clear from the figure, the breaking strength after heating for 7.5 seconds is (
The best value is obtained when the si)/(Mn) ratio is around 0.85, and the breaking strength decreases significantly when the reading ratio is smaller than 0.5 or larger than 1 or 2.

Regarding the fracture morphology, =)/(In) ratios of less than 0.5 and greater than 1.2 indicate fractures other than morphology (Q).

Therefore, the range of (S”/(In) ratio is 0.85±
The need to adjust between 0.85 and 0.85 is obvious.

This is because there is an appropriate balance between solute atomic weight and carbon solubility to eliminate the striped structure, and (si)/(Mn)
The ratio 0.85 indicates this balance amount ratio.

Next, we will show the glue laxation value at high temperatures when curing PC piles etc. in an autoclave.

Test conditions were maintained at 180° C. for 8 hours.

Table 2 shows the relaxation values.

Table 2 Relaxation 7 value (180°X 3 Hr retention The relationship between the tempering temperature and the relaxation m (here, the value at an initial load of 104 kg/2 and room temperature for 10 hours) is calculated.

The relaxation value increases rapidly at tempering temperatures below 280°C. From this, the tempering temperature is 280"
It is particularly advantageous to have a value of C or higher.

Effects of the Invention According to the present invention, when a high-tensile steel bar that has been subjected to hot working, quenching, and tempering is subjected to warm working during use, the properties of the bar deteriorate due to the working. Since these causes have been eliminated by appropriate compositional adjustment of the steel material, the reliability of the properties in this type of use is increased and this makes a significant contribution to the increase in its use.

[Brief explanation of drawings]

Fig. 1 is an explanatory diagram of the fracture test procedure for the head section, Fig. 2 is a schematic diagram of the fracture form, Fig. 3 is a graph showing the influence of warm processing heating time on the fracture strength, and Fig. 5 is the tempering temperature. and relaxation relationship graph. Patent applicant: Kawasaki Steel Corporation Applicant: Kawasaki Steel Wire Industry Co., Ltd.

Claims (1)

[Scope of Claims] 1. As a high tensile strength steel material that is subjected to hot rolling, tempering heat treatment, and then warm working, a: 0.20 to 0.50 weight-i Si: 0.5 to 2.0% by weight In: 0.9 to 2.0% by weight and Or:
0.10 to 0.60% by weight, 0.005% by weight or less of B, or even 0.0% by weight
10 to 0.100 Li content of molten steel containing AI and/or Ti is melted, and the sl and Mn contents are (Si) and (Mn).
3i)/(H,) = 0.85±0.85 using steel whose composition is adjusted to satisfy the condition, hot rolled, quenched, and then tempered at a temperature of 280-0 or higher. Method for producing high tensile strength bar material characterized by -0