JPS5852460A - High strength chromium steel with superior weathering resistance and weldability - Google Patents

Abstract

PURPOSE:To obtain a high strength Cr steel with superior weathering resistance and weldability by adding specified percentages of C, Si, Mn, Cr, Ni, Cu, N, O, Nb, V and Ti to Fe. CONSTITUTION:A Cr steel consisting of, by weight, <=0.08% C, <=1.0% Si, <=4.0% Mn, 10-13.5% Cr, <0.1% Ni, <=1.0% Cu, <=0.02% N, <=0.007% O, one or more among 0.005-<0.05% Nb, 0.02-0.1% V and 0.005-0.05% Ti, and the balance Fe with inevitable impurities and having <=10.5% Cr equiv. represented by the equation is prepared. The structure of the heat-affected zone becomes a single martensite phase and the toughness of the zone is enhanced.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a high-strength chromium steel with excellent weather resistance and weldability, particularly a 10 to 1a 5% chromium steel that exhibits stable high strength in the base metal part and high toughness in the welded part until welding. Regarding.

Various types of chromium steel have been known, but because of their generally poor weldability, they are often used as tool steels or bearing steels, and have rarely been used as structural steels.

However, since chromium steel exhibits excellent weather resistance due to its nature, if it can be used for structural grooves, it is expected to be in high demand due to its relatively low price. However, the weldability of chrome steel, especially the heat affected zone (h
eat affected ZOne1 and below,)IAZ
Several proposals have been made to improve the toughness of

For example, Special Publication No. 51-18468 and “Iron Manufacturing Research”
No. 292, 1977, pages 21-28, 10-
Regarding 18% Cr martensitic stainless steel, (C
It is disclosed that the ductility and toughness of the weld zone are improved by lowering the +N) content, and that the toughness is further improved by adding up to 34% Ni. Also, JP-A-55-215
No. 66 and [Kawasaki Steel Technical Report J Vol. 1142.
On pages 159 to 167, it is stated that simply lowering the (C+N) content does not necessarily improve the toughness of the weld, and that Ni
Recognizing that the presence of nickel actually accelerates the deterioration of the material due to hydrogen absorption during pickling, the (C+N) content was reduced to 0.02% or less, and the amount of Ni as an impurity was reduced to 0.01%.
Chromium steels limited to less than % are disclosed.

However, according to the findings of the present inventors, although it is important for welded structural steel to obtain good HAZ toughness as welded, it is not possible to simply make it low carbon and low nitrogen as in the conventional technology. It was not possible to obtain sufficient toughness.

Furthermore, an important point for structural steel is that it is possible to stably produce high-strength steel with good radius and - while maintaining good HAZ toughness. Since the strength after annealing rapidly decreases, it is difficult to stably produce high-strength steel of a desired level.

Here, in order to solve the problems of the conventional technology, the present inventors have conducted research and development for many years, and have decided to add appropriate amounts of one or more types of N h + V and Ti. It has been found that it is highly stable and exhibits round annealing characteristics, and furthermore, the amount of oxygen in the product steel is strictly limited, and as mentioned above, Nb and V.

The present invention was completed based on the discovery that stable high strength can be obtained and at the same time excellent HAZ properties can be obtained by adding Ti lal or 2'81 or more to an appropriate amount.

Thus, the present invention has the following characteristics by weight: C: 008% or less, Si: 10% or less, Mn540
% or less, Cr: 10-1a5%, Ni: less than 0.1%, Cu: 10% or less, N: 0.02% or less, 0
: (1007% or less, and Nb: 0005% or more, 1 or more, less than 005%, ■ = 002 to 01%, and Ti: α0
05 to α05%, and furthermore, the Cr equivalent represented by the following formula is 1α5% or less, and the C
r equivalent = % Cr + 4% Si + 2x% Ti ÷ ZVt
α5x%Nb-27x%C-04x%Mn-14X%N
1-05

In the present invention, the reason why the steel composition range is limited as described above is as follows.

C: In high chromium steel, carbon is effective as an element to increase the strength of the base metal, but on the other hand, it increases the hardness of the HAZ and at the same time decreases the HAZ toughness.

In addition, since the cold cracking susceptibility of welds increases with an increase in carbon content, the upper limit has been set to α08 from the viewpoint of weldability and weld performance.
%.

Si: Si is an effective element in steelmaking technology as a deoxidizing element, but if it exceeds 10%, HAZ toughness decreases, and it also coarsens ferrite grains that are partially formed during annealing after rolling, reducing toughness. lower. Therefore, in the present invention, St
The upper limit is set to 10%.

Mn: Mn expands the austenitization range at high temperatures and transforms the HAZ structure into martensite. therefore,)
Although it is effective in preventing the formation of ferrite that is harmful to the IAZ toughness, when added in excess, the martensite structure becomes brittle and furthermore, austenite remains untransformed in the HAZ, resulting in a decrease in toughness. For this reason, M
The upper limit of n was set to 4%.

Cr: Cr is an extremely effective element for weather resistance, which is one of the main purposes of this @Meiko steel. Weather resistance, especially in environments where seawater is splashed, is dramatically improved by adding 10% or more of Cr. , 14
When it exceeds 5%, the effect is saturated. Furthermore, when added in excess, the austenitization temperature range at high temperatures is narrowed, and the structure in the HAZ in particular comes to include coarse ferrite grains, resulting in a significant decrease in HAZ toughness. For this reason,
As detailed below, it is necessary to limit the Cr equivalent to 1α5% or less, and from this point of view, the upper limit of Cr is set to 1a5%.

Ni: Ni Vil (A7.I11 is effective as an element for stabilizing martensite in the weave, but when Ni is added, it tends to cause pitting corrosion in the pickling process during steel plate manufacturing, and the surface texture becomes rough. It is better to have as little Ni as possible, and the allowable content was set to less than 11%.

0: Oxygen is an element that is inevitably contained in the steelmaking process, but when QOO exceeds 7%, the IAZ toughness decreases significantly. In addition, when the oxygen content increases, the number of nonmetallic inclusions increases, and the number of surface defects in the rolled material increases, and when the rolled material is sheared, two-piece cracking defects originating from nonmetallic inclusions occur. This causes the occurrence of From these points, in the present invention, oxygen is strictly limited to Q007% or less.

CU: (Similar to Ni, U expands the austenite formation temperature range at high temperatures) and advantageously promotes the formation of martensite, which is effective for improving IAZ toughness. However, when added in excess, grain boundary cracking tends to occur at high temperatures, which not only reduces hot workability but also tends to cause hot cracking in the HAZ. Therefore, from these points, the upper limit of the intermediate Cu is set to LOLy in the present invention.

And so.

Nb, V, Ti: All of these elements are elements that form nitrides and carbides, and when added in appropriate amounts, these carbides and nitrides partially form a solid solution at high temperatures, and precipitate finely during the cooling process. Therefore, the softening resistance of the hot-rolled material is dramatically improved by such fine precipitates, and it becomes possible to easily maintain high strength even after annealing at a temperature higher than the recrystallization temperature. . Furthermore, compared to steel materials to which these elements are not added, the precipitation of Cr and N, which are harmful to toughness, is reduced, so HAZ toughness is improved. Excessive Nb causes the formation of intermetallic compounds during annealing and reduces toughness, so the amount added is set to 0005% or more and less than Q05%.

Moreover, since excessive Ti causes coarsening of TiN precipitated in the HAZ and deteriorates the toughness, the amount of Ti added was set to (1005% to 005%).

In order to cause an excessive decrease in Vti ductility and deterioration of bending performance, the amount of addition of ■ was set to α02 to 01%.

N: Like C, N increases the hardness of the HAZ and increases the sensitivity to cracking, so its upper limit was set at 0.02%.

Other elements: P and S decrease toughness, so the lower the content, the more desirable. Generally P and S are each 004%
0.008% or less and 0.008% or less, particularly preferably 0.008% or less and (101% or less.)
It is desirable to add it so that it is 0.007% or less, and the remaining HL amount is preferably 0.008 to 1%, but
These are not particularly restrictive.

Cr equivalent: Cr equivalent is expressed by the following formula.

Cr per t=%cr+-(i4x%S i +2X%T
i +-%V+ct5X%Nb-27X%C-04X%
Mn-14x%N1-Q5X%Cu-85X%N10~
In steel containing 1a5% Cr, it is possible to control the structure from a ferrite single phase to a ferrite/martensite mixed structure to a martensite single phase by changing the content of other components. . In particular, the toughness of the HAZ varies greatly depending on the components, that is, the structure, and since ferrite present in the HAZ reduces the toughness, it is necessary to control the structure so that it becomes a single phase of martensite. Cr determined by the above formula
When the t is IQ 5% or less, the HAZ composition area becomes a single phase of halaltensite, which improves the HAZ toughness.
In the present invention, the upper limit of Cr content is set to 105%.

The invention will now be further described in connection with examples.

Example 1 A test steel having the steel composition shown in Table 1 was melted in a small high-frequency vacuum melting furnace with a capacity of t17, and then subjected to hot forging and hot rolling to a plate thickness of 8 Ill, and then softened and annealed. and pickling treatment. Next, the weldability of the test piece cut out along the rolling direction was 1ffldfiL. The welding test was performed by MIG welding the abutted portions of the grooves using an 809 type welding rod with a core wire diameter of L6-. The test results are summarized in Table 1.

As can be seen from the results shown in Table 1, all the steels within the scope of the present invention give satisfactory results, but the comparative steels, such as samples 'l1f447 to 8, are particularly
It can be seen that the toughness is considerably reduced when the violet is not 105% or less. Also, as in sample copper 9, Ni content is 0.
When the amount is relatively high at 84%, the HAz toughness is slightly improved, but the surface roughness after pickling is significant. Although the oxygen content and nitrogen content of Test Copper Metals 10 and 11 exceed the range of the present invention, no improvement in HAZ toughness is observed in either case. In the test i@A12, V
is added in an amount exceeding the limit in the present invention, but 18
Cracking occurred in the 0 degree close bending test, indicating that the ductility of the base material had decreased.

Example 2 In this example, Example 1 was repeated, but in this example, Nb was used for the reference material having the composition shown in Table 2.

The mechanical properties such as the yield strength (Kff/-) and the HAZ toughness after softening annealing of the obtained steels were tested by changing the amounts of (1) and Ti added.

The results are summarized in Table 8.

As can be seen from the results shown in Table 8, Nb.

(2) and Ti both exhibit the effect of improving mechanical strength and toughness by their addition.

In the case of Nb, if it is added in an amount of 0.06% or more, the effect will be saturated, and the LAZ toughness will actually decrease. , v is also 01%
A similar trend is seen when exceeding . On the other hand, in the case of Ti, this tendency becomes remarkable when it exceeds 0.005%.

Example 8 This example was conducted to examine the effect of oxygen on the toughness of the HAZ. Example 1 was repeated except that the oxygen content was changed within the composition range shown in Table 4. A sub-size impact test piece with a 2lv notch was prepared to evaluate HAZ toughness. The results are summarized and shown in graphs in the attached drawings. Note that the absorbed energy is a value converted to a standard test piece.

As is clear from the graph shown in the numbered drawing, it can be seen that the HAZvJ properties are significantly improved by reducing the oxygen content to 0007% or less.

Table 4 Cr equivalent≦105 (%)

[Brief explanation of the drawing]

The attached drawing is a graph showing the influence of oxygen on HAZ toughness at -80°C.

Claims (1)

[Claims] By weight: C: 008% or less, st: Lo96 or less, Mn: 4
0% or less, Cr: 10-1a5% N1: α
Less than 1%, Cu: LO% or less, N: (L02
% or less, 0: αoorz or less, and Nb: α00
5% or more, α less than 05%, 72002-01% and T
1: Contains one or more of α005 to α06%, and further has a Cr equivalent represented by the following formula of 105% or less, Cr equivalent = %Cr + α4X%S i +2X%T
i÷%V+α6×%Nb-Nb-27x9α4×%M
n-L4x%Ni-α5X%Cu-85X%N High-strength chromium steel with excellent weather resistance and weldability, with the remainder consisting of iron and inevitable impurities.