EP0170598A1 - Process for manufacturing hot-rolled martensitic stainless steel rods or rod wire, and products obtained - Google Patents

Abstract

A process for the manufacture of martensitic stainless steel rods or machine wire, and the products thus obtained, by hot rolling, which steel comprises by % weight: C=0.015 to 0.090 N=0.015 to 0.080 with C+N=0.05 to 0.120 Cr=9.0 to 14.0 Nb=</=0.1 V=</=0.1 S=</=0.35 Si=</=1.0 Mn=</=1.0 Ni=</=2.0 Mo=</=1.0 P=</=0.040 Cu=</=1.0 Fe and impurities=balance and having the following mechanical properties: R=900 to 1100 MPa; E 0.2=650 to 850 MPa; A>/=10%; and resilience KCU>/=40 J/cm2, and wherein the preheating or the end of the hot rough preliminary processing preceding the final hot rolling brings the products to a temperature between 1000 DEG C. and 1160 DEG , and wherein the final hot rolling which is effected at a temperature below or equal to 1150 DEG C. produces a section reduction "S/s" which is at least equal to 3, and which reduction is followed by homogenous cooling and when S</=0.08% the rods or machine wire have the following mechanical properties: R=900 to 1100 MPa; E 0.2=650 to 850 MPa; A=12 to 16%; resilience KCU=80 to 140 J/cm2, and the rods or machine wire of the invention are particularly used for the manufacture of corrosion resistant mechanisms.

Description

The method of the present invention relates to a method of manufacturing bars or wire rod from stainless steel.

KNOWN STATE OF THE ART

The usual martensitic stainless steels with good mechanical resistance correspond to the designations according to standard NF A 35-575 _: "Z12C13". "Z20C13" - "Z30C13", with overall C 0.08 to 0.34% and Cr 11.5 at 14.0%. They are martensitic steels, that is to say having a predominantly martensite structure.

• - chauffage à 950 à 1050°C
• - trempe à l'huile
• - revenu entre 550 et 650°C

et on aboutit aux caractéristiques typiques suivantes :

• R - 900 à 1100 MPa - E 0,2 - 650 à 850 MPa - A - 12 à 16 % et résilience KCU - 20 à 60 J/cm2.

After hot rolling and cooling, they are hard and brittle, and to give them good mechanical resistance, a quenching treatment is applied to them, followed by tempering, such as:

• - heating to 950 to 1050 ° C
• - oil quenching
• - tempering between 550 and 650 ° C

and we end up with the following typical characteristics:

• R - 900 to 1100 MPa - E 0.2 - 650 to 850 MPa - A - 12 to 16% and resilience KCU - 20 to 60 J / cm2.

This resilience which partly accounts for the toughness is poor, and it can only be improved by remaining in the field of stainless steels by using more expensive steels such as "Z6CND 16-04" (standard NF A 35 -581) and "Z6CNU 17-04-01" (standard NF A 35-574), which in the quenched-tempered state give characteristics (R, E, A) of the same level as the previous ones with improved KCU resilience : 80 to 140 J / cm2.

EXPOSURE OF THE PROBLEM

We sought to obtain bars at around 13% Cr having mechanical characteristics of the same level as those of the more expensive stainless steels "Z6CND 16-06" and "Z6CNU 17-04-01", by simplifying or avoiding the heat treatment. quenching + tempering. We have thus sought to obtain martensitic bars of stainless steel at about 13% Cr having at the same time a good mechanical resistance, a good ductility, and in a new way a good resilience, with economical manufacturing conditions.

STATEMENT OF THE INVENTION

The invention consists in a selection of composition and in the setting of hot rolling conditions which surprisingly lead to the desired levels of properties in the raw hot rolling state.

The conditions of composition and the conditions of rolling are both necessary for obtaining these properties. The metallurgical examinations carried out make it possible to give qualitative indications on the effects of these conditions, effects which appear complex, and to indicate practical limits for the conditions of the process of the invention and the characteristics of the corresponding products.

• - C - 0,015 à 0,090 % et de préférence 0,030 à 0,060 %
• - N - 0,015 à 0,080 % et de préférence 0,020 à 0,050 % avec C + N - 0,05 à 0,120 % et de préférence C + N - 0,050 à 0,100%
• - Cr - 9,0 à 14,0 % et de préférence 11,0 à 14,0 % et de préférence encore 11,5 à 13,5 %
• - Nb ≤ 0,1 %
• - V ≤ 0,1 %
• - S ≤ 0,35 % avec 3 intervalles préférentiels

• S ≤ 0,03 % caractéristiques optimales
• S - 0,03 à 0,08 % caractéristiques mécaniques peu modifiées, usinabilité améliorée
• 0,08 < S ≤ 0,35 %, résilience moins bonne, usinabilité renforcée

• - Si ≤ 1,0 % - Mn ≤ 1,0 % - Ni < 2,0 % et de préférence 1,0 % - Mo ≤ 1,0%
• - P ≤ 0,040 %
• - Cu ≤ 1,0 %
• - autres éléments et Fe : le solde.

The stainless and semi-stainless steels of the invention have the following compositions (% by weight), the preferred ranges indicated can be taken separately or in any combination:

• - C - 0.015 to 0.090% and preferably 0.030 to 0.060%
• - N - 0.015 to 0.080% and preferably 0.020 to 0.050% with C + N - 0.05 to 0.120% and preferably C + N - 0.050 to 0.100%
• - Cr - 9.0 to 14.0% and preferably 11.0 to 14.0% and more preferably 11.5 to 13.5%
• - Nb ≤ 0.1%
• - V ≤ 0.1%
• - S ≤ 0.35% with 3 preferential intervals

• S ≤ 0.03% optimal characteristics
• S - 0.03 to 0.08% slightly modified mechanical characteristics, improved machinability
• 0.08 <S ≤ 0.35%, poorer resilience, enhanced machinability

• - If ≤ 1.0% - Mn ≤ 1.0% - Ni <2.0% and preferably 1.0% - Mo ≤ 1.0%
• - P ≤ 0.040%
• - Cu ≤ 1.0%
• - other elements and Fe: the balance.

The "other elements" are in the usual contents for an elaboration in electric steelworks from scrap, their total is usually less than 0.5%. In particular, the residual Al content is less than 0.1%.

The adjustment of the total content "C + N" is an essential point of the invention: it makes it possible to increase the mechanical resistance (R, E 0.2) of the product obtained and to maintain good resilience (KCU). An example will show the harmful influence on the resilience of a too high "C + N".

When the steel is loaded with S, and especially when its S content is between 0.08 and 0.30%, the method of the invention makes it possible to obtain raw hot rolling bars or wire rod which still have very interesting mechanical characteristics. With improved machinability. these products indeed have mechanical properties (R, E) of a very good level with a resilience that is lower the higher the S content, but overall greater than 40 J / cm2.

Any additions Nb ≤ 0.1% and V ≤ 0.1% have a hardening effect, essentially resulting in an improvement in the breaking load "R" and especially in the elastic limit at 0.2% "E 0.2".

An addition of nickel can be made if the cost is not considered excessive, mainly to improve the resilience. Such an addition tends to decrease the proportion of ferrite in the martensite / ferrite structure.

The rolling conditions necessary to obtain the mechanical characteristics of the bars or machine wires according to the invention are as follows: after the possible hot coarse-graining of the product, whether or not followed by a cooling, the product must be brought to a temperature between 1050 ° C and 1160 ° C before undergoing the final hot rolling, this preliminary setting to temperature being obtained either by preheating or reheating, or by roughing conditions causing this temperature when the product arrives at the final rolling. The final hot rolling of the product thus brought to between 1050 and 1160 ° C is then carried out in practice at a temperature less than or equal to 1150 ° C (the product cools down by 10 ° C or more during the initiation of this rolling), and it must produce a reduction in section "S / s", where "S" is the cross section of the product at the initiation of this final hot rolling and "s" the cross section obtained at the end of said final hot rolling , at least equal to 3. Tests have shown that the final hot rolling should preferably be finished between 1050 and 950 ° C, product temperatures. Finally, the final hot rolling must be followed by homogeneous air cooling. Accelerated cooling means such as blown air or mists (water + air) can be used, provided that the cooling remains homogeneous, that is to say that the cooling rates must differ little from one section right to another of the product. Preheating before the final hot rolling can also be carried out below 1050 ° C, for example between 1000 ° C and 1050 ° C, but the process while remaining applicable becomes more difficult to implement.

• . si S ≤ 0,08 % - R = 900 à 1100 MPa - E 0,2 - 650 à 850 MPa - A - 12 à 16 % . résilience KCU = 80 à 140 J/cm2
• . si S est supérieur à 0,08 % et inférieur ou égal à 0,35 % - R - 900 à 1100 MPa - E 0.2 - 650 à 850 MPa - A ≥ 10 % - résilience KCU ≥ 40 J/cm2.

As will be seen in connection with the tests, the adjustment of the rolling temperature is important at the same time as the adjustment of the composition for adjusting the ferrite content, the dissolved (C + N) content and the grain size on the product, factors all particularly important for the direct obtaining of the very surprising compromise of mechanical characteristics of the rough bars or wire rod of hot rolling according to the invention:

• . if S ≤ 0.08% - R = 900 to 1100 MPa - E 0.2 - 650 to 850 MPa - A - 12 to 16%. resilience KCU = 80 to 140 J / cm2
• . if S is greater than 0.08% and less than or equal to 0.35% - R - 900 to 1100 MPa - E 0.2 - 650 to 850 MPa - A ≥ 10% - resilience KCU ≥ 40 J / cm2.

The rods or wire rod according to the invention are recognizable by all of their mechanical characteristics and their analysis. these mechanical characteristics being singular for such an analysis. They are also characterized in addition by a proportion of ferrite in martensite less than 30% and typically between 15 and 25%, as well as by an average grain or phase diameter (martensite and ferrite) equivalent to 5 to 10 ASTM (specification ASTM E 112 ), i.e. 65 µm to 11 µm. These structural characteristics are largely responsible for the mechanical characteristics.

The bars according to the invention are in the form of raw hot rolling bars, or hot rolled then erected with optionally a surface finish, of diameter or thickness between 15 and 250 mm and preferably between 15 and 120 mm .

The wire rod with a diameter between 5 and 35 mm according to the invention is usually in the form of a crown or erect bars. At the end of the final hot rolling, the cooling is mainly done in offset crowns or turns.

TESTS AND EXAMS

• La figure 1 représente l'évolution de la proportion de ferrite en fonction de la température de préchauffage, dans le cas de la coulée (A) correspondant à la première série d'essais.
• La figure 2 situe les limites élastiques E 0,2 en fonction des % ferrite pour les barres obtenues dans les première et troisième séries d'essais.
• La figure 3 représente les domaines de teneurs extrêmes (C %, N %) des barres ou fils machine de l'invention.

The results of four series of tests and additional examinations will allow a better understanding of the invention and its various aspects.

• FIG. 1 represents the change in the proportion of ferrite as a function of the preheating temperature, in the case of the casting (A) corresponding to the first series of tests.
• Figure 2 locates the elastic limits E 0.2 as a function of the% ferrite for the bars obtained in the first and third series of tests.
• FIG. 3 represents the domains of extreme contents (C%, N%) of the bars or machine wires of the invention.

. First series of tests

• C - 0,038 - N - 0,029 donc C + N = 0,067
• Cr - 12,36 - V - 0,032 - S - 0.016 - Si - 0,27 - Mn - 0,42
• Ni - 0,28 - Mo - 0,07 - P - 0,019 . Cu - 0,11 impuretés inevitables et Fe : le solde.

A casting (A) was carried out in 250 mm squares having for analysis (% by weight):

• C - 0.038 - N - 0.029 so C + N = 0.067
• Cr - 12.36 - V - 0.032 - S - 0.016 - Si - 0.27 - Mn - 0.42
• Ni - 0.28 - Mo - 0.07 - P - 0.019. Cu - 0.11 unavoidable impurities and Fe: the balance.

The hot roughing was carried out in blooming as usual at about 1200-1250 ° C, transforming the 250 mm squares into 148 mm squares.

The cooled 148 mm squares were then preheated in an oven at various temperatures as indicated in the table and then hot-rolled in the final by successive passes to the diameters shown in the table and cooled in air. From the measurements carried out with an optical pyrometer, it was estimated that the temperature of the bars at the end of the rolling was in all cases between 950 and 1000 ° C.

RESULTS TABLE

These results show the excellent level of mechanical characteristics obtained for the bars A1 to A5 preheated depending on the case between 1080 and 1160 ° C and rolled according to the case between 1150 ° C and 950 ° C (bar temperature), with a reduction of minimum S / s section in the case of "A5" bars and then equal to:

The maximum section reduction is that of the A1 20 mm Ø bars with S / s - 69.

The case of "A6" bars preheated to 1240 "C and therefore hot rolled from 1220-1230 ° C clearly shows the harmful effect on the ductility (A%, Z%) and on the KCU resilience of this relative overheating It is above all the grains which are much coarser at mid-radius and at the core than in the preheated and rolled bars under the conditions of the invention which explain these poor results. By comparison with the bars "A5", it can be seen that the limit elastic obtained with these conditions of preheating and hot rolling is also reduced and that the breaking load also seems slightly affected.

. Second series of tests

A large bar from casting A was transformed by rolling and forging into 20 mm thick dishes, samples of which were brought 30 min to various heating temperatures ranging from 850 to 1300 ° C and then quickly cooled by quenching at 1 'water. On each sample, the proportion of "α%" ferrite in martensite was determined on a micrographic section. The results are shown in FIG. 1. The curve (f) which connects the figurative points calls for the following comments: for this flow A and for samples thus heated and quenched, α% passes through a minimum (2%) for the temperature 1050 ° C. The ferrite contents α% <10 are obtained for preheating temperatures between 950 and 1150 ° C. There is thus a qualitative indication: the adjustment of the preheating and rolling temperature is certainly important for the adjustment of the ferrite content. The ferrite content must be minimized to obtain good resilience and good mechanical resistance, therefore preheat and laminate in a limited temperature range depending on both of this effect under real dynamic conditions and other very important factors: dissolution and maintenance of C and N in solution to strengthen the matrix. obtaining a fine-grained structure (preheating and recrystallization during rolling).

. Third series of tests

• C - 0,050 - N - 0.038 donc C + N - 0,088 - Cr - 12,55
• S - 0,06 - Si - 0,34 - Mn - 0,45 - Ni - 0,19 - Mo - 0,05 - P - 0,019
• Cu - 0,11 - impuretés inévitables et Fe : le solde.

An analysis flow (B) was developed (% by weight):

• C - 0.050 - N - 0.038 so C + N - 0.088 - Cr - 12.55
• S - 0.06 - Si - 0.34 - Mn - 0.45 - Ni - 0.19 - Mo - 0.05 - P - 0.019
• Cu - 0.11 - unavoidable impurities and Fe: the balance.

The cast squares of 250 × 250 mm were hot transformed as before and similar tests were carried out starting from squares of 148 mm, with air cooling of the bars obtained. The temperature of the bars at the end of the rolling was between 950 and 1000 ° C. The preheating temperatures and the mechanical characteristics obtained on the bars are collated in the Table below:

These tests corresponding to a higher content (C + N) than in the first tests are a confirmation of this. The preheating temperature of 1180 ° C for "B4" strongly affects the resilience KCU and more weakly but clearly: first the ductility (A% and Z%), and then especially E 0.2. Reconciling these results with "B4" and "A6" clearly shows the progressive effect "overheating" with respect to the limit preheating temperature of 1160 ° C. (bars "A3") according to the invention.

Additional tests

The elastic limits of the bars of the two flows (A) and (B) (first and third series of tests) have been plotted on the graph in FIG. 2.

This graph shows that in the range of ferrite content α% from 18 to 35%, a reduction of α% of 10% corresponds on average to an increase of E 0.2 of approximately 100 MPa.

. Fourth series of tests

• C = 0,105 - N = 0,039 donc C + N = 0,144 - Cr - 12,19 - Nb - 0,073 - V = 0,073 - S - 0,015 - Si - 0,41 . Mn - 0,92 - Ni - 0,18 - Mo - 0,46 - P - 0,021 - Al - 0,02 - impuretés et Fe : le solde.

A steel (D) with a higher (C - N) content than those of the invention was tested, for analysis:

• C = 0.105 - N = 0.039 therefore C + N = 0.144 - Cr - 12.19 - Nb - 0.073 - V = 0.073 - S - 0.015 - Si - 0.41. Mn - 0.92 - Ni - 0.18 - Mo - 0.46 - P - 0.021 - Al - 0.02 - impurities and Fe: the balance.

• R - 1210 MPa - E 0,2 - 1060 MPa - A - 15 % striction Z % - 60 - KCU 5-10 J/cm2.

This steel was rolled into Ø 80 mm bars at 1100 ° C with an "S / s" ratio of 4. The mechanical characteristics obtained on these bars are:

• R - 1210 MPa - E 0.2 - 1060 MPa - A - 15% necking Z% - 60 - KCU 5-10 J / cm2.

We thus obtain with (C + N) also a very low resilience, and when (C + N), and even more at the same time (C + N) and S, increase, there is added to this very low resilience a risk of tapure for the bars.

Comments: effect of (C + N)

Figure 3 locates on the one hand the figurative points (C%, N%) of the flows (A) - (B) and (D), on the other hand the domain (E) of contents (C%, N%) according to the invention as well as the narrowest preferential domain (F).

The hardening of the bars, reflected by the increase in R and E 0.2, depends significantly on the content of (C + N) dissolved. Small additions of Nb ≤ 0.1% and / or V ≤ 0.1% also play a hardening role. In the case of the bars (D), the good characteristics R and E 0.2 are linked to the high content of (C + N) and to the small additions of Nb and of V, but the contents in (C. N) are outside the scope of the invention and the resilience is very low.

We have also seen in the first and third series of tests the influence of the preheating temperature and final hot rolling when one is in the field of compositions of the invention. By comparison with the conventional quenching treatment, which typically comprises a heating for dissolving between 950 and 1050 ° C., the effect of this preheating and of this hot rolling according to the invention is to dissolve at best (C and N ) and to keep them as well as possible in solution. While the increase in C also slightly increases the hardenability, the increase in N can contribute to the fineness of the recrystallization grain during rolling thanks to the precipitation of small nitrides.

• - la taille de grain,
• - la proportion de ferrite dans la martensite,
• - la dureté de la matrice,

dont le rôle paraît important pour comprendre qualitativement les résultats surprenants de l'invention. L'ajustement de (C + N) est un facteur particulièrement important.The metallurgical effects of C, N, C + N and the preheating and rolling temperatures thus appear complex and "entangled", because they influence in varying degrees:

• - grain size,
• - the proportion of ferrite in martensite,
• - the hardness of the matrix,

whose role seems important to understand qualitatively the surprising results of the invention. The adjustment of (C + N) is a particularly important factor.

The process is particularly suitable for the production of rods or wire rod with continuous hot rolling means.

The bars or machine wires of the invention are used in particular for the manufacture of mechanical parts resistant to corrosion, working in contact water, steam, wine or beer: such as trees, pistons, liners, valves or hardware.

Claims (10)

1. A method of manufacturing bars or wire rod in martensitic stainless steel, in which a steel is produced and it is cast in the form of products, then in which the cast products are transformed by hot working including a possible hot roughing, a possible preheating and a final hot rolling, characterized in that a steel of composition (% by weight) is produced: C - 0.015 to 0.080% and N - 0.015 to 0.080% with C + N - 0.05 to 0.120% ; Cr 9.0 to 14.0%; Nb ≤ 0.1%; V ≤ 0.1%; S ≤ 0.35%; If ≤ 1.0%; Mn <1.0%; Ni ≤ 2.0%; Mo ≤ 1.0%; P ≤ 0.040%, other elements and Fe: the balance, and in that the preheating or the end of hot roughing preceding the final hot rolling brings the product to a temperature between 1000 ° C and 1160 ° C, and in that the final hot rolling carried out at a temperature less than or equal to 1150 ° C. produces a reduction in section "S / s" at least equal to 3 and is followed by homogeneous cooling in air or with a mist (water + air). 2. Method according to claim 1, characterized in that the steel produced contains: Cr - 11.0 to 14.0%. 3. Method according to any one of claims 1 or 2, characterized in that the steel produced contains: C - 0.030 to 0.060% and N - 0.020 to 0.050% with C + N ≤ 0.100%. 4. Method according to any one of claims 1, 2 or 3, characterized in that the preheating or the end of hot roughing preceding the final hot rolling brings the product to a temperature between 1050 ° C and 1160 ° C . 5. Method according to any one of claims 1 to 4, characterized in that the final hot rolling is finished between 1050 and 950 ° C (product temperature). 6. Martensitic stainless steel bar or wire, characterized in that it has the following composition (% by weight): C - 0.015 to 0.090% and N - 0.015 to 0.080% with C + N - 0.05 to 0.120% Cr - 9.0 to 14.0%; Nb ≤ 0.1%; V ≤ 0.1%; S ≤ 0.35%; If ≤ 1.0%; Mn 1.0%; Ni ≤ 2.0%; Mo ≤ 1.0%; P ≤ 0.040%; Cu ≤ 1.0%; other elements and Fe: the balance, and in that it has mechanical characteristics: R - 900 to 1100 MPa; E 0.2 - 650 to 850 MPa; A ≥ 10%; KCU resilience ≥ 40 J / _cm ² _. 7. Rod or wire rod according to claim 6, characterized in that it or it contains: C - 0.030 to 0.060% and N - 0.020 to 0.050% with C + N ≤ 0.100%. 8. Rod or wire rod according to any one of claims 6 or 7, characterized in that it or it contains S ≤ 0.08% and in that it or it has as mechanical characteristics: R - 900 to 1100 MPa; E 0.2 - 650 to 850 MPa; A - 12 to 16%; KCU resilience - 80 to 140 J / cm ² . 9. Rod according to any one of claims 6 to 8, of diameter or thickness between 5 and 35 mm, in erect bars or in crowns. 10. Bar according to any one of claims 6 to 8, of diameter or thickness between 15 and 250 mm.

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